Wheelabrator
Safety & Performance: Shot Peening for Rail
This article first appeared in the Railway-News magazine Issue 5 2021.
Rail operators know metal fatigue is dangerous.
The disaster at Eschede, Germany, in 1998, caused by a single broken train wheel, killed over 100 passengers. And that was just one incident in a string of serious accidents due to fatigue failure since the dawn of railway travel.
To improve the fatigue performance and other characteristics of metal components, the rail industry has relied on peening for over a century. The industry has two major peening applications: first, as an anti-fatigue measure for critical parts like wheels and, second, to make springs “spring”, increase their operating life and improve their fatigue strength too.
But first: what is peening?
Peening for Performance and Safety
Peening is the process of working a metal’s surface to improve its material properties. Simply hitting metal with a hammer (cold working) causes plastic deformation and induces a residual compressive stress in the peened surface.
That compressive stress makes crack formation less likely and delays some types of corrosion. Peening also hardens a metal’s surface layer, further reducing the likelihood of crack formation and increasing abrasion resistance too.
However, manual peening is a slow, laborious affair. Shot peening is far faster, with each spherical shot acting as a miniature ball-peen hammer. Shot is accelerated towards the workpiece surface using airblast (driven by compressed air) or wheel blast (shot is introduced into the centre of a spinning wheel and flung out by centrifugal force) technology.
Airblast, Wheel Blast, Process Control
Airblast machines are more suitable for peening smaller quantities of components; manual airblast control is common but automation is possible too. Wheel blasting is ideal for a fully automated process where blasting intensity must be set and maintained to match the desired Almen value and component coverage.
