Effect of martensite to austenite reversion on the formation of nano/submicron grained AISI 301 stainless steel

The martensite to austenite reversion behavior of 90% cold rolled AISI 301 stainless steel was investigated in order to refine the grain size. Cold rolled specimens were annealed at 600–900 °C, and subsequently characterized by scanning electron microscopy, X-ray diffraction, Feritscope, and hardness measurements. The effects of annealing parameters on the formation of fully-austenitic nano/submicron grained structure and the mechanisms involved were studied. It was found that annealing at 800 °C for 10 s exhibited the smallest average austenite grain size of 240 ± 60 nm with an almost fully-austenitic structure.     

Effect of reversion of strain induced martensite on microstructure and mechanical properties in an austenitic stainless steel

Effect of reversion of strain induced α′ martensite on mechanical properties of an austenitic stainless steel has been examined. The α′ martensite formed by cold rolling (40%) at 0 °C has been reverted to austenite by carrying out annealing in the temperature range of 300–800 °C for 1 h. Microstructural investigation has demonstrated the enhanced reversion with increasing annealing temperature without any perceptible grain growth up to 800 °C. X-ray diffraction (XRD) analysis has revealed that 40% cold rolling has resulted in the formation of 32% martensite. The residual martensite content has been found to be about 8% after reversion at 800 °C. Different stages of reversion behavior has been examined by differential scanning calorimetric measurement. The variation of dσ/dε with ε is examined to identify different stages of work hardening of the investigated steel. Both tensile strength and percent elongation values increase with increasing annealing temperature up to 500 °C. Beyond that annealing treatment results in the drop of tensile strength value with the consequent increase in percent elongation. Attractive strength–ductility combination (22.80 GPa%) has been achieved after annealing of the 40% cold deformed specimen at 800 °C for 1 h. The fractographic observation corroborates the tensile results.     

Effect of prior austenite on reversed austenite stability and mechanical properties of low carbon medium manganese steel heavy plate

The effect of prior austenite on reversed austenite stability and mechanical properties of Fe‐0.06C‐0.2Si‐5.5Mn‐0.4Cr (wt.%) annealed steels was elucidated. With the decrease of austenitizing temperature from 1250 °C to 980 °C, the prior austenite changed from complete recrystallization to partial recrystallization, and the average austenite size was reduced. The volume fraction of reversed austenite was increased from 26.32 % to 30.25 % because of high density of grain boundaries and dislocations. The martensite transformation temperature of annealed steels was increased from ～115 °C to ～150 °C, and both of thermal and mechanical stability of reversed were reduced. There was no significant different in tensile properties, however, the impact toughness was enhanced from 100 J to 180 J at ?60 °C. The excellent impact toughness in annealed steel (austenitized at 980 °C) was obtained because of higher density of high misorientation grain boundaries, more volume fraction of reversed austenite and reduced segregation at grain boundaries.     

The influence of reversion annealing behavior on the formation of nanograined structure in AISI 201L austenitic stainless steel through martensite treatment

Martensite treatment is one of the known thermo-mechanical processes that can be used for the grain refinement of metastable austenitic stainless steels. In this work, the martensite to austenite reversion behavior as well as its effect on the processing of nanocrystalline structure in an as-cast AISI 201L austenitic stainless steel was investigated. The as-cast specimens were first homogenized and then hot forged in order to prepare a suitable microstructure for the subsequent martensite treatment. The cold rolling was carried out to various reductions between 10% and 95% followed by annealing at temperature range of 750–900 °C for different times of 15–1800 s. The microstructure characterization was performed using optical and scanning electron microscopies, X-ray diffraction and Feritscope. Hardness measurements were also used for evaluating the mechanical properties of the experimental material. The results indicated that the specimen which was reversion-annealed at 850 °C for 30 s exhibited the smallest average austenite grain size of 65 nm with more than 86% austenite.     

Tensile properties of iron-based P/M steels with ferrite + martensite microstructure

The effect of intercritical heat treatments on the tensile properties of iron-based P/M steels was investigated. For this purpose, atomized iron powder (Ancorsteel 1000) was admixed with 0.3 wt.% graphite powder. Tensile test specimens were cold pressed at 700 MPa and sintered at 1120 °C for 30 min under pure argon gas atmosphere. After sintering, ∼20% pearlite volume fraction in a ferrite matrix was obtained. To produce coarse ferrite + martensite microstructures, the sintered specimens were intercritically annealed at 724 and 760 °C and quenched in water. To obtain fine ferrite + martensite microstructures, the sintered specimens were first austenitized at 890 °C and water-quenched to produce a fully martensitic structure. These specimens were then intercritically annealed at 724 and 760 °C and re-quenched. After the intercritical annealing at 724 and 760 °C and quenching, martensite volume fractions were ∼ 18% and 43%, respectively, in both the coarse- and fine-grained specimens. Although the intercritically annealed specimens exhibited higher yield and tensile strength than the as-sintered specimens, their elongation values were lower. Specimens with a fine ferrite + martensite microstructure showed high yield and tensile strength and ductility in comparison to specimens with a coarse ferrite + martensite microstructure. The strength values of specimens increased with increasing martensite volume fraction.     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

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.     

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℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃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 fer-rite 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 discontin-uous 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 Liiders strain.     

Effect of hot working on microstructure evolution of as-cast Nickel Aluminum Bronze alloy

In this work, influences of temperature and hot working on microstructure evolution of a Nickel Aluminum Bronze alloy (NAB) were studied. First, as-cast NAB alloy was annealed and subsequently cooled in air for obtaining homogeneous structure. Microstructure and mechanical properties of NAB specimens before and after annealing were characterized by tensile test, hardness test, optical microscopy and scanning electron microscopy. Then, annealed NAB samples were heat treated at different temperatures between 750 °C and 1000 °C and rapidly cooled down to room temperature. The results showed that amounts and types of emerged microstructures and corresponding hardness strongly depended on the applied temperatures. Additionally, hot compression tests during the temperature range of 800 °C and 950 °C were performed for the annealed NAB alloy. After forming, specimens were cooled down with two different cooling rates of 40 °C/s and 100 °C/s. Developed microstructure and resulting hardness of the deformed NAB alloy were discussed regarding to the heat treating conditions.     

Reverse transformation mechanism of martensite to austenite and amount of retained austenite after reverse transformation in Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel

Abstract

The reverse transformation mechanism of martensite to austenite and the volume fraction of retained austenite have been studied in an Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel by means of dilatometry, transmission electron microscopy and X-ray diffraction. Below a heating rate of 10 K s^-1, the reverse transformation of α' to γ occurs by diffusion, whereas it occurs by a diffusionless shear mechanism above 10 K s^-1. After reversion treatment at low temperatures, filmlike retained austenite is observed along α' lath boundaries, while reversion treatment at high temperatures produces granular retained austenite inside the α' laths in addition to filmlike retained austenite. The volume fraction of retained austenite at room temperature increases with increasing reversion treatment temperature, exhibiting a maximum at ~625^° C, above which it decreases with increasing reversion temperature.     

High temperature mechanical properties of triple phase steels

A 0.15% C–1.2% Si–1.7% Mn steel was intercritically annealed at 780 °C for 5 min and then isothermally held at 400 °C for 4 min followed by oil quenching to room temperature and the annealed microstructure consist of 75% ferrite , 15% bainite and 10% retained austenite was produced. Samples of this steel with triple phase structure were tensile tested at temperature range of 25–450 °C. Stress–strain curves showed serration flow at temperature range of 120–400 °C and smooth flow at the other temperatures. All of the stress–strain curves showed discontinuous yielding at all testing temperatures. Both yield and ultimate tensile strength decreased with increasing temperature, but there exists a temperature region (120–400 °C) where a reduction of strength with increasing temperature is retarded or even slightly increased. The variation in the mechanical properties with temperature was related to the effects of dynamic strain aging, high temperature softening, bainite tempering and austenite to martensite transformation during deformation.     

The effects of homogenization and recrystallization heat treatments on low-grade cold deformation properties of AA 6063 aluminum alloy

In this study, the effect of cooling rate during homogenization treatment of AA 6063 aluminum alloy on the low-grade cold deformation–recrystallization properties has been investigated. For this purpose tapered tensile test specimens (one side 10 mm width and the other side 18 mm) were prepared from the material taken from billet produced by semi-continuous casting method. These specimens were homogenized at 560 °C for 6 h and cooled down at five different rates. Each group has three specimens. These specimens were then tensile tested to obtain deformation gradient and annealed at 450 °C, 500 °C, and 550 °C for 1 h. Following these treatments specimens were subjected to metallographic examination.Experimental findings have shown that an increase in cooling rate caused an increase in critical strain and a decrease in maximum grain size. Metallographic observations revealed that the particle size, interparticle spacing and volume fraction of particles are directly related to cooling rate during homogenization treatment and cooling rate decreases them.     

Effects of post-annealing on the microstructure and mechanical properties of friction stir processed Al–Mg–TiO2 nanocomposites

Aluminum matrix nanocomposites were fabricated via friction stir processing of an Al–Mg alloy with pre-inserted TiO₂ nanoparticles at different volume fractions of 3%, 5% and 6%. The nanocomposites were annealed at 300–500 °C for 1–5 h in air to study the effect of annealing on the microstructural changes and mechanical properties. Microstructural studies by scanning and transmission electron microscopy showed that new phases were formed during friction stir processing due to chemical reactions at the interface of TiO₂ with the aluminum matrix alloy. Reactive annealing completed the solid-state reactions, which led to a significant improvement in the ductility of the nanocomposites (more than three times) without deteriorating their tensile strength and hardness. Evaluation of the grain structure revealed that the presence of TiO₂ nanoparticles refined the grains during friction stir processing while the in situ formed nanoparticles hindered the grain growth upon the post-annealing treatment. Abnormal grain growth was observed after a prolonged annealing at 500 °C. The highest strength and ductility were obtained for the nanocomposites annealed at 400 °C for 3 h.     

Effect of hot compression and annealing on microstructure evolution of ZK60 magnesium alloys

Microstructural evolution of ZK60 magnesium alloys, during twin roll cast (TRC) and hot compression (HC) with a strain rate of 0.1 s⁻¹ at 350 °C and subsequent annealing at temperatures of 250–400 °C for 10²−5 × 10⁵ s, has been observed by optical microscopy (OM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The distribution of average grain size and recrystallized grain size at different annealing conditions were calculated. Activation energy and recrystallized volume fractions during annealing were discussed using analysis of static recrystallization (SRX) kinetics. Based on examination of microstructure evolution during annealing, it was found that several SRX mechanisms were co-activated. Subgrains with high misorientation angles to surrounding grains were formed by dislocation rearrangement, and they seemed to evolve into newly recrystallized grains.     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Initial microstructure and retained austenite in 8 Mn steel controlled by cooling rate

The present work investigates the effect of the initial microstructure on phase transformation after intercritical annealing by measuring the amount of austenite, which was obtained by X-ray diffraction and saturation magnetisation. Pieces of 8?Mn steel were austenitised at 1100°C for 1?h followed by different cooling rates: water, air, and furnace. Samples of each piece were subsequently intercritically annealed from 600 to 800°C followed by air cooling. The microstructure was characterised using scanning electron microscopy and electron backscatter diffraction. Results show how changing the cooling rate affects the temperature of intercritical annealing at which the highest content of retained austenite was obtained.     

Effect of annealing treatment on the microstructure and mechanical properties of ultrafine-grained aluminum

Microstructural and property evolution of commercial pure Al subjected to multi-axil compression (MAC) and subsequent annealing treatment were investigated. After series of MAC pressings up to 15 passes, the samples were annealed at different temperatures. The deformed and deformed with sequent annealing treatment samples were characterized by X-ray diffraction, electron back scatter diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests. The present results showed that on annealing the grain structures coarsen and transform from lamellar to equiaxed ones. Remarkably, the fraction of high angle grain boundaries drastically increases from 29.3% to 76.3% after annealing at 60 °C. Meanwhile, a significant decrease of lattice microstrain is observed after annealing, from 0.0839% to 0.0731% at 130 °C. A controlled 30 min annealing treatment on ultrafine-grained (UFG) Al at 60 °C can result obviously in a higher strength and a lower elongation, which may be associated with the nucleation and subsequent motion of dislocations in grain boundaries. As the annealing temperature is above 60 °C, the yield strength decreases and elongation increases gradually, which is attributed to the grain coarsening and microstructural enhancement.     

The role of grain size in static and cyclic deformation behaviour of a laser reversion annealed metastable austenitic steel

Different grain sizes were created in a metastable 17Cr‐7Mn‐7Ni steel by martensite‐to‐austenite reversion at different temperatures using a laser beam. Two fully reverted material states obtained at 990°C and 780°C exhibited average grain sizes of 7.7 and 2.7 μm, respectively. The third microstructure (610°C) consisted of grains at different stages of recrystallization and deformed austenite. A hot‐pressed, coarse‐grained counterpart was studied for reference. The yield and tensile strengths increased with refined grain size, maintaining reasonable elongation except for the heterogeneous microstructure. Total strain‐controlled fatigue tests revealed increasing initial stress amplitudes but decreasing cyclic hardening and fatigue‐induced α′‐martensite formation with decreasing grain size. Fatigue life was slightly improved for the 2.7‐μm grain size. Contrary, the heterogeneous microstructure yielded an inferior lifetime, especially at high strain amplitudes. Examinations of the cyclically deformed microstructure showed that the characteristic deformation band structure was less pronounced in refined grains.     

Hydrogen effects in a dual-phase microalloy steel

A dual-phase steel containing niobium, vanadium and titanium as microalloying elements was tested for hydrogen embrittlement (HE). The susceptibility to HE was observed to be closely related to the microstructural state. Hydrogenated specimens intercritically annealed at relatively low temperatures to develop martensite islands in a ferrite matrix basically exhibited quasi-cleavage fracture with some ductile dimpling. The mode of fracture in charged specimens quenched from higher intercritical annealing temperatures was predominantly intergranular fracture along prior austenite grain boundaries and cracking of martensite laths. The detrimental role of residual stresses, retained austenite and microalloying carbides in the process of HE is discussed.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.