A Detailed Discussion of Steel Balls (Part 2) — Wear Failure Mechanisms of Steel Balls and Factors Influencing Steel Ball Wear

“He who knows the enemy and knows himself will never be endangered in a hundred battles; he who knows himself but not the enemy will win one battle and lose another; he who knows neither the enemy nor himself will be doomed in every battle.” This well-known, time-honored maxim originates from The Art of War, one of China’s earliest and most celebrated military treatises. For two millennia, it has served as an indispensable strategic guide for generations of strategists and military leaders, enabling them to plan meticulously and secure victory from afar. Today, it continues to inspire countless business leaders to overcome obstacles and thrive in the marketplace, while also acting as a golden key for accomplished scientists and engineers to explore the unknown and surmount technical challenges. Similarly, in the context of “Talking About Steel Balls,” thoroughly analyzing the operating conditions and service environments that steel balls endure in practical applications—and thereby elucidating the mechanisms and root causes of wear and other failure modes—undoubtedly underscores the critical importance of this endeavor.

First, let us examine the operating conditions and service characteristics of steel balls within the mill, as well as their primary failure modes and quality issues.

In the coarse grinding chamber of the mill, the steel balls predominantly exhibit a “falling” motion: they are lifted to the highest point by the lining plates and then dropped onto the material, while some also strike the exposed lining on the opposite side—a phenomenon known as “empty striking.” In the fine grinding chamber, however, the steel balls are carried to relatively lower positions by lining features with weaker lifting capability, such as flat or inclined liners, resulting in a “cascading” motion that effectively “rub” and “grind” the material. During this process, the lining transfers energy, driving the grinding media to continuously fall, roll, and slide within the mill, thereby generating wear. The wear failure mechanism is as follows:

Cutting and chipping wear: During the ascending phase of the mill’s motion, the steel balls slide relative to the feed material and other grinding media. The sharp, hard particles present in materials such as limestone, sandstone, clinker, slag, shale, and coal cut into the surface of the grinding media, producing relatively deep grooves; by contrast, softer, blunter particles produce shallower grooves, resulting in a heterogeneous pattern of groove depth across the grinding-media surface. In addition to variations in groove depth, groove width also varies depending on the hardness of the feed material as well as the shape and size of the grinding media.

The grinding media (balls) experience three-body abrasive wear. Because the balls can roll, the grooves vary in length and are crisscrossed in both longitudinal and transverse directions. When the balls fall, their impact on the material at a certain velocity and angle results in localized chiseling wear, leading to the formation of chisel-like pits.

Deformation wear: When steel balls slide and impact relative to the material, in addition to direct cutting and chiseling, plow-groove deformation and chiseling also occur simultaneously. The metal is pushed and squeezed toward the outer edges of the grooves and pits; under repeated loading and cyclic deformation, strain fatigue leads to crack initiation. These cracks propagate and coalesce, forming thin plow chips that eventually spall off from the surface.

Brittle spalling: During the mutual impacts among steel balls, as well as the reaction forces from the impact of steel balls against the lining plates, the brittle phases in the steel balls’ microstructure—such as carbides—crack and fracture, spalling off from the surface and thereby causing wear.

Fretting wear: During the rotation of the mill and the lifting phase of the grinding media—when the steel balls are lifted by the mill lining—repeated sliding and rolling occur; during the falling phase, repeated impacts take place. Under the combined action of cyclic impact stress, contact pressure, and shear stress, a fatigue process is initiated. Typically, fatigue cracks nucleate in the subsurface region, propagate parallel to the surface, and extend toward the surface, ultimately forming a fatigue spalling layer. These cracks may nucleate at subsurface inclusions or at brittle carbide phases, or they may nucleate at the interface between the surface-hardened layer and the dynamically softened zone. When crack initiation and propagation are facilitated by casting defects and inclusions located near the surface, macroscopic fatigue spalling occurs, resulting in the formation of thin microscopic layers and spalling pits. The foregoing describes the wear-related failure mechanism of steel balls under normal operating conditions; in fact, besides wear, other failure modes include fracture and loss of roundness.