P 787 – Welded threaded studs with a tensile strength of 800 MPa
Welded threaded studs have to consist of a weldable material because of the joining process. It is for this reason that currently only a few austenitic steels and carbon steels of strength class 4.8 acc. to ISO 898-1, with a maximum carbon content of C = 0.20%, are accredited for threaded studs.
Steel with a higher ultimate tensile strength normally indicates a maximum carbon content of C ≥ 0.20% and therefore these materials has a limited weldability. At the current market there is an existing demand for thread studs with a higher strength. In order to meet the resulting demand the manufacturer currently uses the cold headed steel 20MnB4 (1.5525) which approximately achieves the strength class 8.8 due to thermally post – tempering. For welding the 20MnB4 with a maximum carbon content of C = 0.23% is suitable to only a limited extent and due to additional costs for thermally posttempering processes the 20MnB4 stud has not prevailed in the market.
Another alternative material like the 20MnB4 out of the group of cold headed steels is the 8MnSi7 (1.5113). It has a maximum carbon content of C = 0.10% which suspects a good weldability. The ultimate tensile strength of 8MnSi7 wire rod is approximately between 520 and 620 MPa and only by cold forging it reaches a level of 800 MPa. The mechanical properties and the suspected good weldability would lead one to assume, that 8MnSi7 has the capability for an economic manufacturing of high strength threaded studs.
To verify the assumption the research project was initiated with two main objectives: verification of the weldability and classifying the studs made of 8MnSi7 acc. to the strength classes defined in ISO 898-1. To verify the weldability 91 axial tension tests with a statically load, using threaded studs with partial thread (PD) and reduced shaft (RD) with nominal diameters of M8 and M20 were performed. To avoid failure in the base material high strength steels with a minimum yielding strength of 680 MPa and a thickness between 2 and 40 mm were used.
The test results confirm the good welding properties of threaded studs made of 8MnSi7, but the ultimate tensile strength of the studs didn’t meet the minimum requirements for a classification as strength class 800 (8.8) for all stud types and diameters. Based on statistical evaluations of the test data acc. to DIN EN 1990 a required partial safety factor γ*M2 was determined in order to calculate the design resistance of the threaded studs based on nominal tensile strength of 800 MPa without a reduced safety level.
Now the usage of welded threaded studs made of 8MnSi7steel (1.5113) is possible. The corresponding design rules are developed and enable a safe an economical use.
The research project was carried out at Versuchsanstalt für Stahl, Holz und Steine of the Karlsruher Instituts für Technologie (KIT) and at Labor für Stahl- und Leichtmetallbau der Hochschule München, Fakultät Bauingenieurwesen. FOSTA has accompanied the research work and has organized the project funding from the Foundation for Steel Application Research.
Only available in german language.
D. Ungermann, S. Schneider