Why preheat metal before welding

When preheating is specified, the entire weld joint should be heated evenly through the material thickness to the desired minimum temperature. To obtain a uniform temperature through the material thickness, it is desirable to apply the heating sources to one side of the material surface and to measure the material temperature on the opposite side. Whenever the heating and temperature measurement must be conducted from the same surface, the inspector must assure that more than just the surface of the material has been heated.

It is important to ensure that the entire material thickness has been heated to a uniform temperature. In addition to establishing a preheat temperature, an interpass temperature limitation may need to be considered for some applications. This information should be shown in the welding procedure specification. When an interpass temperature is specified, the weld area must be inspected prior to depositing the next weld bead. Welding may not continue if the measured temperature exceeds the maximum interpass conditions specified in the welding procedure.

The weldment must be permitted to cool down to the specified upper limit of the interpass temperature before continuing with the weld. For instance, a procedure for welding mild steel, which has a low carbon content, relatively low hardenability, and is used in an application with no special service requirements, may consider a minimum preheat and interpass temperature based on the material thickness.

Welding procedures used for the heat-treatable low alloy steels and chromium-molybdenum steels with impact requirements will normally specify a minimum and maximum requirement for preheat and interpass temperatures.

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Print Article Share. Welding preheat is important in many applications. Learn about the benefits and drawbacks of four common welding preheat methods. When to use welding preheat Determining if a welding application requires preheat depends on several factors, including the type and thickness of the base material. Methods of preheating. There are four common welding preheat methods: Induction How it works: Induction creates a magnetic field that generates eddy currents within the base metal, heating it internally from within.

Induction accessories, such as cables or blankets, are placed on the part to generate the magnetic field. Pros: Induction offers quick setup, often in less than five minutes, and a fast time to temperature. Preheat can be applied through various means. The choice of method of applying preheat will depend on the material thickness, weldment size and the heating equipment available at the time of welding. The methods can include furnace heating for small production assemblies or, for large structural components, arrays of torches, electrical strip heaters, induction heaters or radiation heaters.

It is important to apply preheat correctly, with appropriate monitors and controls, and also to monitor the interpass temperature the temperature of the workpiece between welding the first and subsequent passes , to ensure that it does not fall below the preheat temperature.

Common techniques for monitoring preheat are temperature indicating crayons see FAQ: What is a Tempil stick? 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. There are tables and equations that can be used to calculate the preheat temperature precisely.

Generally, for higher carbon steels, a maximum preheat of degrees F degrees C is acceptable. For alloy steels, degrees F degrees C will do. Warming up to welding temperatures must be controlled properly.

Preheating can be accomplished several ways. For shop welding, an open flame from a propane torch is the most common method.