Heat Treatment & Hardening Explained

Hardening is a heat-treatment process that uses controlled heating and cooling to make steel harder and more wear-resistant — without changing the part’s shape.

The key is the steel’s microstructure. Heat it to a certain temperature and cool it rapidly, and the iron atoms rearrange into a very hard phase called martensite — the source of hardness.

For small parts like screws, nuts and bolts, hardening keeps threads from stripping, heads from cracking, and lets fasteners reach their rated torque (e.g. grade 8.8, 10.9, 12.9).

Steel is iron with carbon. That carbon is what makes hardening possible. Steel that is too low in carbon (below ~0.25%) cannot be through-hardened and needs case hardening instead.

Heated to 800–900°C, steel becomes austenite, which dissolves a lot of carbon. Quench it quickly and the carbon is trapped, distorting the structure into hard, brittle martensite.

More carbon means higher peak hardness — but also more brittleness, which is why a tempering step always follows.

Standard hardening has three steps:

• 1) Austenitizing — heat the part to its austenitizing temperature and soak it through.
• 2) Quenching — cool rapidly in oil, water or polymer to form martensite. At this point it is hardest but very brittle.
• 3) Tempering — reheat to 150–650°C to cut brittleness and restore toughness, trading a little hardness for a usable balance.

The tempering temperature sets final hardness: low temper = very hard, less tough (wear parts); high temper = tougher (impact parts).

• Through hardening — hard all the way through; for medium/high-carbon steel like S45C, SCM435; used on bolts and springs.
• Case hardening — hard surface, tough core; for low-carbon steel (see the Carburizing/Nitriding article).
• Induction hardening — electromagnetic heating of specific zones such as screw tips and localized pin zones.
• Tempering / annealing / normalizing — processes to adjust toughness, relieve stress or refine grain.

High-count lots of screws, nuts and bolts bring their own challenges: hardness consistency across every piece, distortion of thin parts, and preventing decarburization during heating.

V.S. Heat Treatment runs a gas-atmosphere mesh-belt furnace where small parts flow continuously for even heating — ideal for high volumes — and random-samples Vickers (HV) hardness on every lot to confirm all pieces fall inside the customer’s window.