br Experimental methods br Results and discussion br

Experimental methods
Results and discussion
Conclusions
Acknowledgments This work was sponsored by the Centre for Industrial Photonics, Institute for Manufacture, Department of Engineering, University of Cambridge; the Natural Science Foundation of China (51271170); China International Science and Technology Cooperation Project (2011DFR50540); Major Scientific and Technological Special Key Industrial Project of Zhejiang Province (2012C11001).
Introduction The high yield strength and excellent fracture toughness of maraging steel make it histone demethylase suitable for defence applications such as missile, rocket motor casing and ship hull. Age hardening is the main strengthening mechanism which develops yield strength of 2400 MPa, of low carbon martensite with nickel, cobalt and molybdenum precipitates [1]. The components for the defence applications are fabricated using various welding processes [2–4]. One of the primary requirements of fabricated components is long storage life. The corrosion is one of the major problems during storage. It is now well established that the high strength steels suffer from poor stress corrosion cracking (SCC) resistance and their SCC resistance decreases with increase in strength [5]. Previous investigations revealed that the threshold yield strength value for SCC in high strength steels is about 1400 MPa [6]. Hence it can be concluded that ultra high strength maraging steels also suffer from SCC problem. Although SCC behaviour of maraging steel in wrought condition has been studied in some detail, work available on the SCC behaviour of maraging steel welds is relatively scarce. Hence, any attempt made to understand SCC behaviour of maraging steel welds used for making defence components is important. The mechanisms proposed for SCC of maraging steel are anodic path dissolution and hydrogen embrittlement [7]. Cracking was found to occur in a plane and inclined to precracking [8]. Crack growth velocity was found to be much faster in water than in oil [9]. Jha et al. [10] studied SCC of maraging steel and found that an intergranular mode of cracking occurs. Study made on 250 grade maraging steel at elevated temperature in steam has shown that it offers better protection compared to manganese phosphate treatment [11]. SCC susceptibility of steel was caused by acid dip step in the pretreatment of phosphating process [12]. Brook et al. studied the role of oxygen in SCC of maraging steel and found that cracking occurred along prior austenite grain boundaries [13]. Mellor et al. found the mechanism of hydrogen embrittlement in SCC failure of maraging steel components [14]. SCC behaviour of wrought maraging steel was studied in detail, but the investigations on welds are very limited. Kenyon et al. revealed that the maraging steel welds have poor SCC resistance compared to base metal [15]. One of the problems in fusion welding of maraging steel is the segregation of alloying elements in the interdendritic arm spacing. This may cause the formation of reverting austenite in the fusion zone of the welds. Both the segregation and reverted austenite affect the toughness and SCC resistance of maraging steel welds. Studies on the influences of reverting austenite and segregation on corrosion behaviour of maraging steel welds have not been available in the existing literature. Understanding of the corrosion behaviour of these welds is important in exploring a remedy to overcome the problem of cracking during storage of welded components made of maraging steel. The present study is aimed at studying the pitting and stress corrosion behaviours of MDN 250 (18% Ni) steel and its welds.
Experimental MDN 250 (18% Ni) steel of 5.2 mm thick sheet was used in the present study. The preparation details of the weld joint are shown in Fig. 1. Welding was carried out using gas tungsten arc welding process. Table 1 gives the welding variables. Table 2 gives the chemical composition of the base metal and filler wire. The steps of post weld heat treatment are (i) ageing at 480 °C/3 h, followed by air cooling; (ii) Solutionizing at 815 °C/1 h/air cooling + ageing at 480 °C/3 h/air cooling; and (iii) Homogenizing at 1150 °C/1 h/air cooling + ageing at 480 °C/3 h/air cooling in the study of corrosion behaviour. Optical microscopy was used to study the microstructural changes of MDN 250 (18% Ni) steel during welding. Stress Tech 3000 X-ray system using CrKα radiation was used to measure the retained austenite content and its fraction in welds and base metal.