On the Contradiction of Applying Rolled Threads to Bolting Exposed to Hydrogen-Bearing Environments
Numerous industries continue to experience bolting failures as a re- sult of hydrogen stress cracking (HSC) when exposed to hydrogen- bearing environments such as seawater with cathodic protection (CP) or as a result of insufficient baking after plating operations. This paper describes the mistaken, but long-held, belief that be- cause rolled threads are beneficial for fatigue resistance, they are at best not injurious to the performance of bolting from other causes of failure such as environmental cracking.
The process for manufacturing external threads on bars of steel and other alloys is accomplished typically by grinding, machining cutting, or rolling of the threads. Historically, the benefit of thread rolling on fatigue resistance of threaded fasteners has been demonstrated in practice and from laboratory testing. This benefit has become so second nature that most standards and practices recommend or simply expect that rolled threads will be applied to fasteners for fatigue resistance in service with no apparent downside. However, recent failures of bolting in many applications have highlighted the lack of understanding of and the disadvantages of rolled threads when environmental cracking is a factor and the risk of failure increases significantly compared with that of machined threads.
This paper presents an examination of the effect of rolled threads on the cracking susceptibility of bolting from hydrogen cracking as a result of plating operations and external sources of hydrogen, such as CP. Only external threaded fasteners are considered in this paper.
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20 April 2018