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It is quite often mentioned within the metal finishing industry that you cannot compare cleaning tests performed in the laboratory with the same tests conducted under the actual conditions as found in production cleaning systems. On the one hand you'll hear the boys in the white coats state loud and long that their tests and findings in the laboratory will determine how cleaning should be performed out in the shop—whereas the boys in the shop just as loudly proclaim that what is done in the laboratory does not necessarily mean that it will work out in the plant. They both have something going for them. Actually, there is no mystery connected with cleaning when some of the variables between the two situations are reviewed in detail. In addition to the variables of age of soil, dry oil deposits, etc., there are several significant differences in the actual construction of the laboratory washer, and in the relative location of parts in comparison to production washers. The following are just a few of the differences between laboratory washers and production spray washers which can have a pronounced effect in fostering erroneous results:
Although the differences in alloys and surfaces of metals have a profound effect on their cleanability, the cause and effect relationship of this aspect is most dramatically demonstrated with steel. For instance, steel is most commonly designated by its carbon content: normally the last two digits of the SAE alloy number represent carbon content. For example; SAE 1010 steel represents the carbon content as 0.10%. The SAE number for most cold and hot rolled sheet steel is 1014. Steel plate is usually 1020, while steel rod is B1113 and 1020. Most steel which is used for tools will be as high as 1040 and up. Generally the higher the carbon content of a steel being phosphated will tend to have more phosphate coating produced on the surface in a given cycle. But in general cleaning, the higher carbon content could cause cleaning problems due to greater quantities of carbon being leached out onto the surface of the metal.
Two major categories are to be considered from the metal cleaning stand-point as far as the steel surface is concerned; namely cold-rolled steel and hot-rolled steel. All steel is originally hot-rolled, while some of it goes through the cold rolling process later. Basically, hot-rolled means that the steel is heated in the furnace to a cherry-red temperature and then passed through a series of rolling mills to reduce it to the desired size. When cold-rolled steel is wanted, the hot-rolled steel is reduced only part of the way in gage and then allowed to cool and then finally passed through a series of reducing mills without further heat being applied to the metal. As metal is worked from one reducing mill to the next, the grain structure is rearranged in such a manner that the steel becomes harder and more brittle. To offset this embrittlement the steel must be periodically "softened' or annealed between cold working operations. Annealing is accomplished by passing the metal through a furnace again, heating it to a cherry-red temperature. These extremely high heats actually burn the surface of the metal thereby producing scaly deposits that makes it necessary to pass the metal through an acid pickling tank after annealing and for a second time just before the final cold-working. The second pickling operation is a 'must' if an even and brighter surface is to be expected. The burning or oxidation can be largely prevented, however, by passing the metal through what is known as an atmospherically controlled furnace. In this type of furnace all oxygen has been removed, thereby eliminating any possible scale production. Without oxygen no oxidation can take place, therefore the metal emerges in much the same condition surface wise as when it entered the furnace.
There is a definite difference in the amount of reactivity to chemicals between hot-rolled and cold-rolled surfaces. Cold-rolled, steel has a much finer grained, smoother and less porous surface. This is so because cold rolling tends to 'fold in' and close up the pores. The less porous the surface, the less surface area is presented to chemical attack by acids, alkalis, and phosphating compounds. By the same token, soft hot-rolled steel is more porous and therefore more reactive to chemicals than is hard, tempered steel. With soft steel the metal is cold-worked. and then annealed just before the final pass which is a light 'skin pass' through the rolls which reduces thickness no more than a couple of thousands of an inch. This amount of final cold reduction is not enough to close up the pores as much as hard tempering, in which the steel is reduced by several cold rolling after annealing, giving it a shinier surface.
