Speaker
Description
Fasteners are quite simple products but critical in most sectors. Bolt manufacturing is a large, well-established industry worldwide with a long history however, due to the low value nature of this product type, the production process has not been fully metallurgically optimised, increasing yield and production rates have instead been main areas of improvement. With modern analysis tools and understanding of recrystallisation, it is now much faster to implement these metallurgical principles into the simplest of parts that aid optimisation.
The nature of the bolt itself provides a complex distribution of temperature and strain with just the bolt head being heated and very little strain being imparted on the threaded section. This results in a barrelling/bulging type deformation of the bolt head during deformation, the strain pattern is dependent on the size of the head and the temperature at which deformation takes place (due to the differing constraints of the threaded section). Choosing an inappropriate temperature would result in very little strain in critical regions such as the shoulder and therefore result in a lower yield strength and therefore overall performance.
Through the combination of an instrumented power hammer and FE simulations, an understanding of the flow patterns of 316 stainless steel 20mm bolts is obtained. Variables such as temperature and deformation rate have allowed a systematic review of the spatial distribution of the grain growth and recrystallisation kinetics.
A series of mechanical tests, hardness mapping and microstructure examination were performed on a manufactured bolt. These results were then compared to the simulation and commercially available comparisons allowing for the bolt process to be optimised.