Effect of test velocity on the tensile strength of high strength steels using the small punch test in a hydrogen environment

This study aims at evaluating the effect of test velocity associated with the Small Punch Test on the ultimate tensile strength. The velocity factor was investigated on two levels (1.0 μm/s and 0.1 μm/s) based on a single factor design. This design was applied to three high strength steels (AISI 4130 M, 4137 M and 4140) widely used in the oil and gas offshore industry. The tests were performed in a cathodic protection environment, which consisted of a 3.5 wt% NaCl aqueous solution to simulate hydrogen embrittlement. For each material, one specimen in air at the conventional rate (0.02 mm/s) was also tested. An increasing trend was noted toward the susceptibility to hydrogen embrittlement when the test velocity was reduced from 1.0 μm/s to 0.1 μm/s. However, the Analysis of Variance showed that the test velocity factor caused statistically significant effects on the ultimate tensile strength only for 4137 M steel. The distinct behavior shown by the 4137 M, 4130 M and 4140 steels, under the experimental conditions investigated, was attributed to the combination of several factors: test velocity in the investigated H-charged environment; shape and volume fraction of non-metallic inclusion; calcium addition; tensile strength and hardness in air at the as-received condition.