Microstructure and properties of ultra low carbon bainitic steel weld metal for HSLA-100

Traditionally, carbon has been one of the most effective contributors to strength in structural steels. However, increasing the weld metal carbon content is known to increase; (1) the susceptibility to hydrogen assisted cracking (HAC) and (2) sensitivity to cooling rate. Similarly, for ultra low carbon contents, the weld metal strength does not decrease with increasing heat input. Steels with sufficiently low carbon levels are readily weldable without preheat, whereas, steels with similar carbon equivalent numbers, but higher carbon levels require preheating. The low carbon copper-precipitation strengthened HSLA-100 type steels can attain the strength and toughness levels of the higher carbon HY-100 quenched and tempered steels without the need for preheating. However, filler metals specifically designed to take advantage of welding without preheat for the HSLA-100 type steels have not yet been developed. The susceptibility of the weld metal to HAC in the current commercially available filler wires necessitates costly preheating.; The purpose of this work was to characterize the strength, toughness and microstructure of Ultra Low Carbon Bainitic (ULCB) steel weld metal deposits and to demonstrate that ULCB steel filler metal can be used to weld HSLA-100 without preheating, while meeting the stringent requirements for weld metal strength and toughness. Gas tungsten are welding (GTAW) and gas metal are welding (GMAW) was carried out on HSLA-100 using ULCB steel filler metal to evaluate the weld metal strength, toughness and microstructure. GTA welds were deposited on 12 mm (0.5 in) thick plates machined from 13 vacuum induction melted (VIM) ingots and one argon oxygen decarburized (AOD) ingot using matching filler metal at 3.5 kJ/mm (90 kJ/in) and 2 kJ/mm (51 kJ /in) heat input, with cooling times from 800 to 500{dollar}spcirc{dollar}C (t{dollar}sb{lcub}8-5{rcub}{dollar}) of 35 s and 15 s, and cooling rates at 538{dollar}spcirc{dollar}C (1000{dollar}spcirc{dollar}F) of 6{dollar}spcirc{dollar}C/s (11{dollar}spcirc{dollar}F/s) and 18{dollar}spcirc{dollar}C/s (32{dollar}spcirc{dollar}F/s), respectively. Ar-5%CO{dollar}sb2{dollar}, Ar and Ar-25%He were used as shielding gases. GMA welds on HSLA-100 plate and GTA welds on matching VIM plate were made using 1.6 mm (1/16 in.) diameter wires of three different compositions with Ar-5%CO{dollar}sb2{dollar} shielding gas. The GMA welding parameters were adjusted to attain same cooling rates as GTA welds.; Weld metal tensile, hardness and Charpy V notch (CVN) toughness testing as well as microstructural studies using transmission electron microscopy were conducted. Depending on the alloy content, microstructures varied from granular bainite with no evidence of laths, to carbide free bainite with well developed laths. Laths traversed across the prior austenite grains but did not have an appreciable effect on the CVN toughness of the weld metal. Because ULCB weld metal was relatively insensitive to cooling rate, good strength and toughness could be maintained for a t{dollar}sb{lcub}8-5{rcub}{dollar} as long as 35 seconds. No decrease in strength was observed at the higher weld heat input. The absence of second phase particles and inclusions was found to be the most effective factor for maintaining high toughness. Unlike conventional high strength weld metals, the toughness and strength of ULCB weld metal did not have a strong dependence on either the grain size or the lath width, and the weld metal inclusions did not have a marked effect on austenite decomposition products. (Abstract shortened by UMI.)...