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Almost every load-bearing component takes at least one trip to the heat-treat furnace. And one of the most common ways to harden a steel to meet performance goals is through hardening.
In this post, we’ll look at the basics of through hardening, then explore what happens at the microstructural level and how to select the best grade for a particular application.
The Fundamentals of Through Hardening
Just like it sounds, through hardening, which is also known as “quench & temper,” makes a component harder all the way through.
Before comparing steel grades for through hardening, it helps to separate hardness from hardenability. Hardness describes how resistant the steel is to deformation at a given location, while hardenability describes to what depth you can achieve a desired hardness (i.e., the higher the hardenability, the deeper the material will harden).
For example, the modified 52100 grades, such as A485-1, were designed to boost the hardenability of standard 52100, meaning you can through harden larger cross sections with the mod grades than you can with regular 52100.
Many people are already familiar with through hardening’s three basic steps:
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Heating the steel to a specified temperature. By holding it at temperature for a specified time, some or all of its carbon content dissolves. (Note: Homogeneous high carbon grades (e.g. 52100) are austenitized such that not all the carbides are fully dissolved)
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Quenching the steel in a suitable cooling medium.
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Tempering (reheating) the steel. The tempering temperature is generally (well) below the temperature used in the initial heating step
Things get even more interesting when you examine what’s happening inside a through hardened steel as it undergoes treatment.
A Deeper Look at Through Hardening
A typical unhardened, or “green,” part starts out in its annealed condition and is relatively soft, ductile, and machinable. Annealed high carbon grades have a microstructure made up of spheroidized carbides in a ferrite matrix. The ferrite has a body-centered crystal lattice structure with the carbon atoms primarily tied up in carbides.
That structure changes when the part is heated (step one from above) and held at its austenitizing temperature. The steel’s matrix phase transforms from ferrite to austenite, which has a face-centered cubic crystal lattice, and a portion of the carbides dissolve and go into solid solution. The carbon atoms (which were dissolved from the carbides) readily fit into the interstitial spaces of the austenite lattice.
When this austenite is quenched (step two) the temperature drops very quickly and the carbon atoms become trapped within the lattice, transforming the steel’s matrix to martensite. However, some of the carbon atoms may find themselves positioned between iron atoms, which strains the iron lattice.
During tempering (step three) the martensite is heated (generally far) below the temperature in step one, which allows some carbon to escape the lattice and relieve residual stress. This makes the martensite more stable and less brittle.
The Influence of Carbon Content
Through hardening for bearing applications is most successful with high carbon steel grades. These grades can form a fine carbide, tempered martensite microstructure that is very hard, very wear resistant, and not subject to core deformation.
Medium and medium-high carbon steels are also an option for through hardening and have been successfully through hardened for many mechanical components.
Learn more about microstructure in the blog: Martensitic Stainless Steels: Achieving High Through Hardening Strength with Corrosion Resistance.
How Through Hardening Affects Performance
| Characteristic | Hardness | Tensile Strength | Temperature Resistance | Wear Resistance | Fatigue Resistance | Corrosion Resistance |
| Effect | Increased | Increased | Increased (depends on alloying) | Increased | Generally decreased | Unchanged |
Altering one property of a steel always has an impact on others, making it important to consider how the through hardening process will change a steel component’s overall performance.
For example, a through hardened part will achieve a higher hardness and durability, but it will have diminished toughness. A through hardened component may also have diminished fatigue resistance in certain applications.
If both hardness and toughness are needed, a “case hardening” method, which allows a part to retain a soft core, may be a better choice than through hardening. However, this decision warrants a detailed engineering review (something that Sullivan’s experts are able to support).
Read about other case hardening methods in the blog: Hardening Options for Steel: Through Hardening vs. Nitriding vs. Carburizing vs. Induction.
Applications for Through Hardening and Case Hardening
Essential factors you should determine before selecting a hardening method or material include:
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How you plan to manufacture or create the part
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Forging, machining, from bar stock or tube stock, etc.
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- The loading characteristics of your part
- Expected loading and stress -- stress profile vs strength profile, magnitude, position, cyclic or constant, etc.
Craig also notes that, “In a roller bearing or a spherical bearing, the load is spread out over a line, or over a larger area, which makes for lower stress at specific points on the component.” In these applications, a case-hardened part has microstructural advantages as well as a softer core, and may resist fatigue better than a through hardened part.
Sullivan's Support for Through Hardening Grades
See our line card for a complete list of through hardening steels.
When it comes to choosing a through hardening grade or a hardening method, Craig concludes, “It’s very difficult to say ‘this is always going to work for this. This is always going to work for that.’ You have to take a look at the application and consider all the engineering factors that are involved.”
That’s why at Sullivan, we choose to work closely with our customers to provide tailored technical support. Our experts can:
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Examine your heat treatment procedure and make practical recommendations.
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Help you zero in on the right grade for your performance goals.
We match our recommendations to each customer’s application and process. You bring the requirements; we supply the metallurgical expertise.
Start a live chat or contact Sullivan today.
References
https://www.sullivansteel.com/blog/nitriding-vs-carburizing
https://www.sullivansteel.com/blog/case-hardening
Krauss, George, STEELS - Processing, Structure and Performance, ASM International, Metals Park Ohio, 2005
https://www.boneham.co.uk/news/whats-the-difference-casehardening-throughhardening
https://www.thermexmetal.com/through-hardening-q-t
https://steeltreating.com/hardening/
https://www.youtube.com/watch?v=OwmgNEay0iM
https://www.youtube.com/watch?v=FE40FawJDkg
https://www.youtube.com/watch?v=Hy8zhkyhWP0
