Thermal Treatment of Steel Bars
Steel bars are often furnace treated to provide a desired microstructure and hardness level for subsequent machining or cold working operations, or to obtain specific in-the-bar mechanical properties for parts application, or to make feasible cold drawing or turning. Of the more common furnace treatments, annealing, normalizing, heat treating (quenching and tempering) precede cold finishing while stress relieving is done afterwards.
Annealing is the heating in a furnace at a predetermined temperature usually above the transformation temperature and then allowing the steel to cool at a controlled rate in the furnace. Its purpose is to control the microstructure and hardness of the bars for improved machinability or subsequent cold working operations. Except for the higher carbon alloy steels (over 0.50% C) and carbon steels (over 0.60%) a lamellar pearlitic structure is preferred for machinability, while for severe cold working, a spheroidized anneal is preferred for all carbons.
Special purpose anneals are sometimes employed to produce grain structure suitable for cold heading or other special operations.
All carbon steels with a maximum of the carbon range of over 0.55% and all alloy steels with a maximum of the carbon range over .38% require annealing or some other form of furnace treating before cold finishing.
Solution annealing is a process performed on steels. In our case, these are primarily the 300 series stainless. The process consists of heating the material up to a temperature above 1950ºF and holding it long enough for the carbon to go into solution. After this, the material is quickly cooled to prevent the carbon from coming out of solution. Solutio-annealed material is in its most corrosion-resistant and ductile (farmable) condition.
Normalizing consists of heating the bars in a furnace to a temperature above the transformation range and cooling them in the air. It is rarely specified for cold finished bar steels.
Heat treating has a specific and restricted meaning when applied to cold finished bars. It consists of heating the bars beyond the "red" temperature (usually 1500 degrees F), quenching in water or oil from this temperature and then tempering or drawing back in the furnace at some lower temperature.
The quenching from these high temperatures hardens the steels while the tempering softens them to the desired hardness or strength. Tempering also relieves the stresses that are set up in the bar in the very rapid cooling quenching. Tempering is always done at temperatures lower than the quenching temperature.
The purpose of heat treating is the attainment of required mechanical properties usually at the expense of machinability. For more precise control of these mechanical properties, heat treated bars are often stress relieved after cold finishing.
Stress relieving is variously known as stress relieving, strain annealing, strain tempering, etc. It consists of furnace treatment wherein the steel bars are heated at relatively low temperatures rarely higher than 1150 degrees F. Its object may be to relieve the stresses in the bar that are responsible for warping or distortion in machining or it may be employed in combination with cold drawing to obtain the desired physical properties. Stress relieving can be applied to any grade of steel after a furnace treatment or after cold finishing.