In this post, we are going to discuss the fundamentals of case hardening and why it is used. In addition to this, we will dig deeper into the 3 most popular methods of case hardening to help develop your base knowledge on the process.

What is Case Hardening?

Case hardening is a heat treatment process that hardens the surface of a material while keeping the centre unaffected from the process. The process develops a hardened ‘case’ around the material, giving rise to the name. The case hardening process takes places once the product or component takes its final shape from manufacturing. This is a result of hardening processes reducing the machinability and formability of the material. Therefore, its a lot smoother process completing case hardening after manufacturing. 

The entire material is not hardened because this results in huge costs and low efficiencies in the manufacturing process. Providing a hardened surface to just a portion of the material is a lot more cost-effective. Additionally, a component that is ductile in the centre and a hard exterior is advantageous in many scenarios. A good example is components that require a large amount of wear resistance while also having to absorb impacts without fracturing.

How Does Case Hardening Work?

As mentioned above, case hardening is a heat treatment process meaning the material is heated to elevated temperatures in the process. Heating the metal results in the crystal structure of the metal changing. The structure alters by allowing other atoms to enter and alter the hardness. The depth of casing is determined by the ability of additional atoms to diffuse into the structure, e.g. carbon, which can be affected by the temperature used. In modern-day (carburising) uses of case hardening, the typical thickness of the case is approximately 1.5 mm.

The 3 Most Popular Methods of Case Hardening

Flame or induction hardening

Flame and induction hardening (as the name suggests) heats the metal using a flame or by induction heating. The heating is very rapid which is followed by very quick cooling, usually by quenching in water. As a result, the metal as a case of martensite on the surface. Learn more about martensite towards the bottom of the article here. Induction heating is more straightforward than using a flame for heating the metal. Using a flame requires operator skill and, it is harder to control the temperature the metal is being heated to, making it more difficult. Induction hardening is a process that is easily automated, making it easier to case harden more components.

Carburising

Carburising is a popular process used to introduce a case hardening by exposing the metal to a carbon-rich environment at high temperature ( approximately 790 to 925°C, depending on the process). Following the heating process, the metal is quenched, locking the carbon in the crystal structure. Carburising is a diffusion-controlled process, so the longer the metal is exposed to a high carbon environment, the more carbon will penetrate and diffuse into the crystal structure.

The simplest carburising method is to place the metal in a box packed with a high carbon material, e.g. charcoal. The box is then sealed and placed in an oven, between 2 and 36 hours, to give the desired results. This process is known as solid carburising.

The other method of carburising is gas carburising. Gas carburising involves heating metal in a furnace in the presence of methane and propane along with neutral carrier gases. The neutral carrier gas has the role of controlling and increasing the amount of carbon available in the atmosphere. The carrier gas also neutralises active and flammable gases to reduce the number of flames and minimise soot deposits. At high temperatures, the methane or propane decomposes (into carbon and hydrogen) at the surface of the metal resulting in carbon diffusing into the surface. Gas carburising can range from 2 hours, giving a case of 1 mm, to 36 hours, resulting in a case of approximately 4 mm. This process is used to create a hardened case for gears.

Nitriding

Nitriding is a process that has the same fundamentals as carburising however, it is nitrogen that diffuses into the structure. Typical products that undergo nitriding are camshafts, crankshafts, valve parts and many more.

The predominant method of nitriding is gas nitriding. Gas nitriding, like gas carburising, involves heating the metal in the presence of a nitrogen-rich gas, for example, ammonia. When ammonia is the gas nitriding gas, the process is also known as ammonia nitriding. When the ammonia comes in contact with the surface, it separates into nitrogen and hydrogen. Following this, the resulting nitrogen diffuses into the crystal structure providing the nitride layer. Developments in the process allow the thickness of the nitriding layer to be selected and, the environment parameters are implemented accordingly.

Carburising and Nitriding are popular methods of Case Hardening
A modern computerised nitriding/carburising furnace, Image found here, Credit: S zillayali 

An advantage of this process is the ability to accurately control the amount of nitrogen in the furnace by controlling the gas flow rate. A computer can be connected to the system to monitor the nitrogen levels and uses a feedback loop to adjust accordingly.

However, ammonia can be harmful if large volumes are inhaled. For this reason, secondary and tertiary sealing mechanisms are in place during the process. An additional risk of working with ammonia is the risk of explosion. Heating ammonia in the presence of oxygen requires a high level of care to minimise the start of an explosion.

What Types Of Metals Can Be Case Hardened?

Case hardening is generally limited to only ferrous metal. However, it is performed on other metals in special cases such as nitriding aluminium and titanium allows.

Common metals that are case hardened include:

  • Low carbon steel
  • High carbon steel
  • Stainless Steel
  • Cast iron

Common components that are case hardened include:

  • Gears 
  • Fasteners
  • Crankshafts
  • Camshafts
  • Rods
  • Pins

Summary

Case hardening is a central mechanism for providing strength to a metal. The process provides an important property to the material, a hardened exterior while maintaining a ductile core. Common examples of case hardened products are gears, bearings and crankshafts.

We have discussed the 3 most popular methods of case hardening used in the industry: Flame or induction hardening, Carburising and Nitriding. I hope you now have a solid fundamental understanding of the individual process but also understand case hardening as a whole!

I hope you enjoyed this post. Please leave your comments and feedback below.

Similar Posts: What Is The Annealing Heat Treatment Process?, Why Do We Heat Treat Metals, Which Manufacturing Process Is Better: Casting or Forging?

Follow My Social Media: Twitter, LinkedIn, Facebook