Case Hardening Steel: Process, Grades, and When to Use It

What Is Case Hardening?

Case hardening is a group of surface treatments that produce a hard, wear-resistant outer layer (the “case”) over a tough, ductile core. The most common method is carburizing, which diffuses carbon into the surface of low-carbon steel. The related diffusion methods are carbonitriding (carbon and nitrogen) and nitriding (nitrogen only). All of them harden the surface by changing its chemistry, then quenching the part where the process requires it.

Case hardening should not be confused with induction or flame hardening. Those also harden a surface layer, but they do it by locally heating and quenching steel that already contains enough carbon, without adding any new element. This guide covers diffusion-based case-hardening processes.

A case-hardened part is designed to deliver two properties at once:

• A hard, wear-resistant surface, typically 58 to 64 HRC
• A tough, shock-resistant core, typically 30 to 45 HRC

A steel hardened throughout to 60 HRC would be too brittle for a shock-loaded part, while a core left at 35 HRC would wear quickly if exposed at the surface. Case hardening separates the two requirements into two zones of the same part.

How Case Hardening Works

Carburizing (most common)

Carburizing adds carbon to the surface of low-carbon steel (typically 0.10 to 0.25% carbon), which is not high enough to harden on its own.

1. Carbon diffusion: the part is held at 900-950°C in a carbon-rich atmosphere (gas, liquid salt, or solid compound). Carbon dissolves into the austenite at the surface and diffuses inward.
2. Case depth: typical effective case depth is 0.5 to 2.0mm. Growth follows the square root of time rather than a constant rate, so a deeper case takes disproportionately longer. Roughly doubling the case depth requires about four times the carburizing time at a given temperature.
3. Quenching: the part is quenched in oil or water. The high-carbon surface transforms to hard martensite at 60 to 64 HRC, while the low-carbon core stays softer and tougher.
4. Tempering: a low-temperature temper at 150 to 200°C relieves stress with little loss of surface hardness.

Carbonitretação

Carbonitriding is essentially carburizing with nitrogen added to the atmosphere. The nitrogen increases the hardenability of the case and lowers the hardening temperature, so the part can run at around 800 to 870°C and be oil-quenched. Case depths are shallower than carburizing, usually 0.1 to 0.5mm, and the cycle is faster. It suits smaller parts and high-volume wear components.

Nitretação

Nitriding diffuses nitrogen into the surface at low temperature, around 500 to 550°C, in ammonia gas or a salt bath. No quench is needed because the hardness comes from hard nitrides that form in place. The case is thin, roughly 0.1 to 0.5mm, but extremely hard, around 700 to 1100 HV, and distortion is minimal because there is no quench and no phase change in the core.

Nitriding is not used on the plain low-carbon carburizing grades. It needs steels that contain nitride-forming elements such as chromium, molybdenum, aluminum, or vanadium. In practice, that means medium-carbon alloy steels (for example, 4140 and dedicated nitriding steels) and many tool steels. H13, for instance, is routinely nitrided to improve surface wear and erosion resistance in die-casting and extrusion tooling.

Steels Suitable for Case Hardening

Carburizing and carbonitriding require a low-carbon base steel. The alloying additions in the higher grades raise core strength and case hardenability for thicker sections.

8620 is the most widely used alloy carburizing grade, balancing core toughness with moderate hardenability. 20CrMnTi adds titanium for grain refinement and dominates Chinese automotive gear production. The plain carbon grades 1018 and 1020 are the cheapest options but offer limited core strength and hardenability, so they are best suited to lighter-duty applications.

Case Hardening vs Through Hardening

The compressive residual stress left in a carburized case is a real advantage for parts loaded in bending or rolling contact, because it suppresses fatigue crack initiation at the surface.

Choosing Between Case Hardening and Tool Steel

Case hardening is the right choice when

• Parts that need a tough core to withstand impacts or bending include gears, shafts, and camshafts.
• Wear is confined to the surface, and the part can tolerate a limited depth of wear.
• The shape is complex, since carburizing follows contoured surfaces and gives a uniform case.
• Bending or contact fatigue life is critical, with compressive surface stress providing a direct benefit.

Through-hardened tool steel is the right choice when

• The part is a cutting tool that will be resharpened, so the edge must have consistent properties through its depth.
• Wear can penetrate beyond a thin case, as in heavy blanking, shearing, and extrusion.
• High hot hardness is required, for example, H13 in die casting.
• The whole cross-section must resist plastic deformation, as in punches and forming dies.

Why case hardening cannot replace tool steel in dies and cutting tools

A case is shallow, typically 0.5-2mm. Once it wears through, the soft core is exposed, and the part fails quickly. Dies are also reground several times over their service life, and every regrind removes hardened material, so a case-hardened die would eventually expose its soft core. On top of that, the wear resistance of a carburized case comes from high-carbon martensite alone, while a grade like D2 carries a high volume fraction of hard chromium carbides, roughly 12 to 18% by volume, which resist abrasion far better. For a cutting edge, the edge retention of through-hardened tool steel is in a different class.

Case hardening is excellent for gears, shafts, and bearings. It is the wrong choice for blanking dies, punches, forming tools, and cutting tools, which require through-hardened tool steel.

Quality Control for Case-Hardened Parts

• Case depth: measured by a hardness traverse from surface to core, or by microscopic examination, against the specified effective case depth.
• Surface hardness: Rockwell C or Vickers at a specified load.
• Core hardness: usually measured at mid-radius or at the center.
• Microstructure: fine martensite in the case, no continuous grain-boundary carbides, and controlled retained austenite.
• Distortion: dimensional check after heat treatment.

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