Monday, December 30, 2013

Removing Fire Scale

Another point about firescale which follows creating it, is removing it.  
That discussion sort of breaks down into, “‘Why are we doing it?’ and ‘How much are we talking about?’”

Often the first step is just wire brushing vigorously and frequently during the forging process.  I use three types of butcher block brushes, short wire, long wire and blade.  For the handled scratch brushes I prefer the stainless steel wire type over the carbon steel type.

Brushing might not be necessary when forging large stock and in the early forging steps but with smaller pieces and when forging toward the finished state the brushing keeps the surface clean and visually appealing.  Lightly hammering the total surface when hot or even cold can pop off a lot of scale.

There are a lot of power devices which will remove scale and I try to avoid most of them.  The wire cup brush seems to be one of the most dangerous things in the shop and I don’t recall using one for a year or two.  First, I used a stainless steel knotted cup on an angle grinder, but rather quickly added a router speed control to slow down the rpm.  Later someone suggested using a buffer rather than an angle grinder.  It naturally ran at lower speed and had a variable speed control built into the paddle switch.  However, it was a lot heavier and bulkier.

During one phase I used some wire wheels mounted on a bench grinder.  It only works for small surface areas and I didn’t like the way it flung off little wires.

Grinding wheels, flap sanding wheels and mounted stones of various types and grits work well  and I use them a lot on small ares but they don’t have much use is large area scale removal.  Other types of composite strip discs extend the options but purchasing all these abrasive consumables can add up to a lot of money.

In my situation the tumbler proved to be the most economical way to remove coal fire scale, rolling mill scale and other surface contaminants - wholesale.

Another, wholesale, scale removal method is pickling.  I usually use hydrochloric acid.  To make sure the acid activity stops it requires rinsing and perhaps neutralizing with sodium bicarbonate and more rinsing to get a clean bare surface.  The only time I’d do that is in the process of creating a patina finish - copper coating, perhaps.

For smaller, more delicate scale removal torch heating and more wire brushing by hand serves me best.

This is one old reference about industrial cleaning.

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