Hydrogen Embrittlement of Dissimilar Metals and Ultra-High Strength Steel Welds

Abstract

Hydrogen embrittlement (HE) can give rise to catastrophic failures of metal structures. This is particularly so for weldments of ferrous alloys because of the susceptible microstructures possibly found in the weldments. An example is the Dissimilar Metal Welds (DMWs), which are especially prone to accelerated corrosion and HE because of the differences in chemical compositions, mechanical and metallurgical properties of different weld regions. HE tends to have a more pronounced effect on high strength steel; an example is the ultra-high strength maraging steel, which may be subjected to HE even in air if not properly heat-treated (or aging). The main objective of this study is to investigate the HE susceptibility of different types of DMWs and heat-treated T-300 maraging steel weldments. Microstructural, mechanical and corrosion properties have also been studied, these together with the welding parameters, are then related to the HE susceptibility of the weldments. In the first part of the study fifteen different DMW combinations have been prepared and evaluated. The parent metals used in this study include AS/NZS 3678-250 carbon steel, 1.25Cr-0.5Mo steel, SAF 2205 duplex stainless steel, 304 austenitic stainless steel and 316L austenitic stainless steel. These dissimilar metals were welded using different filler metals include low carbon steel ER70S-2, duplex ER2209, austenitic ER316L and ER309, and Inconle82. The results showed that narrow hardened transition zones were formed along the fusion lines of carbon steel (HAZ) due to the steep property variations between weld metals and carbon steel. Close investigations to this zone showed ferritic grain growth and steep composition gradients of carbon and chromium due to the heat/cooling cycles during welding. The formation of this hardened narrow zone (associated with the high ferrite content in the adjacent carbon steel HAZ with large grain sizes) reduce the mechanical properties and increased its susceptibility to hydrogen embrittlement. The results show that most of the DMWs involving carbon steel lost more than 70% of their ductility after 48 hours of hydrogenation. This is because of the relative high hydrogen diffusion and accumulation in the ferrite structure of carbon steel HAZ. The accumulated hydrogen with high pressure generated near the hardened zone, caused HE. In addition to the microstructural and compositional gradient, the large electrochemical potential difference between carbon steel HAZ and weld metal created localized galvanic cells, hence the corrosion rate in this zone has been increased by 25% as compared with that of the base metal. The hydrogen permeation behaviour of the base metal and HAZs of carbon steel, 1.25Cr-0.5Mo steel and 2205 stainless steel were evaluated in terms of hydrogen permeation flux, diffusion coefficient and trapping behaviour. In the hydrogen permeation test, the permeation current in the carbon steel took 1 hour to reach a steady state flux, as opposed to the hydrogen permeation through 1.25Cr-0.5Mo steel, which took 5 minutes only. This is an indication of the large amount of hydrogen trapping sites in the former. The scanning electron microscopy results showed many defects including microvoids and cracks. The hydrogen permeation rates in the weldments (include fusion zone and HAZs) of these two dissimilar steel were 25% faster than that in the carbon steel base metal to reach a hydrogen permeation flux steady state. This was due to the increase in the amount of ferritic grains formed at HAZ, as hydrogen diffused faster in ferrite than in pearlite Evaluation of 12 different stainless steel HAZs shows that higher ferrite content was resulted, owning to the large heat input in the welding. In addition, because of the grain growth in this zone, the stainless steel HAZs suffered losses in the reduction in ductility and corrosion resistivity. As an example, the corrosion rate of 2205 HAZ increased by 55%, as compared with the corrosion rate of 2205 base metal. The dissimilar stainless steel welds have also been evaluated in hydrogen environment using different parent and filler metals, and the effect degree of HE was measured by the ductility loss in tensile testing, and the fracture morphology of the specimen surface after hydrogenation. The results showed a significant reduction in the elongation and tensile strength after cathodic hydrogen charging. In the 2205/304 DMW with Inconel82 filler metal, the ductility loss was 70% after only 24 hours of hydrogenation, and 30% drop in ER2209 weldment of the same combination. This was possibly due to the presence of hydrogen of relatively low amounts of hydrogen, and the brittle secondary cracks on the surfaces. For the T-300 maraging steel, the effects of different aging treatments on the microstructure, mechanical properties and corrosion behaviour of its weldments have been investigated. The susceptibility of T-300 maraging steel welds to HE has also been evaluated with different hydrogenation periods. The results showed that the over-aged (538 ᴼC (1000 ᴼF) for 4 h) maraging steel had the higher resistance to HE, compared with peak-aged steel which had better combination of strength and toughness, due to the extensive reverted austenite after heat treatment. The HAZ in peak-aged weldment has the higher HE susceptibility by showing a 90% loss in reduction in ductility. The ductility (in terms of elongation) of over-aged maraging steel was reduced by 60% after 2 hours of hydrogenation. The TEM results showed coherent Ni3Ti precipitates within the matrix, as well as grain boundaries in the HAZ and fusion zone of the peak-aged maraging steel. This work has identified the causes and detail mechanism of the HE failures, and related them to the changes in compositions, microstructures, corrosion resistance of different regions in the DMWs and T-300 maraging steel welds.