Cronidur 30

Citation

(2001), "Cronidur 30", Anti-Corrosion Methods and Materials, Vol. 48 No. 1. https://doi.org/10.1108/acmm.2001.12848aad.009

Cronidur 30

Cronidur 30

Keywords: FAG, The Barden Corporation, Steel, Bearings

Cronidur® 30, a new material employed in the manufacture of spindle bearings, was developed by FAG in co-operation with Bochum University and VSG Energie- und Schmiedetechnik Essen. The development of Cronidur 30 achieved recognition in the form of the Steel Innovation Award of the German Stahlinformationszentrum (steel information centre).

The steel is reported to have proved its reliability and toughness both in the extreme high temperature conditions of the main engines of the space-shuttle and in the low temperature conditions encountered on the flaps of commercial airlines. Its use in bearings is said to be a natural consequence of the market requirement for a material with performance that claims to exceed that of standard bearing steel SAE52100 (100Cr6), according to FAG, by accepting higher loads and higher temperatures, and also in terms of higher speed capability, enhanced corrosion resistance and longer life.

The steel is said to be the ideal bearing material to improve the performance and reduce the life cycle costs of today's machine tools. The new steel also claims substantial operational benefits in bearings used in vacuum pumps in the electronics industry, aerospace, canning applications, Formula 1 racing cars and medical applications.

We are informed that proof of the improved fatigue life and higher load capacity of Croniduro 30 has been supplied in comparative tests with standard 100Cr6 and M50 (a steel of considerable high-temperature strength, which is used in engine bearings) under EHD (elasto-hydro dynamic) conditions and with a contact pressure of 2,800MPa. The calculated fatigue life was exceeded by all the materials. However, the Cronidure 30 bearings reportedly had still not failed after 80 times the calculated life. FAG believes that in view of this Cronidur 30 can be assumed to be practically fail-safe at contact pressures below 2,800MPa.

In a given system this facility for higher permissible contact pressure means a significantly reduced susceptibility to unforeseen operating conditions. It also permits designers to assume higher loads for new designs.

Complementing the life and load characteristics of Cronidure 30 spindle bearings is their claimed capability of achieving much higher constant operating speeds. In tests, installed in the spindle of a machining centre, a grease-lubricated (70mm ID) Cronidure 30 bearing is reported to have run at a constant speed of 20,000rpm while exhibiting good even temperature behaviour.

The importance of the higher speed is thought to be that it provides machine tool builders, rebuilders and machine users with a relatively simple and low cost method of achieving greater performance from their machine spindles. Added to this, the even temperature operation at higher speeds claimed of the Cronidure 30 bearings provides opportunities for substantial cost savings on spindle systems, simply by changing from air-oil to grease lubrication.

In addition to its rolling bearing properties, Cronidur 30 also claims an excellent, resistance to corrosion. A measure of a material's corrosion resistance is its passive, current density value, in this respect Cronidur 30 is stated by FAG to be better, by about a factor of 100, than AISI 440C, the standard corrosion resistant bearing steel. With this property, and the material's good operating characteristics under starved lubrication conditions, Cronidur 30 can in many cases be used with other lubricants: liquid hydrogen, liquid oxygen and many others.

We also understand from FAG that Cronidur 30 has been used particularly successfully in pumps with highly inflammable liquids. The company reports that tests under conditions that were additionally aggravated for the Cronidur 30 bearings were terminated, after twice the life reached by 440C or 100Cr6 TDC-plated (thin dense chromium plated) bearings, when other pump components failed.

Croniduro 30 is described as a martensitic through-hardened steel that can also be induction case hardened. Its extremely fine grain structure is believed to minimise the risks of failure under mechanical loading and results in improved noise and vibration behaviour. In addition, the heat treatment process used with Cronidur 30 reportedly gives the material excellent hardness values, even at higher temperatures. Because of this, it is thought that bearings manufactured from Cronidure 30 can generally be used at higher temperatures than other bearings steels.

FAG informs us that the large carbides and carbide networks (80-100mm) which are a feature of the structure of AISI 440C, do not exist in Cronidure 30. Instead they report that there is a homogeneous structure of finely dispersed carbonitrides, usually less than 10gm in size. As a result, Cronidure 30 is thought not have the problem of large carbides concentrating at grain boundaries and providing potential "weak spots" which are susceptible to mechanical stressing and corrosion. This FAG believes is an exceptionally important factor because damage with spindle bearings almost always originates at the surface, whereas classical rolling bearing fatigue usually originates below the surface.

The contrasting micro-fine structure of Cronidure 30 is said to have a positive effect not only on component fatigue strength, but also in the micro-range of the Hertzian contact area. The corollary of this is that Cronidure 30 is said to be considerably less susceptible to plastic deformations resulting from indentations. In practice, dirt particles that penetrate into the bearings can leave indentations in the raceways, but these indentations do not propagate premature fatigue damage. Moreover, any bearing damage that does develop spreads more slowly than is the case with spindle bearings manufactured from standard bearing steel.

One of the major benefits of a material structure that minimises damage in this way is that, with few wear particles, there is hardly any contamination of the lubricant. Furthermore, as the few wear particles do not corrode there is no catalytic effect – both with grease and oil lubrication – so the quality of the lubricant remains at a constant level over a longer period.

Another major advantage claimed when specifying Cronidure 30 spindle bearings is their behaviour under conditions of mixed friction. Mixed friction is the result of bearings being subjected to repeated running and stopping (the operation of tool changes in machining centres is a very good example of this). It occurs if set transition speeds are exceeded and there is not a sufficient amount of lubrication in the contact areas. In tests carried out under a defined condition of mixed friction (po = 2,500Mpa) spindle bearings manufactured from Croniduro 30 are reported to have achieved a tenfold service life. Added to this, the wear behaviour of the bearing which is said to be considerably improved with Cronidure 30. This fact becomes particularly apparent in the case of hybrid bearings, using Cronidure 30 rings and ceramic balls, where the wear rate is considerably below the former usual values.

Further details are available from The Barden Corporation. Tel: +44 (0)1752 735555, Fax: +44 (0)1752 733481.