Austenite Memory Research Articles

Multiscale in-situ observations of the micro- and nanostructure of a PH 13-8 Mo maraging steel during austenitization

The aim of the present work is to elaborate on the microstructural evolution of a PH 13-8 Mo maraging steel during austenitization by using multiscale in-situ techniques, which range from high-temperature electron backscatter diffraction and high-energy X-ray diffraction to high-temperature transmission electron microscopy. In order to supplement these in-situ experiments, samples quenched from different temperatures after isochronal heating are subjected to an in-depth microstructural characterization by means of atom probe tomography.The results indicate diffusion-dominated kinetics for the α’ to γ transformation in the investigated heating rate range. Moreover, high-temperature electron backscatter diffraction experiments confirm the occurrence of the austenite memory effect. In addition to the inheritance of grain size and crystallographic orientation of the prior austenite grains, a lath-like substructure remains in austenite, which consists of thermally stable, geometrically necessary dislocations. It is suggested that these dislocations play the key role for enabling recrystallization at higher temperatures without prior cold deformation. This phenomenon is evidenced for the first time in a PH 13-8 Mo maraging steel. However, it is believed that excessive amounts of carbides at austenite grain boundaries or an alteration of the dislocation structure could impede this abnormal recrystallization.

Evolution of ultrafine bainite microstructure during rapid heating process and the role of retained austenite

Rapid heat treatment is an important method to improve the properties of steel. Moreover, the study of the phase transformation process plays an important role in the formulation of rapid heat treatment. This paper investigates phase transformation behavior and microstructure evolution of ultrafine bainite at varying heating rates, and reveals the influence mechanism of retained austenite on austenitizing. Results showed that the decomposition of retained austenite was influenced by the diffusion of Si element. Therefore, the decomposition temperature of retained austenite is severely affected by the heating rate. Retained austenite completely transformed into cementite and ferrite when heating rate was below 5 °C/s. A small amount of retained austenite was kept when the heating rate was higher than 5 °C/s. The kept austenite effectively suppressed the hysteresis of Ac1 and resulted in a unique mechanism of austenite grain formation. When the heating rate range was 5–10 °C/s, some new austenite skipped the nucleation process and grew directly on the kept austenite, forming the large austenite grains. And other new austenite was obtained via nucleation and growth process. Thus, the grain size of bimodal distribution is formed. This leads to the phenomenon of austenite memory effect. When the heating rate was increased to 50 °C/s and higher, the austenite grain size was less than 14 μm. Therefore, rapid heating makes the phase transition lag. When the heating rate is high enough, part of the austenite is kept, which has an important influence on austenitizing process of the steel.

Creep rupture properties and microstructures of 9Cr–3Co–3W-Nd-B steel welded joints

The creep rupture properties of welded joints and microstructures before and after creep rupture were evaluated for the 9Cr–3W–3Co-Nd-B steel, and simulated HAZ test was conducted to evaluate the creep strength and microstructures of HAZ. The creep strengths of welded joints were almost equivalent to the average strength of base metal, and the welded joints ruptured in HAZ in a similar manner as conventional steels in low-stress tests. The results of creep strength evaluation for simulated HAZ exhibited that welded joints of 9Cr–3W–3Co-Nd-B steel were the least strong in the areas heated to a temperature near 1050 °C, indicative of being the cause of HAZ rupture at the long-term region. A reduction in the creep strength was lessened at temperatures immediately above the AC3 transformation temperature, likely due to a combined effect of suppression of grain refinement by austenite memory effect and maintenance of the amount of grain boundary precipitates partly affected by large prior austenite grains.

The microstructure evolution and tensile properties of medium-Mn steel heat-treated by a two-step annealing process

The martensitic hot-rolled 0.3C-6Mn-1.5Si (wt%) steel was annealed at 630 °C for 24 h to improve its cold rollability, followed by cold rolling and annealing at 670 °C for 10 min. The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines. A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally, proved the above process is feasible. “Austenite memory” was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite, leading to lath-shaped morphology of austenite and ferrite grains. “Austenite memory” disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization, resulting in globular microstructure and refinement of both austenite and ferrite grains. The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation, but the dislocation strengthening played a decisive role on the yielding behavior. The tensile curves change from continuous to discontinuous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement. A method by controlling the cold-rolled reduction is proposed to avoid the Lüders strain.

A Review of Austenite Memory Effect in HAZ of B Containing 9% Cr Martensitic Heat Resistant Steel

During the welding process of B containing 9% Cr martensitic heat resistant steel (9Cr-B steel), austenite memory effect (referred to that the prior austenite grains in the heat affected zone (HAZ) after welding inherit the shape and size of prior austenite grains before welding) occurs in its normalized sub-zone of HAZ and the grain refinement is suppressed, which can effectively prevent type IV crack, and improve the service life of the welded joint at high temperatures. In the present article, α/γ reverse transformation behavior in the normalized sub-zone of 9Cr-B steel HAZ is reviewed. Austenite memory effect of 9Cr-B steel is derived from B addition. The main mechanisms of austenite memory effect during α/γ reverse transformation are discussed. Various models of boron causing austenite memory effect are discussed in detail. Matrix microstructure also plays an important role in austenite memory effect. Effects of heating rate, peak temperature, and holding time at peak temperature on austenite memory effect are also discussed.

Austenite memory during reverse transformation of steels at different heating rates

The austenite (γ) structure reversely transformed from martensite in Fe–0.55C mass% C alloy was investigated at high temperatures using in-situ an electron backscatter diffraction (EBSD) method. When the heating rate was varied, a rapidly heating produced fine γ grains, in contrast, a slow heating induced coarse γ grains having the same crystal orientations as those of the prior γ grains; known as “γ memory”. The most likely mechanism for the γ memory could be variant restrictions owing to cementite (θ). To investigate the effect of θ on the γ memory, the sample was tempered at 700 °C for 24 h and θ was precipitated. The tempered sample exhibited the γ memory irrespective of the heating rate. To clarify the origin of the γ memory, we focused on the reversely transformed γ formed adjacent to the θ and directly observed the orientation relationships (ORs) using the in-situ EBSD. A crystallographic analysis on the between neighboring ferrites (α), θ, and γ revealed the existence of Kurdjumov–Sachs OR between γ and two α (α1 and α2), Isaichev OR between α and θ, and Pitsch OR between θ and γ. In the all possible combinations, the reversely transformed γ satisfying the K–S OR for two α variants was limited to four variants. The θ variant satisfying the Isaichev OR for α1 and α2 was limited to one variant, which was close to one of the four variants by Pitsch OR, which suggests the γ memory is due to these variant restrictions among α-θ-γ.

Evidence of austenite memory in PH 15-5 and assessment of its formation mechanism

Austenite memory and subsequent spontaneous recrystallization during austenitization is stated for some types of soft martensitic stainless steels such as X4CrNiMo16-5-1 or 13Cr5Ni but have not been proposed for the commercial alloy PH15-5 until now. However, the understanding of the austenitization behavior is defined to be crucial as it influences grain size, dislocation density of austenite and thus the final mechanical properties of the material.Therefore, this study investigates the austenitization behavior of PH15-5 by means of in-situ high temperature electron back scatter diffraction of continuous austenitization and directly evidences austenite memory also in this alloy. In order to characterize the austenite formation mechanism in detail, dilatometer, in-situ X-ray diffraction and confocal laser scanning microscope experiments have been carried out. Corroborated with thermokinetic DICTRA simulations, a diffusion controlled transformation exhibiting austenite memory is evidenced. The in-situ high temperature electron backscatter diffraction data in comparison with recent literature indicates the inheritance of geometrically necessary dislocations from martensite to austenite. Therefore, a basic model is postulated in order to describe the inheritance of geometrically necessary dislocations due to a diffusion controlled transformation process.

Effects of cold rolling reduction ratio on microstructures and tensile properties of intercritically annealed medium-Mn steels

The relationship between the cold rolling reduction ratio of medium-Mn steel before annealing, and the microstructure and tensile properties after annealing was investigated. The model alloy (Fe8Mn–0.2C3Al (wt.%) steel) was cold-rolled at different reduction ratios (between 0% and 60%) prior to intercritical annealing (IA) at 720 °C for 30 min. An increase in the cold rolling reduction ratio caused the nanolaminate morphology (α′ tempered martensite (α′temp) and retained austenite (γR) phases) to change to a nanoscale globular morphology (ferrite (α) and γR phases). However, the volume fraction of the γR phase, width of the nanolaminate phase, and the diameter of the nanoscale globular phase remained essentially unchanged, regardless of the cold rolling reduction ratio. The nanolaminate morphology exhibited a crystallographic orientation characterized by the absence of α′ recrystallization and austenite memory effect, that is preferable to the nanoscale globular morphology. The steel that was cold-rolled at a higher reduction ratio had a higher yield strength due to the increased slip resistance of the phases (consisting of a reduced area of the same orientation). In addition, with an increase in the cold rolling reduction ratio, the tensile strength and total elongation were improved owing to the more dynamic transformation- and twinning-induced plasticity in the nanoscale globular γR phase than the nanolaminate γR phase. Therefore, we have concluded that an optimized cold working prior to IA, will lead to improved mechanical properties of industrially produced medium-Mn steel.

Microstructure evolution during austenite reversion in Fe-Ni martensitic alloys

The change of microstructure during reverse transformation by continuous heating and isothermal holding above Af temperature were studied in Fe-11, 18 and 23 Ni (mass %) alloys. In-situ observation by using confocal laser scanning microscopy (CLSM) and in-situ/ex-situ electron backscatter diffraction (EBSD) analysis were used for direct observation of reverse transformation. It was found that the start temperatures (As) for austenite reversion decrease with increasing of Ni content while they are higher than T0 temperatures. Reverse transformation in the Fe-23 Ni alloy is accompanied with a sharp surface relief indicating that reverse transformation occurs martensitically in this alloy. EBSD measurements show that reversed austenite grains in this alloy are formed with nearly identical crystallographic orientations to the prior one, which means orientations and boundaries of prior austenite grains are preserved due to the austenite memory effect. By further holding above Af temperature spontaneous recrystallization of reverted austenite proceeds. The Fe-18 Ni alloy also shows similar microstructure change during reversion. Near Kurdjamov-Sachs (K-S) orientation relationship is found between reversed austenite and initial martensite during reversion of the Fe-18 and 23 Ni alloys. However, when the Ni content is decreased to 11%, no specific orientation relationship is found between reversed austenite and initial martensite, indicating that the reversion mechanism is changed from martensitic to partitionless diffusional (massive) mechanism.

Observation austenite memory and significant enhancement of tensile properties during cyclic reverse martensite transformation in a Fe-Ni-C TRIP steel

In this study, the influence of reverse martensite transformation (reverse transformation) on microstructure development and mechanical properties of Fe-24Ni-0.3C metastable austenitic TRIP steel was investigated. Microstructural characterization by electron backscatter diffraction (EBSD) system proved that large amount of low angle boundaries appeared after 1-cycle of reverse transformation (γ→α→γ). It is also found that the 1-cycle reversely transformed austenite and original austenite exhibited similar shape, size and orientations indicating that austenite memory appeared during reverse transformation. By increasing the number of reverse transformation cycle, fraction of low angle boundaries significantly increased. Uniaxial tensile test exhibited that yield and ultimate tensile strengths significantly improved even by 1-cycle reverse transformation comparing to the starting material. In addition, further continuation of reverse transformation up to 5- or 7-cycle causes gradual increase in yield and ultimate tensile strengths as well. The significant improvement in yield strength should be originated from increasing the dislocation density that are introduced during reverse transformation.

Phase-field simulation of austenite growth behavior: Insights into the austenite memory phenomenon

Austenite memory phenomenon impedes the application of reverse austenitic transformation to refine grains in steels. In this work, a phase-field model is employed to understand the austenite memory mechanism in terms of austenite growth behaviors under different mechanical boundary conditions, using the Fe–23Ni (wt.%) alloy as an example. The effect of defects formed during martensitic transformation on reverse austenitic transformation is considered by introducing a “stored energy” term. Kurdjumov–Sachs (K–S) variants of each phase are divided into three groups based on the crystallography analysis. Results show that different combinations of mechanical boundary conditions during the austenite→martensite→austenite transformation cycle have different effects on the austenite memory phenomenon, which can be attributed to the minimization of strain energy induced by phase transformations, as well as the inhomogeneous distribution of stored energy (energy of defects).

Austenite Memory and Variant Selection in a Novel Martensitic Welding Alloy

The crystallographic orientation relationship between the reverse austenite and corresponding martensite in a new residual-stress-reducing weld metal was predicted, using the general theory of martensitic transformation, and verified experimentally. The theoretical and experimental results of this work also provided an evidence of the likelihood of austenite memory phenomenon.

Effect of retained austenite on austenite memory of a 13% Cr–5% Ni martensitic steel

Abstract The effect of retained austenite on the microstructure evolution from martensite to austenite in relation to austenite memory of a 13% Cr–5% Ni martensitic steel was studied. In situ observations by confocal laser scanning microscope suggested that austenite memory occurred during the reverse transformation from martensite (α) to austenite (γ) in the presence of retained austenite, but was absent in a fully martensitic sample. The microstructure evolution during intercritical tempering treatment in the α + γ two phase region indicated that, reversed austenite formed in the retained austenite containing steel by using the pre-existing retained austenite as a substrate; it also bore the Kurdjumov–Sachs (K–S) orientation relationship with the martensite matrix and was stabilized by Ni enrichment. For the fully martensitic sample, a certain amount of martensite re-transformed to austenite while the rest transformed to tempered martensite; the tempered martensite then decomposed to ferrite with (Cr, Mo)23(C, N)6 precipitates. Austenite memory was absent due to the decomposition of the tempered martensite in the austenization process. Effect of the retained austenite on the reversion process from martensite to austenite was discussed in detail. A mechanism by which the retained austenite affected austenite memory was proposed.

An in situ study on austenite memory and austenitic spontaneous recrystallization of a martensitic steel

The phenomenon of austenite memory in 13Cr–5Ni martensitic steel during the reverse transformation from martensite to austenite has been observed in situ by confocal laser scanning microscope. The mechanisms of austenite memory and austenitic spontaneous recrystallization in relation to the austenite nucleation and growth characteristics from martensite during the reverse transformation have been studied. The results indicated that, acicular austenite led preferably to the recovery of the initial grain size and orientation and exhibited austenite memory when heated to A f temperature. Furthermore, it is proposed that the coherent interface formed between acicular austenite and lath martensite was the direct cause of austenite memory. Grain refinement was induced by nucleation of globular austenite based on austenitic spontaneous recrystallization, which occurred from 900 °C to 950 °C in austenite single phase region driven by phase transformation stored energy. 950 °C was the most appropriate temperature for grain refinement of 13Cr–5Ni steel.

Variant Selection of Reversed Austenite in Lath Martensite

The microstructure of partially reversed lath martensite in 13%Cr–6%Ni steel was examined by electron backscatter diffraction, and the crystallographic character of the reversed austenite is discussed in relation to the mechanism of ‘austenite memory’. Most of the reversed austenite grains had the same orientation as the original austenite matrix before martensitic transformation. However, some austenite grains had a different orientation in a twin relationship to the other major austenite grains, although all the reversed austenite grains retained a Kurdjumov–Sachs relationship to the martensite matrix. On the basis of the crystallographic relationships among the habit plane, the close packed direction of austenite and the martensite lath boundary, we suggest that the austenite variants are theoretically limited to two kinds within one packet and five kinds within one original austenite grain. In addition, we found that internal stress introduced by martensitic transformation plays an important role in determining the austenite variant: internal stress operates so that reversed austenite selects the same variant as that present in the original austenite matrix before martensitic transformation. This phenomenon is understood as the austenite memory.

Austenite memory effect in 1 Cr–1 Mo–0·75V(Ti, B) steel

Abstract A study of the austenite nucleation and growth characteristics of bainitic and martensitic 1 Cr–1 Mo–0·75V(Ti,B) steel has been made. Two forms of austenite are observed: these are termed ‘globular’ and ‘acicular’. Globular nucleation and growth leads to grain refinement whereas the acicular mode leads to reconstitution of the original austenite structure. The proportions of acicular to globular austenite are dependent upon heating rate, prior heat treatment, and chemical composition. Acicular nucleation occurs on stabilized retained austenite at bainitic or martensitic lath boundaries. Subsequent growth is restricted by these boundaries. Acicular grains within a given prior austenite grain have a common orientation owing to growth from retained austenite, and they coalesce on impingement, leading to a reconstitution of the original prior austenite grain structure. This effect has been termed the austenite ‘memory’. The presence of titanium carbide in the steel stabilizes the lath boundaries suff...

Direct Observation of an Austenite Memory Effect in Low-Alloy Steels

Heating cycles around the civil-type γ→α transformation were performed in the photoemission electron microscope (PEEM) on a variety of low-carbon steels.On some of these alloysa powerful memory effect for the details of the austenitic structure was detected, in that the latter was faithfully reconstructed on reheating the transformed ferritic structure into the austenite range. When the ferrite was supercooled below a critical temperature, TαcM, however, the memory effect was completely destroyed. This phenomenon was observed only in manganese-bearing alloys and did not depend on carbon or on carbide precipitation in the austenite. In addition, the results suggest that the memory effect exerts a control not only over the orientation of the γ-nuc1eiin a given ferrite region, but also on the extent of their subsequent growth.