The Influence of Thermal Cycles on the Microstructure of Grade 92 Steel

Abstract

The microstructure in the heat-affected zone (HAZ) of welds made from the 9 wt pct chromium martensitic Grade 92 steel is complex and has not yet been completely understood. There is a lack of systematic microstructural investigations to define the different regions of the microstructure across the HAZ of Grade 92 steel welds as a function of the welding process. In this study, the microstructure in the HAZ of an as-fabricated single-pass bead-on-plate weld on a parent metal of Grade 92 steel was systematically investigated by using an extensive range of electron and ion-microscopy-based techniques. A dilatometer was used to apply controlled thermal cycles to simulate the microstructures in the different regions of the HAZ. A wide range of microstructural properties in the simulated materials were then characterized and compared with the experimental observations from the weld HAZ. It was found that the microstructure in the HAZ of a single-pass Grade 92 steel weld can be categorized as a function of a decreasing peak temperature reached as (1) the completely transformed (CT) region, in which the original matrix is completely reaustenitized with complete dissolution of the pre-existing secondary precipitate particles; (2) the partially transformed (PT) region, where the original matrix is partially reaustenitized along with a partial dissolution of the secondary precipitate particles from the original matrix; and (3) the overtempered (OT) region, where the pre-existing precipitate particles coarsen. The PT region is considered to be the susceptible area for damage in the commonly reported HAZ failures in weldments constructed from these types of steels.

Microstructural Characterization of the Heat-Affected Zones in Grade 92 Steel Welds: Double-Pass and Multipass Welds

“…Some stringers of particles have also been observed in region 6, which suggests that these particles are distributed on lath and grain boundaries, as has been observed in previous studies. [3] These particles, although with small sizes of less than 100 nm, are likely to be the M 23 C 6 carbides consistent with those observed in the HAZ-simulated material that was exposed to weld thermal cycles with peak temperatures of 1523 K + 1103 K (1250°C + 830°C) (Figure 11). …”

“…The black boxes indicate the samples that were evaluated: OT: overtempered region, PT: partially transformed region, CT: completely transformed region. [3] B. Hardness Test and Microstructural Characterization…”

“…[23] Although some segments of the PAGBs in this case were also observed to have a misorientation in the range of 2 to 20 and 50 to 60 deg, the lengths of these segments were negligible compared to the length of the substructure boundaries ( Figure 5(d)). It was, thus, considered that the classification of boundaries with distinctly different misorientation distribution characteristics, [3] in combination with quantitative measurements of the lengths of boundaries [12] in different misorientation ranges (e.g., 2 to 20, 20 to 50, and 50 to 60 deg), could effectively indicate the variation of the martensitic substructure and PAG structure properties as a function of the different weld thermal cycles experienced.…”

“…The Ac 1 and the Ac 3 temperatures were, thus, measured at 1150 K (877°C) ± 9 K and 1330 K (1057°C) ± 30 K, respectively. [3] The transformation temperatures were also determined from the double-pass simulation thermal cycles in the dilatometer, as illustrated in Table II.…”

“…The microstructure in the heat-affected zone (HAZ) arising from the deposition of a single-pass weld bead was divided into three distinct regions: the completely transformed (CT) region, the partially transformed (PT) region, and the overtempered (OT) region. [3] These regions were linked to the peak temperature the microstructure experienced as a result of the welding cycle, such that the HAZ of an as-welded Grade 92 single-pass weld was categorized as follows:…”