Carburizing is one of the most common types of Case Hardening. The process involves adding carbon to the surface layer of steel by heating the steel below its melting point in a high-carbon atmosphere. It is traditionally performed in an endothermic atmosphere, ensuring that the material is harder on the surface than it is at the core. Carburizing creates a wear-resistant surface while maintaining a tough, ductile core. Steel materials such as 8620, 1018, and 12L14 steel are ideal for carburizing.
Nitriding is a type of Case Hardening. Performed over a long period of time at a comparatively low temperature, the process adds nitrogen to the surface of steel to form carbon and iron nitrides. Like carburizing, nitriding is traditionally performed in an endothermic atmosphere, ensuring that the material is harder on the surface than it is at the core.
Nitriding creates a wear-resistant surface while maintaining a tough, ductile core.
Hardening & Tempering
Hardening of steel is performed in an endothermic or vacuum atmosphere. We harden the entire part, from the outside to the core (often called "through hardening"), without reducing the carbon on the surface of the part (called "de-carb", which is a bad thing). Hardening includes heating the part to temperature, and then quenching it.
Steels such as 4140 and 420 are commonly hardened by heating and quenching. Steels such as 17-4 and 15-5 are precipitation hardening steels, which are heated to lower temperatures and "aged" to a specified hardness.
Aluminum doesn't have carbon, but can be hardened by heating and quenching.
Tempering, or "drawing", slightly reduces the hardness of the part after quenching, and significantly reduces the brittleness of the part. All hardnesses listed on a print are post-tempering.
Annealing & Stress Relief
Annealing and Stress Relief are terms used to describe the heating of an object to a specified temperature, holding it at that temperature, and then cooling the part at a relatively slow rate. The purpose of both of these processes is to reduce some of the stress in the material that can result from cutting the stock from the mill, machining, and cold working.
Stress Relief is a low-temperature heat treating process, usually under 1000F for steel or copper (under 400F for aluminum), while Annealing, which reduces a greater amount of stress in the material, is done at higher temperatures (1600F + for steel and copper, 600F + for aluminum).
We relieve stress, via annealing or low temperature stress relief, in all metals and plastics.
Cryogenic Processing involves freezing metals to low temperatures, in order to increase wear resistance and reduce retained austenite in steels after quenching. Cryogenic services include “sub zero” cooling (nominally -100F), to facilitate precipitation hardening and “cryo” treatments at colder temperatures (nominally -200F or colder). Cryogenic treatment tends to increase hardness, toughness and wear-resistance. Typical applications include steel tools, cutting tools, and high performance racing parts. Click here to read an interesting article on cryogenic heat treating.
In Aluminum, cryogenic processing is used to reduce stress and to prepare the part for a cold environment, such as the arctic or space.