Cyaniding

CYANIDING

1. What is Cyaniding?

✓ Cyaniding, also called liquid carburising, is a process of introducing both nitrogen and carbon to obtain hard surface of the steel components.

✓ Cyaniding is done by immersing the workpiece in a cyanide bath, as in liquid carburising described in Section 2.17.

✓ Metals usually hardened by cyaniding process are plain carbon or alloy steels containing about 0.20% carbon.

2. Procedure

In cyaniding process, the steel components are heated in a bath of molten sodium cyanide and sodium carbonate at a temperature of about 950°C. During this treatment both carbon and nitrogen diffuse into the surface of the steel. The formation of hard iron nitrides contributes to the surface hardening of the material.

3. Reactions During Cyaniding Process

The following reactions occur during the cyaniding process.

2 NaCN + O2 (air) → 2 NaCNO

2 NaCNO+ O2 (air) → Na2CO3 + CO + 2 N (atomic)

2 CO → CO2 + C (atomic)

The atomic nitrogen and carbon diffuse into steel. After cyaniding, the components require quenching to obtain hardness. The case depths of 0.1 mm to 0.5 mm may be readily obtained by this process. 

4. Difference Between Cyaniding and Liquid Carburising

The salt bath composition for cyaniding gives a case high in nitrogen, whereas liquid carburising gives a case rich in carbon.

5. Applications

The cyaniding process is applied to produce automobile parts such as gears, shafts, pins, brakes, cams, sleeves, springs, steps, etc. 

6. Advantages of Cyaniding

The advantages of cyaniding over carburising are as follows:

1. It requires less time and lower temperatures than carburizing.

2. It provides lower warping and distortion of workpiece than carburizing.

3. Resistance to corrosion and wear is high.

7. Disadvantages

The disadvantages of cyaniding as compared with carburising are as follows:

1. It is costly process.

2. Since cyaniding bath is toxic, workers require due protection.