Steel with an extremely low-carbon content re- quires the highest annealing temperature. As the carbon content increases, the annealing temperatures decrease. Nonferrous  Metal Copper becomes hard and brittle when mechani- cally worked; however, it can be made soft again by annealing.  The  annealing  temperature  for  copper  is  be- tween  700°F  and  900°F.  Copper  maybe  cooled  rapidly or slowly since the cooling rate has no effect on the heat treatment.  The  one  drawback  experienced  in  annealing copper is the phenomenon called “hot shortness.” At about 900°F, copper loses its tensile strength, and if not properly  supported,  it  could  fracture. Aluminum reacts similar to copper when heat treat- ing. It also has the characteristic of “hot shortness.” A number of aluminum alloys exist and each requires special heat treatment to produce their best properties. NORMALIZING Normalizing is a type of heat treatment applicable to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forg- ing, forming, or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, you should normalize it first to ensure the maximum desired results. Usually, low-carbon steels do not re- quire normalizing; however, if these steels are normal- ized, no harmful effects result. Castings are usually annealed, rather than normalized; however, some cast- ings require the normalizing treatment. Table 2-2 shows the approximate soaking periods for normalizing steel. Note that the soaking time varies with the thickness of the  metal. Normalized steels are harder and stronger than an- nealed steels. In the normalized condition, steel is much tougher  than  in  any  other  structural  condition.  Parts subjected  to  impact  and  those  that  require  maximum toughness with resistance to external stress are usually normalized. In normalizing, the mass of metal has an influence on the cooling rate and on the resulting struc- ture. Thin pieces cool faster and are harder after normal- izing than thick ones. In annealing (furnace cooling), the hardness of the two are about the same. HARDENING The  hardening  treatment  for  most  steels  consists  of heating the steel to a set temperature and then cooling it rapidly by plunging it into oil, water, or brine. Most steels  require  rapid  cooling  (quenching)  for  hardening but  a  few  can  be  air-cooled  with  the  same  results. Hardening increases the hardness and strength of the steel, but makes it less ductile. Generally, the harder the steel, the more brittle it becomes. To remove some of the brittleness, you should temper the steel after hard- ening. Many nonferrous metals can be hardened and their strength increased by controlled heating and rapid cool- ing. In this case, the process is called heat treatment, rather  than  hardening. To harden steel, you cool the metal rapidly after thoroughly soaking it at a temperature slightly above its upper  critical  point.  The  approximate  soaking  periods for hardening steel are listed in table 2-2. The addition of alloys to steel decreases the cooling rate required to produce hardness. A decrease in the cooling rate is an advantage, since it lessens the danger of cracking and warping. Pure iron, wrought iron, and extremely low-carbon steels have very little hardening properties and are dif- ficult to harden by heat treatment. Cast iron has limited capabilities for hardening. When you cool cast iron rapidly, it forms white iron, which is hard and brittle. And when you cool it slowly, it forms gray iron, which is soft but brittle under impact. In  plain  carbon  steel,  the  maximum  hardness  ob- tained by heat treatment depends almost entirely on the carbon  content  of  the  steel.  As  the  carbon  content  in- creases, the hardening ability of the steel increases; however, this capability of hardening with an increase in carbon content continues only to a certain point. In practice, 0.80 percent carbon is required for maximum hardness. When you increase the carbon content beyond 0.80 percent, there is no increase in hardness, but there is an increase in wear resistance. This increase in wear resistance is due to the formation of a substance called hard cementite. When you alloy steel to increase its hardness, the alloys make the carbon more effective in increasing hardness and strength. Because of this, the carbon con- tent  required  to  produce  maximum  hardness  is  lower than it is for plain carbon steels. Usually, alloy steels are superior  to  carbon  steels. 2-4