Overheating phenomenon and control during heat treatment

We know that overheating in the heat treatment process is most likely to cause the austenite grains to be coarse, which reduces the mechanical properties of the parts. 
1. General overheating: The heating temperature is too high or the holding time at high temperature is too long, which causes the austenite grains to coarsen and is called overheating. Coarse austenite grains will reduce the strength and toughness of the steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is that the furnace temperature meter is out of control or mixing (usually caused by not knowing the process). The superheated structure can be annealed, normalized or tempered at high temperature for several times, and then re-austenitized under normal conditions to refine the grains. 
2. Fracture inheritance: steel with overheated structure, after reheating and quenching, although the austenite grains can be refined, sometimes there are still coarse granular fractures. There are many theoretical controversies about the generation of fracture inheritance. It is generally believed that the heating temperature was too high to cause impurities such as MnS to dissolve into the austenite and concentrate in the crystal interface, and these inclusions will precipitate along the crystal interface when cooling. It is easy to fracture along the coarse austenite grain boundary when subjected to impact.

3. Inheritance of coarse structure: When metal parts with coarse martensite, bainite, and Widmanite structures are re-austenized, they are heated to the conventional quenching temperature at a slow speed, or even lower, and their austenite crystals The grains are still coarse, and this phenomenon is called tissue heredity. To eliminate the heritability of coarse tissues, intermediate annealing or multiple high temperature tempering treatments can be used.

Excessive heating temperature will not only cause coarse austenite grains, but also local oxidation or melting of grain boundaries, which leads to weakening of the grain boundaries, which is called overburning. The performance of steel deteriorates severely after overburning, and cracks are formed during quenching. The burned tissue cannot be recovered and can only be scrapped. Therefore, avoid over-burning during work. 

When the steel metal is heated, the surface carbon reacts with oxygen, hydrogen, carbon dioxide, and water vapor in the medium (or atmosphere), reducing the surface carbon concentration, which is called decarburization. The surface hardness, fatigue strength and resistance of decarburized steel after quenching The abrasiveness is reduced, and the residual tensile stress on the surface is easy to form surface mesh cracks.

Post time: Dec-16-2021