Dissertations / Theses on the topic 'Ferritic steel. Welding'

Stainless steels are a classification of materials that have been available for over 100 years and over that time manufacturers have created variations on chemical composition and manufacturing route, to create materials that meet specific criteria set by the consumer. One type of stainless steel, ferritic, is restricted in applications as a result of a reduction in properties, namely toughness, when it is welded as a result of grain coarsening in the heat affected zone. Welding equipment manufacturers are constantly incorporating new technologies and capabilities into welding equipment, to make welding easier and create better welds, which then gives that manufacturer a competitive advantage. Cold Metal Transfer (CMT) welding is one such innovation and is claimed by the manufacturer to be a lower heat input process. This research project examines the effects of this lower heat welding process, on the joining of ferritic stainless steels to determine if CMT can reduce the detrimental effects, seen in this material, through welding. The research examines the mechanical and metallurgical effects of using the Cold Metal Transfer (CMT) welding process to weld various grades of ferritic stainless steel including, EN1.4016, EN1.4509, EN1.4521 and EN1.4003 and compares them to welds created using a standard Gas Metal Arc Welding (GMAW) technique, with comparisons made using tensile testing, hardness testing, impact testing, fatigue testing and microstructural characterisation. Experimental results show that grades such as EN1.4016 and EN1.4003 are more sensitive to the welding process due to a phase change to martensite present within the heat affected zone. Work has been conducted to determine the temperature at which ferrite transforms to austenite, prior to transformation to martensite under non equilibrium cooling. Some of the findings from this work included; Fatigue testing and microstructural characterisation has shown a benefit in properties for using CMT over the conventional GMAW process for the EN1.4003 material. A relationship has also been proposed which examines the effect of the percentage of fusion zone defects on the fatigue life of the welded joints. Overall it was found that there was variation in the voltage and current by 1.9 Volts and 15 Amps respectively through a 400mm weld. The ALC settings from -30% to +30% affected the net heat input by 6J/mm NDT techniques utilised in the study were ineffective at detecting the lack of side wall fusion evident in some of the welds.

This study aimed to investigate the similar welding of several ferritic and austenitic stainless steels (AISI 304L, AISI 316L, AISI 410S and AISI 444) by the friction stir welding process (FSW), evaluating operational and metallurgical aspects to produce joints without defects. The FSW welding of four materials in this study was performed at the Helmholtz-Zentrum Geesthacht (Germany) due to the establishment of a cooperation agreement with the Universidade Federal do CearÃ. The welding parameters range were based on papers found in literature, for the other kinds of stainless steel, different this study, changes the parameters were changed to determine an acceptable combination of surface finish, absence of cracks and good penetration. The better welding conditions based on surface finish and defects free for each welded steels were subjected to mechanical evaluation through tensile test, bending test and microhardness test. In the same way, for the better welded conditions, samples were extracted to metallographic preparation and evaluated by light microscopy and scanning electron microscopy. The results from microscopy techniques allows the correlation between microstructure with the microhardness profiles, as well as the causes of low mechanical properties for same welds by the identification of defects in the stir zone such, as voids and lack of penetration. The ferritic stainless steels welds showed the best results in mechanical assessment due to grain refinement that occurred in the stir zone and thermomechanical affected zone. The calculation of equivalent heat input showed that for stainless steel AISI 304L and AISI 410S, higher values of heat input determine the highest incidences of chromium carbide precipitation and other possible deleterious phases in the stir zone, however DL-EPR technique and mechanical testing have shown that it is possible to weld the stainless steels by FSW process with excellent corrosion resistance and good mechanical properties.
Este trabalho teve como objetivo, investigar a soldagem similar de diversos aÃos inoxidÃveis ferrÃticos e austenÃticos (AISI 304L, AISI 316L, AISI 410S e AISI 444) pelo processo friction stir welding (FSW), avaliando aspectos operacionais e metalÃrgicos para a produÃÃo de juntas sem defeitos. A soldagem FSW dos quatro materiais em estudo foi realizada no Helmholtz-Zentrum Geesthacht (Alemanha) devido ao estabelecimento de um acordo de cooperaÃÃo com a Universidade Federal do CearÃ. Os parÃmetros de soldagem foram baseados em trabalhos anteriores, para outros tipos de aÃos inoxidÃveis, diferentes deste estudo, e a variaÃÃo realizada na busca de parÃmetros que determinassem uma combinaÃÃo entre acabamento superficial aceitÃvel, ausÃncia de trincas e boa penetraÃÃo. As melhores condiÃÃes soldadas para cada aÃo, com base no acabamento superficial e na ausÃncia de defeitos, foram submetidas a avaliaÃÃo das propriedades mecÃnicas atravÃs dos ensaios de dobramento, microdureza e traÃÃo. Da mesma forma, para as melhores condiÃÃes soldadas, amostras foram extraÃdas para preparaÃÃo metalogrÃfica e avaliadas por microscopia Ãtica e microscopia eletrÃnica de varredura. Os resultados da anÃlise microscÃpica possibilitaram a correlaÃÃo entre microestrutura com os perfis de microdureza, bem como as causas do baixo desempenho mecÃnico para os aÃos inoxidÃveis austenÃticos, com a identificaÃÃo de defeitos na zona de mistura, tais como vazios e falta de penetraÃÃo. Os aÃos inoxidÃveis ferrÃticos apresentaram os melhores resultados na avaliaÃÃo mecÃnica devido ao intenso refino de grÃo que ocorreu na zona de mistura e na zona termomecanicamente afetada. O cÃlculo do aporte tÃrmico equivalente mostrou que para os aÃos inoxidÃveis AISI 304L e AISI 410S, elevados valores de aporte tÃrmico determinam maiores incidÃncias de precipitaÃÃo de carbonetos de cromo e de outras possÃveis fases deletÃrias na zona de mistura, entretanto a tÃcnica EPR-DL e os ensaios mecÃnicos comprovaram que à possÃvel soldar os aÃos inoxidÃveis pelo processo FSW com excelente resistÃncia a corrosÃo e boas propriedades mecÃnicas.

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This thesis explores the processing-microstructure-property relationships in friction stir welded (FSW) HT9A ferritic-martensitic steel. HT9 has previously been studied as a structural component for fusion/fission based reactors; however, the changes in material microstructure and properties after friction stir welding have not been considered. HT9A steel plate was friction stir welded with a series of increasing heat inputs. The microstructure of this welded material was characterized using optical and electron microscopy. The mechanical properties of the welded material were determined using nanoindentation and microhardness measurements. In addition, electrochemical impedance spectroscopy (EIS) in molten lithium fluoride was used to assess the high temperature corrosion resistance of the welded material in the harsh environments found in fusion reactors. The quality of the friction stir welds was excellent, and the basic ferritic-martensitic microstructure was maintained for all of the conditions used. Some reduction in hardness was observed in the welded material, particularly in the heat affected zones. The high temperature corrosion response of the welded material was comparable to, or slightly better than, the base plate material.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico
The use of stainless steels has intensified with the industrial demand growing, which extends its application for various sectors such as the oil and gas, desalination equipment in industry, sugar industry, among others. In that context, the use of ferritic stainless steels has grown in recent years on account of its excellent relationship between corrosion resistance and cost, and a great option in substitution of austenitic stainless steels. Intending to study the connection of dissimilar stainless steels by means of a welding process, this paper will lay the submit the ferritic stainless steel AISI 444 and AISI 316L austenitic stainless steel with TIG welding autogenous (without filler metal) with pulsed current. That union seeks to get a fused zone with better mechanical properties together with the correction of possible related to welding those steels problems, such as grain growth in ferritic steels, to which its refinement is possible through the use of pulsed current during the procedure. The choice of these two materials was based on the characteristics of each one separately for they possess closest properties, despite having different classifications, allowing the combined use of both, and thus ferritic act in order to partially replace the austenitic stainless steel in situations where the combination of high corrosion resistance and mechanical strength are not relevant. That action combined, and does not affect the characteristics of the set of negative way is to use lower cost benefit, because the presence of nickel austenitic stainless steels by more expensive finishes them. Thus, it is expected to provide, through this work, further deepening the respect of dissimilar welding between stainless steel AISI 444 ferritic and austenitic stainless steel AISI 316L, evaluating operational parameters such as the pulse of current and heat input on obtained microstructure and mechanical properties.
A utilizaÃÃo dos aÃos inoxidÃveis tem se intensificado juntamente com a crescente demanda industrial, em que sua aplicaÃÃo se estende pelos mais variados setores, como por exemplo, na indÃstria de petrÃleo e gÃs, em equipamentos de dessalinizaÃÃo, na indÃstria sucroalcooleira, entre outros. Neste contexto, o uso de aÃos inoxidÃveis ferrÃticos tem crescido nos Ãltimos anos devido a sua excelente relaÃÃo entre resistÃncia à corrosÃo e custo, sendo uma Ãtima opÃÃo em substituiÃÃo aos aÃos inoxidÃveis austenÃticos. Objetivando estudar a uniÃo de aÃos inoxidÃveis dissimilares por meio de um processo de soldagem, o presente trabalho submeterà o aÃo inoxidÃvel ferrÃtico AISI 444 e o aÃo inoxidÃvel austenÃtico AISI 316L à soldagem TIG autÃgeno (sem metal de adiÃÃo) com corrente pulsada. Essa uniÃo visa obter uma zona fundida com melhores propriedades mecÃnicas juntamente com a correÃÃo dos possÃveis problemas relacionado à soldagem desses aÃos, como por exemplo, o crescimento de grÃo nos aÃos inoxidÃveis ferrÃticos, em que seu refinamento se torna possÃvel atravÃs da utilizaÃÃo de corrente pulsada durante o procedimento. A escolha desses dois materiais baseou-se nas caracterÃsticas inerentes a cada um separadamente e tambÃm por possuÃrem propriedades muito prÃximas, apesar de possuÃrem classificaÃÃes diferentes, permitindo a utilizaÃÃo combinada de ambos e dessa forma o aÃo inoxidÃvel ferrÃtico atuarà de forma a substituir parcialmente o aÃo inoxidÃvel austenÃtico nas situaÃÃes em que a combinaÃÃo de elevada resistÃncia à corrosÃo e resistÃncia mecÃnica nÃo sÃo tÃo relevantes. Essa utilizaÃÃo combinada, alÃm de nÃo afetar as caracterÃsticas do conjunto de maneira negativa tem como benefÃcio reduzir custos, visto que a presenÃa de nÃquel nos aÃos inoxidÃveis austenÃticos acaba por encarecÃ-los. Dessa forma, espera-se fornecer atravÃs deste trabalho um maior aprofundamento a respeito da soldagem dissimilar entre o aÃo inoxidÃvel ferrÃtico AISI 444 e o aÃo inoxidÃvel austenÃtico AISI 316L, avaliando os parÃmetros operacionais, como a pulsaÃÃo da corrente e a energia de soldagem sobre a microestrutura obtida, bem como as propriedades mecÃnicas.

Ferritic stainless steel is typically used in the automotive industry to fabricate welded tube that is plastically deformed for flanging, bending and necking. The effect of welding parameters during autogenous gastungsten arc welding (GTAW) of thin sheet on the weld metal structure and tensile properties were determined. Two grades of ferritic stainless steels, a titanium-containing Grade 441 and a titanium-free molybdenum-containing Grade 436, were used as base metal. Statistical analysis was used to determine the influence of welding parameters on the microstructure of autogenous GTAW welds. The results of Grade 441 indicated that the welding speed and peak welding current had a statistically significant influence on the amount of equiaxed grains that formed. For Grade 436, the same welding parameters (welding speed and peak welding current) had a statistically significant influence on the grain size of the weld metal grains. The ductility of a tensile test coupon machined parallel to the weld direction, for both base metal grades, was unaffected by the welding parameters or the weld metal microstructure. The elongation was determined by the amount of weld metal in the gauge area of a tensile coupon. The titanium content of the base material seems to have the most significant effect on the formation of equiaxed grains.
Dissertation (MEng)--University of Pretoria, 2019.
Metallurgical Engineering
MEng
Unrestricted

This master thesis was carried out at Scania CV AB. The focus for this thesis is the prediction of welding distortions that can cause problems in the manufacturing process of Scania's after-treatment system. The after-treatment system is mainly assembled by sheet metal plates of the ferritic stainless steel EN 1.4509. The plates are welded together. When welding, distortions and residual stresses occur, and they also depend on the sequence in the component was welded together. The distortions and residual stresses can cause tolerance related issues and a lower lifetime for the welded components. Experiments are expensive and therefore it is desirable to simulate the welding process, thereby controlling distortions and optimizing welding sequences. To simulate the welding process and predict the welding distortions a thermo-mechanical FE-model was created for two typical welds found on the after-treatment system. The first scenario was two thin plates welded onto each other in an overlap weld joint and the second scenario was a thin plate welded onto a thick plate in a overlap weld joint. After the FE-model was compared to the experiments. An optimization of the welding sequences was also made on a larger component typically found on the after-treatment system. The FE-model can predict the distortion shape with good accuracy for the T-fillet weld, while the model predicted a more symmetric distortion shape on the overlap weld compared to a more asymmetric shape found on the experiments, but the error is still not very large. The Fe-model can also be used to optimize the welding sequence for bigger components on the after-treatment system within a reasonable time span compared to doing the opimization manually in an experiment.
Detta examensarbete gjordes för Scania CV AB. Fokus for detta examensarbete har varit kvarvarande deformationer efter svetsning som kan skapa problem vid tillverkningen av Scanias avgasefterbehandlingssystem. Avgasefterbehandlingssystemet är till mesta dels konstruerat av stålplåtar av det ferritiska rostfria stålet EN 1.4509, plåtarna är svetsade ihop och då uppstår kvarvarande deformationer. När komponenter svetsas samman uppstår deformationer och restspänningar. Dessa deformationer och restspänningar är också beroende på i vilken sekvens komponenterna har svetsats ihop. Deformationerna och restspänningarna kan skapa problem med toleranser och sänka livslängden för komponenterna som sammanfogats. Experiment är kostsamma och därför är det önskvärt att simulera svetsprocessen, och därav kontrollera deformationerna som uppstår och optimera i vilken sekvens som komponenterna ska svetsas ihop. För att simulera svetsprocessen och prediktera de kvarvarande deformationerna efter svetsning så gjordes termo-mekanisk FE-model för två vanliga svetsscenarion för avgasefterbehandlingssystemet. Det ena scenariot är två tunna plåtar som svetsas ihop i en överlappande position och det andra var en tunn plåt som svetsas på en tjockare plåt. Ett experiment gjordes sedan för båda svetstyperna. Efter att svetstyperna hade jämförts med experimentet så gjordes en optimering av svetssekvensen för en större komponent likt komponenter funna på avgasefterbehandlingssystemet. Den termomekaniska FE-modelen kunde prediktera de kvarnvarande deformationerna och deras form med bra noggrannhet jämfört med experimentet med undantag för en deformationsform på de tunna plåtarna som var mer symmetrisk i FE-modellen jämfört med den asymmetriska formen i experimentet. FE-modellen kunde också användas för att optimera svetssekvensen för den större komponenten inom en rimlig tidsrymd.

Abstract Stabilized ferritic stainless steel grades are attractive alternatives to common austenitic grades in sheet metal applications. Compared with older unstabilized ferritic grades, the mechanical and corrosion properties are usually improved. The impurity level, mainly the amount of interstitial carbon and nitrogen, plays an important role in these steels. There are notable issues in the welding of these steels, the most apparent difference to austenitic steels is the susceptibility to brittle failure. This research focused on the influence of minor elements, especially aluminium, calcium, silicon, titanium, niobium, nitrogen and oxygen, on the weldability of modern intermediate purity level stabilized ferritic stainless steels. The research proceeded in several stages. At first, the general characteristics and performance data about the welds in currently manufactured 11 to 21 mass percent chromium ferritic stainless steels in Europe was obtained. The research then focused on novel high chromium stabilized ferritic stainless steels. Lastly, the influence of various steelmaking practices on weldability were investigated. The results showed that in stabilized ferritic stainless steels, slag islands are frequently seen in the molten weld pools. These islands can have many origins, e.g. deoxidation, calcium treatment and stabilization practices, and they can be roughly assessed from the chemical composition of the steel. The nature and the influence of these slags varies and can be related to irregularities in the weldability and molten metal fluid flow. Large grain size and titanium carbonitride particles impair the toughness of the heat-affected zone. Generally, stabilization with niobium is preferred. However, solely niobium stabilized steel welds run the risk of forming coarse columnar grains in welds deteriorating some of the properties. A breakdown of the columnar grains is possible to achieve in autogenous welds with minor titanium and aluminium alloying, provided that small amounts of nitrogen and oxygen are induced from the shielding gas. However, grain refinement may not improve the properties, if it is accomplished with an increase in the total interstitial content
Tiivistelmä Stabiloidut ferriittiset ruostumattomat teräkset soveltuvat korvaamaan tavanomaisia austeniittisia ruostumattomia teräksiä ohutlevysovelluksissa. Näillä teräksillä keskeiset mekaaniset ja korroosio-ominaisuudet ovat usein paremmat kuin varhaisilla, stabiloimattomilla ferriittisillä teräksillä. Hiili ja typpi ovat näissä teräksissä kuitenkin epäpuhtauksia. Toisin kuin austeniittiset teräkset, ferriittiset teräkset ovat alttiita haurasmurtumalle, erityisesti hitsatuissa rakenteissa. Tässä väitöstutkimuksessa keskityttiin mikroseosaineiden ja epäpuhtauksien vaikutukseen keskipuhtaiden stabiloitujen ferriittisten teräslajien hitsauksessa. Tutkimus kohdistui erityisesti alumiinin, kalsiumin, piin, titaanin, niobin, typen ja hapen vaikutuksiin. Aluksi tutkittiin kaupallisten terästen hitsien keskeisiä ominaisuuksia. Tämän jälkeen tutkittiin uusia ns. korkeakromisia stabiloituja ferriittisiä teräslajeja. Lopuksi tutkittiin teräksen valmistuksen vaikutuksia stabiloitujen ferriittisten ruostumattomien terästen hitsattavuuteen. Tutkituilla teräksillä hitsauksen aikana muodostui runsaasti kuonalauttoja. Näillä kuonilla on monta alkuperää, esim. deoksidointi, kalsiumkäsittely ja stabilointiaineet. Hitsien kuonaisuutta voidaan karkeasti arvioida teräksen kemiallisen koostumuksen perusteella. Muodostuvilla kuonilla on useita vaikutuksia hitsauksessa, mm. epäjatkuvuuksiin ja sulan virtauksiin. Hitsauksessa muodostuva suuri raekoko ja stabiloinnin titaanikarbonitridipartikkelit heikentävät oleellisesti hitsin muutosvyöhykkeen sitkeyttä. Stabilointi käyttäen pääasiassa niobia on toivottavaa, mutta jos stabilointiin käytetään vain niobia, tulee hitsin mikrorakenteesta karkea ja hitsin ominaisuudet voivat heikentyä. Karkean mikrorakenteen hienontaminen on mahdollista käyttäen suojakaasuna argonia, jossa on hieman typpeä ja happea, mikäli teräkseen on seostettu hieman alumiinia ja titaania. Raerakenteen hienontaminen ei kuitenkaan yksiselitteisesti paranna hitsin ominaisuuksia, mikäli hienontaminen saavutetaan kasvattamalla epäpuhtauspitoisuutta tarpeettoman korkeaksi

Grade 91 (Gr.91) is a common structural material used in boiler applications and is favored due to its high temperature creep strength and oxidation resistance. Under cyclic stresses, the material will experience creep deformation eventually causing the propagation of type IV cracks within its heat-affected-zone (HAZ) which can be a major problem under short-term and long-term applications. In this study, we aim to improve this premature failure by performing a computational thermodynamic study through the Calculation of Phase Diagram (CALPHAD) approach. Under this approach, we have provided a baseline study as well as simulations based on additional alloying elements such as manganese (Mn), nickel (Ni), and titanium (Ti). Our simulation results have concluded that high concentrations of Mn and Ni had destabilized M23C6 for short-term creep failure, while Ti had increased the beneficial MX phase, and low concentrations of nitrogen (N) had successfully destabilized Z-phase formation for long-term creep failure.

Conselho Nacional de Desenvolvimento Científico e Tecnológico
The use of active flux in TIG process can be an alternative to increase the productivity since this technique increases the depth of penetration of weld bead maintaining the same parameters of conventional TIG welding. The results obtained indicate that the use of the traditional way of flux application tend to produce slag, which is hard to remove. In order to avoid this problem, are approach is by applying two flux strips alongside the joint, separated by a predetermined distance, defined as parameter \"a\". This technique is called Contraction Controlled Technique (CCT). The result with this methodology was satisfactory, allowing increasing the depth of penetration and ensuring a good weld bead surface finish. The objective of this research is to verify and analyze the influence of the use of Contraction Controlled Technique with the A-TIG process in the welding of ferritic stainless steel. It also aims to identify and quantify the main phenomena involved such as increase of the penetration, the changes in the arc, and changes in mechanical and metallurgical properties of the ferritic steel. For this purpose, it were done welding of ferritic stainless steels surfaces using the Contraction Controlled Technique, varying the width of the parameter \"a\", the distance between the piece and the electrode, the electrical current and welding speed. It was observed that the geometry of the weld beads has suffered modifications such as reduction in width and increased of the penetration of the weld beads for all cases where the active flux was used. The use of the active flux in TIG welding process is a technique that allows the increased of penetration depth of weld maintain keeping the same parameters used in the conventional TIG. However, it was not possible to increase the speed of the welding within the parameters used. When it was employed the technique of controlled contraction in a disalignment joint, the data showed that it is possible to improve results of arc direction, direct it to the edge of the board, using active flux and modifing both the parameter \"a\" and the deviation of the torch (dt). The application of this technique did not cause changes in the toughness or in the microstructure of the fusion zone the ferritic stainless steel.
O uso de fluxo ativo no processo de soldagem TIG pode ser uma alternativa para aumentar a velocidade de soldagem do processo, pois esta técnica permite o aumento da profundidade de penetração do cordão de solda mantendo os mesmo parâmetros empregados no TIG convencional. A principal desvantagem desta técnica reside principalmente na propensão à formação de uma camada oxidada sobre o cordão de solda o que pode também acarretar num acabamento superficial ruim. Uma forma de tirar proveito desta técnica garantindo um bom acabamento do cordão é aplicar o fluxo de tal maneira que, ao invés de uma camada que cubra toda a superfície da chapa, sejam feitas duas faixas de fluxo, separadas por uma distância predeterminada, definida como parâmetro a . Esta técnica é chamada neste trabalho de Técnica da Constrição Controlada (TCC). Desta forma, a camada de fluxo isola eletricamente certas regiões da superfície da peça direcionando a região de incidência do arco e, como a diluição do fluxo na poça de solda é menor, alia-se a vantagem do aumento de penetração com o bom acabamento do cordão de solda. O objetivo desta pesquisa é verificar e analisar a influência do uso da Técnica da Constrição Controlada com o processo A-TIG na soldagem do aço inoxidável ferrítico; identificar e quantificar os principais fenômenos envolvidos como, aumento da penetração, alterações no arco voltaico e as possíveis modificações nas propriedades mecânicas e metalúrgicas do aço ferrítico. Para isso, realizou-se a soldagem do aço inoxidável ferrítico empregando a técnica da constrição controlada variando-se a largura do parâmetro a , a distância eletrodo-peça, a corrente de soldagem e a velocidade de soldagem. Foi possível observar que a geometria do cordão de solda sofre modificações como redução na largura e aumento da penetração do cordão de solda para todos os casos em que se empregou o fluxo ativo. Porém, não foi possível um aumento da velocidade de soldagem com os parâmetros empregados. Quando empregada a Técnica da Constrição Controlada em uma junta desalinhada, os dados comprovaram que é possível conseguir resultados de desvios satisfatórios no arco, ou seja, direcioná-lo para a borda da junta, com a utilização de fluxo ativo e a modificação tanto do parâmetro a quanto do desalinhamento na tocha (dt). A aplicação desta técnica não provocou alterações nem na tenacidade nem na microestrutura da zona fundida.
Doutor em Engenharia Mecânica

AgÃncia Nacional do PetrÃleo
Superduplex stainless steels (SDSS) may be defined as a family of steels having a two-phase ferritic-austenitic microstructure and the good mechanical properties and high corrosion resistance of this alloy are attributed to this microestrutural balance. These excellent qualities attribute to SDSS great employability in the oil sector, where manufacturing and equipment maintenance are performed by welding, which if not executed properly, can have a negative effect on the metallurgical properties and this problem becomes more critical in multipass welding due to repeated thermal cycles. Among the several welding processes employed in the welding of SDSS, fell to this work was to evaluate the FCAW process in multipass welding joints of SDSS UNS S32750, regarding the selection of appropriate welding parameters and the influence of these parameters on microstructural transformations, mechanical properties and corrosion resistance of welded joints. Then, this work was divided into three steps: Stage 1 was the characterization of the base metal in as-received condition. In Stage 2, weldings were accomplished using a bead on plate (BOP) technique to determine the control factors and their levels to be used in the subsequent stage, in which an experimental design was conducted by Taguchi method with Alloy, Stick out, Shielding gas, welding gun orientation, Arc oscillation, Energy technique and Heat input used as control factors and quality characteristics were evaluated the ratio R/L, bead penetration, ferrite content and inclusions content and microhardness. Finally, in Stage 3 were performed multipass welding in joints and afterwards was executed: A microstructural characterization in three regions of Fusion Zone (Root, Filler and Cap) and Heat Affected Zone by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) and were also performed a measurement of the ferrite content; measurement of inclusions content; microhardness tests and Critical Temperature Pitting (CPT) tests according to ASTM G150. The results showed, ferrite contents ranging from 44% and 54.9% and their highest values were observed when employed the alloy 2507, heat input of 1.6 kJ/mm and pulsed current. The lowest inclusions contents were obtained by using of shielding gas with the mixture of 96%Ar + 4%CO2 (Stage 2) and heat input of 1,6kJ/mm (Stage 3). The microhardness values in FZ of joints were not affected significantly by any of the control factors. Already, the results of CPT tests showed that pits nucleated preferentially within α, at α/γ grain boundaries and at interfaces areas between large diameter inclusions and the metallic matrix and propagated predominantly into α phase. The CPT values ranging from 47 ÂC to 78 ÂC, with highest were obtained for the alloy 2507, heat input of 1.6 kJ/mm and continuous current into Root region and for the alloy 2507, heat input of 1.6 kJ/mm and the pulsed current in Filler and Cap regions. From the three regions of FZs, the Root showed the best results with no defects, satisfactory ferrite contents, lowest inclusions contents and highest CPT values.
Os aÃos inoxidÃveis superduplex (AISDs) possuem uma microestrutura bifÃsica constituÃda por ferrita (α) e austenita (γ) e quando devidamente balanceadas conferem ao material, boas propriedades mecÃnicas e elevada resistÃncia à corrosÃo. Qualidades que atribuem aos AISDs grande empregabilidade no setor petrolÃfero, onde a fabricaÃÃo e manutenÃÃo de equipamentos sÃo realizadas por soldagem e se executada inadequadamente, pode afetar negativamente as propriedades destes aÃos, o que se torna mais crÃtico quando hà a imposiÃÃo de inÃmeros ciclos tÃrmicos durante uma soldagem multipasse. Dentre os inÃmeros processos de soldagem utilizados na soldagem dos AISDS, coube a este trabalho avaliar o processo arame tubular na soldagem multipasse em juntas do AISD UNS S32750, levando em consideraÃÃo a seleÃÃo dos parÃmetros de soldagem adequados, bem como a influÃncia destes parÃmetros nas alteraÃÃes microestruturais, microdureza e resistÃncia à corrosÃo das juntas soldadas. Desta forma, dividiu-se este trabalho em trÃs etapas: A Etapa 1 consistiu na caracterizaÃÃo do metal de base na condiÃÃo como recebido. Jà na Etapa 2 foram realizadas soldagens por simples deposiÃÃo para selecionar os fatores de controle e seus nÃveis a serem utilizados na etapa posterior, e o planejamento experimental foi realizado pelo mÃtodo Taguchi com a Liga, GÃs, DBCP, TÃcnica da Tocha, Tecimento, TÃcnica de Energia e Energia foram utilizados como fatores de controle e como variÃveis de resposta escolheu-se a razÃo R/L, a penetraÃÃo, a fraÃÃo de ferrita, a fraÃÃo das inclusÃes e a microdureza. E por fim, na Etapa 3 foram realizadas soldagens multipasse em juntas e posteriormente efetuou-se: uma caracterizaÃÃo microestrutural em trÃs regiÃes da Zona Fundida (Raiz, Enchimento e Acabamento) e Zona Afetada pelo Calor atravÃs de Microscopia Ãtica (MO) e Microscopia EletrÃnica de Varredura (MEV) e Espectroscopia de Energia Dispersiva de Raios X (EDS); quantificaÃÃo da fraÃÃo de ferrita; quantificaÃÃo da fraÃÃo das inclusÃes; ensaios de microdureza e uma avaliaÃÃo da resistÃncia à corrosÃo por pites atravÃs dos ensaios de temperatura crÃtica de pite (CPT) seguindo a norma ASTM G150. Como resultados, obteve-se fraÃÃes de ferrita entre 44% e 54,9% com seus maiores valores observados quando utilizou-se a liga 2507, a energia de 1,6 kJ/mm e a corrente contÃnua pulsada. As menores fraÃÃes das inclusÃes foram obtidas pela utilizaÃÃo do gÃs de proteÃÃo com 96%Ar + 4%CO2 (Etapa 2) e da energia de 1,6kJ/mm (Etapa 3). As microdurezas na ZF das juntas nÃo apresentaram diferenÃas significativas. Jà os ensaios de CPT revelaram que os pites nuclearam preferencialmente no interior da α, nos contornos α/γ e nas interfaces entre inclusÃes de grande diÃmetro e a matriz metÃlica, propagando-se exclusivamente atravÃs da α. Os valores de CPT apresentaram uma faixa de 47ÂC à 78ÂC, com os maiores valores obtidos para a liga 2507, a energia de 1,6 kJ/mm e a corrente contÃnua constante na regiÃo da Raiz e nas regiÃes do Enchimento e Acabamento ao utilizar-se a liga 2507, a energia de 1,6 kJ/mm e a corrente contÃnua pulsada. Dentre as trÃs regiÃes das ZFs, a Raiz apresentou os melhores resultados com ausÃncia de defeitos, fraÃÃes de ferrita satisfatÃrias, menores fraÃÃes das inclusÃes e maiores valores de CPT.

Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.

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O emprego de aços inoxidáveis duplex na indústria atual vem aumentando devido as suas características que combinam boas propriedades mecânicas e de resistência à corrosão. Esses aços são constituídos por uma estrutura bifásica com frações volumétricas de 50% ferrita e 50% austenita. Portanto, o desenvolvimento de técnicas para melhorar a soldagem dos aços duplex se faz necessário, de modo a garantir que o balanço de fases e as propriedades do material não sejam severamente prejudicados neste processo. Este trabalho tem o objetivo de caracterizar juntas soldadas com o processo GTAW (Gas Tungsten Arc Welding) no aço inoxidável duplex UNS S32205. Foram realizadas soldagens na condição autógena com aporte térmico variando entre 261,00 J/mm e 652,50 J/mm. Seguiu-se com a análise do metal base e dos cordões de solda mediante ensaios de dureza Vickers, EDS (Espectroscopia por Dispersão de Energia), microscopia eletrônica de varredura (MEV) e microscopia óptica (MO). Os resultados apresentaram a região do metal de solda com fração volumétrica de austenita variando entre de 21,71% e 33,89%. Essa variação se deve às menores taxas de resfriamento dos aportes térmicos com valores maiores, que permitem uma maior formação de austenita. Quanto a geometria do cordão, foi observado que valores maiores de aporte térmico, provocaram cordões mais largos e profundos, variando entre 3,9 e 5,9 mm de largura e 0,65 a 1,29 mm de profundidade. Na zona termicamente afetada mais próxima do metal de solda, a dureza Vickers apresentou valores maiores que o metal base, devido a presença de maior percentual de ferrita nessa região. Não se identificou a presença de fases frágeis no material.
The use of duplex stainless steels in industry is increasing due to their characteristics that combine good mechanical and corrosion resistance properties. These steels are comprised of a biphasic structure volume fraction of 50% ferrite and 50% austenite. Therefore, the development of techniques to improve the welding of duplex steels it is necessary, in order to ensure that the phase balance and the material properties are not severely harmed in the process. This work aims to characterize welded joints with the GTAW process in UNS S32205 duplex stainless steel. Autogenous welds were performed with heat input ranging from 261.00 J/mm and 652.50 J/mm. The next step was the analysis of base metal and weld beads by Vickers hardness testing, EDS, electronic and optical microscopy. The results showed the weld metal region with volume fraction of austenite ranging between 21.71% and 33.89%. This variation is due to the lower cooling rates of high heat input values, wich allow a greater formation of austenite. About the geometry, it was observed that higher values of heat input, led to wider and deeper welded joints, ranging between 3.9 and 5.9mm width and 0.65 and 1.29mm depth. In the nearest heat affected zone of the weld metal, Vickers hardness showed higher values than the base metal due to the presence of higher percentage of ferrite. Was not identified brittle phases in the material.

Orientador: Vicente Afonso Ventrella
Resumo: O emprego de aços inoxidáveis duplex na indústria atual vem aumentando devido as suas características que combinam boas propriedades mecânicas e de resistência à corrosão. Esses aços são constituídos por uma estrutura bifásica com frações volumétricas de 50% ferrita e 50% austenita. Portanto, o desenvolvimento de técnicas para melhorar a soldagem dos aços duplex se faz necessário, de modo a garantir que o balanço de fases e as propriedades do material não sejam severamente prejudicados neste processo. Este trabalho tem o objetivo de caracterizar juntas soldadas com o processo GTAW (Gas Tungsten Arc Welding) no aço inoxidável duplex UNS S32205. Foram realizadas soldagens na condição autógena com aporte térmico variando entre 261,00 J/mm e 652,50 J/mm. Seguiu-se com a análise do metal base e dos cordões de solda mediante ensaios de dureza Vickers, EDS (Espectroscopia por Dispersão de Energia), microscopia eletrônica de varredura (MEV) e microscopia óptica (MO). Os resultados apresentaram a região do metal de solda com fração volumétrica de austenita variando entre de 21,71% e 33,89%. Essa variação se deve às menores taxas de resfriamento dos aportes térmicos com valores maiores, que permitem uma maior formação de austenita. Quanto a geometria do cordão, foi observado que valores maiores de aporte térmico, provocaram cordões mais largos e profundos, variando entre 3,9 e 5,9 mm de largura e 0,65 a 1,29 mm de profundidade. Na zona termicamente afetada mais próxima do metal de sold... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The use of duplex stainless steels in industry is increasing due to their characteristics that combine good mechanical and corrosion resistance properties. These steels are comprised of a biphasic structure volume fraction of 50% ferrite and 50% austenite. Therefore, the development of techniques to improve the welding of duplex steels it is necessary, in order to ensure that the phase balance and the material properties are not severely harmed in the process. This work aims to characterize welded joints with the GTAW process in UNS S32205 duplex stainless steel. Autogenous welds were performed with heat input ranging from 261.00 J/mm and 652.50 J/mm. The next step was the analysis of base metal and weld beads by Vickers hardness testing, EDS, electronic and optical microscopy. The results showed the weld metal region with volume fraction of austenite ranging between 21.71% and 33.89%. This variation is due to the lower cooling rates of high heat input values, wich allow a greater formation of austenite. About the geometry, it was observed that higher values of heat input, led to wider and deeper welded joints, ranging between 3.9 and 5.9mm width and 0.65 and 1.29mm depth. In the nearest heat affected zone of the weld metal, Vickers hardness showed higher values than the base metal due to the presence of higher percentage of ferrite. Was not identified brittle phases in the material.
Mestre

Super duplex stainless steel (SDSS) has found a wide use in demanding applications such as offshore, chemical and petrochemical industries thanks to its excellent combination of mechanical properties and corrosion resistance. Welding of SDSS, however, is associated with the risk of precipitation of secondary phases and formation of excessive amounts of ferrite in the weld metal and heat affected zone. The present study was therefore aimed at gaining knowledge about the effect of multiple welding thermal cycles on the microstructure and possible sensitization to corrosion of welds in SDSS.Controlled and repeatable thermal cycles were produced by robotic welding. Oneto four autogenous TIG-remelting passes were applied on 2507 type SDSS plates using low or high heat inputs with pure argon as shielding gas. Thermal cycles were recorded using several thermocouples attached to the plates. Thermodynamic calculations and temperature field modelling were performed in order to understand the microstructural development and to predict the pitting corrosion resistance. Etching revealed the formation of different zones with characteristic microstructures: the fused weld zone (WZ) and the heat affected zone composed of the fusion boundary zone (FBZ), next to the fusion boundary, and further out Zone 1 (Z1) and Zone 2 (Z2). The WZ had a high content of ferrite and often nitrides which increased with increasing number of passes and decreasing heati nput. Nitrogen content of the WZ decreased from 0.28 wt.% to 0.17 wt.% after four passes of low heat input and to 0.10 wt.% after four passes of high heatinput. The FBZ was reheated to high peak temperatures (near melting point) and contained equiaxed ferrite grains with austenite and nitrides. Zone 1 was free from precipitates and the ferrite content was similar to that of the unaffected base material. Sigma phase precipitated only in zone 2, which was heated to peak temperatures in the range of approximately 828°C to 1028°C. The content of sigma phase increased with the number of passes and increasing heat input.  All locations, except Z1, were susceptible to local corrosion after multiplere heating. Thermodynamic calculations predicted that a post weld heat treatment could restore the corrosion resistance of the FBZ and Z2. However, the pitting resistance of the WZ cannot be improved significantly due to the nitrogen loss. Steady state and linear fitting approaches were therefore employed to predict nitrogen loss in autogenous TIG welding with argon as shielding gas. Two practical formulas were derived giving nitrogen loss as functions of initial nitrogen content and arc energy both predicting a larger loss for higher heat input and higher base material nitrogen content. A practical recommendation based on the present study is that it is beneficial to perform welding with a minimum number of passes even if this results in a higherheat input as multiple reheating strongly promotes formation of deleterious phases.

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Turbinas hidráulicas usadas em usinas hidrelétricas podem apresentar desgastes por cavitação e trincas em sua operação, as quais em alguns casos reparadas por soldagem a arco elétrico, por ser um processo de menor custo em comparação à troca da turbina. O aço inoxidável martensítico A743 grau CA6NM é empregado em serviços que requerem boa resistência mecânica associada a resistência à corrosão, principalmente em equipamento para extração de petróleo nos rotores e componentes de turbinas hidráulicas. Este trabalho tem por escopo analisar a microestrutura formada após processo de soldagem TIG (Tungsten Inert Gas) de um aço ASTM A743 CA6NM, usando material de adição similar, avaliando a presença da estrutura ferrita delta, austenita retida e carbonetos. Variando as espessuras das amostras e parâmetros de soldagem como passe único de soldagem e multipasse, mantendo temperatura de pré-aquecimento e interpasse de150ºC. Estimando a influência dos parâmetros na microestrutura com a dureza dos cordões e da ZTA. Os experimentos revelaram que um único passe de solda é suficiente para a formação de ferrita delta na forma de rede ao longo do cordão de solda e nas regiões expostas a altas temperaturas na zona termicamente afetada (ZTA), assim como o perfil de durezas da seção transversal do cordão de solda apontam durezas que variam desde 344 ± 6 HV para a região fundida, passando por um pico de dureza de 379 ± 4 HV na ZTA e terminando no metal base com 290 ± 5 HV, na solda multipasse a origem das diferentes morfologias de ferrita é múltipla, havendo uma diferença da taxa de resfriamento nos cordões,sendo que entre a base e o topo da solda pode ser percebido acumulo de ferrita delta na região interpasse e no topo do ultimo cordão de solda e a variação de largura da ZTA pelos passes subsequentes, a morfologia da ferrita delta zona termicamente afetada (ZTA) na condição multipasse, assim como o perfil de durezas da seção transversal dos cordões de solda apontam durezas que variam desde 311 ± 24 HV para a região fundida, passando por um pico de dureza de 326 ± 33 HV na ZTA.
Hydraulic turbines used in hydroelectric plants may present wear by cavitation and cracks in their operation, and in some cases repaired by electric arc welding, being a process of lower cost compared to the turbine exchange. A743 grade martensitic stainless steel CA6NM is used in services that require good mechanical resistance associated with corrosion resistance, especially in oil extraction equipment, rotors and hydraulic turbine components. This work aims to analyze the microstructure formed after TIG (Tungsten Inert Gas) welding of an ASTM A743 CA6NM steel, using similar addition material, evaluating the presence of the delta ferrite, retained austenite and carbides structure. By varying the sample thicknesses and welding conditions as single pass welding and multipass, maintaining preheating temperature and interpass of 150ºC. Estimating the influence of the parameters in the microstructure with the hardness of the strands and the ZTA. The experiments revealed that a single soldering pass is sufficient for the formation of delta ferrite in the form of a net along the weld bead and in the regions exposed to high temperatures in the thermally affected zone (ZTA), as well as the hardness profile of the section cross section of the weld beads indicate hardness ranging from 344 ± 6 HV to the molten region, passing through a hardness peak of 379 ± 4 HV in the ZTA and ending in the base metal with 290 ± 5 HV, in the multipass weld the origin of the different In the present study, the ferrite morphologies are multiple, with a difference in the cooling rate in the cords, between the base and the top of the weld it is possible to detect the accumulation of delta ferrite in the interpass region and at the top of the last weld bead and the ZTA The thermally affected zone delta ferrite (ZTA) morphology in the multipass condition, as well as the cross section hardness profile of the weld beads, point to hardnesses that vary from 311 ± 24 HV for the molten region, passing through a hardness peak of 326 ± 33 HV in the ZTA.

L'objectif de ce travail de thèse, est de développer un matériau d'apport et un mode opératoire de soudage associé permettant d'assembler des tôles d'acier K44X pour la fabrication de collecteurs d'échappement automobiles. Les propriétés de l'acier K44X ayant été optimisées pour répondre au mieux aux contraintes de l'application, les conditions de soudage recherchées devront, dans la mesure du possible, éviter de dégrader les caractéristiques de l'acier, en particulier en termes de tenue mécanique à haute température et de résistance à l'oxydation et à la fatigue thermique. Ce mémoire est divisé en quatre chapitres. Le premier chapitre est consacré à une présentation synthétique des évolutions dans le domaine de la fabrication des collecteurs d'échappement automobiles, et des connaissances actuelles dans les domaines des aciers inoxydables ferritiques et sa soudabilité, des procédés de soudage à l'arc. Le second chapitre présente les caractéristiques de l'acier K44X et la problématique de l'étude, puis décrit le travail d'élaboration des matériaux d'apport de différentes compositions. Ainsi que les résultats d'une caractérisation préliminaire des soudures obtenues avec les différents matériaux. Le chapitre 3 traite de la caractérisation de la tenue en service de l'assemblage retenu à l'issu du chapitre précédent. Les tests d'oxydation, de traction à chaud sur zone fondue des soudures ou sur assemblages complets, et de fatigue thermique, utilisés pour réaliser cette caractérisation sont décrits, et les résultats sont discutés. La fin de ce chapitre est consacrée à la caractérisation des précipités formés dans les zones fondues. Enfin, le dernier chapitre est consacré à la modélisation thermique du soudage et à la modélisation de la solidification, dans le but de tenter de prédire le type de microstructure de zone fondue formée lors d'une opération de soudage, en fonction des paramètres procédés. Cette modélisation, qui s'appuie sur les résultats d'un essai de soudage instrumenté, doit notamment permettre de prédire si les conditions de soudage, pour une composition d'acier donnée, permettent ou non de former une structure de grains équiaxe en zone fondue des soudures
The objective of this work is to develop a filler metal and an associated welding procedure allowing to join sheets of steels K44X for the manufacturing of exhaust manifolds for automotive. The properties of the steel K44X having been optimized to answer at best the constraints of the application, the welding conditions will have to, as possible, avoid degrading the characteristics of the steel, in particular in terms of mechanical strength with high temperature, oxidation resistance and in thermal fatigue.This report is divided into four chapters.The first chapter is dedicated to a synthetic presentation of the evolutions in the field of the manufacturing of the automotive exhaust manifolds, and current knowledge in the domains of ferritic stainless steels and its weldability, and in arc welding processes.The second chapter presents the characteristics of the steel K44X and the problem of the study, then described the work of elaboration of the filler metals with various compositions. As well as the results of a preliminary characterization of the welds obtained with the various materials.The chapter 3 is about the characterization of the in-service behavior of the assembly stemming from the previous chapter. The tests of oxidation, hot traction on molten zone of the welds or on complete assemblies, and of thermal fatigue, used to realize this characterization are described, and the associated results are discussed. The end of this chapter is dedicated to the characterization of precipitates formed in the molten zones.Finally, the last chapter is dedicated to the thermal modelling of the welding and to the modelling of the solidification, with the aim of trying to predict the type of microstructure of molten zone formed during a welding operation, according to the process parameters. This modelling, based on the results of a instrumented experimental test of welding, has to allow in particular to predict if the welding conditions, for a given composition of steel, allow or not to form a structure of grains equiaxed in molten zone of the welds

Abstract This research work focuses on how to improve the toughness of heat affected zones (HAZs) of low heat input welds in the case of high strength thermomechanically processed (TMCP) and recrystallization controlled rolled and accelerated cooled (RCR) plates with yield strengths of 355–500 MPa. Experimental work was aimed at the investigation of the intragranular nucleation of acicular ferrite or bainite in hot-rolled plates and the evaluation of the Charpy V and CTOD toughness of the most critical sub-zones of the weld HAZ using simulated specimens with a cooling time t8/5 = 5 s. The zones studied were the coarse grained HAZ (CGHAZ), the intercritically reheated coarse-grained HAZ (ICCGHAZ) and the intercritical HAZ (ICHAZ), the metallographical analyses consisted of microstructural investigations complemented with hardness measurements. Optical, scanning and transmission electron microscopy techniques were employed together with image and electron backscatter diffraction (EBSD) analysis. The test results showed that the toughness of the various sub-zones of the HAZ is improved by promoting intragranularly nucleated ferritic-bainitic (acicular) microstructure in both the CGHAZ and in the base plate. In this way, the sub-zones subjected to intercritical thermal cycles (the ICCGHAZ and the ICHAZ) develop evenly distributed M-A constituents between ferrite and bainite laths. These favourable microstructures can be achieved by using titanium killing or by avoiding niobium microalloying by using copper plus nickel alloying instead. In the laboratory experiments titanium killed steel, containing titanium-manganese oxide/manganese sulphide inclusions with a number density of 300–750 particles/mm2, develops a largely acicular ferritic microstructure in the base plate provided the austenite grain size is greater than about 120 μm and the cooling rate is in the range 6–11 °C/s down to 500 °C. Under plate mill conditions, no significant amount of acicular ferrite could be obtained, because it was not possible to achieve austenite grain sizes larger than about 70 μm after rolling. However, a significant fraction of acicular ferritic-bainitic microstructure was achieved in the CGHAZ, when the austenite grain size exceeded 90 μm. When achieved, a uniform distribution of M-A particles in an acicular ferritic-bainitic microstructure improves toughness. Cracks nucleate at numerous sites on M-A/ferrite boundaries or bainite packet interfaces, but they are initially arrested in the acicular matrix. Crack growth finally occurs by linking of the numerous arrested microcracks.

High strength low alloy (HSLA) steels have been developed to simultaneously have high yield strength and high fracture toughness. However, in practical applications steel must be welded. Traditional arc welding has proven detrimental to the fracture toughness of HSLA steels. Friction stir welding has recently shown mixed results in welding HSLA steels. The range of welding parameters used in these recent studies however has been very limited. With only a few welding parameters tested, the effect of spindle speed, travel speed, and heat input on the fracture toughness of friction stir welded HSLA steel remains unknown. To understand how the friction stir welding process parameters affect fracture toughness, double sided welds in API X80 were performed and analyzed. Results show that at room temperature friction stir welded API X80 exceeded industry minimum fracture toughness requirements in both the API Standard 1104 and DNV-OS-F101 by 143% and 62%, respectively. The process parameters of spindle speed and HI have been shown to effectively control the fracture toughness of the stir zone. Relationships have been established that show that fracture toughness increased by 85% when spindle speed decreased by 59% and heat input decreased by 46%.

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Integration of AHSS into the automotive architecture has brought renewed challenges for achieving acceptable welds. The varying alloying content and processing techniques has further complicated this initiative. The current study examines resistance spot welding of high strength and advance high strength steels including high strength low alloy (HSLA), dual phase (DP) and a ferritic-bainitic steel (590R). The mechanical properties and microstructure of these RSW welded steel alloys are detailed. Furthermore a relationship between chemistries and hardness is produced. The effect of strain rate on the joint strength and failure mode is also an important consideration in the design of welded structures. Current literature, however, does not explain the effects of weld microstructure and there are no comprehensive comparisons of steels. This work details the relationship between the joint microstructure and impact performance of spot welded AHSS. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower, respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes are detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties are developed using observed microstructures in the fusion zone and heat-affected-zone. Friction stir spot welding (FSSW) has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has also been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn-coated DP600 AHSS (1.2mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process.

碩士
國立虎尾科技大學
材料科學與工程系材料科學與綠色能源工程碩士班
105
In this paper, beryllium copper alloy and ferritic stainless steel were welded by micro-gas tungsten arc welding. The working ranges of welding parameters were fixed by conducting trial runs and satisfactory results obtained were used to conduct an experimental study. Experiments were conducted applying ultrasonic vibration during the arc welding process and their effects on microstructure and mechanical properties of welds were presented. The observations of microstructure revealed that Fe-Cu compound in the weld metal was confirmed and no precipitation was recorded in weld metal or heat-affected zone. Spherical Fe-Cr sigma-phase precipitates of different diameters were observed in weld metal and heat-affected zone under ultrasonic vibratory condition. The weld joints fabricated with ultrasonic output power 55W condition were found to possess comparatively high hardness value and high ultimate tensile strength, but loss in the ductility. These results were closely associated with the precipitation behavior of sigma-phase.

碩士
國立屏東科技大學
機械工程系所
99
This study is focused on the use of Nd: YAG laser for 439 ferrite stainless steel sheet in butt welding process, such that the effect of each laser welding parameter for the tensile strength and the weld hardness are investigated. Then two single and a multiple optimization models are established. An experimental work is associated with those optimization models in this study. Firstly, an orthogonal array of Taguchi method is used arrange experimental tests. The selected laser welding parameters are peak power, pulse frequency, pulse width and welding speed. Tensile strength and hardness of the weld parts are measured. The average and variance analysis of the Taguchi method are adopted in order to identify the most important laser welding parameter and the best combination of laser welding parameters. A gray relational analysis is used in order to find the optimized parameters combination in a multiple model. The results showed that in the single objective 99.2% in tensile strength can be obtained and 99.1% in weld metal hardness is achieved. In order to ensure accuracy of the analysis Taguchi method, associated with Gray relational analysis for the multiple objective to find the optimization parameters, has been performed and compared with the single objective model results. At the same time the trends for the relationship between hardness and tensile strength of the weld is developed. As a result, the related process can provide a more convenient mode of quality control inspection.

碩士
國立高雄第一科技大學
機械與自動化工程所
91
Y309L stainless with high Cr and Ni contents is becoming an high additional value core material for welding wires. It contains the austenite, δ-ferrite, and the other elements such as Mo, Si, Al, Ti, Nb at the same time. The Y309L stainless steel has excellent characteristics in corrosion resistance, welding, toughness, stregth and few cracking at welding. These high alloyed materials are difficult for hot rolling, many kinds of defects can be happened during its production, i.e. rolling crack, cobble and sliver that can damage rolling facilities and take much time and cost. The result comes from high the content of δ-ferrite. Its production and hot rolling condition to investigate the hot workability at different content of δ-ferrite. The content of δ-ferrite in hot workability influence (1) the character of δ-ferrite with tendency to melt in alloyed material or other element will result in the milting decreasing. (2) δ-ferrite in 600-950℃ will resolve and compose γ and σ phase, which will result in mechanical properties of material and corrosion resistance decreasing. (3) The existence of δ phase interact with carbide, so that N2, H2 molecule will have possibility of congregation. That will lead to H2 cracking. (4) Increasing content of δ-ferrite in stainless will influence intensity, tenacity of stainless material, and character of hot workability. Therefore, the content of δ-ferrite will influence quality during the process of rolling and milling. The research is to study the influence of equivalent ratios of Creq, Nieq, the rolling temperature, and reduction ratio. The result of research showed (1) The higher equivalent ratio of Creq/Nieq and high content of δ-ferrite, the more increasing diminished ratio of wirerod. (2) The δ-ferrite content is increased by increasing the temperature of rolling mill. As temperature in 1100℃, there exists a maximum value of δ-ferrite content. In contract, there is a minimum value of δ-ferrite content. (3) When temperature of experiment is between 990 and 1100, there is no tendency of increasing δ-ferrite content with increasing cross reduction ratio with isothermal. Under the condition of experiment process δ-ferrite content keeping in 10% or so will have high quality of wire rod, the result will offer reference of improving product for manufacture.

The research focused on the influence of welding inter-pass temperature in 304H type austenitic stainless steel weld joints in the as-welded condition. The shielded metal arc welding process was used to weld the joints. The following was evaluated: the theoretical and measured ferrite numbers, solidification mode and delta ferrite morphology, as well as the evolution and precipitation of secondary phases i.e. sigma phase in the weld, chromium carbides in the heat affected zone. After the evaluation, it was clear that the inter-pass temperature had an effect on solute distribution during cooling and subsequent calculated ferrite numbers of the welds. The calculated ferrite numbers, that were determined using the weld metal chemistry of each joint and the WRC-1992 constitution diagram, increased from FN of 1 to FN of 3 with the increase in welding inter-pass temperature from 105°C-100°C and to 195°C-200°C respectively. The measured ferrite number showed no correlation with the increases in interpass temperature. The highest measured ferrite number of 3.8 was obtained when welding at an inter-pass temperature of 135°C – 140°C which was closest to the FN of 5 required minimum, as specified by the SAPREF Refinery, to prevent solidification cracking. No solidification cracking was observed in any of the specimens evaluated in this study even though all the specimens had ferrite contents well below FN 5. This observation supports research that indicates that controlling of the primary solidification mode as delta ferrite is more important a factor in preventing solidification cracking than trying to control the actual ferrite content of the weld metal. The primary solidification mode of the weld was a combination of the austenite-ferrite (AF) to predominantly ferrite-austenite (FA) with the FA solidification mode dominating with the increase in inter-pass temperature. The nature of the carbides formed due to low temperature sensitization in the heat affected zone of the base metal changed with the increase in inter-pass temperature. The precipitated chromium carbides only formed discontinuous carbide networks at the interpass temperature of 195°C-200°C. The transformation of sigma from delta ferrite was not observed in the columnar dendritic and mushy zones of the weld metal. This research revealed the optimum welding inter-pass temperature for welding 304H austenitic stainless steel with 308H electrode to be 135-140°C.