42crmo Steel Forging White Dot Study
42CrMo steel belongs to the alloy structure steel chromium steel. Because of good comprehensive mechanical properties, high quenching permeability, and good thermal processing technology advantages, this material is widely used for manufacturing locomotive traction gear, booster transmission gear, rear axle, spring, engine cylinder, and so on.
However, the practice has proved that 42CrMo steel is easy to produce white dot. The existence of a white dot seriously damages the hydraulic properties of steel. It results in the plasticity of large forgings reduced, while heat treatment quenching can easily lead to forging cracking. This is because it becomes brittle and easy to fracture.
The production of the 42CrMo steel forging is a bit long. The production steps are as follows: EAF furnace smelting →LF refining → pouring → into the pit slowly cold→ release → heating→ forging → positive fire → tuning treatment.
The specific process of 42CrMo steel is:
1) The ingot is not de-stressed after release
2) Pre-forge heating
3) Forging process: the amount of pressure of about 20%, directly pull up to the × 580mm × 6000mm;
4) Forging end directly for positive fire and tuning treatment (water quenching, air cooling).
In order to determine the real cause of brittle fracture in the use of the 42CrMo steel core rod, the typical defect site is sampled and analyzed by slicing in the core bar section. After low acid immersion, the naked eye observed that the slender serrated cracks distributed at a certain distance from the surface of the specimen or near the center were radioactive concentric circles or irregular shapes. The results of the low-acid immersion test were typical white-dot crack defects.
The generation of white dots is an inherent defect in medium carbon alloy structural steels, especially Cr and Mo steels. It often appears in large cross-sectional billets or forgings, indicating that 42CrMo steels are indeed white-dot sensitive steels. Core rod forging belongs to the shaft type forging length. The surface layer is in the difficult deformation area and small deformation area, while the surface below a certain depth to the center range is a large deformation area.
During the forging process, due to the flip of the workpiece, the surface layer in the difficult deformation area and small deformation area exchange. The surface layer must be deep to the heart range has been in the large deformation area. Due to uneven deformation, the middle part of the section is subjected to two-way pressure stress – the edge part on both sides by axial pull stress and thickness direction pressure stress.
It can be clearly seen that the large deformation area causes crack concentration. Hydrogen (H) gas gathers at the crack, and because of the induced diffusion of residual stress, hydrogen pressure increases to equal the metal atomic bond binding force. The atomic bond breaks, resulting in the growth of the crack and forming a white spot. That’s simply how the white dots on 42CrMo steel forging form.
Can this white do problem be fixed? Well, the answer is yes. Through the analysis of 42CrMo steel white dot defects, the three main methods of control and repair are summarized:
(1) Measures to reduce the H content in the steelmaking process
1) Improve the steel finishing point content, and eliminate peroxide operation
2) Increase the stirring strength and the flow of argon under vacuum
3) Improve the capacity of the equipment, and increase the pumping capacity of the vacuum equipment.
(2) From the process to reduce the sensitivity of white point forging compaction methods, such as high temperature forging and shorten the distance between the heart of the forging to the surface, increase the spread speed of H and the relative area of the forging. This makes it easy to extract H from the inside of the forging, reduce the possibility of white spot generation of steel. Note that hydrogen gas is a critical component in the formation of white spots.
(3) The quality of raw materials must be strictly controlled. This is crucial for the content of inclusions is greatly reduced. According to the H content and the size of the cross-section of forging forgings, you need to determine a reasonable heat-treatment process. This will help eliminate the cooling of the temperature stress and uneven deformation caused by residual stress.