Understanding Cyaniding for Surface Hardening

Cyaniding is a thermochemical heat treatment process applied primarily to low-carbon steel. Its main goal is to increase the hardness of the surface layer of a component while retaining a relatively soft and tough core. This combination of properties is highly desirable for parts subjected to wear and fatigue stresses.

The Role of Nitrogen in Cyaniding

The cyaniding process involves heating steel in a molten salt bath containing cyanides (like sodium cyanide) or in a gaseous atmosphere containing carbon- and nitrogen-bearing compounds. At elevated temperatures, usually between 850°C and 950°C (for liquid cyaniding) or slightly lower (for gas cyaniding), the decomposition of these compounds releases nascent carbon and nitrogen atoms.

These released atoms diffuse into the surface layers of the steel workpiece. The process introduces both carbon and nitrogen into the surface. The question specifically asks about what is added for hardening the surface.

While carbon is a primary hardening element in steel, the presence of nitrogen during cyaniding contributes significantly to the surface hardness. Nitrogen atoms diffuse into the iron lattice and form hard nitrides, or they dissolve interstitially, contributing to solid solution strengthening. The combination of carbon and nitrogen diffusion results in a hard surface layer composed of complex carbonitride phases or nitrogen-rich martensite after quenching.

Why Nitrogen is Added for Surface Hardening

During the cyaniding process, both carbon and nitrogen are intentionally diffused into the steel's surface. The absorption of nitrogen, alongside carbon, leads to the formation of a hard case. Nitrogen plays a crucial role in lowering the critical cooling rate required for martensite formation and also contributes to solid solution hardening and nitride precipitation, enhancing wear resistance and fatigue strength.

Therefore, nitrogen is a key element added during cyaniding for the purpose of surface hardening.

Analyzing the Options

Let's look at why the other options are not correct in the context of cyaniding for surface hardening:

• Nichrome: Nichrome is an alloy (typically Nickel and Chromium) known for its high electrical resistance and resistance to oxidation at high temperatures. It is commonly used in heating elements. It is not added to steel during cyaniding for surface hardening.
• Niobium: Niobium is a refractory metal often used as an alloying element in steel to improve strength, toughness, and weldability, often through precipitation strengthening. It is not typically added during surface heat treatment processes like cyaniding to diffuse into the surface for hardening.
• Neon: Neon is a noble gas. It is chemically inert and does not react with steel or diffuse into its structure under heat treatment conditions for hardening purposes.
• Nitrogen: As explained above, nitrogen is deliberately introduced into the surface of steel during cyaniding, along with carbon, to achieve a hard surface layer through the formation of nitrides and carbonitrides and solid solution strengthening.

Based on the process of cyaniding, the addition of nitrogen is essential for achieving the desired surface hardness.

Mechanism of Surface Hardening by Cyaniding

When nitrogen and carbon diffuse into the steel surface during cyaniding, they occupy interstitial positions in the iron lattice. Upon subsequent quenching, this enriched surface layer transforms into a hard structure. Nitrogen enhances the hardenability of the surface layer, allowing for the formation of martensite even with less severe quenching than plain carbon steel might require for through hardening. Furthermore, nitrogen can form hard iron nitrides ($\text{Fe}_2\text{N}$, $\text{Fe}_3\text{N}$) or carbonitrides ($\text{Fe}_{x}(\text{C},\text{N})_{y}$), especially if tempering occurs or if the nitrogen content is high. These phases significantly contribute to the high hardness, wear resistance, and fatigue strength of the cyanided case.

Revision Table: Cyaniding Process and Inputs

Additional Information: Case Hardening Methods

Cyaniding is one of several case hardening (or surface hardening) processes used to create a hard, wear-resistant surface layer (the "case") on a softer, tougher core. Other common case hardening methods include:

• Carburizing: Diffusion of carbon into the surface of low-carbon steel at high temperatures, followed by quenching and tempering.
• Nitriding: Diffusion of nitrogen into the surface of steel (often alloy steels containing nitride-forming elements) at lower temperatures (typically 480-580°C), resulting in the formation of hard nitrides. This process does not require quenching for hardening.
• Carbonitriding: Similar to cyaniding but typically uses a gaseous atmosphere containing both carbon and nitrogen sources. It is also performed at temperatures similar to carburizing, followed by quenching. It achieves a case with properties similar to cyaniding.

Cyaniding provides a fast rate of carbon and nitrogen penetration, making it a relatively quick case hardening process, particularly liquid cyaniding. However, the use of cyanide salts requires careful handling due to their toxicity.