Through hardening is the process of using a rapid quench to increase hardness throughout a steel alloy for the purpose of increasing its strength. As opposed to case hardening, which gives steel a hard outer layer while preserving a softer, more ductile core, through hardening diffuses carbon throughout the entire section of steel.

Through hardening typically consists of three steps: the heating of an alloy to alter its physical properties, a rapid quench in a medium, such as oil, salt or caustic, and a reheating, or tempering, to eliminate excess brittleness from the treated alloy.

Through-hardened steel is commonly referred to as “tempered steel,” but this is a bit of misnomer despite the fact that tempering does, in reality, occur. Tempering is simply the process of reheating a material following initial heating and quenching to combat brittleness. It’s conducted in many heat treating processes and does not necessarily indicate that an alloy has been through hardened.

As carbon, alloy and tool undergo the heat treating process, cracks and distortion can occur – and that’s simply unacceptable. At Paulo, we understand these risks and have proven, sophisticated computer-controlled systems in place to assess and mitigate risks. This ensures consistent quality across large production orders. Our experienced metallurgical staff oversees every step, allowing for constant refinements to keep everything on time and in spec.

Through hardening applications

Through hardening is useful for applications where strength, hardness and wear-resistance are desired. It’s a process that’s used in a wide range of engineering applications, including:

• Seat frames and seat belt buckles

• Hand tools

• Parts that will need to sustain heavy loads during their service life, such nuts and bolts, brackets, chains, nails and hooks

• Springs, axles, bearings, blades, scrapers and other miscellaneous components used in industry

Case hardening is a term used to describe several, more specific procedures which involve the addition of carbon or carbon and nitrogen to the surface of steel. This is done to give the material a hard, wear-resistant outer layer while preserving a softer, more ductile core that is better able to respond to stress without cracking. Case hardening allows manufacturers to work with softer materials and still meet basic requirements for hardness required by an application.

Raw, untreated steel is pliable and soft all the way through — which is not much use if you need a part to be hard at the surface but strong and ductile (or tough) in the middle. Though case hardening is not necessarily a strategy for adding material strength, it does effectively increase the hardness of the outer layer of the material, making it more wear resistant than it would otherwise be.

A steel cutting screw, for example, needs edges hard enough to cut through materials. The trouble is that hard steel is brittle and prone to breaking — so the case hardening provides this cutting edge on the screw while the tougher core holds everything together without breaking.

By diffusing either carbon or nitrogen and carbon case hardening is a suitable strategy for adopting relatively soft materials to applications where a hard exterior is paramount.  These are a just a few of the common uses of case hardening:

• Fasteners

• Bearings and gears

• Screw machine parts

• Cutting tools

• Engine parts

How does case hardening work?

Case hardening works by diffusing carbon or carbon and nitrogen through the surface of a metal by adding them to the atmosphere within a furnace. In case hardening, this is done at high temperatures. With other techniques, such as with ferritic nitrocaborizing, these higher temperatures are avoided to reduce the stress that can accompany phase changes.

Regardless of which specific case hardening technique is used, it’s ultimately a function of diffusion. That is, case depth behaves predictably over time and at given temperatures. In general, carbon and hardness in case hardened materials are high at the surface and gradually decrease with depth.