Heat treatment normalizing process

Abstract

The invention discloses a heat treatment normalizing process suitable for small-sized parts made of 40CrH steel and similar alloy structural steel. The process comprises: firstly, normalizing to make the hardness of a material be larger than or equal to 26HRC and make a pearlite and an upper bainite appear in the structure of the material, wherein the material is heated to a first temperature, and the first temperature is at least 50 DEG C higher than a critical temperature Ac3 of the material; secondly, tempering to decompose the pearlite and the upper bainite in the normalized structure, and regulating the hardness of the material to be within the range of 20-26HRC, wherein the tempering temperature is a second temperature; and thirdly, carrying out water cooling after tempering and taking the material out of a furnace. By using the heat treatment normalizing process, the normalizing hardness of each of the parts made of the 40CrH steel and similar alloy structural steel can be controlled within the range of 20-26HRC, unbalanced structures can be eliminated, and the stable strength, toughness and mechanical property combination can be realized.

Description

technical field [0001] The invention belongs to the category of metal heat treatment, and in particular relates to a heat treatment normalizing process suitable for high normalizing hardness technical requirements of small-sized parts of 40CrH steel and similar alloy structural steel. Background technique [0002] 40CrH steel and similar alloy structural steel are widely used in the manufacture of gears, shafts, transmission shafts, steering knuckles and other parts. When it is used as a transmission part for mechanical service after surface heat treatment, it has high requirements for the strength and performance of the core. A company's turbine shaft material is 40CrH steel, the blank size is Φ24×270 (mm), the material technical requirement is normalizing after cold drawing, and the normalizing hardness is 20-26HRC (Rockwell hardness). The Ac3 critical temperature of 40CrH (the final temperature at which ferrite transforms into austenite during heating) is 788°C. If it is...

AI Technical Summary

Problems solved by technology

The Ac3 critical temperature of 40CrH (the final temperature at which ferrite transforms into austenite during heating) is 788°C. If it is heated to Ac3+30-50°C according to the normal normalizing process, the hardness range obtained by forced air cooling is 195-50°C. 220HB (Brinell hardness), converted into Rockwell hardness or actual Rockwell measurement, the hardness is less than 20HRC, which does not meet the product design requirements

If the isothermal normalizing process is adopted, the specific heat capacity of air is 1.3KJ / M3°C, the thermal conductivity is 0.024W / M°C, the specific heat capacity of steel is about 4290KJ / M3°C, and the thermal conductivity is 50W / M°C. There is a huge gap between the two. 100Kg steel is cooled from 900°C to 650°C, and the flow rate of 550°C hot air is about 60-80m3 / s. Obviously, this process parameter is difficult to control and realize under the current technical conditions, and it is impossible for heat treatment equipment manufacturers to produce practical isothermal normalizing equipment

Therefore, normalizing 40CrH directly to a hardness of 20-26HRC requires that it cannot be operated in actual production.