Effect of alloy carbides on hydrogen embrittlement of tempered martensitic steel

Abstract

Tempered martensitic steel has high strength and is easy to mass-produce because the heat treatment is simple. Therefore, this steel is widely used in industries requiring high strength. However, high-strength steels are affected by hydrogen embrittlement (HE), which is a phenomenon that mechanical properties are degraded by internal hydrogen; thus, HE is a serious problem in developing high-strength steels. In particular, tempered martensitic steel has very low HE resistance compared to other steels due to its high defect density and the film-shaped cementite precipitated during the tempering process. The HE resistance of tempered martensitic steel has been improved by controlling the microstructure using a heat treatment process and precipitating carbides that can trap hydrogen strongly. Many studies have investigated the effect of simple addition of alloy elements or the effect of carbide in ternary steels such as Fe-C-X. However, opposite HE behavior has been reported despite the presence of the same type of carbide. Therefore, this study investigated the effect of alloy carbides on HE of tempered martensitic steels. Firstly, the opposite HE behaviors for the same carbide were investigated by changing the characteristics of the carbides through heat treatment with different tempering times. In the vanadium-added steel, the hardness and strength were maintained with the initial increase of the tempering time due to the precipitation hardening effect of the carbides but tended to decrease after a certain time. Similar to the previous properties, HE resistance increased and then decreased with increasing tempering time. At this time, the tempering times for decreasing the mechanical properties and HE resistance were the same. This tendency was caused by the change of characteristics of V carbide. The hydrogen-trapping capability of the V carbide, which is a strong hydrogen trap site, increased and decreased in the same tendency as the HE resistance. This means that the stronger the precipitation hardening effect was, the more hydrogen was trapped by carbide, and the more the hydrogen trapping of carbide, the more the HE resistance increased. Therefore, it was found that the greater the precipitation-hardening effect of the carbide, the higher the hydrogen-trapping capability of the carbide, so the proportion of hydrogen trapped at the weak hydrogen-trapping site decreased, thereby improving the HE resistance. Secondly, the HE behavior was investigated by varying the tempering time in molybdenum-added steel through a similar method to the previous study. The precipitation-hardening effect by carbide was small, so the hardness and strength gradually decreased as the tempering time increased. However, since precipitation hardening by carbide did occur, hardness did not decrease linearly. The HE resistance had a maximum at 4 hr tempering time. Observing the internal hydrogen behavior showed that the hydrogen contents slightly increased with the precipitation of Mo carbides, and in particular, the hydrogen diffusivity decreased. The HE resistance and hydrogen diffusivity changed with opposite tendencies according to tempering time. This meant that the low hydrogen diffusivity due to Mo carbide precipitation improved the HE resistance. Therefore, it was found that in molybdenum-added steels, the reduction of hydrogen diffusivity could improve the HE resistance, and when the Mo carbide was precipitated with the lowest hydrogen diffusivity, the HE resistance was the best. Finally, the effect of V addition and Mo addition in tempered martensitic steel on HE was compared, V-add steel and Mo-added steel had different properties and HE resistance. In the V-added steel, the precipitation-hardening effect was sufficiently large, but in the Mo-added steel, the precipitation-hardening effect was not great. This difference affected the HE resistance. Regardless of the tempering time, the V-added steel had better HE resistance than the Mo-added steel because of the difference in the hydrogen behavior of each steel. When the V carbide was precipitated, the total hydrogen contents increased than when the Mo carbide was precipitated, but the strongly trapped hydrogen contents increased and the hydrogen diffusivity decreased, so the HE resistance was improved. In addition, the effect of various carbide-forming elements on HE was investigated. Firstly, the effect of the addition of V and Mo on HE was studied with three steels: Base without V and Mo added and two steels with V and Mo added in different V/Mo ratios on Base. The Base was tempered at a low temperature because it was difficult to secure sufficient strength by high-temperature tempering. In the V and Mo added steels, sufficient strength could be secured by the precipitation hardening effect, so the steels were prepared through high-temperature tempering. The addition of V and Mo enabled high-temperature tempering to segment the film-like cementite vulnerable to HE and enabled the precipitation of carbide which traps hydrogen strongly, so the HE resistance was improved. The carbide precipitated by the addition of V and Mo was identified as V carbide. In particular, when the V/Mo ratio was high, more V carbide was precipitated, so more hydrogen could be trapped. In addition, when the V/Mo ratio is high, the dissolution temperature of V carbide was high, so that V carbides were not dissolved in the austenitization process, so they refined the prior austenite grains (PAGs). As a result, with the increase of the V/Mo ratio, the HE resistance was improved due to the large hydrogen-trapping of V carbide and the PAG refinement. Finally, the effect of undissolved Nb carbide on HE of Nb added steel was investigated. In particular, this study evaluated the resistance to HE in uniaxial tensile and high-cycle fatigue environments in consideration of the use environment of these steels. Steel without Nb added and steel with Nb added were prepared. These steels were heat-treated with the general condition and special condition to make the PAG size the same. Second heat treatment was conducted to separate and analyze the effect of undissolved Nb carbide and the PAG refinement by this carbide on HE. The precipitation of undissolved Nb carbide by the addition of Nb improved the HE resistance regardless of the load environment, but this effect did not appear when the PAG size was similar. In the analysis of hydrogen behavior, undissolved Nb carbide did not trap hydrogen, so it did not affect the amount of hydrogen. However, the refining of PAGs could improve the HE resistance by reducing the amount of hydrogen per unit area of PAG boundaries. Therefore, when undissolved Nb carbides exist, the HE resistance was improved regardless of the load environment due to the refinement of PAGs. In this study, alloyed carbides were used to improve the HE resistance of tempered martensitic steel, and the effects of various alloy carbides on HE were investigated. The HE resistance of V carbide was changed in proportion to the hydrogen-trapping capability of carbide, and Mo carbide was changed in inverse proportion to the hydrogen diffusivity. When V and Mo were added together, with increasing V/Mo ratio, the hydrogen-trapping capability of the V carbide increased and the PAGs were refined, thereby improving the HE resistance. In addition, the precipitation of undissolved Nb carbide did not trap hydrogen, but the PAGs were refined, thereby improving the HE resistance.