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1.
采用电化学充氢及慢应变速率拉伸(SSRT)实验研究了真空渗碳热处理后20Cr2Ni4A齿轮钢的氢脆敏感性,并与常规淬火+回火处理(QT)的20Cr2Ni4A齿轮钢进行了对比.结果表明,渗碳试样渗碳层中的残余奥氏体含量(约13.8%,体积分数,下同)远高于渗碳试样心部和QT试样(约4.6%),前者主要呈多尺度的块状分布在原奥氏体晶界及板条界处.渗碳试样与QT试样中的室温可扩散性H含量相当,但前者组织中较多的残余奥氏体和渗碳体含量使得其室温非扩散性H含量明显高于后者,H扩散系数明显低于后者.QT试样呈现出优异的强塑性配合,以相对断后伸长率损失表征的氢脆敏感性指数(HEI)为54.3%.与QT试样相比,渗碳试样的抗拉强度提高了34.6%,但塑性显著降低,断后伸长率及断面收缩率分别降低了66.5%和92.4%;充氢后在屈服之前就发生了脆性断裂,呈现出很高的氢脆敏感性,HEI高达90.9%.SSRT断口分析表明,充氢QT试样与最大H扩散距离大体相当的表层脆性区为沿晶+准解理的混合断裂,而充氢渗碳试样则在距表面一定距离的渗碳层内呈现一定宽度的沿晶断裂脆性区,且在接近有效渗碳层深度处出现了一条大体沿渗碳层圆周方向扩展的长裂纹.造成渗碳试样与QT试样氢脆敏感性显著差异和独特氢脆断裂特征的主要原因与2者的微观组织、强度水平及渗层残余压应力等因素有关. 相似文献
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通过宏观观察、金相组织检查、扫描电镜及硬度分析,对SCM15钢轿车变速器行星齿轮断裂原因进行分析。结果表明,行星齿轮承受瞬间过载,导致韧性较低最薄弱的渗碳层部位达到萌生裂纹临界尺寸,发生沿晶脆性断裂。 相似文献
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采用显微组织观察、断口形貌分析、显微硬度测试和能谱分析,对1Cr11Ni2W2MoV钢渗碳层中孪晶组织形成原因及其对性能的影响进行了研究.结果表明,1Cr11Ni2W2MoV钢渗碳层中出现孪晶的原因可能与渗碳时工件周围碳势较低,且渗碳温度较高有关;渗碳层中孪晶组织的存在,使其断裂模式发生改变,脆性增加,应视为缺陷组织. 相似文献
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采用真空低压渗碳技术对304和316L奥氏体不锈钢进行表面强化,利用光学显微镜、扫描电镜、Thermo-Calc热力学软件、X射线衍射仪和显微硬度计等对渗碳层显微组织、相组成及硬度分布进行分析表征,计算了奥氏体不锈钢渗碳层中不同衍射峰的偏移量及渗碳前后晶格常数的变化量。结合钼对奥氏体不锈钢渗碳过程的影响,对比研究了304和316L奥氏体不锈钢渗碳后,在渗碳层深度、表面硬度及碳化物的析出规律等方面的差异。结果表明,经750 ℃真空渗碳2.6 h后,304和316L奥氏体不锈钢晶格常数分别增加了1.33%和1.14%,形成了由膨胀奥氏体和Cr23C6组成的渗碳层,Cr23C6在渗碳层中主要以条状沿膨胀奥氏体晶界析出,表面硬度较基体硬度均提升了两倍以上。 相似文献
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采用低温超饱和气体渗碳技术对316L奥氏体不锈钢进行表面强化,测量了渗碳层沿深度方向的碳含量、残余应力及纳米硬度分布。通过单轴拉伸试验,测量了渗碳层表面开裂伸长率,计算了断裂韧性,并采用分离法研究了低温超饱和气体渗碳表面强化层的平均抗拉强度。结果表明,经470℃,30h低温超饱和气体渗碳处理后,奥氏体不锈钢表面形成一层厚度约30μm的表面渗碳强化层,渗碳层表面碳质量分数高达约2.4%,纳米硬度达到12.6 GPa,残余应力达到-2.2 GPa;渗碳层表面断裂韧性约19 MPa·m~(1/2),断裂应变约1.5%;渗碳层平均抗拉强度为1.4 GPa;渗碳层在提高材料整体抗拉强度的同时,降低了屈服强度和伸长率。 相似文献
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离子渗碳温度对316L不锈钢渗层组织和性能的影响 总被引:1,自引:0,他引:1
利用低温离子渗碳技术.在不同温度下对AISI 316L奥氏体不锈钢进行渗碳处理.利用光学显微镜、显微硬度计、XRD以及电化学测试技术研究了渗碳温度对不锈钢表面显微组织和性能的影响.结果表明,渗碳温度显著影响AISI 316L奥氏体不锈钢渗碳层的组织结构与性能.渗碳温度在400~550℃之间时,可以获得无碳化物析出的、具有单一γ_c相结构的渗碳层;渗碳温度在550℃时,渗碳层为γ相+Cr_(23)C_6+Cr_7C_3+Fe_3C+Fe_2C的混合组织.渗碳层的厚度与硬度均随渗碳温度的升高而增加.550℃是AISI 316L奥氏体不锈钢中铬的碳化物析出的临界温度.为了避免铬的碳化物析出而降低不锈钢的耐蚀性能.奥氏体不锈钢渗碳必须在低于550℃的渗碳温度下进行. 相似文献
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设计了一种新型的铁基二元合金—Fe-3%Si合金,并对其进行表面渗碳处理.借助于SEM、XRD、EPMA、硬度测定及摩擦实验等分析手段,研究了合金渗碳层的组织及其性能.结果表明:Fe-3%Si合金渗碳层组织中形成大量SiO2颗粒,基体主要由马氏体和残余奥氏体组成.渗碳初期,Si元素促进了碳在奥氏体中扩散并有利于奥氏体稳定性提高.但氧原子沿奥氏体晶界扩散并与晶界及其附近的Si结合形成网状SiO2,阻碍碳原子进一步扩散.渗碳处理后的Fe-3%Si表面硬度达到66 HRC,干摩擦条件下抗平面滑动摩擦磨损性能优于高铬铸铁. 相似文献
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This paper deals with the cause of intergranular fracture occurred in the retained austeniticregion in plasma carburized layer.The results show that the presence of retained austenite,which has a good effect on the impact toughness,has no relation to this embrittlement.Analy-sis by Auger electron spectroscopy shows that the impurities S and P segregate at the grainboundaries is the main reason of the intergranular embrittlement in carburized layer.However,the segregation of P and S can be removed by reheating and quenching treatment. 相似文献
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The steel of a higher bulk carbon content shows the denser precipitation distribution of carbides after the solution treatment followed by tempering. Such a carbide distribution produces the smaller prior austenite grain size after the welding simulation at a high temperature. Because the equilibrium segregation concentration of phosphorus decreases with decreasing prior austenite grain size, the specimen of the higher bulk carbon content shows therefore the longer intergranular rupture life. The rupture life is also increased by the partitioning of phosphorus pre-segregated at prior austenite grain boundary/carbide interfaces onto the fresh surface of precipitates formed on the surface of pre-formed carbides. The intergranular rupture life is additionally increased by the repulsive segregation between carbon and phosphorus which decreases the overall phosphorus segregation concentration at the prior austenite grain boundaries. 相似文献
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As a result of the phase transformation of austenite to martensite during steel quenching, weakened structural regions, specifically the boundaries of the original austenite grains, have been formed. They are weakened because of microstructural factors, such as the residual internal microstrains and segregation of embrittling impurities. The joint effect of microstructural factors, namely, residual microstrains and segregation of phosphorus and carbon at grain boundaries, on reducing the local strength of the boundaries of the initial austenite grains in martensitic steels is quantitatively evaluated, and the impacts of these microstructural factors have been separated. The dependences of the local grain-boundary strength on the ratio of various levels of residual microstrains and on the atomic concentration of phosphorus impurities at the grain boundary in segregation spots have been determined. It has been shown quantitatively that the adsorption enrichment of the austenite grain boundaries with phosphorus leads to a decrease in the intergrain adhesion and facilitates the emergence and development of cracks along the boundaries of the initial austenite grains. The quantitative dependence of the local strength of grain boundaries on the concentration ratio of carbon and phosphorus in them has been shown. Carbon in concentrations of up to 0.04% reduces the embrittlement of the boundaries due to the segregation of phosphorus and loses its neutralizing effect on the phosphorus segregation at concentrations of more than 0.04%, so the phosphorus concentration at the grain boundaries increases and the embrittlement resistance of the latter decreases. The applicability of the developed technique for the quantitative evaluation of the local strength of hardened steel grain boundaries by using tests on delayed fracture and applying the method of finite elements to determine the local strains has been shown. 相似文献
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The grain boundary embrittlement in a binary Fe–12Mn is due to the grain boundary segregation of Mn. During tempering at 400 °C (higher than the equilibrium eutectoid reaction temperature 247 °C), reverted austenite particles were formed at lath and grain boundaries through the equilibrium reaction of lath martensite to ferrite + austenite. Surprisingly, hydrostatic pressure, which is induced by the transformation of epsilon martensite to austenite during heating at the tempering temperature, resulted in the nonequilibrium eutectoid reaction producing α-Mn precipitates at the interface between lath martensite and the transformed austenite during the tempering. The segregation concentration kinetics of Mn formed a convex profile due to the active grain boundary precipitation of the reverted austenite particles and the α-Mn particles, which act as a sink for the segregated Mn. Finally, the convex segregation profile of Mn corresponded to the concave profile of intergranular fracture strength. 相似文献
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Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation
of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in
combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation
rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat
treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under
different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused
during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot
produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular
fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement
and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element
segregation provided that the grain boundary segregation level exceeds a certain critical value. 相似文献
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M.A. Islam 《Journal of Materials Engineering and Performance》2007,16(1):73-79
Low alloy steels serving for a long time at high temperature, e.g., around 500 °C, are very sensitive to temper embrittlement
due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type
of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue
crack propagation rate with subsequent change in fracture morphology of low alloy steels. This article describes the segregation
behavior of various elements in 2.25Cr-1Mo pressure vessel steel investigated by AES, FEG-STEM, SEM, and EDS analyses. As
confirmed by AES and FEG-STEM, phosphorus is found to be the main embrittling element for isothermal embrittlement. Sulfur
and Mo segregation is only evident after longer embrittlement times. In the step-cooling embrittlement, phosphorus is still
found to be the main embrittling element, but heavy segregation of sulfur in some isolated intergranular facets was also observed.
For P segregation, a Mo-C-P interaction is observed, while sulfur segregation is attributed to site competition between sulfur
and carbon atoms. 相似文献
18.
Kaveh Meshinchi AslJian Luo 《Acta Materialia》2012,60(1):149-165
The effects of impurities on the intergranular penetration of Bi based liquids in polycrystalline Ni at 700 °C were systematically investigated. In comparison with a nominally pure Ni (99.9945%), the presence of a total amount of <0.5 at.% impurities of Mn, Fe and Si in the Ni increased the penetration length by six times when a near-equilibrium Bi-Ni liquid was applied; when an initially pure Bi liquid was applied, this increment further enlarged to ∼20 times in the initial penetration stage. In a second set of controlled experiments, the addition of Mn, Sn and Fe to the liquid Bi-Ni all enhanced the intergranular penetration, but produced different kinetics and morphologies. We extended a concept that was initially proposed in the Rice-Wang model for grain boundary embrittlement to explain our observations of the impurity-enhanced intergranular penetration based on a theory that segregation of an impurity could reduce the grain boundary energy more rapidly than the solid-liquid interfacial energy. Correspondingly, a new analytical model for the effect of adding a third impurity on changing the equilibrium dihedral angle and the associated intergranular penetration kinetics has been derived for the dilute solution limit. Furthermore, we demonstrated that the interplay of bulk phase equilibria, interfacial segregation, transport (dissolution, precipitation and diffusion) processes and stress generation could effectively explain a variety of different intergranular penetration behaviors and morphologies that have been observed in the experiments conducted using 10 different combinations of the solid and liquid metals. The framework for understanding the impurity effects on intergranular liquid penetration developed in this study can be applied to other materials systems. This study has practically importance for understanding and controlling liquid metal corrosion and embrittlement. 相似文献
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稀土低温高浓度气体渗碳工艺及其在20Cr2Ni4A钢上的应用 总被引:4,自引:5,他引:4
20Cr2Ni4A钢由于渗碳层奥氏体十分稳定,无法渗碳后直接淬火,而需经过复杂的热处理,本文采用稀土低温高浓度大气体渗碳,使渗层过共析区沉淀析出大量细小弥散的碳化物,奥氏体的稳定性大幅度下降,实现了渗后直接淬火,同时使组织和性能得到进一步改善。 相似文献