Steel is an alloy composed primarily of iron and a small percentage of carbon, which gives the material its distinct strength and utility. Not all steel is the same, and the answer to whether one piece of steel can cut another is definitively yes. However, the cutting tool must possess a significantly superior material composition and preparation compared to the workpiece. Successful machining requires exploiting differences in hardness achieved through precise metallurgical manipulation.
The Fundamental Principle of Differential Hardness
The ability of one steel to cut another relies entirely on the principle of differential hardness, determined primarily by carbon content and subsequent heat treatment. Plain carbon steel contains between 0.02% and 2.14% carbon by weight. Increasing this percentage directly enhances the potential hardness of the alloy because carbon atoms inhibit the movement of dislocations within the iron lattice structure. High-carbon steels, with over 0.6% carbon, are the foundational materials for cutting tools.
The transformation into a cutting material occurs during heat treatment, which includes quenching and tempering. Quenching rapidly cools the heated steel, trapping carbon atoms in a highly strained structure called martensite. This martensite microstructure is exceptionally hard and wear-resistant. Alloying elements such as chromium, tungsten, and vanadium are also added to improve the steel’s response to heat treatment and enhance properties like high-temperature strength.
Specialized Steels and Materials Used for Cutting
The most common steel alloy used for cutting applications is High-Speed Steel (HSS), engineered to maintain hardness even when the cutting edge becomes hot. HSS contains significant amounts of tungsten or molybdenum, along with vanadium and cobalt. These elements form hard carbides that resist softening at elevated temperatures, allowing HSS tools to operate at much higher cutting speeds than conventional high-carbon steels. HSS is used for general-purpose machining of softer steels and applications involving moderate impact.
For heavy-duty industrial machining of hard steels, the material shifts away from steel entirely to cemented carbide inserts. Cemented carbide is a composite material, not an alloy, made from extremely hard tungsten carbide particles bound together with a metallic binder, usually cobalt. Cemented carbide is significantly harder and more heat-resistant than HSS, often reaching 80 to 95 HRA. These inserts allow for faster cutting speeds and longer tool life, making them the preferred method for efficiently removing large volumes of material from hardened steel workpieces.
Mechanics of the Cutting Process
The physical process of cutting steel involves forcing the harder tool edge into the softer workpiece material, causing localized plastic deformation. As the tool advances, the material ahead of the cutting edge is compressed until it yields and shears, forming a continuous ribbon of metal known as a chip. This shearing action occurs across the primary shear zone, where high force is concentrated to cause the metal to flow. The process generates substantial heat, primarily through plastic deformation and friction as the chip slides across the tool face.
Uncontrolled friction and heat can quickly lead to tool failure by softening the cutting edge, even in specialized materials like HSS. To manage this, cutting fluids—commonly oil- or water-based coolants—are constantly applied to the cutting zone. These fluids serve the dual purpose of cooling the tool and workpiece, dissipating up to 50% of the generated heat, and providing lubrication to reduce friction. Maintaining a stable, cool cutting environment is required for the hard tool to successfully operate against the workpiece without immediate wear.