Stainless steel thread forming screws and their susceptibility to stress corrosion cracking

Abstract

Thread forming fasteners incorporate both drilling and hole tapping features and are commonly used in the construction industry to fix together steel sheets of different material types. With this practical application in mind, fasteners manufactured from martensitic and austenitic stainless steels have been subjected to alternating corrosion conditions in accordance with test standards DIN 50021-SS and DIN 50018-K WF 2·0. The torque applied to the screws during these tests was controlled to place the fasteners under equal tensile loads, independent of their tensile strength. Thus, the results provided information on their relative susceptibilities to stress corrosion cracking. At the end of the tests, up to 80% of the martensitic stainless steel drilling and tapping screws had failed due to hydrogen induced stress corrosion cracking. The fasteners manufactured from austenitic materials withstood identical test conditions without any evidence of cracking or crack initiation. It is concluded that fasteners manufactured from modified martensitic stainless steel are more susceptible to stress corrosion cracking under the conditions of test than those made from cold worked austenitic stainless steels. This suggests that in practical applications the potential for catastrophic failure due to stress corrosion cracking could be considerably higher in modified martensitic fasteners in comparison with austenitic stainless steel fasteners, including those with hardened carbon steel drill points.