Heat treatment process for high-strength bolts

Under different climatic conditions, the problem of substandard performance of fasteners used to fix wind turbine towers has gradually emerged. The wind and sand in western Inner Mongolia are relatively large, and the erosion of wind and sand is also serious. Wind turbine tower fasteners often fail to meet the requirements of comprehensive performance such as strength and hardness.

This paper aims at the problem that the comprehensive performance of 42CrMo steel high-strength bolts cannot meet the requirements in the use environment of wind turbine tower fasteners in windy and sandy areas. From adjusting the heat treatment process parameters of 42CrMo steel high-strength bolts, the carbon potential during quenching heating and quenching cooling Start with the concentration of water-soluble quenching agent and tempering temperature at the time.

By measuring the mechanical properties of high-strength bolts after heat treatment and scanning electron microscope microstructure analysis, the comprehensive performance of 42CrMo steel is maximized to meet the requirements of high-strength bolts. 42CrMo steel has high requirements in a specific use environment.

The influence of different heat treatment process parameters on the microstructure of 42CrMo steel

The bainite structure is carbon-free bainite, but there are big differences in morphology.

Under the carbon potential (0.40), water-soluble quenching agent concentration (6%-12%) and tempering temperature of 530℃ heat treatment process parameters, the bainite structure of 42CrMo steel is mostly granular and there are adjacent bainite structures connected to each other This phenomenon indicates that the early formation of carbon-free bainite is not the usual granular shape, but mostly needle-like and uniformly distributed. This is because there are higher energy and composition fluctuations at the grain boundary, which is beneficial to Bainite Body ferrite nucleates. In Figure a and Figure c, the carbon-free bainite is not fully grown, forming a bunch of bainite ferrite that grows parallel from the grain boundary to the grain.

The formation of carbon-free bainite is mainly due to the strong diffusion ability of carbon in ferrite and austenite when 42CrMo steel is quenched and cooled in an appropriate concentration of water-soluble quenching agent. When the bainitic ferrite is shaped After nucleation, the supersaturated carbon can quickly diffuse out, and can diffuse into austenite for a long distance, which ensures that there is no carbide precipitation in the bainite ferrite during and after it grows.

At the same time, it also shows that the carbon content of austenite around bainite ferrite is not too high, otherwise carbides will precipitate, which will hinder its growth. Therefore, under appropriate heat treatment process parameters such as carbon potential, water-soluble quenching agent concentration and tempering temperature, the structure of 42CrMo steel is more refined and uniform, and the overall performance is better.

Effect of water-soluble quenching agent on mechanical properties of 42CrMo steel

The quenching cooling rate is determined by the concentration of the water-soluble quenching agent. The lower the concentration, the faster the quenching cooling rate. When the concentration of the water-soluble quenching agent is 1% to 6%, the quenching cooling rate is too fast and the martensite structure The transformation is not sufficient, and defects such as segregation and distortion are likely to occur in the 42CrMo steel structure; and when the concentration is 6% to 12%, the quenching cooling rate is moderate, which provides sufficient time for martensite transformation, thus effectively reducing the workpiece During quenching, there are opportunities for defects such as distortion and cracking, the structure is more uniform, and the overall performance is better; when the concentration of the water-soluble quenching agent continues to increase, the quenching cooling rate is too slow, causing the martensite to become coarser. The mechanical properties are reduced.