Properties of Metals Affected by Welding and Heat
Every metal contains certain physical properties that can be altered or changed when welding occurs. Understanding what these properties are and how they may change will help us make adjustments to our process when we encounter problems. While each material has many different physical properties, which can change after welding, we will discuss the ones that are most altered by welding.
Strength, can be altered drastically by welding. If the weld is made with too little heat, little penetration will occur. If the weld is made with too much heat we could destroy the chemistry of the base material. If the metal has prior heat treating history welding can have major effects. If the weld is made correctly, with the proper filler metal, it should meet or exceed the strength of the base material. The amount of heat the material sees will depend upon the welding process selected. For example, Gas Tungsten Arc Welding or TIG tends to put more heat into the material than a Gas Metal Arc Welding or MIG type process for the same size weld.
Hardness is the resistance of being scratched or indented by another material. Welding can have great effects on the hardness of the metal. If the metal was heat-treated prior to welding to increase hardness, in general the material becomes softer, and loses the heat treatment in the weld heat effected zone. Depending upon the alloy some materials get harder in the weld zone after welding and then have a subsequent heat treating process to anneal, stress relieve, or normalize the base material. A hard metal is usually a strong metal so something that affects the hardness is also going to affect the strength
Ductility is the ability of a material to be permanently deformed or stretched by loading. Ductility is also related to the strength of the material. In general, stronger material is going to be more rigid than a weaker material, for the same cross section. Adding a weld that rapidly cools off can cause some materials to loose ductility. On the other hand, many welds on a part that gets it really hot and keeps it hot for a sufficient amount of time can cause softening in the weld zone, especially something that had previously heat treated to increase strength or hardness. We need to pay close attention to any weld seam that will be bent or formed at a later time. As mentioned before, Gas Tungsten Arc Welding or TIG tends to put more heat into the material than a Gas Metal Arc Welding or Shielded Metal Arc Welding type process for the same size weld.
Corrosion Resistance can be effected greatly by welding. Stainless Steels are a great example for how welding can impact corrosion resistance. If we overheat Stainless Steel when welding, the alloy elements will separate and form carbide precipitation or what some people call sugaring. Stainless becomes sensitized between about 800-1600 degrees Fahrenheit when the chromium combines with carbon to precipitate out in the form of black badness on the back side of our weld, and heat affected area.
Other corrosion resistant metals can be susceptible to stress corrosion cracking as well. This happens when metals are under stress in a corrosive environment, and overheating has occurred in and around the weld. Once again selecting between, Gas Metal Arc Welding, Shielded Metal Arc Welding, Gas Tungsten Arc Welding or Flux Core Arc Welding will make a big difference. Selecting the proper filler metal will also help.
Longevity has a wide range for welding machines to fit all your welding needs. Check out their website (www.longevity-inc.com) or YouTube channel (www.youtube.com/longevitywelding) for more details and information about welding.