This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The facility with which steel may be welded to steel diminishes as the metal approximates to cast iron with respect to the proportion of carbon; or, what amounts to the same thing, it increases as the metal approximates to wrought iron with respect to absence of carbon. Hence in welding together 2 pieces of steel - caeteris paribus - the more nearly their melting points coincide - and these are determined by the amount of carbon they contain - the less should be the difficulty. (Percy.) Puddled steel welds very indifferently, and so does cast steel containing a large percentage of carbon. The mild cast steels, also shear and blister steel, can be welded with ease. In welding cast steel, borax or sal-ammoniac, or mixtures of them, are used as fluxes. Another used for mining drills in America is a mixture of 6 qt. powdered limestone and 1 qt. sulphur; heat very carefully with frequent turnings, take from the fire and brush with a short besom, dip into the mixture, and return to the fire, 4 or 5 times, before the heat is on. (See also Workshop Receipts, Third Series, pp. 293-303.)