still air. Drafts will result in uneven cooling, which will again set up strains in the metal. Prolonged soaking of the metal at high temperatures must be avoided, as this practice will cause the grain structure to enlarge. The length of time required for the soaking temperature will depend upon the mass of metal being treated. CASE HARDENING.—In many instances, it is desirable to produce a hard, wear-resistant surface or “case” over a strong, tough core. Treatment of this kind is known as “case hardening.” This treatment may be accomplished in several ways; the principal ways being carburizing, cyaniding, and nitriding. Carburizing.—When steel is heated, the pores of the metal expand, allowing it to absorb any gases to which it is exposed. By heating steel while it is in contact with a carbonaceous substance, carbonic gases given off by this material will penetrate the steel to an amount proportional to the time and temperature. The carburizing process may be applied to plain carbon steels provided they are within the low-carbon range. Specifically, the carburizing steels are those that contain no more than 0.20 percent carbon. The lower the carbon content in the steel, the more readily it will absorb carbon during the carburizing process. The amount of carbon absorbed and the thickness of the case obtained increase with time; however, the carburization progresses more slowly as the carbon content increases during the process. The length of time required to produce the desired degree of carburization and depth of the case depend upon the composition of the metal, the kind of carburization material used, and the temperature to which the metal is subjected. It is apparent that in carburizing, carbon travels slowly from the outside toward the center; therefore, the proportion of carbon absorbed must decrease from the outside to the center. A common method of carburizing is called “pack carburizing.” When carburizing is to be done by this method, the steel parts are packed with the carburizing material in a sealed steel container to prevent the solid carburizing compound from burning and retaining the carbon monoxide and dioxide gases. The container should be placed in a position to allow the heat to circulate entirely around it. The furnace must be brought to the carburizing temperature as quickly as possible, and held at this heat from 1 to 16 hours, depending upon the depth of the case desired and the size of the work. After carburizing, the container should be removed and allowed to cool in the air, or the parts removed from the carburizing compound and quenched in oil or water. The air coding, although slow, reduces warpage, and is advisable in many cases. In another method of carburizing, called “gaseous carburizing,” a carbonaceous material is introduced into the furnace atmosphere. When the steel parts are heated in this carburizing atmosphere, carbon monoxide combines with the iron to produce results that are practically the same as those described under the pack carburizing process. Cyaniding.—Steel parts maybe surface hardened by heating while in contact with a cyanide salt, followed by quenching. Only a thin case is obtained by this method; therefore, it is seldom used in connection with aircraft construction or repair. However, cyaniding is a rapid and economical method of case hardening, and maybe used in some instances for relatively unimportant parts. The work to be hardened is immersed in a bath of molten sodium or potassium cyanide from 30 to 60 minutes. The cyanide bath should be maintained at a temperature of 760° to 899°C ( 1,400° to 1,650°F). Immediately after removal from the bath, the parts are quenched in water. The case obtained in this manner is due principally to the formation of carbides on the surface of the steel. The use of a closed pot is required for cyaniding, as cyanide vapors are extremely poisonous. Nitriding.—This method of case hardening is advantageous because a harder case is obtained than by carburizing. Nitriding can only be applied to certain special steel alloys, one of the essential constituents of which is aluminum. The process involves the soaking of the parts in the presence of anhydrous ammonia at a temperature below the critical point of the steel. During the soaking period, the aluminum and iron combine with the nitrogen of the ammonia to produce iron nitrides in the surface of the metal. Warpage of work during nitriding can be reduced by stress-relief annealing, and by exposure to nitrogen at temperatures no higher than 538°C (1,000”F). Growth of the work is similarly prevented, but cannot be entirely eliminated, and some parts may require special allowance in some dimensions to take care of growth. The temperature required for nitriding is 510°C (950°F), and the soaking period from 48 to 72 hours. An airtight container must be used, and it should be provided with a fan to produce good circulation and even temperature throughout. No quenching is required, and the parts may be allowed to cool in air. 15-43