Showing papers on "Austenite published in 2014"

Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

Abstract: Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (μ phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations.

Microstructure and Mechanical Behavior of 17-4 Precipitation Hardenable Steel Processed by Selective Laser Melting

Abstract: The mechanical behavior and the microstructural evolution of 17-4 precipitation hardenable (PH) stainless steel processed using selective laser melting have been studied. Test coupons were produced from 17-4 PH stainless steel powder in argon and nitrogen atmospheres. Characterization studies were carried out using mechanical testing, optical microscopy, scanning electron microscopy, and x-ray diffraction. The results show that post-process heat treatment is required to obtain typically desired tensile properties. Columnar grains of smaller diameters (<2 µm) emerged within the melt pool with a mixture of martensite and retained austenite phases. It was found that the phase content of the samples is greatly influenced by the powder chemistry, processing environment, and grain diameter.

Effect of SLM Parameters on Transformation Temperatures of Shape Memory Nickel Titanium Parts

Abstract: Selective laser melting (SLM) is used to manufacture dense nickel titanium (NiTi) parts. The reversible martensitic transformation of the NiTi parts is investigated with various SLM parameters. The parameters are in the same energy density range, composed of high laser parameters (HP: high laser power adjusted to high scanning speed) and low laser parameters (LP: low laser power adjusted to low scanning speed). The results are linked to the mechanical behavior and shape memory response achieved from compression and dilatometry tests. It is shown that the products may exhibit distinct transformation temperatures depending on the used SLM parameters. The atomized powders and the HP SLM parts with dominant pseudoelastic properties contain austenite at room temperature (due to their lower transformation temperatures), in contrast to the large thermal memory of the LP parts originating from martensitic phases (corresponding to higher transformation temperatures). The post-annealed samples undergo transformations in a comparable temperature range, implying no significant effect of SLM on composition of the originally used powder. The possible origin of the above findings is postulated and discussed.

Effect of fresh martensite on the stability of retained austenite in quenching and partitioning steel

Abstract: Restrictions on fuel consumption and safety in the automotive industry have stimulated the development of quenching and partitioning (Q&P) steel. This steel is expected to have very high strength in combination with acceptable ductility owing to its microstructure consisting of martensite with a considerable amount of retained austenite. The effect of retained austenite on the mechanical properties and its transformation stability were determined by stepwise uniaxial micro-tensile testing and subsequent electron backscatter diffraction (EBSD) study of a pre-selected region. The austenite fraction evolution with increasing plastic deformation and the influence of fresh martensite on the local strain distribution were quantified based on the orientation data. The decrease of the retained austenite as a function of the applied strain was described by an exponential function with the pre-exponential and exponential factors related to the starting austenite fraction and its transformation stability respectively. It was proven that the presence of fresh martensite has a negative influence on this austenite transformation stability due to its constraining effect on the strain distribution. This effects the mechanical properties manifested by changes in the strain hardening behavior and total elongation. The results suggest that the ductility of the Q&P steels can be improved by an appropriate design of the heat treatment schedule in order to ensure high retained austenite fractions without the presence of fresh martensite in the final microstructure.

Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels

Abstract: We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and 10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement.

Influence of prior austenite grain size on martensite–austenite constituent and toughness in the heat affected zone of 700MPa high strength linepipe steel

Abstract: Structure–mechanical property relationship studies were carried out on Gleeble simulated intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of 700 MPa linepipe steel microalloyed with Nb. The design of experiments was aimed at varying reheat temperature in the first pass to obtain different coarse grain size in the HAZ. This enabled the study of the effect of prior austenite grain size on martensite–austenite (M–A) constituent during the second pass reheating and its consequent influence on impact toughness. We elucidate here the role of phase transformation and the fraction, size, shape, distribution, and carbon content of M–A constituent on impact toughness. The data suggests that the fraction of M–A constituent is not influenced by grain size, but the size of M–A constituent is influenced by the prior austenite grain size, which consequently governs toughness. Coarse austenite grain size increases the size of M–A constituent and lowers the HAZ toughness. Coarse austenite grain associated with coarse M–A constituent along grain boundary is the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M–A constituent is favorable in obtaining high toughness. Good toughness is obtained on refining the prior austenite grain size in the CGHAZ during first pass and hence ICCGHAZ in the second pass.

Correlations between nanoindentation hardness and macroscopic mechanical properties in DP980 steels

Abstract: Nanoindentation measurements were obtained on eight commercially-produced DP980 dual-phase steels to quantify the hardness of the individual constituents, ferrite and martensite, in each steel. Each microstructure was also evaluated to determine grain size, martensite volume fraction (MVF), and retained austenite content. Nanoindentation hardnesses and quantitative microstructural measurements were correlated with tensile properties and performance in hole expansion tests to assess the importance of the individual constituent properties. Hole expansion samples were prepared with both sheared edges produced by mechanical punching, and non-deformed edges produced by electric discharge machining (EDM). Average material hardness based on nanoindentation data correlated directly to Vickers hardness measurements, verifying the capability of the nanoindentation technique to produce data consistent with traditional hardness measurements. Yield strength (YS) correlated directly to ferrite hardness indicating that, for a similar MVF and microstructural morphology, the YS is controlled by the strength of the softer matrix phase (ferrite). Hole expansion ratios (HER) on EDM samples decreased with an increase in both martensite and ferrite hardness, indicating that EDM HER values can be enhanced by softening both constituents. Punched-hole HER values decreased with increasing martensite hardness and martensite-to-ferrite hardness ratio, but were independent of ferrite hardness, indicating that softening the martensite while increasing the ferrite hardness could produce a higher HER.

Study on microstructure and mechanical characteristics of low-carbon steel and ferritic stainless steel joints

Abstract: In this work, examinations on the microstructure and mechanical properties of plain carbon steel and AISI 430 ferritic stainless steel dissimilar welds are carried out. Welding is conducted in both autogenous and using ER309L austenitic filler rod conditions through gas tungsten arc welding process. The results indicate that fully-ferritic and duplex ferritic–martensitic microstructures are formed for autogenous and filler-added welds, respectively. Carbide precipitation and formation of martensite at ferrite grain boundaries (intergranular martensite) as well as grain growth occur in the heat affected zone (HAZ) of AISI 430 steel. It is found that weld heat input can strongly affect grain growth phenomenon along with the amount and the composition of carbides and intergranular martensite. Acquired mechanical characteristics of weld in the case of using filler metal are significantly higher than those of autogenous one. Accordingly, ultimate tensile strength (UTS), hardness, and absorbed energy during tensile test of weld metal are increased from 662 MPa to 910 MPa, 140 Hv to 385 Hv, and 53.6 J m −3 to 79 J m −3 , respectively by filler metal addition. From fracture surfaces, predominantly ductile fracture is observed in the specimen welded with filler metal while mainly cleavage fracture occurs in the autogenous weld metal.

Effect of laser parameters on microstructure and hardness of laser clad and tempered AISI H13 tool steel

Abstract: In the present study, laser cladding of AISI H13 tool steel powder has been undertaken on hardened and tempered AISI H13 tool steel components using a 6 kW fiber coupled diode laser both in continuous wave and pulsed mode with coaxial powder feeder to explore the possibility of reclamation/refurbishment of large AISI H13 tool steel components. Laser parameters in terms of laser power, processing speed and powder feed rate have been optimized to achieve acceptable clad-substrate integrity with optimum deposition height and width without any surface cracks and porosity. A detailed study of the effect of post-cladding heat treatment (both by laser assisted surface heating and conventional tempering at 550 °C) on the microstructure, phase aggregate, crystallite size, micro-strain and residual stress was carried out. Subsequently, the microhardness of the clad zone was evaluated. Pulsed laser cladding (heat input controlled) with 50 Hz frequency and 50% duty cycle with 10 ms ON-time developed a microstructure comprising martensite, retained austenite and mixed carbides. The maximum improvement in hardness (considered as a convenient index of strengthening) by laser cladding reached up to 650 VHN, which is about 45% higher than the hardness of hardened and tempered substrates. Post-cladding tempering at 550 °C for 2 h led to the development of uniform microstructure without fine carbide precipitated along grain boundaries or inter-dendritic regions taking the hardness to 600–650 VHN level. Laser reheating (tempering) of the clad zone with a 1000 W laser power showed a more refined microstructure with maximum hardness up to 680–700 VHN. Residual stress, developed on the surface after laser cladding, was compressive in nature and measured higher in magnitude after pulsed laser cladding. Magnitude of this residual compressive stress marginally decreased after post-cladding tempering either by conventional or laser assisted route. Thus, the novelty of the present study lies in the continuous or pulsed mode laser cladding assisted reclamation of AISI H13 hot working dies with similar composition powder, followed up with conventional or laser assisted tempering, leading to the development of about 45% higher hardness (than as hardened and tempered conditions) and retention of residual compressive stress on the surface after cladding.

Tensile Behavior of Ti,Mo-added Low Carbon Steels with Interphase Precipitation

Abstract: Tensile behavior and structure-property relationship of ferritic steels with nano-sized carbide dispersion were invesigated using Ti-added steel and Ti,Mo-added low carbon steels By austenitizing followed by isothermal heat treatment at 700°C, polygonal ferrites containing very fine carbides of TiC and (Ti,Mo)C were obtained in the Ti-added and the Ti,Mo-added steels, respectively The size of such carbides was finer in the Ti,Mo-added steel than in the Ti-added steel at the same isothermal holding The results of tensile tests for these samples showed that the strength is higher as the carbide size is smaller The structure-based strength calculation led to a good agreement with the experiments, when it was assumed that the Ashby-Orowan mechanism is dominant for precipitation strengthening of nano-sized alloy carbides It was also suggested that a relatively large tensile ductility is related to enhanced recovery during the tensile deformation, accompanied with promotion of secondary slips or cross slips in a finer scale due to the nano-sized particles

Microstructure–properties relationship in a one-step quenched and partitioned steel

Abstract: To comprehensively analyze the relationship between the microstructure and mechanical properties in a one-step quenched and partitioned (Q&P) steel, Q&P processes with different partitioning time were applied to a low-carbon steel. Microstructures were characterized by means of EPMA, XRD, EBSD and TEM. The dislocation density of martensite was calculated using the Williamson–Hall method. Mechanical properties were measured by uniaxial tensile tests. Results show that the microstructures consist of lath martensite accompanying with both film-like inter-lath retained austenite and blocky retained austenite. Martensite laths broaden with prolonged partitioning time. The amount of retained austenite increases first and decreases with the critical partitioning time of 100 s. The relation between the microstructure and properties was clarified by analyzing the stress–strain curves stage by stage combining with the substructure of martensite and the condition of retained austenite. The presence of retained austenite decreases the elastic limit and influences on the yield strength for its early plastic deformation. Two kinds of nano-scaled carbide appearing in the specimens partitioned longer than 100 s promote the austenite decomposition and play the main role in increasing the yield strength. The interaction of the dislocations in martensite and the transformation induced plasticity (TRIP) effect of retained austenite increase the work hardening rate and improve both the ultimate tensile strength (UTS) and the uniform elongation.

Structure–property correlation in laser surface treated AISI H13 tool steel for improved mechanical properties

Abstract: The present study concerns laser surface hardening (LSH) and melting (LSM) of AISI H13 tool steel using a high power continuous wave diode laser. Depth of surface hardened or melted layer increases with increase in incident laser energy density. Surface melting occurs at a higher laser energy density (>75 J/mm 2 ) and leads to the formation of inhomogeneous microstructure comprising non-uniform distribution of retained austenite, carbides (along inter-dendritic boundary) and martensite with their respective volume fractions varying with depth. Application of intermediate laser energy density (50–75 J/mm 2 ) yields a hardened layer with dispersion of ultrafine mixed carbides (M 23 C 6 , M 7 C 3 , MC or M 2 C). Laser treatment with a very low laser energy density ( 2 ) leads to formation of an over-tempered microstructure consisting of low carbon martensite and coarse carbide precipitates. Micro-tensile studies with specially machined samples from the surface melted zone following LSM with a laser energy density of 100 J/mm 2 records a high yield strength of 1310 MPa along with poor ductility, marked by brittle failure. On the other hand, a similar sample from laser surface hardened zone treated with a laser energy density of 62.5 J/mm 2 yielded even higher yield strength of 1460 MPa with a maximum elongation of 3.6%. Though both LSH and LSM produced higher yield strength compared to hardened and tempered AISI H13 tool steel, LSH yielded a combination of higher elongation (3.6%), than that after LSM (0.97%), with high yield strength and hence was considered a better option.

High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides

Abstract: We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered κ-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L′12 ordered precipitates (so called κ-carbides) resulting in high strength (yield stress ∼1·0 GPa) and ductile (elongation to fracture ∼30%) steels. Novel insights into dislocation–particle interactions in a Fe–30·5Mn–8·0Al–1·2C (wt-%) steel strengthened by nanoscale κ-carbides are discussed.

Young’s Modulus of Austenite and Martensite Phases in Superelastic NiTi Wires

Abstract: Young’s moduli of superelastic NiTi wires in austenite and stress-induced martensite states were evaluated by three different experimental methods (tensile tests, in situ synchrotron x-ray diffraction, and dynamic mechanical analysis) and estimated via theoretical calculation from elastic constants. The unusually low value of the Young’s modulus of the martensite phase appearing in material property tables (<40 GPa) is generally ascribed in the literature to the fact that stress-driven martensitic transformation and/or twinning processes continue even beyond the transformation range and effectively decrease the value of the tangent modulus evaluated from macroscopic stress-strain curve. In this work, we claim that this low value is real in the sense that it corresponds to the appropriate combination of elastic constants of the B19′ martensite phase forming the polycrystalline wire. However, the Young’s modulus of the martensite phase is low only for wire loaded in tension, not for compression or other deformation modes. It is shown that the low value of the martensite Young’s modulus in tension is due to the combination of the unique coincidence of elastic anisotropy of the B19′ martensite characterized by the low elastic constant C55, austenite drawing texture, and strong martensite texture due to the martensite variant selection under tensile stress.

Tensile Behavior of Intercritically Annealed 10 pct Mn Multi-phase Steel

Abstract: The exceptional elongation obtained during tensile testing of intercritically annealed 10 pct Mn steel, with a two phase ferrite–austenite microstructure at room temperature, was investigated. The austenite phase exhibited deformation-twinning and strain-induced transformation to martensite. These two plasticity-enhancing mechanisms occurred in succession, resulting in a high rate of work hardening and a total elongation of 65 pct for a tensile strength of 1443 MPa. A constitutive model for the tensile behavior of the 10 pct Mn steel was developed using the Kocks–Mecking hardening model.

Co-optimization of wrought alumina-forming austenitic stainless steel composition ranges for high-temperature creep and oxidation/corrosion resistance

Abstract: A series of candidate alumina-forming austenitic (AFA) stainless steels designed to evaluate the effects of variation in Al, C, Cr, Mn, Nb, and Ni content on high-temperature tensile properties, creep, and oxidation/corrosion resistance were studied. The compositions assessed were based on medium Ni (20–25 wt%) and low Ni (12 wt%) AFA variations strengthened primarily by MC and/or M23C6 carbide precipitates, and a high Ni (32 wt%) AFA superalloy variation strengthened primarily by γ′-Ni3Al intermetallic precipitates. Tensile and creep properties were measured at 650 and 750/760 °C, oxidation resistance from 650 to 900 °C in air with water vapor and steam environments, and sulfidation–oxidation resistance in Ar–20%H2–20%H2O–5% H2S at 550 and 650 °C. Optimized composition ranges for different use temperatures ranges based on these evaluations are presented.

Cladding of aluminum on AISI 304L stainless steel by cold roll bonding: Mechanism, microstructure, and mechanical properties

Abstract: The AA1050 aluminum alloy and AISI 304L stainless steel sheets were stacked together to fabricate Al/304L/Al clad sheet composites by the cold roll bonding process, which was performed at temperatures of ~100 and 23 °C to produce austenitic and austenitic–martensitic microstructures in the AISI 304L counterpart, respectively. The peel test results showed that the threshold reduction required to make a suitable bond at room temperature is below 10%, which is significantly lower than the required reduction for cold roll bonding of Al sheets. The tearing of the Al sheet during the peel test signified that the bond strength of the roll bonded sheets by only 38% reduction has reached the strength of Al, which is a key advantage of the developed sheets. The extrusion of Al through the surface cracks and settling inside the 304L surface valleys due to strong affinity between Al and Fe was found to be the bonding mechanism. Subsequently, the interface and tensile behaviors of three-layered clad sheets after soaking at 200–600 °C for 1 h were investigated to characterize the effect of annealing treatment on the formation and thickening of intermetallic compound layer and the resultant mechanical properties. Field emission scanning electron microscopy, X-ray diffraction, and optical microscopy techniques revealed that an intermediate layer composed mainly of Al 13 Fe 4 , FeC and Al 8 SiC 7 forms during annealing at 500–600 °C. A significant drop in tensile stress–strain curves after the maximum point (UTS) was correlated to the interface debonding. It was found that the formation of intermediate layer by post heat treatment deteriorates the bond quality and encourages the debonding process. Moreover, the existence of strain-induced martensite in clad sheets was found to play a key role in the enhancement of tensile strength.