`Copper-bearing carbon steel railroad application`

1880s discovery that small copper additions (<0.2%) doubled atmospheric corrosion resistance in railroad steels. Used to motivate why "carbon steels" carry more than just carbon.

Today I wanted to talk about hardenability — I've mentioned it, but I haven't really discussed why we have this need for it. It goes back to a question a student asked me last year: why do you have so many steels? If you go through a book on steels, you'll find for the carbon steels — carbon, manganese, phosphorus, sulfur, silicon — and then these others: copper and lead can be up there in very small percentages. Copper is there for corrosion resistance, going back to the 1880s. People found that some steels with a little bit of copper had better corrosion resistance in the railroads. So the railroads specified a copper-bearing carbon steel with less than two-tenths of a percent copper, and got double the corrosion resistance in the atmosphere. Lead is sometimes put in steel for machinability — it forms little inclusions. Iron and lead don't mix, so you form these little lead inclusions, and when you're machining, the lead melts and gives you a liquid lubricant. Now we're getting away from lead in what we call free-machining steels.