Carbon  steels  are  usually  quenched  in  brine  or water, and alloy steels are generally quenched in oil. When hardening carbon steel, remember that you must cool the steel to below 1000°F in less than 1 second. When you add alloys to steel, the time limit for the temperature to drop below 1000°F increases above the l-second limit, and a slower quenching medium can produce  the  desired  hardness. Quenching   produces   extremely   high   internal stresses in steel, and to relieve them, you can temper the steel just before it becomes cold. The part is removed from the quenching bath at a temperature of about 200°F and allowed to air-cool. The temperature range from 200°F down to room temperature is called the “cracking range” and you do not want the steel to pass through it. In the following paragraphs, we discuss the differ- ent methods of hardening that are commercially used. In the Seabees, we use a rapid surface hardening com- pound called “Case” that can be ordered through the Navy supply system. Information on the use of “Case” is located in the Welding Materials Handbook,  P-433. Case  Hardening Case hardening produces a hard, wear-resistant sur- face or case over a strong, tough core. The principal forms of casehardening are carburizing, cyaniding, and nitriding.  Only  ferrous  metals  are  case-hardened. Case hardening is ideal for parts that require a wear-resistant surface and must be tough enough inter- nally  to  withstand  heavy  loading.  The  steels  best  suited for case hardening are the low-carbon and low-alloy series. When high-carbon steels are case-hardened, the hardness penetrates the core and causes brittleness. In case hardening, you change the surface of the metal chemically by introducing a high carbide or nitride content.  The  core  remains  chemically  unaffected.  When heat-treated,  the  high-carbon  surface  responds  to  hard- ening,  and  the  core  toughens. CARBURIZING.— Carburizing is a case-harden- ing process by which carbon is added to the surface of low-carbon steel. This results in a carburized steel that has a high-carbon surface and a low-carbon interior. When the carburized steel is heat-treated, the case be- comes  hardened  and  the  core  remains  soft  and  tough. Two methods are used for carburizing steel. One method  consists  of  heating  the  steel  in  a  furnace  con- taining  a  carbon  monoxide  atmosphere.  The  other method has the steel placed in a container packed with charcoal  or  some  other  carbon-rich  material  and  then heated in a furnace. To cool the parts, you can leave the container in the furnace to cool or remove it and let it air cool. In both cases, the parts become annealed during the  slow  cooling.  The  depth  of  the  carbon  penetration depends  on  the  length  of  the  soaking  period.  With  to- day’s  methods,  carburizing  is  almost  exclusively  done by  gas  atmospheres. CYANIDING.—   This process is a type of case hardening that is fast and efficient. Preheated steel is dipped into a heated cyanide bath and allowed to soak. Upon removal, it is quenched and then rinsed to remove any residual cyanide. This process produces a thin, hard shell that is harder than the one produced by carburizing and can be completed in 20 to 30 minutes vice several hours. The major drawback is that cyanide salts are a deadly  poison. NITRIDING.— This  case-hardening  method  pro- duces the hardest surface of any of the hardening proc- esses. It differs from the other methods in that the individual parts have been heat-treated and tempered before nitriding. The parts are then heated in a furnace that has an ammonia gas atmosphere. No quenching is required so there is no worry about warping or other types of distortion. This process is used to case harden items, such as gears, cylinder sleeves, camshafts and other engine parts, that need to be wear resistant and operate in high-heat areas. Flame  Hardening Flame hardening is another procedure that is used to harden the surface of metal parts. When you use an oxyacetylene flame, a thin layer at the surface of the part is rapidly heated to its critical temperature and then immediately quenched by a combination of a water spray and the cold base metal. This process produces a thin, hardened surface, and at the same time, the internal parts retain their original properties. Whether the proc- ess is manual or mechanical, a close watch must be maintained, since the torches heat the metal rapidly and the temperatures are usually determined visually. Flame hardening may be either manual or automat- ic. Automatic equipment produces uniform results and is more desirable. Most automatic machines have vari- able travel speeds and can be adapted to parts of various sizes  and  shapes.  The  size  and  shape  of  the  torch  de- pends on the part. The torch consists of a mixing head, straight  extension  tube,  90-degree  extension  head,  an adjustable  yoke,  and  a  water-cooled  tip.  Practically  any shape  or  size  flame-hardening  tip  is  available  (fig.  2-1). 2-5