Ythärdning

Yt-härdning, often written ythärdning or yt-härdning, is a Swedish term for surface hardening. It denotes a family of heat-treatment processes that harden only the outer surface of a metal part while keeping the core relatively soft and tough. The goal is to improve wear resistance and fatigue strength at the surface while limiting distortion and preserving core ductility.

Methods used for yt-härdning include diffusion-based treatments such as carburizing, carbonitriding, and nitriding, which introduce carbon and/or nitrogen into the surface at elevated temperatures and are followed by quenching or tempering. Non-diffusion methods, like induction hardening and flame hardening, heat the surface rapidly and then quench, producing a hardened shell with a softer core. Case depth and surface hardness vary with process parameters, alloy content, and subsequent tempering.

Materials commonly treated are low- to medium-carbon steels and certain alloy steels; some stainless steels can also be surface hardened with specialized processes. The resulting microstructure features a hard outer layer and a tougher, more ductile core. Surface hardness is typically reported in hardness scales such as Rockwell (HRC), with case depths ranging from fractions of a millimetre to a few millimetres, depending on the method and component requirements.

Applications include gears, camshafts, shafts, bearings, and other wear-prone components in automotive, industrial, and mechanical systems. Advantages of yt-härdning include increased wear resistance, improved fatigue life, and localized hardness without compromising core toughness. Limitations include limits on achievable case depth, potential residual stresses, and distortion if processing is not carefully controlled.

Quality control involves hardness testing across the surface and, where needed, cross-sectional metallography to verify case depth and microstructure.