Product category:
Bearings
News Release from: NSK UK | Subject: Super-TF bearing material
Edited by the Engineeringtalk Editorial
Team on 07 May 2001
Big advantages in bearings for steel
industry
Super-TF bearing material greatly improves the life of bearings used in all industrial sectors including steel making and can be applied to a wide range of bearing types
Increasing pressure to reduce the cost of steel means that failures in the continuous production cycle are very expensive Bearings, of course, are fundamental components in this cycle
This article was originally published on Engineeringtalk on 4 Jul 2008 at 8.00am (UK)
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Therefore, despite the many operating difficulties, bearing manufacturers are constantly under pressure to provide ever-increasing levels of reliability and operating life from their products.
NSK is a major supplier of rolling element bearings to the steel industry.
The company has studied the many operating problems of bearings in this industry and through an extensive research programme have made improvements in bearing materials.
One such improved bearing material is Super-TF, which greatly improves the life of bearings used in all industrial sectors including steel making.
For many years, NSK has been engaged in the development of long life bearing technology which includes the development of new materials.
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This development has been based on the results of investigations carried out on bearings used in the field.
As a result, it was discovered that the fatigue mechanisms that occur in bearings depend on the application and operating condition.
This means it is necessary to investigate the fatigue type that takes place in each application and chose the appropriate material to combat it.
NSK has investigated the types of fatigue in several applications and developed enhanced materials for each application.
For example, in applications were the lubrication conditions are relatively good, the fatigue type is subsurface originated.
In this case, the fatigue life is mainly related to the steel cleanness and therefore an improvement in steel cleanness is beneficial.
NSK has investigated the effect of the various types of non-metallic inclusions on fatigue life and clarified that oxides above a certain size have the most detrimental effect.
The traditional methods of determining the inclusion content of steel, based on visual assessment, are not adequate for today's bearing steels.
Therefore, NSK have developed a method that uses image analysis equipment together with software developed in-house, to give a better indication of the inclusion content of a steel.
When we investigate the inclusion rating for three different casts of the same bearing steel obtained using the traditional ASTM method and the NSK method, the ASTM method infers that the inclusion content of cast C is less than casts A and B.
Using the NSK method, however, shows that cast C actually has the highest oxide inclusion content and that cast A is the cleanness.
Using this method NSK has developed a steel whose oxide size and distribution is controlled for applications were subsurface originated type fatigue is the main cause of failure.
On the other hand, in applications were the lubrication conditions are poor, for example, contaminated lubricant or boundary lubrication conditions, the main cause of failure is from surface originated type fatigue.
In the case of this type of fatigue, defects produced in the surface as a result of the poor lubrication act as fatigue initiation sites.
Surface originated type fatigue is the most common type that occurs in bearings used in the steel industry.
With regard to surface originated type fatigue, NSK has investigated what material factors have an effect on this type of fatigue.
The investigation revealed that changes to a material, such as composition and microstructure, can significantly improve its resistance to surface originated type fatigue.
As mentioned earlier, in the case of bearings used in the steel making industry the lubrication conditions are often poor resulting in surface originated type fatigue.
To combat surface originated type fatigue Super-TF bearings have been developed by NSK.
These bearings are manufactured using a new bearing steel and new heat treatment technology.
As part of the development of a new material resistant to surface originated type fatigue, NSK investigated the fatigue mechanism.
In most cases, the failure was caused by the stress concentration generated around a surface defect such as a dent.
Stress is concentrated around these dents and eventually leads to cracking and to surface-originating flaking.
NSK examined what kind of material factors minimise the stress concentration around surface defects.
The examination revealed that high levels of retained austenite is an extremely effective means of maximising the r/c value around surface dents in the bearing material.
TF technology includes a unique heat treatment process developed by NSK to optimise the level of retained austenite in bearing materials.
Austenite itself, however, has a soft structure and reduces the hardness of the bearing material.
To maintain the high hardness required of a bearing material, a material was developed containing appropriate quantities of chromium and molybdenum.
These promote the formation of a uniform distribution and finer carbide and carbonitride particles in the bearing material thereby maintaining the high hardness required.
In addition to maintaining the required hardness, the uniform distribution of fine carbide and carbonitride particles improves the materials resistance to wear and seizure.
In a Sawin-type wear test, showing the degree of wear and the seizure limit for different types of bearing material, the results revealed that Super-TF material has superior wear resistance to both SUJ2 and TF material.
Super-TF bearings are also 40% more resistant to seizure than SUJ2.
In comparison with bearing made of conventional carburised steel, Super-TF bearings offer extended life under both contaminated lubrication conditions and boundary lubrication condition.
The results of service life tests conducted under contaminated lubrication conditions using taper roller bearings show the service life (L10) of a Super-TF bearing is up to 10 times that of the ordinary carburised bearing.
A further benefit of Super-TF material is its superior service life at high temperatures compared to standard SUJ2 bearing material.
Fig.
10 shows the results of 6206 type ball bearing tests carried out under clean lubrication conditions at 160C.
The results show that Super-TF bearings (heat resistant specification) have approximately 4 times the service life of equivalent SUJ2 bearings.
Super-TF technology can be applied to a wide range of bearing types such as, cylindrical roller bearings, spherical roller bearings, tapered roller bearings, deep groove ball bearing and angular contact ball bearings.
Today Super-TF bearings are widely used and highly appreciated in a variety of applications where the lubrication conditions are poor, including the steel making industry. Request a free brochure from NSK UK ...
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