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Wednesday, 24 February 2016

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


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