The hardness and softening behavior of HAZ in Ti and Nb precipitation strengthened 600 MPa wheel steel
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摘要: 采用连续闪光对焊技术，焊接600 MPa级Ti，Nb析出强化车轮钢，制造22.5×9.0车轮的轮辋。采用气体保护焊技术，焊接轮辋与轮辐的搭接接头。轮辋闪光对焊接头在扩口工序出现开裂、辊型工序出现减薄、扩张工序出现开裂，综合报废率37%。车轮径向疲劳试验80万转时，轮辋与轮辐搭接焊缝发生焊根部位的疲劳开裂。对车轮钢接头的断口、组织、硬度、析出相进行分析。结果表明，对焊接头硬度呈现“马鞍形”局部软化特征，搭接接头硬度呈现“盆地形”局部软化特征。热影响区中，过热区晶粒长大，20 nm以下第二相粒子大量溶解，正火区晶粒长大，20 nm以下第二相粒子消失伴随20 nm以上第二相粒子熟化，造成热影响区软化。Abstract: A 22.5 × 9.0 wheel rim made of Ti and Nb precipitation strengthened 600 MPa wheel steel was manufactured by continuous flash butt welding process to. The lap joint of rim and spoke was welded under gas shielded welding process. The flash butt welded joint of the rim cracked during the flaring process, thinned during rolling forming process and cracked during expansion process. Consequently, the comprehensive scrap rate was 37%. When the wheel was subjected to radial fatigue test for 800000 cycles, fatigue cracking occurred at the weld root of the lap between the rim and the spoke. The fracture, microstructure, hardness and precipitated phase of wheel steel joint were analyzed. The results show that the hardness of butt welded joints presents "saddle" local softening characteristics, and the hardness of lap joints presents "basin" local softening characteristics. In the CGHAZ the grains coarsen, while the second phase particles below 20 nm dissovle significantly. In the FGHAZ the grains grow, with the second phase particles below 20 nm disappear and particles above 20 nm coarsen, leading to softening behavior of HAZ.
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图 7 闪光对焊接头组织
(a)焊缝中心；(b)距焊缝中心 2 mm；(c)距焊缝中心4 mm；(d)距焊缝中心8 mm；(e)距焊缝中心10 mm
Figure 7. Microstructure of the butt welding joint
图 8 搭接接头组织
(a)焊缝金属；(b)熔合线；(c)粗晶区；(d)正火区
Figure 8. Microstructure of the circumferential lap weld
图 9 热影响区第二相粒子
(a)粗晶区；(b)正火区
Figure 9. TEM photos showing second phase particles in HAZ
图 10 轮辋扩口开裂断口形貌
(a)断口截面； (b)韧窝状断口；(c)脆性断口
Figure 10. SEM picture showing fracture morphology of cracking rim during flaring forming
表 1 试验钢板化学成分及力学性能
Table 1. Chemical composition and mechanical properties of material investigated
w/% 屈服强度/MPa 抗拉强度/MPa 伸长率/% C Si Mn P S Nb+Ti Ceq 0.090 0.21 1.63 0.009 <0.002 0.075 0.368 570 630 25 注：Ceq=C+Mn/6+(Ni +Cu)/15+(Cr+Mo+V)/5 
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