Observe the above samples tempered at different temperatures with optical metallographic microscope. Start at 250 ° C and take every 50 ° C as the step. The metallographic structure of air-cooled samples is shown in Figure 1.
It can be seen from Fig. 1a-e that when the tempering temperature is between 250 ° C and 450 ° C, the morphology characteristics of acicular or flake tempered martensite in the structure are obvious, so the experimental steel ensures high hardness in this temperature range.
According to Fig. 1e-h, when the tempering temperature starts at 450 ° C, especially in the range of 500 ° C to 600 ° C, the acicular or flake tempered martensite gradually disappears, and the microstructure presents a typical tempered sorbite. At this time, the ferrite morphology changes to equiaxed, and there is basically no carbon supersaturation. At this time, the hardness corresponding to the sample also decreases significantly.
From 250 ° C, every 50 ° C is a step, and the optical metallographic structure of the furnace cooled sample is shown in Figure 2. As can be seen from Fig. 2a-f, compared with the air-cooled sample, the lath size of tempered martensite in the tempered structure of furnace cooled test steel increases significantly. When the tempering temperature is between 250 ° C and 500 ° C, the acicular or flake tempered martensite in the structure is obvious, so a higher hardness level is guaranteed in this temperature range.
From 2f-h, it can be seen that when the tempering temperature is between 500 ° C and 600 ° C, the tempered martensite gradually changes to tempered sorbite, and the ferrite shape changes to equiaxed shape. At the same time, there is basically no carbon supersaturation, and the hardness corresponding to the sample also decreases significantly. The initial temperature at which the hardness begins to decrease is slightly higher than that of the air-cooled sample, which is related to the size of martensitic lath.