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

Jia Jia,Li Li,Xu Xu,Sun Sun,Zhang Zhang

doi:10.3390/met9070798

Abstract

Hot-stamped steel has been widely used in automobile bumper and other safety components due to its high strength. Therefore, this paper investigates the effect of hydrogen content and strain rate on hydrogen-induced delay cracking (HIDC) behavior. The results showed that the plasticity of the steel significantly decreased with an increase in hydrogen content or a decrease in the strain rate. Fractography was analyzed after tensile tests. It was found that all of the pre-charged specimens cracked at large-sized inclusions when stretched at a strain rate of 1 × 10−3 s−1, which indicates that, in this case, the defect itself in the material had great influence on the extend properties. No inclusions were found at the main fracture origin area for hydrogen steady-state specimens, when stretched at a strain rate of 1 × 10−6 s−1, which demonstrated that a slower strain rate causes greater influence by hydrogen. However, for the non-pre-charged samples, the fractures surface showed that cracking originated from the defect near the sample surface, which was independent of strain rates.

Highlights

Light weight, low pollution, and high safety are the main goals for the new generation of cars, which make use of advanced, high-strength steel an inevitable trend [1,2,3,4,5]
It can be seen that the small-sized inclusions were mainly roundish Al-Mg-O-CaS, individually cubic Ti(C, N), and compound inclusions of both; while, the large-sized inclusions mainly consisted of long strip inclusions and aggregates of small-sized inclusions
The role of inclusions in hot-stamped steel on hydrogen-induced delay cracking (HIDC) was studied by changing the hydrogen content or strain rate and by using tensile tests

Summary

Introduction

Low pollution, and high safety are the main goals for the new generation of cars, which make use of advanced, high-strength steel an inevitable trend [1,2,3,4,5]. When the strength of the material is greater than 1000 MPa, the material will undergo hydrogen-induced fractures in service environments, and the sensitivity of hydrogen-induced delayed cracking generally increases as the strength of the material increases, as is well-known; for ultra-high strength steel, hydrogen-induced delayed cracking is more likely to occur [8]. There are many factors that influence hydrogen delay cracking, which range from the material composition design and heat treatment process to the service environment of the material in later periods. Among these factors, the material composition and micro-structure are the key factors that determine the properties of the material. As for specific materials, their compositions are usually fixed, so the influence of the material micro-structure on hydrogen-induced delayed cracking is important [9]

Methods

Results

Discussion

Conclusion