Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Abstract

Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds have not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fractured surface and intergranular fractured surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at the crack tip.

Effects of Diffusible Hydrogen on Fatigue Life of Spot Welds in High-tensile-strength Steel Sheets

“…Our samples were martensite steel sheets with a tensile strength class of 1 500 MPa, which were also used in the previous study. 19) Tables 1 and 2 show the mechanical properties and chemical compositions of these materials, respectively.…”

“…However, few studies have assessed the hydrogen embrittlement of spot welds in high-tensile-strength steel sheets and the mechanism of hydrogen embrittlement. [17][18][19] In a previous study, 19) we used the tensile shear test, which is used to evaluate the strength of spot welds, to assess the hydrogen embrittlement of spot welds in high-tensile-strength steel sheets and reported the results of introducing diffusible hydrogen under a cathodic charge. The results showed that as the diffusible hydrogen content of the test specimens increased, the tensile shear strength decreased.…”

“…Therefore, in this study we assessed the effect of the diffusible hydrogen content on the cycles to failure, fatigue limit, and fracture surface of spot welds in high-tensilestrength steel sheets using the tensile shear fatigue test (fatigue test hereafter). In addition, we used test specimens that are commonly used in a static test, the tensile shear test; 19) we compare the results of these two tests and report on the differences.…”

Effects of Diffusible Hydrogen on Tensile–shear Fatigue Life of Spot Welds of Advanced High-strength Steel Sheets

“…水素脆化感受性が増大するため 3) ，スポット溶接部におい ても水素脆化の懸念がある。 水素脆化メカニズム解明に向けた材料試験や高張力鋼板 の活用のための水素脆化評価法として多くの先行研究 [4][5][6][7][8][9][10][11][12][13][14][15] が実施されている。U 曲げ試験 [4][5][6][7] では，板材の加工ひず みの影響を考慮した評価法であり，しばしば実施されてい る。また，材料の水素脆化評価としては，高力ボルトの評 価 8) と同様に，4 点曲げ 9,10) や 3 種の引張試験；低ひずみ速 度 法(SSRT：Slow Strain Rate Technique) 11,12) ，通常の引張 試 験(CSRT：Conventional Strain Rate Technique) 11,13) ，およ び定荷重試験(CLT：Constant Load Test) 11,14,15) が実施され ている。各試験法については，メリット，デメリットがあ り，鋼板を評価するための最適な試験法についての結論は 出ていないが，各種試験法による評価結果の差異 11) ，拡散 性水素量や転位と水素の集積など水素脆化を支配する因 子 16) については明らかになりつつある。一方，高張力鋼板 をスポット溶接したときのスポット溶接部の水素脆化評価 や水素脆化挙動に関する研究は少ない [17][18][19] 。著者ら 19)…”

“…3. Diffusible hydrogen content of spot welds after tensileshear fatigue tests and slow rate tensile-shear tests(S-TST)19) under different hydrogen charging conditions.…”