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Causes of White Spot Defects in 43CrMo Steel Forgings

43CrMo steel forgings belong to chromium-molybdenum steel in alloy tool steels. Because of their excellent comprehensive physical properties, high hardenability and good hot working process, they are mainly used for manufacturing with higher compressive strength requirements or aging treatment. Forgings with large cross-sections, such as large gears for electric locomotive traction belts, supercharger casing gear shafts, rear axles, spring yellows, diesel engine cylinders, etc. However, according to practical experience, 43CrMo steel forgings are very easy to cause small white spots. The existence of small white spots damages the physical properties of the steel, which reduces the plastic deformation of large forgings. It is very easy to cause forgings to crack during quenching and heat treatment, which is very easy in application. produce brittle fracture.

Causes of White Spot Defects in 43CrMo Steel Forgings

The production process line of 43CrMo steel forgings is: EAF furnace smelter → LF refining → pouring → slow cooling in the pit → die release → heating → forging → quenching + heat treatment. The actual processing process of 43CrMo steel forgings is as follows:

1) Stress relief annealing is not carried out after the ingot is ejected;

2) Heating before forging;

3) The whole process of forging: the reduction amount is about 20%, and the length is immediately drawn to Φ780mm×7000mm;

4) After forging, quenching + heat treatment (water quenching, air cooling) is carried out immediately.

In order to clarify the real reason for the brittle fracture of the 43CrMo steel forging mandrel in the whole process of application, the cross section of the mandrel was cut, and the typical defect positions were sampled and analyzed. After high-power acid leaching, the human eye observed that the long and thin cracks in the shape of zigzags spread out at a certain distance from the surface of the test piece or near the management center, in the shape of radial concentric circles or arcs, and the test results of high-power acid leaching were typical. Sexual small white spot crack defects.

43CrMo steel forgings

The transformation of small white spots is a disadvantage of medium carbon alloy tool steel, especially Cr and Mo steels often appear in billets or forgings with large cross-sections, indicating that 43CrMo steel forgings are indeed more sensitive to small white spots. The mandrel forging belongs to the drawing length of the main shaft forging of the machine tool. The surface is in the difficult deformation area and the small deformation area. The depth below the surface layer must be within the range of 1 to the management center. During the whole forging process, due to the rotation of the steel parts, the surface is exchanged between the difficult deformation area and the small deformation area, and the surface must always be in the large deformation area within the range of depth 1 to the core, because the deformation is not symmetrical, and the cross section is positive. The middle part is subjected to biaxial compressive stress, and a part of both edges is subjected to radial tensile stress and thin-thickness azimuthal compressive stress. It can be clearly seen that the large deformation area causes the concentration of cracks, and H accumulates at the cracks. At the same time, due to the external diffusion induced by internal stress, the hydrogen pressure expands to the equivalent of the molecular bonding of metal materials, and the molecular bonds are broken, resulting in the growth of cracks. , resulting in small white spots.

According to the analysis of the white spot defect of 43CrMo steel forgings, the methods of its manipulation and repair are summarized:

(1) Countermeasures to reduce the H water content in the whole process of ironmaking:

1) Improve the water content of C at the tapping terminal to avoid the actual operation of overoxidation;

2) Improve the compressive strength of mixing and increase the total flow of argon under the vacuum pump;

3) Improve the working capacity of machinery and equipment, and expand the vacuum working capacity of vacuum equipment.

(2) Select a forging and tamping method that reduces the sensitivity of small white spots from the level of processing technology, such as forging at high temperature and reducing the distance from the core to the surface of the forging as much as possible, expanding the external diffusion rate of H and the relative total area of the forging , so that H is very easy to dissolve from the inside of the forging, reducing the probability of the transformation of small white spots in the steel.

(3) It is necessary to strictly control the quality of raw materials, so that the water content of the inclusions can be greatly reduced. According to the H water content and the size of the forging section after forging, an effective heat treatment method should be defined, and the temperature-in-situ stress and deformation caused by water cooling will be removed. Internal stress caused by symmetry.

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