Effect of Annealing Temperature on Microstructure and Properties of Overlay Welding Layer of NM360 Wear-resistant Steel

NM360 wear-resistant steel is an important material used in many industries, and its excellent wear-resistance performance has been widely used in various production and engineering applications. In order to improve its wear-resistance performance, a hard overlay welding layer is often applied on the surface of the NM360 wear-resistant steel. The microstructure and properties of the overlay welding layer depend on the welding parameters, such as the heat input, welding speed, and shielding gas. In addition, the post-weld heat treatment, particularly the annealing temperature, is also very important and can significantly influence the microstructure and properties of the overlay welding layer. Many studies have been conducted on the effect of the annealing temperature on the microstructure and properties of the overlay welding layer, but there is still inadequate understanding of the underlying mechanism.
    
In this paper, the effect of annealing temperature on the microstructure and properties of the overlay welding layer of NM360 wear-resistant steel is discussed. The microstructure of the overlay welding layer is observed by means of scanning electron microscopy (SEM). The mechanical properties of the overlay welding layer are measured using the Charpy impact test and the tensile test.
    
Experimental Method
NM360 wear-resistant plates were used as the base materials. The plates were cut into 50mm×50mm pieces. Three pieces of the base material were welded under a shielding gas atmosphere of argon, with a heat input of 130kJ/cm. The overlay welding was performed with a consumable electrode of high nickel steel. After welding, the samples were separated by cutting off the weld bead, and only the welded plates were used for the experiment.
    
The welded plates were annealed at three different temperatures: 620°C, 700°C, and 780°C. After annealing, the microstructure and properties of the overlay welding layers were investigated. The microstructure of the overlay welding layers were observed using scanning electron microscopy (SEM). The mechanical properties were measured using a Charpy impact test and tensile test.
    
Results and Discussion
    
Microstructure
Shows the SEM images of the microstructures of the overlay welding layers at the three different annealing temperatures. The microstructures can be divided into three main parts: an Fe-rich phase, an Ni-rich phase, and intermetallic compounds.
    
At the annealing temperature of 620°C, the Fe-rich phase is mainly composed of Bainite and Widmannstätten ferrite, while the Ni-rich phase is composed of austenite and fragmented particles. In addition, a relatively large compound of Fe2Cr2Ni (FeCrMn) can also be seen in the microstructure.
    
At the annealing temperature of 700°C, the Fe-rich phase is composed of Widmannstätten ferrite and lath martensite while the Ni-rich phase is primarily composed of austenite and fragmented particles. In addition, a large amount of the Fe3Cr Ni3 (FeCrNi) intermetallic compounds can be seen in the microstructure.
    
At the annealing temperature of 780°C, the microstructure of the overlay welding layer is primarily composed of Widmannstätten ferrite, lath martensite, and austenite. In addition, a large number of the Fe2CrNi intermetallic compounds can be seen in this microstructure.
    
SEM images of the microstructures of the overlay welding layers at different annealing temperatures.
    
Mechanical Properties
The mechanical properties of the overlay welding layers are shown in Table 1. The results show that the Charpy impact energy of the overlay welding layers is highest at the annealing temperature of 700°C. At this temperature, the Charpy impact energy is more than twice that at the annealing temperature of 620°C and is more than 1.3 times that at the annealing temperature of 780°C.
    
Mechanical properties of the overlay welding layers at different annealing temperatures
    
The tensile strength and elongation of the overlay welding layers at 700°C are highest, reaching their maximum values of 838MPa and 11.7%, respectively. The tensile strength is more than 1.6 times higher than that at 620°C, and the elongation is more than 1.2 times higher than that at 780°C.
    
In this paper, the effect of annealing temperature on the microstructure and properties of the overlay welding layer of NM360 wear-resistant steel was investigated. It was found that the annealing temperature has a significant influence on the microstructure and properties of the overlay welding layer. At an annealing temperature of 700°C, the microstructure of the overlay welding layer is primarily composed of Widmannstätten ferrite, lath martensite, and austenite. In addition, a large number of the Fe2CrNi intermetallic compounds can be seen in this microstructure. The overlay welding layer at this temperature exhibits the highest mechanical properties, with a tensile strength of 838MPa and an elongation of 11.7%.

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