DIY Metallurgy: Making your own metals and alloys

For many, Formula One is the pinnacle. Not only does it attract some of the best drivers in the world, but its engineers are widely regarded as some of today's most motivated and talented.

In an industry where the day job is to push back boundaries, it is unsurprising that many of the materials used are a long way from what you might call, 'standard'.

The desire for something better than commercially available materials is what first prompted transmission specialists Xtrac to develop its own unique set of steels. The company which has supplied transmissions to Formula One teams and other classes of motorsport for over two decades made the bold move to do it themselves and develop steels tailored for its specific requirements.

"From the early 1990s Xtrac has been producing its own unique recipes of specialist steel," says Martin Halley, chief engineer at Xtrac. "Components require wear resistance, differing load bearing capacities and toughness, and as transmission technology has developed so has the need for dedicated materials."

Xtrac teamed up with Corus Engineering Steels (now Tata Steel) to develop high strength steel for its transmissions. The materials needed to combine high strength to cope with the extremely high operating speeds, yet be the minimum weight possible. And as engines have got more powerful, so too has the demand on the transmissions.

Individual gear teeth were coming up against increasingly high contact pressures from the torque of engines. Compared with the 1980s where gearboxes were coupled with engines typically rated at 500bhp at 11,000rpm and weighing 50kg, the teams saw new requirements to cope with higher bhp, at higher revs, but weighing less. Modern F1 engines produced over 900bhp and can reach over 18,000rpm, yet transmissions must target a weight of less than 40kg.

Xtrac conducted tests to analyse typical failure modes of gear teeth. It found that the teeth bending through plastic deformation of the material could be solved not only by re-designing tooth geometry but also by increasing the material's yield strength. Another concern with increasing demand was the impact of interconnecting teeth as the gears rotate.

However, increasing the toughness of the material and reducing the transient peak loads experienced on the teeth largely mitigates this effect. These, with more normal failure modes such as bending fatigue as well as rolling contact fatigue, can be overcome by increasing the yield stress, surface hardness and lubrication of the materials used. The combination of improved materials and smarter design now provide the gearboxes with their most important feature, reliability.

As gearboxes have to last multiple races under extreme conditions, Xtrac and Tata Steel developed processes to ensure the microstructure and properties were exactly what was required. The company purchases steel in 5 to 100 tonne yields which are then processed with the appropriate melting and re-melting route to formulate the steel in a highly repeatable manner.

The ability to better control all the variables throughout the vacuum melting process has led to step change improvements to the materials' properties. This has seen Xtrac crossover recently from using Air Melting as its main method to Vacuum Melting.

"Although traditional air melting routes have improved to create much cleaner materials, primary vacuum melting and secondary vacuum re-melting technologies have provided a stepped improvement in steel purity," explains Halley. "This is essential where components are designed with thin sections as any inclusion, however small, will increase the risk of failure."

This has led to the production of Xtrac's XM031 and XM033 Nickel-Chromium-Molybdenum based steel alloys. The XM031 steel is unique and has been developed for high temperature (<350°C), high performance, transmission and differential applications including gear ratios, dog rings, hubs, crown wheels and pinion. The similarly unique XM033 steel has been developed for ultra-high strength applications including drive shafts and output hubs.

For extreme motorsport applications Xtrac use Vacuum Induction Melting (VIM) and Vacuum Arc Re-melting (VAR) as a processing route. This double vacuum melting increases the chemical and mechanical homogeneity of the microstructure.

"The combination of high alloy content and thin component sections reduce the problems associated with hardenability," says Halley. "The range of Xtrac materials provides choices in bending fatigue, rolling contact fatigue, temper resistance - to permit high temperature coatings - and torsional strength for shafts. A trade off that we have to manage is that increasing tensile strength can result in less fracture toughness and therefore a case hardening heat treatment is employed."

What this has led to is bespoke, double vacuum melted materials becoming the standard process for the company, as it strives to increase strength while improving fatigue life. In addition, by optimising heat treatment, the inherent brittleness of high h­ardness steels can be minimised.

"The important point is to design within the increased elastic limit," says Halley.

The metallurgical developments of the XM0 range of steels belong, categorically, to Xtrac and are made for them exclusively by Tata Steel. The company understandably wants to maintain its competitive edge internationally, and it is unlikely to allow third party use of the materials.

Adrian Moore, technical director at Xtrac, concludes: "They are not available to anybody else, and never will be. It's one of Xtrac's unique selling propositions and I'm pretty sure we're the only transmission manufacturer to have developed its own unique steels. That's why our transmissions are sought after by the world's leading motorsport teams."

Benefits of a Vacuum Furnace
A vacuum furnace is a type of furnace that can heat materials, typically metals, to very high temperatures and carry out processes such as brazing, sintering and heat treatment with high consistency and low contamination. Heating metals to high temperatures normally causes rapid oxidation, which is undesirable. A vacuum furnace removes the oxygen and prevents this from happening.
In a vacuum furnace the product in the furnace is surrounded by a vacuum. The absence of air or other gases prevents heat transfer with the product through convection and removes a source of contamination.

Vacuum Induction Melting & Vacuum Arc Re-Melting
Vacuum induction melting (VIM) utilises electric currents to melt metal within a vacuum and is used extensively as a Primary Melting process. One of the only ways to induce a current within a conductor is through electromagnetic induction. Electromagnetic induction induces eddy currents within conductors by changing the magnetic field. Eddy currents create the heating effects to melt the metal. VIM is usually used for refining high purity metal and alloys. VIM is now becoming more popular due to the variety of uses now available.
Vacuum Arc Re-Melting (VAR) is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. For alloy steels the VAR process has become the preferred processing route.

Author
Justin Cunningham

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Can you recommend a company that can make small amounts of special alloys. For example Cooper 80%, Zinc 15%, and Lead 5%. Thank You!


Comment Cornelius Butler, 16/03/2018
Why would the transmission vendor for example, but also anyone seeking higher strength and lighter weight not switch to electrically heated stamp formed metallic glass?

Comment James McAllister, 27/12/2015
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