Steel Heat Treatment: Fundamentals for Buyers and End Users
What is heat treatment, why is it used in steel manufacturing, and how do different heating processes affect finished properties?
In this edition of Ask the Metallurgist, I take a look at the fundamentals of steel heat treatment and provide a concise tutorial on common processes, key variables, and practical specification tips.
This article is designed to familiarize steel buyers with how heat treatment can tailor strength, hardness, toughness, and machinability to a part’s specific requirements.
The information presented below is provided as a general instructional guide only and is not intended to be comprehensive. Please reach out to Jade Sterling Steel if you have questions about your steel project and its requirements.
What Is Heat Treatment of Steel?
Heat treatment is the controlled heating and cooling of steel to alter its microstructure and, consequently, its mechanical properties. By moving steel through specific time–temperature paths, manufacturers can increase hardness and strength, improve toughness, relieve internal stresses, enhance machinability, or develop a hard wear-resistant “case” over a tougher core.
Why Heat Treat?
Increase hardness and strength for wear resistance and load-bearing performance.
Improve toughness to help parts absorb energy without cracking.
Stabilize dimensions and relieve stress after machining or forming.
Optimize machinability before finishing operations.
Create case-hardened surfaces for parts that need a hard exterior and tough interior.
How Does Heat Treating Work?
At elevated temperatures, most steels transform to austenite, a face-centered cubic structure that can dissolve carbon. What happens upon cooling determines the final properties. Slow cooling allows ferrite and pearlite to form and gives a softer, more machinable condition. Rapid cooling (or quenching) traps carbon and creates martensite, a hard, brittle structure that must be tempered to balance strength and toughness.
Two ideas help frame expectations: hardness is the measured resistance to indentation, while hardenability describes how deeply a steel can harden through its thickness during quenching. Chemistry and section size control hardenability, while process control sets the final balance.
Options like boron steel can increase hardenability to augment the heat-treating process, while a variety of cold finishing processes may be applied after heat-treating to obtain the desired characteristics.
Which Steels Respond to Heat Treatment?
Most plain carbon and low-alloy steels respond well to quench-and-temper and surface-hardening processes. Tool steels and martensitic stainless grades are engineered for specific heat treatments.
Common Heat-Treatment Processes
Annealing is used when the primary goals are softness, stability, and machinability. In a full anneal, the steel is heated above its critical range and cooled slowly to produce coarse pearlite. Stresses relax and cutting becomes easier. A subcritical or “stress-relief” anneal takes the steel to a lower temperature to reduce residual stress without major changes to hardness. When the need is very high machinability or cold-forming response, a prolonged hold just below the critical range promotes spheroidized carbides, which shear more cleanly during machining.
Normalizing is a clean-up step after hot working or forging. Heating slightly above the critical range and air cooling refines grain size and evens out variations caused by prior processing. The result is a more uniform, fine-pearlitic structure that machines predictably and serves as a stable starting point for subsequent hardening.
Quench and temper is the workhorse route for many carbon and alloy steels. The part is austenitized to dissolve carbides and equalize chemistry, then quenched to form martensite. Tempering follows immediately to restore toughness and set the final hardness. Lower tempering temperatures preserve more hardness; higher temperatures trade hardness for ductility. In thicker sections or complex geometries, alloy content (and, in specific grades, very small boron additions) raises hardenability so the interior cools fast enough to transform.
Case hardening creates a hard skin over a tougher core for components that see sliding or rolling contact.
Heat treatment is one of many value added processes that Jade Sterling Steel can apply: learn more about our full range of options.
Specifying Heat Treatment: What to Tell Your Supplier
For through-hardened parts, specify a hardness range (for example, HRC 28–32) appropriate to the section size and service conditions, and note any tensile, yield, or impact requirements and where they should be measured (surface vs. core). For case-hardened parts, define effective case depth and the measurement basis along with any surface finish or grind-stock expectations.
If dimensional stability is critical, state the project’s allowable distortion and whether martempering, austempering, or induction hardening is preferred.
When surface integrity is key (ie. for gears, or bearings) include decarb limits and inspection methods.
For safety-critical applications, call out nondestructive testing and any certification or furnace uniformity documentation you require.
In our experience, most heat-treat headaches can be traced to mismatches between geometry, hardenability, and quench severity. Sharp corners and sudden section changes concentrate stress; generous radii and uniform walls behave better. We recommend working with an experienced steel supplier to choose the mildest quench possible to achieve the required target while limiting distortion and cracking.
Source Heat-Treated Steel from a Dependable Supply Chain Partner
Heat treatment is a powerful tool for aligning steel properties with the demands of real-world parts. Getting reliable results depends on selecting the appropriate process for the grade and geometry, controlling key variables, and working with a trusted partner.
The information presented here is intended as a general instructional guide and is not comprehensive or definitive.
For assistance in selecting the right steel grade, heat-treat process, and property targets for your application, please contact Jade Sterling Steel.
