Historically, two kinds of greases have been used in steel mills – clay and aluminum complex greases – neither of which was ideal. Clay greases have good higher temperature properties, but are incompatible with other thickeners and many grease additives systems. Aluminum complex greases have similar issues with additive compatibility, but have excellent water and high temperature resistance. It is not easy to formulate either of these two thickener types to have good extreme pressure (EP) and antiwear properties. Lithium complex greases fortified with polymers have also been used in steel mill applications, but they can be problematic. These types of greases need antiwear and EP additives to increase their load carrying capacity. Many of these formulations are not stable in the presence of significant amounts of water ingress, which is typical of the steel mill operating environment. A polymer-fortified lithium complex grease may resist water spray-off or washout, but may lose performance through additive hydrolysis. For steel mill applications, calcium sulfonates greases are superior because they have good high temperature resistance, good water resistance and inherent EP and antiwear properties.
Steel mill grease requirements:
Taking these typical steel mill requirements into consideration, the calcium sulfonate grease technology needed to meet them can be defined. Most typical sulfonates come in low-to-medium viscosity fluids. Adding bright stock to them will only increase the viscosity to approximately ISO VG 150; well below the required ISO VG 460. Adding high viscosity polyalphaolefin (PAO) fluids such as PAO 100 will increase viscosity to ISO VG 320 (still too low), but will improve water spray-off and washout characteristics. Rather than adding high molecular weight polymers (>50,000 Daltons), the optimum solution is to add a polyisobutylene (PIB) to the base oil. By blending a suitable mineral oil, such as a 600 neutral paraffinic, with a lower molecular weight PIB (2000 to 4000 Daltons), the correct viscosity can be achieved without negatively impacting the pour point. After adding an anti-oxidant, a passivated polysulfide and a corrosion inhibitor to the gelled sulfonate grease, the results shown in the table were obtained (see Table 1). Further improvements to water resistance can be achieved by incorporating additional polymers. Adding functionalized polymers or a styrene isoprene polymer can reduce the spray-off to <4.0%.
Greases used in steel mill processes typically encounter a significant amount of water contamination. In addition to water spray-off and washout resistance, grease compatibility with water is critical. The miscibility with water is usually checked by measuring the grease’s shear and roll stability in the presence of water. The effect on shear stability is determined by comparing the 100k stroke penetrations of a sample to which 10%wt water has been mixed in with a dry sample. Roll stability with up to 20% added water at temperatures up to 80°C and durations up to 100 hours is also compared to dry samples. Calcium sulfonate steel mill greases perform very well in both of these types of water miscibility testing and, with added polymer, outperform other grease thickeners.
Table 1: Properties of Steel Mill Grease Formulation
Gareth Fish is a Technical Fellow & Strategic Technology Manager Industrial Additives at The Lubrizol Corporation. You can find his contact information in our membership database.
Other articles in this issue: