High Performance Gearbox Steels

This article was first published in 2010.

This article is based on a presentation to the Aerospace Industries Division of the UK’s Institution of Mechanical Engineers by Xtrac’s chief metallurgist, Steve Vanes. It is used here with permission.

Motorsport, and in particular Formula One, has always made use of new technology. In the early part of this decade the emphasis was on reducing weight and increasing performance, requiring the transmission of greater power through thinner or smaller components. At that time, the typical life of a gearbox mainshaft gear may only have been ~350 thousand cycles.

In recent years, however, the technical regulations have gone through major changes, with many aimed at lowering the cost of racing. Achieving extended life (and so lower costs) has meant the standardisation of gearbox design, and introducing minimum face width and weight requirements for gear ratios.

For 2008/9, transmission components were required to achieve a four-race life, which was quite a considerable jump in terms of life expectancy from earlier in the decade. For the future (2011 onwards) the requirement is likely to be extended still further to six races, which will equate to a life of more than 2.5 million cycles.

This article aims to review some of the developments in gear steels introduced into Formula One and sports car transmissions over the last decade, and others which are anticipated to be adopted in the future.

Development of Motorsport Steels

From the early 1990s the primary gear steel used in motorsport applications was EN36C (832M31), and designs and heat treatment followed those typically applied to aerospace components.

Towards the end of the 90’s, the focus in Formula One transmissions was centred largely on increasing power and reducing weight. It was the effect of these running conditions that caused bending fatigue to become the over-riding material limiting factor. Typical failures resulted from the initiation of fatigue cracks in the fillet radius of gear tooth roots, often leading to the loss of one or a number of gear teeth, as shown here. The focus of activity was therefore on how to increase bending fatigue resistance.

As well as the steel alloy’s chemistry being a performance variable, so also is the heat treatment. Virtually all transmission components are case hardened (by carburising), so this in turn offers a number of other variables such as case depth and surface hardness.

To be able to quantify the effects that material and heat treatment (and their interrelationship) have, a means of evaluating properties was required.

Tensile test data, although used extensively in the past for design, was considered of little practical importance when trying to predict and evaluate component life and performance. Consequently, at Xtrac the basic evaluation tool that has been used is a cantilever bend test, which allowed the elastic and plastic properties of a material and its heat treatment to be evaluated.

Initially, we established baseline performance for mechanical properties and how these may be influenced by case hardening depth, surface and core hardness. This included characteristics of yield strength, fracture strength and strain hardening.

From these evaluations it was possible to ascertain how alloy type and heat treatment affected mechanical properties. Once promising configurations had been identified, a four square rig test was used to evaluate the bending fatigue, rolling contact fatigue and performance of lubricated test gear components subjected to environments similar to those experienced in service.

Gear Steels – Old and New

Over the last 100 years, engineering steels have been extensively developed and researched, creating a wide and varied range of specifications. However a relative few have found favour in motorsport applications and a list of the most notable is shown below.

Material Type	Description	Applications	Comments
En36C (VAR) 14NiCrMo13	3%NiCrMo case hardening steel	Gear, shaft and bearings	Low core strength (UTS:1350MPa) Low tempering temperature 140/160 degrees C
S156 (VAR) 16NiCrMo17	4¼%NiCrMo case hardening steel	Gear and bearings	Low core strength (UTS:1380MPa) Low tempering temperature 160/180 degrees C
20NiCrMo13 (VAR)	3%NiCrMo case hardening steel (increased core strength)	Gears and shafts	Higher core strength achieved by increased core carbon content. UTS: 1450 MPa Low tempering temperature ~150 degrees C
35NiCrMo16 (VAR)	4%NiCrMo alloy steel	Shafts and gears	High core strength (1950MPa) Low tempering temperature ~150 degrees C
S155 (VAR) 40NiSiCrMoV7	Ultra high strength alloy steel (Si modified 4340)	Cross shaft, driveshafts	High core strength (1930MPa) Tempering range 250/300 degrees C
Hy-Tuf* (VAR) 25NiSiMnMoCr7	High strength alloy steel	Gears and shafts	Tempering temperature 140/180 degrees C UTS: 1550MPa
40SiNiCrMoV10 (VIM/VAR)	Ultra high strength alloy steel	Driveshafts, highly stressed gears	Tempering temperature 300 degrees C UTS: 2050MPa

*Trade name of Crucible Steel

Essentially all these materials are used in the case hardened condition and cover a wide range of core hardness and strength. Interestingly, most of the materials listed above were developed without motorsport applications in mind!

Consequently, while many alloys were readily and commercially available, there was a need for custom materials designed principally for motorsport applications. In collaboration with Corus Engineering Steels (CES), this led to the development of the new Xtrac materials XM023, XM031 and XM033, as shown in the table below.

Material	Description	Applications	Comments
XM023 (VIM/VAR)	High strength alloy steel	Gearbox internals	Improved bending fatigue and bearing properties. Tempering 200/250 degrees C
XM031 (VIM/VAR)	High molybdenum alloy steel	Gearbox internals	Improved bearing properties Tempering temperature 350/400 degrees C
XM033 (VIM/VAR)	Ultra high strength alloy steel	Driveshafts, highly stressed gears	High core strength (2000MPa) Improved bearing properties Tempering 300/350 degrees C

The new steel XM023 included nominal alloy additions to provide the required bearing and bend strength properties. By optimising the heat treatment variables, case depth and surface hardness, an approximate 30% increase in bending fatigue life over En36C-VAR was achieved which helped to secure the Formula One drivers' and constructors' championships in 2005 and 2006.

As well as gear ratios, XM023 was also used on the majority of internals in the gearbox including layshaft, mainshaft, dog rings, hubs, selector forks, cross shaft and final drives.

Following the success of XM023, and also because of rule changes where endurance became increasingly more important, a second steel designated XM031 was developed which took advantage of the temper-resistant properties of molybdenum. This enabled the tempering temperature to be raised from ~220 degrees C to 350/400 degrees C while maintaining bearing hardness, thereby opening up greater alternatives with respect to physical vapour deposition coatings (including low friction and solid lubricants).

In addition, because molybdenum is also a strong carbide-forming element, greater rolling contact fatigue resistance and improved bearing properties can be developed. Bending fatigue resistance is also improved.

The third variant and latest alloy in the material range to be introduced is an ultra high strength (2000MPa) shaft and gear steel, which offers improved bearing properties (to comparable strength steels) due to its higher alloy content.

XM033 offers the advantage of high core strength for shafts, but also improved bearing properties for integrated rolling contact surfaces. Due to the balance of alloying elements, properties of surface hardness (>700Hv), core strength (2000MPa) and elevated tempering temperature (300 degrees C) means the alloy is ideally suited for arduous applications such as cross shafts, integral bearing drive-and clutch-shafts and highly stressed gears.

Properties are generated as a result of a nominal 0.4% core carbon content, an increased silicon level and the addition of vanadium. The result is a readily carburisable material able to simultaneously exhibit bearing properties and high core strength. This material is readily suited to driveshafts with integral bearings, now becoming popular in many motorsport applications.

Conclusion

A family of three new motorsport gear steels has been developed to meet the requirements for increased operating temperature and improved bending and rolling contact fatigue resistance relevant to gear, bearing and shaft component types. Unlike most other steels which have been historically used, these materials were specifically designed to accommodate the requirements encountered in current and foreseeable motorsport applications.