June 10, 2014
Q: We are expanding our job shop and adding the capability to work with much heavier structures, including considerably thicker materials. Some of these jobs specify high-strength materials and preheating. Some of our welders claim preheating is performed to remove water from the steel. Can you expand on this?
A: This is an excellent topic for discussion! Preheating and postweld stress relief are key points when it comes to welding thicker material sections or high-strength steels (HSS). More often than not, the reasons given for these requirements are misunderstood or simply incorrect.
Let’s start by clarifying one fundamental point: Steel does not contain water. When you heat steel with a torch, water will form several inches away in all directions from the flame. This occurs because the gas being burned to produce the flame is a hydrocarbon, which is composed of hydrogen, oxygen, and carbon. When consumed in a chemical reaction such as a flame, the byproducts are mainly carbon dioxide and water. As the hot gases from the flame containing this water get farther away from the flame, the cooler temperatures of the ambient air and steel surface cause the water to condense out of the gases and collect on the surface of the steel plate. This phenomenon appears to be water coming out of the steel, which we know is impossible. Unfortunately, common phrases such as "sweating the steel" or "drive the moisture out" mislead people in the industry as to what is actually occurring.
Preheating steel before welding is necessary for two reasons: First, it raises the overall temperature of the material, which results in a slower cooling rate of the base materials and the weld. Thicker sections of steel, generally 1/2 inch or more, and HSLA (high-strength low-alloy) steels are susceptible to formation of brittle microstructures if the weld cools too quickly. This brittle microstructure we are trying to avoid is called martensite, which is a brittle crystalline structure.
In welding applications, martensite may form in the deposited weld metal; base material being welded; or the heat-affected zone (HAZ), which is the base metal adjacent to the fusion zone. Applying the appropriate preheat before welding will help prevent the martensite from forming. Preheat temperatures vary by the thickness, type, grade, and carbon equivalency of the base material. Note: If preheating is required, be sure to do it before any welding—including tack welding—takes place.
Second, preheating results in removing moisture (condensate) before welding. If this is not done, the moisture will change phase during welding, and hydrogen will be absorbed into the weld metal. As hydrogen diffusion has a time and temperature relationship, removing most of the moisture before welding is recommended. In short, if hydrogen molecules gets trapped in the solidified weld metal, they can wedge their way between the grain boundaries, possibly resulting in a welding failure.
Thoroughly preheating the material and covering it with an insulating medium after welding to reduce the cooling rate even more this allows the hydrogen to diffuse out of the weld joint, which prevents hydrogen cracking. Residual weld stresses and over constrained weld joints can contribute to this phenomenon as well. Sometimes you’ll need to conduct postweld stress relieving to undermine these potential issues.
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