What is case hardening?

Case hardening is a heat treatment for low-carbon case-hardening steels such as 16MnCr5 (1.7131) or 18CrNiMo7-6 (1.6587). The part is held in a carbon-rich atmosphere, usually at around 880 to 950 °C, so that carbon diffuses into the surface: carburising. It is then quenched and tempered low. The carburised case becomes hard, often in the region of 58 to 62 HRC, while the low-carbon core stays tough. Case depth ranges from a few tenths of a millimetre to more than a millimetre.

What does case hardening depend on?

Case depth, surface hardness and core strength depend on the grade, carburising time and temperature, quench medium and section size. The quench causes distortion and dimensional change; how much depends on geometry, wall thickness, grade and how the part is hung or laid in the furnace. Distortion can be limited but not predicted exactly in advance. Areas that must stay soft are masked with paste or copper plating, or left with extra material that is removed after carburising and before hardening.

What does the standard say about case hardening?

The drawing gives the surface hardness and the case hardening depth, CHD, for example 0.8 to 1.1 mm. Under the usual definition, used in ISO 18203 among others, that is the depth at which hardness has fallen to 550 HV, unless another limit is stated. ISO 15787 covers how heat treatment is indicated on drawings. Note where the requirement applies: usually on the finished part, so after grinding. The heat treater then has to carburise deeper to cover the grinding allowance; otherwise grinding removes too much of the hard case.

How does case hardening work in practice?

Case hardening splits the route. The part is first machined with grinding allowance on its functional faces, goes to the heat treater and comes back for grinding or hard turning of bearing seats, fits or gear flanks, often on a different machine at a different rate. Where threads or faces still to be machined must stay soft, they are masked, or the carburised layer is turned away between carburising and hardening; the second option means an extra trip back to the shop. The number of setups and subcontracted steps this adds, and the extra lead time, depend on the number of functional faces, the tolerances and what you can grind in house.

What is case hardening often confused with?

Case hardening is not through hardening: through hardening hardens steel with enough carbon across the whole section, case hardening only the carburised outer layer. Nor is it induction or flame hardening: those harden the surface of steel that already has enough carbon, without adding any. And nitriding runs at a lower temperature without a quench, giving a thinner case and less distortion.

How do you see case hardening in Blake?

Case hardening goes into the Blake estimate as a subcontract operation at the price held in your subcontract library; until that price is filled in, the line stays open. You enter the setups needed before and after hardening per route and then accept them. More on estimating.