High carbon martensitic steel balls are quenched using ultrasonic induction quenching equipment, the causes of cracking defects and their improvement processes.

The steel ball is an important part of the ball mill, and its material is T7 or T8 steel. Ball mill steel balls require high wear resistance and good toughness, and the surface hardness of the steel balls is >60HRC. A factory produces steel balls with a diameter of 120mm. They usually use residual heat quenching after forging. The forging temperature is 1050°C, the final forging temperature is 850°C, and then the steel balls are quenched into 40-60°C water. During production, it was found that more than 50% of the steel balls processed in this way suffered from cracking and failure, seriously affecting product quality and economic benefits.

Macroscopic inspection of the cracked steel ball found that the section of the raw material billet was flat and brittle; the fracture surface of the workpiece was located in the center of the steel ball; the cracks on the center line of the steel ball were straight, and the crack lines in the upper and lower directions were arc-shaped; a wood grain pattern was found in the middle of the section The fracture is flush at a distance of 20 mm from the surface, has a metallic luster, and exhibits the characteristics of a brittle fracture with coarse grains. Scanning electron microscopy was used to observe the fracture morphology microscopically. The microscopic fracture of the cracked steel ball showed a cleavage fracture morphology; in the area 20mm from the surface, brittle fracture was dominant, with a small amount of ductile fracture. Observation of another fractured steel ball revealed that cracks appeared near the fracture surface, and the local fracture surface was brown-red. The fracture mode was intergranular fracture. Its grains are very coarse. This indicates that there are hot brittleness or over-burning defects during the forging and heating of the steel ball.

Analysis shows that quenching cracking of steel balls is closely related to forging and heat treatment processes. Improper forging heating time or temperature control may cause overheating or overburning of the steel ball, resulting in coarse grains and reduced toughness of the workpiece. On the other hand, the center part of the 120mm diameter steel ball has small forging deformation and low cooling rate. Therefore, the recrystallized grains in this part are coarse, causing cracks and fractures of the steel ball to occur first in the middle part. From the analysis of the heat treatment process, due to the post-forging residual heat quenching, the steel ball was not spheroidized annealed, so the grains are coarse and there is a banded structure. At the same time, the carbon content in the quenched martensite is very high, and the water temperature in the post-forging quenching is On the high side, the quenching stress of the steel ball increases. Analysis shows that the 120mm diameter steel ball is quenched by water cooling, and the temperature distribution is very uneven, forming a high organizational stress. The surface is in a state of compressive stress and the interior is tensile stress. The internal tensile stress is the main reason for the fracture of the workpiece. In addition, the water quenching ability of the steel ball is strong, and the lamellar structure inside the workpiece is coarse, and there is a hard structure near the center, which makes the core of the workpiece poor in toughness and poses the risk of cracking. During the production of the workpiece, the workpiece was not tempered in time after quenching. The stress after quenching was very high and was not eliminated and released, resulting in cracking and damage to the steel ball.

Based on the above analysis, the following measures are proposed to improve the process of preventing cracking of high carbon martensitic steel balls:

(1) Forging. Increase the forging ratio and strictly control the forging process to prevent the core structure from becoming enlarged after forging.

(2) Before using ultrasonic induction quenching equipment for quenching, spheroidizing annealing should be added to refine the structure so that the workpiece becomes fine needle-like (fine flake-like) martensite after quenching to prevent the formation of coarse structure after quenching.

(3) Lowering the inlet temperature of the steel ball quenching water and increasing the outlet temperature can significantly reduce and ease the quenching stress of the workpiece.

(4) Temper in time after quenching to eliminate quenching stress, stabilize the structure, and further eliminate the hidden danger of cracking in the workpiece.

The editor briefly talked about the reasons for quenching cracking of high carbon martensitic steel balls and process improvements. After many of the above process improvements were adopted, the failure phenomenon of steel ball quenching cracking was eliminated. The quality of the workpieces after heat treatment was excellent and the production ran well. The editor also hopes that after reading the theoretical knowledge in this article, you can apply it to your work in time to produce high-quality high-carbon martensitic steel balls.