Role of the Ni-based filler composition on microstructure and mechanical behavior of the dissimilar welded joint of P22 and P91 steel

https://doi.org/10.1016/j.ijpvp.2021.104473Get rights and content

Highlights

  • Study the role of dissimilar ERNiCrMoCo-1 filler on microstructure and mechanical properties of the P22/P91 dissimilar welds joint.

  • Study the effect of PWHT on the mechanical properties of the dissimilar welded joint.

  • Characterization of the interface region and soft zone.

Abstract

The manuscript described the effect of Ni-based filler on microstructure and mechanical properties of the dissimilar welded ASTM A335 grade P22 and P91 alloy steel plate. The dissimilar welds joint (DWJ) was produced using the Gas Tungsten Arc Welding (GTAW) process. The DWJ was characterized for metallographic and mechanical testing for as-welded and post-weld heat treatment (PWHT) conditions. For mechanical properties, microhardness, tensile test and Charpy impact test were performed. The DWJ showed the heterogeneity in microstructure and mechanical properties (microhardness, impact toughness) along the welded joint. The weld metal showed the formation of the austenitic microstructure consisted of the equiaxed dendrite along with segregation of the alloying elements like Cr, Mo, and Ti at the inter-dendritic boundaries, while P91 and P22 HAZ showed the formation of coarse-grained, fine-grained, and inter-critical heat-affected region depends on the temperature experienced during the welding cycle. The region of P91 HAZ was characterized by untempered and tempered martensite, while P22 HAZ was characterized by coarse and fine bainitic microstructure. The PWHT showed a negligible effect on the microstructure of the weld metal, while a significant change in microstructure of P91 HAZ and P22 HAZ was observed. PWHT also led to the formation of the soft and hard zone along the interface of the P22 steel due to the carbon diffusion. At the interface of the weld metal and base material, a sharp gradient was observed for the elements Cr, Fe, Ni and Mo. The weld metal near the interface showed the columnar dendritic structure having segregation of the alloying elements Mo, Ti and Cr along the inter-dendrite boundaries. A variation in hardness was measured along the weldments for as-welded and PWHT conditions. The peak hardness was measured in P91 coarse-grained heat-affected zone (CGHAZ) for both as-welded and PWHT, while the poor hardness of 202 HV and 195 HV was measured for weld metal in as-welded and PWHT conditions. The strength of the welded joint was measured higher than the strength of P22 steel for both conditions; however, the fracture was observed in P22 base metal for the as-welded joint, while in PWHT, it was in P22 HAZ near the interface. A variation in Charpy impact toughness was observed along the weldment for both as-welded and PWHT conditions. In as-welded, the minimum Charpy impact toughness (CIT) of 95.8 J was measured in weld metal, while the maximum was 152 J in P91 HAZ. After the PWHT HAZ region of P22 and P91 steel showed a drastic increase in CIT, however reduction in CIT of the weld metal was measured.

Introduction

The Cr–Mo steel is extensively used as the structural component in the power and chemical processing industries due to its attractive mechanical properties at room and elevated temperature. In the power section, the component operating at high temperatures, i.e., steam header's are mainly made of modified 9Cr–1Mo steel (Grade 91 steel), while the components operating at low temperatures are manufactured by low Cr 2.25Cr–1Mo (P22) steel. In order to save the material and cost, the dissimilar joining of these two different grade steels is employed extensively in power plants. The grade 91 steel offers better creep strength and mechanical properties at elevated temperature as compared to P22 steel. A boiler in a power plant consists of several components like superheaters, economizer, reheaters, reheaters, water panel etc. and to connect these components to each other, several junctions are made, which are called headers. Junction or header are the sections where working fluid is received at a particular temperature and pressure from one component and then further distributed to other components. Generally, the header is a large diameter pipe in which several small diameter tubes (nipples or stubs) are welded. A supercritical boiler mainly consists of the 40–50 header having different operating temperature and pressure. A boiler is primarily consisting of two types of headers: (1) low-temperature header, (2) high-temperature header. The headers at the outlet of the superheater and reheater are classified as high-temperature headers.

Hence, welding of the header section made of P91 steel with tubes made of the P22 steel is commonly used in power plants. The effect of the welding process parameters, welding process, filler composition, welding sequence, and repair procedure has been studied by many researchers [[1], [2], [3], [4], [5], [6]]. Sultan et al. [3] had studied the effect of the varying PWHT in the mechanical behavior of the dissimilar welded joint of P22 and P91 steel produced by using the 9018 B9 electrodes. The PWHT has reported a significant effect on carbon migration along the interface of weld metal and P22 steel and is mainly governed by the temperature and time of the PWHT. A formation of the carbon enriched (CE) and carbon depleted (CD) zone was observed near the interface and the size of CE zone was measured lower than CD zone for each condition of heat treatment. A similar observation has also been made by Sudha et al. [7], and a detailed mechanism of the CD and CE zone formation is discussed by Sudha et al. [8]. The formation of the CD zone in P22 steel and CE zone in P91 steel affects the creep rupture life of the welded joint and the creep life of the welded joint was measured lower than the P91 and P22 base metal [9]. Laha et al. [9] also reported that during the creep exposures, the higher concentration of the creep cavities was mainly observed in the inter-critical heat affected zone (ICHAZ) of P22 steel and in the CD soft zone. The formation of the soft zone in the region of inter-critical HAZ of P91 and P22 steel also reduces the creep rupture life of the component, and failure from the ICHAZ is terms as Type IV cracking [[10], [11], [12], [13], [14]]. Laha et al. [9] also reported the failure of the dissimilar welded joint from the region of ICHAZ. In actual working conditions also, most of the failure was observed in the region of the ICHAZ [15]. Sunil Kumar et al. [5] had studied the effect of the welding process on the formation of the soft and hard zone as well as the mechanical behavior of the dissimilar welded joint of P22 and P91 steel. An inferior mechanical property was reported for the Friction stir welded joint as compared to the multi-pass GTA welded joint. Dittrich et al. [16] had studied the failure mechanism in dissimilar welded joints of P22 and P91 steel after the ex-service of 79,000 h at a temperature of 570 °C. The research reported several changes in the microstructure near the interface of the weld metal and P22 steel as a result of carbon diffusion from the low Cr side, i.e., P22 to high Cr side, i.e., P91.

The dissimilar Ni-based filler effect on mechanical behavior and creep performance of the dissimilar welded joint was also studied by the researchers [6,15]. TAMMASOPHON et al. [6] performed the combined study of the effect of dissimilar Ni-based IN625 filler and PHT on the mechanical behavior of the dissimilar welded joint of P22 and P91 steel. The optimum PWHT condition for homogeneous microstructure and minimum hardness variation along the weldments was reported 750 °C for 2 h. Laha et al. [15] performed the creep study on the dissimilar welded joint of P22 and Alloy 800, which was prepared by the SMAW process with IN182 electrode. For dissimilar welded joints, higher creep-rupture life was measured for the applied stress of more than 130 MPa. For a similar joint of P22 and dissimilar welded joint of P22 and alloy 800 steel, the failure was reported in the region of P22 ICHAZ for applied stress more than 150 MPa. However, for applied stress lower than 150 MPa, the failure for the dissimilar welded joint was reported in the region of the interface. Sirohi et al. [17] had also studied the effect of the Ni-based IN617 filler on mechanical and microstructural behavior of the dissimilar welded joint of LDX2101 and P22 steel and reported the formation of the soft and hard zone after the PWHT [18].

After a detailed literature study, it has been found that a lot of work has been done in the field of dissimilar joining of the P22 and P91 steel for 9018 B9 electrodes. A major study has been found related to mechanical properties, microstructure evolution, and creep behavior of the welded joint. However, the work related to the effect of the filler composition (Ni-based filler like IN617, IN82, IN625) has not been reported yet. The aim of the present investigation is to study the combined effect of the Ni-based IN617 filler and PWHT on microstructure evolution in the weld metal and at the interface and mechanical properties of the dissimilar welded joint of the P22 and P91 steel.

Section snippets

Experimental details

A steel plate of P91 (normalized and tempered) and P22 of dimensions 140 mm × 80 mm × 10 mm was used to make the dissimilar Gas Tungsten Arc (GTA) weld joint by using the Ni-based filler ERNiCrMoCo-1. The heat-treatment condition employed on the P91 base plate is given in Table 1. The composition of the base plate and filler metal is given in Table 2. A conventional V-groove geometry (Fig. 1a–b) having a groove angle of 75° and root height and root gap of 1.5 mm was used to prepare the

Characterization of received material

Fig. 4a shows the typical bainitic-ferritic microstructure for P22 steel having a grain size of 10 ± 4 μm. The fraction of the bainite and ferrite was measured 65% and 35%, respectively. The SE image (top of Fig. 4a) shows the white precipitates of the carbides decorated along the boundaries and inside the bainitic matrix. The carbide precipitates in P22 steel were confirmed as the type of M6C, M23C6, M7C3, and Mo2C [19]. Kulkarni et al. [4] had also reported the type of M6C and M23C6

Conclusion

  • A heterogeneous microstructure was developed across the welded joint. The weld metal showed the austenitic microstructure consisted of equiaxed dendrites along with segregation of the alloying element at inter-dendritic boundaries and SGBs. The hardness of the weld metal was measured 202 HV. The microstructure of the weld metal near the interface showed columnar dendrites having dendrite core, inter-dendritic boundaries, and alloying elements segregation at boundaries. The P92 CGHAZ showed the

Credit author statement

Sarbesh Kumar: Data curation, Formal analysis, Methodology, Project administration, Roles/Writing - original draft, Validation. Sachin Sirohi: Conceptualization, Formal analysis, Methodology, Project administration, Roles/Writing - original draft, Validation, Writing - review & editing. R. S. Vidyarthy: Conceptualization, Formal analysis, Methodology, Roles/Writing - original draft, Validation, Visualization, Writing - review & editing. Ankur Gupta: Conceptualization, Methodology, Supervision,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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