Microstructural Characterization of the Heat-Affected Zones in Grade 92 Steel Welds: Double-Pass and Multipass Welds
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GRADE 92 steel is currently in use in the power generation industry in product forms such as tubes, pipes, and forgings. It has been specified for use in new plants and also as a replacement material for lower alloy steel grades and another 9 wt pct Cr steel, Grade 91, because of its superior creep strength and the ability to use thinner section material for the same, or improved, mechanical properties. The material is normally supplied in a normalized and tempered condition used to achieve a tempered martensitic matrix stabilized by grain boundary M23C6 carbides and MX carbonitrides.[1] Typical normalization heat treatments are performed in the range of 1313 K to 1353 K (1040 C to 1080 C), whereas tempering is usually performed in the range of 1023 K to 1053 K (750 C to 780 C).[1]
X. XU, G.D. WEST, and R.C. THOMSON are with the Department of Materials, Loughborough University, Loughborough LE11 3TU, UK. Contact email: [email protected] J.A. SIEFERT and J.D. PARKER are with the EPRI, 1300 Harris Boulevard, Charlotte, NC 28262. Manuscript submitted October 29, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The vast majority of Grade 92 components, such as headers or main steam piping systems, are constructed using the traditional arc welding process, such as gas tungsten arc welding, shielded metal arc welding (SMAW), or submerged arc welding (SAW).[2] The complexity and thickness of the components specified in a thermal power plant mean that it is necessary to manufacture weldments in this steel, which require the use of multipass welding techniques.[2] The microstructures of multipass welds can be very complex, with multiple heat treatment cycles to different peak temperatures applied to individual weld beads as a result of the manufacturing process. Therefore, in a previous work by the authors, a single-pass Grade 92 steel weld has been studied in order to fully understand the transformations that can take place in both the matrix and the resulting precipitate distribution. 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:
(1)
(2)
(3)
the CT region (i.e., peak temperature > Ac3), in which the original matrix of the parent metal is fully reaustenitized with a complete dissolution of the pre-existing secondary precipitate particles; the PT region (i.e., peak temperature between Ac1 and Ac3), in which the original matrix is only partially reaustenitized together with a partial dissolution of the pre-existing precipitate particles; and the OT region (i.e., peak temperature < Ac1), in which the grain structure remains similar to the original matrix in the parent metal (however, the pre-
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