Effects of cold metal transfer welding on properties of ferritic stainless steel

Abstract

Stainless steels are a classification of materials that have been available for over 100 years and over that time manufacturers have created variations on chemical composition and manufacturing route, to create materials that meet specific criteria set by the consumer. One type of stainless steel, ferritic, is restricted in applications as a result of a reduction in properties, namely toughness, when it is welded as a result of grain coarsening in the heat affected zone. Welding equipment manufacturers are constantly incorporating new technologies and capabilities into welding equipment, to make welding easier and create better welds, which then gives that manufacturer a competitive advantage. Cold Metal Transfer (CMT) welding is one such innovation and is claimed by the manufacturer to be a lower heat input process. This research project examines the effects of this lower heat welding process, on the joining of ferritic stainless steels to determine if CMT can reduce the detrimental effects, seen in this material, through welding. The research examines the mechanical and metallurgical effects of using the Cold Metal Transfer (CMT) welding process to weld various grades of ferritic stainless steel including, EN1.4016, EN1.4509, EN1.4521 and EN1.4003 and compares them to welds created using a standard Gas Metal Arc Welding (GMAW) technique, with comparisons made using tensile testing, hardness testing, impact testing, fatigue testing and microstructural characterisation. Experimental results show that grades such as EN1.4016 and EN1.4003 are more sensitive to the welding process due to a phase change to martensite present within the heat affected zone. Work has been conducted to determine the temperature at which ferrite transforms to austenite, prior to transformation to martensite under non equilibrium cooling. Some of the findings from this work included; Fatigue testing and microstructural characterisation has shown a benefit in properties for using CMT over the conventional GMAW process for the EN1.4003 material. A relationship has also been proposed which examines the effect of the percentage of fusion zone defects on the fatigue life of the welded joints. Overall it was found that there was variation in the voltage and current by 1.9 Volts and 15 Amps respectively through a 400mm weld. The ALC settings from -30% to +30% affected the net heat input by 6J/mm NDT techniques utilised in the study were ineffective at detecting the lack of side wall fusion evident in some of the welds.