Demands on rolling bearing reliability and performance are continually increasing. Bearings are required to
last longer and transmit ever higher loads often under arduous conditions. This places similar pressure on steel manufacturers to ensure consistency and quality in the products delivered for bearing applications.Clean steel technology is the heart of bearing steel requirements and this has long traditions within SKF and Ovako Steel. In essence, high quality components such as bearings require high- quality raw materials. Bearing steel is made from raw materials that vary in composition and overall consistency.
Bearing steel is one of the most demanding of the high quality steel applications. Important criteria are cleanliness of the steel and the reduction of non-metallic inclusions – particles that could cause stresses and hence potential failure of the steel component. Through careful attention to these factors, coupled with the introduction of a number of key techniques including vacuum degassing, argon shrouding of molten steel and inductive stirring during secondary steel refining, the fatigue performance of engineering steels has improved progressively.
Steel-making development work is critically dependent upon the combination of process know-how and profound knowledge of the requirements of the application for which the steel is intended.
Final product property requirements, component-forming operations and the raw material production all need to be aligned and constantly addressed in product and productivity development work. Control of the entire production chain, from scrap selection to final product marketing activities, is an effective means of achieving this.
Steel making stages
Steel is made in three stages: melting, refining and teeming.
Molten steel is poured twice during processing. The first pouring takes place after melting, when the steel is transferred from the melting to the ladle furnace. This procedure is called tapping. After refining, the steel is poured into moulds. This procedure is called teeming, to distinguish it from the other type of pouring operation.
The ladle furnace technology which Ovako Steel uses today was invented and developed in Hällefors, Sweden, one of Ovako’s plants. However, “Bearing Quality” steel became synonymous with high-quality, clean steel much earlier than this with the application of the acid open hearth process used up until the late 70s. This iron ore based process was uniquely suited for the production of high-carbon low-alloy steels, and consistently gave high-quality steel.
Only when ladle furnace and vacuum degassing technology was well developed could electric arc melted steel compete in quality.
Today, high precision ladle metallurgy is the heart of all quality steel-making.
Melting is the process stage where raw material is converted to molten metal. During this process several important features of the bearing quality steel are set.
It is vital that the molten steel delivered in the secondary steel making stage is under full control and that factors which can be affected by scrap selection and oxidation during the melt down are properly set. After this point, many significant properties of the steel cannot be altered.
Refining sets quality
Refining is the processing stage which determines the factors which are most often associated with metallurgical quality. Refining is performed in a ladle furnace and incorporates all the steps required to create high-quality steel (fig. 1).
Basically, the steps involved are:
- alloying – where the steel is given the composition required by addition of certain alloying materials such as chromium, molybdenum and nickel
- deoxidation – where the oxygen always present after melting is removed
- desulphurisation – where excess sulphur is removed from the melt as desired
- vacuum treatment – to reduce as far as possible the amount of harmful, dissolved gases
- temperature adjustment – to provide the right conditions for the teeming of the steel.
During alloying, the chemical composition is set. All quality-sensitive component producers have specifications which set the maximum and minimum levels allowed for a specific alloying element. It has been possible to improve all the industry standards with the advent of modern production technology.
Target values assigned to each alloying element and minimising variation in chemical composition have been obtained. This means that instead of trying to stay within a specific range, the intention is to pin-point the same contents time and time again. This improves consistency in forming, in heat treatment and in final product properties. Moreover, using the “target value” approach helps in continuously reducing variation in every processing step (fig. 2).
Staying within a specified range is a quite different matter from hitting the nail every time. Composition precision is essential in many of the steps used after steel production, in order to attain high and consistent product quality. One problem is that very often elements interact in their influence on a certain steel property. For example, hardenability is a function of the combination of a large number of elements such as carbon, silicon, chromium and nickel present in the steel. Hardenability is an important property for engineering steels. It is the ability of a steel to harden under a given set of heat treatment conditions and is vital to bearing performance.
Deoxidation is the processing stage where the oxygen always present on tapping of the steel melt into the ladle furnace is reduced. Most elements are able to dissolve in solidified steel. Oxygen cannot. This means that oxygen must find an element with which to combine. Oxygen is very reactive, and will combine in the melt with whatever element seems most attractive. It is vital to create oxygen compounds which are easily removed from the melt by the stirring motion given to the melt. The ease with which particles can be removed depends upon their chemical composition.
The total oxygen content of the steel is a simple and understandable measure of the steel’s cleanliness (fig. 3). All oxygen present in the steel will form oxide particles on solidification, and oxide particles are hard, brittle, and harmful to product properties.
The classic way bearing components fail was by sub-surface initiated fatigue. Such fatigue failures initiate at weak spots in the material, and the most dangerous kind of steel imperfections present in steel are hard, brittle oxide inclusions.
Oxide removal is achieved by applying inductive stirring of the steel melt throughout the entire process. Inductive stirring is an effective method of controlling the movement of molten steel inside the ladle furnace. It uses a powerful electrical transformer outside the furnace wall that generates electromagnetic fields which influence the flow of steel inside the ladle. Additional agitation is achieved by argon bubbling through the melt during the vacuum degassing operation.
It is vital that the steel products delivered are as free as possible from deoxidation products. This requires continuous improvements not only in the steel making operations but also in the testing procedures applied.
Steel making precision has long surpassed the existing measurement standards, and conventional inclusion rating procedures no longer provide significant information on steel quality. Developments are underway for new standards, and some semi-quantitative methods are in use today. Significant work is going on using ultrasonic and spectro-chemical methods for inclusion ratings, and promising results have been obtained (fig. 4). It seems certain that new, quantitative methods can be introduced in the near future.
Desulphurisation and degassing are achieved by exposing the steel melt to a combination of argon bubbling through the melt and inductive stirring under vacuum. This reduces the sulphur contents to the desired level and completes the deoxidation procedure, leaving the melt ready for teeming (the second pouring phase).
Teeming is another operation important to final product quality. During teeming, the risk of external oxidation and contamination by external particles is at its highest (fig. 5).
The melt has to be protected from oxygen during teeming. Ovako Steel’s ingot teeming procedure uses a teeming shroud, which in combination with the “uphill teeming” and the high-grade auxiliary materials used, ensures that the oxygen pick-up from surrounding air is minimised.
Other quality bearing steel producers today use a different teeming procedure. Instead of producing ingots, they produce continuously cast blooms. The difference between ingot casting and continuous casting is that segregation (areas in the steel where the chemical composition is different from the average composition) is generated in a different way, and solidification rates differ.
Large external particles (macro-inclusions) are of crucial importance to product properties which are determined in the teeming operation. These are entrapped in the steel during the casting operation. Such macro-inclusions are few, difficult to detect and highly detrimental to component performance. Macro-inclusions are frequently caused by auxiliary elements used during teeming such as casting powders and ceramics. It is vital that these inclusions be avoided in this area, as the situation regarding standard rating methods is even worse than for the deoxidation products.
For micro-inclusions or small particles, standardised methods have been in use for a long time and can at least sort out bad steel from good. For macro-inclusions, the methods in general use today (such as blue-fracture or step-down tests) have given zero readings for a long time due to the randomness and low frequency of macro-inclusion occurrence, which does not help with overall quality issues.
Recent developments in laboratory-based ultrasonic inclusion rating techniques have resulted in a major step forward in the ability to detect and statistically measure large particle occurrence in steel. The ultrasonic method for cleanliness assessment is a focused probe system. The sample is a section of billet which is scanned. The focused sound beam can count the particles in the particular volume of steel. This method is applied during the billet stage and extensive investments and development work have proven that it is quite feasible to provide a good understanding of the relationship between metallurgical processing and the occurrence of macro-inclusions (fig. 6).
This allows better conformity and control of changes in the steel processing route and in the auxiliary material selection, to improve quality quickly and reliably.
Steel making has come a long way since the old blacksmith days, and today much can be considered ‘high-tech’ as regards process control, process capability, consistency and quality assurance.
It is fair to claim that the difference between the bearing steel of thirty years ago and today is almost as large as the difference between the bulky, punch-card computers of the 70s and the lap-top on which this article was written.