In which of the following process does mild steel absorb carbon and nitrogen to obtain a hard surface?
Cyaniding
Understanding Mild Steel Surface Hardening Processes
Mild steel, while having good bulk properties, often requires a hard surface layer to improve wear resistance. This is achieved through various heat treatment processes that modify the chemical composition and structure of the surface.
Analyzing Surface Hardening Options
Let's look at the processes mentioned in the options to understand how they affect the surface of mild steel:
- Carburizing: This is a heat treatment process where the surface layer of steel absorbs carbon. The steel is heated in a carbon-rich atmosphere (like carbon monoxide) or placed in contact with carbon-rich materials. This increases the carbon content at the surface, which can then be hardened by quenching and tempering. Carburizing primarily adds carbon, not nitrogen.
- Nitriding: This process involves diffusing nitrogen into the surface of steel, usually at lower temperatures than carburizing. The steel is heated in a nitrogen-rich atmosphere, often using ammonia gas. The nitrogen reacts with elements in the steel to form hard nitrides, creating a very hard surface layer. Nitriding primarily adds nitrogen, not carbon.
- Cyaniding: This is a heat treatment process that involves diffusing both carbon and nitrogen into the surface of steel. It is typically carried out by heating the steel in molten cyanide salts or in a gas mixture containing carbon and nitrogen sources. The simultaneous absorption of carbon and nitrogen leads to the formation of hard carbonitride compounds at the surface, resulting in a hard and wear-resistant case.
- Annealing: This is a heat treatment process primarily used to soften steel, improve ductility, relieve internal stresses, and refine grain structure. It involves heating the steel to a specific temperature, holding it there for a period, and then slowly cooling it. Annealing does not result in a hardened surface layer or the absorption of carbon or nitrogen for hardening purposes.
Cyaniding: Absorbing Carbon and Nitrogen for Hardness
The question asks for a process where mild steel absorbs both carbon and nitrogen to obtain a hard surface. Based on the analysis of the options:
- Carburizing adds primarily carbon.
- Nitriding adds primarily nitrogen.
- Annealing softens the material.
- Cyaniding adds both carbon and nitrogen.
Therefore, Cyaniding is the process that fits the description of absorbing both carbon and nitrogen to achieve a hard surface layer on mild steel.
| Process | Elements Absorbed | Primary Effect |
|---|---|---|
| Carburizing | Carbon | Surface Hardening (requires subsequent heat treatment) |
| Nitriding | Nitrogen | Surface Hardening (forms nitrides directly) |
| Cyaniding | Carbon & Nitrogen | Surface Hardening (forms carbonitrides) |
| Annealing | None | Softening, stress relief, ductility improvement |
Conclusion on Mild Steel Hardening
The process by which mild steel absorbs both carbon and nitrogen to obtain a hard surface is known as Cyaniding.
Revision Table: Steel Heat Treatment
| Process Name | Purpose | Mechanism |
|---|---|---|
| Annealing | Soften, increase ductility, relieve stress | Heating and slow cooling |
| Normalizing | Refine grain structure, improve strength | Heating and air cooling |
| Hardening | Increase hardness and strength | Heating and rapid cooling (quenching) |
| Tempering | Reduce brittleness after hardening | Reheating hardened steel to lower temp and cooling |
| Carburizing | Surface hardening by adding carbon | Diffusion of carbon into surface |
| Nitriding | Surface hardening by adding nitrogen | Diffusion of nitrogen into surface |
| Cyaniding | Surface hardening by adding carbon & nitrogen | Diffusion of carbon and nitrogen into surface |
Additional Information: Case Hardening of Steel
Carburizing, Nitriding, and Cyaniding are all types of "case hardening" or "case carburizing" processes. These processes create a hard outer layer (the "case") while leaving the core of the material relatively soft and tough. This combination of properties is desirable for many applications where components need to withstand surface wear while also being resistant to fracture from shock or impact.
Cyaniding is often considered a faster process than simple carburizing or nitriding for achieving a shallow case depth. However, it involves highly toxic cyanide salts, which necessitates strict safety precautions.
Important Questions from Case Hardening
Cyaniding involves the addition of ___________ for the hardening of surface.
Cyaniding is carried out at a temperature of ___________.
Quenching is not necessary when hardening is done by-
Which of the following is used as energiser in pack carburising?
The behaviour of mild steel under creep is a result of ________
