Some fastener manufacturers have indicated that the materials some body engineers describe as ‘hard’, they would call quite soft. The steels used are significantly stronger than mild or cold-rolled steel; the aim of using stronger steel is to make fastener products that are thinner and lighter, but with no loss of strength. However, harder metals can present their own problems and fastener manufacturers appear to be more concerned about chemical/metallurgical challenges such as hydrogen embrittlement phenomenon, compared to the producers of steels for BIW stamps and panels.
Hydrogen embrittlement phenomenon becomes something to watch out for in bolts with tensile strength higher than 1,000N/m2. Electroplating can introduce hydrogen at the atomic level and at the granular level, it wants to expand. The typical manifestation is that the head of the bolt drops off. Unpredictability is something over which there is no final control. The same bolt can be made under exactly the same conditions for years without failure and then, for no apparent reason, problems arise. Manufacturers are also seeking to minimise trouble by using a plating process that does not require acid cleaning or electroplating. If electroplating is used, then another method of overcoming the phenomenon is to bake after plating, at 200°C, which should drive out the hydrogen. However, there is no absolute guarantee; in a batch of 10,000 bolts, maybe 100 might suffer from embrittlement; but it cannot be forecasted. Recently, manufacturers and steel suppliers have been experimenting with higher-carbon steels and with additives including chrome and molybdenum. Higher-strength steels can be thinner for the same performance and there has been a trend towards the use of alternative joining methods. While spot-welding remains the primary means of joining, the higher the proportion of AHSS and UHSS, the greater the challenges. Higher contents of carbon and molybdenum, for example, lead to reduced ‘weldability’.
It is known that welds involving different materials do not necessarily work effectively, so there is a limit to the amount of additives that can be put in before the materials become impossible to weld. There has been a trend towards wider use of adhesives but there is a continuing and essential role for nuts and bolts. One problem with adhesives is that structures cannot easily be taken apart and put back together again. While roof panels, for example, can be stuck to side elements with adhesives, long-established nuts bolts are actually doing more than they did 20-30 years ago. Tightening regimes are more sophisticated and smaller bolts can be used for generating higher clamping forces; they can take more load, even though they are lighter.
The use of UHSS bolts has had a positive impact on weight reduction and improved safety. However, fasteners made of UHSS are more expensive than conventional products, which has had an effect on acceptance. On the other hand, if the objective was a rust-free, ultra-strong and lighter vehicle then, ideally, cars would be made entirely from stainless steel; however, the cost would be prohibitive. A stainless steel bolt costs about 10 times as much as its regular equivalent. The trend to increased use of UHSS is driven – at least in part – by such cost considerations. Lighter weight alternatives, such as aluminium, are difficult to get to the same strength levels as steel. Exotic alloys, including materials like titanium, are also hindered by cost. While the essential design of fasteners has not changed much, their usage has. UHSS has enabled reductions in size and weight, and continued development will see wider application in the future.