Case Hardening Heat Treatments

posted On Monday, December 18, 2023 in Blog

What is Case Hardening?

Case hardening heat treatments, which includes nitriding, nitrocarburizing, carburizing, and carbonitriding, alter a part’s chemical composition and focus on its surface properties. These processes create hardened surface layers ranging from 0.01 to 0.25 in. deep, depending on processing times and temperatures. Making the hardened layer thicker incurs higher costs due to additional processing times, but the part’s extended wear life can quickly justify additional processing costs. Material experts can apply these processes to provide the most cost-effective parts for specific applications.

Case Hardening with Carburization and Carbonitriding

Carburizing

Carburization is ideal for parts requiring extra hardening on the surface for wear resistance but need a softer core.

Carburization is a high temperature process (900 to 950°C) that involves the addition and diffusion of carbon into the steel. Those temperatures are above steel’s critical temperature, so subsequent quenching lets the carbon-rich surface form martensite while the core remains a softer ferrite and/or pearlite structure. Hardened depths can be as thick as 0.25 in., depending on the amount of time the part spends soaking at carburization temperatures.

As mentioned, the advantage of carburization is a deep wear resistance layer with high hardness. This is ideal for gears, blades, and cutting tools. Carburization creates hard, durable parts from lower cost alloyed steels and low carbon steels, such as 1008, 1018, and 8620. For alloys with higher carbon content (>0.3wt% carbon), carburization has minimal or even detrimental effects because the carbon in the original alloy could lead to a through-hardened, or bulk martensite structure. It should be noted also, that carburization temperatures cause some part distortion.

Carbonitriding

For lower carbon steels without significant amounts of alloying elements that promote hardening, adding nitrogen to the process can increase surface hardness. Adding nitrogen is called carbonitriding. Carbonitriding is commonly performed at slightly lower temperatures than carburizing (850°C), so distortion is less, but it also reduces hardening depths (for comparable processing time). The hardened surface created during carbonitriding, while thinner, does have greater hardness and resistance to elevated processing temperatures (such as tempering and stress relieving.)

Case Hardening with Nitriding and Nitrocarburizing

The alternative to the high temperature carburizing/carbonitriding is nitriding/nitrocarburizing. It also produces hardened surface layers and similar wear resistances, but it diffuses nitrogen throughout the surface layer (not carbon), and it uses sub-critical processing temperatures. Typical temperature ranges for nitriding range from 450° to 575°C. This means parts can be processed in their final machined state and undergo little to no distortion, so little post-nitriding machining is required (if any). The lower temperatures also maintain the desired core microstructure and physical properties while modifying the surface layer for the given application.

One note to consider when selecting nitriding: Inform the heat treater as to any stress relief, aging, or tempering temperatures to prevent altering core properties.

Unlike carburization, which is limited to lower-carbon-content steels, a broad range of alloys can be given surface hardnesses of 600 to 1,200 Hv via nitriding. But alloys best-suited for nitriding typically contain nominal amounts of the microalloying elements: Cr, V, Ti, Al, and Mo. Nitriding can be extremely beneficial for stainless and tool steels containing large amounts of chromium (10+wt%). These nitrided steels can have surface hardness well above 70 HRC equivalent, perfect for long-term wear resistance.

Nitriding is not limited to these types of ferrous alloys either, as low carbon steels can be hardened as well. In addition to creating a hardened, wear resistant surface, nitriding also forms a compound zone. Compound zones are nitrogen-rich layers formed on the surface during nitriding which are hard, wear-resistant (>60 HRC equivalent), and corrosion-resistant. This benefits low carbon and low alloyed steels which would not be considered for harsh environmental conditions if not for the presence of a compound zone.

Depth of hardening for nitrided/nitrocarburized alloys typically range from 0.005 to 0.030 in., depending on the process’s time and temperatures. Deeper hardened layers require more time. Compound zone thicknesses can be up to 0.002-in. thick, and it’s a function of which alloy is being nitrides, the time, and temperature. How the part is nitrided also affects zone depth. Nitriding can be performed via gas or ion (plasma).

Gas Nitriding

Gas nitriding uses cracked ammonia as the nitrogen source and is done in a positive-pressure environment. It’s ideal for large quantity batch processing and is also excellent with regards to temperature uniformity and nitriding parts with deep holes or channels. Gas nitriding is not recommended for porous parts because gas flowing through pores can cause severe embrittlement.

Ion Nitriding

Ion nitriding is excellent for selectively nitriding, since parts can be masked off from the plasma to prevent nitriding. Ion nitriding is performed by applying a potential electrical difference across an anode and the part (the cathode) in a vacuum. This potential difference forms a nitrogen plasma (a unique purple glow) which forces nitrogen atoms into the part’s exposed surfaces.

Plasma nitriding is well-suited to alloys, such as stainless steels, since it quickly breaks down passive oxide surfaces. Typically, ion nitrided steels have thinner compound zones than their gas nitrided counterparts due to the plasma’s constant sputtering. But this can be ideal for certain applications, such as gears, where contact stresses could harm surfaces with excessive compound zones.

Nitrocarburizing

In comparing nitriding and nitrocarburizing, the latter is typically performed at higher temperatures (575°C) and a source of carbon is used. The addition of carbon forms a harder, more wear-resistant, and higher lubricious layer. Thicker compound zones can also be formed by nitrocarburization. For comparison, a pure nitrogen nitriding environment forms a hard and wear-resistant layer, but less so than nitrocarburization.

So why not always use nitrocarburization?

Introducing carbon can increase the surface porosity, which is bad for parts with large contact stresses. The resulting layer is also less ductile. Material selection also drives which processing techniques are best for an application.

Questions to Ask Prior to Case Hardening / Heat Treating

This general guideline explains an array of case hardening heat treatments. But it is important for engineers to keep in mind the following questions about their part design when considering heat treatments:

What forces are my parts subjected to?
What environment are they working in?
Does the application require distinct properties for the surface, core, or particular surface regions?

The answer to the above questions will guide the selection.

Need Assistance Figuring Out Which Heat Treatment is Right for Your Case Hardening Needs?

Article adapted from Advanced Heat Treat Corp.'s "How to Determine the Best Heat Treatment for Your Parts."