Question

1. In carbonitriding of an iron-carbon alloy (steel) initially containing 0.05 wt% C and no nitrogen is subjected to carbonitriding at an elevated temperature and in an atmosphere that gives a surface carbon concentration constant at 0.5 wt% and surface nitrogen concentration constant 0.2 wt%. Find the ratio of the diffusion coefficients ‘C in steel’/‘N in steel’ if after 36 h the concentration of carbon at a position 0.4 mm below the surface is 0.35 wt%, and the concentration of nitrogen at the same depth is 0.10 wt%.

2. Using your knowledge on diffusion mechanisms in solids, would you expect the ratio ‘C in steel’/‘N in steel’ you found in (1) ? (Would you expect DC in steel or DN in steel to be larger?) 3. Sketch expected trends for: a) the concentration of carbon vs depth and the concentration of nitrogen vs depth (on the same graph), b) overall density vs depth, c) the number of substitutional vacancies vs depth.

4. What is the temperature in (1)? Qd for C in steel is 148 J/mol and Do = 2.3 x 10-5 m2 /s 5. In cyaniding, another case-hardening process, carbon source is NaCN. The part to be hardened is heated above 850 C is immersed in a bath of aqueous NaCN. The process produces more efficient diffusion of carbon in just 20-30 min. In comparison to carburization - which takes hours for depositing same amount of carbon at the same depths – cyaniding seems more cost-effective. However, it is not widely used, why? 6. Write down two applications of case-hardening

Answer 1

Initially vacancy will be more as diffusion occurs vacancies occupied by atoms which are diffusing. If diffusion is fast vacancy will fill fast and if diffusion is slow vacancy will fill slow and number of vacancies decreases slowly as compared to fast diffusion case.

For density as atoms occupies the vacant place density increases for diffusing atom. So the fast will be the diffusion more increase in density.

Answer 5- Even cyaniding is cost effective but due to liberation of toxic gases in this process leads this process not to use widely. Because gases liberated from NaCN are highly toxic for atmosphere.

Answer 6- Two applications of case hardening.

A- In making machine parts.

B- In Carbon steel forgings.

Mainly used to provide hardness to the surface.

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