Lubricants for Cold Forging of Stainless Steel

One of the major problems related to cold forging of stainless steel is to secure sufficient lubrication during the process. The severe conditions during the process (i.e. high normal pressures and large surface expansion) usually requires a two component lubricant system consisting of a porous carrier coating lubricated with a suitable lubricant.
This project deals with the development of two new lubricant systems for cold forging of stainless steel. The lubricant systems are based on zinc phosphate and iron(lll) chloride respectively. The zinc phosphate coating serves as a porous “carrier coating” for the actual lubricant (i.e. sodium stearate or MoS2). Iron(lll) chloride can be used alone or in combination with MoS2 or graphite. The work has been focusing on application methods, characterisation and tribological tests.
An electrochemical method for depositing zinc/calcium phosphate coatings on stainless steel from an aqueous nitric acid solution has been developed. The method involves hydrogen evolution at the surface of the specimen and the deposition occurs due to decreasing solubility of the metal phosphates at increasing pH.
The lubrication mechanism of iron(lll) chloride is believed to involve the reaction between iron(lll) chloride and the stainless steel surface to form iron(ll) chloride. Results from backward can extrusion tests confirm that this reaction contributes significantly to the lubrication properties.
Backward can extrusion tests of zinc/calcium phosphate (lubricated with sodium stearate) and iron(lll) chloride show that a height/diameter ratio of more than two can be obtained. A height/diameter ration of 1.2 can be obtained with the current State of The Art lubricant system (Iron oxalate combined with MoS2).