Fine-grain heat affected zone softening of G115/Sanicro25 dissimilar steel welded joints after post-weld heat treatment

https://doi.org/10.1016/j.ijpvp.2020.104253Get rights and content

Highlights

  • The deformation of the joint is uneven.

  • After PWHT, dynamic recovery occurred in the FGHAZ, dislocation density reduced.

  • Decreased dislocation strengthening effect is the main cause of fracture.

Abstract

The mechanical properties of the G115/Sanicro25 dissimilar steel welded joints were characterized by room temperature tensile and microhardness. The microstructures of fine-grain heat affected zone (FGHAZ) and base metal (BM) in the G115 steel side were characterized by optical microscopy, SEM, TEM, and XRD. The results show that as the strain increases, weld beam first yields and hardens, the stress on the specimen is further increased, then Sanicro25 steel yields and hardens, and G115 steel yields finally, the specimen broken in the FGHAZ of G115 steel side. The FGHAZ of G115 steel side softens after post-weld heat treatment (PWHT). The size of the heat-affected zone (HAZ) on the G115 side after welding is 3.7 mm. After PWHT, dynamic recovery occurs in the FGHAZ of the G115 side. The precipitates increase, martensite laths are polygonal, dislocation density is reduced, residual austenite disappears. The FGHAZ with large residual stress after welding undergoes stress relaxation during PWHT. Residual stress in the FGHAZ is reduced, the dislocation density is reduced, which reduces the effect of dislocation strengthening and martensite lath strengthening. The strength of the FGHAZ is lowered than BM.

Introduction

The application of G115 and Sanicro25 as the next-generation candidate materials for T92 and Super304H, respectively, can meet the requirements of high steam parameters for the material properties of boiler tubes. However, joints made of martensitic/austenitic dissimilar steel often fail prematurely during long-term high-temperature service, which seriously affects the normal operation of the power plant. Before welding, it is generally needed to preheat the martensitic steel to prevent cold cracks in the welded joint. Therefore, a low-temperature tempering will be undergone in the coarse-grain heat affected zone (CGHAZ), the fine-grain heat affected zone (FGHAZ), and the base metal zone of the martensitic steel. During the welding process, the filler metal and part of base metal are melted firstly by the arc and then solidified to form a weld, which is equivalent to undergo a casting process and obtain the as-cast microstructure finally. The temperature range of the CGHAZ in the martensite steel side is from the carbide dissolution temperature (about 1100 °C) to the melting point of martensitic steel [1], which is equivalent to undergo normalizing heat treatment. The temperature range of FGHAZ in the martensite steel side is from AC1 temperature to carbide dissolution temperature (about 1100 °C) [1], equivalent to experience incomplete normalization heat treatment. During the PWHT process, the joint experienced tempering heat treatment. At the same time, the stress state of the different zone at different temperatures is also different due to the difference in the thermal expansion coefficient of the austenitic base metal, the martensitic metal, and the weld metal [2]. Therefore, the martensite/austenitic dissimilar steel welded joint can be regarded as composed of regions of different materials and different material states, and the stress states of different zones at different temperatures are also different. The transition zone formed by the difference in structure, composition, and properties of G115/Sanicro25 dissimilar steel welded joints will be the weak link for the safe operation of high-temperature conditions in ultra-supercritical units and will be the source of the initial failure of superheater tubes and reheater tubes.

Therefore, many scholars have studied dissimilar steel welded joints. The literature [[3], [4], [5], [6], [7], [8], [9]] examined the T92/Super304H dissimilar steel welded joints, which are carried out by two aspects of microstructure and performance. In the aspect of microstructure, after PWHT, the T92 steel is lath-shaped tempered martensite, and the excess carbon is distributed in the grain and at the grain boundaries in the form of carbides M23C6 and MX. The microstructure of the Super304H steel is austenite, and annealed twins are distributed in the grain. At the same time, NbC, NbN, and copper-rich phases are distributed in the matrix as second phase particles. The microstructure of FGHAZ and CGHAZ in the joint is tempered sorbite, and carbides are distributed in the grains and grain boundaries. The microstructure of CGHAZ is relatively large. The number of precipitated phases in the T92 steel is more than that in the heat-affected zone. The welds are coarse dendritic austenite. In terms of performance, the tensile results show that the fracture of the heat-treated strong joint is located in the weld, and the fracture of the joint after heat treatment is located in the T92 base metal. The results of the impact test show that the weld is the weakest area of joint toughness. The results of the hardness test show that the proper heat treatment process after welding can improve the toughness of T92 HAZ.

The literature [10,11] studied the influence of filler metal on the microstructure and properties of T92/Sanicro25 dissimilar steel welded joints. It was found that the filler metal had a significant effect on the ferrite content of CGHAZ in the T92 side. Li Linping [12,13] explored the welded joints between G115 steel and T92 steel. G115/T92 dissimilar steel welded joint was joined by different filler metal, and the microstructure and influence on performance were studied. Some scholars have explored other welding methods, such as friction welding, given the high-temperature creep rupture in the HAZ common to fusion weldings such as TIG and SMAW. Literature [14,15] used continuous drive friction welding technology to weld Super304H and T92 steel tubes to study the microstructure and mechanical properties of the joints. It is found that the HAZ in the T92 side is no longer divided into the CGHAZ and the FGHAZ, and the fracture occurs in the Super304H base metal, but the tensile strength at room temperature is lower than that of the TIG weld.

G115 and Sanicro25 have less application experience as a new type of heat-resistant steel. In particular, the research on the properties of dissimilar steel joints has rarely been reported, which restricts the development of dissimilar steel welding technology in power plant. This brings inconvenience to the metal supervision work of the safety management of the power plant. Therefore, this paper attempts to join G115 steel and Sanicro25 steel by TIG welding, studies the room temperature tensile properties of G115/Sanicro25 welded joints, takes the fracture location as the breakthrough point, focuses on the difference in microstructure between G115 base metal and FGHAZ. Explain the reason for the fracture of the welded joint in FGHAZ, and provide technical reference for the new martensitic/austenitic steel dissimilar steel joint in the higher-parameter unit.

Section snippets

Experimental

The experimental materials are G115 and Sanicro25 small-diameter steel tubes. The outer diameter and wall thickness are 44 mm and 7 mm respectively, and the steel tube length is 40 mm. The filler metal is a nickel-based welding wire of the ErNiCrCoMo-1 type, which has a diameter of 2.4 mm. The chemical composition of G115 steel tubes, Sanicro25 steel tubes, and welding consumables is shown in Table 1.

The weld joints were prepared by manual TIG method with V-bevel 70° and preheating the G115

Mechanical property

Fig. 2 shows the tensile curves of G115 steel, Sanicro25 steel, welded joints without PWHT, and welded joints after PWHT. It can be seen from the tensile curves of the two base metals that the properties between them are very different. G115 steel is martensitic heat-resistant steel with a high yield strength (687.09 MPa), but the strengthening effect is limited and the plastic is poor. While Sanicro25 steel is austenitic heat-resistant steel, its yield strength is low (356.13 MPa), but it has

Conclusion

In this paper, the mechanical properties of G115/Sanicro25 dissimilar steel welded joints were characterized by room temperature tensile and microhardness. The microstructures in FGHAZ and BM of the G115 side were characterized by optical microscopy, SEM, TEM, and XRD. The important conclusions of the study are listed below:

  • 1.

    The deformation behavior of G115/Sanicro25 dissimilar steel welded joints at room temperature after PWHT is: as the strain increases, weld beam first yields and hardens, the

CRediT authorship contribution statement

Maohong Yang: Formal analysis, Data curation, Writing - original draft, Writing - review & editing. Zheng Zhang: Conceptualization, Methodology, Supervision, Project administration. Yanrong Liu: Investigation. Linping Li: Resources, Funding acquisition. Jiankang Huang: Validation.

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.

Acknowledgments

The present study was financially supported by the National Key R&D Program of China (2016YFC0801902), and the Shenhua Guohua (Beijing) Electric Power Research Institute Co., Ltd.

References (20)

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