In this article, we are going to describe one of the vital applications of steelmaking plants. You can understand the criticality of the application by knowing that companies claim caster is the heart of steel plants. As this application describes as the heart of steelmaking plants, it’s also critical as the human heart. As the study says, approximately 75 to 80% of application breakdowns due to bearing failure happen due to improper lubricants selection & improper lubrication. The selection of Lubricants & lubrication process is not as simple as it looks like. As we have stated that Caster application is the heart of steelmaking plants, proper lubrication & proper lubricants selection is undoubtedly a critical task for application engineers & maintenance peoples.
There are mainly 3 types continues casters we can see in mainly all integrated steel plants as per the end products & caster criticality.
- Billet Caster
- Bloom Caster
- Slab Caster
As per criticality, Slab Casters are the most critical continuous caster application in steel plants. For decades lubrication scientists & tribologists are doing research & studies to find a permanent solution for line clogging issues & as an aftereffect of line clogging caster rolls bearing failure. All major steel-making plants face these issues resulting in high downtime of application, production failure, & considerable person-hour loss. To avoid these types of problems, modern continuous casting process (CCP) segments are equipped with multiple split rollers, additionally increased the number of lubrication points, but that is not enough to avoid caster rolls bearing failure due to improperly lubrication &improper lubricants selection.
Nowadays, casting machines have multiple & separate automatic & centralized lubrication systems to lubricate different types of caster application machine elements, mostly rolls bearing. These centralized lubrication systems pump a relatively small amount of grease at frequent intervals to the bearing. The grease quantity is typically less than 5cc (0.15 oz.), but the frequency is typically 6 to 10 times per hour. This frequent cycle can keep the bearing full with new grease while also clearing old grease and contaminants.
The bearings located at the runout table area in the continuous casting machine are continuously exposed to high radiation heat (considering exit billet temp is more than 900°C). The bearing temp can go up to more than 150°C, which is not a favorable condition for both bearing lubrication and lubrication method.
In Slab Caster application, machine elements, mainly bearings & grease lines, are exposed to higher radiation temperature because of higher slab width and greater slab thickness. That is the reason for lubrication line clogging or solidification of grease inside the pipeline. Due to grease chocking inside the grease pipeline, lubricants are not able to reach the lubrication points. When rolls bearing starving for lubrication for a prolonged period, the result is early premature bearing failure & system shutdown.
There is a way to fight this kind of problem. Usually, anyone can avoid this problem by using high temp grease with a frequent or shorter lubrication cycle. Still, these will increase the lubricants consumption drastically as well as lubrication cost will also increase uncontrollably. In addition, frequent roll jamming and caster segment replacement also increase the cost, which can not be controlled by using non-effective high temp lubricants.
An alternative is an air-oil system; The Air Oil system uses compressed air to drive a film of oil through distribution pipes to the bearings. These systems can operate continuously to keep fresh required grease flowing while clearing out the old grease and contaminants. In addition, modern air-oil systems can operate with significantly reduced lubricant consumption.
Common problem faced by the customer
Because of continuous expose to high temp,the problems are as follows:
- The grease feed line (from distributor to bearing) clogs very frequently because of hardening of grease inside the narrow pipe, which leads to frequent replacement / cleaning of the pipes.
- The grease inside the bearings sometimes gets hardened which also attracts premature bearing failure.
- Because of lubrication starvation inside the bearing, the problem can occur.
- Bearing life can be shortened because of improper or inadequate lubrication.
Required properties of the grease
- Should not harden at higher temperature.
- Should be able to reduce wear on the bearings.
- Reduces lubricant consumption.
Lubricants / Greases for Caster Application in Major Steel Plants
For applications operating at very high temperatures, the type of lubrication used must also be evaluated. For example, suppose the caster application temperatures exceed the lubricant’s temp rating. In that case, the effectiveness of the lubrication will be compromised, which will be the reason for increased friction in the rolls bearings used in the caster. This increased friction increases wear and reduce the life of the bearing components.
You probably know the importance of THICKENERS for any grease making. Actually, without a thickener, it is not possible to make grease from oil. Because to make any grease, first we need to take lubricating oil (Mineral or Synthetic). Then, at a certain temp, we need to mix thickeners ( lithium, lithium complex, calcium or aluminum complex soap), additives & solid lubricants to make lubricating grease from a lubrication oil. You can read the fundamental difference between Lubricating grease vs. lubricating oil article.
Polyuria thickened greases is also very popular in some types of specific applications. In past & recent years, polyurea thickened greases have gained popularity, particularly in steel mill & continuous castors applications. Greases used in caster applications should have good thermal stability as well as good load-carrying capacity. So, some polyurea-based greases with added extreme-pressure (EP) additive packages provide good load-carrying capability without degrading the thermal stability of the grease at high temperatures. Also, polyuria-based greases have value-added properties like excellent corrosion protection, good mechanical oxidation and high-shear stability, low wear, low volatility and low oil bleed.
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