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