Microstructural evolution and corresponding property changes after deep cryotreatment of tool steel

The microstructural evolution and structure–property correlation subjected to deep cryotreatment of tool steel were studied. The results show that the retained austenite continues to transform into martensite almost but not complete at low temperature. The topography of retained austenite exhibits as a nanoscale thin film with a thickness range of 20–60?nm between the martensite laths. The changes of internal friction peaks have been explained well by the coupling model, which indicates that deep cryotreatment is not only removing retained austenite but also promoting the interstitial carbon atoms segregated to nearby dislocations under the shrinking strain energy. In addition, more carbides precipitated from the matrix during tempering in cryotreated samples and were verified by analyses of transmission electron microscopy.     

Residual stresses and fracture mechanics analysis of a crack in welds of high strength steels

This study presented the characteristics of residual stresses in welds of high strength steels (POSTEN60, POSTEN80) whose tensile strengths were 600 MPa and 800 MPa, respectively. Three-dimensional thermal elastic-plastic analyses were conducted to investigate the characteristics of welding residual stresses in welds of high strength steels through the thermal and mechanical properties at high temperatures obtained from the elevated temperature tensile tests. A finite element analysis method which can calculate the J-integral for a crack in a residual stress field was developed to evaluate the J-integral for a centre crack when mechanical stresses were applied in conjunction with residual stresses.The results show that the volumetric changes associated with the austenite to martensite phase transformation during rapid cooling after welding of high strength steels significantly influence on the development of residual stresses in the weld fusion zone and heat-affected zone. For a centre crack in welds of high strength steels where only residual stresses are present, increased tensile strength of the steel, increased the J-integral values. The values of the J-integral for the case when mechanical stresses are applied in conjunction with residual stresses are larger than those for the case when only residual stresses are present.     

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.     

Effect of deep cryogenic treatment on the microstructure and wear performance of Cr–Mn–Cu white cast iron grinding media

The phase transformation and grinding wear behavior of Cr–Mn–Cu white cast irons subjected to destabilization treatment followed by air cooling or deep cryogenic treatment were studied as a part of the development program of substitute alloys for existing costly wear resistant alloys. The microstructural evolution during heat treatment and the consequent improvement in grinding wear performance were evaluated with optical and scanning electron microscopy, X-ray diffraction analysis, bulk hardness, impact toughness and corrosion rate measurements, laboratory ball mill grinding wear test etc. The deep cryogenic treatment has a significant effect in minimizing the retained austenite content and converts it to martensite embedded with fine M₇C₃ alloy carbides. The cumulative wear losses in cryotreated alloys are lesser than those with conventionally destabilized alloys followed by air cooling both in wet and dry grinding conditions. The cryotreated Cr–Mn–Cu irons exhibit comparable wear performance to high chromium irons.     

The influence of tempering temperature on the reversed austenite formation and tensile properties in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steels

The influence of tempering temperature on the reversed austenite formation and tensile properties are investigated in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steel in the temperature range of 550-950 °C. It is found that at the temperatures below 680 °C, the reversed austenite formation occurs by diffusion. Amount of the reversed austenite is determined by the tempering temperature and the holding time. The segregation of Ni is the main reason for the stability of the reversed austenite. When the temperatures are above 680 °C, the reversed austenite formation proceeds by diffusionless. The reversed austenite will transform back to martensite after cooled to room temperature. The tensile properties are most strongly influenced by the amount of the reversed austenite obtained at room temperature. The excellent combination of good strength and ductility is at 610 °C.     

Microstructural stability of austempered ductile iron after sub-zero cooling

Abstract

The present article investigates the stability of the retained austenite, present in austempered ductile iron (ADI) after cooling at sub-zero temperatures, considering that the austenite could transform into martensite when austempered parts are exposed to low temperatures or stresses or strains. Optical microscopy with oblique illumination, X-ray diffraction techniques and microhardness tests were used to analyse the transformation of the austenite on samples with different austempering thermal cycles. The results indicated that the martensitic transformation took place mainly at the unreacted austenite present at the last to freeze areas of samples austenitised and austempered at the highest temperatures. On the other hand, the reacted austenite, present in the bulk of all the investigated samples, remains unchanged after cooling. Tensile tests were performed in order to evaluate the influence of the martensitic transformation, promoted by the sub-zero cooling, on strength and ductility.     

Deviation of residual stress patterns in 52 100 bearing steel due to inherent microstructural transformations after rolling contact

A previous publication proved empirically that residual stresses in 52 100 bearing steel are caused mainly by microstructural transformations during rolling contact. It also introduced a model estimating the magnitude of residual stresses resulting from microstructural transformations. The results from this model prompted this study because the model showed a discrepancy between the measured results and the magnitude of the estimated residual stresses derived from the model. This study uses metallurgical analysis and modeling to explain why the measured and estimated magnitudes of residual stresses did not coincide. Metallurgical analysis explains the roles of retained austenite transformation and martensite decay in the ultimate magnitude of residual stresses at the surface of the bearing balls after rolling contact. The model estimates the magnitude of expected residual stresses due to martensite decay and suggests the number of cycles that bearing balls can operate under the beneficial effects of compressive residual stresses without spalling.     

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.     

动态碳配分对先进高强钢组织与力学性能的影响

目的 为了使钢表现出更好的吸能特性,以具有较高的强度以及较好的塑性。方法 提出了一种新型一步法成形碳配分一体化工艺,即热冲压-动态碳配分(HS-DP)工艺。所提出的HS-DP工艺采用盐浴热处理的方式进行物理模拟。采用扫描电子显微镜(SEM)、X射线衍射(XRD)和拉伸试验等方法,研究了新工艺中的冷却速率对低碳先进高强钢的微观组织和力学性能的影响。针对冷却速率对残余奥氏体含量的影响进行了分析,重点研究了残留奥氏体的体积分数和碳含量对钢伸长率的影响。结果 经过HS-DP工艺处理的钢显微组织主要由初始淬火态马氏体相、最终淬火态马氏体相和残余奥氏体相共存组成。结论 实验钢表现出优异性能,说明了热冲压动态碳配分工艺前景广阔。     

In situ synchrotron study on the interplay between martensite formation, texture evolution and load partitioning in low-alloyed TRIP steels

We have studied the micromechanical behaviour of two low-alloyed multiphase TRIP steels with different aluminium contents by performing in situ high-energy X-ray diffraction experiments at a synchrotron source under increasing tensile stress levels. A detailed analysis of the two-dimensional diffraction data has allowed us to unravel the interplay between the martensite formation, the texture evolution and the load partitioning, and to correlate the observed behaviour to the macroscopic response of the material. The high aluminium content TRIP steel grade presents a higher volume fraction of retained austenite at room temperature that transforms more gradually into martensite under deformation, providing a larger uniform elongation. The comparison between the observed transformation behaviour and the texture evolution indicates that the 〈1 0 0〉 component along the loading direction corresponds to a low critical stress for the transformation. The evolution of the elastic strains revealed the occurrence of a significant load partitioning before reaching the macroscopic yield strength, which becomes more pronounced in the plastic regime due to the progressive yielding of the different grains in the polycrystalline material. This opens the door to tailor the austenite stability by altering the distribution in grain size, local carbon content, and grain orientation in order to produce the optimal load partitioning and work hardening for improved combinations of strength and formability in low-alloyed TRIP steels.     

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.     

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb-V microalloyed steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb-V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.     

Low temperature fracture properties of DIN 22NiMoCr37 steel in fine-grained bainite and coarse-grained tempered embrittled martensite microstructures

An as-received (AR) DIN 22NiMoCr37 nuclear reactor pressure vessel steel has been heat treated for 1 h at austenitising temperatures of 1373 and 1473 K to obtain different austenite grain sizes. After austenitising, the samples were water quenched, tempered for 2 h at 923 K, water quenched and then held isothermally at 793 K for 180 h before final air-cooling. The AR condition had a tempered bainite microstructure and a prior austenite grain size of 30 μm, whereas the heat treated conditions were tempered martensite and had a prior austenite grain size of approximately 100 μm for the 1373 K condition and ‘extraordinary’ large austenite grains (>1 mm diameter) for the 1473 K condition. Their low temperature fracture properties were determined and were related to the susceptibility to segregation induced embrittlement. Despite the heat treated conditions having a larger prior austenite grain size compared to the AR condition, at a given testing temperature, the tempered martensitic 1373 K condition generally exhibited higher strength and higher fracture toughness values at 123 K. The heat treated conditions generally exhibited higher local fracture stress (σ_f) values in 0.2 mm blunt notch SE(B)-0.4T specimens at 123 and 77 K.     

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.     

High strength-elongation product of Nb-microalloyed low-carbon steel by a novel quenching-partitioning-tempering process

In this article, a novel quenching-partitioning-tempering (Q-P-T) process was applied to a Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt%) hot-rolled steel, and its optimized parameters were obtained by a Gleeble-3500 thermal simulator and salt baths, respectively. Mechanical property results of the as-treated Q-P-T samples show that the Nb-microalloyed low-carbon steels subjected to Q-P-T processes cover a wide spectrum of strength (1200-1500 MPa) and elongation (14-18%), and exhibit excellent product of strength and elongation (21,000-22,000 MPa%). Microstructural characterization indicates that high strength results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ?-carbides in martensite matrix and good ductility is attributed to transformation induced plasticity (TRIP) effect from plenty of retained austenite flakes between martensite laths.     

Effect of epitaxial ferrite on yielding and plastic flow in dual phase steel in tension and compression

Abstract

A low carbon, microalloyed steel was heat treated to obtain dual phase microstructures containing constant levels of 18 and 25 vol.-% martensite at two levels of microstructural refinement and with varying epitaxial ferrite content. Tensile and compression tests were conducted at a strain sensitivity of 2 × 10^-5. Elastic limits in tension and compression were indistinguishable and very low, suggesting that mobile dislocations were present in the ferrite as a consequence of stress relaxation processes. These mobile dislocations accommodated the volume increase accompanying the austenite to martensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0·2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the martensite and ferrite following heat treatment. The residual stresses calculated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the temperature at which the martensite formed. Epitaxial ferrite significantly increased uniform elongation in tension with a small decrease in tensile strength for both levels of martensite in the finer microstructure but only at the 18 vol.-% martensite level in the coarser microstructure. The cause of the increased ductility was the effect of epitaxial ferrite on the work hardening rate between approximately 0·5 and 3% strain; epitaxial ferrite reduced the work hardening rate in this range of strain.     

双相钢中的残余奥氏体

研究了热－机械处理中各种因素对双相钢中的残余奥氏体的影响，发现：双相区热处理前的冷加工使奥氏体成核密度提高，且使残余奥氏体分布均匀和含量增加；微观结构的观察表明，空冷过程中奥氏体颗粒在不断缩小，颗粒的尺寸效应和碳原子向奥氏体中偏聚均使奥氏体变得稳定。因此，恰当地控制冷加工量、冷却速度、加热温度和时间，可使双相钢含有数量和稳定度都合适的残余奥氏体。此外还发现，从双相区温度空冷所得马氏体必须经低温短时     

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.     

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy were investigated according to martensitic transformation kinetics, morphology, magnetism of both austenite and martensite phases and also in terms of martensitic transformation start temperatures (M_s) for different austenite grain sizes of alloy. Kinetics of the transformation was found to be athermal. Also only lenticular martensite morphology was observed during investigations. On the other hand, Mössbauer spectra revealed a paramagnetic character for austenite phases and a ferromagnetic character for thermally induced martensitic phases. Determined M_s temperatures were found to be at − 128 °C for large grained samples and − 135 °C for small grained samples.     

Correlation of austenite stability and ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn austenitic steels

Ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn austenitic steels containing different contents of Ni, Mo, Cu as well as nitrogen is discussed in terms of austenite stability and associated deformation-induced martensitic transformation (DIMT). Electron back-scattered diffraction and transmission electron microscopy analyses of cross-sectional area of the Charpy impact specimens fractured at −196 °C indicated that the brittle fracture planes were almost parallel to one of {1 1 1} slip planes and some metastable austenites near the fracture surface were transformed to α′-martensite by localized plastic deformation occurring during crack propagation. Quantitative evaluation of deformation-induced martensite together with characteristics of true stress-strain and load-displacement curves obtained from tensile and Charpy impact tests, respectively, supported that DIMT might take place in high-nitrogen austenitic steels with relatively low austenite stability. The occurrence of DIMT decreased low-temperature toughness and thus increased largely ductile-to-brittle transition temperature (DBTT), as compared to that predicted by empirical equations strongly depending on nitrogen content. As a result, the increased DBTT could be reasonably correlated with austenite stability against DIMT.