Austenite Content Research Articles

Mechanical Properties and Failure Mechanism of Resistance Spot Welding Joint of 0.13C – 6.98Mn Medium Manganese TRIP Steel

A single-pulse spot welding test was performed on 0.13C-6.98Mn medium manganese TRIP steel. The test results showed that the joint performance increases with a rise in welding heat input before splashing occurs. Owing to the fusion zone (FZ) being closest to the heat source, its austenite content was completely transformed and its performance was worse than that of the heat affected zone comprising austenite. The segregation bands of C and Mn observed in the FZ were considered to be the main reason for the deterioration of grain boundary strength. These characteristics of the weld zone led to evident intergranular cracking and transgranular fracture in the tensile test, and all samples in this test showed partial or complete interfacial fractures.

Predictive modelling of cryogenic hard turning of AISI 52100 based on response surface methodology for the use in soft sensors

This paper focuses on the development of an empirical model for the use in soft sensors to predict the surface layer properties of AISI 52100 machined via cryogenic hard turning. By using response surface methodology, a multiple regression is performed to achieve correlations between the process parameters, the process forces and the temperature, as well as the residual stresses and the austenite content. The results show a predictive R2 of over 0.75 and indicate that the empirical model can be used in soft sensors for a process control of cryogenic hard turning.

Study of change in the austenitic phase content of two-phase magnetic material based on silicon steel when changing the gas mixture composition

Study of change in the austenitic phase content of two-phase magnetic material based on silicon steel when changing the gas mixture composition

Impact toughness of an electron-beam welded 0.2C direct-quenched and partitioned steel

Third generation advanced high-strength steels, e.g., quenched and partitioned steels, are forthcoming structural materials, which consist of a martensitic matrix and a substantial proportion of stabilized residual austenite for improved deformability. A novel less energy-intensive processing route of direct-quenching and partitioning advances this concept by facilitating carbon partitioning to untransformed austenite directly from the quench-stop temperature. However, a major challenge also with these steels is how to maintain structural integrity in the welded end-products after additional heat-input reaching above a temperature where given microstructure is still stable. Heat-input limiting beam welding processes are a solution to this by minimizing degradation of the heat-affected zone (HAZ) and producing even-strength welded joints for S1100 and above. In this study, we report toughness properties of an electron-beam (EB) welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) direct-quenched and partitioned steel (DQ&P) having a yield strength of ∼1100 MPa, and a direct-quenched (DQ) was used as a reference. Low-temperature post-weld heat treatment (PWHT) was considered, too. Weld seam, coarse-grained HAZ, and the base materials were tested for impact toughness. Both the DQ and DQ&P base materials have excellent impact toughness transition temperatures T28J below -100°C. The weld seam has very good low-temperature toughness already at this stage of optimisation with T28J of -66°C, which shows robustness of the chosen alloy. Increased residual austenite content increased upper shelf toughness but not T28J. Furthermore, both the DQ and DQP HAZs have T28J below -70°C, pointing to the weld seam as the weakest link. PWHT reduced low-temperature impact toughness in all the cases with T28J being above -40°C, clearly demanding reassessment of its feasibility.

Use of structural anomalies in steel gas-thermal coatings during increased wear-out

The structure of gas-thermal coatings made of wire materials has been studied by determining the most efficient methods of controlling the process of structure formation to achieve the highest physical and mechanical properties of renewable surfaces of vehicle parts. The effect of formation of anomaly amount of residual austenite in sprayed steel coatings was established. Technologies of application of the “austenitic effect” is suggested here to increase a coating wear-resistance. It is determined that the main factors influencing the content of residual austenite in hardened steel are the cooling rate of steel, the concentration of alloying elements in the austenitic phase, as well as thermal stabilization of austenite during self-tempering. It is shown that to ensure the formation in the structure of sprayed coatings of alloy structural, tool and corrosion-resistant steels of metastable austenite, which has a low flow temperature of deformation gamma-alpha transformation, which corresponds to the operating temperatures of sliding friction units, it is necessary to achieve certain coating conditions. wire spraying, cooling rate of molten particles and the degree of their oxidation). One of the most probable reasons for the appearance of the "austenitic effect" in coatings is the heating of the surface layer to a temperature that promotes thermal stabilization of austenite, as well as saturation of melt droplets with alloying elements (primarily chromium) and impurities (carbon, nitrogen) in flames. The relatively low flight speed of molten steel particles and the high concentration of propane containing carbon in the combustion products contribute to the deep saturation of the melt droplets with carbon. It is likely that these circumstances are associated with a high content of residual austenite in the coatings obtained by gas-flame spraying. An additional factor that increases the resistance of austenite in the sprayed coating may be the saturation of the droplets of the melt with carbon during melting and spraying using a propane flame. The studies under discussion have suggested that both for the method of gas-flame spraying and for the method of electric arc spraying, there are modes and steels for spraying that allow the formation of large amounts of metastable austenite in coatings, which in the process of tribocoupling will turn into martensite.On the basis of the carried-out researches technologies of restoration of details of vehicles by drawing multipurpose coverings in which the choice of a method of heating of a wire at spraying is carried out depending on temperature of the beginning of martensitic transformation of a wire material are offered.

Modelling and Simulation of Phase Formations in Martensitic Stainless Steels

Modelling and simulation of solidification processes and solid-state phase transformation have become key instruments in the field of alloy development and heat treatment optimization. Apart from equilibrium-controlled processes, also diffusion-based effects need to be considered. This contribution presents some typical approaches at the example of martensitic stainless steels. Important aspects affecting the production and properties of these steels, such as alloying limits with nitrogen, the formation of ledeburitic structures, or the retained austenite content after heat treatment, can be predicted with reasonable accuracy.

Cold work induced stability of retained austenite at elevated temperature in a medium carbon high silicon steel

In the present work, a potential solution has been suggested for arresting the depletion of the retained austenite during prolonged holding at elevated temperature. A medium carbon high silicon steel specimen was austenitized and then air cooled for 20s followed by austempering at 350 °C for 10 min. 20% retained austenite was measured from X-Ray diffraction studies in the specimen and 23.4 ± 1.7% uniform elongation was achieved. On holding the similar austempered specimen at 350 °C for 120 min, all the RA austenite depleted from the microstructure and uniform elongation values dropped to 5.7 ± 0.6%. In further thermomechanical treatments, austempered specimens were cold worked by 4, 7 and 10% at room temperature. The cold worked specimens were further held at elevated temperature (350 °C) for prolonged duration (120 min). Retained austenite content was observed to increase from 14 to 18% with the amount of cold work and tensile elongation value of up to 14% was observed. The decomposition of the retained austenite from the microstructure of cold worked specimens was arrested due to the increase in the dislocation density during cold work applied before holding at the elevated temperature. First principle calculations indicate migration of large number of carbon atoms towards the dislocations at elevated temperature thereby inhibiting the carbide precipitation during decomposition of retained austenite. Application of cold work to the austempered specimens could arrest the depletion of retained austenite at elevated temperature.

Influence of shielding gas nitrogen content on the microstructure and mechanical properties of Cu-reinforced maraging steel fabricated by wire arc additive manufacturing

This study investigated the effects of using shielding gases with different nitrogen concentrations on the microstructures and mechanical properties of SHER-120G maraging steel fabricated by wire arc additive manufacturing (WAAM). With increases in the nitrogen concentration, the contents of nitrogen and austenite in the bulk specimens increased significantly, while δ-ferrite formation was inhibited. In addition, nitrogen was absorbed by the molten pool and decomposed into N atoms during WAAM. Some N atoms formed a solid solution with the matrix and the rest reacted with Cr to form Cr2N, which plays an important role in the process of martensite lath refinement. The optimal shielding gas composition was 94% Ar + 2% O2 + 4% N2, which resulted in a tensile strength and micro-hardness of 1324 MPa and 441 HV, respectively. These values are 12.3% and 12.5% higher, respectively, than those of samples fabricated without nitrogen in the shielding gas.

Microstructure, Mechanical Properties, and Corrosion Behavior of Laser‐Welded Dissimilar Joints Between DP980 and QP980 Steel

Herein, fiber laser welding experiments are performed to evaluate the microstructure, mechanical properties, and corrosion behavior of dissimilar/similar joints between DP980 and QP980 steel. Results show that the fusion zone (FZ) of all joints consists of lath martensite with high microhardness (≈480 HV); the retained austenite content in the subcritical heat‐affected zone (HAZ) in the QP980 side of the dissimilar weld joint (WJ) is less than base metals (BMs). Lower hardness in the subcritical HAZ is found compared with BMs in all joints. The microstructure of softening zones is tempered martensite, and the reduction in hardness is about 8.5−10.8% of BMs, which has a great influence on the tensile fracture position of WJs. All joints fracture at softening zones with a typical feature of ductile fracture, and the joint efficiencies are above 96%. The electrochemical corrosion test reveals that the welded samples exhibit a higher corrosion rate compared with the BMs due to the increase in martensite content and area fraction of grain boundary.

Effect of Cryogenic Treatment on Microstructure, Mechanical Properties and Distortion of Carburized Gear Steels

The effects of cryogenic treatment and low temperature tempering on the microstructure, mechanical properties and distortion of the 20Cr2Ni4A and 17Cr2Ni2MoVNb carburized gear steels were investigated. The results showed that the case hardness of the experimental steels was increased after the cryogenic treatment, due to the decrease of the retained austenite content and the precipitation of the tiny carbides. The wear resistance of the two steels after cryogenic treatment was improved, although the wear mechanisms were different for 17Cr2Ni2MoVNb and 20Cr2Ni4A steels. The distortion of the Navy C-ring specimens underwent shrinkage before expansion during the cryogenic process, and the distortion of 17Cr2Ni2MoVNb steel was smaller than that of 20Cr2Ni4A steel.

Influence of Quenching and Partitioning Parameters on Phase Transformations and Mechanical Properties of Medium Manganese Steel for Press-Hardening Application

It has been proven that through targeted quenching and partitioning (Q & P), medium manganese steels can exhibit excellent mechanical properties combining very high strength and ductility. In order to apply the potential of these steels in industrial press hardening and to avoid high scrap rates, it is of utmost importance to determine a robust process window for Q & P. Hence, an intensive study of dilatometry experiments was carried out to identify the influence of quenching temperature (TQ) and partitioning time (tp) on phase transformations, phase stabilities, and the mechanical properties of a lean medium manganese steel. For this purpose, additional scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDX) examinations as well as tensile testing were performed. Based on the dilatometry data, an adjustment of the Koistinen–Marburger (K-M) equation for medium manganese steel was developed. The results show that a retained austenite content of 12–21% in combination with a low-phase fraction of untempered secondary martensite (max. 20%) leads to excellent mechanical properties with a tensile strength higher than 1500 MPa and a total elongation of 18%, whereas an exceeding secondary martensite phase fraction results in brittle failure. The optimum retained austenite content was adjusted for TQ between 130 °C and 150 °C by means of an adapted partitioning.

Understanding microstructural influences on hydrogen diffusion characteristics in martensitic steels using finite element analysis (FEA)

A two-fold approach is considered to study hydrogen (H) diffusion characteristics in martensitic steels. Initially, a multi-trap stress coupled H diffusion finite element model was developed to investigate the role of various trap states on effective H trapping during a four point bend test. The calculations show that high angle boundaries are more influential in controlling H diffusion in presence of low initial (bulk) H concentrations, while dislocations can have more pronounced impact, when the bulk H concentration is higher. A microstructural model comprising of prior austenite grains and packets was further developed. The study highlights the importance of packet boundaries (PBs) moderating H diffusion in martensite microstructure. The presence of retained austenite content affecting H diffusion paths was also studied. Overall, this parametric study presents complementary techniques in numerical modeling, as well as implications on the role of various microstructural entities affecting H diffusion.

Comparison of microstructure and mechanical properties of plastic mould steel for physical application

In this paper, plastic mold steel S136 and S136 SUP were studied for microstructural observation and mechanical properties through metallographic, SEM, EDS, XRD, hardness test and impact test. The results showed that after the same heat treatment, the martensite structure of S136 SUP was denser, the carbides were more uniform and finer, and the hardness was slightly lower but the toughness was greatly increased compared with that of S136. the residual austenite content of S136 and S136 SUP were 2.46% and 10%, respectively, and the heat treatment hardness was 50.1 HRC and 49.1 HRC, respectively. The impact toughness was 90.6 J and 299.1 J.

Effect of a two-step annealing process on deformation-induced transformation mechanisms in cold-rolled medium manganese steel

In this work, the relationship between the mechanical properties and deformation-induced transformation mechanisms in transformation-induced plasticity (TRIP) medium manganese steel subjected to a two-step annealing process is comprehensively examined. Two-step annealed samples have bimodal microstructures, which are reversely transformed from partially recrystallized thermal martensite into lath- and globular-shaped grains of austenite (γ)–ferrite (α). This process widens the distribution of austenite stability, increasing the product of strength and elongation (PSE) from 35.5 GPa% to 46.9 GPa% at 600 °C annealing. When the annealing temperature is further increased to 640 °C, the difference of partitioning Mn content in austenite transforms the strain-induced TRIP effect into the stress-assisted TRIP effect. The yield strength and PSE are significantly reduced to 596 MPa and 30.5 GPa%, respectively. In addition, different degrees of stability in austenite occur in different martensite transformations (i.e., γ → ε martensite, γ → ε martensite → α′ martensite) at the same deformation stage, and the deformation mechanism of γ is generally based on the cooperation of the TRIP effect and twinning. For the coarse metastable austenite grains annealed at 640 °C, the α’ martensite transformation is assisted by the applied stress. An intergranular fracture occurs because of a high stress concentration mismatch between the deformed martensite and ferrite, leading to premature fracture.

Influence of reversed austenite on the properties of a 13Cr4NiMo ultra‐low carbon martensitic stainless steel in different environments

AbstractIn this study, the influence of reversed austenite on the mechanical and corrosion resistance properties of 13Cr4NiMo martensitic stainless steel was investigated. The results reveal that the reversed austenite content in the sample increases with the increase of tempering times. The presence of an appropriate amount of reversed austenite not only improves the comprehensive mechanical properties of martensitic stainless steel but also reduces the susceptibility to pitting corrosion.

Temperature Dependence of Fracture Characteristics of Variously Heat-Treated Grades of Ultra-High-Strength Steel: Experimental and Modelling.

The temperature dependence of tensile characteristics and fracture toughness of the standardly heat-treated low-alloyed steel OCHN3MFA along with three additionally heat-treated grades was experimentally studied. In the temperature range of 〈−196; 22〉 °C, all the additional heat treatments transferred the standard steel from a high- to ultra-high strength levels even with improved tensile ductility characteristics. This could be explained by a reduction of the inclusion content, refinement of the martensitic blocks, ductile retained austenite content, and homogenization of the shape ratio of martensitic laths as revealed by metallographic, X-ray, and EBSD techniques. On the other hand, the values of the fracture toughness of all grades were found to be comparable in the whole temperature range as the cause of a high stress triaxiality in the pre-cracked Charpy V-notch samples. The values of the fracture toughness of the standard steel grade could be predicted well using the fracture model proposed by Pokluda et al. based on the tensile characteristics. Such a prediction failed in the case of additionally heat-treated grades due to the different temperature dependence of the fracture mechanisms occurring in the tensile and fracture-toughness tests. While the tensile samples fractured in a ductile-dimple mode at all temperatures, the fracture-toughness specimens exhibited a transition from the ductile to quasi-brittle fracture mode with decreasing temperature. This transition could be interpreted in terms of a transfer from the model proposed by Rice and Johnson to the model of Tvergaard and Hutchinson.

Processing of a newly developed nitrogen-alloyed ferritic-austenitic stainless steel by laser powder bed fusion – Microstructure and properties

In this work, a novel alloy design of a stainless steel with a ferritic-austenitic microstructure is derived for PBF-LB/M (powder bed fusion-laser beam/metal). The alloy was developed based on X2CrNiMo17-12-2 steel, for which an austenite volume content of approx. 54 vol% in the PBF-LB/M state was achieved using a reduced Ni equivalent. Partial substitution of Ni by Mn increases the N solubility of the alloy. By melting and further gas-atomizing this melt in an N2 atmosphere, an N content of 0.27 mass% was set in the produced steel powder. This leads to both high strength and high corrosion resistance of the PBF-LB/M-processed steel. However, microstructural investigations in the PBF-LB/M state confirm a microstructure consisting of ferrite, austenite, and Mo- and Cr-rich nitrides of M2N type. The nitrides were not completely eliminated by a subsequent heat treatment of the PBF-LB/M samples. As a result of the solution annealing, the microstructure approaches the thermodynamic equilibrium so that the austenite volume content increases from 54.2 vol% to 92.7 vol%. The higher Cr and N contents result in a higher corrosion resistance of the investigated steel compared to PBF-LB/M-processed X2CrNiMo17-12-2, regarded as the reference material. In addition, the measured strengths are significantly higher due to the larger amounts of austenite/ferrite interfaces and the N-induced solid-solution strengthening effect compared to X2CrNiMo17-12-2.

Microstructural characteristics of fusion zone in continuous wave fiber laser welded Nb-modified δ-TRIP steel

Welding processes were accomplished by means of a 600 W Continuous Wave (CW) fiber laser instrument in bead on plate manner with 60, 80 and 100 J mm−1 heat inputs. δ-TRIP Steel samples were similar in chemical composition and only contained different Nb (40Nb: 0.040 & 72Nb: 0.072 wt.% Niobium) amounts. Elevation of heat input resulted in changes in microstructure and hardness properties, which are desirable for welders to guarantee safe joints in engineering structures. An increase in the heat input can lead to reduction of the austenite in weld metals of both steel sheets. The reduction of the austenite could be attributed to the increase in δ-ferrite volume fraction which was induced by availability of sufficient time for segregation of aluminum and formation of more dendrite structured δ-ferrite at lower cooling rates. Increase in δ-ferrite content results in decreasing austenite content required for transformation of austenite to martensite. The direct consequence of a decrease in the martensite volume fraction is reduction of hardness. It is shown that an increase in the heat input from 60 to 100 J mm−1 results in reduction of hardness difference between weld and base metals by 40% and 37% for 40Nb and 72Nb steels respectively.

Carbide-Free Bainite Transformations Above and Below Martensite Start Temperature Investigated by In-Situ High-Energy X-Ray Diffraction

Carbide-free bainitic advanced high-strength steels (AHSS) exhibits multiphase microstructures made of fine ferrite and retained austenite. The carbon partitions between these constituting phases during the bainitic transformation were studied above and below the martensite start temperature (MS) by in-situ high-energy X-ray diffraction experiments. The time-resolved analysis of the phase fraction, austenite lattice parameter, and ferrite tetragonality offer the possibility of establishing phase-scaled consistent and precise carbon balances. In the studied steel, the carbon content in ferrite ranges between 0.1 and 0.15 wt.% after isothermal treatments, meaning that the available carbon for austenite stabilization is reduced. The stasis conditions are close to the T0′ criterion below 400°C but are not reached at higher temperatures. Our multi-step experiments also prove that the transformation below MS is very sensitive to the carbon content in austenite and auto-catalytic nucleation processes inherited from prior transformations (martensite and bainite at higher temperature). The bainitic transformation thus does not follow additivity rules.

Evaluation of Thermal Damage Impact on Microstructure and Properties of Carburized AISI 9310 Gear Steel Grade by Destructive and Non-Destructive Testing Methods

Advanced aircraft gearboxes operate under high mechanical loads. Currently, aircraft gears are manufactured from chromium–nickel–molybdenum steel grades such as AISI 9310 or Pyrowear 53. The major causes of gear failure are wear and fatigue cracking. As the crack initiation occurs predominantly on the component surface, the gears are routinely subjected to surface hardening processes such as low-pressure carburizing and case hardening. The gears are manufactured in a multiple operation process, in which teeth grinding is a crucial step. Selection of improper grinding conditions can lead to local heat concentration and creation of grinding burns, which are small areas where microstructure and properties changes are induced by high temperature generated during grinding. Their presence can lead to significant reduction of gear durability. Therefore destructive and non-destructive (NDT) quality-control methods such as chemical etching or magnetic Barkhausen noise (MBN) measurements are applied to detect the grinding burns. In the area of a grinding burn, effects related to the over-tempering or re-hardening of the carburized case may occur. In this paper, the results of the studies on the characterization of microstructure changes caused by local heating performed to simulate grinding burns are presented. The areas with the over-tempering and re-hardening effects typical for grinding burns were formed by laser surface heating of carburized AISI 9310 steel. Analyses of the microstructure, residual stresses, retained austenite content, and non-destructive testing by the MBN method were performed. The correlation between the MBN value and the properties of the modified surface layer was identified. It was also found that the re-hardened areas had similar characteristics of changes in the Barkhausen noise intensity, despite the significant differences in the width of the overheated zone, which depended on the laser-heating process conditions.