表面脱碳对Al-Si镀层热冲压钢弯曲韧性的影响

Al-Si镀层热冲压钢因为其超高的强度而被广泛应用于车身,研究发现镀层会严重恶化热冲压钢的弯曲韧性。基于“界面富碳致脆”理论,以国内某钢厂生产的22MnB5热冲压钢为研究对象,研究了表面脱碳对表面镀层组织和力学性能的影响,为提高Al-Si镀层热冲压钢的弯曲韧性提供参考。结果表明,在920℃保温6 min后,试验钢的镀层组织由FeAl相和铁素体层构成,基体组织均为马氏体。表面脱碳对Al-Si镀层热冲压钢的拉伸性能影响较小,但对弯曲性能影响较大。钢材表面脱碳后,降低了热冲压后镀层/基体界面碳富集程度,提高了界面马氏体的韧性,进而提高了热冲压钢的弯曲韧性。随着表面脱碳厚度的增加,试验钢的弯曲性能逐渐提高,完全脱碳层为30.3μm时,最大载荷弯曲角为82.4°±2.9°。在弯曲过程中,裂纹扩展分成2类,一类是裂纹穿过铁素体层后,在镀层/基体界面钝化,随后基体发生变形及断裂;另一类是裂纹穿过铁素体层后,进入到马氏体基体,在马氏体基体中钝化,随后基体发生变形和断裂。裂纹扩展路径取决于界面马氏体的强度,马氏体强度高时,发生第一种扩展;马氏体强度低时,发生第二种扩展。     

不同表面处理条件下身管烧蚀研究

通过模拟发射试验对表面镀铬、氮碳共渗两种表面处理条件下的身管进行了烧蚀模拟测试,研究在模拟工况下身管烧蚀情况.镀铬身管由于镀铬层固有的脆性,且受到高温高压火药气体的冲击作用,铬层内易产生显微裂纹,裂纹扩展至铬层与基体界面处,并沿着镀层与基体界面扩展,从而导致镀层剥落.氮碳共渗身管在烧蚀过程中,表面产生大量较深且较宽的裂纹,裂纹直接贯穿到基体使基体严重地被火药燃烧气体腐蚀,从而导致身管失效.在上述研究基础上,提出了两种不同处理方式下身管的失效模式.      

B级船板钢形变断裂过程的原位研究

 在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明：在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。     

钴磷镀层表面热疲劳裂纹的萌生及扩展机理

在自约束型热疲劳试验机上对化学镀钴磷合金镀层进行了热疲劳试验,采用光学显微镜、扫描电子显微镜研究了在热应力作用下镀层表面热疲劳裂纹萌生、扩展的方式和形态。结果表明:200次冷热循环后,V型缺口处发生塑性变形,并且随冷热循环的进行,热疲劳裂纹由缺口底部萌生并沿着循环方向扩展。重点分析了钴磷合金镀层表面热疲劳裂纹的萌生及扩展机理。     

WC系钢结硬质合金断裂过程与断裂机制的研究

用金相显微镜观察了WC系钢结硬质合金在三点弯曲载荷作用下的一断裂过程。试验结果表明,在外加载荷作用下,裂纹茵先在WC相粗大粒子和碳化物偏集区WC相粒子界面及碳化物偏集区的孔隙处萌生;随着应变量增大,已开裂WC相粒子中的解理裂纹张开粗化,与此同时较小的WC相粒子发生开裂;裂纹沿WC相与WC相粒子界面扩展,并穿过WC相粒子间的钢基体相薄层,然后裂纹沿WC相或二次碳化物粒子与钢基体相交界面扩展并撕裂钢基体相,最终导致试样完全断裂。作者根据对断裂过程的观察和断裂试样断口的分析结罘,讨论了WC系钢结硬质合金断裂的微观机制,并提出了提高材料抗脆性断裂能力和防止早期破裂的途径。     

45钢拉伸实验样棒开裂原因分析

通过对45钢拉伸开裂试样进行分析后发现,偏析带处铁素体含量极少,偏析带处存在夹渣、魏氏组织及网状碳化物。受到拉伸应力时,以夹渣处为裂源,产生了穿晶开裂,裂纹扩展至基体时,由于基体组织塑性较好,在受力作用下产生塑性变形,偏析处裂纹扩展至基体后沿晶开裂。     

预合金化时间对热成形钢锌基镀层组织和成分的影响

 利用扫描电子显微镜及附属EDS能谱仪研究了不同预合金化时间对热成形钢22MnB5锌基镀层组织形貌及成分分布的影响规律。结果表明,试样在加热之后,镀层表面氧化物主要为ZnO和Al2O3;在550 ℃预合金化时,随着时间的延长,基体中的铁逐渐向镀层中扩散,镀层组成由η相、ζ相、δ相和Γ相逐渐转变为只有Γ相;奥氏体化之后,镀层中的组织几乎全部为α-Fe（Zn）,只在表面有极少的Г相;预合金化保温时间为30 min时,在900 ℃保温5 min奥氏体化后能得到质量较好的镀层。     

钛-钢复合板卷制裂纹分析

通过金相分析等手段研究钛-钢复合板卷制裂纹产生的原因,结果表明:钛-钢复合板冷卷制裂纹产生的根源在于钢中存在A类非金属夹杂(硫化物),该类夹杂物破坏基体的连续性。钢板内部在后续卷制和焊接过程中发生变形,使裂纹继续扩展。当裂纹没有受力扩展时,钛-钢复合板的性能和探伤指标均能满足标准要求。     

Q235B钢中夹杂物与基体协调变形的工艺相关性研究

利用光学显微镜和扫描电镜,对Q235B钢中夹杂物与基体在轧制过程中协调变形相关性进行了研究。结果表明:采用控轧控冷工艺,因钢板在未再结晶区轧制时塑性相对较差,钢中夹杂物与基体在轧制过程中易产生相对滑动并生成微裂纹,微裂纹成为后续拉伸过程中裂纹扩展的源头,进而影响钢板的塑性;采用直接轧制工艺,轧制在再结晶区完成,钢板的塑性较好,钢中夹杂物与基体在轧制过程中不易产生微裂纹,对塑性影响较小。     

拉伸载荷作用下AIN夹杂物对航空用超高强度钢中裂纹萌生的影响

采用扫描电镜原位观测法,跟踪观察了航空用超高强度钢中长方型夹杂物导致裂纹萌生与扩展的微观行为。结果表明,拉伸载荷作用下,航空用超高强度钢中裂纹萌生的方式与夹杂物尺寸及夹杂物周围孔洞的大小有关。当夹杂物面积小于一定值时,无论夹杂物周围有无孔洞,裂纹均以夹杂物／基体界面开裂的方式萌生;当夹杂物面积大于一定值后,若夹杂物／基体界面基本完好,则裂纹易以夹杂物自身开裂的方式萌生;若夹杂物周围孔洞面积较大,则裂纹易以夹杂物／基体界面开裂的方式萌生。     

Residual stress analysis in galvannealed coating

The galvannealing process results in thermal residual stress in zinc–iron alloy coating on steel substrate due to differential thermal behaviour of the coating and the substrate. The magnitude of biaxial tensile residual stress generated just after cooling has been estimated theoretically and the value is found to be high enough to cause through-thickness cracks in the coating. Energy model has been adopted to determine the distribution of cracks and crack spacing in the galvannealed (GA) coating. The same was investigated from the microstructural analysis using scanning electron microscope. It is shown that the estimated crack spacing is always lower than the actual values. Subsequently a stress field develops around each crack tip which induces a non-uniform triaxial stress state in the coating.     

Liquid impact erosion mechanism and theoretical impact stress analysis in TiN-coated steam turbine blade materials

Coating of TiN film was done by reactive magnetron sputter ion plating to improve the liquid impact erosion resistance of steam turbine blade materials, 12Cr steel and Stellite 6B, for nuclear power plant application. TiN-coated blade materials were initially deformed with depressions due to plastic deformation of the ductile substrate. The increase in the curvature in the depressions induced stress concentration with increasing number of impacts, followed by circumferential fracture of the TiN coating due to the circular propagation of cracks. The liquid impact erosion resistance of the blade materials was greatly improved by TiN coating done with the optimum ion plating condition. Damage decreased with increasing TiN coating thickness. According to the theoretical analysis of stresses generated by liquid impact, TiN coating alleviated the impact stress of 12Cr steel and Stellite 6B due to stress attenuation and stress wave reactions such as reflection and transmission at the coating-substrate interface.     

82B高速线材拉拔过程中显微组织的演变分析

借助扫描电子显微镜(SEM)观察分析研究了82B线材在拉拔过程中显微组织和芯部马氏体的演变规律。分析认为:由于索氏体属于细片状珠光体,拉拔变形时,承受滑移的铁素体相不易引起应力集中;渗碳体相为细薄片层形态,也能够发生塑性变形,经过多道次的拉拔,索氏体基体变形量较大,没有出现任何裂纹。而芯部由偏析形成的马氏体,其塑性变形能力明显低于索氏体,基本无变形,在基本拉应力和附加拉应力的共同作用下,在早期拉拔时就在芯部出现裂纹,在后期拉拔过程中,裂纹沿着拉拔方向不断扩展。为减少断丝,可采用加大吹风量、扩大连铸坯、控制钢水过热度、增加电磁搅拌等工艺,来提高索氏体含量和细化渗碳体片层及预防马氏体的形成,以提高拉拔性能。     

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part II. Damage modes

Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the damage modes at work in three different microstructures of a zinc coating on an interstitial-free steel substrate under tension, planestrain tension, and expansion loading. Plastic-deformation mechanisms are addressed in the companion article. Two main fracture mechanisms, namely, intergranular cracking and transgranular cleavage fracture, were identified in an untempered cold-rolled coating, a tempered cold-rolled coating, and a recrystallized coating. No fracture at the interface between the steel and zinc coating was observed that could lead to spalling, in the studied zinc alloy. A complex network of cleavage cracks and their interaction with deformation twinning is shown to develop in the material. An extensive quantitative analysis based on systematic image analysis provides the number and cumulative length of cleavage cracks at different strain levels for the three investigated microstructures and three loading conditions. Grain refinement by recrystallization is shown to lead to an improved cracking resistance of the coating. A model for crystallographic cleavage combining the stress component normal to the basal plane and the amount of plastic slip on the basal slip systems is proposed and identified from equibiaxial tension tests and electron backscattered diffraction (EBSD) analysis of the cracked grains. This analysis requires the computation of the nonlinear stress-strain response of each grain using a crystal-plasticity constitutive model. The model is then applied successfully to other loading conditions and is shown to account for the preferred orientations of damaged grains observed in the case of plane-strain tension.     

Factors influencing the strength differential of high strength steels

Room temperature tensile and compressive true stress-true strain curves of various high strength steels (quenched and tempered 4340 steel, 410 martensitic stainless steel, and H-11 steel; and aged 300-grade 18 Ni maraging steel) were analyzed to determine the effect of the various microstructures, on what has been termed the strength differential (SD),i.e., the strength level difference between the tensile and compressive flow curves. Care was taken to insure that the compressive deformation was homogeneous. Regardless of the amount of plastic deformation, the quenched and tempered steels exhibited a higher flow stress in homogeneous compressive deformation than for tensile deformation. The extent of the SD was dependent on tempering temperature. This observation is consistent with what others have observed regarding yield strength behavior of quenched and quenched-and-tempered steels. Despite the low carbon content, aged maraging steel also showed a greater resistance to homogeneous compressive deformation. Metallographic examination of the maraging steel revealed the banding that is indicative of segregation. However, homogenization had little effect on the SD despite a change in austenite grain size, reverted austenite content, and the austenite-to-martensite transformational strains shown by Goldberg to be present in segregated material.     

Cavitation Erosion Behavior and Mechanism of HVOF Sprayed WC-10Co-4Cr Coating in 3.5 wt% NaCl Solution

A WC-10Co-4Cr coating was prepared by high-velocity oxygen-fuel (HVOF) thermal spraying process. The cavitation erosion (CE) characteristics of the coating as well as the stainless steel 1Cr18Ni9Ti were investigated in 3.5 wt% NaCl solution. The coating exhibited higher CE resistance than that of the stainless steel 1Cr18Ni9Ti. After being eroded for 20 h, the CE volume loss of the stainless steel 1Cr18Ni9Ti is 3.22 times to that of the coating. The removal mechanism for the coating was erosion of the binder phase first, followed by brittle detachment of hard phases as a result of the action of corrosion and mechanical effect. The cracks were found to initiate at the carbide-binder interface and the edge of the pores, leading to craters on the surface and accelerating the damage of the coating. Fatigue and plastic deformation were found to be the material removal mechanism for the substrate steel 1Cr18Ni9Ti.     

Multiple-cracking phenomenon of the galvannealed coating layer on steels under thermal and tensile stresses

The multiple-cracking phenomenon of the Fe-Zn intermetallic coating layer on the hot-dip galvannealed (GA) steels under thermal and tensile stresses was studied experimentally by tensile tests and analytically by means of the finite-element analysis. The multiple cracking of the coating layer had occurred in the as-supplied samples, and it progressed with increasing applied strain. Based on the calculated dependence of the stress of the coating layer on the crack spacing and applied strain, the multiple cracking in the as-supplied samples was accounted for by the thermally induced residual stress, and the further multiple cracking with increasing applied strain was accounted for by the increased stress of the coating layer. The experimentally observed decrease of the average crack spacing with increasing applied strain was described well, and the tensile strength of the coating layer was estimated to be 260 MPa, by application of the calculated relation between the increased stress of the coating layer and applied strain. The influences of the thickness of the coating layer and the substrate material on the multiple cracking were discussed based the stress analysis. It was revealed that the thinner the coating layer and the higher the flow stress of the substrate, the higher the stress of the coating layer becomes and, therefore, the smaller the crack spacing becomes.     

Effect of substrate surface polishing on galvannealing characteristics of a mild IF and an HSIF steel

Abstract

Mechanical properties and visual appearance of galvannealed steel sheets are strongly influenced by steel substrate surfaces before dipping into molten Zn. It has been known that oil contamination, iron fragments and surface enrichment of oxides deteriorate the Fe–Zn reaction and result in various harmful surface defects on the galvannealed coating. In order to prepare the optimal galvannealed coating for automotive exposed panels, the steel substrate surfaces were mechanically polished by SiO₂ polishing brushes.

The types of brush and pressure between the brush roll and strip are major factors for removing the appearance of visual defects on the surfaces of galvannealed coatings. The microstructure of the polished substrate, galvannealed coating and whiteness index on the surface of steel substrate were characterised by optical microscopy, scanning electron microscopy and colour difference meter.     

硬质合金金刚石涂层的残余应力分析

采用有限元方法分析硬质合金金刚石涂层的热残余应力,采用轴对称二维几何模型和自由边界条件,考虑了涂层厚度及不同基体对应力场的影响。分析表明,在多晶金刚石涂层中存在压应力,在基体中出现了拉应力,这种应力结构分布对材料性能的影响是有利的。在明锐界面出现了大的剪切应力(415MPa),应力奇异场出现在明锐界面靠近自由边界处。涂层压应力随基体中钴含量的增加而增加,随涂层厚度的增加而减小。因此可以借助这两个因素来适当控制和调节材料中的应力场以得到性能优良的产品。     

基板划痕对热镀锌及合金化镀层组织结构的影响

研究了IF钢表面划痕深度对热镀锌镀层及合金化镀层表面缺陷的影响、划痕部位镀层的生长过程、组织结构及合金化行为.试验及分析结果表明,热镀锌时锌液能够有效填充划痕凹陷,从而降低基板表面的划痕缺陷,但划痕缺陷可以遗传到合金化镀层表面;在镀层合金化过程中,与正常部位相比缺陷部位的合金化速度快;随着镀层合金化的进行,缺陷部位的Z...