To investigate the micro-hardness, microstructural and chemical composition behaviour of API X70 SAW multipass beads using ternary phase diagrams

The success of SAW process in pipeline applications depends not only on the welding parameters and filler wires but also critically on the flux composition. In present paper an attempt has made to study the chemical composition, mechanical and microstructural behaviour of twenty five multipass beads fabricated by using submerged arc welding technique. Flux composition has been chosen by ternary phase diagrams and mixture design approach. Twenty five SAW fluxes were formulated by mixture design approach of design of experiments using SiO2, TiO2, CaF2, MnO, BaO and CaO as primary flux ingredients. Statistically significant regression models (R2 = 0.76–0.95, p < 0.05) were developed to predict elemental transfer and properties. Individual oxides SiO2 and MnO were identified as having a positive effect on carbon content, while their interaction effects with other components may produce either negative or positive influences. Carbon content increased from base metal value (0.058%) to 0.032–0.050% in weld beads, microhardness ranged from 175–226 HV and carbon equivalent (CE) ranged from 0.19–0.31. Individual flux ingredients such as SiO2, TiO2, CaF2, MnO and CaO enhance the carbon content as compared to the binary and ternary mixture. Acicular ferrite with carbide inclusions was observed in F4 optimized flux. F4 optimized flux yielded microhardness of 220 HV, CE of 0.29, and fine acicular ferrite microstructure at low Ti levels (9–30 ppm).