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Vacuum Heat Treatment of Fasteners

We all know that the vacuum heat treatment of fasteners can make them have a certain strength, good plasticity, toughness, etc., and then they can be used better, while reducing the phenomenon of relaxation and ensuring the quality and reliability of fasteners. Vacuum heat treatment is the most critical process of fasteners, and its process design will directly affect the final quality of fasteners.

The purpose of vacuum heat treatment is to obtain the strength, toughness and plasticity that meet the design requirements through vacuum heat treatment of high-strength fastener materials. The commonly used vacuum heat treatment method for high-strength fasteners is quenching and tempering treatment, that is, using continuous vacuum heat treatment equipment to complete vacuum quenching and high-temperature vacuum tempering at one time, which is a comprehensive vacuum heat treatment process.

The process of continuous quenching and tempering treatment is: feeding→cleaning (degreasing, dephosphorizing)→vacuum quenching→cleaning→high temperature vacuum tempering→water cooling→discharging.

1.Vacuum quenching process

Vacuum quenching is a process that heats the product to a temperature above AC₃ or AC₁, and after holding it for a period of time, rapidly cools it at a rate greater than the critical cooling rate to obtain the required martensite or bainite structure.

Vacuum quenching process parameters In order to ensure that vacuum quenching can meet the final requirements of the required products, the two process parameters of vacuum quenching temperature and vacuum quenching time must be reasonably designed.

1.1 Vacuum quenching temperature

The vacuum quenching temperature refers to the austenitizing temperature of the steel. The basic principle of selecting the vacuum quenching temperature is to obtain a uniform and fine austenite structure. Whether the vacuum quenching temperature is set correctly or not is an important factor affecting the quality of vacuum quenching products.

The determination of the vacuum quenching temperature is mainly based on the chemical composition of the product material, the original structure of the product before vacuum quenching, the product geometry, size, vacuum quenching medium, vacuum quenching method, continuous cooling transition curve, etc., and then according to the Fe-Fe₃ C phase diagram To preliminarily determine the vacuum quenching temperature, and then according to the process verification results, finally determine the vacuum quenching temperature.

1.2 Vacuum quenching time

The vacuum quenching time refers to the time when the product reaches the vacuum quenching temperature and completes the transformation of the structure, the dissolution of carbides and the homogenization of the austenite components, and it is also an important parameter of the vacuum quenching process.

The holding time must be set reasonably. If it is too long, resources will be wasted, and it will easily lead to coarsening of austenite grains. After vacuum quenching, the hardness will be low and the brittleness will increase. If it is too short, it cannot meet the product requirements. Therefore, under the premise of ensuring that the product structure is fully transformed, set the shortest holding time.

The determination of the vacuum quenching time is generally based on the size of the product and the thickness of the material, first calculated through empirical formulas, and then adjusted and revised according to process verification.

1.3 Vacuum quenching process

Mainly control the three process parameters of vacuum quenching temperature, vacuum quenching time and carbon potential, as well as the process parameters and oil temperature of vacuum quenching oil. The carbon potential should be set according to the middle and upper limit of the carbon content of the material.

It is recommended to check the furnace temperature uniformity and vacuum quenching oil every 3 months. After one year of use, the vacuum quenching oil should be sent to the manufacturer for refining to ensure that the sediment in the oil is less than 0.5%.

2.Vacuum tempering process

The vacuum tempering process refers to the vacuum heat treatment process in which the steel is heated to a temperature below Ac1 after vacuum quenching, kept for a period of time, and then cooled to room temperature.

Its main purpose is to obtain the final required hardness, strength and toughness of the product, while eliminating the residual internal stress generated during the vacuum quenching process.

The vacuum tempering process is mainly divided into three processes: low temperature tempering (150°C~250°C), medium temperature tempering (350°C-500°C) and high temperature tempering (500°C-650°C). Strength threaded fasteners.

2.1 Vacuum tempering process parameters

Vacuum tempering temperature is an important factor in determining the hardness, strength and toughness of the vacuum tempering structure and products. It is usually determined according to the product material and hardness requirements, and according to the “temperature-hardness” relationship curve or chart of the vacuum tempering material.

2.2 Vacuum tempering time

The vacuum tempering time refers to the time from when the product enters the furnace to the vacuum tempering temperature to when it comes out of the furnace. The vacuum tempering time is related to factors such as product material, size, vacuum tempering temperature and heating medium. The setting of the time must ensure that the metallographic structure of the product is fully transformed.