There are several methods of hardening steel, and your choice can determine whether the final component develops the strength, longevity, and corrosion resistance required. Ultimately, the technique you decide to go with (be it nitriding, carburizing, or some other process) should match the performance needs of your application.
However, there’s no direct path to determining the “ideal” hardening method, as it depends on several factors, including:
- Steel grade
- Part size
- Part shape
- Desired properties (corrosion resistance, depth of case, etc.)
There are two ways to make a steel part harder: case hardening (also called surface hardening) and through hardening; each method imparts different properties for different steels.
This post discusses industry hardening methods and their advantages (for example, nitriding vs. carburizing) and includes examples of when and where to use them.
Through Hardening
As the name suggests, through hardening creates a uniform hardness throughout a steel, whereas nitriding, carburizing, and induction scanning form a hard surface case. The through hardening process includes heating, a rapid quench, and tempering or annealing. The hardness achieved depends on the treatment parameters. Typically, through hardening is an industrialized process carried out at volume.
Through hardening is excellent for applications where a part is subject to heavy loading. However, since hardness is uniform throughout the metal, it increases a part’s brittleness—a through-hardened steel is not as fracture resistant as case-hardened steel.
This hardening method is best for medium and high carbon steels; CX13VDW, XD15NW, XD16N, and 440C can all be through hardened.
Through hardening is commonly used for:
- Bearings
- Nuts & bolts
- Blades
Nitriding
Nitriding involves adding nitrogen atoms to a steel part’s surface using ammonia gas or nitrogen ion plasma. This procedure creates a hardened case layer with very high strength and corrosion resistance, while the core of the metal stays relatively ductile. Nitriding requires a specialized furnace to nitride high-performance steel grades and (in general) takes longer (and is more costly) than other methods of hardening.
Before nitriding, steel is heat treated for through hardening (discussed below) and then machined if necessary. An advantage of nitriding is the ability to mask certain areas during the process, so they remain soft (to allow for additional machining, for example).
Some steel grades accept nitriding more readily than others, such as low-carbon, low-alloy steels, including:
When comparing nitriding vs. carburizing, a nitrided steel’s surface (case) gets harder but not as deep as a carburized steel part. However, nitriding does offer superior corrosion resistance. Nitriding also provides better abrasion resistance, as it creates a compressive surface stress that counters forces that act upon the part.
Nitrided steel parts have a high case hardness and core strength, which is ideal for gears and transmission components.
Carburizing
Carburizing is a case hardening method that adds carbon to the surface of a steel part. Carburizing improves wear resistance and can create a deeper case than nitriding. Unlike nitriding, steel is typically carburized before heat treatment.
During carburizing, a carbon atmosphere is introduced into a furnace where steel is heated. Afterward, the metal must be quenched and tempered to create the desired hardness properties. It’s possible to maintain a soft surface by masking certain areas of a part during carburizing.
Steels that readily accept carburizing are low carbon and high alloy grades, such as:
Because carburizing introduces carbon, it makes a steel susceptible to corrosion—carburize stainless steel, and it will no longer be stainless. Also, as carburization takes place at a high temperature, it may lead to distortion of a part (which can create a challenge when working with tight tolerances).
Parts that are suitable for carburizing include bearings, bearing races, gears, and pinion shafts, which require a hard outer surface and a tough, fatigue-resistant core.
Induction Hardening
Induction hardening (or induction scanning) is an electrical case-hardening process that uses a specially designed copper coil. An alternating current passes through the coil, which heats the steel. Then, the steel is quenched to create the desired surface hardness.
Induction hardening is a clean, repeatable process that can be carried out on many pieces simultaneously. It is typically less expensive than either nitriding or carburizing while creating a hard, wear-resistance surface with a deeper case than other methods of hardening steel.
The induction hardening method is ideal for stainless steels—with no introduction of carbon onto the metal’s surface, corrosion resistance is maintained and the metal stays completely stainless. 4340 steel is a good candidate for induction hardening.
Before induction hardening begins, steel is typically heat treated and through hardened. However, masking is impossible—induction scanning case hardens the entire surface of a part.
XD15NW and XD16N can be induction case hardened to 58+ HRC after a core through hardening process that brings the core to a medium hardness.
Typical applications for induction hardening include bearings, bearing races, gears, and ball screws.
Nitrocarburizing & Carbonitriding
Two techniques for hardening a metal to the limits of its properties include nitrocarburizing and carbonitriding. When the usual methods of hardening steel can’t achieve high enough hardness for your application, nitrocarburizing or carbonitriding may be an option.
Nitrocarburizing is similar to nitriding but with the addition of carbon onto a steel’s surface. There are two types of nitrocarburizing: austenitic and ferritic. Ferritic nitrocarburizing uses lower temperatures than austenitic, which limits the distortion of the steel.
Carbonitriding is like nitrocarburizing but uses less nitrogen. It is carried out in the austenitic temperature range, followed by quenching the steel. The case depth produced by carbonitriding is typically deeper than nitrocarburizing.
You can use both techniques on low alloy steels and mild steels to create a hard surface with a relatively soft core and high fatigue strength.
Examples of parts made using nitrocarburizing include automotive drive train components and heavy equipment, while wood screws and stamped automotive parts may be carbonitrided.
Which Steel Hardening Method Should I Use?
To determine the right method of hardening steel, an engineer or metallurgist considers the performance characteristics they need for a final component. They will often look at the benefits of nitriding vs. carburizing vs. induction hardening, etc.
Hardness is just one of many factors for choosing a hardening method:
- Masking Requirements. If you only want to harden a portion of a part, nitriding or carburizing is ideal, as you can apply a mask to keep certain areas soft.
- Case Depth. Nitriding imparts a relatively shallow case depth, whereas carburizing or induction scanning creates a case depth of over a millimeter.
- Corrosion Resistance. Carburizing adds carbon to a part’s surface, making it susceptible to corrosion. Alternatively, induction scanning or nitriding will maintain the inherent corrosion-resistant properties of a metal.
- Cost. Through-hardened steel may perform as well as case-hardened steel; however, case hardening methods take longer and add cost. It may be an option to through harden a specialty steel rather than case harden a lower-cost metal.
Sullivan Steel’s Hardening and Heat Treatment Expertise
At Sullivan Steel, we have a unique approach to customer support—if you have questions about hardening steel, call us. Our metallurgists and steel experts will walk you through your options and support you throughout production.
We can work with your heat treater to provide advice and guidance for steel hardening and heat treatment, or even work to develop a heat treatment process. We’re familiar with heat treaters nationwide and can recommend a company experienced with your chosen steel grade.
Are you ready to work with the experts?
