Investigation of Stress Corrosion Cracking and Hydrogen Embrittlement Behaviors of High Strength Stainless Steels Grade 15-5 PH

Abstract

Based on the steel grade, its elemental composition, and thermal treatment parameters governed by manufacturing procedures, high strength stainless steel (SS) alloys can be designed and established with desired surface microstructures containing different phases with appropriate integration of mechanical features in the steel structural components. In recent years, Precipitation Hardening (PH) SS alloys have been utilised as structural components in several applications, including aerospace, military, and marine oil and gas industries in which excellent corrosion resistance, excessive toughness and strength are prerequisite. PH SS alloys contain chromium and nickel, which deliver features of both martensitic and austenitic phases. However, one of the major impediments for such practice is their proneness to stress corrosion cracking and hydrogen embrittlement. Aging treatment can further hardened the PH SS alloys which offers good toughness and strength with good corrosion resistance. Necessary information regarding the general microstructure characteristics depending on aging parameters is needed further to enhance the material performance of martensitic PH SS alloys. Besides, a quantifiable description of the volume fraction, dimension distribution, and elemental configuration of the formed precipitates are also required. In the current thesis work, an in-depth microstructure evaluation has been performed to evaluate the influence of heat treatment parameters on the development of different phases and precipitates, and its responsibility over the resultant mechanical characteristics. This thesis also targets to correlate the altered microstructural features with corrosion issues, including SCC and HE. Appropriate characterization procedures, such as X-ray diffraction and scanning electron microscopic analyses, have involved relating the microstructural changes with the obtained hardness test results and drawing the qualitative and quantitative assumptions. Moreover, the influence of surface microstructural changes on the mechanical characteristics have been evaluated using slow strain rate testing and described the impact of hydrogen charging on the mechanical performance of 15-5 PH SS alloy for further enhancement of the mechanistic of high strength SS against stress corrosion cracking (SCC) and hydrogen embrittlement (HE). The obtained hardness testing results revealed that the maximum hardness value is found at ageing temperature of 540 ℃ due to the precipitation of nano-sized copper (Cu) rich particles. XRD patterns of ageing treated samples indicated that the amount of γ-austenitic phases increases with ageing temperature up to 650 ℃. From the calculated austenitic phase fraction and hardness values of heat-treated samples, it is inferred that the hardness primarily increases in the low ageing temperature of 540 ℃, and it was regulated by the second phase precipitate hardening. The higher ageing temperatures of 700 and 750 ℃ produces a considerable reduction in the austenitic phase fraction, increasing the hardness values. The tensile testing results revealed that a strong knockdown in the elongation to failure occurred on 15-5 PH alloy after reaching cathodic potential and further, the yield stress and ultimate tensile stress (UTS) were not altered in the tests done in air and cathodic potentials. SEM images of the fracture surface displayed micro-void coalescence failure at second phase precipitates along with broad plasticity and 45° grain rotation to principal loading direction, in addition to the secondary cracking. SEM images of the hydrogen charged samples exhibit brittle quasi cleavage transgranular cracking across the martensitic lathes with some intergranular cracking characteristics. The brittle type failure was also demonstrated by the absence of substantial necking. Therefore, the HE performance at H1025 w