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Vacuum heat treatment process for steel parts

Definition of vacuum heat treatment of steel parts:

The process of heating, insulating and cooling solid steel parts in a vacuum to obtain the desired structure and properties. Objective: To improve the mechanical properties of steel and improve the technological properties of steel.

Vacuum annealing

The vacuum heat treatment process is to heat the workpiece to around the critical line of austenitization, keep warm and cool slowly to obtain a pearlite-based structure. Vacuum annealing is mainly used for casting, forging, welding blanks or semi-finished parts. The purpose of vacuum annealing: reduce the hardness of steel, improve plasticity, improve its cutting performance; uniform steel composition, refine grain, improve structure and performance; eliminate internal stress of workpiece, prevent deformation and cracking; prepare for final heat treatment .

Vacuum quenching

A vacuum heat treatment process in which the steel is heated to 30-50°C above the critical line of austenitization, and then cooled rapidly after heat preservation to obtain a structure dominated by martensite or lower bainite. Quenching materials include water, mineral oil, salt water, alkaline water, etc.

In order to obtain a room temperature structure dominated by martensite, the ideal cooling rate should be before the transformation start curve and as slow as possible near the Ms point. In this way, the quenching stress can be reduced and cracking of the workpiece can be avoided. The hardenability of steel refers to the ability of the steel sample to achieve the highest hardness that can be achieved by vacuum quenching and hardening under specified conditions. The hardenability of steel mainly depends on the carbon content in the steel, the higher the carbon content in the steel, the better the hardenability.

The stress caused by inconsistent thermal expansion and contraction due to temperature differences in different parts of the steel is called thermal stress. The internal stress caused by the different phase transitions of various parts of the workpiece is called phase transition stress. Therefore, vacuum tempering must be carried out after vacuum quenching.

Vacuum tempering

Vacuum tempering of vacuum quenched steel parts at 350~500 °C generally results in tempered troostite structure, which is different from vacuum annealed troostite. Tempered troostite is a ferrite matrix distributed with extremely fine spherical grains. organization composed of carbides.

Purpose: To make the workpiece obtain higher elasticity and strength, proper toughness and hardness. Tempered organization: tempered troostite. Mechanical properties: 35~45HRC, higher elastic limit, yield point and certain toughness. Application range: various elastic components and hot forging dies, etc.