Feasibility Analysis of Cold Heading for Wind Turbine Bearing Steel Balls​

# Cold Heading Forming Process for Wind Turbine Bearing Steel Balls

## Introduction to Cold Heading Forming

In recent years, wind power technology has developed rapidly. The power of wind turbine generator sets has increased from 2-3 MW to 6-10 MW. Consequently, the life requirement has also increased from 10 years to 20 years. As an important part of wind turbine bearings, the processing quality of steel balls directly affects the rotational accuracy, noise, and life of the bearings. The original casting and hot heading forming process for large-size steel balls of wind turbine bearings is inefficient. It consumes a lot of energy and results in a non-dense microstructure. This cannot meet the high strength and long life requirements for bearing steel balls of current ultra-large power wind turbine generator sets.

Cold heading forming offers a solution. Steel balls formed by this method have a denser microstructure and higher strength. Therefore, research on the cold heading forming process for wind turbine bearing steel balls is essential.

## Research Overview

Research on cold heading forming of steel balls includes various studies. Literature has conducted numerical analysis on the cold heading forming process of steel balls with diameters less than 10 mm. They optimized the mold structure for steel ball cold heading forming. Other studies have used simulation software to analyze the cold heading of different billet shapes. The results indicate that the optimal cone angle for specific shapes can significantly influence the cold heading force and material utilization rate.

Additionally, studies have explored the heat treatment process of steel balls formed by cold rolling. By appropriately controlling the rolled microstructure, the strength of cold-headed steel balls can be improved. A calculation model for the volume of cold-headed ball billets has also been established. Based on this research, I have developed a cold heading forming model for wind turbine bearing steel balls and analyzed its feasibility.

## Theoretical Basis of Steel Ball Cold Heading Forming

Technical Requirements for Steel Balls

Wind turbine bearing steel balls require high reliability, long life, and high precision. The material is generally GCr15SiMn or GCr15. The crushing load and hardness of the steel balls after heat treatment must meet GB/T 34891-2017, which outlines the heat treatment technical conditions for high carbon chromium bearing steel parts. The diameter of wind turbine bearing steel balls ranges from 30 mm to 70 mm. This paper focuses on steel balls with diameters of 50 mm and 65 mm. The selected material is GCr15, characterized by an elastic modulus of 219 GPa, a density of 7,830 kg/m³, a yield strength of 518.42 MPa, and a Poisson's ratio of 0.3.

Volume of Steel Ball Billet

The ball billet after cold heading of the bar stock consists of three parts: a spherical segment, a ring band, and two poles. Its volume can be expressed mathematically.

Theoretical Calculation of Bar Stock Size

The material volume remains unchanged before and after cold heading of the bar stock. The bar stock volume can be calculated using specific formulas. During the cold heading forming process, the cavity diameter equals the ball billet diameter. To head the billet into a sphere, the bar stock length must exceed the cavity diameter, while the bar stock diameter must be less than the cavity diameter.

For a steel ball with a diameter of 50 mm, the calculations show that the bar stock diameter should be selected appropriately to ensure efficient filling of the cavity. For a steel ball with a diameter of 65 mm, similar calculations are performed to determine the optimal bar stock size.

## Simulation of Steel Ball Cold Heading Process

A 3D cold heading model was established and imported into Deform simulation software. Tetrahedral elements were used for meshing, resulting in 8,000 elements. The downward moving speed of the upper die was set at 12.7 mm/s, with a minimum step size of 0.03 mm.

The required cold heading force and the shape of the ball billet after cold heading were analyzed. The ball billet exhibited distinct two poles and a ring band. For steel balls with diameters of 50 mm and 65 mm, the required cold heading forces were calculated. The equivalent stress distribution of the ball billet was uniform, indicating that the selected bar stock size can theoretically cold-head the specified ball billet.

## Calculation of Theoretical Crushing Load of Steel Ball

The steel ball after cold heading forming undergoes heat treatment processes such as quenching and tempering to enhance its properties. The microstructure transforms from austenite to martensite, increasing strength while reducing hardness. To determine the suitability of the cold heading forming process for wind turbine bearing steel balls, it is essential to calculate whether the crushing load meets the specified requirements.

During the crushing test, the internal energy of the steel ball converts into external work. The work done during the crushing process can be calculated using specific formulas. A steel ball model was established in ABAQUS simulation software, and the crushing load was determined. The results indicated that the crushing load of the 50 mm diameter steel ball met the requirements, confirming the feasibility of using cold heading for steel balls ranging from 50 mm to 65 mm in diameter.

## Experimental Verification

Cold heading tests were conducted on steel balls with diameters of 50 mm and 65 mm. The results showed that the ball billet after cold heading maintained its structural integrity, with smooth spherical surfaces and no cracks. The billets were subjected to further processing and heat treatment to produce finished steel balls. The hardness measurements of the finished steel balls met the requirements outlined in GB/T 34891-2017.

Three steel balls of the same specification were selected for the crushing test. The load-deformation curve during the crushing process was analyzed. The crushing load of the 50 mm diameter steel ball was found to be 1,328.7 kN, while the crushing load of the 65 mm diameter steel ball was 1,803.9 kN. The internal microstructure of the cold-headed steel ball was examined, revealing a fine acicular martensite structure that met the inspection requirements.

## Conclusion

This paper presents a comprehensive analysis of the cold heading forming process for steel balls with diameters of 50 mm and 65 mm. The theoretical calculations and simulations demonstrate that the cold heading forming process is feasible for wind turbine bearing steel balls. The actual processing verification confirms that the crushing loads meet the required standards, and the internal microstructure of the steel balls exhibits high strength. Future research will focus on optimizing the bar stock size to minimize machining allowances for the ball billets.

## Future Directions in Cold Heading Forming

As we look to the future, it is essential to continue innovating the cold heading forming process. This includes exploring new materials and techniques that can further enhance the properties of steel balls. By investing in research and development, we can ensure that our products meet the evolving demands of the wind energy sector.

Additionally, collaboration with industry partners can lead to breakthroughs in manufacturing efficiency. Sharing knowledge and resources will allow us to stay at the forefront of precision steel ball production.

In conclusion, the cold heading forming process is not just a method; it is a pathway to achieving excellence in steel ball manufacturing. I am committed to this journey and look forward to the advancements we will make together.

Diameter/mm	After Quenching	After Conventional Tempering	After Tempering at 350°C
50	62~64	61~62	52
65	62~64	61~62	52