Annealing is a heat treatment process in which the sheet metal is altered on a molecular level altering its strength and hardness. The metal is heated above its recrystallization temperature, maintained at this temperature and then cooled in a controlled environment. The annealing process is used to improve ductility, relieve stress in the material, refine its crystal structure and most importantly for us improve its bending properties. The annealing process improves bending consistency and reduces the chances of cracking.
Types Of Annealing
Normalization is a form of annealing which uses the air to cool the sheet metal. It is not uncommon that this air is a controlled atmosphere to prevent oxidation of the surface. The sheet metal is brought to a temperature above the transformation range, soaked for a brief period of time and then cooled well below this range in the air. This can improve a piece’s toughness after bending. Normalization grants a higher toughness and tensile strength compared to standard annealing.
Intermediate annealing, sub-critical annealing and in-process annealing all refer to what is known as process annealing. This process is used to improve the ductility of the work piece. This allows the part to be bent and cold worked further without cracking or breaking. It is used in sheet metal that is going to be rolled, drawn and spun. The temperature range for process annealing is typically below its austenizing temperature. Key to this method is holding the piece at this temperature for an extended period of time, then cooling it slowly at room temperature.
Full annealing achieves a very ductile state for the sheet metal alloy. In this process the metal is heated to about 50 ° above the austenic range and then held for a sufficient amount of time allowing the sheet metal to fully austentize. A slow cooling process allows the material to maintain its structure. Furnace cooling is often the preferred method.
Short Cycle Anneal
Short cycle annealing is a method which heats, cools, reheats and re-cools the method in short cycles for up to 8 hours. This increases the malleability of the steel.
Basic Annealing With A Torch
Many of the more practical applications for annealing are found in the work shop where a kiln is not available, but a torch is. Be sure that theres is nothing in contact with, or around the sheet metal you’re annealing before beginning. Use a large torch tip with a softer flame and try to heat the area you wish to anneal as quickly and evenly as possible. The metal should just begin to glow red under dim lighting. If you have a bright environment then you can draw concentric lines with a permanent marker around the area you are annealing. The lines will turn white or disappear entirely once the temperature of the steel is correct. Try to maintain the part at this temperature for a minute or so. Then quench the area with water. If may be necessary to clean with acid in order to restore the metal’s color.
Basic Kiln Annealing
Thinner sheet metal responds well to conventional shop kilns. Set the temperature of your kiln to 1100 ° Fahrenheit and allow it to come up to temperature. Place the sheet metal part into the kiln and allow it to heat for approximately 10 minutes, larger pieces with heavier gauges may require more time. Remove the part from the kiln and quench in water. As with torch annealing some cleaning may be necessary.
Uses For Annealing
Beyond the high end uses of annealing, ie treating finished pieces to achieve properties required by your customer there are some basic low tech uses for annealing in your sheet metal shop. When working with metals a sheet metal worker can easily begin to work harden a piece. Work hardening is means that each time you bend the metal it becomes stiffer and harder. Over time this will cause your piece to crack. In order to ‘reset’ the piece and soften it the worker can anneal a work piece by taking a torch to it and heating it until it glows brightly. Then remove the torch and allow it to cool in the air. For non ferrous metals you can typically quench the piece with water or oil. Doing this will soften the metal and allow you work continue working with it without worrying about it snapping or cracking. This can potentially save a workpiece in which an operator made a bad bend which needs to be flattened and re-bent. Without annealing the piece the work involved in flattening and re-bending can cause cracks to form.