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.     

Deformation induced martensite characteristics in Fe–29Ni–2Mn alloy

Abstract

Deformation induced martensite characteristics in the austenite phase of Fe–29Ni–2Mn alloy were studied for different austenite grain sizes of alloy. Scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and also differential scanning calorimetry techniques were applied to study in order to clarify the deformation induced martensite characteristics from morphological, crystallographical, magnetical and thermal points of view. Scanning electron microscope revealed that the increasing of deformation amount also increased the amount of existed martensite. Transmission electron microscope observations showed that the crystal structure of these deformation induced martensites morphology was lenticular plates with a bcc crystal structure. Also the magnetism of both austenite and martensite phases were determined with Mössbauer spectroscopy. Mössbauer spectrometer measurements showed paramagnetic character for austenite phases and ferromagnetic character for martensite phases in all samples. According to obtained differential scanning calorimetry cooling curves, deformation induced martensite start temperature M _d was found to be higher (?128°C) for larger grained samples than for smaller grained samples (?135°C).     

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.     

Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).     

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.     

Isothermal transformations in advanced high strength steels below martensite start temperature

Abstract

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M_S are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.     

Fracture toughness of steels with nickel content in respect of carbide morphology

Abstract

This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2?h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor K_Ic significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.     

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract

The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T _NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.     

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.     

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.     

Characterisation of severely deformed austenitic stainless steel wire

Abstract

The microstructure of 8 μm diameter wire produced by the severe deformation of 316L austenitic stainless steel has been examined using TEM and X-ray diffraction. The deformation imparted amounts to a true strain of 6·3. Data from previous studies on strain induced transformation of this steel have been combined with new results to show that true strains >2 are required in order to observe mechanical stabilisation, i.e. the cessation of martensitic transformation when the martensite/austenite interfaces are unable to propagate through the dislocation debris created in the austenite.     

Study of heat treatment parameters and kinetics of quenching and partitioning cycles

Abstract

Cold rolled sheets of a low carbon quenching and partitioning (Q&P) steel grade were subjected to heat treatment cycles, which were designed by dilatometric experiments and optimised with respect to the quenching temperature, partitioning temperature and partitioning time. Characterisation of the retained austenite was carried out by electron backscattered diffraction, whereas the carbides were studied by scanning electron microscopy (SEM) and differential scanning calorimetry. The mechanical properties were evaluated by tensile testing and linked with retained austenite fractions and carbon contents, determined by X-ray diffraction. Conclusions are drawn concerning the influence of the kinetics of partitioning on the microstructure in terms of optimal austenite fraction in the martensitic matrix, its C content and ensuing mechanical properties.     

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.     

Micro-defect effect on gigacycle fatigue S-N property and very slow crack growth of high strength low alloy steel

Abstract

Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.     

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.     

Investigation of the evolution of retained austenite in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering

The microstructure and amount of retained austenite (the austenite remained at room temperature) evolved in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering at 620 °C have been investigated. The amount of retained austenite showed a parabolic trend with increase in tempering time, which can be attributed to the gradual decrease in the thermal stability of the reversed austenite (the austenite formed at high temperature). The influences of chemical composition, morphology of reversed austenite, and mechanical constraints originating from tempered martensite matrix on the thermal stability have been discussed. The precipitation and growth of M₂₃C₆ in reversed austenite dilute the carbon concentration in reversed austenite. The spheroidization of lathy reversed austenite during tempering decreases the interfacial energy barrier to the phase transformation of reversed austenite to martensite. Furthermore, the decrease in the strength of martensite matrix lowers the strain energy associated with the transformation of reversed austenite to martensite. All these factors during tempering weaken the thermal stability of reversed austenite and facilitate the phase transformation of reversed austenite to martensite during the cooling step of intercritical tempering.     

Plastic accommodation of martensite in disordered and ordered iron–platinum alloys

Abstract

The degree of order in iron–platinum austenite can be changed by heat treatment. Highly ordered austenite tends to transform into thermoelastic martensite, whereas non-thermoelastic martensite isformed from disordered austenite. This is because the ordered austenite is able to accommodate elastically the shape deformation brought about by the growth of martensite. In the present study, atomic force microscopy has been used to establish the nature of the shape deformation caused by both types of martensite. It is confirmed that the extent of plastic accommodation is larger when disordered austenite is induced to transform into martensite.

MST/3232     

前驱体对含Cu低碳钢I&Q&P处理后组织性能的影响

采用IQP工艺和EPMA、SEM和XRD等手段,研究了3种前驱体对含Cu低碳钢残余奥氏体含量及力学性能的影响。结果表明,双相区保温初期试验钢奥氏体长大由C配分控制,后期由合金元素Mn、Cu配分控制;双相区保温奥氏体化后,双相区配分后形成弥散分布的局部高浓度Mn、Cu区域仍保留富集效果,在随后的淬火-碳配分阶段易于形成残余奥氏体。经IQP处理后,前驱体为P+F的钢室温组织中马氏体板条较粗,原始奥氏体晶界并不明显;前驱体为F+M钢得到的马氏体板条有序细密;前驱体为M的钢室温组织中马氏体板条更加细密。其中,前驱体组织为M的钢中残余奥氏体量最高,延伸率为24.1%,强塑积可达25 338 MPa·%,综合性能最好。     

X-ray diffraction and magnetic characterization of the retained austenite in a chromium alloyed high carbon steel

In the present work the amount of retained austenite present in quenched and tempered high carbon–chromium alloyed steel was quantified by X-ray diffraction and magnetization saturation measurements. The steel was forged and directly quenched. The retained austenite partially transformed into martensite on cooling down to −196 °C. The M_f temperature of about −150 °C was found by thermomagnetic analysis. Tempering at low temperatures (220 °C and 270 °C) promoted the stabilization effect of austenite. The intrinsic magnetization of the ferromagnetic martensite used in the phase quantification was 206.4 A² m/kg. The increase of the tempering temperature above 320 °C slightly decreases the m _s value of the martensite due to tempering reactions.     

Thermal stability of retained austenite in Cr–Ni weld metals at low temperatures

Abstract

As environmental temperature decreases, the amount of retained austenite is more likely to greatly reduce due to the thermal austenite–martensite transformation caused by the decreased thermal stability of retained austenite, probably making its amount lower than the required content. In the present study, the thermal stability of retained austenite in Cr–Ni weld metals was investigated to see whether sufficient retained austenite can be maintained at low temperatures. The specific experimental procedure is as follows: briefly, the samples were cooled in turn from room temperature to 0, ?20, ?40, ?60, ?80, ?100 and ?196°C; the amount of retained austenite at the above temperatures was measured using X-ray diffraction. Through investigating the dependence of the content of retained austenite on temperature, it was revealed that when the content of retained austenite is <20%, retained austenite can be maintained until ?196°C.