Effect of Boron on the Kinetics of Low-Temperature Decomposition of Martensite in Quenched Medium-Carbon Steel

The effect of boron on the microstructure, microhardness, and kinetics of low-temperature decomposition of martensite in the 40Kh and 30KhRA steels quenched at different cooling rates has been studied. It has been shown that the low-temperature decomposition of martensite in the boron-containing steel after quenching from 1050°C at a high cooling rate is strongly decelerated at the initial stage of decomposition. At low quenching cooling rates, the martensite decomposition in the steels under investigation is characterized by a similar kinetics.     

Fracture toughness and microstructure of 780 MPa class high-tensile strength steel plates: improvement of steel structural performance by laser beam welding for mid-thick section steel plate

Fracture toughness and microstructure of laser weld metal of 780 MPa class steels are investigated and compared with those of SM490A and SM570Q.

In SM490A and SM570Q, Charpy energy transition temperature of laser weld metals is 60–90°C higher than that of base metal, and upper bainite microstructures are observed in the laser weld metals.

In 780 MPa class steels, difference of Charpy energy transition temperatures between laser weld metal and base metal are only 10–30°C, and no upper bainite microstructures are observed in the laser weld metals. Hardness of the laser weld metals of 780 MPa class steels is identical to that of martensite microstructure. A superior toughness of the laser weld metal of 780 MPa class steels would be owing to the martensite microstructure resulted from a high carbon equivalent.     

Acoustic emission in bearing steel during isothermal formation of midrib

The first attempt of an acoustic emission (AE) application for the control of isothermal transformation, at the temperature near the martensite start (M _S ) in 100CrMnSi6-4 bearing steel, is made herein. A unique method for recording the signals of the AE was developed. The Short Time Fourier Transform algorithm was used for displaying the signal values on the spectrogram graphs. Acoustic emission showed the nucleation and growth of the isothermal martensite in the residual austenite. The maximum power spectral density of signals was determined using the relative intensity of the RMS values. The observed energy of separated signals during the second stage of residual austenite decomposition is comparable to the energy of the initial stage at lower temperatures. It was proposed as a new process concept under paraequilibrium that involves the midrib formation and swing back in the kinetics of lower bainite.     

Low-temperature martensitic transformation in tool steels in relation to their deep cryogenic treatment

The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mössbauer spectroscopy, X-ray diffraction, mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of ?100 down to ?170 °C with its largest intensity near ?150 °C. No transformation is observed at ?196 °C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels.     

基于信息熵的马氏体相变声发射信号分析

以信息熵理论为基础,针对马氏体相变过程声发射信号,利用基于频域分析的小波包能谱熵,得到相变过程声发射信号频率空间的能量分布信息。提取出不同类型信号的能谱熵特征值,并利用基于时域分析的吉布斯概率熵,以能谱熵特征值作为幅度谱的子区间划分参数,实现马氏体相变过程声发射信号吉布斯概率熵变化过程的准确描述。分析表明,这些信息熵特征值可以准确评价马氏相变过程的声发射信号变化规律。     

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 借助于X射线衍射，研究了C、Mn、Cr和Ni 含量对304奥氏体不锈钢拉伸力学性能和应变诱发 马氏体相变倾向的影响。结果表明：C、Mn、Cr和Ni在允许的成分范围内变化，应变诱发α′马氏体相变倾向差异很大，这导致屈服强度和抗拉强度复杂的变化，尽管应变诱发α′马氏体相变使加工硬化速率提高，相变可以诱发塑性，但相变速率较快，相变倾向较大的钢塑性反而下降，此外，由于室温变形还增大热诱发马氏体相变倾向，从而限制了C、Mn、Cr和Ni下限钢在高精度和低温环境下构件的应用。     

Characterization of a New Fe-C-Mn-Si-Cr Bearing Alloy: Tempered Martensite Embrittlement Susceptibility

Bearing steels containing 1% C and 1.5% Cr have been the usual material of choice for machine components submitted to rolling and contact fatigue, for more than a century. As a rule these steels are quenched from the intercritical gamma + carbide region and tempered at low temperatures (less than 250 °C), in order to retain the high hardness of the martensite matrix and avoid the tempered martensite embrittlement (TME) phenomena, which compromise the toughness of steels tempered at higher temperatures. A new high Si alloy was developed for bearing applications. The inhibiting and/or retarding effect of Si on the kinetics of cementite precipitation leads to a higher temperature of TME occurrence, allowing the tempering of the components at a higher temperature, thus increasing the toughness, without sacrificing the high hardness. The purpose of this work was to confirm the TME resistance of the new alloy. In this work, impact tests result for commercial SAE/AISI 52100 (0.25% Si) and for a modified 52100 containing 1.74% Si, were compared. No evidence of TME was detected on the Si-modified steel.     

304不锈钢应变诱发α''''马氏体相变及对力学性能的影响

借助于C射线衍射，研究了C、Mn、Cr和Ni含量对304奥氏体不锈钢拉伸力学性能和应变诱发马氏体相变倾向的影响。结果表明：C、Mn、Cr和Ni在允许的成分范围内变化，应变诱发α’马氏体相变倾向差异很大，这导致屈服强度和抗拉强度复杂的变化，尽管应变诱发α’马氏体相变使加工硬化速率提高，相变可以诱发塑性，但相变速率较快，相变倾向较大的钢塑性反而下降，此外，由于室温变形还增大热诱发马氏体相变倾向，从而限制了C、Mn、Cr和Ni下限钢在高精度和低温环境下构件的应用。     

Influence of Martensite Volume Fraction on Impact Properties of Triple Phase (TP) Steels

Ferrite-bainite-martensite triple phase (TP) microstructures with different volume fractions of martensite were obtained by changing heat treatment time during austempering at 300 °C. Room temperature impact properties of TP steels with different martensite volume fractions (V _M) were determined by means of Charpy impact testing. The effects of test temperature on impact properties were also investigated for two selected microstructures containing 0 (the DP steel) and 8.5 vol.% martensite. Test results showed reduction in toughness with increasing V _M in TP steels. Fracture toughness values for the DP and TP steels with 8.5 vol.% martensite were obtained from correlation between fracture toughness and the Charpy impact energy. Fractography of Charpy specimens confirmed decrease in TP steels’ toughness with increasing V _M by considering and comparing radial marks and crack initiation regions at the fracture surfaces of the studied steels.     

Effect of carbon on the plastic strain ratio of low carbon dual-phase steels

The effect of carbon on the plastic strain ratio of low-carbon dual-phase steels was investigated in a series of low C-1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon contents ranging from 0.021 to 0.048%. The r_m value of dual-phase steel could be increased by controlling both the carbon content and the martensite morphology. The highest r_m value of 1.23 was obtained in 0.021% C steel annealed at 790 °C according to the typical galvannealing heat cycle. The martensite volume fraction was 5.4%, which was sufficient to eliminate the yieldpoint elongation in as-annealed steel, and this had little deteriorating effect on the r_m value. The fine martensite particles between 0.5 μm and 2.0 μm in size were desirable for a high r_m value in dual-phase steel.     

Continuous cooling transformation behavior and impact toughness in heat-affected zone of Nb-containing fire-resistant steel

Simulated heat-affected zone continuous cooling transformation diagram was developed for advanced fireresistant steel. Over a wide range of cooling rates, corresponding to t_8/5 from 6 s to 150 s, granular bainite was the dominant transformation constituent, while the morphology of less dominant martensite-austenite (M-A) constituent changed from film-like to block-type constituent; but the hardness remained similar to the average value of 190-205 HV (0.2). The start and finish transformation temperature was high at 700 °C and 500 °C, and is different from the conventional high strength low alloy steels. It is believed that the high-content (0.09 wt%) of Nb may promote bainite transformation at relatively high temperatures. Martenistic matrix was not observed at high cooling rate and the film-like M-A constituent and blocky M-A constituent with thin film of retained austenite and lath martensite were observed on slow cooling. Excellent impact toughness was obtained in the heat-affected zone with 15-75 kJ/cm welding heat input.     

Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.     

The attack of molten zinc on steels

A new technology test method has been developed to determine the attack of molten zinc on different steels. The tests were carried out with low-alloy steels, tool steels and stainless steels. These materials were tested in pure molten zinc and in a zinc alloy containing 4% aluminium. The test temperatures were 400–600 °C. The effect of the superficial oxide layer that protects steel against zinc attack has been investigated and the attack mechanism have been discussed. The relationships between zinc attack, time and temperature have been established for low and high-alloy steels. Small additions of aluminium to the zinc melt have no effect on the attack rate. In a zic melt contaning 4% aluminium the rate of the zinc attack on steel is greatly reduced. In this case the time law is found to be parabolic at 500 °C and linear at 600 °C. In a pure zinc melt the unalloyed steel has a better resistance to zinc attack than tool steel.     

The Effects of Alloying Elements on the Continuous Cooling Transformation Behavior of 2¼Cr-1Mo Steels

The continuous cooling transformation (CCT) behavior of eight 2¼Cr-1Mo steels from a statistically designed matrix was determined. These steels contained two levels of carbon (0.07 and 0.16 wt.%), manganese (0.35 and 0.85%), chromium (1.5 and 2.8%), and molybdenum (0.3 and 1.25%). Each steel was tested in a quenching dilatometer at five to six cooling rates between 725 and 1.2 °C/min. For each CCT sample, the change in length, microstructure, and macrohardness were determined. The ferrite content was also measured for samples cooled near the ferrite nose. Pearson correlation and multiple regression analyses were performed for various CCT diagram parameters. The correlation analysis showed that carbon and chromium contents significantly affected the critical temperatures and the bainite and martensite transformation temperatures. Increasing carbon content significantly increased the hardness for the bainite and martensite range of cooling rates, but hardness at slower cooling rates was unaffected by alloying elements. Regression equations were obtained for the critical temperatures and the ferrite nose cooling rate.     

碳含量对5Cr-2Mo-V系列钢的连续冷却转变行为的影响

通过测定不同碳含量的钢种在不同冷速下的相变点曲线、金相组织和显微硬度,得到了不同碳含量的钢种的过冷奥氏体连续冷却转变曲线(CCT曲线)及其相变点和组织形貌演变;比较分析了不同碳含量下5Cr-2Mo-V系列钢CCT曲线的关系.结果表明:随着碳含量的增加,3种钢Ac1点变化不大,Ac3点略有降低,Ms点降低幅度较大;当冷却...     

Work hardening induced by martensite during transformation-induced plasticity in plain carbon steel

Transformation-induced plasticity (TRIP) steels are becoming increasingly exploited for industrial applications because they show high strength and high uniform elongation (ductility). Despite this interest, the relative contributions of the various strengthening and straining mechanisms are often poorly understood. In this study, neutron diffraction is employed to quantify the contribution of different mechanisms to ductility and work hardening for a 0.25 wt.% C steel. Differences in stress–strain response at different temperatures are related to the extent of the transformation of metastable austenite into martensite during deformation. At room temperature (RT) the transformation of austenite occurs gradually with straining, while at ?50 °C the transformation occurs almost from the onset of loading. The associated transformation strain is reduced, comprising nearly half the total strain, lowering the apparent elastic modulus and explaining the relatively low work hardening compared to RT straining. By contrast, deformation at RT after pre-straining at ?50 °C results in larger work hardening than for solely RT straining due to the higher martensite levels introduced at ?50 °C. This is due to composite load transfer to the strong constituent from the soft matrix. The extent of the transformation is quantified as a function of strain at both temperatures as well as its effect on the work hardening and elongation.     

Microstructure,Mechanical Properties,and Unlubricated Sliding Wear Behavior of Air-Cooled MnSiCrB Cast Steels

Two medium carbon low-alloy MnSiCrB cast steels containing different Si contents (0.5 and 1.5 wt.%) were designed, and the effects of Si contents on the microstructure, mechanical properties, and unlubricated sliding wear behavior of the cast steels after air-cooling from 850 °C and subsequent tempering at 220 °C was studied. The results show that the microstructure of the cast steel containing 0.5 wt.% Si consists of granular bainite and lower bainite/martensite multi-phase. In the cast steel containing 1.5 wt.% Si, granular bainite was not observed. The microstructure consists of carbide-free bainite/martensite multi-phase. Excellent hardenability can be obtained at both low and high Si levels. The cast steel containing 0.5 wt.% Si exhibits excellent combination of strength, ductility, and impact toughness superior to the cast steel containing 1.5 wt.% Si. Also, the wear-resistance of the former steel is better than that of the latter in the unlubricated sliding wear condition. The air-cooled MnSiCrB cast steel containing low Si levels, with excellent mechanical properties and wear-resistance, is a potential high-performance and low-cost wear-resistant cast steel for unlubricated sliding wear condition.     

The Effect of Dynamic Strain Aging on Subsequent Mechanical Properties of Dual-Phase Steels

Dual-phase (DP) steels with different martensite contents were produced by subjecting a low carbon steel to various heat treatment cycles. In order to investigate the effect of dynamic strain aging (DSA) on mechanical properties, tensile specimens were deformed 3% at 300 °C. Room temperature tensile tests of specimens which deformed at 300 °C showed that both yield and ultimate tensile strengths increased, while total elongation decreased. The fatigue limit increased after pre-strain in the DSA temperature range. The effects of martensite volume fraction on mechanical properties were discussed.     

Effects of Temperature on Microstructure and Wear of Salt Bath Nitrided 17-4PH Stainless Steel

Salt bath nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 610, 630, and 650?°C for 2?h using a complex salt bath heat-treatment, and the properties of the nitrided surface were systematically evaluated. Experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt bathing nitriding, the main phase of the nitrided layer was expanded martensite (????), expanded austenite (??_N), CrN, Fe₄N, and (Fe,Cr) _x O _y . In the sample nitrided above 610?°C, the expanded martensite transformed into expanded austenite. But in the sample nitrided at 650?°C, the expanded austenite decomposed into ??_N and CrN. The decomposed ??_N then disassembled into CrN and alpha again. The nitrided layer depth thickened intensively with the increasing nitriding temperature. The activation energy of nitriding in this salt bath was 125?±?5?kJ/mol.     

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.