Abstract A new technique called micro-needle peening was proposed to improve the fatigue strength of arc-welded ultra-high strength steel joints. Use of a small tip pin (micro needle) makes it possible to introduce compressive residual stress and work hardening effects without causing the crack-like defect which is inevitable with conventional needle peening. The fatigue strength of micro-needle peened joints was assessed in comparison with those of needle peened, shot peened and un-peened joints of ultra-high strength steel with a tensile strength grade of 980 MPa. The results showed that micro-needle peening and shot peening improved fatigue strength at 3 × 106 cycles from 275 MPa (un-peened) to 600 MPa, while the conventional needle peening improved it to 300 MPa. The fracture surfaces near the toes of the micro-needle peened and shot peened joints displayed a “smooth surface.” An electron back scattered diffraction study of the surface layers of the micro-needle peened and shot peened joints revealed that the top layer was so deformed that the crystal orientation could not be identified confidently, and the second layer was distorted and presented high kernel average misorientation and grain orientation spread values. The total depth of these layers was comparable to that of the smooth fracture surface. The effects of residual stress and work hardening induced by micro-needle peening were evaluated separately by comparing the results of different types of post-heat treatment. That study revealed that both residual stress and work hardening make comparable contributions to the improvement of fatigue strength by micro-needle peening. However, the effect of the change in weld toe geometry was negligible. The micro-needle peening method is expected to contribute to enhanced fatigue strength in welded parts.

中文翻译：

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微针喷丸法提高弧焊超高强钢接头疲劳强度

摘要 为提高弧焊超高强钢接头的疲劳强度，提出了一种微针喷丸新工艺。使用小尖端销（微针）可以引入压缩残余应力和加工硬化效应，而不会导致传统针刺不可避免的裂纹状缺陷。将微针喷丸接头的疲劳强度与抗拉强度等级为 980 MPa 的超高强度钢的针刺、喷丸和未喷丸接头的疲劳强度进行比较。结果表明，微针喷丸和喷丸在 3 × 106 次循环时将疲劳强度从 275 MPa（未喷丸）提高到 600 MPa，而传统针喷丸将疲劳强度提高到 300 MPa。微针喷丸和喷丸喷丸接头脚趾附近的断裂表面显示出“光滑表面”。对微针喷丸和喷丸接头表面层的电子背散射衍射研究表明，顶层变形严重，无法确定晶体取向，第二层扭曲并呈现高内核平均取向差和晶粒取向分布值。这些层的总深度与光滑断裂表面的总深度相当。通过比较不同类型后热处理的结果，分别评估了微针喷丸引起的残余应力和加工硬化的影响。该研究表明，残余应力和加工硬化对通过微针喷丸提高疲劳强度的贡献相当。然而，焊趾几何形状变化的影响可以忽略不计。预计微针喷丸法有助于提高焊接部件的疲劳强度。