Heat Treatment of Fasteners: Through-Hardening, Case Hardening and Decarburization
A fastener's property class is a promise about microstructure — and microstructure is set in heat treatment. The furnace decisions determine whether a 12.9 marking is honest.
Two hardening philosophies
Through-hardening (quench and temper) treats the whole cross-section: the standard route for medium- and high-tensile screws, giving uniform strength with tempering tuned for toughness. Case hardening hardens only the surface of low-carbon steel, leaving a tough core — typical for small self-tapping screws where surface hardness drives thread-forming life and the core must not snap.
Tempering: the toughness dial
Quenching without tempering yields glass-brittle steel. Tempering temperature trades hardness for toughness — higher temper, lower strength, much better impact tolerance. Property classes fix the windows; running at the edge of one to chase hardness readings buys brittleness.
Decarburization: the hidden defect
- Furnace atmosphere that steals surface carbon leaves a soft or weak surface layer exactly where threads bear load.
- It is invisible without metallographic sectioning — which is why sampling plans must include them.
- Standards limit decarb depth per class; a合格 report checks it, not just hardness.
Topprecision heat-treats through controlled partner lines with atmosphere monitoring, verifies hardness and decarb per lot, and publishes process capability for temper-critical characteristics.
