高Cr-Co-Mo轴承钢强韧机制及抗疲劳特性的多尺度研究

发布时间：2018-04-17 17:02

本文选题：高Cr-Co-Mo轴承钢 + 多尺度　；参考：《昆明理工大学》2015年博士论文

【摘要】：为满足航空航天领域的特殊苛刻环境对轴承提出具备高强度、高硬度、耐腐蚀、韧性良好和优异高温性能的新要求,开发了满足更高指标的新型高Cr-Co-Mo系轴承钢。过去,对轴承钢强-韧性能关系和强韧化机理的研究大多围绕着相对简单的结构体系,其特征结构单一并且尺度单一。基于复杂“成分-结构”而设计开发的高Cr-Co-Mo系轴承钢具有多级的组织结构要素和多尺度、多相、以及多层次耦合等特点,使高Cr-Co-Mo系轴承钢在获得高强度的同时仍保持充足韧性,并兼备其他综合性能的改盖。本论文运用多尺度的研究方法,并结合轴承钢抗疲劳能力受多重因素影响的特点,通过系统地研究热处理后各相、各尺度的组织以及各类机制对高Cr-Co-Mo马氏体轴承钢强韧性和疲劳性能的影响,采用SEM、EBSD及TEM等观察手段,以定量表征的方式整合试验钢中精细组织结构的演化规律。以揭示由多相、多尺度控制的组织所对应的本征力学性能及强韧化机制,阐明复杂结构中调控强韧性能的组织控制单元,进而优化其综合性能。此外,根据项目指标要求设置强韧性能略有差异的对比试验组,从表面状态、应力及组织缺陷等方面揭示高Cr-Co-Mo轴承钢疲劳破坏的决定因素及工艺可控的相应临界指标范围。基于深入系统的理论分析研究与实际结果相结合,找出充分发挥试验钢应用潜能的组织调控方法,获得了具有一定创新性和工程应用价值的研究成果。研究得出了高Cr-Co-Mo低碳马氏体轴承钢在不同热处理工艺下的微观组织变化规律。-82℃冷处理2h使得马氏体晶格收缩导致间隙碳原子迁出并富集于M/A相界面附近,在随后的回火过程中扩散进入并稳定化残余奥氏体,最终少量残余奥氏体以薄膜状稳定存在于马氏体板条间。对碳化物进行统计分析发现,当固溶温度升高至1060℃时,少量存在于大角度晶界处的未溶M6C碳化物,其平均尺寸约0.3μm,能一定程度钉扎晶界以抑制高温阶段奥氏体的粗化。淬火态析出的M2C碳化物面积百分数1%,尺寸集中分布于30-40nm之间；而冷处理后M2C碳化物的面积百分数增加至7%左右,尺寸集中分布于20nm的区间；回火使M2C面积百分数进一步提高至7.33%,尺寸集中分布于20-30nm之间,尺寸过小(10nm)的析出相略微长大以增加对位错阻碍的有效位置。在马氏体亚结构中,板条领域尺寸由原奥尺寸决定：马氏体板条束由大角晶界包围,束间取向差约600而体现孪晶关系；通过晶体学分析发现,板条领域内包含三种变体群、6种变体,变体之间有特定结合模式。定量分析发现,固溶温度对M2C纳米相的析出强化影响不显著,但对细晶强化影响较大；淬火态试样屈服强度受控于板条束宽度,控制板条束宽度在10μm以下并使M6C碳化物的体积分数低于0.5%,可获得较好的强塑性配合。在冷处理-回火产生二次硬化以前M2C对屈服强度的贡献仅7.6%；冷处理和回火阶段析出尺寸40nm的M2C充分发挥了尺度效应,与位错产生交互作用,使屈服强度的增加超过800MPa,其强度贡献比例增至75%。经二次冷处理-回火循环后,试验钢最终屈服强度达到1532MPa,冲击吸收功保持在52J。与常规合金钢相比,晶粒尺寸的细化未能有效改善冲击韧性。根据Griffith脆断理论分析M6C碳化物对试验钢韧性的影响发现,大角晶界处的M6C降低韧性但不是决定因素,原因在于M6C的尺寸远低于导致脆性解离裂纹萌生的临界尺寸3μm。进而分析特征晶界的分布情况发现,低能重位点阵(CSL)晶界所占比例直接决定试验钢的韧性,∑3晶界是所有CSL晶界中最主要的晶界；并且所有低能晶界所占比例之和与板条束宽度存在线性的数学关系,马氏体板条束宽度的增加使束内的马氏体变体数量增加从而低能晶界比例随之提高。通过研究解理裂纹的扩展路径发现,裂纹的传播路径单元是板条领域(Packet);定量统计表面,解理平台尺寸与板条领域尺寸数值相互吻合。因此,板条领域尺寸是试验钢的韧性控制单元。根据项目指标要求,试验测定高强-Q组和表面渗碳-ST组试样通过107次应力循环的极限疲劳强度均超过600MPa。观察疲劳断口发现,高韧-R组主要为表面缺陷导致疲劳断裂,ST组经表面渗碳处理既提高了加工表面的平整度,又提供了由外内的压应力以抑制裂纹的萌生和扩展,从而抗疲劳能力大幅提高。根据定量计算得出,试验钢加工精度的临界粗糙度指标为0.53μm。而ST组主要由非金属夹杂物导致疲劳断裂,其临界夹杂物尺寸为5.5μm。夹杂物所处位置和尺寸均影响疲劳寿命；并且疲劳强度还受制于夹杂物所处位置的基体硬度。夹杂物深度越深、尺寸越小且基体硬度值越高,抗疲劳能力可得到最优化。对于试验钢而言,控制距离试样自由表面100μm以内的夹杂物尺寸将有效提高试验钢的抗疲劳能力。此外,Q组由于较大的晶粒尺寸易发生表面驻留滑移现象,由基体表层挤出微米级台阶,最终使疲劳裂纹源于挤出台阶两侧的高应力集中区域。
[Abstract]:In order to meet the needs of aerospace special harsh environment put forward with high strength, high hardness of bearing, corrosion resistance, good toughness and excellent new requirements of high temperature performance, developed to meet the new high Cr-Co-Mo bearing steel with higher index. In the past, the strong relationship between bearing steel toughness and strengthening and toughening mechanism of most of the studies focus on a relatively simple structure, the structure characteristics of single and single scale. Based on the complex "component structure and design of high Cr-Co-Mo bearing steel has developed organizational structure elements multi-level and multi-scale, multi phase characteristics and multi-layer coupling, high Cr-Co-Mo bearing steel still maintain adequate toughness in high the strength at the same time, to cover and combine with other comprehensive performance. This research method using multi scales, and combined with the characteristics of Bearing Steel Anti fatigue ability affected by multiple factors, through the systematic study of heat After each phase, the scale effect, the organization and all kinds of mechanism of high Cr-Co-Mo martensitic steel bearing strong toughness and fatigue performance of the SEM, EBSD and TEM in the observation methods, quantitative characterization of integration evolution of fine steel structure. As revealed by multiple phases, corresponding to multi scale control the organization of the intrinsic mechanical properties and toughening mechanism of complex structure, strength and toughness of the organization to clarify regulation control unit, and then optimize its comprehensive performance. In addition, according to the contrast test group project indicators requirements set slightly different from the properties of strength and toughness, surface state, corresponding critical index range of stress and tissue defects of determinants the fatigue failure of high Cr-Co-Mo bearing steel and process controllable. Systematic theoretical analysis and actual results based on the combination, give full play to find potential application organization adjustment test of steel Control method, obtained the innovative research achievements and application value. The research shows that the high Cr-Co-Mo low carbon martensite bearing steel under different heat treatment process of.-82 under the variation of microstructure at cold treatment 2H makes martensite lattice contraction leads to interstitial carbon atoms move out and enrichment near the M/A interface, diffusion into the and the stability of retained austenite during tempering, then in the final with a small amount of residual austenite film exist in lath martensite. The carbides for statistical analysis found that when the solid solution temperature to 1060 DEG C, undissolved M6C carbides exist in large angle grain boundaries less, the average size of about 0.3 m, can to some extent coarse grain boundary pinning to suppress the high temperature stage. The percentage of M2C 1% austenitic quenched carbide precipitation, concentrated in size between 30-40nm and M2C carbon after cold treatment; Area percentage of compounds increased to about 7%, the interval size mainly focus on 20nm M2C; tempering area percentage increased to 7.33%, concentrated in size between 20-30nm size (10nm) precipitates grow up slightly to increase the effective dislocation obstacle position. In the structure of the martensite lath. The field size is determined by the original Olympic size: martensite by grain boundary surrounded by beam orientation difference between about 600 and reflects the twin relationship; through crystallographic analysis found that in the field of slab contains three kinds of variant group, 6 variants have a specific binding mode between variants. Quantitative analysis showed that the temperature of solid solution precipitation M2C nano strengthening effect is not significant, but the influence of fine grain strengthening; quenching yield strength is controlled by the lath width, lath width control below 10 m and the M6C carbide volume fraction is less than 0.5% And can obtain better combination of strength and plasticity. Two tempering hardened in the cold treatment before the M2C contribution to the yield strength of only 7.6%; cold treatment and tempering stage precipitation size 40nm M2C give full play to the scale effect, interact with dislocation, increase the yield strength of more than 800MPa, its contribution ratio increased to strength 75%. after two times of cold treatment tempering cycles, the final test of steel yield strength up to 1532MPa, keep the impact absorbing energy in the 52J. compared with the conventional alloy steel, grain size refinement could not effectively improve the impact toughness of Griffith. According to the theory of brittle fracture analysis of the influence of M6C carbides on the steel toughness test found that the angle grain boundaries of M6C decrease toughness but it is not the determining factor, because that distribution of the size of M6C is far lower than the critical size of brittle crack initiation leads to dissociation of 3 m. and the analysis of the characteristics of the grain boundary, low CSL (CS L) grain boundary proportion directly determines the steel toughness test, 3 Sigma is the main grain boundaries in the grain boundaries of all CSL; and all the proportion of low energy grain boundary and lath width linear mathematical relations of the martensite beam in martensite increases the width of the body so as to increase the number of low energy variable the proportion of grain boundaries increased. By extending the path of the cleavage cracks found crack propagation path unit is lath field (Packet); quantitative statistics of surface, the size of the platform and the numerical solutions agree with each other. Therefore the field size of lath lath, field size is the steel toughness test control unit. According to the project requirements, test of high strength -Q group and surface carburizing of -ST groups of samples by 107 times should limit the fatigue strength of stress cycles were more than 600MPa. observation of fatigue fracture finds that high toughness -R group mainly for surface defects and fatigue fracture of ST. After carburizing treatment can improve the smoothness of the surface, and provides the initiation and propagation by internal and external compressive stress to prevent crack, and anti fatigue ability has been greatly improved. According to the quantitative calculation, the critical processing precision of test steel roughness index is 0.53 M. and the ST group is mainly composed of non metals inclusion and fatigue fracture, the critical inclusion size of 5.5 mu m. inclusion location and size effect and fatigue life; fatigue strength is also subject to the inclusion of the location of hardness. The inclusion of the deep, the smaller the size and matrix hardness value is higher, the ability of anti fatigue test for steel can be optimized. In terms of control size of inclusion free surface within the sample distance of 100 m will effectively improve the ability of anti fatigue test of steel. In addition, the Q group due to surface persistslip prone to the phenomenon of large grain size, from the base The surface layer extruded the microscale step, and finally the fatigue crack originated from the high stress concentration area on both sides of the extrusion step.

【学位授予单位】：昆明理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

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