ABSTRACT

High-strength structural bolting assemblies of the HV system according to EN 14399–4 and EN 14399–6 and DASt-Richtlinie 021 with large diameters up to M72 are increasingly used as preloaded bolting assemblies in highly loaded steel structures. In addition to static and fatigue loads, the connections are exposed to low temperatures. To close the lack of knowledge on low temperature behaviour of these bolts, systematic investigations were carried out to determine the tendency of brittle fracture of high-strength bolts with large diameters. This paper presents the main results of the experimental investigations on the fracture toughness behaviour of the bolts material.

INTRODUCTION

High-strength structural bolting assemblies of the HV system according to EN 14399–4 and EN 14399–6 or DASt-Richtline 021, respectively, with large diameters up to M72 are increasingly used as preloaded bolting assemblies in highly loaded steel structures. In some of these structures, e. g. on- and offshore wind towers, the connections are exposed to low temperatures additionally to the regular static and fatigue loads. It is well known that the risk of brittle fracture at low temperatures rises with increasing thickness of the steel material. In principle, this relation also applies to bolts of large diameters. So far, no investigations on the low temperature behaviour of these bolts exist and the choice of steel material for high strength bolts is not covered by EN 1993–1-10. As a precaution, high strength bolts of large diameters are typically made of higher alloyed steels without knowing whether the higher alloy of the bolt base material is sufficient.

Usually, high-strength bolts according to EN 14399–4 are made of quenched and tempered steel 32CrB4 (1.7076) or 36CrB4 (1.7077) according to EN 10263–4, respectively. For bolts with diameters greater than M39, rod material 34CrNiMo6 (1.6582) or 30CrNiMo8 (1.6580) according to EN 10083–3 is used. EN 10263–4 does not specify any minimum Charpy impact toughness values for base materials 32CrB4 and 36CrB4 while the required minimum values according to EN 10083–3 are 45 J for 34CrNiMo6, regardless the diameter, and between 30 J and 45 J for 30CrNiMo8 depending on the diameter, all values determined at room temperature.

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