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What is case hardening steel? The ABRAMS Steel Guide® explains!

Case-hardening steel is unalloyed or low-alloyed structural steel which is suitable for case-hardening due to its low carbon content of 0.1% to max. 0.25%. Carbon is diffused into the material surface by carburising, and then the steel is given its hard and wear-resistant surface by hardening and tempering, while the core retains its toughness.

Due to the relatively high surface hardness (approx. 60 HRC), workpieces or components have a more wear-resistant surface, while the level of core hardness is determined by the level of alloy content.

FAQ - Questions about case hardening steel

The name “case hardening steel” is derived from the so-called “case hardening” process. It is a special heat treatment to which this type of steel is subjected. Through this process, the material in a sense combines two types of steel in one due to the different carbon content and can thus have a hard surface and retain a tough core at the same time.

Case-hardening steels are a group of alloy steels developed specifically for the case-hardening process. They can be classified into different groups depending on the specific alloying elements they contain. The groups of case-hardening steels are:

  • Unalloyed case-hardening steel
  • Chromium and chromium-manganese alloyed case hardening steel
  • Chromium-molybdenum alloyed case hardening steel
  • Nickel-chromium alloyed case hardening steel
  • Chromium-nickel-molybdenum alloy case hardening steel

It is important to note that many case-hardening steels contain a combination of these alloying elements to achieve a range of desired properties. The specific composition of each steel depends on the requirements of the application for which it is intended.

Case hardening is the most used heat treatment for case hardening steel. It describes the process of surface hardening, which consists of the heat treatment processes carburising and hardening, followed by tempering. Hardening and tempering provide the optimum properties (e.g., high wear resistance) of a hard surface layer and a tough core.

At temperatures of 800°C – 1050°C, carbon enrichment (carburising) takes place, e.g., by means of powder / granules, gas, or salt bath. The carbon enrichment increases the hardness of the surface layer. The carbon diffuses into the workpiece surface to a depth of 0.1 – 2.5 mm. Carbon contents of up to 0.8 % can be achieved. The choice of temperature depends on the case hardening steel. The carbon content decreases with the penetration depth.

Case-hardening steel and heat treatable steel are combined in a separate group of steels (DIN 10027). Both types of steel are hardened by heat treatment. The difference lies in the type of hardness. Case-hardened steel has a hardened surface while the core remains tough. Heat treatable steel, on the other hand, is through-hardened, which means that the surface as well as the core are hardened equally.

Case hardening steel has a wide range of applications. It is often used in the automotive industry, mechanical engineering, and plant construction. Among other things, it is used for the construction of cardan shafts, coupling parts, gears, and bolts. But case-hardened steel can also be used in the construction industry for the manufacture of components that have to withstand high stresses.

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