What is the Difference Between Case Hardening and Flame Hardening

The key difference between case hardening and flame hardening is that case hardening can increase the hardness of the surface of the metal by infusing elements into the surface of the material, forming a thin layer of harder alloy, whereas flame hardening can selectively harden specific areas on the surface of a part.

Case hardening and flame hardening are two types of surface hardening processes that have many important applications in industries.

1. Overview and Key Difference
2. What is Case Hardening
3. What is Flame Hardening
4. Case Hardening vs Flame Hardening in Tabular Form
5. Summary – Case Hardening vs Flame Hardening

What is Case Hardening?

Case hardening process is the hardening of a metal surface that allows the deep underneath of the metal to remain soft while forming a thin layer of harder metal at the surface. There are different forms of case hardening processes, including cyaniding, carbonitriding, carburizing, nitriding, flame or induction hardening, and ferric nitrocarburizing.

Cyaniding is a type of case hardening process that uses sodium cyanide. This is a very fast and efficient process that is mainly useful on low-carbon steel. In this process, we need to heat the metal object or part of it at a high temperature in a bath of sodium cyanide. Thereafter, we need to quench the metal part, followed by rinsing it in water or oil, in order to remove any remaining sodium cyanide on the metal surface.

Carbonitriding is a type of case hardening in which a gaseous atmosphere is used for the hardening process. We can observe that carbonitriding process is very similar to the cyaniding process, except this process uses a gaseous atmosphere.

What is Flame Hardening?

Flame hardening is a process of heat treatment where oxyfuel gas flames are directly impinged onto the gear-tooth surface area to be hardened, which is then subjected to quenching. This process can result in a hard surface layer of martensite over a softer interior core. It has a cost that is considerably less than induction hardening.

Basically, there are two types of flame hardening techniques; they are spin hardening and tooth-at-a-time methods. Spin hardening is best for gears having enough mass to absorb the excessive heat that is applied in the aforementioned method without much distortion. When considering the tooth-at-a-time method, we need to heat the gear and quench it using a machine that can limit the amount of heat going into the gear. The two major methods we can use for this heating are tooth-to-tooth method and the method in which only the flank is hardened, leaving the root area untreated.

In the flame hardening process, a gas flame consisting of a single or multiple flame heads moves across the surface of the part, which can raise the temperature up to about 850 degrees Celsius. This step needs to be followed by the water quenching head that sprays water at the surface previously heated.

What is the Difference Between Case Hardening and Flame Hardening?

There are different forms of case hardening processes, such as cyaniding. Both case hardening and flame hardening are useful in hardening the surface of an object. The key difference between case hardening and flame hardening is that case hardening can increase the hardness of the surface of the metal by infusing elements into the surface of the material, forming a thin layer of harder alloy, whereas flame hardening can selectively harden specific areas on the surface of a part. Case hardening is common for low-carbon metal alloys such as mild steel, while flame hardening is used for many pieces of equipment made from steel.

The below infographic presents the differences between case hardening and flame hardening in tabular form for side-by-side comparison.

Summary – Case Hardening vs Flame Hardening

The key difference between case hardening and flame hardening is that case hardening can increase the hardness of the surface of the metal by infusing elements into the surface of the material, forming a thin layer of harder alloy, whereas flame hardening can selectively harden specific areas on the surface of a part.