Characterization of the soft zone in dissimilar welds joint of 2.25Cr-1Mo and lean duplex LDX2101 steel
Introduction
A thermal power plant boiler is a complex structure as it involves the design and manufacturing of the complex piping system. Water walls, superheater tubes, steam drum, headers, economizer, reheater tubes are a few parts of the complex piping system. This complexity necessitates the use of different steels and different geometries, and this inevitably leads to a dissimilar weld joint (DWJ) and multiple weld sections. DWJs between ferritic and austenitic steels exhibit localized changes in microstructure, strength, and fatigue properties. These localized variations can cause failures after operating at extended durations at high temperatures and pressure conditions [1]. The failures occur due to differences in the chemical composition of the weld constituents. The difference in chromium content between the ferritic and austenitic steel is one of the major driving force for diffusion [2]. DWJs result in the formation of a carbon depleted zone in the low chromium side (ferritic side), a corresponding carbide rich zone is formed in the austenitic side of the DWJ. The migration of carbon atoms is attributed to the difference in the activity of carbon in the two steels [3]. The ferritic steel and austenitic stainless have certain advantages making them an ideal selection for thermal power plant application. Austenitic stainless steel irrespective of its benefits, has a lower resistance to stress corrosion cracking, leading to the development of duplex austeno-ferritic alloys [4].
A 2.25Cr-1Mo is the most commonly used steel in power plant applications. It is a low carbon ferritic steel which shows the presence of a few metastable carbides that are formed as a result of material processing. Depending on the processing route, 2.25Cr-1Mo can show a variety of microstructures. These microstructures include ferrite-pearlite, ferrite-bainite, full bainite, and tempered martensite microstructure. The ferrite-pearlite, ferrite-bainite, is the most commonly observed microstructure in 2.25Cr-1Mo steel [5]. The normalizing and tempering of 2.25Cr-1Mo steel results in the formation of a ferrite-bainite microstructure. A ferrite-pearlite microstructure results from a slow cooling rate after austenitization. A tempered microstructure shows the presence of metastable M23C6, M2C carbide precipitates. The size of these precipitates varies with the duration and tempering temperature [6].
The austenitic-ferritic steel was first described by Bain and Griffith in 1927 and instantly gained popularity due to their higher strength and high corrosion resistance [7,8]. A lean duplex steel has chromium 20–24 %, nickel 1–5 %, molybdenum 0.1−0.3 % and nitrogen 0.10−0.22 %. Lean duplex LDX 2101 has chromium, manganese, and nickel content of around 21 %, 5 %, and 1.5 %, respectively, 0.22 % nitrogen content is added to allow the rapid formation of austenite in the weld metal and heat-affected zone (HAZ) [8,9]. The development of the LDX 2101, researchers have been able to achieve a stable phase (35–65 %) of either phase, similar corrosion resistance, but enhanced stress corrosion crcking resistance than that of SS304 L. The duplex microstructure in LDX2101 (F69) provides the benefits of both ferritic and austenitic microstructure. The ferrite phase contributes to high strength, resistance to stress corrosion cracking while the austenite provides ductility to the steel [9]. The LDX2101 has good weldability with higher austenite reformation in the HAZ [10]. This steel has twice the yield strength than the conventional AISI 304 L with enhanced corrosion resistance in a chloride environment. The standard SS304 L grade is being used in construction, pressure vessels, storage tanks, and bridges. The chromium content combined with a low level of molybdenum is a good alternative in paper and pulp application, storage of alkaline pulping liquor, domestic water heater application [11]. The lean duplex steels are relatively young, the widespread popularity in the near future is undeniable [10], and hence the examination of a dissimilar joint with ferritic steel, conventional stainless steel, and other commonly used steels becomes mandatory. Sandor et al. [12] examined the effect of four different types of cold wire, the joint gap on the ferrite content of TIG and ATIG welded DWJ of LDX 2101 and SS304. The ferrite content was found to be evenly distributed in the transverse direction of the weld joint. The ferrite content of the weld tended to be similar to the consumable used [12].
In the current research work, an LDX 2101 was joined with 2.25Cr-1Mo steel using IN617 filler. The microstructure of the DWJ was analyzed to study the heterogeneity and identify the soft zone formation at the interface of weld metal and HAZ. Various heat treatment procedures were carried out to reduce the heterogeneity in the microstructure and improve the overall properties of the weldment. The role of heat-treatment on the morphology of the soft zone was also analyzed.
Section snippets
Experimental details
The dissimilar welds joint of 2.25Cr-1Mo and lean duplex LDX2101 steel of dimensions 125 mm × 75 mm × 10 mm were developed by using the Gas Tungsten Arc Welding (GTAW) process. The Inconel grade IN617 filler was used for joining. The chemical composition of 2.25Cr-1Mo, LDX2101 and IN617 filler is listed in Table 1. The detailed experimental analysis including groove design and welding process parameters is explained in my previous work [13]. Three sets of the welds joint have been prepared, and
As-received material
The typical optical micrograph and SE image of the 2.25Cr-1Mo and LDX2101 steels are shown in Fig. 2. LDX2101 shows the typical banded structure having both ferrite and austenite phases. The light region (γ) and dark region (α) is clearly marked in the optical image, as shown in Fig. 2(a). In the dual-phase duplex steel LDX2101, the ferrite and austenite content was measured by using the Image J software (Fig. 2(c)) and it was 51 % and 49 %, respectively. SE image also distinguishes the
Conclusions
A dissimilar welds joint of 2.25Cr-1Mo and lean duplex LDX2101 steel was produced by using the GTAW process and In617 filler. The interface region of the weld metal and 2.25Cr-1Mo steel was characterized in detail for microstructure and mechanical properties. From the research work, the following main conclusions can be drawn:
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A heterogeneous microstructure formation was observed along the weldments for both AW and PWHT condition.
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The weld metal micrograph showed the equiaxed dendritic structure
CRediT authorship contribution statement
C. Pandey: Supervision, Validation, Visualization, Writing - review & editing. J.G. Thakare: Supervision, Validation, Visualization, Writing - review & editing. P.K. Taraphdar: Conceptualization, Methodology, Supervision, Investigation, Project administration, Validation, Writing - original draft. P. Kumar: Conceptualization, Methodology, Supervision, Investigation, Project administration, Validation, Writing - original draft. A. Gupta: Data curation, Formal analysis, Methodology,
Declaration of Competing Interest
The authors report no declarations of interest.
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