Surface hardening

Surface hardening, treatment of steel by heat or mechanical means to increase the hardness of the outer surface while the core remains relatively soft. The combination of a hard surface and a soft interior is greatly valued in modern engineering because it can withstand very high stress and fatigue, a property that is required in such items as gears and anti-friction bearings. Surface-hardened steel is also valued for its low cost and superior flexibility in manufacturing.

The various methods of surface hardening are:

1. Case hardening
• Pack hardening
• Liquid carburizing
• gas carburizing
2. Nitriding
3. Cyaniding
4. Flame hardening
5. Induction hardening.

          Mechanical means of hardening the surface of steel parts include peening, which is the hammering of the heated surface, as by iron pellets shot onto the surface or by air blasting, and cold-working, which consists of rolling, hammering, or drawing at temperatures that do not affect the composition of the steel.

Cyaniding

          Cyaniding is a superficial case-hardening process that utilizes carbon and nitrogen. The part to be cyanide case hardened is heated in a molten sodium cyanide at about 850°C  followed by quenching. The cyanide-casing operation is the fastest type of casing that can be applied to a mild steel core, less distortion due to use of salt bath; however, its disadvantages are that the cyanide salts are extremely toxic and the process is messy. Special protective equipment must be on by personnel operating in the vicinity of the cyanide salts. Parts must also be limited to a size that can be handled by one or two people. Not suitable for components subjected to shock, fatigue and impact because nitrogen has adverse effect on these properties.

Process:

Medium: Parts immersed in liquid bath containing NACN varying between 25% and 90%.

Bath heated in a range of 800 to 950°C.

Measured amount of air passed through the molten bath.

Reactions: 

                   2NaCN + O2 ----> 2NaCNO 

                   2NaCNO + O2 ----> Na2CO3 + CO + 2N

                   2CO  ---->  CO2 + C 

C and N2 so formed diffuse into steel and give thin wear resistant layer of carbonitride  ϵ  phase.

Quenched in oil or water.

Cyaniding time of 1.5 to 6hrs for case depth of 0.13 to 0.35mm @ 850° C 

Higher the temperature, higher the C diffusion (0.8 to 1.2%) on surface as compared to N (0.2 to 0.3% )

Case hardness: 850 VHN