Heat treatment normalizing process

Abstract

The invention discloses a heat treatment normalizing process suitable for small-size parts of 40 CrH steel and similar alloy constructional steel, which comprises the following steps of: step 1, enabling the rigidity of materials to be more than or equal to 26 HRC through normalizing, so that pearlite and upper bainite occur in material tissue, wherein the materials are heated to a first temperature which is at least above 50 DEG C more than the critical temperature Ac3 of the materials; step 2, decomposing the pearlite and the upper bainite in the normalizing tissue through tempering to adjust the rigidity of the materials to 20 to 26 HRC, wherein the tempering temperature is a second temperature; and step 3, cooling the materials through water-cooling after tempering and tapping. The heat treatment normalizing process can realize that the normalizing rigidity of the parts of the 40 CrH steel and the similar alloy constructional steel is controlled to be between 20 and 26 HRC, eliminate imbalanced tissue and realize the combination of the stable strength, toughness and mechanical properties of the materials.

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). 40CrH of Ac 3 The critical temperature (the final temperature at which ferrite transforms into austenite when heated) is 788°C. If it is ...

AI Technical Summary

Problems solved by technology

40CrH of Ac 3 The critical temperature (the final temperature at which ferrite transforms into austenite when heated) is 788°C. If it is heated to Ac 3 The hardness range obtained by forced air cooling after heat preservation at +30~50°C is 195~220HB (Brinell hardness), converted into Rockwell hardness or actual Rockwell measurement, the hardness is less than 20HRC, which does not meet the product design requirements

But normalizing in actual production, the specific heat capacity of air is 1.3KJ / M 3 ℃, the thermal conductivity is 0.024W / M℃, the specific heat capacity of steel is about 4290KJ / M 3 ℃, the thermal conductivity coefficient is 50W / M℃, the difference between the two is huge, to cool 100Kg steel from 900℃ to 650℃, it takes about 60-80m of hot air at 550℃ 3 / s flow rate, 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.

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