Showing papers on "Austenite published in 2021"

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

Abstract: Over many decades, significant efforts have been made to improve the strength-elongation product of advanced high strength steels (AHSSs) by creating tailored multi-phase microstructures. Successive solid-state phase transformations for steels with a well selected chemical composition turned out to be the key instrument in the realisation of such microstructures. In this contribution, we first provide a brief review of the desired microstructures for Transformation-induced plasticity (TRIP), Carbide-free Bainitic (CFB), Quenching & Partitioning (Q&P) and Medium Manganese steels followed by comprehensive discussions on the phase transformations to be used in their creation. The implications for the steel composition to be selected are addressed too. As the presence of the right amount and type of metastable retained austenite (RA) is of crucial importance for the mechanical performance of these AHSSs, special attention is paid to the important role of successive solid-state phase transformations in creating the desired fraction and composition of RA by suitable element partitioning (in particular C and Mn). This critical partitioning not only takes place during final cooling (austenite decomposition) but also during the back transformation (austenite reversion) during reheating. This review aims to be more than just descriptive of the various findings, but to present them from a coherent thermodynamic / thermo-kinetic perspective, such that it provides the academic and industrial community with a rather complete conceptual and theoretical framework to accelerate the further development of this important class of steels. The detailed stepwise treatment makes the review relevant not only for experts but also metallurgists entering the field.

The effect of post-processing heat treatment on the microstructure, residual stress and mechanical properties of selective laser melted 316L stainless steel

Abstract: Additively manufactured 316L austenitic stainless steel typically displays a hierarchical microstructure consisting of fine columnar grains, cellular dislocation tangles and nano-inclusions, which provides a combination of exceptional strength and ductility. However, the rapidly solidified microstructure often contains significant residual stress and various post-processing heat treatments are generally used to relieve the residual stress and to alter the microstructure and properties. In this work, a 316L austenitic stainless steel additively manufactured by a laser-based powder bed fusion process (selective laser melting, SLM) was for the first time subjected to various heat treatments to systematically study the evolution of residual stress, microstructure and mechanical properties. Significant compressive residual stress was revealed in the core volume of the as-built condition, whilst moderate to full stress relief of 24%, 65% and ~90% was achieved upon 2 h post-processing annealing at 400 and 650 °C and solution annealing at 1100 °C for 5 min, respectively. The extent of stress-relieving is closely associated with the evolution of substructure (i.e., dislocation tangles), which also affects the yield strength. Marked alteration from the as-built metastable microstructure was seen except for the low-temperature treatment at 400 °C. This includes the precipitation of embrittling σ phase or its precursors at 650–800 °C which results in a reduction in ductility. Unlike conventional wrought 316L, no carbide formation was seen in the treatment temperature regime. Recrystallization of columnar grains and coarsening of nano-inclusions took place over time upon solution annealing at 1100 °C, causing softening and unexpected reductions in ductility. This work led to the establishment of heat treatment-property relationships and corresponding microstructural changes, which are of great significance for the component design and structural application of SLM 316L.

The effect of pulse current changes in PCGTAW on microstructural evolution, drastic improvement in mechanical properties, and fracture mode of dissimilar welded joint of AISI 316L-AISI 310S stainless steels

Abstract: In this paper, the effect of pulse current changes in the Pulsed Current Gas Tungsten Arc Welding (PCGTAW) on the various properties of dissimilar welding of AISI 316L-AISI 310S stainless steels was investigated. 10 mm thickness steel sheets were joined by the PCGTAW process with the background current (Ib) of 55, 70 and 85A, as well as the peak current (Ip) of 110, 130 and 150A. Then, optical microscopy (OM) and Field Emission Scanning Electron Microscopy (FE-SEM) techniques were used to study the microstructural evolution in different areas of the welded joints. Also, tensile, Charpy impact and Vickers microhardness tests were used to evaluate the effect of the pulsed current changes on mechanical properties. Finally, the fracture surfaces of Charpy impact and tensile tests samples were studied by FE-SEM. The weld metal (WM) microstructure consisted of austenite dendrites together with a low amount of delta ferrite in the grain-boundaries. Results also showed that by increasing Ib and decreasing Ip, the microstructure of the WM was changed from columnar dendritic to coaxial, and a very fine, dendritic one. This condition led to the reduction of the size of the dendrites and the amount of delta ferrite ingrain boundaries of the WM, as well as a reduction in the width of Unmixed Zone (UMZ) area. Moreover, all the welded joints were fractured from the AISI 316L stainless steels side. However, the results of the Charpy impact and microhardness tests showed that with the above variation in the welding parameters, hardness value and fracture energy of the WM increased significantly. Fractography of the surfaces showed a completely ductile fracture for both tensile and Charpy impact tests samples.

Dislocation densities in a low-carbon steel during martensite transformation determined by in situ high energy X-Ray diffraction

Abstract: The evolution of the dislocation densities in martensite and in austenite during the quench of a low-carbon (0.215 wt% C) steel is investigated in situ by the mean of a High Energy X-Ray Diffraction experiment on a synchrotron beamline. The line configuration offers an excellent time resolution well adapted to the studied martensitic transformation kinetics. The mean density of dislocations in martensite increases as the transformation proceeds confirming that dislocations are not homogeneously distributed between the laths in agreement with some recent post-mortem observations. The resulting spatial distribution of dislocations and the associated strain-hardening support the views assuming that lath martensite is a heterogeneous microstructure and behaves as a “multiphase” aggregate. In austenite, the increase in dislocation densities is even more significant meaning that austenite in martensite is also a hard phase, contradicting some recent theories attributing to films of retained austenite a major role in the plasticity of martensite.

Role of bainitic microstructures with M-A constituent on the toughness of an HSLA steel for seismic resistant structural applications

Abstract: Fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of fusion welded HSLA steel is severely reduced at low temperatures with the combination of deleterious microstructures such as bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF), and the martensite-austenite (M-A) constituents. Toughness is one of the basic requirements for seismic resistant steels. The three heat input conditions (different cooling rate) were thermo-mechanically simulated, and the combined effect of M-A constituents with bainitic microstructures on toughness was examined. As the heat input changes from low to high, a combination of acicular ferrite (AF) and BF microstructures changes to a combination of PF, GB and M-A constituent. The former microstructure combination provides better toughness (ductile), whereas the latter makes the samples more brittle. The coarse BF, GB, and PF microstructures with low angle grain boundaries (LAGBs) make them brittle by easy crack propagation. Moreover, harder microstructures such as M-A can potentially act as cleavage initiation sites. However, a high frequency of high angle grain boundaries (HAGBs) in the AF microstructure can suppress the crack propagation, which turns the fracture to completely ductile. Also, it is observed that the M-A constituent in CGHAZ was abundant in martensite rather than the austenite.

Duplex Stainless Steels—Alloys for the 21st Century

Abstract: Duplex stainless steels were first manufactured early in the 20th century, but it was the introduction in the 1970s of the argon-oxygen decarburisation (AOD) steel making process and the addition of nitrogen to these steels, that made the alloys stronger, more weldable and more corrosion resistant. Today, duplex stainless steels can be categorised into four main groups, i.e., “lean”, “standard”, “super”, and “hyper” duplex types. These groups cover a range of compositions and properties, but they all have in common a microstructure consisting of roughly equal proportions of austenite and ferrite, high strength, good toughness and good corrosion resistance, especially to stress corrosion cracking (SCC) compared with similar austenitic stainless steels. Moreover, the development of a duplex stainless-steel microstructure requires lower levels of nickel in the composition than for a corresponding austenitic stainless steel with comparable pitting and crevice corrosion resistance, hence they cost less. This makes duplex stainless steels a very versatile and attractive group of alloys both commercially and technically. There are applications where duplex grades can be used as lower cost through-life options, in preference to coated carbon steels, a range of other stainless steels, and in some cases nickel alloys. This cost benefit is further emphasised if the design engineer can use the higher strength of duplex grades to construct vessels and pipework of lower wall thickness than would be the case if an austenitic grade or nickel alloy was being used. Hence, we find duplex stainless steels are widely used in many industries. In this paper their use in three industrial applications is reviewed, namely marine, heat exchangers, and the chemical and process industries. The corrosion resistance in the relevant fluids is discussed and some case histories highlight both successes and potential problems with duplex alloys in these industries. The paper shows how duplex stainless steels can provide cost-effective solutions in corrosive environments, and why they will be a standard corrosion resistant alloy (CRA) for many industries through the 21st century.

Sensitivity to hydrogen induced cracking, and corrosion performance of an API X65 pipeline steel in H2S containing environment: influence of heat treatment and its subsequent microstructural changes

Abstract: In this investigation, the effect of microstructural changes and phase equilibria on corrosion behavior and hydrogen induced cracking (HIC) sensitivity of an API X65 pipeline steel was studied. For this purpose, heat treatment was performed at 850 °C, 950 °C, 1050 °C and 1150 °C to engineer the desired microstructure of this pipeline steel. Then, the microstructural evolution was performed by optical microscopy, and Field Emission Scanning Electron Microscopy (FE-SEM) equipped with Energy Dispersive X-Ray Spectroscopy (EDS). Corrosion properties were evaluated in H2S environment by open circuit potential (OCP), Potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS). As well, HIC sensitivity of the API X65 pipeline steel was assessed by hydrogen charging of the cathode and immediately conducting the tensile test. Microscopy analyses showed that the microstructure of the steel is ferritic-pearlitic together with the islands of martensite/austenite constituents. Increasing the heat treatment temperature reduced the amount of pearlite and increased ferrite grain size. It also stabilized the ferrite content. Corrosion results indicated that no active layer was formed on the surface of this pipeline steel. Also, increasing the heat treatment temperature increased the corrosion resistance and reduced sensitivity to micro-galvanic localized corrosion. As well, results suggested that the sensitivity to HIC in the API X65 pipeline was substantially increased with increasing the amount of pearlite and reducing the amount of ferrite; i.e. at lower heat treatment temperature.

Nitrogen-induced hardening in an austenitic CrFeMnNi high-entropy alloy (HEA)

Abstract: Nitrogen is a well-known gamma-stabiliser in austenitic steels, also responsible for significant solid solution hardening of these materials. Yet, only few papers have studied its impact on austenitic high-entropy alloy (HEA) matrixes. This study focuses on a cobalt-free, non equimolar CrFeMnNi HEA doped with nitrogen. A series of alloys was cast under a nitriding atmosphere to promote nitrogen absorption into the liquid alloy. Study of as-cast alloys has shown nitrogen presence in solid solution up to 0.3 wt % (1.2 at. %). Over the whole range of compositions, a linear increase of hardness (134 HV/wt. % of N) was measured as well as an expansion of the lattice parameter of Δa/a = 1.01/wt. % N due to nitrogen addition in the interstitial sites of the lattice. Tests on forged and annealed samples showed that the increase of hardness with nitrogen addition is higher than in as-cast state (210 HV/wt. % of N) surely due to presence of other strengthening mechanisms. Tensile tests confirmed that the presence of dissolved nitrogen increases yield strength and ultimate strength and enhances strain-hardening, without any modification of ductility.

Evaluation of strain-induced martensite formation and mechanical properties in N-alloyed austenitic stainless steels by in situ tensile tests

Abstract: In this study, the influence of various nitrogen contents (0.12–0.23 wt.%) on the mechanical properties, especially the strain-induced α'-martensite formation behavior, of the austenitic stainless steel X3CrMnNiMoN17-8-4 was investigated by temperature dependent in situ tensile tests. With the aid of in situ magnetic measurements during tensile test, the correlation between the strain-induced α'-martensite formation and the inflection points in the true stress-strain curve could be verified. In addition, a connection between the in situ measured α'-martensite formation rate and the strain hardening curve was established. As the temperature decreases, the formation of a large strain-induced α'-martensite fraction allows a strong increase in strength accompanied by a simultaneous decrease in elongation. The α'-martensite volume fraction increase and the triggering stress for the strain-induced martensite formation decreases with decreasing nitrogen content from to 0.23 wt.% to 0.12 wt.%. The deformation mechanisms taking place at various temperatures during tensile test were analyzed by microstructure analysis. As expected, a transition from Transformation Induced Plasticity to Twinning Induced Plasticity behavior was observed with increasing temperature. Compared to the other examined steels, the steel with 0.19 wt.% nitrogen has the highest tensile strength of 856 MPa accompanied by an excellent total elongation of 75 % at RT.

Effect of heat treatments on 316 stainless steel parts fabricated by wire and arc additive manufacturing : Microstructure and synchrotron X-ray diffraction analysis

Abstract: Different geometrical features and intricate parts can now be fabricated by wire and arc additive manufacturing (WAAM). Even though a broad range of applications rises with this technology, the processed metallic materials still follow metallurgy rules. Therefore, undesired phases may appear during the multiple thermal cycles affecting the fabricated part. One of the most used stainless steel in the industry is the 316 L, which provides a combination of high corrosion resistance and mechanical properties. In this study, 316 L stainless steel walls were fabricated by WAAM and submitted to several heat treatments to understand the precipitation kinetics of secondary phases and observe the δ-ferrite dissolution with synchrotron X-ray diffraction measurements. The as-built samples presented δ-ferrite dendrites in an austenite (γ) matrix. In-situ observations showed σ precipitation during the first minutes of isothermal holding at 950 °C, from direct precipitation on the δ-ferrite islands. Solubilization heat treatments at 1050 and 1200 °C resulted in an undissolved amount of ferrite of approximately 6.5% and 0.4%, respectively. The amount of δ-ferrite showed a direct relationship with the hardness values. This work combined advanced materials characterization and thermodynamic calculations to rationalize the microstructure evolution upon the use of heat treatments in WAAM-fabricated 316 L stainless steel parts.

Tensile properties and deformation behavior of ferrite and austenite duplex stainless steel at cryogenic temperatures

Abstract: The tensile properties and deformation and fracture behaviors of a commercial ferrite (body-centered cubic) and austenite (face-centered cubic) duplex stainless steel were investigated at cryogenic temperatures, and the mechanism underlying the improvement in the tensile properties of the steel at cryogenic temperatures was discussed. The strength of the steel increased continuously with lowering temperature, and the elongation hardly decreased with a decrease in temperature from 293 K to 77 K. At 40 K, the elongation of the steel decreased drastically, however, it increased significantly at 8 K. The serration of the steel was measured during its tensile test at 8 K. The volume fraction of the deformation induced martensite was high at 77 K and 8 K, while it was low at 40 K. The strain accumulated in the ferrite and austenite phases during the deformation process remained almost constant at all the test temperatures. The ductile fracture occurred 293 K, 200 K, 77 K, and 8 K, while brittle fracture occurred at 40 K. The low volume fraction of the deformation induced martensite and the occurrence of brittle fracture can explain the low elongation of the steel at 40 K. At 8 K, the occurrence of serration may increase the temperature in the deformation region, which led to an increase in the volume fraction of the deformation induced martensite, as well as the suppression of brittle fracture, resulting in high elongation. The occurrence of deformation induced martensitic transformation, simultaneous deformation of ferrite and austenite, ductile fracture contribute to improvement of tensile property of ferrite and austenite duplex stainless steel at cryogenic temperatures.

Effect of Heat-Treatment on the Microstructure, Mechanical Properties and Corrosion Behaviour of SS 316 Structures Built by Laser Directed Energy Deposition Based Additive Manufacturing

Abstract: This paper reports investigation on the effect of heat-treatment on the microstructure, mechanical, tribological and corrosion characteristics of laser directed energy deposition (LDED) built stainless steel (SS 316) bulk structures. LDED built SS 316 structures are subjected to solution treatment at 1073 K (HT1073) and 1273 K (HT1273) and reduction in ferrite phase with heat-treatment is observed from microstructure. X-ray diffraction and microstructure shows that the austenite phase is observed at all conditions and reducing ferrite phase intensity is noticed with an increase in heat-treatment temperature. Improvement in the plasticity retaining capacity and reduction in micro-hardness by 72.8% and 6.75% are noticed with heat-treatment, respectively. It is observed that the corrosion rate and specific wear rate increases after heat-treatment. The maximum specific wear rate of 0.19375 × 10–4 mm3/min is observed in the HT1273 sample with wide and deep grooves noticed on the worn out surface of heat-treated samples. The SEM images of wear track is characterized with abrasive wear mechanism for as-built sample, while heat-treated samples shows plastic deformation, followed by spalling effect. The work paves a way to understand the effect of heat-treatment on LDED built SS 316 bulk structures.