Formation Of Athermal Martensite Research Articles

Role of Austenite Stability in Elevated Temperature Mechanical Properties of Gas Metal Arc-Directed Energy Deposition Austenitic Stainless Steels

A gas metal-directed energy deposition process was used to fabricate builds using two commercial weld fillers used in power generation applications, 16-8-2 and 316H. Microstructure stability and mechanical properties were investigated through room-temperature and elevated temperature tensile testing and creep testing at 650°C, 750°C, and 825°C. 16-8-2 exhibited reduced austenite stability which resulted in athermal martensite formation after aging at 650°C for 1000 h and strain-induced martensite formation during room-temperature tensile testing. 316H exhibited relatively higher austenite stability due to increased alloying content, resulting in no athermal martensite or strain-induced martensite. Due to lower austenite stability, ferrite formed during creep at 650°C in 16-8-2, which resulted in reduced creep life and lower creep ductility compared to 316H. At 750°C and 825°C, when ferrite is no longer thermodynamically stable, 16-8-2 exhibited longer creep life and similar creep ductility as 316H. The formation of ferrite in 16-8-2 appears to have a greater impact on creep performance than the formation of embrittling topologically close-packed phases like the σ phase, as 316H exhibited superior creep performance while predicted to form 14 vol.% σ phase at 650°C.

Modeling of Mechanical Properties of Metastable Austenitic Cr–Mn–Ni Steels with TRIP Effect

The mechanical properties of two metastable austenitic cast steels are examined by performing tensile tests in a temperature range of −70 to 300 °C. The 16‐6‐6 steel has an Ms temperature of −30 °C and an Md temperature of 100 °C. Athermal martensite formation down to −196 °C can be excluded for X3CrNiCuN17‐6‐4 steel. The Md temperature of X3CrNiCuN17‐6‐4 steel is 130 °C. The results obtained by a semiempirical thermodynamic‐mechanical calculation model are linked to experimental results from the tensile tests. The presented model combines chemical energy amounts from thermodynamic databases with mechanical energy amounts obtained by tensile tests with the aid of the conversion rule proposed by Patel and Cohen. Thermodynamic calculations are performed using the Thermo‐Calc software with the database TCFE10. As a result, the stress–temperature–transformation diagram showing the temperature dependence of proof stress, tensile strength, triggering stress for martensite formation, and the associated critical transformation temperatures (Ms, Md) is obtained. With the aid of the semiempirical thermodynamic‐mechanical model, it can be shown that the mechanical properties and critical temperatures of a metastable austenitic steel can be determined at very low experimental effort.

Characterization of the metastable austenite in low-alloy FeCMnSi TRIP-aided steel by neutron diffraction

Abstract The detailed analysis of X-ray diffraction data obtained from intercritically annealed and isothermally transformed low-alloy FeCMnSi TRIP-aided steels reveals that the microstructure contains athermal plate martensite and Fe2C η carbide in addition to ferrite, bainite and residual austenite. Neutron diffraction shows that athermal plate martensite can be formed at room temperature in the isolated austenite phase. Whereas the formation of athermal martensite leads to compressive strains in the austenite, the formation of strain-induced martensite results in tensile straining of the austenite. The strain-induced transformation leads to the formation of a martensite of low tetragonality. Low-temperature annealing leads to the formation of η carbide in both the athermal and strain-induced martensite.

Effect of Austempering below and above Ms on the Microstructure and Wear Performance of a Low-Carbon Bainitic Steel

The microstructure and wear performance of a low-carbon steel treated by austempering below and above martensite start temperature (Ms) were investigated. The results show that the bainite, fresh martensite (FM) and retained austenite (RA) were observed in samples austempered above Ms. Except for the three above phases, the athermal martensite (AM) was also observed in samples austempered below Ms. The bainite transformation was accelerated and finer bainite was obtained due to the AM formation in samples austempered below Ms. In addition, the strength and hardness were improved with the decrease of the isothermal temperature and time, whereas the total elongation decreased with the increasing isothermal time and the decreasing isothermal temperature. Moreover, the materials austempered below Ms exhibited better wear performance than the ones treated above Ms, which is attributed to the improved impact toughness by the finer bainite and the enhanced hardness by AM. The best wear resistance was obtained in the samples austempered at 300 °C below Ms for 200 s, due to the highest hardness and considerable impact toughness.

Role of the annealing twin boundary on the athermal α′-martensite formation in a 304 austenitic stainless steel

The preferential formation of α′-martensite at the annealing twin boundary (TB) was identified in a 304 austenitic stainless steel. The α′-martensite holds the N-W orientation relationship with austenite (matrix or twin). Atomic-level analysis reveals that the unidirectional shear into the <112>γ forms α′ in the annealing TB-containing austenite matrix. Formation of athermal martensite leaves stacking fault and ε-martensite at the surrounding matrix. The role of the annealing TB on the α′ formation was further discussed.

The Use of Acoustic Emission and Neural Network in the Study of Phase Transformation below MS.

Acoustic emission and dilatometry were applied to investigate the characteristics of phase transformations in bearing steel 100CrMnSi6-4 during austempering below the martensite start temperature (MS 175 °C) at 150 °C. The aim of this study is to characterize the product of transformation occurring below the MS temperature using various research methods. Analysis of the dilatometric curves shows that, after the formation of athermal martensite below the MS temperature, the austenite continues to undergo isothermal transformation, indicating the formation of bainite. Additionally, tests were carried out with the use of acoustic emission during isothermal hardening of the adopted steel. The obtained acoustic emission signals were analyzed using an artificial neural network. The results, in the form of a graph of the frequency of acoustic emission (AE) event occurrence as a function of time, make it possible to infer about the bainite isothermal transformation. The results of this research may be used in the future to design optimal heat treatment methods and, consequently, may enable desired microstructure shaping.

Tempering effects of athermal martensite during quenching and reheating of a SAE 52100 bearing steel

Athermal martensite formation is normally described as temperature-dependent. By dilatometer experiments, it will be shown that time-dependent effects occur during martensite formation of a bearing steel SAE 52100 (DIN EN 100Cr6). A series of different cooling rates or isothermal holding stages below martensite start temperature were applied. The nature of the time-dependent effects during cooling is enlightened by analyzing the tempering effects during a subsequent reheating. It was found that in particular the low temperature tempering effects were activated during cooling. Mainly the formation of transition carbides and the loss of tetragonality depended on the cooling condition. Retained austenite transformation was only affected by austenite stabilization and cementite formation was unaffected by the cooling condition.

Unravelling the mechanical behaviour of advanced multiphase steels isothermally obtained below Ms

The initial formation of athermal martensite was proven to accelerate the subsequent bainite formation kinetics during isothermal holdings below the martensite-start temperature (Ms). The presence of prior athermal martensite (PAM) within the phase mixture is expected to modify the overall mechanical response of these newly-designed multiphase steels. Differences stem not only from the balance of product phases, but also from the effect of tempering of the PAM with variations in the applied holding time. This work investigates the effect of tempering time on the mechanical behaviour of the PAM and, as consequence, on the overall mechanical response of these microstructures. Results show that, for short holding times (several minutes), PAM yields similar to as-quenched martensite while, for longer holding times, its yielding behaviour becomes comparable to the one exhibited by typical tempered martensite. Furthermore, the use of Kocks-Mecking curves for the analysis of the mechanical performance confirms the bainitic character of the product phase isothermally formed below Ms. Tailoring the bainitic-martensitic microstructure with variations of the holding time below Ms enables to obtain advanced multiphase steels with comparable mechanical properties to those exhibited by conventional bainitic steels, but in shorter processing times due to the acceleration of bainite formation.

Influence of the prior athermal martensite on the mechanical response of advanced bainitic steel

The accelerated formation of bainite in presence of martensite is opening a new processing window for the steel industry. However, for a feasible industrial implementation, it is necessary to determine the mechanical behaviour of the steels developed under such conditions. This study focuses on analysing the effects of the formation of athermal martensite, followed by the formation of bainitic ferrite, on the mechanical response of a low-C high-Si steel. For this purpose, microhardness measurements and tensile tests have been performed on specimens that were thermally treated either above or below the martensite-start temperature (Ms). Specimens isothermally treated below Ms exhibit a good combination of mechanical properties, comparable with that of the specimens heat treated by conventional treatments above Ms, where there was no prior formation of martensite. Investigations show an increase of the yield stress and a decrease of the ultimate tensile strength as the isothermal holding temperature is decreased below Ms. The formation of prior athermal martensite and its tempering during the isothermal holding leads to the strengthening of the specimens isothermally heat treated below Ms at the expense of slightly decreasing their strain hardening capacity.

On the prediction of martensite formation in metals

A new approach to predict athermal martensite formation in metals is presented. It is based on computing the driving force of the transformation including a strain energy term induced by atomic shear displacements and energy terms due to substitutional and interstitial lattice distortions. The model is applied to prescribe the martensite and austenite start temperatures in Fe-, Ti- and Co-based alloys with no adjustable parameters. Expressions for Ms variations with composition are derived for multicomponent systems. The transformation temperature hysteresis is predicted in Co alloys showing that this approximation can be used to design alloys with the shape memory effect.

Experimental Quantification of the Austenite‐Stabilizing Effect of Mn in CrMnNi As‐Cast Stainless Steels

In this study, the austenite‐stabilizing effect of manganese in as‐cast Fe–(12–16)Cr–(0–13)Mn–6Ni (concentrations in wt%) stainless steels was investigated by means of light optical microscopy (LOM) and magnetic scale measurements. The well known Schaeffler‐diagram was used for the prediction of the microstructure at room temperature. Major deviations were observed between the Schaeffler‐diagram predictions and the experimentally determined phase fractions. Manganese contents above 2% were found to stabilize austenite more than expected on the basis of the Schaeffler‐diagram. To make that, the Schaeffler‐diagram matches the experimental observations using CrMnNi steels, the boundary between austenite and austenite + martensite regions, which denotes the beginning of the athermal martensite formation, must be displaced to lower nickel‐equivalents. Alternatively, the efficiency coefficient of manganese in the Ni‐equivalent formula can be increased which is preferred as it does not involve displacement of the boundaries in the Schaeffler‐diagram. The best match between observed microstructures and the Schaeffler‐diagram was obtained by defining the manganese efficiency coefficient in the Ni‐equivalent formula as a quadratic equation of the Cr‐equivalent.

A study on martensitic phase transformation in 9Cr–1W–0.23V–0.063Ta–0.56Mn–0.09C–0.02N (wt.%) reduced activation steel using differential scanning calorimetry

The salient characteristics of martensite formation kinetics in 9Cr–1W–0.23V–0.063Ta–0.09C–0.02N (wt.%) reduced activation steel have been investigated using differential scanning calorimetry and supplemented by microscopy characterisation. Accurate determination of the martensite start ( M S) and finish ( M f) temperatures as a function of cooling rate (1–99 K min −1) and holding time (1–120 min) in the γ-austenite phase field have been made. The critical cooling rate to form nearly 100% martensite is found to be of the order of 5–6 K min −1. The measured latent heat for the martensite transformation is found to be in the range 62–75 J g −1. In the case of austenitisation at 1323 K, it is found that the grain size of the parent austenite exhibited significant growth and the observed M S temperature too exhibited a systematic increase with austenite grain size. However, for those samples that are solution treated at a slightly lower temperature of 1253 K, the M S is found to be initially insensitive to the increase in holding time for up to about 30 min. This trend is followed later on by the usual increasing character of M S for further rise in the holding periods, up to about 120 min. This unusual initial behaviour is also reflected by the corresponding variation in the hardness values of the quenched in martensite. Moreover, the growth of parent austenite grains has also been restricted during the initial phase of solution annealing at 1253 K. This is attributed to the presence of undissolved carbide particles, which exert a pinning influence on the movement of austenite grain boundaries. The observed athermal martensite formation kinetics is described well by the Koistinen–Marburger relation.

Effect of composition on kinetics of athermal martensite formation in plain carbon steels

The kinetics of martensite formation in three Fe–C–Mn alloys has been determined using dilatometry, and is compared with the data for other steels reported in literature. Each curve can be well described by the Koistinen–Marburger equation using a composition dependent start temperature and rate parameter αm. The empirical relationship derived for αm as a function of the chemical composition can improve predictions with the Koistinen–Marburger model of the volume fraction martensite at a certain temperature.

Experimental evidence for bainite formation below Ms in Fe–0.66C

The isothermal decomposition of austenite in the temperature range 220–300 °C in Fe–0.69Mn–0.66C has been studied using dilatometry and scanning electron microscopy (SEM). Analysis of the dilatometry curves shows that after athermal martensite formation is interrupted below Ms = 264 °C, the austenite continues to decompose isothermally at a certain rate, which is in agreement with the kinetics of bainite formation above Ms. This strongly indicates that isothermal bainite is formed below Ms, which is supported by SEM images showing a mixed martensite–bainite microstructure.

New observations on the formation of athermal martensite in Fe–Ni–Mo alloys

The thermally induced γ–ɛ and γ–α′ martensitic transformation in Fe–30%Ni– x%Mo alloys with different Mo contents have been studied by transmission electron microscopy (TEM). Observations reveal that α′ martensite is found in alloys containing 0.8, 1.8, and 2.6% Mo, and ɛ martensite in the alloy containing 5%Mo. The first three display the Kurdjumov–Sachs (K–S) orientation relationship between γ and α′, while the other displays the Shoji–Nishiyama (S–N) orientation relationship between γ and ɛ.

Effect of a dc magnetic field on the isothermal martensitic transformation in the Fe-24%Ni-4%Mn alloy

Effects of a dc magnetic field on the isothermal martensitic transformation in the region of the kinetic maximum of the Fe-24%Ni-4%Mn alloy and on the marternstic transformation at a liquid-helium temperature were studied. It was shown that a dc magnetic field markedly affects the isothermal martensitic transformation. In particular, it shifts the temperature range of the martensitic transformation and the kinetic maximum toward higher temperatures and initiates the nucleation of isothermal martensite crystals and their subsequent growth upon isothermal holding. A dc magnetic field of a sufficiently high strength causes the intense formation of athermal martensite at the liquid-helium temperature, at which no transformation develops without a magnetic field. A comparison of the effects of the pulse and dc magnetic fields on the martensitic transformation in the alloys with martensite transformation isothermal kinetics was carried out.

Effects of grain size on development of athermal and strain induced ɛ martensite in Co–Cr–Mo implant alloy

In the present work, the development of athermal ɛ martensite during quenching of a low carbon Co–Cr–Mo alloy was investigated as a function of the grain size. In addition, a strain induced transformation (SIT) from fcc to hcp was exhibited during compressive plastic straining. It was found that grain size exerts a strong influence on the resultant volume fractions of athermal and strain induced ɛ martensite. In particular, fine grain sizes inhibit the formation of athermal martensite while promoting appreciable volume fractions of ɛ martensite through the SIT mechanism. Moreover, X-ray diffraction analyses indicated that in 10 μm grained structures the volume fraction of strain induced ɛ martensite reaches a saturation level of approximately 0·65 just before compressive fracture. In contrast, increasing grain sizes result in the formation of up to 0·9 volume fraction of SIT martensite. Moreover, the alloy yield strength was found to decrease down to 592 MPa (approximately half the yield strength of the as received alloy). Annealing gave rise to appreciable improvements in the compressive strength (242 MPa) and ductility (0·41) when compared with the as received alloy (2141 MPa and 0·295 respectively). The alloy hardness initially drops from 42 to 29 HRC as the grain sizes increase from 10 to 90 μm. A further reduction in alloy hardness did not occur for grain sizes between 90 and 324 μm. Compression straining did not have a significant effect on the exhibited hardness of the as received alloy and only a moderate effect was found in coarse grained alloys. Alloys with grain sizes of 117 μm exhibited an increase in hardness from roughly 29 to 46 HRC through compression straining up to 0·407. Probable mechanisms are considered to account for the role of grain size on the development of athermal and SIT ɛ martensite.

Carbon Diffusion and Kinetics During the Lath Martensite Formation

Carbon Diffusion and Kinetics During the Lath Martensite Formation

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the Mb temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the Mb temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain.