Explaining Through-Hardened vs Case-Hardened Forgings

What case hardening forging actually changes

Case hardening forging is a heat treat method that alters only the surface of a forged steel part. The core stays relatively soft and ductile. The surface becomes hard, which helps resist wear, galling, and surface fatigue. The depth of that hardened layer is measured in millimeters or inches and is one of the first numbers an engineer must lock down.

The base forging is usually made from lower carbon alloy steel. After the part is forged, it is heated in a carburizing atmosphere. Carbon diffuses into the outer surface. Then the part is hardened in quench and temper. The result is a gradient. The outer skin is hard. The core is tougher. This gradient is what gives the part its service behavior.

Through hardening works differently. The whole forging is heated and quenched so the entire cross section reaches a high hardness level. The material becomes strong throughout. But the core is also harder and usually less ductile. That can be good for some applications. It can be bad for others, especially where the part sees shock, bending, or impact.

The basic difference is depth. Case hardening forgings are designed so the hard layer covers the zone where the part fails. Through hardening forgings are designed so the whole part shares the load equally. The sourcing decision starts there. If the part wears only on a small bearing surface, a surface treatment may be enough. If the whole part sees stress, full-depth hardening may be required.

How the two methods affect sourcing and heat treat specs

When an engineer specifies case hardening forging, the heat treat spec must call out the target case depth, hardness range, and core hardness range. The carbon content of the starting steel matters. Too little carbon and the case layer may be too thin or too soft. Too much and the part may crack during quench.

The forging supplier must also confirm the starting material. A lower carbon alloy steel is typical for good case hardening. The supplier needs to know the part geometry because thin sections can take case too quickly and thick sections can stay soft too long. The heat treat shop must control atmosphere, temperature, and quench medium.

Through hardening specs are simpler in one way. The engineer specifies the final hardness across the section. The supplier still needs to confirm the starting steel, but the heat treat process is more uniform. The quench medium and tempering temperature control the balance between hardness and toughness.

The cost difference is not always large. A case hardening process can take longer because it involves carburizing, hardening, and tempering. Through hardening can be faster. But the material cost and inspection requirements can shift the balance. A through hardened part may need more careful quenching to avoid distortion. A case hardening part may need more inspection of the case depth profile.

When surface treatment is enough

Surface treatment is enough when the failure mode is surface driven. That includes rolling contact, sliding contact, and abrasion. The part does not need full-depth strength because the core is not the weak link. The core only needs to stay tough enough to carry the reaction loads.

A typical example is a gear. The gear teeth wear and crack in the surface zone. The core of the tooth must be tough enough to handle bending, but it does not need to be as hard as the tip. Case hardening forging gives the gear a hard tip and a tough root. That is why many gears are case hardened.

Another example is a camshaft lobe. The lobe wears against a follower. The wear is surface driven. The shaft body must stay tough to handle torsion and bending. A through hardened shaft may be stronger, but it can be less forgiving if it sees a shock load.

The key check is the depth of the stress zone. If the engineer can define the zone where the part actually fails, and that zone is small, a surface treatment is usually sufficient. If the engineer cannot define that zone, or if the part sees high bending stress, through hardening is often the safer path.

When full depth hardening is required

Full depth hardening is required when the part sees stress across the section. That includes high bending loads, impact loads, and cyclic loading that reaches deep into the material. The core must be strong enough to resist the load. A soft core will not hold up.

A typical example is a crankshaft. The crank throw sees high bending stress. The core of the throw must be strong. A case hardening treatment may not be enough because the stress does not stay near the surface. Through hardening gives the whole throw a stronger profile.

Another example is a high speed reducer pinion. The pinion root sees high bending stress. The core must be tough and strong. A surface treatment alone may not be enough if the pinion is small and the load is high. Through hardening is often used, with careful quenching to avoid cracking.

The engineer must also consider the geometry. Sharp corners and thin sections are more likely to crack during quench. Through hardening requires better control of quench and temper. The heat treat shop must know the part shape and the target hardness.

A worked example in plain words

Consider two forged parts: a spur gear and a small pinion. Both are made from the same alloy steel. Both are used in a gearbox. The gear has a larger diameter. The pinion has a smaller diameter and a higher gear ratio.

The gear sees rolling contact on the tooth tips. The wear is surface driven. The core of the tooth must be tough enough to handle bending, but the surface is the main concern. The engineer specifies case hardening forging. The target case depth is set to cover the wear zone. The core hardness is set to keep the root tough. The part is inspected for case depth and hardness.

The pinion sees high bending stress at the tooth root. The core must be strong. The engineer specifies through hardening. The whole tooth is hardened. The quench is controlled to avoid cracking. The temper is set to keep the core tough. The part is inspected for hardness across the section.

Both parts are forged. Both parts are heat treated. The heat treat specs are different. The sourcing decision is different. The gear supplier must confirm the case depth capability. The pinion supplier must confirm the quench capability. The engineer must specify the right method.

How to verify the heat treat result

Verification is where many projects go wrong. A case hardening forging must be checked for case depth, surface hardness, and core hardness. A through hardening forging must be checked for hardness across the section. The inspection method depends on the part size and the heat treat spec.

For case hardening, the engineer should ask for a hardness profile. This shows how hardness changes from the surface to the core. It also shows the case depth. The core hardness must be in the specified range. If the core is too hard, the part may be brittle. If the core is too soft, the part may deform under load.

For through hardening, the engineer should ask for hardness readings at multiple points. The readings should be consistent across the section. If one area is much harder than another, the quench may have been uneven. The part may need to be rejected or reworked.

The supplier should provide mill certs and heat treat certs. The engineer should check that the certs match the spec. The certs should show the starting material, the heat treat process, and the inspection results. This is where the supplier and the engineer must agree on the acceptance criteria.

How to choose between the two methods

The choice comes down to the service conditions. The engineer should ask three questions. What is the failure mode? What is the depth of the stress zone? What is the cost and schedule impact of each method?

If the failure mode is surface driven, case hardening forging is usually the better choice. If the failure mode is bulk driven, through hardening is usually the better choice. If the engineer is unsure, the safer path is to start with through hardening and then test. A through hardened part can often be modified by tempering to get a softer core. A case hardening part cannot be easily changed once the case is formed.

The cost difference is often small. The bigger difference is in the heat treat capability and inspection. The engineer must choose a supplier that can do both. The supplier must be able to show the heat treat capability and the inspection results.

The final decision is not just technical. It is also a sourcing decision. The engineer must specify the method, the depth, the hardness range, and the inspection requirements. The supplier must confirm it can meet those requirements. The heat treat shop must control the process. The part must be inspected. That is how the right forging gets to the customer.