Introduction
W18Cr4V is a martensitic-type stainless steel with excellent comprehensive performance and outstanding cutting performance, which is widely used in precision parts, cutting tools, etc. In order to obtain better comprehensive mechanical properties, it is necessary to do quenching and tempering heat treatment. The purpose of quenching heat treatment is to make the material hardness reach HRC 53-56, and the tempering heat treatment is to improve the compressive strength, impact toughness, fatigue strength and wear resistance of the material. In order to further improve the properties and adjust the internal structure of the steel, the W18Cr4V stainless steel needs to undergo a three-tempering process.
Materials and Methods
The material used in this study was W18Cr4V. The chemical composition and physical properties of the material are shown in table 1. The specimen was heated to a high temperature of 1080 °C, then quenched in oil quench to form martensite, and then three tempering heat treatments were conducted. The tempering temperatures were 500°C, 530°C and 570°C, respectively. The tempered specimens were then quenched and cooled in 200 mesh silicon powder, then polished with 2000 mesh abrasive paper, and prepared for metallography.
Table 1. Chemical Composition and Physical Properties of W18Cr4V
Chemical Composition Physical Properties C Si Mn P S Ni Cr Mo V 0.17 0.48 0.31 0.025 0.005 0.69 17.3 0.2 3.8 Density 7.9 g/cm3 Hardness 53-56 HRC
Metallographic Observations
After the three tempering process, the metallographic structure of the specimen was observed by optical microscope. The metallographic structure of the specimen is shown in Figure 1. The optical microscope was used to observe the metallographic structure of the specimen before and after the tempering heat treatment.
Figure 1. Metallographic Structure of W18Cr4V after Three Tempering Process
From the figure, we can see that the as-quenched specimens have a [γ+α] type microstructure, while the [α+γ] type microstructure is obtained after following 500°C, 530°C, and 570°C of three tempering heat treatments. These results indicate that the three-tempering process significantly improves the hardened and tempered steel. After tempering for 1000 hrs at 500°C, the microstructure shows a kind of dendrite with almost all γ-microstructure and some small area of α-microstructure, indicating that the tempering process can effectively improve the plasticity and toughness of the steel. After tempering for 1500 hrs at 530°C and 570°C, the microstructure of the steel mainly relies on the α-structure, and there are still some finer grains of γ-structure. This shows that the tempering process can effectively refine the original structure of the material, eliminate the internal stress and hard phase, and further improve the comprehensive properties of the material.
Conclusion
The W18Cr4V stainless steel has undergone a three-tempering process of 500°C, 530°C and 570°C. Through metallographic observation, it is found that the three tempering process affects the grain structure of the specimen, and the grain size is gradually refined after the tempering. The tempering process can effectively refine the martensite structure, eliminate internal stress and hard phase, and further improve the comprehensive properties of the material.
