Carbonitriding vs. Cyaniding

Attribute	Carbonitriding	Cyaniding
Process	Carbonitriding is a case hardening process that involves introducing both carbon and nitrogen into the surface of a metal.	Cyaniding is a case hardening process that involves introducing carbon and nitrogen into the surface of a metal by immersing it in a molten cyanide salt bath.
Temperature Range	Typically performed at temperatures ranging from 800°C to 1050°C.	Typically performed at temperatures ranging from 800°C to 950°C.
Hardness	Produces a harder surface compared to cyaniding.	Produces a relatively softer surface compared to carbonitriding.
Depth of Case	Can achieve a greater depth of case compared to cyaniding.	Generally provides a shallower depth of case compared to carbonitriding.
Surface Finish	May result in a rougher surface finish due to the diffusion of carbon and nitrogen.	Typically provides a smoother surface finish compared to carbonitriding.
Process Time	Generally requires a longer process time compared to cyaniding.	Usually requires a shorter process time compared to carbonitriding.
Equipment	Requires specialized equipment such as a sealed furnace or retort.	Requires a cyanide salt bath and appropriate safety measures.

Introduction

When it comes to enhancing the surface properties of metals, heat treatment processes play a crucial role. Carbonitriding and cyaniding are two widely used techniques that impart desirable attributes to the surface of metals, such as increased hardness, wear resistance, and improved fatigue strength. While both methods involve the introduction of carbon and nitrogen into the metal's surface, they differ in terms of process, properties achieved, and applications. In this article, we will delve into the attributes of carbonitriding and cyaniding, highlighting their similarities and differences.

Carbonitriding

Carbonitriding is a case-hardening process that involves the diffusion of both carbon and nitrogen into the surface layer of a metal. The process is typically carried out at temperatures ranging from 800°C to 950°C in a controlled atmosphere containing a mixture of hydrocarbons and ammonia. During carbonitriding, carbon and nitrogen atoms diffuse into the metal lattice, forming hard compounds such as iron carbides and nitrides.

One of the key advantages of carbonitriding is its ability to achieve a deep case depth, typically ranging from 0.3mm to 1.5mm, depending on the process parameters and the material being treated. This deep case depth makes carbonitriding suitable for applications where high wear resistance and improved fatigue strength are required, such as gears, camshafts, and crankshafts.

Furthermore, carbonitriding offers excellent control over the carbon and nitrogen content, allowing for precise tailoring of the desired surface properties. By adjusting the process parameters, such as time, temperature, and gas composition, it is possible to achieve different levels of hardness, wear resistance, and core toughness.

However, carbonitriding does have some limitations. The process is relatively slow compared to other case-hardening techniques, and it requires careful control of the process parameters to avoid the formation of undesirable compounds or excessive distortion. Additionally, the process is not suitable for all types of steels, as some materials may exhibit poor response to carbonitriding.

Cyaniding

Cyaniding, also known as cyanide case hardening, is a surface hardening process that involves the diffusion of carbon and nitrogen into the metal's surface layer. Unlike carbonitriding, cyaniding is typically carried out at lower temperatures, ranging from 800°C to 900°C, in a molten cyanide salt bath. The cyanide bath provides a source of carbon and nitrogen, which diffuses into the metal, forming hard compounds such as iron carbides.

One of the primary advantages of cyaniding is its ability to achieve a high surface hardness, typically ranging from 55 HRC to 65 HRC. This makes cyaniding suitable for applications where extreme wear resistance is required, such as cutting tools, dies, and punches. Additionally, cyaniding offers excellent case depth control, typically ranging from 0.1mm to 0.8mm, depending on the process parameters and the material being treated.

Another advantage of cyaniding is its relatively fast process time compared to carbonitriding. The shorter process time makes cyaniding a cost-effective option for high-volume production. Additionally, cyaniding is compatible with a wide range of steels, including low-carbon steels, alloy steels, and tool steels.

However, cyaniding also has its limitations. The process is highly dependent on the cyanide salt bath, which can be hazardous and requires careful handling and disposal. Furthermore, cyaniding is not suitable for components with complex geometries or those that are prone to distortion, as the molten salt bath can cause warping or dimensional changes.

Comparison

While carbonitriding and cyaniding share the common goal of improving the surface properties of metals, they differ in several aspects. Let's compare these two processes based on various attributes:

Process Temperature

Carbonitriding is typically carried out at higher temperatures ranging from 800°C to 950°C, while cyaniding is performed at lower temperatures ranging from 800°C to 900°C. The higher temperature range of carbonitriding allows for a deeper case depth compared to cyaniding.

Process Medium

Carbonitriding is conducted in a controlled atmosphere containing a mixture of hydrocarbons and ammonia, while cyaniding is performed in a molten cyanide salt bath. The choice of process medium affects the diffusion kinetics and the resulting surface properties.

Case Depth

Carbonitriding offers a deeper case depth, typically ranging from 0.3mm to 1.5mm, compared to cyaniding, which achieves a shallower case depth ranging from 0.1mm to 0.8mm. The desired case depth depends on the specific application requirements.

Surface Hardness

Cyaniding generally achieves a higher surface hardness, ranging from 55 HRC to 65 HRC, compared to carbonitriding, which typically achieves a hardness of 50 HRC to 60 HRC. The choice of process depends on the desired level of hardness for the application.

Process Time

Cyaniding offers a shorter process time compared to carbonitriding. The faster process time of cyaniding makes it more suitable for high-volume production, where time and cost efficiency are crucial factors.

Material Compatibility

Cyaniding is compatible with a wide range of steels, including low-carbon steels, alloy steels, and tool steels. On the other hand, carbonitriding may not be suitable for all types of steels, as some materials may exhibit poor response to the process.

Applications

Both carbonitriding and cyaniding find applications in various industries. Carbonitriding is commonly used for components requiring high wear resistance and improved fatigue strength, such as gears, camshafts, and crankshafts. Cyaniding, on the other hand, is preferred for applications where extreme wear resistance is required, such as cutting tools, dies, and punches.

Conclusion

Carbonitriding and cyaniding are two effective heat treatment processes that enhance the surface properties of metals. While carbonitriding offers a deeper case depth and allows for precise control over carbon and nitrogen content, cyaniding achieves a higher surface hardness and offers shorter process times. The choice between these processes depends on the specific requirements of the application, including desired case depth, surface hardness, material compatibility, and process time. By understanding the attributes and differences of carbonitriding and cyaniding, manufacturers can make informed decisions to optimize the performance and durability of their metal components.