01.20: Preloading of stainless steel bolting assemblies

Abstract

ABSTRACTIn modern steel constructions, the use of stainless steel becomes more important, especially in corrosive environments. But: the use of stainless steel bolting assemblies is restricted to non preloaded assemblies by EN 1090‐2 due to several reasons. Stainless steel tends to creep and relaxation which might result in severe preload losses in bolting assemblies. In addition, the susceptibility of stainless steel to galling respectively cold welding causes problems in bolting assemblies because of undefined coefficients of friction. Herewith, qualification procedures are necessary for every possible application scenario. Within the European RFCS‐research project “Execution and reliability of slip resistant connections for steel structures using CS and SS” (SIROCO), stainless steel components and stainless steel bolting assemblies made of austenitic, super duplex, duplex and lean duplex materials have been investigated in tightening tests according to EN ISO 16047 and EN 14399‐2. Furthermore, relaxations tests of bolted connections with bolts according to EN ISO 4014 and EN ISO 4017 have been carried out. The presented paper intends to provide an initial insight into the behaviour of stainless steel bolts based on first results achieved from SIROCO. Regarding the tightening tests, in a first step the evaluation was chosen similar to EN 14399‐2 which specifies essential properties and characteristics of carbon steel bolting assemblies for preloading which should be fulfilled for tightening. The objective of the investigations will be to define a secure preload level and a corresponding tightening procedure for preloaded stainless steel bolting assemblies. The tightening tests are compared with results of the well‐known carbon steel preloading systems HR according to EN 14399‐3 and HV according to EN 14399‐4. Within this contribution, the so far achieved results of the tightening and relaxation tests will be summarized and discussed. For now, the susceptibility to galling and the tendency to creep and relaxation can be confirmed even though galling can be significantly reduced by the use of suitable lubricants which could be determined by experimental testing. Furthermore, secure preload levels could be defined depending on the stainless steel grade and strength class.