Hydrogen Induced Cracking Characteristics Of High Strength Martensitic Steel Research

Typically, high strength martensitic steels have a high susceptibility to hydrogen embrittlement, and hydrogen induced cracking is one of the main failure modes of high strength martensitic steels. Generally, with the increase of the tensile strength of martensitic steels, the susceptibility to hydrogen embrittlement also increases. As high strength martensitic steels, hydrogen induced cracking also occured in service of AF1410steel and40CrNi3MoV steel. But both of the hydrogen embrittlement has not been exact comparison. Characteristics of hydrogen induced cracking have been compared in this work for the AF1410steel with the strength level of1700MPa and for the40CrNi3MoV steel with strength level of1500MPa. Hydrogen effect on ductility of the steels has been studied by slow strain rate tensile (SSRT) tests, and the fracture surfaces have also been observed on scanning electron microscope (SEM). Hydrogen desorption and diffusion behavior has been studied by thermal desorption spectrometry (TDS) test. The main results are as follows:(1) Microstructures of the AF1410and40CrNi3MoV steels are both lath martensite after quenching and tempering, with the existence of M2C and Fe3C carbides, respectively. Because of the higher contents of Ni, Co, and Mo, the strength and toughness of AF1410steel are higher than those of40CrNi3MoV steel. The Charpy impact absorbed energy and fracture toughness of the1700MPa AF1410steel are about twice of those of the1500MPa40CrNi3MoV steel.(2) Slow strain rate tensile tests after electro-chemical hydrogen charging show that: the reduction of area decreased linearly with the hydrogen content in the two steels investigated; when the hydrogen content reached a critical value the fracture of the tensile specimen changed to brittle mode, and the reduction of area decreased to zero; if the hydrogen content further increased, the sample fractured prematurely and the tensile strength decreased. The critical hydrogen contents corresponding to the zero reduction of area of AF1410and40CrNi3MoV steels are6.0ppm and0.8ppm, respectively. The AF1410steel has a lower susceptibility to hydrogen embrittlement than the40CrNi3MoV steel. (3) Thermal desorption spectrometry experiments show that there are two hydrogen desorption peaks below300℃in the TDS curve of the AF1410steel, and the hydrogen desorption activation energy values are20.2kJ/mol and24.6kJ/mol, which are presumably related to hydrogen trapping sites of dislocations and grain boubdaries in the AF1410steel. There is only one hydrogen desorption peak below300℃in the TDS curve of the40CrNi3MoV steel, and the hydrogen desorption activation energy value is13.9kJ/mol, which is presumably related to hydrogen trapping sites of dislocations in the40CrNi3MoV steel.(4) Hydrogen diffusion coefficients have been measured by hydrogen release at room temperature from round bar specimens after electro-chemical hydrogen charging, and the hydrogen diffusion coefficients in the40CrNi3MoV and AF1410steels are measured to be3.11×10-8cm2s-1and9.3×10-8cm2s-1, respectively. Hydrogen can diffuse more easily in the40CrNi3MoV than in the AF1410steel. There are more hydrogen trapping sites in the AF1410steel than the40CrNi3MoV steel, and a lower hydrogen diffusion coefficient in the AF1410steel, the AF1410steel has a lower hydrogen embrittlement susceptibility than the latter40CrNi3MoV steel. The hydrogen embrittlement susceptibility of martensitic steels does not necessarily increase with the increase of tensile strength, it is also related to the hydrogen trapping sites and hydrogen diffusion coefficient of the steels.