Multi-Scale Characterization of Heat-Affected Zone in Martensitic Steels

Due to the heat induced by welding process, Heat-affected zone (HAZ) microstructure and properties may significantly differ from base metal in steels. Careful designed integrity of original microstructure and composition may not exist in HAZ after welding. As a result, there is need to understand HAZ microstructure from macro scale to nano scale. In current study, various characterization tools will be applied to understand microstructure at different scale. Two steels are studied.;The first steel studied is BlastAlloy 160 (BA-160). BA-160 is a low-carbon martensitic steel strengthened by copper and M2C precipitates. Heat-affected zone (HAZ) microstructure evaluation of BA-160 exhibited softening in samples subjected to the coarse-grained HAZ thermal simulations. 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 Ac1 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. A hybrid in-situ characterization system, which couples the laser scanning confocal microscopy (LSCM) with the time-resolved X-ray diffraction (TRXRD) measurement with synchrotron radiation, was used to characterize the microstructure evolution during heat-affected zone (HAZ) thermal cycling of high-strength and blast-resistant steel. The combined technique has a time resolution of 0.3 seconds that allows for high-fidelity measurements of transformation kinetics, lattice parameters, and morphological features. The measurements showed a significant reduction in the martensite start transformation temperature with a decrease in the prior austenite grain size. In addition, the LSCM images confirmed the concurrent refinement of martensite packet size with smaller austenite grain sizes. This is consistent with dilatometric observations. The austenite grain size also influenced the rate of martensitic transformation (dfm/dT); however, the measurements from the hybrid (surface) and dilatometric (volume) measurements were inconsistent. Challenges and future directions of adopting this technique for comprehensive tracking of microstructure evolution in steels are discussed.;Similar characterization techniques were used to investigate the HAZ of Grade 91 steels. Current research suggest that modification of the ASME A387 Code-specified tempering of Grade 91 [i.e. reducing pre-weld tempering temperature from 760°C to 650°C] has the potential to increase creep life in the welded plate. To understand this improvement, weld samples were made with standard condition [normalizing (1080°C), tempering (760°C), welding and post-weld heat treatment (760°C)], as well as, samples with pre-weld tempering temperature at 650°C. Creep life of welds made with modified tempering was almost 5 times longer than that of standard welds (2414 hours vs. 518 hours). Optical and electron microscopy characterization exhibited higher volume fraction of M23C6 carbides in standard pre-welding tempered condition (1.8 vol.%) than that of the modified pre-welding tempered condition. In the samples subjected to standard pre-weld tempering and welding, carbides at martensite lath and prior austenite grain boundaries did not dissolve completely in the FGHAZ region. During PWHT, growth and coarsening of pre-existing carbides hinders the formation of carbides on prior austenite grain and martensite block boundaries. Since carbides on the boundaries provide creep resistanct, this condition has poor creep strength. In contrast, the samples subjected to modified tempering; nearly 100% dissolution of small M23C6 carbides was observed in FGHAZ. During PWHT, new and fine carbides re-precipitated on grain boundaries in the FGHAZ and thereby providing good creep strength.