plating, anodizing, & finishing Q&As since 1989
How does a mill measure carbon smut on cold rolled
We are having adhesion problem after processing for gaskets.Tom Dudas
Mills measure carbon content of the steel, not surface carbon. High carbon content is an indicator of poor steelmaking process control. Mills have begun eliminating certain steps in their process to lower costs, unfortunately this means we have to really watch the quality of incoming steel.
There is a guy named David Chalk of Nalco who I heard speak on this very subject recently. If you need an expert he's your best bet.
- Pearland, Texas
Smut Testing on cold rolled steel sheet:
Years ago we sheared cold rolled sheets for LTV. We also evaluated the surface cleanliness by performing a smut test. We simply used white index cards and clear tape. A short piece of tape ( 2-3") was placed on the surface of the strip. After lifting it off the strip, it was placed on the card board index card.
We did not wipe or touch the area were the tape was placed. Nor did we try to remove any rust preventive oil.
A clean sheet of steel will result in a clean piece of tape. It will still "look" white on the card. A dirty piece of steel will have a smutty residue. The tape sample will appear grey, not white.
We sampled the strip at the uncoiler. We collected numerous sample cards and had a range from white, to light gray, and finally dark gray. The representative samples were labeled 1 thru 5, with 1 being white and 5 being dark grey. Ones and twos easily passed. Threes turned out to be ok. Fours were held for disposition by LTV. Rarely saw a five or dark grey sample.
The processing of sheets could result in an accumulation of dirty oil, especially if we sheared hot rolled, before the cold rolled. Consequently, we did our sampling at the uncoiler, before the strip was feed into our process or shear line.
At LTV's Cleveland electro plate line, a different test was performed. They secured cold rolled steel samples, from each coil, prior to electro-plating. These samples were about 12" square. Although they could have been smaller. The flat samples were simply placed on a table. A lamp was placed on top of each respective sample. This lamp measured the amount of light reflected back from the sample. A clean sample would reflect almost 100% of the light. A smutty (dirty) sample would reflect less light.
With experience they learned how dirty a strip could be, and still be successfully plated. I don't remember who manufactured the hand held lamp and I don't have the valuations LTV used to make their pass - fail decision.
In general, batch annealed cold rolled steel will be smuttier than continuous annealed.
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