Fig. 1 shows the microstructure of 30cr13nb0.1 steel tempered at different temperatures. It is preliminarily judged that it is martensite and carbide. As shown in Fig. 1 (a), carbides are mainly in the form of dots and fine chains. Fig. 1 (B, c) shows that when the tempering temperature increases from 350 ℃ to 450 ℃, the morphology and quantity of carbides change obviously, that is, the point carbides disappear and the chain and rod carbides increase. It can also be seen from Fig. 1 that with the increase of tempering temperature, the size and quantity of carbides in the steel increase.
Fig. 2 shows the SEM structure of 30cr13nb0.1 steel. The microstructure of the steel tempered at 250 ℃ is tempered martensite and a small amount of granular carbide (see Fig. 2 (a)). For the steel tempered at 350 ℃, as shown in Fig. 2 (b), the carbides are dispersed and the amount of carbides increases significantly. For the steel tempered at 450 ℃, as shown in Fig. 2 (c), the precipitated carbides change from granular to lamellar, and the amount increases. The reason is that metastable ε – carbides are formed at lower tempering temperature. With the increase of tempering temperature, metastable ε – carbides change into stable ones. However, the decomposition degree of quenched martensite is high, and its microstructure is mainly composed of α phase and carbide in the orientation of martensite lath.
In Fig. 2 (a), there are less black phases under SEM. In Fig. 2 (b), there are a lot of white stripes and dots around the black phase, and a small amount of white dots are distributed inside the black phase. As shown in Fig. 2 (c), the structure of the black phase is still unclear at high magnification.
The results show that the white dots are carbides, and the white and black phases are analyzed by energy spectrum. With the tempering temperature increasing from 250 ℃ to 350 ℃ and 450 ℃, the mass fraction of Si in black phase increases from 1.73% to 2.54% and 2.61%, while the mass fraction of Fe decreases from 83.88% to 82.78% and 82.82%, respectively. The Si content in the black phase is higher than that in the steel tempered at the same temperature, while the Fe content is mostly lower than that in the matrix. This shows that Si is more easily accumulated in the black phase, and the content of Si increases with the increase of tempering temperature. Si is a ferrite forming element and repels C. Therefore, as shown in Fig. 2, more carbides are precipitated around the black phase. With the increase of tempering temperature, martensite gradually decomposes, carbide precipitates into black phase, grows up gradually and enriches Si element.
The content of precipitates in steel tempered at different temperatures by electrolytic extraction is shown in Fig. 3. It can be seen from Fig. 3 that the mass fraction of precipitated phase in steel increases from 1.60% to 2.39% when tempering temperature is increased from 250 ℃ to 350 ℃. The content of precipitated phase in the steel tempered at 450 ℃ increases correspondingly. During tempering, the carbon in martensite transforms into carbides and precipitates. With the increase of tempering temperature, the activity of carbon atoms increases, the decomposition of martensite is accelerated, and a large number of carbides are precipitated. However, the content of precipitates in the steel tempered at 350 ℃ increases slowly, which may be related to the transformation from metastable carbides to stable carbides, and the carbides change from fine particles to flakes, and the density of carbides decreases. It can be seen that tempering temperature has a significant effect on the content of precipitates in the steel.
Figure 4 shows the XRD patterns of carbides in 30cr13nb0.1 steel tempered at different temperatures. The results show that the carbides in both steels are in the range of fec6425.
It can be seen from Figure 4 that the precipitates in the steel are mainly cr15.58fe7.42-c6 and NBC. With the increase of tempering temperature to 350 ℃, the peak value and peak area of cr15.58fe7.42c6 carbide increase, which indicates that the precipitation amount increases and the distribution of NBC is more uniform. For the steel tempered at 450 ℃, the peak value of cr15.58fe7.42c6 carbide decreases, but the peak area of NBC increases, and the precipitation amount of cr15.58fe7.42-c6 and NBC increases.
With the increase of tempering temperature, the relative intensity of diffraction peak of cr15.58fe7.42c6 carbide increases first and then decreases, while that of NBC carbide increases first and then decreases. The content of NBC carbide increases all the time, which means that the precipitation amount of carbides in steel tempered at 450 ℃ is the most. The amount and distribution of carbides have a great influence on the corrosion resistance of stainless steel with the same type of carbides. The precipitation and distribution of carbides along the grain will reduce the solid solution strengthening effect of alloy elements, which may affect the mechanical properties and corrosion resistance of the steel.