Microstructure of martensite–austenite constituents in heat affected zones of high strength low alloy steel welds in relation to toughness properties

Abstract

Microstructure and fracture properties relationships have been investigated in the heat affected zones (HAZs) of a high strength low alloy steel used for offshore applications. Metallographic examinations of simulated HAZ microstructures were conducted to investigate the detailed microstructure of the martensite–austenite (M–A) constituents. Using scanning and transmission electron microscopy, various microstructures were found for the M–A constituents. In mixed particles, retained austenite was located at the periphery of the islands. Chemical and/or mechanical effects could possibly account for the stabilisation of this austenite phase. In situ cooling experiments in the transmission electron microscope showed that stacking faults play an important role in the thermal stability of austenite. Impact properties of simulated HAZ microstructures are strongly affected by both the bainitic microstructure and M–A constituents. In particular, freshly transformed high carbon martensite was shown to be much more deleterious than retained austenite.

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

“…Cleavage microcrack arrest micromechanism Arrested cleavage microcracks were investigated after interrupted tests (Figure 8(a)). Crack arrest occurred at high-angle bainite packet boundaries, as shown by EBSD measurements on fracture surfaces (Figure 8(a)), [25] which are difficult in nature, and on polished cross sections (Figures 8(b) and (c)), which only provide in-plane information. Disorientation measurements as exemplified in Figures 8(a) and (c) suggest that the "crystallographic" bainite packet size (i.e., the size of bainite entities delimited by high-angle boundaries, shown in Figure 2) is a relevant parameter to investigate crack propagation conditions.…”

“…More details about M-A constituents in these microstructures can be found elsewhere. [18,19] Here, it is enough to say that the large, "blocky" M-A particles often consist of carbon-enriched martensite islands surrounded by a thin, noncontinuous austenite film.…”

“…The nature of the particles (mostly austenite in elongated particles located between the bainite laths, mostly brittle martensite in more blocky particles) [18] plays a role at low strain (Ͻ5 pct) and low stress triaxiality only. Metallographic analyses of fractured specimens, as well as magnetic measurements on smooth tensile specimens strained by 5 pct both at 20°C and at Ϫ196°C, showed that for fracture strains higher than about 5 pct, or very near the fracture surface, or at very low temperatures, or for a high stress triaxiality ratio (RN0.1 and TPB specimens), all M-A particles were transformed into martensite during the test and became brittle.…”

Effect of Morphological Differences on the Cold Formability of an Isothermally Heat-Treated Advanced High-Strength Steel

“…Zajac et al 28 describe similar microstructures as granular bainite. In all samples with T iso above 350°C, the transformation of austenite to ferrite or bainite was not finished after 600 s, so the final quenching step leads to the formation of the martensite-austenite (M-A) phase, 29,30 which appears as a dark globular constituent in the IQ maps. In the analyzed material, the M-A islands were found mainly in the center of the sheets, and they are also the reason for a lower average IQ 27 and reduced linear intercept lengths in the conditions with an T iso > 450°C.…”

Effect of Controlled Cooling on Microstructure and Tensile Properties of Low C Nb-Ti-Containing HSLA Steel for Construction

“…constituent acting as a mixed hard phase [9][10][11][12] can enhance the stain-hardening capacity of the soft ferrite phase [2,[6][7][8]13] and correspondingly lower the YR, which has been achieved by the addition of nitrogen to V-microalloyed steel [8], and the accelerated cooling followed by heat-treatment on-line process (HOP) of Nb-microalloyed steel [6,7]. Nevertheless, the M/A constituent acting as brittle islands can cause a toughness loss [14][15][16], which can be prevented by grain refinement of soft ferrite phase [17]. Mixed microstructure of suitably fine-grained QPF and/or GBF as the soft phase and an increasing amount of M/A constituent as the mixed hard phase, may also form in a low carbon Nb-Ti-containing HSLA steel, and the tensile properties of this type of multiphase plate steel are characterized by a combination of high strength and low YR.…”