Basic Microstructural Characterization of Second Phases in Homogeneous Weld Joint Made of X6CrNiNbN25-20 Steel After Long-Term Exposure Time at 973 K

Abstract

New blocks of fossil fuel power plants designed for steam temperatures above 873 K require advanced stainless steels as material for superheater or reheater systems. Weld joints are critical parts in fossil power units. Great attention is paid to the exploitation of new steel grades with higher material properties. In the austenitic steels family, the superior grade is undoubtedly HR3C steel (X6CrNiNbN25-20). A detailed knowledge on stability and microstructure composition during thermal exposure of the weld joints made from HR3C is necessary in order to use them in fossil fuel power plants with ultrasupercritical (USC) and new advanced ultrasupercritical (A-USC) steam parameters. The aim of the paper is to identify critical minor phases in HR3C steel, which allow acceleration of creep damage. The σ-phase and rough carbides M23C6 type is considered as such a phases in this steel. In this study, the σ-phase is identified and studied in more detail in relation to the development of creep damage at 973 K. Experimental material of the homogeneous HR3C weld joints in two states: in the as-welded state (AW) and after the postweld heat treatment (PWHT). Weld joints were manufactured by orbital welding using the gas tungsten arc welding (GTAW) method, heat input Qs = 1600 J/mm, interpass 423 K, three beads. Nickel-base alloy UTP A6170 Co (equivalent to Thermanit 617) was used as a filler material. The PWHT was carried out at the temperature of 1503 K for 15 min. Stress rupture tests were performed on the cross-weld (CW) joints of tubes ø 38 × 6.3 mm at 973 K with times to rupture up to nearly 22,000 h. The polished surface of the longitudinal sections was subjected to color etching in Murakami (30 g K3(Fe(CN)6), 30 g KOH, 60 ml H2O) in order to highlight the σ-phase. Several microscopic techniques were used for the study. The results were supplemented by creep, grain size, and microhardness data hardness vickers (HV) 0.5. The PWHT specimens exhibited an average σ-phase size of approximately 5 μm as well as AW specimens in specimens with short time to rupture (tr). However, tr such as 20,000 h, the average σ-phase size already reached dangerous border 10 μm. The AW specimens as opposed to the PWHT specimens did not show a noticeable growth of austenitic grains in the heat-affected zone (HAZ). In specimens after PWHT, the average grain size in HAZ was more than twice that of the body material (BM). It is worth noting that creep ductility values of specimens in the state after PWHT are very low, which is the result of coarse-grained structure in the HAZ and accelerated precipitation of σ-phase particles along grain boundaries during creep at 973 K.