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通过化学成分检测、金相组织分析、氢含量的测定、断口的宏微观观察、大直径高强螺栓的原型件拉伸试验以及有限元分析等手段从材料、结构、制造工艺三个方面上探究了ITER磁体支撑718螺栓低强度断裂的原因。材料分析结果表明,国产718螺栓材料的性能满足要求。工艺分析结果表明,螺栓低强度断裂的主要原因为滚压螺纹后的热处理导致螺纹根部脆性。结构分析结果表明,由于应力集中的影响,718螺栓的强度低于原材料强度,而且随着螺栓直径的增大而降低。基于以上分析结论,提出了能显著提高718螺栓强度的新工艺。通过原型件的拉伸试验,新工艺螺栓可以有效地提高螺栓的强度160MPa,达到1230MPa。

Failure analysis and process improvement of ITER inconel 718 bolts

Chemical composition analysis, metallographic observations, and hydrogen content test was used to inspect raw material properties of ITER inconel 718 bolts. The finite element analysis and the tensile test of the bolts were adopted to investigate mechanical properties of the bolts. Macro and micro observation of bolt fractured surface was used to verify the manufacturing processes of the bolt. The results showed that domestic 718 raw material has meet the property requirements, and the fracture of the bolts was caused by temper brittleness of the rolling layer under the condition of concentrates stress. The results of tensile test of bolts and FEA analysis show that due to the effect of stress concentration, the strength of the bolts was lower than raw material. Meanwhile the strength of bolts decreases with the increase of bolts diameter. Based on the above analysis the new technology was development which can significantly improve the strength of the bolts to 1230MPa, the increment is 160MPa.