Issue 36

F. Z. Liu et alii, Frattura ed Integrità Strutturale, 36 (2016) 139-150; DOI: 10.3221/IGF-ESIS.36.14 142 the decrease of tensile strength or the increase of tempering temperature. With the decrease of strength of notch samples, critical fracture stress and delay fracture strength ratio of the samples increased first and then decreased. Though the strength of the samples processed at a tempering temperature of 200 °C reduced for 6.65%, the critical fracture stress improved for 51.14% and the delay fracture strength ratio improved for 61.97%. It indicated that, proper tempering processing after quenching could greatly improve the delay fracture performance when the strength was slightly weakened. Figure 3 : Curves for stress – time of failure of the experimental materials in different states in notched tensile experiment. Quenching state 100 °C tempering state 200 °C tempering state 400 °C tempering state Smooth tensile strength R m , MPa 1488 1466 1399 1102 Notch tensile strength σ N , MPa 1759 1781 1642 1455 Critical fracture stress σ c , MPa 874 1162 1321 1134 Delay fracture strength ratio σ c /σ N 0.497 0.652 0.805 0.780 Table 2 : Results of delay fracture experiment. Hydrogen absorption and effusion behaviors of the experimental materials Delay crack of high-strength steel is correlated to the hydrogen in steels and the hydrogen absorbed from the environment and in corrosion process. Hence we explored hydrogen absorption and effusion behaviors of round bar and plate samples. Hydrogen absorption and hydrogen effusion behaviors of non-bearing experimental materials before and after hydrogen charging. After the round bar samples in different states were charged with hydrogen for 72 h at a current density of 4mA/cm 2 , the content of hydrogen was measured using TDS at a heating rate of 100 °C/h. Besides, the content of hydrogen in samples without hydrogen charging was also measured (Tab. 3). Generally, hydrogen released at a temperature below 400 °C is called as diffusible hydrogen, whereas hydrogen released at a temperature higher than 400 °C is called as non-diffusible hydrogen [15, 16]. As the delay fracture is the most obvious when the temperature is near to room temperature, delay fracture is considered to be induced by diffusible hydrogen released at room temperature rather than non-diffusible hydrogen released at room temperature [17, 18]. In this study, hydrogen released at a temperature below 300 °C was regarded as diffusible hydrogen and hydrogen released at a temperature above 300 °C was as non-diffusible hydrogen. Therefore, the hydrogen corresponding to the first hydrogen effusion peak of materials processed at a relatively low temperature was diffusible hydrogen and the hydrogen corresponding to the second hydrogen effusion peak of materials processed at a relatively high temperature was non- diffusible hydrogen. Obviously, the content of diffusible hydrogen in the samples in quenching state was quite low, so was