Showing papers on "Austenite published in 2016"

The microstructure, mechanical properties and corrosion resistance of 316 L stainless steel fabricated using laser engineered net shaping

Abstract: The mechanical properties and corrosion resistance of 316 L stainless steel fabricated using the Laser Engineered Net Shaping (LENS) technique have been studied. The crack-free, full density samples made using SS316L alloy powder and the LENS technique are characterized by an unusual distinct dual-phase microstructure. STEM analysis revealed a significant increase of Cr and Mo content and a decrease of Ni in the grain boundaries. Based on the Cr and Ni content (austenite stabilizing elements), the Schaeffler diagram and the EBSD results, the existence of intercellular delta ferrite on subgrain boundaries and austenite in the fine-grains are observed. The XRD patterns, in addition to the FCC austenite phase, revealed the second BCC ferrite phase. Moreover, the sigma (FeCr) phases are present in the analyzed 316 L stainless steel. The occurrence of ferrite, which does not occur in regular stainless steel fabricated using conventional metallurgical methods, improves the mechanical and corrosion properties of the LENS-fabricated sample made using 316 L stainless steel powder. The obtained results prove that the microstructure of the SS316L fabricated using LENS is heterogeneous; its impact on the mechanical properties is visible. The analyzed samples are characterized by anisotropic mechanical properties that are favorable. For both the perpendicular and parallel directions of tensile tests, samples had a ductile fracture with many dimples inside of the larger dimples. The corrosion potential of SS316L LENS and classically manufactured steel is similar. The SS316L fabricated using LENS is characterized by a relatively low value of corrosion current density, which translates into much smaller corrosion rates.

Deformation Mechanisms in Austenitic TRIP/TWIP Steel as a Function of Temperature

Abstract: A high-alloy austenitic CrMnNi steel was deformed at temperatures between 213 K and 473 K (−60 °C and 200 °C) and the resulting microstructures were investigated. At low temperatures, the deformation was mainly accompanied by the direct martensitic transformation of γ-austenite to α′-martensite (fcc → bcc), whereas at ambient temperatures, the transformation via e-martensite (fcc → hcp → bcc) was observed in deformation bands. Deformation twinning of the austenite became the dominant deformation mechanism at 373 K (100 °C), whereas the conventional dislocation glide represented the prevailing deformation mode at 473 K (200 °C). The change of the deformation mechanisms was attributed to the temperature dependence of both the driving force of the martensitic γ → α′ transformation and the stacking fault energy of the austenite. The continuous transition between the e-martensite formation and the twinning could be explained by different stacking fault arrangements on every second and on each successive {111} austenite lattice plane, respectively, when the stacking fault energy increased. A continuous transition between the transformation-induced plasticity effect and the twinning-induced plasticity effect was observed with increasing deformation temperature. Whereas the formation of α′-martensite was mainly responsible for increased work hardening, the stacking fault configurations forming e-martensite and twins induced additional elongation during tensile testing.

Comparison of microstructure and mechanical properties of 316 L austenitic steel processed by selective laser melting with hot-isostatic pressed and cast material

Abstract: Besides the chemical composition, the manufacturing route primarily determines a material's properties. In this work, the influence of the manufacturing process of the 316 L grade austenitic steel on the microstructure and the resulting material properties were investigated. Thus, the microstructure and mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM) and SLM+HIP, were compared. A SLM parameter study illustrates that the porosity of SLM-densified specimens can be reduced with direction of a higher exposure time and a smaller point distance. With an additional treatment by HIP, the porosity scarcely changes, while cracks are reduced. The mechanical properties were investigated depending on the manufacturing process, and the influence of the sample build up by SLM was examined. High mechanical values have been obtained; in particular, the yield strength in the SLM-densified condition is much higher than in cast or HIP condition, as a result of the smaller grain size.

Additive Manufacturing of 17-4 PH Stainless Steel: Post-processing Heat Treatment to Achieve Uniform Reproducible Microstructure

Abstract: 17-4 precipitation hardenable (PH) stainless steel is a useful material when a combination of high strength and good corrosion resistance up to about 315°C is required. In the wrought form, this steel has a fully martensitic structure that can be strengthened by precipitation of fine Cu-rich face-centered cubic phase upon aging. When fabricated via additive manufacturing (AM), specifically laser powder-bed fusion, 17-4 PH steel exhibits a dendritic structure containing a substantial fraction of nearly 50% of retained austenite along with body centered cubic/martensite and fine niobium carbides preferentially aligned along interdendritic boundaries. The effect of post-build thermal processing on the material microstructure is studied in comparison to that of conventionally produced wrought 17-4 PH with the intention of creating a more uniform, fully martensitic microstructure. The recommended stress relief heat treatment currently employed in industry for post-processing of AM 17-4 PH steel is found to have little effect on the as-built dendritic microstructure. It is found that, by implementing the recommended homogenization heat treatment regimen of Aerospace Materials Specification 5355 for CB7Cu-1, a casting alloy analog to 17-4 PH, the dendritic solidification structure is eliminated, resulting in a microstructure containing about 90% martensite with 10% retained austenite.

Aging Behaviour and Mechanical Performance of 18-Ni 300 Steel Processed by Selective Laser Melting

Abstract: An 18-Ni 300 grade maraging steel was processed by selective laser melting and an investigation was carried out on microstructural and mechanical behaviour as a function of aging condition. Owing to the rapid cooling rate, the as-built alloy featured a full potential for precipitate strengthening, without the need of a solution treatment prior to aging. The amount of reversed austenite found in the microstructure increased after aging and revealed to depend on aging temperature and time. Similarly to the corresponding wrought counterpart, also in the selective laser-melted 18-Ni 300 alloy, aging promoted a dramatic increase in strength with respect to the as-built condition and a drop in tensile ductility. No systematic changes were found in tensile properties as a function of measured amount of austenite. It is proposed that the submicrometric structure and the phase distribution inherited by the rapid solidification condition brought by selective laser melting are such that changes in tensile strength and ductility are mainly governed by the effects brought by the strengthening precipitates, whereas the concurrent reversion of the γ-Fe phase in different amounts seems to play a minor role.

Comparison of Maraging Steel Micro- and Nanostructure Produced Conventionally and by Laser Additive Manufacturing.

Abstract: Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure—in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. Here, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition—i.e., the intrinsic heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing.

Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties

Abstract: To achieve high thermal efficiency, modern day thermal power plants operate at higher operating temperature and pressure which necessitates use of steels with high creep rupture strength such as modified 9Cr-1Mo steels. In the present study, the evolution of phases in modified 9Cr-1Mo P91 steel and their effects on microstructural stability and mechanical properties have been studied for specimens that were subjected to different thermal heat treatment conditions. The main focus has been to study the effect of heat treatment temperature ranging from 623 K to 1033 K (350–760 °C) on P91 steel. Further, the effect of furnace cooling, water quenching, tempering at 1273 K (1000 °C) and austenitizing on the mechanical properties and microstructure has been studied. The techniques used for material characterization were scanning electron microscopy (SEM), optical microscopy (OM) and X-ray diffraction. For low tempering temperature, i.e. 623 K (350 °C), M 23 C 6 , M 3 C, M 7 C 3, and MX precipitates have been observed with high yield strength (YS), tensile strength (UTS), hardness and low toughness. In the high temperature range, 923–1033 K (650–760 °C), fine MX, M 7 C 3 , M 23 C 6 , M 2 X, and M 3 C precipitates have been observed with low YS, UTS, hardness and high toughness. The steel tempered at 1033 K (760 °C) was observed to be having best combination of YS, UTS, hardness, toughness and ductility.

Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

Abstract: Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

Grain-resolved analysis of localized deformation in nickel-titanium wire under tensile load.

Abstract: The stress-induced martensitic transformation in tensioned nickel-titanium shape-memory alloys proceeds by propagation of macroscopic fronts of localized deformation. We used three-dimensional synchrotron x-ray diffraction to image at micrometer-scale resolution the grain-resolved elastic strains and stresses in austenite around one such front in a prestrained nickel-titanium wire. We found that the local stresses in austenite grains are modified ahead of the nose cone–shaped buried interface where the martensitic transformation begins. Elevated shear stresses at the cone interface explain why the martensitic transformation proceeds in a localized manner. We established the crossover from stresses in individual grains to a continuum macroscopic internal stress field in the wire and rationalized the experimentally observed internal stress field and the topology of the macroscopic front by means of finite element simulations of the localized deformation.

Texture and anisotropy in selective laser melting of NiTi alloy

Abstract: Selective laser melting (SLM) is used to manufacture dense NiTi parts. The microstructure and texture are assessed (before and after annealing followed by furnace cooling) and linked to the compression behaviour and shape memory response. It is shown that SLM strongly orients the fine austenite subgrains towards the building direction. This texture induces the highest spring back along the building (vertical) direction and the lowest along the horizontal direction after compression. The compressive stiffness, on the other hand, is the highest along horizontal direction and the lowest in vertical direction. The internal stresses due to SLM processing are another factor that may induce large martensite plates, decreasing the spring back. Although post-annealing (followed by furnace cooling) annihilates these large SLM stress-induced martensite plates, it is unsuccessful to achieve completely isotropic properties. The furnace cooling after annealing may even segregate austenite and martensite within SLM solidified tracks, causing a mixed shape memory response.

Austenite stability and its effect on the toughness of a high strength ultra-low carbon medium manganese steel plate

Abstract: A novel two-step intercritical annealing process was designed for an ultra-low carbon medium manganese steel plate. Excellent mechanical properties with yield strength of 590 MPa, tensile strength of 840 MPa, total elongation of 28.5% and high impact energy of 106 J at −80 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite together with a small amount of martensite after the two-step intercritical annealing. Both lath-like and blocky retained austenite with volume fraction of ~25% and relatively poor stability were obtained. The submicron-sized lath-like retained austenite exhibited Nishiyama-Wassermann (N-W) orientation relationship with the neighboring martensitic ferrite lath. The fine grain size played a crucial role in stabilizing austenite during phase transformation by significantly lowering Ms temperature and increasing the elastic strain energy. The overall stability of retained austenite during deformation was considered to be mainly governed by the chemical composition of the studied steel. The mechanism of toughening was elucidated. The superior low-temperature toughness was associated with TRIP effect of metastable retained austenite, which relieved the local stress concentration, enhanced the ability to plastic deformation and delayed the initiation and propagation of microcracks.

Iterative Determination of the Orientation Relationship Between Austenite and Martensite from a Large Amount of Grain Pair Misorientations

Abstract: An automatic, iterative method to determine the orientation relationship between parent austenite and martensite is described. The algorithm generates the orientation relationship from grain boundary misorientations through an iterative procedure based on correct symmetry operator assignment. The automatic method is demonstrated to work on both martensitic and bainitic steels and to provide comparable results to a manual grain selection method.

Novel ferritic stainless steel formed by laser melting from duplex stainless steel powder with advanced mechanical properties and high ductility

Abstract: Stainless steel bodies with relative density of 99.5% (with the theoretical density being 7.8 gr/cm3) were manufactured by laser melting (LM) of duplex 2507SAF steel powder. The crystalline phases of starting powder were fully ferrite with only a small trace of austenite. The chemical composition was unchanged during laser melting. A unique mosaic-type structure with mosaics of 100–150 µm size was formed after LM. Recrystallized grains with 1–5 µm was formed in between the mosaic boundaries. A great number of entangled dislocation loops resembling a loops with 100–200 nm size were also formed inside each of these mosaics and also within recrystallized micron size grains at the mosaic boundary zones. Nitrogen enriched areas and nitride phase were detected in the inner microstructure of the laser melted samples. The measured tensile strength, yield strength and microhardness were 1214 MPa, 1321 MPa and 450 HV, respectively, which is superior to that of conventional ferritic, austenitic and duplex stainless steels. The Enhanced mechanical properties are due to a number of nano- and microstructure factors such as the nano-sized dislocation loops restricting dislocation movements, different crystalline grain orientation of grains within the mosaics and boundary inclusions and precipitates that inhibit slip/slide effects. Despite of high strength and hardness, the laser melted ferritic steel was very ductile according to stress-strain curves and fracture analysis.

Influence of martensite-austenite (MA) on impact toughness of X80 line pipe steels

Abstract: Varying cooling rates are used to investigate the influence of martensite-austenite (MA) microconstituent on the mechanical properties of X80 linepipe steel after intercritical reheating. It is shown that air and water cooling forms MA, while furnace cooling does not. The increase in cooling rate decreases carbon diffusion to promote MA during air or water cooling. Faster cooling using water results in a change of MA morphology from slender to blocky. Both water and air cooled samples exhibit poor impact toughness in comparison to furnace cooling, with lower Charpy impact toughness occurring with a high percentage of blocky MA following water quenching. It is shown that MA deteriorates toughness by facilitating debonding, cracking and crack initiation during impact testing.

Effect of retained austenite stability and morphology on the hydrogen embrittlement susceptibility in quenching and partitioning treated steels

Abstract: The effect of retained austenite (RA) stability and morphology on the hydrogen embrittlement (HE) susceptibility were investigated in a high strength steel subjected to three different heat treatments, i.e., the intercritical annealing quenching and partitioning (IAQP), quenching and partitioning (QP) and quenching and tempering (QT). IAQP treatment results in the coexistence of blocky and filmy morphologies and both QP and QT treatments lead to only filmy RA. No martensitic transformation occurs in QT steel during deformation, while the QP and IAQP undergo the transformation with the same extent. It is shown that the HE susceptibility increases in the following order: QT, QP and IAQP. Despite of the highest strength level and the highest hydrogen diffusion rate, the QT steel is relative immune to HE, suggesting that the metastable RA which transforms to martensite during deformation is detrimental to the HE resistance. The improved resistance to HE by QP treatment compared with IAQP steel is mainly attributed to the morphology effect of RA. Massive hydrogen-induced cracking (HIC) cracks are found to initiate in the blocky RA of IAQP steel, while only isolate voids are observed in QP steel. It is thus deduced that filmy RA is less susceptible to HE than the blocky RA.