Microstructure and corrosion behaviour of gas tungsten arc welds of maraging steel

Abstract

Superior properties of maraging steels make them suitable for the fabrication of components used for military applications like missile covering, rocket motor casing and ship hulls. Welding is the main process for fabrication of these components, while the maraging steels can be fusion welded using gas tungsten arc welding (GTAW) process. All these fabricated components require longer storage life and a major problem in welds is susceptible to stress corrosion cracking (SCC). The present study is aimed at studying the SCC behaviour of MDN 250 (18% Ni) steel and its welds with respect to microstructural changes. In the present study, 5.2 mm thick sheets made of MDN 250 steel in the solution annealed condition was welded using GTAW process. Post-weld heat treatments of direct ageing (480 C for 3 h), solutionizing (815 C for 1 h) followed by ageing and homogenizing (1150 C for 1 h) followed by ageing were carried out. A mixture of martensite and austenite was observed in the microstructure of the fusion zone of solutionized and direct aged welds and only martensite in as-welded condition. Homogenization and ageing treatment have eliminated reverted austenite and elemental segregation. Homogenized welds also exhibited a marginal improvement in the corrosion resistance compared to those in the as-welded, solutionized and aged condition. Constant load SCC test data clearly revealed that the failure time of homogenized weld is much longer compared to other post weld treatments, and the homogenization treatment is recommended to improve the SCC life of GTA welds of MDN 250 Maraging steel.

Microstructural characterization of grade 300 and grade 350 maraging steels and electrochemical study in hydrofluoric solution

“…These cracks and pits have similarities with the degradation found by Masoumi et al [18], Dai et al [25] and Pawel et al [26]. This degradation arises from the hydrogen reaction and inter-granular corrosion caused by the stress produced by the precipitation of intermetallic compounds in the grain boundaries and the preference of the HF to dissolve these regions, as mentioned in the literature [10,18,[24][25][26][27][28][29][30][31]. Linear EDS analysis was also performed on the surface of the steels.…”

“…14 shows the linear potentiodynamic polarization curves for both maraging steel grades under study and Table 3 shows the corrosion potential (V corr ) and corrosion current density (I corr ). The analysis of these curves shows that none of the maraging steel grades had passivation in the anodic branch as reported by Poormina [10], Sanatkumar [11], El-Mahdy [22] and Madhusudhan [24], who studied the behavior of these steels in acid solutions. The same behavior found in OCP curves can also be found in the linear potentiodynamic polarization curves.…”

“…Changes in the potential and direction of each curve show that the reactions are taking place on the surface, which indicates their electrochemical nature [22]. The solution treated samples (300 Sol and 350 Sol) in general showed a more noble behavior than the aged samples, in accordance with [10,[22][23][24], and the curve of grade 350 maraging steel showed more noble potentials than grade 300 maraging steel. This can be associated with the greater amount of alloying elements such as cobalt and titanium in grade 350 maraging steel.…”

“…According to El-Mahdy [22], the evolution of hydrogen reduction reaction on the steel surface is under activation control, while the anodic branch suggests dissolution of the material. The absence of film formation can be explained by observing the studies [10,11,22,24], where the samples were immersed in solutions with phosphoric acid and hydrochloric acid. These studies mention the reactivity of the phosphorus ions and mainly the chlorine ion as the causes for the breakdown of the formed protective film, allowing the solution to continue dissolving the material.…”

“…The poorer corrosion resistance of the steels aged at 560 °C results from precipitation of austenite inside the material, as mentioned before. The reverted austenite is rich in nickel and molybdenum, forming a new focus of galvanic pair within the metal together with the increase in the interphase between austenite and martensite, which are areas more reactive to corrosion [22,24].…”