The delayed fracture evaluation of high tensile steel sheet, and the influence of experimental methods

Abstract

In order to apply the high strength steel sheet with tensile strength is above 1200MPa, it is necessary to assess the delayed fracture susceptibility caused by hydrogen entering from the environment where steel components are exposed, and also necessary to adopt proper evaluation method of the delayed fracture. In this study, to clarify the influence of various experimental methods on delayed fracture susceptibility of high tensile steel sheets, slow strain rate technique (SSRT), constant load test (CLT) and conventional strain rate technique (CSRT) were demonstrated using commercial high strength steel sheet, and the fracture surfaces were observed. The results show that fracture stress decreased with the increase in the diffusible hydrogen content, and the fracture limit curves of SSRT and CLT were almost the same, whereas that of CSRT shifted to higher stress level. The area fraction of brittle fracture of SSRT, which is the sum of intergranular and quasi-cleavage fracture, increasing with the diffusible hydrogen content, is almost the same as that of CLT, however the area fraction of brittle fracture of CSRT is lower. It is supposed that SSRT and CLT owe to accumulated hydrogen concentration and CSRT owes to accumulated less than that of the others. Thus, both SSRT and CLT are suitable methods to assess the delayed fracture susceptibility; SSRT is superior to the point of testing time, and CLT is superior to the point of reproducing the environments. However, CSRT is suitable to classify materials susceptibility of the delayed fracture immediately on the condition of high diffusible hydrogen content.

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

“…24) Therefore, even at a constant diffusible hydrogen content, specimen strength is lower at slower tensile speeds as more hydrogen accumulates in the stress-concentration region. 19,24) It is considered that the same phenomenon occurred in the tensile-shear testing of spot welds in this study. In S-TST, because hydrogen accumulation occurred due to the stress-induced diffusion of diffusible hydrogen to the stress-concentrated region at the nugget end and to the crack tip and trapping of diffusible hydrogen in dislocations, tensile-shear strength decreased even at low diffusible hydrogen contents.…”

“…AHSS sheets (1500 MPa grade) with a known hydrogenembrittlement sensitivity 19) were used in this study. Their chemical composition is shown in Table 1.…”

“…The former corresponds to the condition generally used in tensile-shear tests and the latter corresponds to the rate used in SSRT tests to evaluate delayed fracture in steel sheets. 19) In this study, the test conducted at 1 mm/min is called C-TST (conventional tensile-shear testing), and that conducted at 0.005 mm/min is called S-TST (slow displacement rate tensile-shear testing). Figure 3 shows a schematic illustration of the S-TST (inhouse designed and manufactured).…”

“…U-bending, 1316) 4-point bending, 17,18) and tensile tests 1719) are often used to test high-strength steel sheets. In particular, from the tensile data of AHSS sheet specimens containing hydrogen (from tests such as the slow strain-rate method (SSRT), which tests at a strain rate of 10 ¹5 /s or lower, 20) conventional strain-rate test (CSRT), 21) which tests at a tensile rate of ³1 mm/min, and the constant load test (CLT), 22,23) which evaluates fracture/unbroken samples by holding at a constant load), it is possible to analyze the critical diffusible hydrogen content or the fracture limit diagram 19,2224) showing the relationship between fracture stress and diffusible hydrogen content. In tensile tests on general AHSS test pieces 19) without a notch, the fracture limit diagrams obtained using SSRT and CLT were quite similar, which is considered to be relatively close to the actual usage environment, such as a corrosive atmospheric environment, and practical values of load applied to the product; thus, it was concluded that SSRT is the most effective rapid evaluation method.…”

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

“…，4 点曲 げ 17,18) および各種引張試験 [17][18][19] が用いられている．特に引張 試験では，10 −5 /s 以下のひずみ速度で試験を行う低ひずみ速 度法 (SSRT：Slow Strain Rate Technique) 20) ，1 mm/min 程度の 引張速度で試験を行う通常のひずみ速度引張試験法 (CSRT： 22,23) がよく用いられ，破断応力と拡散性水素の関係を示 す限界拡散性水素量 (H c ) もしくは破壊限界線図 19,[22][23][24] を得る ことができる．特に，鋼板の平滑試験片 19 .…”