Surface hardening is used to – as the name implies – create a surface that is harder than the original steel. The harder surface makes for higher performing machine components, molds, dies and stamping tools. Knives and blades in for example recycling equipment will stay sharp for longer. Molds and dies that work with abrasive materials will get a longer productive life. Other common applications are crankshafts, camshafts, valve parts, gears, and screws.
There are different methods to achieve the desired hardness of the steel’s surface. One method relies on heating the steel close to its melting point, typically 900-1400 °C. This changes the grain to an austenitic structure. When the heat is removed the steel cools so rapidly that it is quenched, and a fine-grained microstructure called martensite is formed. The martensite zone can be as deep as 2 mm, improving the steel’s hardness and resistance to wear.
There is also a low-temperature method for surface hardening, gas nitriding which will be discussed below.
Laser hardening is a hot method where a laser beam is shot at the steel’s surface. The benefit of this technique is that it introduces as little heat into the surrounding material as possible. Laser hardening has many advantages compared to conventional heat treatment techniques, such as high throughput, reproducibility, and product quality.
The steel part is heated to a high temperature by using an alternating magnetic field, after which the steel is immediately quenched. The fast quenching creates the martensitic structure, which is harder than the base metal. Quenching is performed in water, oil, or a polymer-based liquid.
Gas nitriding is performed at a relatively low temperature, typically 520 °C. It works by diffusing nitrogen into the steel from a nitrogen-rich gas. The gas is usually ammonia (NH3). This method can also be called ammonia nitriding. Gas nitrided steel usually also becomes more resistant to corrosion.
Chemical or photo etching are two terms used to describe the surface etching of metal. It is a type of subtractive metal machining that modifies the surface geometry of a steel item using chemistry, such as when constructing steel molds for automobile interior panels.
In shot peening, spherical metallic, glass, or ceramic particles are struck against the steel surface with enough energy to cause plastic deformation. This compresses the steel's surface, which improves the fatigue resistance and reduces the risk for propagation of microcracks in the surface.
Laser peening is used for the same purpose as shot peening, to introduce compressive stresses in the steel’s surface. The laser pulse creates a pressure wave which penetrates into the metal. The compressive stresses make the steel more resistant to fatigue, cracking, and corrosion.
Toolox® is a clean tool steel with high surface quality and properties that make it highly suitable for surface hardening and other types of surface treatment techniques. It’s a prehardened steel used for machine components, molds, dies and stamping tools.