Phase transformations during the decomposition of austenite below Ms in a low-carbon steel

The purpose of this study is to investigate and understand the phase transformations during the decomposition of austenite, which occurs during isothermal treatments below the martensite start temperature (M_s) in a low-carbon steel. Isothermal holding treatments after rapid cooling to various temperatures (forming a controlled volume fraction of initial martensite) were carried out in a dilatometer. Results obtained by dilatometry, microstructural characterization and hardness were analyzed. This combination of results shows that the microstructures formed below the M_s temperature are mainly bainitic, mixed with tempered martensite. The kinetics of isothermal bainite formation was described by a nucleation-based transformation model. The complex competition and interactions between their transformation mechanisms during the isothermal holding at different temperature regimes are discussed.     

Martensitic transformation and work hardening of metastable austenite induced by abrasion in austenitic Fe-C-Cr-Mn-B Alloy - a TEM study

Abstract

The martensite transformation and work hardening of metastable austenite induced by abrasive wear in an austenitic Fe-C-Cr-Mn-B alloy were studied by TEM. The results show that an α' martensitic transformation occurred to form an elongated and equiaxial cellular dislocation substructure and the untransformed austenite matrix produced an equiaxial cellular dislocation substructure on the abraded surface. Electron diffraction patterns of the abraded material are composed of diffraction rings with series of broken arcs resulting from a fine grain structure and the deformation texture. The work hardening zone of austenite at the subsurface reveals heavy slip bands and deformation faults, at which the dislocations pile up. Examples of ? martensite induced by abrasive wear can be detected. The α' martensite transformation and metastable austenite work hardening should bring about an increase in surface hardness and wear resistance. Additionally, the cellular dislocation substructure of α' and γ have a significant effect on increasing the hardness of the wear surface. Observation by TEM indicates that the α' martensite transformation happens more easily in the austenite matrix near the carbide (Fe, Cr)₇C₃ as a result of the depletion of carbon and chromium.     

Austenite stability for quenching and partitioning treated steel revealed by colour tint-etching method

Abstract

The complex microstructures of quenching and partitioning treated (QP980) steel have been investigated using two-step colour tint-etching method and further verified by X-ray diffraction, electron backscattering diffraction, magnetisation measurements and Mössbauer spectroscopy. The colour tint-etching method can quantitatively discriminate the ferrite, martensite and retained austenite by obviously colour differences. It is found that retained austenite was observed inside both martensite and ferrite, and the fraction of retained austenite in martensite was statistically higher but more scattering than that in ferrite. Moreover, the retained austenite in martensite is a little bit more stable than that in ferrite by comparing the change of volume fraction retained austenite in both phases after tension.     

Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACT

The present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

High strain rate behavior, transformation-induced plasticity and fracture toughness characterization of cast and additionally tempered Fe85Cr4Mo8V2C1 alloy manufactured using a rapid solidification technique

This study focuses on the characterization of the microstructures of an FeCrMoVC alloy in two states (an as-cast and a heat-treated state) as well as the compressive strain rate-dependent material and fracture toughness behavior. Both microstructures consist of martensite, retained austenite and complex carbides. Tempering results in a transformation of retained austenite into martensite, the precipitation of fine alloy carbides, and diffusion processes. High yield stresses, flow and ultimate compressive strength values at a relatively good deformability were measured. The yield and flow stresses at the onset of deformation are higher for the heat-treated state due to higher martensitic phase fractions and fine precipitations of alloy carbides respectively. Compressive deformation causes a strain-induced transformation of retained austenite to α′-martensite. Hence, both high-strength alloys are TRIP-assisted steels (TRansformation-Induced Plasticity). However, the martensitic transformation is more pronounced in the as-cast state due to higher phase fractions of retained austenite already in the initial state. Examinations of strained microstructures showed decreased crystallite sizes with increasing deformation. It is assumed that, during plastic deformation, the amount of low angle grain boundaries increases while the incremental formation of α′-martensite leads to decreased crystallite size. In general, lower microstrains were determined in the heat-treated state as a consequence of stress relaxation during tempering. In comparison to commercially available tool steels, the determined fracture toughness K _Ic of both variants revealed relatively high fracture toughness values. It was found that the lower shelf of K _Ic is already reached at room temperature. Higher loading rates $ \dot{K} $ resulted in lower dynamic fracture toughness K _Id values. Notch fracture toughness K _A measurements indicate that the critical notch tip radii of the examined materials are slightly smaller than 0.09?mm.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 1 – Microstructural analysis

Abstract

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of δ ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475 ° C. The specimens aged at 625 ° C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe₂Mo Laves phase was detected by X-ray analysis. Above 625 ° C, Cr₂₃C₆ and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625 ° C.     

Electron backscattering diffraction study of acicular ferrite,bainite, and martensite steel microstructures

Abstract

This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In conclusion, it is shown that EBSD is a powerful tool for studying phase transformation and fracture mechanisms in steels on a microscopic scale.     

Carbon composition and temperature dependence of relative change in length during isothermal decomposition of Fe–C austenite

Abstract

The carbon composition and temperature dependence of the relative change in length during isothermal decompos ition of Fe–C austenite has been modelled. Decomposition of austenite above and below the A₁ temperature has been considered, as well as a two-step procedure where a specimen decomposes into ferrite and carbon enriched austenite at a temperature above A₁ and is subsequently subjected to transformation below A₁ leading to ferrite and cementite. Analytical expressions have been given for the relative change in length as a function of carbon composition, isothermal decomposition temperature, and degree of transformation. Predicted changes in length have been compared with experimental results.     

Carbon partitioning during bainite transformation in low alloy steels

Abstract

Carbon partitioning in untransformed austenite during bainite transformation has been studied using high speed dilatometry. It was found that in specimens partially transformed to bainite, during subsequent quenching to ambient temperature two martensite start temperatures M _s can be registered. Because M _s depends directly on a carbon content in austenite, the obtained results may indicate that the carbon concentration trapped in films of austenite between parallel subunits of bainitic ferrite is much larger than in the blocks of austenite. It would indicate the necessity of a substantial modification of bainite and martensite regions on the time–temperature–transformation (continuous cooling) diagrams.     

Pressure induced martensite transformation in plain carbon steel

Abstract

In the present study, plain low carbon steel with 0·033 wt-% carbon content was subjected to severe pressure during continuous cooling from austenite region. The pressure increased gradually and then suddenly released by the breakdown of ram under pressure. As a result, a microstructure composed of 80% lath martensite and 20% ferrite was produced. Results showed that the martensite formation is not due to the effect of cooling rate but the effect of hydrostatic pressure on the austenite to ferrite transformation start temperature Ar₃.     

Microstructure evolution during thermomechanical processing in 3Mn-0.1C medium-Mn steel

ABSTRACT

Medium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Influence of α′ martensite on shape memory effect in Fe-14Mn-5Si-9Cr-5Ni alloy

Abstract

The strain induced γ → ε and γ → α′ martensite transformations were examined by X-ray diffraction in an Fe-14Mn-5Si-9Cr-5Ni alloy. From the original microstructures of single phase austenite and two phase austenite and α′ martensite, the alloy was examined after prestraining at room temperature and recovery heating at 673 K, and finally, its shape memory effect was determined. It was shown that the alloy with the original two phase microstructure of autensite and α′ martensite exhibits a higher degree of shape recovery. The presence of α′ martensite in the original microstructure before prestraining can considerably improve the extent of shape recovery.     

Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Abstract

The kinetics of the martensitic transformation in Fe–0·80C has been determined from dilatometry data and shows no significant variation when the cooling rate is changed by two orders of magnitude. All kinetic data can be adequately simulated by the Koistinen and Marburger (KM) equation using a specific start temperature T_KM and rate parameter α_m. This finding supports the suggestion that the transformation is athermal, and moreover, the absence of a time dependence strongly indicates that autocatalytic nucleation does not contribute to the transformation kinetics in plain carbon steels on measurable time scales. Furthermore, dilatometry experiments with different austenitising conditions were conducted to examine the effect of the prior austenite grain size on the overall kinetics of martensite formation. The present results indicate that the progress of martensite formation beyond a fraction f?=?0·15 is independent of the prior austenitising treatment. It is therefore concluded that austenite–austenite grain boundaries have no significant effect on the overall nucleation and growth of athermal martensite, which is consistent with a model proposed by Ansell and co-workers.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

In situ observation on microstructural development of high-speed steel during carbon partitioning process

Quenching–partitioning–tempering (Q–P–T) process was applied to treat high-speed steel. Microstructural development and properties were studied using high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, X-ray diffraction and a drop hammer impact toughness tester. The results of impact toughness test showed that the impact absorption energy increased to 16.2–20.1?J along the Q–P–T process, namely 56–93% higher than that of traditional Q–T approach (10.4?J). The results of in situ HRTEM showed that the interface of martensite/austenite (α/γ) migrated from martensite towards austenite with the increase of holding time, which led to a reduction of retained austenite. One of the migration planes was (110)_α/(111)_γ. The cementite precipitation and slight decomposition of retained austenite were observed during carbon partitioning process.     

FCC to HCP transformation kinetics in a Co–27Cr–5Mo–0.23C alloy

In this work the kinetic aspects associated with the FCC → HCP martensitic transformation in a Co–27Cr–5Mo–0.23C alloy processed by powder metallurgy were investigated. In situ X-ray diffraction during isochronous heat treatments in a hot stage indicated that a fully metastable FCC matrix transforms rather fast at temperatures above 725 °C and reaches a maximum transformation into the HCP phase at 940 °C. Alternatively, when the matrix is HCP, some HCP martensite reverts to metastable FCC. Apparently, at low temperatures carbon excess in the HCP martensite promotes the reversal to metastable FCC. In addition, the volume percent of ε-martensite precipitated from stable FCC was determined as a function of time and temperature during isothermal aging between 675 and 900 °C. From these results, TTT diagrams were plotted for a 1% HCP transformed martensite. Maximum transformation rates were found to occur between 825 and 850 °C and activation energies, Q _s of 41–52 kcal/mol were estimated from the experimental outcome. The aged microstructures indicated that below 800 °C, the isothermal transformation was dominated by a lamellar morphology. Nevertheless, aging above 800 °C promoted carbide nucleation and coarsening along the grain boundaries independently of the FCC → HCP martensitic transformation.     

Modification of microstructure to increase impact toughness of nanostructured bainite–austenite steel

Abstract

To improve impact toughness of the nanostructured bainite–austenite steel, a heat treatment operation was developed to divide prior austenite grains by plates of martensite directly before isothermal transformation. In the investigation, nanostructured steel containing 0·55%C, 1·95%Mn, 1·82%Si, 1·29%Cr and 0·72%Mo was used. It was found that a partial transformation to martensite achieved by cooling to 160°C followed by direct isothermal transformation to bainite at 225°C was the most promising treatment to improve Charpy impact energy of the investigated steel. For each testing temperature: ambient, 0, ?20, ?40 and ?60°C, the specimens subjected to the developed treatment showed a higher averaged impact energy than the specimens subjected to the standard treatment.     

Effect of pre-quenching on martensite-bainitic microstructure and mechanical properties of GCr15 bearing steel

In order to improve the strength and toughness of steel GCr15 (52100), the effect of different amounts of pre-transformed martensite on the kinetics of isothermal bainitic transformation and the strength and toughness of martensite-bainitic (MB) duplex microstructure has been studied by using pre-quenching after conventional 850°C heating to different temperatures (220, 200, 180°C) below M _s and then isothermal treatment at 240°C. The experimental results show that the accelerating effect of pre-quenched martensite on isothermal bainitic transformation principally depends upon the pre-quenching temperature (the amount of pre-quenched martensite). The MB duplex microstructure with 33% pre-transformed martensite has the optimum combination of strength and toughness.     

Age hardening behaviour during reverse transformation from martensite to austenite in Fe–31·1wt-%Ni

Abstract

The effects of thermomechanical treatments on the reverse transformation behaviour from twinned plate martensite to austenite in Fe–31·1%Ni have been studied. The variation of both diffusion controlled and diffusionless reverse transformation with temperature and time was examined. Diffusional reversion was dominant at lower reheating temperatures and led to a fine martensite–austenite duplex microstructure with a grain size of 0·01–0·1 μm, which caused a remarkable hardening ?H_v of 170–230 HV during aging. Cold working of the martensite promoted diffusional reversion and enhanced age hardening. X-ray analysis indicates that the age hardening is caused mainly by elastic strain resulting from coherent precipitation of austenite in martensite.

MST/1414     

Effect of Cux substituted for Gax on the thermodynamic and fracture behavior in magnetic shape memory Ni50Mn30Ga20 − xCux with high transformation temperature

The effect of Cu_x substituted for Ga_x on the thermodynamic characteristic, evolution of phases and fracture behavior in Ni₅₀Mn₃₀Ga_20???xCu_x alloys have been investigated. The results show that the martensitic transformation, reverse transformation temperatures and average interface temperature of Ni₅₀Mn₃₀Ga_20???xCu_x sample increase remarkably with increased content of Cu. The hysteresis temperature of the austenite to martensite transition firstly showed a steep rising trend, then almost reduced to a lower rate with the increased content of Cu up to 4 at.% (the maximum level of copper addition studied). The (202) diffraction peak position was observed to shifted towards high angles, when the Cu content was lower than 1 at.%. Further, the, M₂₀₂ diffraction peak was found to split into two diffraction peaks when the Cu content was more than 1 at.%. In addition, the compressive strength and fracture strain of alloys is clearly enhanced with increased content of Cu. Furthermore, the fracture type of Ni₅₀Mn₃₀Ga_20???xCu_x alloys was found to changes from intergranular fracture for the Ni–Mn–Ga alloys, to transgranular cleavage fracture with increase in Cu content.