Heating to extreme temperatures (or chilling) will harden or soften metal. The process may alter the physical, and sometimes chemical, properties of the metal. Heat treatment is also done to glass and other materials, but we'll focus on metal today. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, and quenching. Metallurgists often employ complex heat treating schedules in the aerospace industry to develop the desired properties. A superalloy may go through five or more heat treatment processes. Heating and cooling that occur during the manufacturing process include hot forming or welding. These are incidental to manufacturing and are not done with the intent to alter properties of the material.
What Happens During Heat Treatment?
The microstructure of metallic materials can be manipulated by controlling the diffusion rate or cooling. Martensite transformation may take place during heat treatment; crystals will be intrinsically deformed. Most crystal structures or lattice are specifically arranged, but will rearrange depending on temperature and pressure. Allotropy or polymorphism may occur at different temperatures. Properties that may change could include dissolving or changing whether a metal is soluble. Metals and non-metals may go through martensite transformation when cooled quickly. The insoluble atoms may not leave the solution fast enough and a diffusionless transformation will take place. Trapped atoms prevent the matrix from changing. Martensite transformation will harden some metals, but soften aluminum alloy. The homogeneity of the alloy will be changed during diffusion mechanism.
Time and Temperature
Time and temperature will affect the results of heat treatment. Proper heat treatment requires precise temperature control. Temperature must be held for the right length of time and the cooling must be done at the proper rate. Most heat treatments begin by heating an alloy beyond the upper transformation temperature. The heat needs to completely penetrate the alloy so it is a solid solution. Smaller grain size will increase toughness, shear strength, and tensile strength. To achieve this, metal will be heated just above the upper critical temperature, keeping the grains from overgrowth. Larger grains have larger boundaries that are weak spots in the metal.
Just as moisture in the air can produce clouds, snow, or hail, precipitation can produce different size particles. Cooling controls precipitation. Different size particles result in different metal properties. Changes in solid solubility and temperature will produce fine particles, impeding dislocation, or defects in the crystal's lattice. Alloys undergoing precipitation hardening must be held at high temperatures for hours to allow precipitation to take place. No wonder the process is also called age hardening. This heat treatment technique will increase the yield strength of aluminum, magnesium, nickel, titanium, and some stainless steels.
Metals that are quenched are cooled very quickly. This is usually done to produce the martensite transformation mentioned earlier. Ferrous alloys are hardened. Non-ferrous are made softer. The process involves heating beyond the upper critical temperature and quick cooling. Cooling may be done with forced air or by liquid cooling. Quenching too fast can result in cracking.
To make martensitic steel less brittle, the metal can be tempered. Many quenched materials are also tempered. Tempering is heating steel to below the lower critical temperature to improve toughness. Some yield strength may be lost. Tempering may be performed by quenching above the martensite start temperature, and holding until internal stress is relieved.
Other heat treatments may include other hardening techniques, annealing, stress relieving, normalizing, cold treating, or cryogenic treating. Watch for a future article on these techniques.
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