INFLUENCE OF HEAT INPUT AND POST WELD HEAT TREATMENT ON HAZ MICROSTRUCTURE AND HARDNESS OF A710 PRECIPITATION STRENGTHENED STEEL

Abstract

The influence of change in the welding heat inputs between the range 1.4 to 3.78 kJ/mm on the ASTM 4710 HSLA steel was investigated with the bead-on-plate welds. The welds were deposited on a plates of 12.5 mm thick, by submerged arc welding process. ESAB (10.71) flux (AWS A 5.17) and 4 mm diameter filler wire conforming to AWS A 5.23 /ASTM SFA 5.23 EM2 were used. The weidments were prepared without and with preheat of 125°C. The hardness of weld metal was found to be more than the HAZ, and as the heat input increases, the mean weld centre hardness also increases. A soft zone having hardness lower than the base materialwas observed in the HAZ of weldments prepared in as-welded condition, with and without preheat. Also it has been found that the softening increases with the increase in the heat input. Further, with increasing heat input the softening width also was increased. The post weld heat treatment (PWHT) at 540°C for 1 hr was carried out to eliminate the softening region and the hardness was increased throughout the HAZ and the base material in both the cases. The maximum HAZ hardness in the ASTM 4710 HSLA steel was obtained experimentally and has been compared with empirical formulae proposed by various investigators. It has been found that, the formula proposed by Yuriok-3 matches reasonably with the experimental values. Therefore, for this particular steel, the formula proposed by Yuriok-3 can be used with acceptable accuracy. The cooling time from 800 to 500°C was calculated and compared with that of the earliar resaearch findings, and it has been found that the equation proposed by Uwer and Degenkolbe is valid. The amount of diffusible hydrogen in the weld of A710 HSLA steel was determined by glycerin method, and it has been found that the flux wire combination provides low hydrogen in the weldment. The base materials microstructure was found to be a mixture of polygonal and acicular ferrite, and some dark areas of bainite.The weld deposite was acicular ferrite and some secondary constituents which increases upon increasing heat input from 1.4 to 3.78 kJ/mm. In the coarse grain heat affected zone (SGHAZ),a mixture of martensite and bainite was found at lower heat inputs of 1.4 and 2 kJ/mm. and a fully bainitic structure was found at higher heat inputs of 3 and 3.78 kJ/mm. iv