Delta Ferrite Content Research Articles

Hot-Deformation Behavior and Hot-Processing Maps of AISI 410 Martensitic Stainless Steel

AbstractThe compressive deformation behaviors of 410 martensitic stainless steel were investigated on a Gleeble-1500 thermomechanical simulator, and the experimental stress–strain data were obtained. The measured flow stress was corrected for friction and temperature. A constitutive equation that accounts for the influence of strain was established, and the hot-processing maps at different strain were plotted. The microstructure evolution of the hot-deformation process was studied on the basis of microstructural observations at high temperatures. Phase-transformation experiments on 410 steel were conducted at high temperatures to elucidate the effects of temperature on the delta-ferrite content. The initial forging temperature and optimum process parameters were obtained on the basis of the processing map and the changes in the delta-ferrite content at high temperatures.

SSC13 주강품의 내부식특성에 미치는 고용화 열처리 영향

Recently, Stainless steels have been increasingly selected as the fitting or the valve materials of water pipes as the human health issue is getting higher and higher. Therefore, the connectors attached at pipes to deliver water are exposed to more severe environments than the pipes because crevice or galvanic corrosion is apt to occur at the fittings or the valves. Effects of the solid solution annealing, cooling rate after this heat treatment, and passivation on the corrosion properties of the shell mold casted SSC13 (STS304 alloy equivalent) were studied. The heating and quenching treatment more or less reduced hardness but effectively improved corrosion resistance. It was explained by the reduction of delta ferrite contents. Independent of heat treatment, the chemical passivation treatment also lowered corrosion rate but the improvement of corrosion resistance depended on temperature and time for passivation treatment indicating that the optimum conditions for passivation treatment were the bath temperature of 34 oC and operating time of 10 minutes. Therefore it is suggested that the corrosion resistance of SSC13 can be effectively improved with the heat treatment, where SSC13 is heated for 10 minutes at 1120 °C and quenched and passivation treatment, where SSC13 is passivated for at least 10 seconds at 34 °C nitric acid solution.

Welding of 316L Austenitic Stainless Steel with Activated Tungsten Inert Gas Process

The use of activating flux in TIG welding process is one of the most notable techniques which are developed recently. This technique, known as A-TIG welding, increases the penetration depth and improves the productivity of the TIG welding. In the present study, four oxide fluxes (SiO2, TiO2, Cr2O3, and CaO) were used to investigate the effect of activating flux on the depth/width ratio and mechanical property of 316L austenitic stainless steel. The effect of coating density of activating flux on the weld pool shape and oxygen content in the weld after the welding process was studied systematically. Experimental results indicated that the maximum depth/width ratio of stainless steel activated TIG weld was obtained when the coating density was 2.6, 1.3, 2, and 7.8 mg/cm2 for SiO2, TiO2, Cr2O3, and CaO, respectively. The certain range of oxygen content dissolved in the weld, led to a significant increase in the penetration capability of TIG welds. TIG welding with active fluxes can increase the delta-ferrite content and improves the mechanical strength of the welded joint.

Avaliação metalúrgica da soldagem de revestimento inox austenítico sobre aço SAE 4130

O objetivo deste trabalho foi realizar uma avaliação metalúrgica da soldagem de revestimento inox austenítico sobre a área de selagem de tubos de Riser fabricados em aço SAE 4130. O estudo foi dividido em duas etapas. A primeira etapa teve como objetivo a determinação da taxa de diluição e taxa de deposição para os níveis máximo e mínimo de energia de soldagem conforme procedimentos de soldagem (EPS) com os processos TIG e ER. A seguir, foi feita análise e estudo com o diagrama de Schaeffler para identificar os consumíveis de solda mais adequados, conforme critérios de qualidade estabelecidos. Este estudo identificou o metal de adição 312 para a primeira camada, o 309 e 308 para a segunda. Todos usando o nível máximo de energia de soldagem e o processo de soldagem ER, com benefícios à produtividade. A segunda etapa teve como objetivo a validação dos resultados empíricos encontrados na 1ª etapa, com a execução de soldas de revestimento com duas camadas cada, e a caracterização metalúrgica das juntas soldadas por ensaios mecânicos e análise microestrutural por MO e MEV. Os resultados dos ensaios mecânicos e da microestrutura foram considerados satisfatórios, atendendo os critérios de qualidade adotados. Foi observada uma microestrutura austeno-ferritica com teor de ferrita delta entre 10 a 15 % nas duas camadas do revestimento. O eletrodo 312 assumiu posição de destaque na aplicação da 1ª camada da solda de revestimento sobre o aço SAE 4130, em função do seu maior teor de ferrita delta e cromo, evitando a formação de trincas a quente. Para a segunda camada, tanto o 309 como o 308 podem ser usados. Todas as soldas foram feitas com elevada energia de soldagem, garantindo o aumento da produtividade, sem alteração significativa nas características físicas e mecânicas.

Prediction of ferrite number in stainless steel gas tungsten arc welded plates using artificial neural networks

The quality of stainless steel weld is highly influenced by delta ferrite content expressed in terms of ferrite number. The quantity of delta ferrite content formed is controlled by the process parameters. This paper discusses the development of artificial neural network model for predicting the ferrite number in 202 grade stainless steel gas tungsten arc welded plates (GTAW). The process parameters chosen for study are welding gun angle, welding speed, plate length, welding current and shielding gas flow rate. The experiments were conducted based on design of experiments – fractional factorial with 125 runs. Using the experimental data, feed forward, back propagation neural models were developed and trained using Levenberg Marquardt algorithm. The training, learning, performance and transfer functions used are trainlm, learningdm, mean square error and tansig respectively. Five networks were developed with five neurons in the input layer, 1 neuron in the output layer and different nodes in the hidden layer. They are 5-3-1, 5-5-1, 5-10-1, 5-11-1 and 5-15-1. It was found that the artificial neural network (ANN) model based on network 5-5-1 predicted ferrite number more accurately than other networks. The prediction helps in identifying the recommended combination of process parameters to achieve a desired ferrite number in GTAW of stainless steel 202 grade plates.

Mechanical properties and microstructural investigations of TIG welded 40 mm and 60 mm thick SS 316L samples for fusion reactor vacuum vessel applications

The development of advanced fusion reactors like DEMO will have various challenges in materials and fabrication. The vacuum vessel is important part of the fusion reactor. The double walled design for vacuum vessel with thicker stainless steel material (40–60mm) has been proposed in the advanced fusion reactors like ITER. Different welding techniques will have to be used for such vacuum vessel development. The required mechanical, structural and other properties of stainless steels have to be maintained in these joining processes of components of various shapes and sizes in the form of plates, ribs, shells, etc. The present paper reports characterization of welding joints of SS316L plates with higher thicknesses like 40mm and 60mm, prepared using multi-pass Tungsten Inert Gas (TIG) welding process. The weld quality has been evaluated with non-destructive tests by X-ray radiography and ultrasonic methods. The mechanical properties like tensile, bend tests, Vickers hardness and impact fracture tests have been carried out for the weld samples. Tensile property test results indicate sound weld joints with efficiencies over 100%. Hardening was observed in the weld zone in non-uniform manner. Macro and microstructure studies have been carried out for Base Metal (BM), Heat Affected Zone (HAZ) and Weld Zone (WZ). Scanning Electron Microscopy (SEM) analysis carried out for the impact fractured specimens show ductile fracture. The microstructural study and ferrite number data indicate the presence of high content of delta ferrite in the weld zone as compared to the delta ferrite in base metal.

Delta ferrite in the weld metal of reduced activation ferritic martensitic steel

Formation of delta(δ)-ferrite in the weld metal, during autogenous bead-on-plate welding of Reduced Activation Ferritic Martensitic (RAFM) steel using Gas Tungsten Arc Welding (GTAW) process, has been studied. Composition of the alloy is such that delta-ferrite is not expected in the alloy; but examination of the weld metal revealed presence of delta-ferrite in the weld metal. Volume fraction of delta-ferrite is found to be higher in the weld interface than in the rest of the fusion zone. Decrease in the volume fraction of delta-ferrite, with an increase in preheat temperature or with an increase in heat input, is observed. Results indicate that the cooling rate experienced during welding affects the volume fraction of delta-ferrite retained in the weld metal and variation in the delta-ferrite content with cooling rate is explained with variation in the time that the weld metal spends in various temperature regimes in which delta-ferrite is stable for the alloy during its cooling from the liquid metal to the ambient temperature. This manuscript will discuss the effect of welding parameters on formation of delta-ferrite and its retention in the weld metal of RAFM steel.

UNS S31603 Stainless Steel Tungsten Inert Gas Welds Made with Microparticle and Nanoparticle Oxides

The purpose of this study was to investigate the difference between tungsten inert gas (TIG) welding of austenitic stainless steel assisted by microparticle oxides and that assisted by nanoparticle oxides. SiO2 and Al2O3 were used to investigate the effects of the thermal stability and the particle size of the activated compounds on the surface appearance, geometric shape, angular distortion, delta ferrite content and Vickers hardness of the UNS S31603 stainless steel TIG weld. The results show that the use of SiO2 leads to a satisfactory surface appearance compared to that of the TIG weld made with Al2O3. The surface appearance of the TIG weld made with nanoparticle oxide has less flux slag compared with the one made with microparticle oxide of the same type. Compared with microparticle SiO2, the TIG welding with nanoparticle SiO2 has the potential benefits of high joint penetration and less angular distortion in the resulting weldment. The TIG welding with nanoparticle Al2O3 does not result in a significant increase in the penetration or reduction of distortion. The TIG welding with microparticle or nanoparticle SiO2 uses a heat source with higher power density, resulting in a higher ferrite content and hardness of the stainless steel weld metal. In contrast, microparticle or nanoparticle Al2O3 results in no significant difference in metallurgical properties compared to that of the C-TIG weld metal. Compared with oxide particle size, the thermal stability of the oxide plays a significant role in enhancing the joint penetration capability of the weld, for the UNS S31603 stainless steel TIG welds made with activated oxides.

Stainless Steel Processing to Meet Advanced Applications

Stainless steel bar and wire products that cater to the high technology application in defence, nuclear, aerospace, oil field and chemical engineering is an area poised for rapid growth in India. The advancing capabilities of alloy steel plants in India have enabled mastering of techniques to make a wide variety of stainless steels. However, there are increasing challenges to meet the advanced property requirements, which call for a basic understanding on the structure property relationship that are influenced by appropriate alloy design and down-stream processing. The special steel industry cater to a wide variety of stainless steels namely ferritic, martensitic, austenitic and precipitation hardenable categories for meeting requirements of high technology. One of the process for making the primary stainless steels is Vacuum Oxygen Decarburisation process. For advanced applications, the primary melted steel is again secondary refined using electroslagremelting for the management of solidification structures and control of inclusions. In the austenitic grades, the hot forged and hot rolled heat treated steels, careful choice of chemistry controls the delta ferrite content and ensures uniformity of the grain size in the product during deformation processing and heat treatment. In the martensitic stainless steel grades, focus is given to delta ferrite, grain size control and appropriate tempering treatment. In the precipitation hardenable steels grades the aging reactions and hot deformation range have to be optimised for deriving specified mechanical properties. Special grades are produced using non ESR and ESR routes to meet high temperature applications such as turbine blades and bolting. In these grades control of delta ferrite content, carbides, carbo-nitrides in the matrix has a deep influence on the mechanical and sub zero fracture properties. In the ferritic stainless steel grade grain size control is critical. The presentation would bring forth the correlation between the alloy design, processing and properties that were achieved in the products mentioned above to meet some of the challenging requirements.

Nitrogen Enhanced 316LN Austenitic Stainless Steel for Sodium Cooled Fast Reactors

For the future sodium-cooled fast reactors (SFRs), which are envisaged with a design life of 60 years, nitrogen-enhanced 316LN austenitic stainless steel (SS) with improved high-temperature properties is being developed. To optimize the enhanced nitrogen content in 316LN SS, the effect of nitrogen on its tensile, creep and low cycle fatigue behavior has been investigated. For different heats of 316LN SS containing 0.07-0.22 wt% nitrogen, the tensile and creep properties increased with increase in nitrogen content, while low cycle fatigue properties peaked at 0.14 wt% nitrogen. Finally, based on the evaluation of the hot cracking susceptibility of the different heats of 316LN SS with varying nitrogen content, using the Varestraint and Gleeble hot-ductility tests, the nitrogen content for the nitrogen-enhanced 316LN SS has been optimized at a level of 0.14 wt%. The 0.14 wt% nitrogen content in this optimised composition shifts the solidification mode of the weld metal to fully austenitic region, including that due to dilution of nitrogen from the base metal, thereby increasing its hot cracking susceptibility. This necessitated development and qualification of welding electrodes for obtaining weld metal with 0.14 wt% nitrogen by optimising the weld metal chemistry so as to obtain the requisite delta ferrite content, tensile properties, and very importantly impact toughness both in the as-welded and aged conditions. Studies on localised corrosion behaviour of nitrogen-enhanced 316LN SS indicated the beneficial effect of nitrogen addition to sensitization, pitting, intergranular corrosion, stress corrosion cracking and corrosion fatigue.

Role of microstructural constituents on surface crack formation during hot rolling of standard and low nickel austenitic stainless steels

The effect of alloy segregation and delta (δ) ferrite contents on surface cracking of three standard (i.e. AISI 304L, AISI 310S and AISI 321) and two low nickel (i.e. LNi-1 and LNi-0.3) austenitic stainless steels (ASS) during hot rolling was investigated using optical microscopy (OM), automatic image analyzer, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analyzer (EPMA). It was observed that the amount of δ-ferrite varied among different grades and also distributed heterogeneously across the width of the steel plates. In general, low nickel ASS showed higher amount of δ-ferrite compared to the standard ASS grades. The tendency to surface cracking during hot rolling gradually increased with increasing δ-ferrite content. Interestingly, carbon and nitrogen exerted maximum effect on δ-ferrite formation. The higher carbon and nitrogen content in the steel decreased δ-ferrite content. In addition, the segregation of Cu and Mn plays significant role in low nickel ASS and Ni-Cr in case of standard ASS has profound effect on surface cracking of the steel plates. A possible cause of surface crack formation/origination in steel plates during hot rolling was discussed.

Effect of Different Oxide Fluxes on the Penetration Depth, Microstructure and Corrosion Behaviour of Austenitic Stainless Steel in A-TIG Welding

Three single oxide fluxes and two combined oxide fluxes were used to investigate the effect of different fluxes on depth/width ratio, microstructure and corrosion resistance in type 304 stainless steel. The result showed that the type of oxide flux has a significant effect on the 304 SS weld penetration, microstructure and corrosion resistance. The uses of oxide fluxes appear to promote the formation of deltaferrite and higher oxygen content resulted fine delta ferrite. Furthermore, the corrosion resistance of austenitic stainless steel welds improved with addition of oxide fluxes.

Application of iron-based powders in tungsten inert gas welding for 17Cr–10Ni–2Mo alloys

This study investigates the influences of specific flux powders, including FeF2, FeO, and FeS on the surface appearance, geometric shape, angular distortion, hot crack susceptibility, and metallurgical properties of 5-mm-thick 17Cr–10Ni–2Mo alloys welded using the tungsten inert gas (TIG) process. Results indicate that TIG welding with FeF2 powder produces a weld of satisfactory appearance. TIG welding with FeO and FeS powders results in a substantial increase in both the joint penetration and weld aspect ratio, thereby reducing angular distortion of the weldment. TIG welding with FeO and FeS powders increases the delta-ferrite content and decreases welding heat input, and tends to reduce hot crack susceptibility of the welds. This study demonstrates that the application of activated powder assisted TIG welding can improve the weldment performances.

Modeling and analysis of ferrite number of stainless steel gas tungsten arc welded plates using response surface methodology

The prediction of delta ferrite content expressed in terms of ferrite number in austenitic stainless steel welds is very helpful in assessing its performance. The final ferrite content determines the properties of weldments such as strength, toughness, corrosion resistance, and phase stability. This paper presents a study on the effect of process parameters on ferrite number in 202 grade stainless steel gas tungsten arc welded plates (GTAW). Experiments were conducted based on response surface methodology. The ferrite number was determined by using a ferrite scope and by using DeLong diagram. A mathematical model was developed correlating the important controllable GTAW process parameters like welding gun angle, welding speed, plate length, welding current, and shielding gas flow rate with ferrite number. The adequacy of the model was checked using analysis of variance technique. The developed model is very useful to quantitatively determine the ferrite number. The main and interaction effects of the process parameters are presented in graphical form that helps in selecting quickly the process parameters to achieve the desired results.

Predicting Delta Ferrite Content in Stainless Steel Castings

The distribution of delta ferrite fraction was measured with the magnetic method in specimens of different stainless steel compositions cast by the investment casting (lost wax) process. Ferrite fraction measurements published in the literature for stainless steel cast samples were added to the present work data, enabling an extensive analysis about practical methods to calculate delta ferrite fractions in stainless steel castings. Nineteen different versions of practical methods were formed using Schaeffler, DeLong, and Siewert diagrams and the nickel and chromium equivalent indexes suggested by several authors. These methods were evaluated by a detailed statistical analysis, showing that the Siewert diagram, including its equivalent indexes and iso-ferrite lines, gives the lowest relative errors between calculated and measured delta ferrite fractions. Although originally created for stainless steel welds, this diagram gives relative errors lower than those for the current ASTM standard method (800/A 800M-01), developed to predict ferrite fractions in stainless steel castings. Practical methods originated from a combination of different chromium/nickel equivalent indexes and the iso-ferrite lines from Schaeffler diagram give the lowest relative errors when compared with combinations using other iso-ferrite line diagrams. For the samples cast in the present work, an increase in cooling rate from 0.78 to 2.7 K/s caused a decrease in the delta ferrite fraction, but a statistical hypothesis test revealed that this effect is significant in only 50% of the samples that have ferrite in their microstructures.

Corrosion resistance improvement in Gas Tungsten Arc Welded 316L stainless steel joints through controlled preheat treatment

In the present study, an attempt has been made to improve the corrosion characteristics of 316L stainless steel weldments through preheating at 450 °C and 650 °C. The infrared and Tungsten–Rhenium thermocouples were utilized to probe the cooling trend of heat affected zone (HAZ) and weld pool, respectively. X-ray diffraction (XRD) patterns, optical microscopy, electron microscopy, Energy Dispersion Spectroscopy (EDS) and ferritscope were also used to investigate the effect of preheating on microstructural characteristics within the weld and HAZ. Moreover, cyclic potentiodynamic test was carried out to evaluate the corrosion features of welds such as corrosion current, passivation current ( I PP), breakdown potential ( E B) and repassivation potential ( E re). Results revealed that preheating reduces the cooling rate of weld pool, accompanied by reduction of delta ferrite content of weldments. Moreover, it was observed that increment of preheat temperature improves corrosion behavior of weldments, including a lower passivation current and a more pitting resistance. These outcomes were mainly ascribed to decrease of austenite/delta ferrite interfaces as vulnerable sites to corrosion attacks, through preheat treatment. Observations showed no evidence of sensitization in preheated samples, which guaranteed the feasibility of suggested heat treatment.

Advances in Fusion Welding Technique of Austenitic Stainless Steels

Activated flux assisted tungsten inert gas (TIG) welding of austenitic 316L stainless steel was investigated. Autogenous TIG welding was used to produce a bead-on-plate weld. The FeO, FeS, and FeF2 were selected as the flux powders. The results showed that the FeS and FeO fluxes produced high aspect ratio welds, while the FeF2 flux produced a relatively low aspect ratio weld. The undercut defect was produced with use of the FeS flux. An experimental result suggested the constriction of arc column as a mechanism in improving activated TIG penetration. In addition, activated TIG process can increase the delta-ferrite content in austenitic stainless steel weld metal.

Influence of Heat Input on the Content of Delta Ferrite in the Structure of 304L Stainless Steel GTA Welded Joints

Welding of austenitic stainless steel has its specific issues, even when the weldability is considered good. The main problems of austenitic stainless steel welding are connected with its metallurgical weldability. The amount of the components presented in the structure of stainless steel welded joint affect its properties, therefore the understanding of the behavior of stainless steel during its welding is important for successful processing and allows the fabricators the possibility to manage the resulting issues. This paper is focused on the influence of heat input on the structural changes in GTA welded joints of austenitic stainless steel designated: ASTM SA TP 304L.

Performance of activated TIG process in austenitic stainless steel welds

Five kinds of oxide fluxes, MnO 2, TiO 2, MoO 3, SiO 2, and Al 2O 3, were used to investigate the effect of activated tungsten inert gas (activated TIG) process on weld morphology, angular distortion, delta-ferrite content, and hardness of Type 316L stainless steels. An autogenous TIG welding was applied to 6 mm thick stainless steel plates through a thin layer of flux to produce a bead-on-plate welded joint. The oxide fluxes used were packed in powdered form. The experimental results indicated that the SiO 2 flux facilitated root pass joint penetration, but Al 2O 3 flux led to the deterioration in the weld depth and bead width compared with conventional TIG process. Activated TIG welding can increase the joint penetration and weld depth-to-width ratio, thereby reducing angular distortion of the weldment. On the basis of the present results, it is considered that the centripetal Marangoni convection and constricted arc plasma as a mechanism in increasing the penetration of activated TIG joint.

Estudo da soldagem de tubos de aço inoxidável duplex e superduplex na posição 5G

Os aços inoxidáveis duplex e superduplex possuem uma microestrutura austeno-ferrítica com fração média de cada fase de cerca 50%. A microestrutura duplex é responsável pelas excelentes propriedades mecânicas, especialmente o limite de escoamento e a tenacidade, e pela elevada resistência a corrosão por pites e sob tensão em meios contendo cloretos. A soldagem destes aços é frequentemente uma operação crítica. Neste trabalho, um tubo de superduplex SAF 2207 foi soldado pelo processo TIG (GTAW) no passe de raiz e eletrodo revestido (SMAW) nos passes de enchimento, e um tubo de duplex SAF 2205 foi todo soldado pelo processo GTAW. A microestrutura do metal base, zona afetada termicamente (ZTA) e metal de solda foi caracterizada e quantificada. As propriedades de tenacidade, resistência a corrosão e composição química foram avaliadas e correlacionadas. Os valores de tenacidade ao impacto Charpy-V foram considerados adequados. Não foi observada a precipitação de intermetálicos, carbonetos e nitretos. O melhor resultado no teste de corrosão da junta soldada de aço inox superduplex, comparado ao duplex, foi atribuído ao baixo teor de ferrita delta na solda e ZTA, e composição química da solda enriquecida em Cr, Mo, W e N.