The draught should be regulated so that the flame will pass around the roof, or so nearly so as to never touch the steel, not even in momentary eddies.
In such a furnace clock-spring wire not more than .01 inch in diameter, or clock-spring strands not more than .006 to .008 inch thick and several hundred feet long, may be annealed perfectly. The steel is scaled of course, but the operation is so quick and so complete that there is no decarbonized surface under the scale.
This plan is better than the Jones method or any closed method, because the big boxes necessary to hold the strands or coils cannot be heated up without in some parts overheating the steel; all of which is avoided in the open furnace, because by means of peep-holes the operator can see what he is about, and after a little practice he can anneal large quantities of steel uniformly and efficiently.
VIII.
HARDENING AND TEMPERING.
For nearly all structural and machinery purposes steel is used in the condition in which it comes from the rolls or the forge; in exceptional cases it is annealed, and in some cases such as for wire in cables or for bearings in machinery, it is hardened and tempered.
For all uses for tools steel must be hardened, or hardened and tempered. The operations of hardening and tempering, including the necessary heating, are the most important, the most delicate, and the most difficult of all of the manipulations to which steel is subjected; these operations form an art in themselves where skill, care, good judgment, and experience are required to produce reliable and satisfactory results. It is a common idea that all that is necessary is to heat a piece of steel, quench it in water, brine, or some pet nostrum, and then warm it to a certain color; these are indeed the only operations that are necessary, but the way in which they are done are all-important.
An experienced steel-maker is often amazed at the confidence with which an ignorant person will put a valuable tool in the fire, rush the heat up to some bright color, or half a dozen colors at once, and souse it into the cooling-bath without regard to consequences. That such work does not always result in disastrous fractures shows that steel does possess marvellous strength to resist even the worst disregard of rules and facts.
On the other hand, the beautiful work upon the most delicate and difficult shapes that is done by one skilled in the art cannot but excite the surprise and admiration of the onlooker who is familiar with the physics of steel, and who can appreciate the delicacy of handling required in the operation.
There are a few simple laws to observe and rules to follow which will lead to success; they will be stated in this chapter as clearly as may be, in the hope of giving the reader a good starting-point and a plain path to follow; but he who would become an expert can do so only by travelling the road carefully step by step. The hair-spring of a watch, or a little pinion or pivot, so small that it can only be seen through a magnifying-glass, the exquisitely engraved die costing hundreds or thousands of dollars, and the huge armor-plate weighing many tons, must all be hardened and tempered under precisely the same laws and in exactly the same way; the only difference is in the means of getting at it in each case.
Referring now to properties mentioned in the previous chapters, we have first to heat the piece to the right temperature and then to cool it in the quickest possible way in order to secure the greatest hardness and the best grain. In doing this we subject the steel to the greatest shocks or strains, and great care must be used.