Carburized mild steel is used to a great extent in the manufacture of automobile and other parts which are likely to be subjected to rough usage. The strength and ability to withstand hard knocks depend to a very considerable degree on the thoroughness with which the carburizing process is conducted. Many automobile manufacturers have at one time or another passed through a period of unfortunate breakages, or have found that for a certain period the parts turned out of their hardening shops were not sufficiently hard to enable the rubbing surfaces to stand up against the pressure to which they were subjected. So many factors govern the success of hardening that often this succession of bad work has been actually overcome without those interested realizing what was the weak point in their system of treatment. As the question is one that can create a bad reputation for the product of any firm it is well to study the influential factors minutely. INTRODUCTION OF CARBON The matter to which these notes are primarily directed is the introduction of carbon into the case of the article to be hardened. In the first place the chances of success are increased by selecting as few brands of steel as practicable to cover the requirements of each component of the mechanism. The hardener is then able to become accustomed to the characteristics of that particular material, and after determining the most suitable treatment for it no further experimenting beyond the usual check-test pieces is necessary. Although a certain make of material may vary in composition from time to time the products of a manufacturer of good steel can be generally relied upon, and it is better to deal directly with him than with others. In most cases the case-hardening steels can be chosen from the following: (1) Case-hardening mild steel of 0.20 per cent carbon; (2) case-hardening 3-1/2 per cent nickel steel; (3) case-hardening nickel-chromium steel; (4) case-hardening chromium vanadium. After having chosen a suitable steel it is best to have the sample analyzed by reliable chemists and also to have test pieces machined and pulled. To prepare samples for analysis place a sheet of paper on the table of a drilling machine, and with a 3/8-in. diameter drill, machine a few holes about 3/8 in. deep in various parts of the sample bar, collecting about 3 oz. of fine drillings free from dust. This can be placed in a bottle and dispatched to the laboratory with instructions to search for carbon, silicon, manganese, sulphur, phosphorus and alloys. The results of the different tests should be carefully tabulated, and as there would most probably be some variation an average should be made as a fair basis of each element present, and the following tables may be used with confidence when deciding if the material is reliable enough to be used. TABLE 16.--CASE-HARDENING MILD STEEL OF 0.20 PER CENT CARBON