Development of Flux assisted Tungsten inert Gas Welding Process for Low Activation Ferritic, Martensitic Steel

Abstract

Activated tungsten inert gas (A-TIG) welding is a relatively novel variant of TIG welding process wherein the weld penetration capability of the autogenous TIG welding is enhanced. This increase in the weld depth of penetration is achieved by depositing a thin layer of chemical powder (termed as flux) on the metal surface to be welded prior to carrying out the autogenous TIG welding. The thin flux paste is melted due to high arc temperatures and the weld depth enhancement occurs primarily due to change in the direction of the fluid flow or due to the arc constriction mechanisms. The A-TIG welding technique has been successfully applied to a variety of steels ranging from stainless steels, low alloy steels and even super alloys. Low Activation Ferritic/Martensitic (LAFM) steel is specially developed steel from conventional Cr-Mo steels by replacing and controlling their certain elements. The elements like Molybdenum (Mo) and Nickel (Ni) in Cr-Mo steels were replaced by Tungsten (W) and Tantalum (Ta) and the content of Vanadium (V) was decreased to develop LAFM steels. This was done to decrease the induced radioactivity phenomenon experienced by the components of the vessels used for the nuclear application. The development of this steel would allow simple and shallow land burial techniques of these components after their service times are over. Given the excellent mechanical properties, reducing swelling potential and high-temperature strength of LAFM steel, it was considered as the primary candidate for the Test Blanket Module (TBM) of International Thermonuclear Experimental Reactor (ITER) which is a joint effort between several countries of the world. Currently, A-TIG welding procedures specifically for the welding of LAFM steel have not been discussed well in the literature. This is due to the fact that the usability of the A-TIG welding is essentially dependent on the type and nature of the flux used for welding. Different studies have reported different values of penetration on different steels with a variety of fluxes and its combinations. Thus, the success of the A-TIG welding in enhancing the weld penetration capability of the steel is dependent on the type of flux used and the chemical composition of the base metal. On the other hand, LAFM steel falls under alloy steels category and is an air hardenable steel prone to formation of hard phases during welding. Thus, a well-investigated weldability studies incorporating the metallurgical implication and subsequently meeting the mechanical properties was required for developing the fabrication procedures of TBM was needed. Thus, the aim of the present research is to investigate the weldability of LAFM steel using single component oxide fluxes and to provide insight into depth enhancing mechanisms responsible for deeper penetration. The tangible benefit of the A-TIG welding lies in enhancing the weld depth penetration of the autogenous TIG welding process which enables the welding of 6 mm thick LAFM steel in a single pass. On basis of this criterion, the appropriate combination of flux, a carrier solvent, and the corresponding welding parameters was applied on the standard size test coupons involving butt welding of 6 mm thick LAFM steel plates. Subsequently, the effect of post-weld heat treatment (PWHT) on mechanical and metallurgical properties such as tensile properties, impact toughness properties as well as ductile to brittle transition temperature (DBTT) has also been analyzed. Furthermore, autogenous TIG welding and laser beam welding (LBW) process has also been carried out and correspondingly analyzed on 6 mm thick LAFM steel plate. Twelve different single component oxide fluxes such as Al2O3, CaO, Co3O4, CrO3, CuO, Fe2O3, HgO, MnO2, MoO3, NiO, TiO2 and ZnO and two different carrier solvent such as acetone and methanol are used for the present study. The macrostructure analysis of the bead on the plate (BOP) trials indicated different weld bead morphological features such as depth of penetration (DOP), Bead width (BW) and width of heat affected zone (HAZ). Out of all the experimental trials, the most appropriate combination, capable of finger-like penetration incorporating reduced bead width and enhanced weld penetration is achieved with flux TiO2 mixed with methanol. Subsequently, from the weld bead profiles and peak welding temperatures, it was analyzed that two different depth enhancing mechanism (reversed Marangoni effect and arc constriction) were prevalent during A-TIG welding. The standard size butt welding coupons were welded with the finalized parameters and fluxcarrier solvent combination and full penetration of 7.8 mm in 6mm thick LAFM steel plate was achieved in a single pass. The coupons were subsequently subjected to post-weld heat treatment (PWHT) at 760°C for 02 hours followed by tempering in still air (referred as single PWHT). In order to accommodate possible weld repair cycle, selected plates were subjected to same PWHT cycle (referred as double PWHT) and the effect of this single, as well as double PWHT on mechanical and metallurgical properties, are analyzed. The yield strength of the A-TIG welded joints after PWHT was improved as compared with base metal, with A-TIG welded joint undergone double PWHT having highest yield strength. The ductility of the A-TIG weld joint undergone double PWHT was improved as compared to ATIG weld joint undergone single PWHT and even greater than the base metal. Similarly, the impact toughness of the welded joints was inferior to the base metal, however, an improvement in these values observed for A-TIG weld joints undergone double PWHT. The ductile to brittle transition temperature (DBTT) values achieved for A-TIG weld joints undergone single and double PWHT were -5°C and -11°C respectively. The presence of delta ferrite and carbides of type M23C6 and MX type are confirmed in microstructures. The microhardness values of the weld joint after double PWHT was similar to base metal indicating that the selected PWHT cycle was appropriate. Autogenous TIG welding was carried out in butt welding configuration on the standard size coupon in double-sided technique. For the comparison, welding parameters selected were exactly same that were used for A-TIG welding, the only change being that no flux was used for welding. The macrostructures indicated full and secure penetration and similar PWHT cycles were imposed on the welded plates. The ultimate tensile strength, as well as the ductility of the TIG, welded joints after single and double PWHT was inferior as compared to base metal and the yield strength values were comparable to base metal. The impact toughness values were subsequently inferior as compared to base metal, however, minor improvement in these values was noticed with TIG welded joints undergone double PWHT. These values were also inferior as compared to the values obtained with A-TIG welding. The DBTT for single and double PWHT TIG welded joints was 3°C and -4°C respectively. The microhardness values of the weld joint after double PWHT was similar to base metal indicating that the selected PWHT cycle was appropriate. In order to check the feasibility of laser beam welding (LBW) process on LAFM steel, standard size butt welded coupon was welded by LBW process. Effect of double PWHT on mechanical and metallurgical properties of LBW welded joint was analyzed. Inferior tensile properties as compared to base metal were observed for these welded joint. Similarly, the toughness values for the welded joint were also somewhat inferior to base metal toughness values. The microhardness values of the weld joint after double PWHT was similar to base metal indicating that the selected PWHT cycle was appropriate. Keywords: A-TIG, Arc constriction, Flux, LAFM, Oxide, PWHT, Reversed Marangoni