Strength Recovery in a High-Strength Steel During Multiple Weld Thermal Simulations

Abstract

BlastAlloy 160 (BA160) is a low-carbon martensitic steel strengthened by copper and M 2 C precipitates. Heat-affected zone (HAZ) microstructure evaluation of BA160 exhibited softening in samples subjected to the coarse-grained HAZ thermal simulations of this steel. This softening is partially attributed to dissolution of copper precipitates and metal carbides. After subjecting these coarse-grained HAZs to a second weld thermal cycle below the A c1 temperature (at which austenite begins to form on heating), recovery of strength was observed. Atom-probe tomography and microhardness analyses correlated this strength recovery to re-precipitation of copper precipitates and metal carbides. A continuum model is proposed to rationalize strengthening and softening in the HAZ regions of BlastAlloy 160.

Investigation of Strength Recovery in Welds of NUCu-140 Steel Through Multipass Welding and Isothermal Post-Weld Heat Treatments

“…The chemical composition of BA-160 as reported by Yu et al [17] is also shown for later comparison. The steel was cast into ingots and homogenized at 1423 K (1150°C) for 3 hours prior to hot rolling at approximately 1223 K (950°C) and air cooled.…”

“…This gives rise to the possibility of a simple low-temperature post-weld heat treatment (PWHT) or ''in situ'' aging with subsequent weld thermal cycles to reverse softening in the HAZ by the re-precipitation of Cu-rich precipitates. The latter approach has been demonstrated on BlastAlloy 160 (BA-160), by Yu et al, [17] where re-precipitation of Cu-rich precipitates was observed in the HAZ after multiple simulated weld thermal cycles. Thus, the objective of this study is to determine if a direct aging PWHT or subsequent welding thermal cycles can be used to restore the strength in the HAZ of NUCu-140 steel.…”

Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

“…In this case, the liquation of Cu-rich precipitates could be promoted at a lower temperature compared with the solidus temperature for the bulk composition. Thermodynamic calculations using the average Cu precipitate composition determined previously for the sub-critical HAZ [9] showed that the Cu-rich precipitates would transform to liquid at approximately 1373 K (1100°C). The microstructure evolution arising from the OH portion of the weld thermal cycle is considered to be in between equilibrium heating rates and rapid heating rates that lead to the extreme condition where longrange diffusion is restricted completely.…”

“…[3,9] The increase in hardness was attributed to the reprecipitation of Cu precipitates and carbides. Because steels strengthened through Cu precipitates and carbides are susceptible to reheat cracking, [10][11][12][13] BA-160 could exhibit susceptibility as well.…”

“…This susceptibility relates to the rapid reprecipitation of Cu precipitates and carbides that occur in the reheat cracking temperature range. [9] Reheat cracking, which is also referred to as stressrelief cracking, occurs during PWHT procedures designed to reduce residual stresses and/or temper martensitic microstructures. The term can also be used to describe solid-state cracking in multipass welds where subsequent weld passes provide the necessary ''reheating'' of the initial weld microstructures into the susceptible temperature range.…”