Hydrogen redistribution under stress-induced diffusion and corresponding fracture behaviour of a structural steel

Abstract

Hydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960 MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10−6 to 10−4 s−1. In the pre-charged sample with a certain hydrogen content of 0.62 ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10−4 to 5 × 10−6 s−1, while it stabilised at the strain rate slower than 5 × 10−6 s−1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material. This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Effect of Hydrogen Content and Strain Rate on Hydrogen-induced Delay Cracking for Hot-Stamped Steel

“…The former was electrically charged in a 3.5% NaCl solution with 2 mA•cm −2 current density before the tensile tests, while the latter was soaked in the same solution during SSRT. It must be mentioned that the pre-charged samples were immediately treated with a cadmium plating after pre-charging, in order to avoid hydrogen desorption into the environment during the SSRT [24].…”

“…Grain boundaries were also preferential hydrogen trap sites, due to their possible role as short-circuit for hydrogen diffusion and high stress [13]. In addition, martensitic steels have a high density of movable dislocation [24]. Therefore, hydrogen could be transported to the prior austenite grain boundary by movable dislocations during the stretching process and, consequently, the cohesion of grain boundary would be reduced, due to hydrogen segregation (i.e., a HEDE-type mechanism) [9,23,[38][39][40].…”

“…The previous work of our research group also studied this phenomenon. Kan et al [24] found that the brittle region size increased with the strain rate slowing down.…”

Effect of Relative Humidity on Mechanical Degradation of Medium Mn Steels

“…In order to avoid the corrosion or passivation of the sample in the solution, nickel coating is usually required on the surface of the sample. The nickel electroplating solution is 250 g NiSO 4 -7H 2 O + 45 g NiCl 2 -6H 2 O + 40 g H 3 BO 4 /L, with a current density of 10 mA/cm 2 for 60 s. The hydrogen-charging solution on the charging side was 0.2 molL −1 NaOH + 0.22 gL −1 thiourea (H 2 NCSNH 2 ) with 5 mAcm −2 charging current density at room temperature, approximately 25 • C, while the solution in the escaping side was 0.2 mol L −1 NaOH [28].…”

The effect of hydrogen concentration on the fracture surface of medium Mn steels

“…Liu [19] has explained hydrogen influence on DP, QP and TWIP steels. Depover [20] has investigated the effect of hydrogen charging on the mechanical properties of TRIP, Ferrite Bainitic (FB), DP and High Strength Low Alloy (HSLA) steels, and Han [21] has examined hydrogen embrittlement under intercritically annealed conditions, while hydrogen redistribution under stress-induced diffusion was investigated in structural steel [22].…”