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Hardening involves heating a steel to its normalising temperature and cooling (Quenching ) rapidly in a suitable fluid e.g oil, water or air. Steel is basically an alloy iron and carbon steels alloys have have various other elements in solution. When steel is heated above the upper critical temperature (about 760oC), the iron crystal structure will change to face centered cubic (FCC), and the carbon atoms will migrate into the central position formerly occupied by an iron atom. This form of red-hot steel is called austentite (γ iron). If this steel form cools slowly, the iron atoms move back into the cube forcing the carbon atoms back out, resulting in soft steel called pearlite. If the sample was formerly hard, this softening process is called annealing. If the steel is cooled quickly (quench) by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel. This steel crystal structure is now a body centered tetragonal(BCT) form called martensite.
Flame Hardening
This process involves direct an oxy acetylene flame on the surface of the steel being hardened and heating the surface above the upper critical temperature before quenching the steel in a spray of water. This is also known as the shorter process.
This is a surface hardening process resulting in a hard surface layer of about 2mm to 6mm deep. The main difference between this process and other surface hardening processes is that the composition of the steel being hardened is not changed. The steel must itself have sufficient hardenability . This limits this process to steels having carbon contents of above 0,35%. Steels with carbon contents of 0,4%-0,7% are most suitable for this process. Steels with higher content and high alloy steels may not be suitable as they a liable to cracking. This process produces similar result to the conventional hardening process but with less hardness penetration.
Induction Hardening:
Induction hardening provides a similar surface treatment regime to flame hardening . The steel component is located inside a water cooled copper coil which has (AC) alternating current through it. This causes the outer surface of the component to heat up. Depending on the AC frequency and current, the rate of heating as well as the depth of heating can be controlled. This process is well suited for surface heat treatment.
Case Hardening
The primary purpose of case hardening is to produce a surface which is resistant to wear while maintaining the overall toughness and strength of the steel core. This type of process is normally used on a steel with a low carbon content and introduces carbon by diffusion (carburising) into the local surfaces requiring treatment. Subsequent heat treatment develops the desired combination of high surface hardness and internal toughness. Another process called Nitriding consists of the diffusion of nitrogen.
Notes on three primary carburising processes (Pack Carburising, Gas Carburising and Liquid Carburising are provided below.
Related References:
- ASTM A564 / A564M Standard Specification for Hot-Rolled and Cold-Finished Age-Hardening Stainless Steel Bars and Shapes
- Normalising Normalizing
- Full Anealing
- heat treatment