br Experimental work br Discussion Solidification cracking g

Experimental work
Discussion Solidification cracking generally occurs slightly above the melting temperature of the lowest melting constituent, which is sometimes referred to as the effective solidus temperature [36]. At this point in the solidification process of weld, the adjacent reverse transcriptase impinge upon each other to form the solidified bridges, which are surrounded by the regions containing lower-melting interdendritic liquid. These solid bridges are subject to the greatest shrinkage-induced strain as the surrounding material cools. A threshold amount of either low-melting liquid or strain may cause the fracture of these solid bridges and the subsequent formation of a weld hot crack [37,38]. Based on the correlation between cracking susceptibility and solidification behaviour developed for the austenitic stainless steels, the susceptibility of the duplex alloys to hot cracking would be expected to be low [39]. The solidification cracking is generally produced during the final stages of solidification and very much depends on the geometrical factors, such as width of weld bead, W, and depth of penetration, D. W and D are two major factors in achieving a good weld and depend on the welding procedure to a large extent. The shape of a weld in terms of width-to-depth ratio known as aspect ratio (ASR) has a marked influence on its solidification cracking tendency, which can be minimized by ensuring that ASR is between 1 and 1.4 and is illustrated in Fig. 6[40]. ASR is a predominant factor that affects solidification cracking in structural steel joints and is applicable to DSS joints also. In this study, ASR is found to be 1.24 (W: 7.12 mm; D: 5.72 mm) for the joint fabricated using optimized process parameters: travel speed (130 mm/min), current (140 A), voltage (12 V) and electrode gap (1 mm). There is no evidence solidification cracking macroscopically and is evident from the macrograph for the above joint (Fig. 5). Max heat input allowed for this grade is 2.5 kJ/mm. For TIG welding, it is desirable to have the heat input from 0.75 to 1.5 kJ/mm. The optimized process parameters of activated GTA welding process in this study yielded a heat input of 0.778 kJ/mm which is within the recommended levels. The goal to weld any duplex stainless steel is to obtain fusion and heat-affected zones having the excellent corrosion resistance of the base metal and sufficiently high impact toughness for application. ASTM/UNS S32205 grade base metal has an annealed structure with the equal proportion of austenite-ferrite phases and is virtually free of intermetallic phases. Welding procedures should be designed to produce this same structure in the weld metal and the heat-affected zones. The weld thermal cycle, filler metal and protection atmosphere, can control this structure. Near the fusion temperature, the structure of duplex stainless steels is entirely ferritic. The desired 30–55% ferrite can be achieved only if the cooling rate is slow enough to allow austenite to re-form as the weld cools. If the cooling rate is too slow, however, embrittling intermetallic phases may form in spite of the presence of the optimum ferrite content. Extremely low heat input followed by rapid cooling may produce a predominant ferritic heat-affected zone with reduced toughness and corrosion resistance [41,42]. In this study, the average ferrite number (FN) in the weld zone for the joints fabricated using the optimized process parameters is 71.62, and the ferrite content is approximately 50.674% which is well within the acceptable range. Hence, the optimized process parameters are justified for welding ASTM/UNS S32205 grade DSS by ATIG welding process.
Conclusions In this study, the ATIG welding process parameters were optimized for ASTM/UNS S32205 DSS joints to obtain desirable aspect ratio, and the results were analysed in detail. We can draw the following conclusions.