Influence of microstructure on the erosive-wear resistance of JIS SKH57 high-speed tool steel

In the harsh world of industrial surface treatment, machines rely on heavy-duty components like blades and liners to blast away rust, paint, and old coatings. These parts endure a relentless assault from steel grit and shot fired at high speeds, a process that grinds them down through a combination of scratching and chipping. For decades, engineers have relied on high-chromium cast iron to survive this punishment, but as the demand for longer-lasting equipment grows, they are turning toward a different class of material: high-speed tool steel. These steels are famous for holding a sharp edge on cutting tools, but their ability to resist the specific kind of battering found in blasting equipment is less understood. The key to their durability lies in their internal architecture, a microscopic landscape of hard particles called carbides embedded in a metal matrix. The behavior of this matrix, particularly how it hardens under stress and how it contains the hard particles, determines whether a component survives for months or fails in weeks.

A team of researchers set out to decode exactly how the internal structure of a specific high-speed tool steel, known as JIS SKH57, influences its ability to withstand this erosive wear. They focused on a material manufactured through a self-hardening mold casting process, a method that allows for complex shapes but creates a unique cooling history. The scientists prepared samples of this steel and subjected them to different heat treatments to see how the internal structure changed. They created samples in their raw, as-cast state, samples that were rapidly cooled to lock in a specific structure, and samples that were subsequently heated to various temperatures to see how they responded. By comparing these states, the team aimed to find the precise recipe that maximizes the material's resistance to being worn away by the flying grit.

The investigation began by examining the steel's hardness and its microscopic makeup. In the raw, as-cast state, the steel contained large, primary carbides along with a network of finer secondary carbides. When the researchers rapidly cooled the steel, its hardness increased, but the most dramatic change occurred when they tempered the material, a process of heating it to a specific temperature and letting it cool. While a similar iron-based alloy lost its hardness when heated above a certain point, the SKH57 steel did something different. When heated to approximately 525 degrees Celsius, it became significantly harder, reaching a peak hardness of 825 on the Vickers scale. This phenomenon, known as secondary hardening, was driven by the formation of fresh, fine carbides within the metal matrix and a corresponding increase in the density of defects in the crystal structure, known as dislocations. These dislocations are essentially lines of strain within the metal that make it more difficult for the material to deform.

To understand why this mattered for real-world use, the team put the different samples through a rigorous test. They blasted each sample with iron grit, 1.2 millimeters in diameter, fired at a pressure of 0.5 megapascals. The results were clear: the sample tempered at 525 degrees Celsius outperformed all others, losing the least amount of weight over twenty cycles of blasting. In contrast, the sample tempered at a higher temperature of 650 degrees Celsius failed the fastest, developing a hole through its thickness after just twelve cycles. The raw, as-cast sample and the rapidly cooled sample fell somewhere in between. The superior performance of the 525-degree sample was not just a matter of being hard to begin with; it was about how the material reacted to the impact.

When the researchers looked closely at the surfaces after the blasting, they found that the grit had caused severe plastic deformation, creating wave-like patterns on the metal. In the weaker samples, this impact caused cracks to form inside the large primary carbides and allowed the metal matrix to deform easily, leading to the detachment of material. However, in the high-performing sample, the matrix was so robust that it held the carbides in place. Even when the hard carbides cracked under the force of the grit, the surrounding metal did not give way. This resistance was linked to a phenomenon called surface hardening. As the grit struck the surface, it triggered a transformation in the metal's structure, increasing the density of dislocations and converting some of the softer, unstable austenite phase into harder martensite. This transformation happened most effectively in the sample that had been tempered at 525 degrees, creating a tough, self-reinforcing skin that resisted the erosion.

The study also clarified what did not work. The researchers found that simply having a high amount of retained austenite, a softer phase of steel, did not guarantee better wear resistance if the matrix itself could not harden under stress. Furthermore, they observed that heating the steel too high, such as to 650 degrees, caused the beneficial secondary carbides to coarsen and the dislocation density to drop, leading to a softer material that could not withstand the impact. The team concluded that the secret to the SKH57 steel's success was not just the presence of hard carbides, but the ability of the metal matrix to become harder when struck. This secondary hardening, driven by the precipitation of fine carbides and the accumulation of dislocations, created a material that could absorb the energy of the blasting grit without crumbling. By identifying the precise heat treatment that maximizes this effect, the researchers provided a clear path for manufacturing components that can endure the most severe wear environments, offering a significant improvement in service life over traditional materials.