Metallographic diagram of 60 steel (annealed at 710°C)

参考文章

Microstructures of SUS630 Steel (Annealed at 710℃)

Stainless steel is increasingly being used in a variety of fields due to its outstanding corrosion resistance and excellent mechanical strength. Among all kinds of stainless steels, the grade 630 stainless steels (i.e. SUS630) contain higher amounts of chromium and nickel than most of conventional stainless steels, therefore its strength and corrosion resistance capability are significantly higher. In this article, two samples of SUS630 steel were heated and annealed at 710℃, and the microstructures of the specimens have been investigated by means of optical and scanning electron microscopy.

Our optical micrographs revealed that the structure of SUS630 steel consisted of ferrite, modified austenite and undissolved precipitates (see Fig.1). The matrix of the steel is in ferritic form with some very fine-grained modified austenite regions. It was also noted that the dispersion of the modified austenite grains was poorer than that of ferrite grains. When observed via SEM, rod-shaped precipitates with a particle size ranging from 4 to 25nm could be detected in SUS630 steel as well (see Fig.2). Moreover, these precipitates were distributed homogeneously along the surface of the matrix, and annealing at 710℃ was confirmed to reduce the size of particles with a diameter below 8nm.

In conclusion, the microstructures of SUS630 steel annealed at 710℃ could be identified as undissolved rod-shaped precipitates dispersed in ferrite and modified austenite matrix. By further controlling the annealing processes of SUS630 steel, the size and distribution of precipitates could be manipulated and adjusted to improve the performance of this type of stainless steel.

Fig.1: Optical microstructure of SUS630 steel

Fig.2: SEM image of rod-shaped precipitates in SUS630 steel

The microstructure of S60 steel is mainly martensite, and a small amount of cementite and carbide solution. After quenching and tempering at 710℃, the martensite is tempered martensite, which has higher hardness and strength; The finegrain size ferrite improves the impact toughness and ductility of the steel; Meanwhile, the uniform distribution of tempered martensite and bainite also contributes to the comprehensive properties of the steel.

Under the optical microscope, the macrostructure of S60 steel is mostly martensite plus a small amount of carbide after quenching and tempering at 710℃, and the cementite can be observed under the metallographic microscope. The average grain size of tempered martensite is about 0.8μm, and a large number of small lath form in the lattice, which is the characteristic of quenching and tempering. The non-undergone transformation in the pearlite region is visible in the extinction area. The boundaries of the tempered martensite and bainite boundaries are clear, indicating good tempering effect. In addition, these regions are finely dispersed and their shapes are uniform, indicating that the properties of the steel are excellent.

In summary, the S60 steel heat treated at 710℃ has excellent comprehensive mechanical properties compared with those of normal quenched and tempered materials. Its main microstructure is tempered martensite which is distributed uniformly. The average grain size of tempered martensite is 0.8μm, and the boundaries of tempered martensite and bainite boundaries are clear, indicating good tempering effect.