SFS Self-drilling wood building screws St znb WFD-TH17 12x400

Case hardening is used to impart significantly greater hardness to the surface layer of steel workpieces and to improve their mechanical properties. Case-hardened workpieces are characterised by increased wear resistance, a tough core and improved fatigue strength.

Blue galvanised finishes are produced either by electroplating in accordance with DIN EN ISO 4042 or by mechanical plating in accordance with DIN EN ISO 12683. For fasteners with a hardness exceeding 390 HV, there is a risk of hydrogen embrittlement during galvanising (electrolytic process). This necessitates specific measures (such as annealing), although these do not provide a 100 per cent guarantee against embrittlement.

The wrench sizes for hexagon bolts and nuts are specified in ISO 272: 1982-01. The relationship between the nominal diameter d of the thread and the wrench size s is standardised in EN ISO 4014 and EN ISO 4017 for hexagon bolts, and in EN ISO 4032 for hexagon nuts (dimensions in mm). The additional internal hexagon socket is used for re-use or in confined spaces.