Fracture analysis of a welded front axle tube structure from a mini-truck

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The failure (fracture) mechanism of a welded front axle tube structure made of C45E4 steel from a mini truck was analyzed. The fracture occurred on the right side of the right support frame with the fracture surface perpendicular to the tube axis. SEM examination showed that the fracture surface could be divided into three areas: intergranular area, cleavage area and dimple area. Crack initiation site of the failed front axle tube was at the front weld joint fixing the right support frame on the axle tube. The crack propagated in two opposite directions along the circumference of the tube and converged at the dimple area. Intergranular fracture was found to be in heat affected zone (HAZ). With higher magnification, fine dimples, intergranular and trans-granular fracture characteristics were observed in the crack initiation site. By metallurgical examination, Widmanstätten ferrites, which could decrease the toughness and strength of the weld joint, were observed in the columnar grains. The hardness of HAZ coarse grain area (623 VHN) was far higher than HAZ fine grain area (310 VHN) and base metal (225 VHN). As the weld process indicates, neither pre-weld nor post-weld treatment was carried out. A non-uniform temperature distribution around the weld joint could generate large thermal residual tensile stress in HAZ; thus, the material was very unstable. It could fracture for very small or even no external stress. Hydrogen atoms would be released during welding and microstructures with the highest hardness are the most susceptible of hydrogen assisted cracking. It is concluded that the fracture was caused by hydrogen assisted brittleness under the induction of weld residual stress. Post-weld aging treatment (PWAT) is recommended to release the residual stress generated during welding process. In this case, PWAT was carried out on the failed weld joint and Vickers hardness of HAZ coarse grain area, HAZ fine grain area and base metal decreased to 232 VHN, 205 VHN and 125 VHN, respectively. That indicates that the PWAT procedure could effectively soften the material and relieve residual stress.

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This work was financially supported by Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014YL003), the National Natural Science Foundation of China (Grant No. 11404192 and 11605106), the Key Research and Development Project of Shandong Province, China (Grant No. 2017GSF220004), the Shandong Province Special Grant for High-Level Overseas Talents and the research fund of Shandong Academy of Sciences (Grant No. 2017QN001, 2019GHPY11 and KJHZ201805), Foundation of Key Laboratory of Applied Technology of Sophisticated Analytical Instruments of Shandong Province (Grant No. 201806).

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Correspondence to Weimin Guo.

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Recommended by Editor Chongdu Cho

Weimin Guo, Ph.D., Assistant Professor, works on microstructure and mechanical properties of steel and other alloys at Shandong Analysis and Test Center, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China.

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Guo, W., Ding, N., Xu, N. et al. Fracture analysis of a welded front axle tube structure from a mini-truck. J Mech Sci Technol 34, 109–116 (2020) doi:10.1007/s12206-019-1210-4

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  • Fracture
  • C45E4 steel
  • Welding
  • Residual stress
  • Hydrogen embrittlement