Microstructure and toughness of welded A710 steel

HSLA copper-bearing, precipitation-aged A710 steels are of increasing interest in the Navy ship-building and structural applications. In this investigation, weldments of ASTM A710 steel plates (grade A, class 3) were studied through mechanical testing and microstructural characterization (including optical, scanning and transmission electron microscopy). From the experimental observations the relationship between the microstructure and mechanical properties of both base materials and welded (or Gleeble simulated) heat affected zones (HAZ) are presented.;As for A710 base metal, its high strength and toughness were a result of fine ferrite grain size, less amount of pearlite compared with conventional HSLA steels, and uniformly distributed small Cu-containing precipitates. In the welded structure of both 19 mm and 32 mm thick A710 plates, the coarse grained heat affected zone had a lower impact toughness than the base metal, and this effect was greater at higher welding heat inputs. The cause for the HAZ toughness deterioration was mainly due to the formation of coarse bainitic microstructures in the coarse grained region. In spite of this toughness loss in the HAZ compared with the base metal, the weldability of A710 steels is very satisfactory, and the impact toughness levels of the HAZ (as well as the weld metal), especially for those made by heat input of 3 KJ/mm or less, are adequate for structural applications.;The effect of post weld heat treatment (PWHT) on the HAZ toughness is not consistent. PWHT provides a small improvement on the HAZ toughness for higher heat inputs (4.9 or 5 KJ/mm), but decreases the toughness for lower heat inputs. From TEM studies, it was found that some of the Cu-containing precipitates are probably in the form of Fe-Cu compounds, and it is suggested that Cu-containing precipitates are responsible for the HAZ toughness changes (compared with as welded condition) alter PWHT.