Conventional theories about metalworking fluids state that they react chemically with the surfaces of metal workpieces during the short time that the two are in contact. These reactions supposedly form metal chlorinate, metal sulfide or metal phosphor layers that are more easily removed by the machine tool.
Researchers at a German supplier of metalworking fluids challenged that model recently, contending that, in most metalworking operations, the fluid and part are not in contact long enough for the complex reactions necessary to form these layers. In a January presentation at the OilDoc conference in Rosenheim, Germany, Wisura GmbH Research and Development Director Joachim Schulz proposed a new theory based instead on the premise that additives adsorb to workpiece surfaces. The new theory, he said, will lead to new approaches to formulating metalworking fluids.
Schulz agrees with one aspect of conventional theories on metalworking fluids: Additives must be interacting with the workpiece in some way to make it more amenable to the metalworking process. Without fluid, metalworking tools generally wear out faster, the finish on workpiece surfaces ends up rougher and the likelihood of adhesion between tool and workpiece increases. [S]ome type of reaction is essential to ensure long tool life and to produce an optimum surface finish, he said.
According to Schulz, research supporting the conventional theories has basic flaws. One difficulty with commonly held theories about metalworking operations, he said, is that most published literature describes tests in which the additives have a lot of time, often hours, to react with the metal. In addition, most studies do not differentiate between different kinds of metals.
But there is a big difference, for example, in the way carbon steel and stainless steel react, he noted. Because they were questioning models of interactions with workpieces, Wisura researchers began by studying the atomic structures of various metals used in metalworking. Using secondary ion mass spectroscopy and secondary neutral particle-mass spectroscopy, they found different types of molecules covering surfaces of different metals. Iron surfaces are typically covered with hydroxyl groups, Schulz said, while carbon steel is covered with a layer of hydroxides and oxides. Oxides either cover the entire area or create islandshaped patches. Stainless steels generally are covered by chrome-oxides and nickel-oxides, with the ratio between the two dependent on the specific alloy. And aluminum is covered by a tenacious oxide layer. Thus, steel, stainless steel and aluminum have quite different surfaces and, therefore, react differently with additives.
Additives interact with oxides, hydroxides or metal ions, said Schulz. Generally, additives are classified as ionic (acidic phosphoric acid esters and passive extreme pressure (PEP) additives) or nonionic (chlorinated paraffins and polysulfides). Tests show that ionic additives react preferentially with metal ions while nonionic additives react with oxide and hydroxide groups, he said.
Wisura studied the effectiveness of chlorine- and sulfurcontaining additives on various metalworking processes. Chlorinated oils work in every kind of forming process and with all kinds of materials, especially stainless steel, he said. In addition, chlorinated oils work well in high-speed processes like tube or bar drawing. Chlorine-free oils, however, typically do not work on stainless steel, even if the oil and metal are in contact for a long period of time.
Thus, chlorinated paraffins and sulfur-containing compounds work well on stainless steel, but not PEP additives or acidic phosphoric esters. On the other hand, PEP additives combined with sulfur-containing compounds perform well on carbon steel, as do chlorinated paraffins.
Lube Report Asia will occasionally include articles originally published in sister publications of LNG Publishing Co. This is an
Lube Report Asia will occasionally include articles originally published in sister publications of LNG Publishing Co. This is an excerpt from an article that originally appeared in the May 2013 issue of LubesnGreases Europe Middle East Africa-Issue 47 – under the headline, A New Model for Metalworking. To continue reading this article, clickhere.
from an article that originally appeared in the May 2013 issue of LubesnGreases Europe-Middle East-Africa, Issue 47, under the headline, A New Model for Metalworking. To continue reading this article, click here.