Steel – powder metallurgyTags : advanced-materials
When we talk of powder metallurgy methods of manufacture, there are two almost unconnected methods. The first, which is used to manufacture near net-shape components, involves filling a mould with powdered metal and sintering it together under pressure to produce a pre-form that may need very little machining before the part can be used in an engine. Some production con rods are produced by this technique.
However, for the purposes of this article, I want to discuss the powder metallurgy method of producing very high quality wrought-steel alloys. Such alloys are now becoming much more widespread, and can offer significant advantages over conventionally manufactured alloys, albeit at a cost penalty. The production method was discussed in the recent article on metals in Race Engine Technology magazine, and in a RET-Monitor article in 2012. We will look here at the production process in more detail, hopefully giving the reader a better understanding of why the steels might offer an improvement, and why the materials are so much more expensive.
As mentioned in the previous RET-Monitor article, powder metallurgy steels offer improved fatigue and fracture properties for the same composition compared to conventionally manufactured steels. Another stated benefit is that they allow more highly alloyed materials to be produced than with conventional production methods.
The material from which the powder is produced is first melted under vacuum. This is done to prevent oxidation of the surface and the danger of oxide contamination spoiling the product. There may be an ‘extended process’ where any impurities are removed through solidification and re-melting; many of the conventionally manufactured steels of very high quality are re-melted products. Where we see the acronyms VIM, VAR, VIM-VAR and ESR applied to steels, these are re-melted products. Such techniques produce clean steels that exhibit better durability compared steels that are not re-melted and of the same nominal composition. In previous ‘Focus’ articles on specific components in Race Engine Technology magazine, suppliers of very highly stressed steel components will often specify single or double re-melted steels because of their improved fatigue strength.
Once the material from which the powder is to be produced is selected, atomisation takes place by melting the material and passing it into high-velocity jets of nitrogen, which break up the stream of molten metal into very small spherical particles that solidify very quickly. The advantage of such rapid solidification is that each particle thus produced is very uniform, with very fine precipitated carbides. Where cooling is slower, the carbides can tend to segregate into larger carbides rather than being finely and evenly dispersed.
The collected powder is put into steel ‘cans’ which are then evacuated and sealed, commonly by welding. The sealed containers are then placed into a furnace where hot isostatic processing (HIP) takes place. You may have heard of the HIP process in relation to castings, where the process results in better quality and stronger castings owing to a lower level of porosity. In the powder metallurgy process, HIP compacts, joins and densifies the powder into a solid, fully dense ‘compact’.
Once the HIP process is complete, the can is removed and the semi-finished material is processed by conventional means such as forging, rolling and drawing. The cost of the process means that most steels produced by this method are high-value tool steels. The technique is also used for superalloys.
Written by Wayne Ward