Valve springs – surface treatments
Valve springs are often the most highly stressed parts in any race engine. While some types of race engine don’t have valve springs – two-stroke, Wankel, four-stroke desmodromic or four-stroke with pneumatic valve return – most of the race engines we encounter will be equipped with some form of wire spring. For modern engines valve springs will generally be the familiar helical spring.
In their unending quest to make the engine perform better through improved breathing, development engineers will try to open and close the valves in an ever more aggressive fashion, and will want to achieve this with the smallest and lightest spring. The stresses in the springs are high as a result, and without careful surface engineering they would have a very short life.
The spring wire itself is very special. It has extremely high strength compared to any ‘normal’ specification of steel, and special attention is paid to the cleanliness of the material. In order to improve the fatigue life of the material, there is an advantage in placing its surface in a state of compressive stress. As the stresses are highest at the surface, any residual compressive stress before service loads are applied effectively reduces the stresses in the component when in use. For example, if there is 500 MPa of compressive stress before loading, and the service load results in a stress of 1800 MPa, the algebraic sum of these is 1300 MPa. While compressive residual stresses don’t reduce the stress amplitude, however, they can beneficially offset the mean stress significantly.
The two main surface treatment processes used in the manufacture of racing valve springs are nitriding and shot-peening. Nitriding is a surface hardening process during which nitrogen is diffused into the surface of the steel. The depth of nitriding on a valve spring will be small compared to that on a larger component such as a crankshaft; while it is possible to nitride small sections completely, we want to maintain a tough, ductile core. Nitriding is known to impart significant residual stress, and the process can be tailored to increase the level of that stress.
The process takes place between 400 and 600 C; the higher the temperature, the greater the rate of diffusion of nitrogen into the surface. The temptation would be to use a higher temperature to shorten the process time, but this is not without penalty. Nitriding can lead to softening of the core material, so some valve spring steels have been developed especially to resist the softening effect of nitriding temperatures.
Shot-peening is a well known technique for increasing the fatigue strength of many components. For the highest specification of valve springs, multi-stage peening is employed using different sized media and peening intensities. The aim here is not only to ensure a high level of compressive residual stress but to make sure that the maximum occurs at or very close to the surface while also having a useful depth of compressive residual stress.
Where both nitriding and shot-peening are used, the nitriding process is done first, as the nitriding temperatures would relieve the stresses introduced through peening. Another advantage of peening after nitriding is that the amount of residual compressive stress due to peening is increased, as the substrate is harder and stronger. According to Suda and Ibaraki*, a combined nitrided and shot-peened material can have a 40% higher fatigue strength compared to the non-nitrided and non-peened state.
* Suda, S. and Ibaraki, N., “The Past and Future of High-strength Steel for Valve Springs”, Kobelco Technology Review, December 2005
Written by Wayne Ward