预应变对16MnR钢断裂韧性的影响SCIEI

本文实验发现预应变对16MnR钢的断裂韧性具有显著的影响,断裂过程中裂尖区域材料的组织结构发生了变化,裂纹在二次珠光体组织上扩展。16MnR钢断裂韧性的大小取决于碳原子的扩散及二次珠光体组织生成的难易程度。     

预应变下焊接接头冲击韧度及韧-脆转变的分析

以常用建筑结构钢(16Mn和Q235B)为研究对象，通过不同温度下的夏比冲击试验研究预应变对其母材和焊缝冲击性能的影响。结果表明，材料在预应变下无论是母材还是焊缝夏比冲击韧度均有所降低，韧一脆转变温度有所提高。5％预应变后的16Mn母材和焊缝其韧一脆转变温度均低于室温(20℃)，因此仍可在室温下安全使用；Q235B钢室温下冲击韧度较16Mn钢低，且预应变对母材影响更大，和未经预应变冲击值相比，降低了三倍有余。可以看出，Q235B钢的抗震性能较16Mn差。研究发现：为建立客观的材料韧度指标和进行更有效的安全分析时，预应变也应该作为一个考虑因素。     

Numerical simulation of prestrain history effect on ductile crack growth in mismatched welded joints

Pipe reeling may lead to plastic pre‐deformation (prestrain) around existing cracks in components; therefore, investigating whether this process accelerates or counteracts ductile crack growth, especially for strength mismatched welded joints, is warranted. This study focused on the effect of prestrain history on ductile crack growth in mismatched welded joints. A single‐edge‐notched tension specimen was selected for numerical study, and the crack was assumed to have existed before a prestrain history was applied. Crack growth resistance curves for plane strain and mode I crack growth under large‐scale yielding conditions have been computed using the complete Gurson model. Meanwhile, symmetrical and non‐symmetrical prestrain cycle modes with different loading levels were applied to the overmatched specimens. The outcome demonstrated that the mismatch ratio (the ratio between the yield stress of the weld metal and base metal) showed a significant effect on fracture resistance regardless of the stage at which the prestrain cycle loading was located. By contrast, the processing of the crack growth was weakened by the increase of prestrain values, and the symmetrical prestrain cycle resulted in greater plastic damage than the non‐symmetrical prestrain cycle did. However, the initial crack length had a non‐significant effect on the ductile fracture considering the prestrain and mismatch effects.     

Microscopic Response of TRIP Steels to Prestrain During Plastic Deformation

In order to uncover the mechanism of elastic modulus degradation during plastic deformation, uniaxial tensile test of transformation-induced plasticity (TRIP) steels under different prestrain levels was carried out. The real elastic modulus unloaded at each prestrain was calculated by linearly fitting. The microstructure evolution with plastic strain and the fracture morphology were monitored by using a scanning electron microscope (SEM). Dislocation density and its distribution were detected under a transmission electron microscope (TEM). Microscopic mechanism of the elastic modulus degradation of TRIP steels was discussed in detail. Experimental results indicated that the investigated TRIP600 steel was of severe elastic modulus degradation during plastic deformation. The new-born martensite distributed among the retained austenite, resulting in the combination of good ductility and high strength for TRIP steels. It was the change of dislocation movement that induced the variation of atomic binding force and finally led to the variation of elastic modulus.     

预应变和超载对低合金钢疲劳性能的影响

本文总结了微量预应变和超载对低合金高强度钢疲劳性能影响的实验结果.实验结果表明,微量预应变和超载的影响与试件几何有关,对光滑试件产生负效应,而对切口试件产生零效应.对超载效应的机制作了实验研究与分析.讨论了Miner定则适用性及应考虑的因素.     

Kinetics of strain aging in bake hardening ultra low carbon steel—a comparison with low carbon steel

The kinetics of the static strain aging process have been analyzed in a vacuum-degassed ultra low carbon bake hardenable (ULC BH) steel with a total carbon content of 20 wt.ppm through measurement of the strength properties. The influence of prestrain and free interstitial carbon content has been studied. The kinetic results were compared with those of a BH low carbon (LC) steel. In the derivation of the time exponent and the activation energy, only the first stage of aging was considered. It was observed that, at all prestrain levels and matrix solute carbon contents, the initial aging process in the ULC steel obeyed the t ^2/3 kinetic law and the kinetics were not influenced by the changes in dislocation structure due to prestrain and the dissolved carbon content. In comparison, the aging process and the kinetics in the LC steel were found to be significantly influenced by the amount of prestrain. The presence of carbide particles in LC steels can modify the aging kinetics.     

预变形及退火处理提高AZ31镁合金的塑性

研究了预变形及退火处理对挤压态AZ31镁合金压缩力学性能的影响,结果表明:沿挤压方向进行应变量为0.086的预压缩变形,随后在300℃下进行0.5小时退火处理,可显著提高镁合金的塑性,其压缩率比一次压缩至破碎的压缩率提高约137%。织构及金相分析结果表明:预变形使(0002)基面发生了近90°的转动,由平行挤压方向变为与挤压方向垂直,且产生了大量孪晶组织。退火处理不改变(0002)基面织构,但消除了孪晶且出现了细小再结晶晶粒,因而提高了镁合金的塑性。     

不同温度预应变TiNi形状记忆合金丝的逆相变行为

研究了不同温度预应变TiNi形状记忆合金丝的逆相变行为。实验结果表明,预应变温度显著影响合金丝的逆相变行为,在253~313K(马氏体相)预应变的样品,其加热DSC曲线上出现单峰,而在333K下预应变的样品,有三个连续的吸热峰出现在小应变样品的加热DSC曲线上,一个吸热峰出现在大应变样品的加热DSC曲线上。在253~313K(马氏体相)预应变样品的可恢复应变率相类似,而在333K预应变样品的可恢复应变率明显低于在马氏体相预应变样品。     

Comparison of low cycle fatigue behavior of 304 stainless steels induced by tensile and torsional prestrain

The effect of tensile and torsional prestrains on fatigue lives of 304 stainless steels is compared. The fatigue life as a function of prestrain amplitudes exhibits an ‘S’ curve, and the inflexion point of the S curve is affected by prestrain modes and cyclic strain amplitudes. An interesting phenomenon is that when the onset stress of secondary hardening reaches 300 MPa, intergranular martensite begins to be formed. Combining martensite distribution, fatigue life and cyclic stress responses, the onset stress of secondary hardening is proposed to reflect the location of martensite nucleation sites. Compared with tensile prestrain, torsional prestrain results in higher stress. In addition, higher prestrain and cyclic amplitudes also lead to higher cyclic stress and hence earlier nucleation of intergranular martensite. Furthermore, the influence mechanism of prestrain modes and cyclic strain amplitudes on the inflexion point of S curve is revealed.