The nominal size of steel balls is a fixed value, but in reality, the size of steel balls is a range. This range depends on the variability of the material and the screening method used.
Steel balls are directly manufactured from molten steel, and this method is the primary cause of the variability in the size of the balls. Molten steel is poured into a high-pressure water jet from a ladle, where the jet disperses the molten steel into small droplets, which then solidify into ball-shaped particles. During the solidification process, the droplets strive to minimize surface energy by reducing the surface-to-volume ratio. A sphere has a smaller surface-to-volume ratio, hence the particles of cast steel balls are approximately spherical. The possible distribution range of steel balls that can be produced from a full scoop of steel is depicted. The curve in the image is very similar to a "normal distribution" curve. The mean of this curve can be controlled through means such as adjusting the speed and shape of the water jet. The variability of the mean can be quantitatively represented by variance (the square of the standard deviation).
Due to the brittleness of metal in the water quenching state, the sand portion of the pellet manufacturing process is particularly prone to breaking. However, typically, before the coarse separation of pellets and sand, all pellets and sand undergo grading, followed by austenitization and quenching. Subsequently, the pellet section is tempered and finely sieved to produce pellets of different specifications that meet the standards.
Through a fine screening process, pellets can be separated into different standardized models. The fine screening process is a manufacturer's secret.
Each model of ball material will include a certain size range. For cast steel balls, the diameter range is from 0.125mm to 0.355mm. The mass of a single ball is the product of its volume and the density of steel. The volume is calculated as πd³/6 (where d is the diameter). Therefore, the mass range is the cube of the diameter range. Taking the S70 ball material mentioned earlier as an example, its mass distribution range is from (0.355/0.125)³ to 1. For S110 ball material, the mass distribution range is from 2.8 to 1, S170 is from 2.9 to 1, and S230 is from 1.7 to 1. If a ball material manufacturer wishes to produce a finer S70 steel ball, the lower limit of the S70 steel ball range should be increased to 0.180mm, also meeting specification J444. For the fine S70, the mass range is from 7.7 to 1.
