Why are valves required to be ductile?
When you visit a large motorsport trade show, as I recently did, there will be a number of engine component suppliers present, some of which will be selling high-quality valvetrain components. In the display cabinets ranged before you, among other things you are likely to see valves of all sizes, shapes and materials, there for you to examine and discuss with the various specialists manning the stands.
Among these exhibits, there is sometimes a single, horribly distorted part, proudly displayed. What possible use can this be? One of the primary requirements for a valve to operate in a satisfactory manner is for it to be very straight. Straightness tolerances are measured in microns over the length of the valve, yet these specimens are severely twisted.
The reason a valve manufacturer or supplier wants you to see the twisted valve is to prove to you that their valves are possessed of a large measure of ductility - in other words, the valve will behave plastically beyond yield. This is of little consequence in normal operation, but can be a real saviour in the event that the unexpected happens.
There are many reasons why a valve might see loads that are far different in terms of magnitude and direction from those we can expect in normal service. Normal operating loads come from the valve being opened (stem compression) and closing - tension in the stem and seating loads, which may be compressive, tensile or bending depending on the location in the component. There are also some loads due to friction in the valve stem. However, none of these require a large degree of ductility.
When something goes really wrong - for example, the inlet swallowing something it shouldn't have, such as a screw in the inlet becoming loose and finding its way down the inlet port - valve loading cannot be predicted. Such debris can come between the valve and its seat as the valve is closing, or it can clatter past the valve and into the combustion chamber. It will inevitably lead to huge bending loads in the valve, causing plastic deformation.
If the valve head then breaks off, the problems are compounded, and there is then a greater chance that the other valves will fail and that one of the pieces will subsequently puncture and pass through the piston. Where materials have little plastic deformation beyond yield before they fail, we commonly term these to be brittle, and overloading will cause them to fail by rupture.
If the valves are ductile and remain in one piece - albeit twisted - the damage to the engine can be limited, and perhaps restricted to replacing the cylinder head in question and a cylinder liner. Where the valve head becomes detached, our next line of defence is the piston, but if this is breached and the valve head reaches the bottom end of the engine, the engine can effectively be scrapped, requiring the replacement of major structural components.
Fig. 1 - The damage to the engine would probably have been much worse if these valves weren't so ductile.
Written by Wayne Ward