Is it safe and wise, then, for steel-users to ignore composition?
Users of tool-steel may do so safely, because the smallest variations will manifest themselves so unmistakably that they give immediate warning, and the steel-maker must keep his product up to a rigid standard of excellence or lose his character and his trade. Many of the ablest users of structural steel take a similar ground, and say, We have nothing to do with method or composition if the material meets our tests.
It is believed that if these men knew how easy it is for a skilful worker to doctor temporarily an off heat by a little manipulation, and how dangerous the same may become by a little off practice in the field, they would be convinced that some limits should be put upon composition, especially if they could realize that a reasonable specification would add nothing to cost, as competition would take care of that.
The reader is referred again to [Chap. X] on impurities.
V.
GENERAL PROPERTIES OF STEEL.
Steel is very sensitive to heat. In general it may be stated that, starting with cold steel, every degree of heat added causes a change in size and in structure, until the limit is reached where disintegration begins. The changes are not continuous; there are one or two breaks in the line, notably at the point where we have what is called recalescence; this is a marked phenomenon and it will be considered later.
The effects of heat are permanent, so that it is a fact that every variation of temperature which is marked enough to be visible to the naked eye will leave a structure, due to that variation, when the steel is cold, which will be observable by the naked eye, and such structure, when not influenced by external force, such as by hammering or rolling, is as invariable and certain as is the structure of an ingot due to the quantity of carbon present.
This property furnishes what may be called the steel-maker’s and the steel-user’s thermometer. By its means the steel-maker can discover every irregularity in heating that may have been perpetrated by the operatives; so also the steel-user can decide whether the steel furnished him has been heated and worked uniformly and properly, and later he can tell whether those who have shaped this steel to its final forms have done their work properly. A thorough knowledge of this property is essential to a steel-maker; until he possesses it he is not fit to conduct his business. It is of great importance to the steel-user, and every engineer should try to acquire a knowledge of it in order that he may not be fooled by the carelessness or rascality of those who have preceded him. The steel-maker acquires this knowledge by daily contact with the facts; the engineer does not have it forced upon him in this way, but he should seek opportunities of observation, which will be abundant in his earlier practice when he is sent upon inspection duty. Like the structure of ingots, this heat-structure cannot be illustrated on paper, and an attempt to do so would be misleading; attempts at description will be made in the hope that by their means the engineer will have a pretty good idea what to look for, and to know when his suspicions should be aroused.
In addition to the ocular observations mentioned it has been shown by specific-gravity determinations, and by delicate electrical tests through small ranges of temperature, that steel is as truly thermometrical as mercury.
Steel passes through or into four general conditions due to heat. First, in the cold state, it is a crystalline solid of no uniform structure, for its structure is influenced by every element that enters into it, and by every irregularity of heat to which it has been subjected.