Investigation of the Microstructural Changes and Hardness Variations of Sub-Zero Treated Cr-V Ledeburitic Tool Steel Due to the Tempering Treatment

The microstructure and tempering response of Cr-V ledeburitic steel Vanadis 6 subjected to sub-zero treatment at ??196 °C for 4 h have been examined with reference to the same steel after conventional heat treatment. The obtained experimental results infer that sub-zero treatment significantly reduces the retained austenite amount, makes an overall refinement of microstructure, and induces a significant increase in the number and population density of small globular carbides with a size 100-500 nm. At low tempering temperatures, the transient M₃C-carbides precipitated, whereas their number was enhanced by sub-zero treatment. The presence of chromium-based M₇C₃ precipitates was evidenced after tempering at the temperature of normal secondary hardening; this phase was detected along with the M₃C. Tempering above 470 °C converts almost all the retained austenite in conventionally quenched specimens while the transformation of retained austenite is rather accelerated in sub-zero treated material. As a result of tempering, a decrease in the population density of small globular carbides was recorded; however, the number of these particles retained much higher in sub-zero treated steel. Elevated hardness of sub-zero treated steel can be referred to more completed martensitic transformation and enhanced number of small globular carbides; this state is retained up to a tempering temperature of around 500 °C in certain extent. Correspondingly, lower as-tempered hardness of sub-zero treated steel tempered above 500 °C is referred to much lower contribution of the transformation of retained austenite, and to an expectedly lower amount of precipitated alloy carbides.     

The Kinetics of Phase Transformations During Tempering in Laser Melted High Chromium Cast Steel

The precipitation of secondary carbides in the laser melted high chromium cast steels during tempering at 300-650?°C for 2?h in air furnace was characterized and the present phases was identified, by using transmission electron microscopy. Laser melted high chromium cast steel consists of austenitic dendrites and interdendritic M₂₃C₆ carbides. The austenite has such a strong tempering stability that it remains unchanged at temperature below 400?°C and the secondary hardening phenomenon starts from 450?°C to the maximum value of 672 HV at 560?°C. After tempering at 450?°C fine M₂₃C₆ carbides precipitate from the supersaturated austenite preferentially. In addition, the dislocation lines and slip bands still exist inside the austenite. While tempering at temperature below 560?°C, the secondary hardening simultaneously results from the martensite phase transformation and the precipitation of carbides as well as dislocation strengthening within a refined microstructure. Moreover, the formation of the ferrite matrix and large quality of coarse lamellar M₃C carbides when the samples were tempered at 650?°C contributes to the decrease of hardness.     

Rapid Tempering of Martensitic Stainless Steel AISI420: Microstructure,Mechanical and Corrosion Properties

In this research, the effect of rapid tempering on the microstructure, mechanical properties and corrosion resistance of AISI 420 martensitic stainless steel has been investigated. At first, all test specimens were austenitized at 1050 °C for 1 h and tempered at 200 °C for 1 h. Then, the samples were rapidly reheated by a salt bath furnace in a temperature range from 300 to 1050 °C for 2 min and cooled in air. The tensile tests, impact, hardness and electrochemical corrosion were carried out on the reheated samples. Scanning electron microscopy was used to study the microstructure and fracture surface. To investigate carbides, transmission electron microscopy and also scanning electron microscopy were used. X-ray diffraction was used for determination of the retained austenite. The results showed that the minimum properties such as the tensile strength, impact energy, hardness and corrosion resistance were obtained at reheating temperature of 700 °C. Semi-continuous carbides in the grain boundaries were seen in this temperature. Secondary hardening phenomenon was occurred at reheating temperature of 500 °C.     

Effect of tempering temperature on the microstructure,corrosion resistance,and antibacterial properties of Cu-bearing martensitic stainless steel

In this study, the effects of tempering temperature on the microstructure, hardness, antibacterial performance and corrosion resistance of Cu-bearing 5Cr15MoV martensitic stainless steel (5Cr15MoV-Cu MSS) were investigated using an optical microscope, a scanning electron microscope, X-ray diffraction, a transmission electron microscope, an antibacterial test, electrochemical measurements, and the salt spray test. The results showed that the hardness curve had a saddle shape and its values reached the peak value after tempering at 500°C, due to the secondary hardening effect by the precipitation of tiny secondary carbides and Cu-rich precipitates. In addition, the antibacterial results also showed excellent antibacterial performance against Escherichia coli at 500°C because of the formation of Cu-rich precipitates. Results of corrosion tests indicated that the corrosion resistance decreased gradually with an increase of the tempering temperature. In particular, the passivation did not occur when the tempering temperature was above 500°C, which may be related to the Cu-rich precipitates and Cr depletion.     

Effect of Tempering and Strain on Decomposition of Metastable Austenite in X210CrW12 Thixo-Cast Steel

Thixoforming of hot rolled X210CrW12tool steel led to the formation of globular austenitic grains (82.4 vol.%) surrounded by eutectic mixture (α-Fe and M₇C₃ carbides). The thixo-cast steel reached compression strength 4.8 GPa at plastic strain 34%. The analysis of pole figures after deformation indicated distinct texturization of microstructure in comparison with undeformed steel. Main texture components for austenite were {101}, 〈010〉, while ferrite did not reveal clearly formed orientation. DSC analysis confirmed that austenitic structure in the X210CrW12 steel was metastable and temperature of decomposition depended on the strain applied at 634 °C for the un-deformed sample and at 599 °C for sample compressed up to 4.8 GPa. Discontinuous transformation of austenite into perlite, that started mainly at grain boundaries and proceeded to the center, was the predominant mechanism responsible for the decomposition of globular grains in thixoformed X210CrW12 steel. The decomposition caused by tempering of supersaturated and severely strained steel led to obtaining characteristic product of transformation of higher hardness in comparison with only tempered sample. In the deformed sample the reaction started on slip bands and twins which revealed high density of defects, promoting precipitation of carbides, followed by local depletion in carbon as a result of α′- Fe formation. In contrast to non-deformed state they covered the area of grains. Two fronts of reaction α-Fe plate +M₃C → mixture of α-Fe and M₇C₃ carbides were also observed.     

回火温度对二次硬化马氏体不锈钢组织和性能的影响

二次硬化对回火温度非常敏感,研究了某含Mo二次硬化马氏体不锈钢在250~650 ℃回火时组织和性能的演变过程,并利用XRD、SEM、TEM以及冲击测试等手段分析了显微组织与力学性能之间的关系,着重讨论了试验钢二次硬化与残留奥氏体增韧机理。结果表明: 480~500 ℃回火时,试验钢同时出现了二次硬化和回火脆性现象,宏观硬度达到了56 HRC以上,冲击性能为14 J·cm^-2左右,显微组织主要由纳米级合金碳化物、板条马氏体以及残留奥氏体构成,其中纳米级合金碳化物弥散强化引起了二次硬化,体积分数约为10%的残留奥氏体有利于提高钢的冲击性能。而在上述温度区之外低温和高温回火时,试验钢均具有较高的冲击性能,但宏观硬度相对较低。     

热处理工艺参数对Cr8冷作模具钢组织和性能的影响

针对Cr8冷作模具钢的二次硬化问题,采用金相显微镜、SEM、EDS和洛氏硬度计,研究了低温回火和高温回火对Cr8冷作模具钢材料组织和性能的影响。结果表明,原材料组织为珠光体和少量长为9~41μm、宽为2~11μm大颗粒共晶碳化物;不同回火温度下,材料的组织为马氏体和少量长为4~25μm、宽为1~8μm的大颗粒共晶碳化物及细小二次析出碳化物;经1030℃淬火、高温520℃回火之后材料出现二次硬化现象,硬度为58. 4 HRC,比低温200℃回火下的硬度高出1. 4 HRC。Cr8钢的二次硬化是由于高温520℃回火之后,二次纳米级小颗粒碳化物析出较多,且颗粒均匀弥散分布于基体中造成的。     

Effect of Si addition on kinetics of martensitic hot-work die steel during tempering

Dilatometry measurement, transmission electron microscope (TEM), and a three-dimensional atom probe (3DAP) are utilized to study how adding Si affects the kinetics of martensitic steel during tempering. The results show that both the tempering transformation temperature and the activation energy of martensitic hot-work die steel increase remarkably when Si is added. Meanwhile, the film-type retained austenite and the Si-rich region adjacent to the carbides are each observed in tempered martensitic steels. The results suggest that coarsening of carbides is efficiently inhibited and softening of steels is sufficiently delayed.     

Evaluation of properties in laser welds of A6061‐T6 aluminium alloy

In order to clarify the effect of tip velocity on the weld solidification process of hot-work tool steel (SKD61) during welding, information about microstructure evolution was obtained by the combination of a liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. From the experimental results, it was found that the solidification mode was changed from FA mode (L → L+δ → L+δ+γ → L+γ → γ) to A mode (L → L+γ → γ) at high tip velocity. Moreover, the effect of tip velocity on the microstructure selection during solidification between the primary δ, ferrite and the primary γ, austenite was theoretically proven by the Kurz, Giovanola and Trivedi model. Therefore, it was understood that the solidification cracking susceptibility of hot-work tool steel (SKD61) weld metal was increased due to the δ to γ transition of the primary phase.     

Precipitation of Secondary Carbides in M2 High-Speed Steel Modified with Titanium diboride

The precipitation of the secondary carbides in high-speed steel of AISI M2 type modified with titanium diboride has been investigated for both the cast and the heat-treated states. The primary focus was on the effect of austenitizing temperatures on the secondary carbide precipitation during tempering. Some differences in origin of the secondary carbides, as well as in their shape and size distribution, were found in the tempered microstructure for the different austenitizing temperatures. After austenitization at 1180 °C and triple tempering at 560 °C, the secondary carbide particles of a spherical shape up to 200 nm in size were identified by selected area electron diffraction as M₂₃C₆. After austenitization at 1220 °C, two types of the secondary carbides were found in the tempered microstructure, M₂₃C₆ with a size up to 200 nm and M₆C with a size up to 400 nm. In both the cases, the carbide particles were slightly angular. After austenitization at 1260 °C, only M₆C secondary carbides were revealed in the tempered microstructure, which occurred as the angular particles up to 350 nm in size. In addition, considerably finer M₂₃C₆ carbide particles with a size of 10-40 nm were found to precipitate in the tempered microstructure.     

离心铸造轧辊用高速钢的热处理

采用差式扫描量热法(DSC)及热膨胀法测定了离心铸造轧辊用高速钢的相变点,根据高速钢的临界转变温度,进行了退火、淬火和回火实验.着重研究了热处理过程中碳化物演变对轧辊用高速钢性能的影响.结果表明:退火温度需高于630℃,才能使铸态高速钢得到软化,硬度降低,便于切削加工.热处理前后,MC型一次共晶碳化物的成分、形态和数量基本没有变化;随着淬火温度的升高,处于亚稳状态的M2C型共晶碳化物的分解数量越来越多.高速钢最终硬度受回火温度影响较大,研究了回火温度与二次硬化之间的关系,确定了二次硬化峰值温度,研究了二次碳化物的溶解与析出对高速钢基体组织与力学性能的影响规律.     

PHASE AND STRUCTURE TRANSFORMATION OF BAINITE IN 3Cr3Mo3VNb STEEL DURING TEMPERING

本文研究了3Cr3Mo3VNb钢等温淬火后贝氏体回火时的相及组织转变规律。实验结果表明,贝氏体的二次硬化机制和马氏体一样,主要是由Mo_2C,V_4C_3,NbC等细小弥散的合金碳化物析出造成的。回火时,贝氏体中粗大的渗碳体溶解较慢,减慢了合金碳化物的析出速率。贝氏体中的残余奥氏体分解和转变的行为与马氏体中的不同。贝氏体中位错密度较低,不利于合金碳化物的聚集粗化。以上诸因素的共同作用,使贝氏体具有比直接淬火所获得的马氏体更高的二次硬化效应、热强性和组织稳定性。     

Microstructural Evolution and Deformation Behavior of a Hot-Rolled and Heat Treated Fe-8Mn-4Al-0.2C Steel

The microstructural evolution following tensile deformation of a hot-rolled and heat treated Fe-8Mn-4Al-0.2C steel was studied. Quenching in the range of 750-800 °C followed by tempering at 200 °C led to a ferrite-austenite mixed microstructure that was characterized by excellent combination of tensile strength of 800-1000 MPa and elongation of 30-40%, and a three-stage work hardening behavior. During the tensile deformation, the retained austenite transformed into martensite and delayed the onset of necking, thus leading to a higher ductility via the transformation-induced plasticity (TRIP) effect. The improvement of elongation is attributed to diffusion of carbon from δ-ferrite to austenite during tempering, which improves the stability of austenite, thus contributing to enhanced tensile ductility.     

Effect of single and double austenitization treatments on the microstructure and mechanical properties of 16Cr-2Ni steel

Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.     

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe?CCr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe?CCr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite?Cspinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N₂ gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.     

Mechanical characterization of the welding of two experimental HSLA steels by microhardness and nanoindentation tests

The microhardness and nanohardness of the welding zone of two experimental HSLA steels were determined. The first steel has a microstructure of martensite and bainite, and the second one has a microstructure of quasipolygonal ferrite and acicular ferrite. In the bainitic - martensitic steel, softening of the heat affected zone was observed. This softening can be attributed to: the formation of polygonal ferrite in the recrystallization subzone, the formation of quasi-polygonal ferrite and the tempering of martensite in the intercritical subzone, and the tempering of martensite in the subcritical subzone. Besides the softening, with nanoindentation technique, hardening was observed at the position where the peak temperature reached the critical temperature A _c1, which can be attributed to a phenomenon of secondary hardening by precipitation of carbides of alloying elements. In the ferritic steel, a softening phenomenon did not appear since there was no martensite in its initial microstructure. Finally, it was noted that both polygonal ferrite and the bainite have similar behavior and nanohardness, this coincidence can be attributed to the effect of grain boundary.     

Evolution of secondary-phase precipitates during annealing of the 12Kh18N9T steel irradiated with neutrons to a dose of 5 DPA

The formation and evolution of thermally-induced secondary precipitates in an austenitic stainless steel 12Kh18N9T irradiated in the core of a laboratory reactor VVR-K to a dose of 5 dpa and subjected to post-radiation isochronous annealings for 1 h in a temperature range from 450 to 1050°C have been studied using transmission electron microscopy (TEM) and microhardness measurements. It has been shown that the formation of stitch (secondary) titanium carbides and M ₂₃C₆ carbides at grain and twin boundaries after annealing at 1050°C is preceded by a complex evolution of fineparticles of secondary phases (titanium carbides and nitrides) precipitated at dislocation loops and dislocations during annealing at temperatures above 750°C.     

Microstructures and Hardness of 8CrWMoV Steel with Multiple Types of Ultra Fine Carbides

The structure and hardness of 8CrWMoV steel with multiple types of ultra fine carbides are studied after annealing, quenching and tempering in this paper. The results show that multiple types of carbides M3C, M7C3, M23C6, M6C and MC were observed in the annealed steel. Nucleation and coalescence of new carbides, partial dissolution of original carbides in 7 phase region during annealing at 800-840℃, result in ultra-fine carbides. Average size of the carbides is 0.33～0.34μm in the steel annealed at 800～840℃. Because M3C and M23C6 dissolve easily in austenite, the high hardness HRC63～65 can be obtained by quenching at 840～860℃. Un-dissolved carbides M6C and MC (VC) can effectively prevent the coarsening of austenitic grain, and conduce to obtain very fine martensite. The retained austenite can be easy to decompose during tempering at low and middle temperature due to the precipitation of multiple types of carbides and the good tempering-resistance of the steel is obtained. The microstructure and property of the steel after heat treatment can be accurately explained by calculating based on phase equilibrium thermodynamic.     

Influence of tempering temperature on both the microstructural evolution and elemental distribution in AISI 4340 steels

In the present study, the influence of tempering temperature on the microstructural evolution and prior austenite grain boundary segregation of AISI 4340 steels was investigated by transmission electron microscope and atom probe. The transmission electron microscopy results showed a variation in the microstructure and the morphology of carbides with a change in tempering temperature. Additionally, the chemical compositions of the prior austenite grain boundaries and carbides were quantified by atom probe tomography. An increase in the tempering temperature led to a decrease in the amount of carbon segregated at the prior austenite grain boundary from 7.9 to 1.3 at.%. It was found that a higher tempering temperature can accelerate the diffusion of carbon from the prior austenite grain boundary into carbide. However, phosphorus atoms were segregated mainly at the prior austenite grain boundary in steel tempered at 400°C (up to 0.18 at.%). It was found that formation of film-like carbide and phosphorus segregation along the prior austenite grain boundary is the main cause of embrittlement in steel tempered at 400°C.     

提高渗碳淬火齿轮硬度的后处理工艺

研究了20Cr2Ni4A钢渗碳淬火齿轮在低温回火后硬度较低时进行后冷处理和后低温回火处理对表面硬度、有效硬化层深度及心部硬度的影响。结果表明,20Cr2Ni4A钢在渗碳淬火低温回火后的残留奥氏体稳定化现象并不明显,此时进行冷处理仍能提高工件硬度,而当残留奥氏体较多时具有低温回火二次硬化现象,提高低温回火温度也能提高表面硬度。据此可采用后冷处理和后低温回火工艺提高硬度,代替常规的重新高温回火+渗碳淬火+低温回火的返工工艺。后冷处理温度可根据Mf点确定,对于渗碳后高温回火并重新加热淬火和低温回火工艺,Ms和Mf点不能按常规方法计算,可根据残留奥氏体含量进行估算。