Heat treatment process for 55 steel operating points

What is 55 steel? According to xianji network small make understand 55 steel for high strength medium carbon steel, after heat treatment after high surface hardness and strength, but plastic, toughness is poor, medium cutting performance, poor welding, hardenability, water quenching has the tendency to form cracks. The following share 55 steel heat treatment process operation points.

Generally used after normalizing or quenching, used for parts requiring higher strength and wear resistance or elasticity, small dynamic load and impact load, such as gear, flange, flat spring, crankshaft, etc., can also be cast.

Steel 55 and 45 have the same hardenability, but both have poor hardenability:

Critical quenching diameter in oil: 10mm (unilateral maximum quenching depth 5mm);

Critical water penetration diameter: 20mm (unilateral maximum penetration depth: 10mm).

The depth of the hardened layer of steel 55 is related to the size of the workpiece, that is, the larger the size of the workpiece, the smaller the depth of the hardened layer: mainly because the larger the size, the more heat capacity stored in the workpiece, the slower the cooling speed, thus reducing the depth of the hardened layer, and also reducing the surface hardness.

The carbon content of 55 steel is generally about 0.55%, and the quenching temperature is generally recommended to be 820-840℃ when the content of the steel is qualified. In actual working conditions, the sources of steel are not uniform, relatively messy, it is difficult to ensure the unity of raw material ingredients, so the quenching temperature is only for reference. It is necessary to find the best quenching temperature and holding time according to the test.

Under the condition of complete quenching, the surface hardness of steel 55 can reach above 55HRC. When the material size is large (the effective thickness is more than 10mm), the surface quenching hardness of the workpiece will decrease due to the size effect.

For carbon steels with poor hardenability such as 55 steel, the most effective way to increase the depth of the hardened layer is to increase the quenching temperature.

When the workpiece is quenched.it is also necessary to cooperate with strong stirring, that is to increase the stirring strength of the oil in the oil tank (it is recommended to install the stirring device of propeller + diversion drum in the mesh furnace); For large size workpiece, the quenching temperature or the holding time shall be increased, and the cooling time in oil shall be prolonged (that is, the lifting speed of the mesh belt shall be reduced).

Before heat treatment, the quality of raw materials should be strictly controlled. On the one hand, the content of ingredients should meet the standard requirements; on the other hand, the surface of the workpiece should be free of carbon. When the workpiece is heated and kept warm in the mesh furnace, a protective atmosphere should be put in to prevent it from releasing carbon. Once the anti-poverty carbon occurs on the surface of the workpiece, the hardness test result will be affected, making the hardness test value low. Hardness test should be conducted after the workpiece is polished.