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Powder metallurgy steels

Steels make up a large part of a typical race engine: while most race engines have the main castings made of aluminium or cast iron, steel remains the favourite for a number of other components, notably camshafts and crankshafts. While cast iron can be used for both of these components, for highly developed, minimum-mass, high-stress applications we invariably find that steels are used. The piston pins in most race engines are still made from steel too.

Despite the fact that steel-making is a very mature technology, steel materials still enjoy a lot of r&d. One technology leading to better performing steels is that of powder metallurgy. There are a number of reasons why it is being used in steel production. One is that it allows materials to be formulated which cannot easily be made using more conventional ingot metallurgy methods. However, a number of powder metallurgy tool steels are made to existing standards and compositions. So, why would a steel company go to the trouble and expense of melting a good piece of tool steel, only to reform it as a powder metallurgy material?

Producers of powder metallurgy steels say the microstructure of the steel is far more consistent, and that the steels produced by this method are more ductile and less prone to distortion during heat treatment, even compared to steels of the same composition produced conventionally.

The method by which powder metallurgy steels are made follows five steps:

" Production of the powder from a melt
" Mixing and blending of powders, where applicable
" 'Canning' of the powder, by filling a steel box with powder and welding it shut
" Consolidation under high pressure and temperature in a hot-isostatic pressure process, and
" Processing of the billet thus produced

The production of the powder is a critical step, and is the subject of much development itself. The production of ever-finer powder stock has its own benefits - it stands to reason that the size of the powdered material limits the size of any non-metallic inclusions or defects. Defect size affects the fracture and fatigue behaviour of materials: those produced from finer powder have a fundamentally better chance of being more durable than those made from a coarser one.

The concept of producing very highly alloyed materials by powder metallurgy methods is attractive. The trends in properties predicted by adding certain elements is limited by the effect of carbide coarsening and segregation in conventional ingot metallurgy materials.

In a study of the high-cycle fatigue behaviour of conventional and powder-metallurgy steels*, fatigue-crack initiation for ingot steels was found to begin at large carbide particles or clusters of carbides, whereas for powder metallurgy grades, fatigue cracks were initiated at non-metallic inclusions. The report notes that the powder metallurgy materials have significantly higher fatigue strengths than their ingot metallurgy counterparts. Powder metallurgy methods mean that the dispersion of carbides within the material is very even, avoiding carbide segregation and coarsening.

These materials would seem to offer the possibility for lower-mass components in motor racing, where equivalent powder metallurgy grades for existing materials are available, and the chance to produce materials that are not possible by ingot metallurgy methods.

* Danninger,H., Sohar, C., Gierl, C., Betzwar-Kotas, A., Weiss, B., "Gigacycle Fatigue Response of PM versus Ingot Metallurgy Tool Steels", Materials Science Forum, vol 672

Written by Wayne Ward

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