Steel Mill Lubrication - Boosting the Grease Performance for Longer Bearing Service Life

Hocine Faci Philip Booker Martin Maass Marcus Totten

Castrol Industrial

Abstract

Calcium Sulfonate Grease has been in use in the steel mill industry for more than 15 years. Its success in these applications is mainly due to a set of performances associated with the thickening material itself, which provides a decent carrying capacity and wear protection, good water resistance, corrosion resistance and thermal stability. Experience has demonstrated however that this technology as currently offered, does not meet the totality of the end users expectations, namely on load carrying capacity, thermal stability and mobility, crucial properties for long service life of the bearings.

The objective of this project was to build on the existing technology of Calcium Sulfonate greases, a more robust product that would boost the load carrying capacity / antiwar properties to higher levels, while keeping the current corrosion inhibition, water resistance, thermal stability, and grease mobility at least at the current level of performance, if not better.

An extensive benchmarking program that involved over 9 different commercially available products has been conducted in the laboratory testing phase where more simulative tests have been performed. This work has led to the development of a product that has met all the specification requirements. The load carrying capacity has been improved as well as the wear protection, the water resistance (under both the sprayoff and washout test conditions), and mechanical stability in the presence of various still mill process waters.

Since undergoing trial in a steel mill caster section, this product has shown great mobility, outstanding thermo-oxidation stability, excellent mechanical stability, excellent grease coverage of the bearing, and more importantly all these benefits were achieved with lower grease consumption. The service life of the bearing was extended from 3 to 4 years, and could be extended further. The detailed work will be compiled in this paper.

Introduction

Prior to the introduction of continuous casting in the 1940/1950s, steel was poured in individual molds to form ingots. Since then, the steel manufacturing has improved drastically. Continuous casting became the process of choice in steel, aluminium and copper industry.

Within the steelmaking process, the steel is cooled to form a semi-finished product in the continuous caster area. In a typical installation, molten steel is tapped from ladles on the ladle turret and fills a mold before entering a continuous caster. The mold is cooled externally and an outer skin is formed around the liquid metal core; this will remain liquid until near the end of the casting process (known as the metallurgical length).

The caster consists of rollers that are in contact with the cast steel, and these are generally cooled internally with process water. Bearings are installed to allow the rollers to rotate, and these are also water cooled in most casters. The steel is spray cooled at a determined rate, to maintain the metallurgical length. Bearings and rollers are also water cooled in the runout table directly after the caster; at this stage, the cast steel can still be in the region of 800°C.

roller and bearing temperatures will rise rapidly. The cast steel is very soft and malleable at this stage, and surface damage can occur that cannot be rectified further downstream in the steelmaking process. This can result in scrap costs, as well as downtime to repair the affected machinery including bearings, rollers and cooling water pipe work.

Specialty greases are available to lubricate high temperature applications in the hot areas of a steel mill, and water resistant greases are available for areas where large volumes of water are used. It is not common to find products that are effective in both high temperatures and extreme conditions such as large volumes of process water.

Balancing the mix of oil, thickener and additives to match the type of application can result in a significant increase in performance compared to conventional greases.

This paper discusses the evaluation, benchmarking and field trial of single grease specifically designed for performing under the harsh and varied conditions found in casting sections in steel mill plants.

Background

The most used greases in the steel industry are those based on aluminium complex, lithium complex and calcium sulfonates. References [1, 2, 3, 4, 5 and 6] cover a comprehensive description of some of the steel mill plants, the typical greases used in these plants and their conditions of application.

Recently a good number of papers have been presented at ELGI and NLGI outlining the benefits of calcium sulfonates or calcium sulfonate complexes; Reference [7] indicates that the calcium sulfonate greases remain the fastest growing grease thickener system, a main reason being that it does not change its characteristics even in the presence of large amounts of water. Reference [8] emphasizes on what makes the calcium sulfonate greases different to the other greases from the structure to the manufacturing process. Reference [9] studies the effect of water on various types of thickening systems, and different types of base oils are evaluated for a good number of characteristics by various test methods. This reference joins the other papers to confirm that calcium sulfonate greases outperform the other greases under high load and high temperature applications. Reference [10] outlines the impact of other advanced thickening systems on the bearing service life.

This study takes into consideration all the work that has been covered earlier using the calcium sulfonate grease technology, and will identify the key advantages of this technology. The laboratory work as well as the field testing approaches will be covered.

Laboratory Development Work

Definition of required tests

The caster section bearings in a steel mill plant as described above require grease with good number of properties in order to withstand the harsh conditions of the application, namely: (1) high load carrying capacity, (2) improved wear protection, (3) excellent mechanical stability, (4) outstanding water resistance, and (5) excellent corrosion inhibition. To evaluate these properties, the following test methods were used:

1. Load Carrying Capacity:

One of the most popular tests for determining the load carrying capacity is Four Ball EP (ASTM D 2596, DIN 51350-4). This test consists of 4 balls arranged in the form of an equilateral tetrahedron. The basic elements of the tetrahedron are 3 balls held stationary in a pot to form a cradle in which the fourth upper ball is rotated around a vertical axis under pre-selected conditions of loads. The rotating speed is 1770 +/- 60 rpm. A series of 10-second runs are made at successively higher loads until welding of the 4th ball occurs (Figure 1).

Wear:

Wear protection is measured by Four Ball Wear method. The test (ASTM D 2266, DIN 51350-5) can be run at a specified rotational speed under a prescribed load at a controlled temperature. The test duration standard is 1 hour. The diameters of the wear scars on the stationary balls are measured after completion of the test (Figure 2).

Mechanical / Shear Stability:

Shear stability is the ability of grease to resist changes in consistency when subjected to mechanical work. The most common laboratory tests used to evaluate shear stability are described below:

Worked Penetration (extended worked stability):

ASTM D 217, DIN 51804

It consists of subjecting the grease in a standard penetration cup in a grease worker to a number of double strokes (1 k, 10 k, 100 k, etc…). The difference between worked penetration before the test and the worked penetration after the test determines the shear stability and is reported as the change from the original figure (Figures 3 and 4). Roll Stability Test: ASTM D 1831

In the Roll Stability Test, a small sample (50 grams) of grease is rolled at 165 rpm for a specified period of time under a given temperature. The difference in worked penetration measured with ¼ scale penetrometer before and after rolling is reported (Figures 5 and 6).

Water Resistance:

This can be evaluated by using three different procedures that complement each other. The following test methods were used in this study:

Water Washout Test: ASTM D 1264

Method consists of a standardized bearing with front and rear shields having a specific clearance. The bearing is packed with 4 grams of grease and rotated at 600 rpm

Figure 2: Four Ball Wear set up Figure 4: Grease Worker

Figure 5: Rolls

Figure 6: Roll Stability Tester

Figure 3: Penetrometer

under a jet of water at a given temperature for one hour. At the end of the hour, the bearing is removed, dried and weighed. The weight loss is reported (Figure 7).

Water Spray-off Test: ASTM D 4049

This test method consists of subjecting a layer of grease at a given thickness on a stainless steel panel to water spray off. The grease is sprayed with 40 psi water at a given temperature for 5 minutes. The panel is then dried and weighed. Spray off resistance is reported as the percentage weight of grease removed by the water spray (Figure 8). Wet Roll Stability: ASTM D 1831 (modified)

Method consists of running the roll stability test, but in the presence of water. Process water may be used instead of distilled water. Visual inspection of the grease and its penetration change after the roll, along with the presence of free water and its quantity, are determining factors for the water resistance characteristics of the grease.

Corrosion Resistance:

In wet applications such as those in steel mill environments, greases are expected to assure protection against steel rusting. Two tests are considered in this program:

Rust Test: ASTM D 1743

Method consists of running a tapered roller bearing packed with 2 grams of the grease. The bearing is rotated under a given thrust load for 60 seconds to distribute the grease uniformly. The bearing is then immersed in distilled water without breaking contact between cup and cone and stored at 52°C and 100% humidity for 48 hours. At the end of the 48 hours, the bearing is removed, cleaned and inspected for rusting. A corrosion spot of 1.0 mm or longer is an indication of failure. Only pass or fail ratings are used. Run in the presence of process water, this test gives an accurate indication of the corrosion resistance characteristics of lubricating grease (Figure 9).

Figure 7: Water Washout Tester

Emcor Rust Test: ASTM D 6138, DIN 51802

This test method is used to assess the ability of grease to prevent corrosion in rolling bearings operated in the presence of distilled or process water. It consists of packing a pair of double row ball bearings with grease and mounting them on a shaft in a housing where a specified amount of water is added. The bearings are subject to 8 hours run and 16 hours off for three days and after that they are left for 4 days. The bearings are disassembled and the bearing race is rated for corrosion (0 to 5: no corrosion to highly corroded surface), (Figures 10 and 11)

Figure 10: Emcor Test Unit Figure 11: Emcor Test housing and bearing

Based on field experience and after consultation with a good number of end users and OEMs, the following targets have been selected (Table 1):

Table 1: Initial Specification Target

Evaluation of Commercially Available Products:

Nine commercially available products based on different thickening systems have been subjected to a series of tests including water washout, water spray-off, roll stability in presence of process water and rust test as per ASTM D 1743. Different process waters are considered in this program. The results for the nine greases and calcium sulfonate grease prototype, P5606 are displayed in Table 2 below:

Table 2: Evaluation of commercially available products vs. prototype (Process Waters)

P5606: Prototype CAP 1, 2,…: Commercially Available Product 1, 2, …. WSO: Water Sprayoff WWO: Water Washout

The graphs of Figures 12, 13 and 14 reproduce the above results, showing the effect of process water on each product. The results showed that none of the commercial greases met all three performance targets set for measuring water resistance and mechanical stability. On the other hand, calcium sulfonate prototype easily met the targets and specification limits were made more stringent as shown in Table 3.

Figure 13: Water Washout Data obtained on Commercially Available Products:

Final Specification Target:

Based on the data obtained above, a new specification target has been established for the prototype grease in terms of water and corrosion resistance. Table 3 regroups these data as well as the results obtained on the prototype:

Table 3: Final Specification Requirement and Prototype Characteristics

The above results indicate that the prototype meets all of the revised requirements. It displays outstanding results in terms of load carrying capacity, wear, water resistance, mechanical stability as well as corrosion resistance.

Field trial

A trial has been conducted for 12 months in the process downstream of the curved section of a six strand billet caster. This covers the extractors, straightener section, runout table to cutting torches, runout table to cooling bed, lifters, cooling bed, walking bed pivots and transfer car.

This demonstrates the flexibility of the grease; one product can be used in many areas, consolidating stock and making lubrication tasks a lot simpler.

Field Trial Results

Grease Consumption

Using the evaluation data in comparison with the performance of a previous grease in application, it was possible to reduce the grease consumption by 50%. This reduction in consumption resulted mainly from the improved mechanical stability as well as water resistance. Further reductions were possible, but the centralised grease system was not flexible enough to further optimize grease consumption without risking the damage of critical components lubricated by the system. This level of consumption set up was maintained until the lubrication system constraints are over-hauled.

This reduction in grease consumption has resulted in a lower pumping rate through the lubrication system pipe work, eliminating the risks of blocking the line, and therefore assuring permanently the dispensing of grease to the bearings. Figure 15 shows the condition of the grease inside a bearing housing before cleaning ready for inspection.

Mechanical Stability & Water Resistance

Although the grease consumption has been significantly reduced at the start of the trial, a generous amount of grease still remains in the bearing 12 months later. This indicates excellent mechanical stability, advanced water resistance and extremely good tackiness to keep the grease in place.

Pumpability, Dispensability, and Mobility

The above picture as well as the field inspection reports indicate that the grease was present in large amounts in all the bearings inspected, demonstrating a good pumpability through the dispensing systems and mobility in the bearing.

Noise Reduction

The specificity of the calcium sulfonate greases, which by definition contain a much larger amount of thickening system in comparison with the conventional greases and when also enforced with solid lubricants, this grease will have the capacity of assuring the presence of a strong film between the lubricated surfaces. This film will contribute to the amortization of the shock loading and therefore reducing or eliminating the noise. Effectively the operators have reported that in the walking beam area, where the plain metal bearings of the pivots used to be very noisy, due to metal on metal contact under high temperatures and low speeds, after using the prototype product, no noises were noticed. During the final inspection after 12 months trial, the end user confirmed

Figure 15: Bearing, Pre Inspection

that the prototype grease had prevented damage within these critical bearings.

High Temperature Performance

On two occasions during the trial period, pipe work for the internal cooling of the continuous caster roller bearings failed.

This can be quite a common occurrence with flexible pipe work, and failures mean that bearing temperatures will raise very quickly, leading to seized bearings. This causes production quality issues, as the billet surface finish will be affected by sliding instead of rolling over the rollers.

The bearing housing and grease pipe work in the immediate vicinity will also be subjected to extreme heat; some greases will form hard deposits in this situation and will require pipe work and bearing housing replacement as well as a bearing change.

During the trial with the prototype grease, the end user reported that cooling water supply to one roller had failed for a period of between 24 and 48 hours. It was not possible to stop production, so the affected bearings had to remain in place after the cooling water system was repaired. Cast product quality was monitored, and no quality issues were reported.

At the next available maintenance opportunity, the bearings were removed from their housings and changed; however, upon further inspection they were found to be in a serviceable condition. Furthermore, grease pipe work did not need to be replaced. Grease within the bearing housings also remained mobile, significantly reducing maintenance time after the cooling water failure.

Pictures of a used bearing running in normal conditions, and one subjected to extreme heat, are shown below (Figures 16, 17).

Discolouration of the brass cage can clearly be seen on the heated bearing, but all roller elements remained free to rotate and clearances were measured and found to be within tolerances defined by the end user.

No scoring marks could be seen on the bearing elements, and the grease did not form hard deposits

Bearing Life

A selection of bearings were inspected jointly with the end user, and the remainder were inspected by the end user’s maintenance team. All bearings were found to be within a specified tolerance and deemed suitable for further use.

Historically, bearings have been replaced every three years as a preventive measure to avoid unplanned downtime. Bearings reaching three year life were also found to be suitable for further use, minimizing bearing replacement costs across the whole of the caster installation. It is fully expected that these three year old bearings will not require replacing during the next 12 monthly maintenance period.

Figure 16: Bearing in normal conditions

Figure 17: Heated Bearing

Even when considering a minimum of 25% life extension then the reduction in spare parts alone is significant, but the end user is confident that bearing life can be extended even further. This will be confirmed after the next 12 month maintenance period.

Corrosion Resistance

No staining or corrosion was visible in any of the bearings inspected during the maintenance period, indicating that the grease performs extremely well in being able to resist corrosion from the chemical aggressiveness of process waters in use in the plant

Conclusion

Key parameters selected to prove the capabilities of the product were high temperature performance, load carrying capability, performance in wet environments and extended component life.

Taking the performance of greases with a similar technology platform, an initial specification was shaped to benchmark a new development product, targeting high load performance, water resistance, corrosion prevention and mechanical stability (also in the presence of process water).

A continuous caster was selected for a field trial. This application is well known to be a critical area requiring high performance in lubrication to avoid costly breakdowns and increased maintenance.

After the 12 month trial period, the performance was evaluated with the end user and the following benefits were observed: • Grease consumption reduced by 50% • Grease still appears to have the same consistency as the new product • No signs of corrosion or surface staining • Bearing tolerances remained within specification • Bearing life extended from three to four years, most likely longer

In addition to the points listed above, the grease did not block pipe work when subjected to extreme heat, giving the added advantage of minimising associated maintenance work.

Acknowledgements

Authors would like to thank Luis Blazquez, Soman Dhar, and the customer for the valued contribution to the product field testing

References

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