Home                                                      Basic Heat Treating To refer to the Iron-Carbon diagram on the Metallurgy page      Click Here Heat treating is a metallurgical process that involves heating and cooling. It includes processes such as quenching, annealing,  normalizing, tempering and surface hardening. (case hardening or carburization)     QUENCHING is a cooling process that causes metals to HARDEN. Before quenching the steel must be heated to a high temperature. The steel is heated then agitated in the quenching medium , and allowed to cool. The quenching medium can be water, oil, air, and some other less common ones. To obtain the maximum hardness, the steel should be heated above the upper  transformation temperature, (line G-S-E in the Iron �Carbon diagram) where the crystal structure turns to Austenite then quenched in water. This also makes the steel brittle.  Oil is used when it is desired to make the steel less hard and brittle.  Quenching can cause cracking and distortion.     NORMALIZING  is a process that involves heating the steel above the upper transformation temperature, (line G-S-E)  and set out to cool in free air. It is left alone until it cools to room temperature. This produces a metal that is softer, more ductile,  and it relieves some internal stresses.     ANNEALING  is a process similar to normalizing except the cooling takes place more slowly. The steel is heated above the upper transformation temperature changing the crystal structure to Austenite (above line G-S-E), and left to soak at that temperature for a period of time. Next the temperature of the oven is lowered SLOWLY. This may take days, depending on the size of the part. This produces steel that is soft, easier to machine or form, free from internal stress, with a refined crystal structure.     TEMPERING is a process of  re-heating steel AFTER HARDENING to a temperature BELOW the LOWER transformation temperature (Line P-S-K in the diagram), then allowed to cool.  This returns some of the toughness and ductility and reduces some of the brittleness and internal stresses caused by hardening. Many people think that tempering hardens steel, THIS IS FALSE! For example, hardened knives are tempered so they wont break if bent. Tempering should be done immediately after hardening.      SURFACE HARDENING  is also called case hardening.  This is a process that creates a thin, hard wear resistant, outer surface, while permitting the inner core to remain soft, ductile, and tough. This is often done to parts like gears, so they have a hard wear resistant outer surface, but the inside is tougher and less brittle so that the teeth don�t break off. This is done after machining. This is usually done by packing the parts in a container full of powdered charcoal or coke. This container is heated in a furnace that is hot enough to convert some of the carbon in the charcoal to carbon monoxide. The carbon monoxide penetrates the surface of the parts, where it is absorbed by the Austenite, and deposits a thin, hard shell of high carbon steel on the part. The temperature of the furnace should be above the upper transformation temperature (G-S-E) so that the crystal structure is changed to Austenite. Case hardening should only be done in a well ventilated area or outside due to the DANGER of  CARBON MONOXIDE POISONING!!!   Flame Hardening is a type of surface hardening where the flame of an oxyacetylene torch is brought into direct contact the surface to be hardened. As soon as the SURFACE is hot, it must be quenched immediately. If the heat is given time to penetrate into the part, the entire part will be hardened, not just the outer surface. An advantage to flame hardening is that the hardened surface can be localized to a specific area of the part. (like ways of a lathe bed).