Cr12模具钢裂纹电磁热止裂的研究

采用脉冲电流发生装置对Cr12模具钢中裂纹进行了脉冲放电止裂。放电后,裂纹尖端金属熔化形成焊口和压应力区,使裂纹尖端钝化,达到了止裂的目的。同时在裂纹尖端处得到了超细化的隐晶马氏体和细粒碳化物等微观组织,极大提高了裂纹尖端处的硬度、强韧性和耐磨性。     

304不锈钢棘轮变形过程中应变诱发马氏体相变行为研究

利用OM, SEM和XRD对单轴非对称应力循环下304不锈钢棘轮变形过程中的微观组织变化进行了实验观察. 结果表明: 304不锈钢棘轮变形过程中,当棘轮应变达到一定值后会 产生应变诱发马氏体相变, 形成板条状 马氏体,并且随循环周次的增加, 形成的应变诱发 马氏体相对量逐渐增加.因马氏体相变而诱发的塑性变形对总的棘轮变形量产生一定的影响,材料的棘轮应变应由两部分组成, 即应力引起的塑性应变和相变诱发的塑性应变.     

残余应力对激光修复紧凑拉伸试件疲劳性能的影响

以304不锈钢制作的含裂纹紧凑拉伸试样为研究对象,开展了裂纹尖端激光修复实验研究。基于热力顺序耦合有限元方法,模拟了激光作用后裂纹尖端修复区残余应力的分布情况,并分析了裂纹扩展对应力重分布和塑性区尺寸的影响;同时,得到了考虑残余应力影响的应力强度因子,并由此建立了改进的裂纹扩展速率预测模型以评价修复试件的疲劳性能。最后通过开展疲劳裂纹扩展实验验证了本文模型的有效性。结果表明,裂纹尖端激光修复会使修复区产生残余拉应力,裂纹的扩展会使残余应力场重新分布,并减小裂纹尖端的塑性区尺寸。残余拉应力的存在会增大试件所受载荷的有效应力比,提高疲劳裂纹扩展速率,降低构件的疲劳寿命。     

基于有限元法和纳米压痕技术的SS304/BNi-2/SS304钎焊接头残余应力分析

利用有限元软件ABAQUS开发了一个顺次耦合的有限元程序对SS304/BNi-2/SS304T形钎焊接头残余应力进行了数值模拟,并采用纳米压痕试验进行了测量.结果表明,由于304不锈钢与镍基钎料的力学性能的差异导致了钎焊接头残余应力的产生.其最大值出现在钎角部位,裂纹易在该处起裂并扩展,成为最薄弱区域.残余应力沿中缝逐渐减小,其它区域应力分布较均匀.纳米压痕试验采用了Suresh模型,并将此模型下的试验测量结果与有限元结果进行了比较分析,两者吻合较好,从而也验证了有限元模拟的有效性与可靠性.     

电磁热效应对T7钢的裂纹止裂

利用电磁热效应原理,在自制的ＺＬ０－１型超强度脉冲电流以发生装置上,对Ｔ７钢的纹止裂效果进行了初步研究。表明：对试件实施放电瞬间,在裂纹尖端处形成焊口使裂纹尖端的同蓄半径增大,达到抑制裂纹扩展的目的。同时：对裂纹尖端热影响区的组织进行了分析,并讨论了因快速加热和冷却作用所获得细小淬火马氏体组织及热压应力和相变压应力对止效果的影响。     

形变诱导马氏体相变循环制备304不锈钢超塑性的研究

提出了形变诱导马氏体相变实现304不锈钢超塑性的构想,研究了利用形变诱导马氏体相变循环实现超塑性的方法。结果表明,304不锈钢通过室温形变可诱导马氏体相变,随后时效时马氏体发生逆相变且伴随产生回复应力,从而间接达到了相变超塑性的实现条件。采用室温拉伸30%后在973~1273 K时效20 min,相变循环2次,该钢可实现相变超塑性,总伸长率随时效温度的增加先增加后下降,1123 K达到最大值177%;在室温拉伸30%后1073K时效30 min时,随相变循环次数增加,总伸长率增加,相变循环7次后达到764%。随相变循环次数增加,晶粒明显细化、均匀化。     

单边裂纹电磁热止裂强化的残余应力分析

建立带有单边裂纹的无限大金属薄板模型,给出脉冲放电瞬间的温度场,应用一维条件下的热弹塑性理论,导出了单边裂纹金属构件电磁热止裂后的一维残余应力解;对带有单边裂纹的导电薄板脉冲放电后裂纹尖端产生的残余应力进行数值分析,理论与数值分析结果基本吻合;对40CrNiMo试件的残余应力超声波无损检测发现,止裂后残余应力变大。研究表明,在对带有单边裂纹的导电薄板进行电磁热止裂时,裂纹尖端周围温度急剧升高,使得裂纹尖端受热膨胀,由于裂尖受到周围材料的刚性约束,在受热和冷却过程中会残留一个压缩塑性应变,其产生的残余应力可有效的抑制裂纹扩展。     

奥氏体304不锈钢在活化状态下的应力腐蚀与马氏体相变

用SSRT和电化学方法研究了304不锈钢在酸性氯化物溶液中的应力腐蚀与形变诱发马氏体的关系。结果表明形变诱发马氏体相不仅有利于应力腐蚀裂纹的形核而且它的选择性溶解构成了裂纹扩展的活性通道。文中讨论了裂纹沿马氏体相扩展的机制。     

GCr15凹模淬火裂纹止裂处的显微组织分析

对GCr15冷冲凹模淬火裂纹尖端进行脉冲放电止裂，放电时裂纹尖端熔化形成焊点和高压应力区，使裂纹尖端钝化，达到止裂目的，由于脉放电超快速加热和冷却，在裂纹尖端处实现了冲击淬火，得到了超细化的隐晶马氏体和细粒碳化物，硬度提高到HV900－1200。     

应力松弛对应变诱发马氏体相变影响的有限元模拟

在基于新型淬火-分配-回火(Q-P-T)钢微观组织的有限元模型中,建立了产生马氏体相变的一维应变等效模型,模拟了单轴拉伸条件下的相变诱发塑性(TRIP)效应,由此揭示了该效应的微观机制.TRIP效应产生的应力松弛有效地缓解了未转变的残余奥氏体和邻近马氏体的应力,阻止了裂纹的形成,并使较多的残余奥氏体在较大的应变下存在,这是TRIP效应的起因;模拟结果还显示,相变形成的新(应变诱发)马氏体比原始(热诱发)马氏体承载更大的应力,由此预测裂纹首先在新马氏体中或其边界处形成.应力松弛效应使应变诱发马氏体断续缓慢地生成,这与实验观察结果相符.通过比较有应力松弛效应和无应力松弛效应的有限元模拟结果发现,无应力松弛效应使应变诱发马氏体相继快速地生成,这与实验不符,由此反证TRIP效应必然产生应力松弛.     

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (ΔK) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test.Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.     

Stress-induced martensitic transformation in metastable austenitic stainless steels: Effect on fatigue crack growth rate

This paper addresses the influence of cyclic stress-induced martensitic transformation on fatigue crack growth rates in metastable austenitic stainless steels. At low applied stress and mean stress values in AISI type 301 stainless steel, fatigue crack growth rate is substantially retarded due to a cyclic stress-induced γ-α′ and γ-ε martensitic transformation occurring at the crack-tip plastic zone. It is suggested that the transformation products produce a compressive residual stress at the tip of the fatigue crack, which essentially lowers the effective stress intensity and hence retards the fatigue crack growth rate. At high applied stress or mean stress values, fatigue crack growth rates in AISI type 301 steels become almost equal to those of stable AISI type 302 alloy. As the amount of transformed products increases (with an increase in applied or mean stress), the strain-hardening effect brought about by the transformed martensite phase appears to accelerate fatigue crack growth, offsetting the contribution from the compressive residual stress produced by the positive volume change of γ → α′ or ε transformation.     

构件中半埋藏裂纹在线止裂及力学性能分析

依据热-电耦合数值分析结果选择合适的脉冲放电电压,应用ZL-2超强脉冲电流发生装置,对带有半埋藏裂纹的45号试件进行拉伸载荷下脉冲放电止裂研究。放电止裂后,裂纹尖端瞬间熔化、钝化,达到了裂纹止裂目的。止裂中,试件在拉伸载荷作用下没有瞬间开裂。对止裂前后的拉伸试件通过冲击试验机、扭转试验机进行了冲击和扭转力学性能测试。研究结果表明,电磁热裂纹止裂可以有效提高试件的抗冲击和抗扭力学性能。     

40GrNiMo试件单边裂纹在线止裂及组织分析

对带有单边裂纹的40GrNiMo金属薄板试件进行了脉冲放电在线止裂实验研究。在超强脉冲电流作用瞬间,单边裂纹尖端熔化、裂尖钝化,曲率半径增加了2～3个数量级,并在裂纹尖端处产生热压应力场,达到了遏制裂纹扩展的目的。止裂前后裂纹前缘的微观组织对比分析发现,止裂后裂尖处组织更加均匀细小,且裂尖处Cr原子数量增加,其他元素与Cr生成更多的合金组织,得到强韧性均高的微观组织,提高了止裂后构件的力学性能。     

Quantitative measurement of deformation-induced martensite in 304 stainless steel by X-ray diffraction

A single X-ray diffraction scan is effectively used for identifying and evaluating deformation-induced transformation in 304 austenitic stainless steel. Variations in grain size influence surface constraint and hence the through-thickness transformation response. The initial stage of transformation in this steel is most likely dominated by –martensite formation.     

裂纹尖端塑性区的声发射特性研究

对裂纹尖端形成塑性区时产生声发射信号的累计数与应力强度因子的关系进行研究。运用线弹性断裂理论和弹塑性断裂理论进行了分析,得出累计数在小范围屈服时与应力强度因子的4m次方成正比,在大范围屈服时与应力强度因子的2m次方成正比。对碳钢和不锈钢的单边切口试样进行了拉伸声发射监测实验,实验测得大范围屈服时,比例系数分别为0.333和0.183,m值为0.5。     

Comparative Study of Hardening Mechanisms During Aging of a 304 Stainless Steel Containing α′-Martensite

Strain aging and hardening behaviors of a 304 stainless steel containing deformation-induced martensite were investigated by examining mechanical properties and microstructural evolution for different aging temperature and time. Introduced age hardening mechanisms of a cold rolled 304 stainless steel were the additional formation of ????-martensite, hardening of ????-martensite, and hardening of deformed austenite. The increased amount of ????-martensite at an aging temperature of 450?°C confirmed the additional formation of ????-martensite as a hardening mechanism in a cold rolled 304 stainless steel. Additionally, the increased hardness in both ????-martensite and austenite phases with aging temperature proved that hardening of both ????-martensite and austenite phases would be effective as hardening mechanisms in cold rolled and aged 304 stainless steels. The results suggested that among hardening mechanisms, hardening of an ????-martensite phase, including the diffusion of interstitial solute carbon atoms to dislocations and the precipitation of fine carbide particles would become a major hardening mechanism during aging of cold rolled 304 stainless steels.     

奥氏体304不锈钢在活化态下的应力腐蚀与马氏体相变

用ＳＳＲＴ和电化学方法研究了３０４不锈钢在酸性氯化物溶液中的应力腐蚀与形变诱发马氏体的关系．结果表明形变诱发马氏体相不仅有利于应力腐蚀裂纹的形核而且它的选择性溶解构成了裂纹扩展的活性通道．文中讨论了裂纹沿马氏体相扩展的机制．