In making fine-tool steel a bad charge in the pot inevitably means a bad piece of steel. It may happen also that an iron of apparently good analysis will not produce a really fine steel; then there must be a search for unusual elements, such as copper, arsenic, antimony, etc., or for dirt, left in the iron by careless working. The refining-test then is as necessary as analysis, for if steel will not refine thoroughly it will not make good tools. Battering-tools, such as sledges, hammers, flatters, etc., should be refined carefully, for although their work is mainly compressive they are liable to receive, and do get, blows on the corners and edges that would ruin them if they were not in the strongest condition possible.
The reasons for refining hot-working tools have been stated already. Engraved dies for use in drop-presses where they are subjected to heavy blows are undoubtedly in the most durable condition when they are refined, but they are subjected not only to impact, but to enormous compression, and therefore they must be hardened deeply. When a die-block is heated so as to refine, and then is quenched, it hardens perfectly on the surface and not very deeply, and it is quite common in such a case to see a die crushed by a few blows: the hardened part is driven bodily into the soft steel below it, and the die is ruined; thus:
To avoid this, such a die should be heated to No. 5, or a dark lemon, and quenched suddenly in a large volume of rushing water.
It will then have the enormous resistance to compression that is so well known in very hard steel, and it will be hardened so deeply that the blow of the hammer will not crush through the hard part. This is the best condition, too, of an armor-plate that is to resist the impact of a projectile.
It will be brittle, a light blow of a hammer will snip the corners, but it cannot be crushed by ordinary work. Dies made in this way have turned out thousands of gross of stamped pieces, showing no appreciable wear.
To harden a die in this way is a critical operation, because the strains are so enormous that a very trifling unevenness in the heat will break the piece, but the skill of expert temperers is so great that they will harden hundreds of dies in this way and not lose one if the steel be sound.
HEATING FOR HARDENING.
A smith can heat an occasional piece for hardening, in his ordinary fire by using care and taking a little time. Where there are many pieces to be hardened, special furnaces should be used.
For thousands of little pieces, such as saw-teeth or little springs, a large furnace with a brick floor, and so arranged that the flame will not impinge on the pieces, is good.