The ability to produce steel is basic to the development of economic, political, and military power. Steel is made by refining either pig iron or scrap steel, or by refining a combination of both materials. In the quest for steels of higher quality, particularly those lower in gas content (dissolved hydrogen, oxygen, and nitrogen) and with fewer nonmetallic inclusions (oxides), various vacuum techniques have been developed. Carbon steels are those in which carbon is the alloying element that essentially controls the properties of the alloys, and in which the amount of manganese cannot exceed 1.65% and the copper and silicon contents must each be less than 0.60%. Alloy steels contain appreciable quantities of alloying elements in addition to carbon. There are three common classes of stainless steel: (1) austenitic, (2) ferritic, and (3) martensitic.