Warming Up to Welding: The Whys and Hows of Preheating
The decision to preheat is something a welder will consider in an industrial setting. But is it something a DIY welder needs to worry about? Like so many things, the answer is: maybe. Preheating can depend on steel grade, base metal thickness, and design.
What is preheating?
The term itself is rather self-evident, being the heating of base metal near the joint before welding.
Now for the why of preheating. It slows the cooling rate of the welded joint and the adjacent metal, producing a tougher joint that resists cracking. It also reduces the shrinkage stresses that can result during cooling, especially if the joint is part of a restrained assembly. Even without these mechanical considerations, preheating will have the effect of removing moisture and greasy residue that could result in poor fusion.
There are a couple of rules of thumb about grades and thickness. Generally, low carbon steel grades with a thickness less than one inch (25 mm) do not require preheating but should be, at minimum, at a temperature greater than standard room temperature. That means that if you are working outside on a cool day, preheating might be considered. If the steel is stored outside and brought into the shop, it needs time to warm to room temperature, or this time can be shortened by preheating.
As carbon content is increased above 0.20%, carbon or alloys like chrome or moly are added, or sections are thicker than one inch (25 mm), preheating should definitely be considered. There are tables and equations that can be used to calculate the preheat temperature precisely. Generally, for higher carbon steels, a maximum preheat of 400 degrees F (200 degrees C) is acceptable. For alloy steels, 250 degrees F (120 degrees C) will do.
How is preheating done?
Preheating can be accomplished several ways. For shop welding, an open flame from a propane torch is the most common method. It has the advantage of being low-cost and easy to use. Another method can be wielding a heat gun that is often used for paint stripping. If you want to be a bit more precise, an electric ceramic blanket will allow you to dial in the desired temperature. You may also consider using an insulated blanket to cover the preheated area to retain heat if you have any delays or interruptions during the course of performing the weld.
As the saying goes, what can’t be measured can’t be controlled. The preheat temperature can be measured using Tempilstiks, contact pyrometers, or infrared (IR) thermometers. Tempilstiks are essentially engineered crayons that will melt at a specific temperature. For example, a line drawn on the base metal near the joint with a Tempilstik rated 400 degrees F will melt when it reaches that temperature, indicating you have exceeded the preheat temperature. Tempilstiks are available at any welding supply store, and they come in many different temperature ratings. Our preferred method for measuring joint temperature is a simple IR thermometer that is available at hardware stores. It has the simplicity of point and shoot to get the temperature.
This about covers the subject of preheating. There is also the consideration of potential postwelding stress-relieving, but that is a story for another day.
Featured image credit: jbolles, Flickr