Microstructure of tempered Crl3 steel

The tempering process is the quenching and tempering step after the previous heat treatment. Although the martensite in the structure after quenching is very “tough”, it is all due to the dislocation distortion formed by the supersaturation of carbides. Such a structure has internal stress and is unstable. In order to obtain a stable structure and eliminate internal stress, the tempering step is also essential.

Tempering process in Crl3 steel, in order to avoid tempering brittleness and obtain the combination of strength and toughness, it is usually tempered at 600-750 ° C at high temperature. Medium and low temperature tempering is generally undesirable. Martensite is obtained by quenching at 2Crl3. The reason why this structure with high hardness is hard, brittle and unstable, Because a large amount of C element is dissolved in supersaturated A-SOLID solution, the degree of lattice distortion becomes larger. Low temperature tempering at 200-350 ° C is only applicable to high carbon Crl3 stainless steel, such as 4Crl3. It is only to obtain corrosion resistance and high hardness. In this state, carbides in martensite precipitate but not much, internal stress decreases to a certain extent, and the structure becomes tempered martensite phase, Because the precipitation of Cr is not complete, most of it is in the solid solution phase, which is very helpful to the corrosion resistance. In terms of 2Crl3 high temperature tempering temperature, tempering at 600 ° C-750 ° C makes the microstructure of Crl3 martensitic stainless steel change into tempered sorbite. 2Crl3 at this tempering temperature can obtain a good match of strength and toughness without losing corrosion resistance.

Why is there less carbide precipitation and good corrosion resistance and hardness in the medium and low temperature tempering state, but the mechanical properties of medium and low temperature tempering will not weaken in the whole line under the high temperature tempering?

According to the research, it is found that the dispersion of alloy carbides is better and more sufficient under the continuous temperature in the high-temperature tempering state. The dispersion of alloy carbides is limited by the initial tempering. Carbides that are initially carburized are affected by temperature and time, The change of alloy carbide in Crl3 steel is roughly the same: (Fe, Cr) 3C → KCR, Fe) 7C3 → (Cr, Fe) 23c6. This phenomenon can be explained as a transition from a small amount of carbide precipitation to non-equilibrium and then to equilibrium. Firstly, the initial (Fe, Cr) 3C type carbide changes to (Cr, Fe) 7C3 type, and carbides are continuously precipitated inside and outside the grain boundary, This transformation leads to the decrease of Cr concentration at the grain boundary. With the increase of holding time, the high concentration Cr in the solid solution will diffuse to the low Cr, and the Cr element in the solid solution will disperse better. Then the (Cr, fe7) C3 carbide becomes (Cr, Fe) 23c6 carbide, This state can reach equilibrium. This transformation can only be achieved by high temperature tempering and medium temperature tempering + holding for hundreds of hours. Tempering makes the quenched supersaturated martensite change from metastable state to stable state. In this process, the dispersion of Cr, C and other alloy elements, the segregation of grain boundary position and atomic phase dislocation are difficult to move, which makes Crl3 stainless steel obtain a set of comprehensive strengthening of phase transformation, fine grain, solid solution and aging.

The change of mechanical properties of Cr13 steel after heat treatment is basically similar to that of alloy steel in general, u which is in line with the objective law. With the increase of tempering temperature, the toughness and plasticity will increase and the strength and hardness will decrease to a certain extent.