Effect of Specimen Cross Section and Notch Radius on the Hydrogen Embrittlement Susceptibility of Tempered and Quenched AISI 4140 Steel

To investigate the effects of the cross section and notch radius of specimens on the susceptibility to hydrogen embrittlement of AISI 4140 steel at different hardness levels, the incremental step loading technique was used. Initially, the effect of the three factors on the fast force fracture strength values was investigated. The study was based on a full factorial design, 2 x 2 x 3, in which the cross section and notch radius factors were investigated on 2 levels and hardness on 3 levels. Tests were performed using two equipment setup configurations. In the first test, a 1-kN load cell was used for standard test specimens with nominal dimensions of 10 x 10 x 60 mm(3), and in the second, a load cell of 20 kN was used for nonstandard test specimens with nominal dimensions of 30 x 30 x200 mm(3). The analysis of variance showed that the cross section factor caused statistically significant effects on the S-FFs values (the stress related to P-FFs), and the hardness and the interaction between the hardness and cross section had statistically significant effects on the hydrogen embrittlement of AISI 4140 steel. When analysis of variance was performed separately for each cross section, it was observed that for specimens with cross sections of 10x10 mm(2), the notch radius and the hardness factors caused statistically significant effects on the P-FFs value, whereas for specimens with 30 x 30 mm(2) cross sections, none of the investigated factors had statistically significant effects because of the larger constraint effect at the notch tip. The incremental step loading technique needs fewer specimens to complete the Pm evaluation when applied using larger strain constraint (larger cross section, smaller notch root, and higher hardness).