GOVERNING SPECIFICATION(S): MIL-H-6875, AMS2759
Normalizing is a process in which the material is heated in an atmospheric furnace to a temperature just above the upper critical line of the iron-iron carbide phase diagram. The actual temperature varies by alloy but is typically around 1600 ° F. The material is then cooled in still or only slightly agitated air. The goal of this treatment is to produce a homogeneous, uniform, austenitic structure. This is of particular importance when dealing with weldments or assemblies made of high strength steels as they tend to crack during the hardening process without first being normalized.
Annealing is a process in which the material is heated in an atmospheric furnace to a temperature just above the upper critical line of the iron-iron carbide phase diagram. The material is then cooled at a very slow rate to form coarse pearlite. This basically softens the material and makes it easier to form or machine.
Hardening is a process in which the material is heated in an atmospheric furnace to the austenitizing temperature. The temperature will vary by alloy but is typically around 1500 ° F. – 1600 ° F. for low alloy steels (4130, 4140, 4330, 4340). Tool steels and martensitic stainless steels will be in the 1800 ° F. – 2000 ° F. range. The material is then rapidly quenched in oil, air, salt baths, water or polymer. This is intended to produce a controlled martensitic microstructure in order to achieve desired hardness, tensile strength or toughness while minimizing residual stress, distortion or the possibility of cracking. After quenching, the material will be at its maximum attainable strength and hardness however brittle due to residual quenching stresses. To alleviate the residual stress and perhaps decrease the hardness or tensile strength, the parts are tempered or re-heated to a temperature suitable to obtain the desired mechanical properties.
Cold treatments are used to enhance the transformation of austenite to martensite. It is basically a continuation of the quenching process. Decreasing retained austenite will produce a more uniform microstructure thus increasing hardness as well as adding dimensional stability. Cold treatment temperatures are typically -100 ° F. – 120 ° F. Soak times are usually 1 – 4 hours. Cold treatments are performed after quenching and before tempering.
Stresses can be created in a number of different ways, ranging from ingot processing in the mill to the manufacturing of the finished product. Some of these include rolling, casting, forging, bending, machining, grinding, welding, and quenching. In ferritic materials, the main result of residual stress is the significant reduction is resistance to fracture, stress corrosion cracking, and warping. For steel, the normal procedure would be to uniformly heat the parts to about 1100 ° F. then slowly cool back to room temperature.
PRECIPITATION HARDENING – AGING
This is a hardening process performed on PH stainless steels i.e. (17-4, 15-5, 13-8, etc.) Parts are heated to a temperature of 900 ° F. – 1150 ° F. and soaked for a period of 1 – 4 hours then cooled to room temperature. After treatment the parts are said to be in a condition of H-followed by the aging temperature i.e. (H-900, H1050, etc.) Each condition will have specified hardness or tensile strength criteria. The conditions will normally be in increments of 25.