Sub-temperature quenching is the heating of hypoeutectoid steel in the two-phase region between Ac1~Ac3 temperature, and quenching after sufficient heat preservation, also known as critical zone quenching. Sub-temperature quenching is based on the process of quenching hypoeutectoid steel after heating to the two-phase region of austenite and ferrite. In order to eliminate the mechanical stress and prepare the organization for quenching. Then normalize before quenching, then quench at <800 ℃ and directly cool through in brine, so that 15~20% undissolved ferrite remains in the quenched structure. It not only ensures the hardness, reduces deformation, but also prevents cracks.
Many new heat treatment processes have been developed as a means of strengthening and toughening steel, and the composite structure using toughness phase is one of them. Sub-temperature quenching is to use the existence of ductile phase ferrite to show the vitality of its process. Sub-temperature quenching can be called incomplete quenching of hypoeutectoid steel, two-phase (α+γ) zone quenching or critical zone quenching. The microstructure obtained by sub-temperature quenching is a martensite-ferrite duplex structure.
According to the concept of classical heat treatment, the quenching of hypoeutectoid steel must be completely austenitized, that is, completely quenched, and ferrite is not allowed to exist. Therefore, the (α+γ) two-phase region was once a forbidden zone for quenching hypoeutectoid steel, that is, incomplete quenching is not allowed for hypoeutectoid steel, because the existence of ferrite deteriorates the performance of steel.
However, research results at home and abroad show that the advantages of sub-temperature quenching not only break the old concept of "forbidden zone", but also become the basis for strengthening and toughening. The sub-temperature quenching of structural steel can obtain a small amount of dispersed ferrite structure on the martensite substrate, and its advantages can be summarized as follows: first, the impact toughness of steel at room temperature and low temperature is improved, thereby expanding The scope of use of materials; second, reduce the cold-brittle transition temperature of steel, and the material can be in a tough state at a lower temperature; third, suppress the reversible temper brittleness of steel, so that the tempering temperature of quenched and tempered parts can be lowered and Restores strength to obtain high toughness without sacrificing strength.
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