1. The natural bar—untreated.
2. Quenched at dark orange or orange red.
3.““medium orange—refined.
4.““bright orange.
5.““dark lemon.
6.““medium lemon.
7.““bright lemon.
8.““very bright lemon or creamy.
Heats 6, 7, 8 will almost invariably produce
cracks although the pieces be evenly heated.

These squares do not represent absolute structures with marked divisions; they are only the steps on an incline, like the temper numbers in the carbon series; thus, the carbon-line is continuous, but the temper divisions represent steps up the incline. So with the series of squares, the changes of grain or structure are continuous, as represented by the doubly inclined line; the squares being only the steps to indicate easily observed divisions. The minuteness of the changes is illustrated by the fact that in a piece heated continuously from creamy to dark orange and quenched, differences of grain have been observed unmistakably on opposite sides of pieces broken off not more than ⅛ inch thick.

In practice the differences due to the colors given in the list above are as plain and surely marked as are the differences in the structure of ingots due to the different temper carbons already described.

In this hardened series each carbon temper gives its own peculiar grain; in low steel, say 40 carbon compared to 1.00 carbon or higher, No. 3 will be larger and No. 8 will be smaller in the low temper than in the high—another illustration of the fact that low steel is more inert to the action of heat than high steel. All grades and all tempers go through the same changes, but they are more marked in the high than in the low steel.

The grain of hardened steel is affected by the presence of silicon, phosphorus, and manganese, and doubtless by any other ingredients, these three being the most common.

It is in the grain of hardened steel that the conditions described in [Chap. V] as “sappy,” “dry,” and “fiery” are the most easily and frequently observed, although the same conditions obtain in unhardened steel in a manner that is useful to an observing steel-user. But it is in this hardened condition that the excellences or defects of steel are brought out and emphasized.

When a piece of steel is heated continuously from “creamy,” or scintillating, down to black, or unheated, and is then quenched, the grain will be found to be coarsest, hardest, and most brittle at the hottest end, and with the brightest lustre, even to brilliancy, and to become finer down to a certain point, noted as No. 3 in the series of squares, or at a heat which shows about a medium orange color; here the grain becomes exceedingly fine, and here the steel is found to be the strongest and to be without lustre. Below this heat the grain appears coarser and the steel is less hard, until the grain and condition of the unheated part are reached. This fine condition, known as the refined condition, is very remarkable. It is the condition to be aimed at in all hardening operations, with one or two exceptions which will be noted, because in this state steel is at its best; it is strongest then, and it would seem to be clear without argument that the finest grain and the strongest will hold the best at a fine cutting-edge, and will do the most work with the least wear, although a coarser grain may be a little harder, the coarser and more brittle condition of the latter more than counterbalancing its superior hardness.

The advantages of this refined condition are so great that it is found to be well to harden and refine mild-steel dies, and battering- and cutting-tools that are to be used for hot work, although the heat will draw out all of the temper in the first few minutes, because the superior strength of the fine grain will enable the tool to do twice to twenty times more work than an unhardened tool.