Hardening
The steps to hardening steel alloys are to austenitize, quench and temper. Other alloys like copper, aluminum and stainless steel, require different methods.
The only way to harden steel is to undergo phase changes. The first thing we need to do is form austenite. You cannot form any other phase without cooling from austenite. If you quench hot pearlite, you will end up with cold pearlite. If you properly quench austenite, you will end up with martensite. Austenite has the ability to dissolve up to 0.8% carbon. This is because the atoms of an austenite crystal are arranged so that they are much further apart than a crystal of ferrite.
If we have as much as 0.8% carbon dissolved in austenite, and we slowly cool it to room temperature, the dissolved carbon will precipitate out of solution, and form carbides, (Fe3C), in little striped plates alternating with the ferrite called pearlite. If we cool or quench the austenite very rapidly, we will trap the carbon atoms in the austenite crystals when they try to change to ferrite. This phase of carbon trapped in the iron crystal is called martensite.
The rate of cooling needed to form martensite varies greatly, depending on the chemistry of the steel. If you quench austenite slower than what is needed to form martensite, you will form bainite. Bainite is a microstructure between pearlite and martensite. For some applications this is desirable due to its toughness.
The rate of cooling needed for each grade of steel can be found on a Continuous Cooling Transformation (CCT) diagram or Isothermal Transformation (I.T.) diagram. Too many to list here. Bainite is a kind of phase that's in between martensite and pearlite. A steel with less than 0.20% carbon cannot form martensite at any rate of cooling. Should I repeat that? No, SAE1018 cannot be hardened.
Steels with carbon contents in the 0.20% to 0.40% range need water or brine (salt water) as a quenchant. The "speed" of a quench media is determined by the rate of heat transfer from the part to the media and is given in degrees per second. The fact that brine was a "faster" quenchant than water was discovered centuries ago when a blacksmith quenched a part in a bucket of urine. (Goat urine is best!) Higher carbon steels can be quenched in oil. There are several "speeds" of oil, but all of them are much slower than water. Some tool steels are so hardenable that the only quenchant needed is air. There are polymer quenchants on the market today but are not readily available. All quenchants work better if they are agitated vigorously.
A fully hardened steel contains martensite. Martensite is very hard. It can be as hard as 65 HRC. This means its also very brittle. The formation of martensite is so violent and brittle that many times the part cracks. This is called a quench crack. To reduce the stresses caused by the formation of martensite, we must temper it. Tempering is done by raising the temperature of the steel to a point LESS THAN the critical temperature, or austenite formation temperature. Tempering temperatures range from 800 to 1200º F. The higher the temperature, the softer the metal
