Preheating Welds

Weld preheating is the purposeful application of heat to warm metal to a desired temperature prior to welding.

Preheating slows down the cooling rate of the weld and gives the metal more time to form a good microstructure, release internal stresses and dissipate hydrogen from the weld.

Keywords: thickness, weld puddle, cooling rate, temperature.

Why preheat welds?

• The cooling rate affects the weld’s final properties such as hardness and ductility. Preheating prior to welding will:
• Reduce the chance of catastrophic cracking in the weld from micro-cracks and trapped hydrogen gas.
• Reduce the hardness and brittleness of the weld due to rapid cooling.
• Reduce the amount of distortion caused by the weld introducing stress into the part.
• Reduce the amount of shrinkage stress from the differential temperatures present between the weld and parent metal.

The metal in all three dimensions around the weld puddle is raised to the predetermined temperature (up to 300oC (570oF) for steel) before welding and then allowed to cool- off slowly once welded.

Without adequate preheat, the cooling would be rapid and intolerably high hardness and brittleness would occur in the weld and the heat-affected zone (HAZ) neighboring the weld.

When do you preheat?

Common materials that require preheat are steels, cast irons, copper (and its alloys) and aluminum.

Often the heat from welding is sufficient to preheat the metal. However, preheating the weld is required when the metal:

• is below 20°C
• conducts heat away very fast (such as aluminum, copper and both thick and thin steel sections)
• requires slow cooling to form the correct microstructure after melting (like cast irons and thin steel sections)
• will be highly stressed when in use (pressure vessels, lifting equipment, etc)

The need for preheat increases as the following factors change:

• The larger the mass being welded
• The lower the temperature of the piece being welded
• The lower the atmospheric temperature
• The smaller the welding rod diameter
• The greater the speed of welding
• The higher the carbon content in the steel
• The higher the manganese content
• The greater the alloy content in air hardening steels
• The more the air hardening capacity of the steel
• The greater the difference in mass between pieces being joined
• The more complicated the shape or section of the part

How to preheat

Preheating can be done with gas torches or banks of torches, by electrical heating elements strapped to the item, in ovens or furnaces, with induction heating by an electric field and by radiant heating from a short distance off the part.

The metal is preheated to the same temperature in all three dimensions for a distance at least equal to its thickness and to a minimum distance of 75 mm all around the weld point.

How much preheat do you apply?

Correct preheating practice considers both the temperature to be reached and the area to be heated in order to produce the required length of cooling time.

The amount of preheat depends on:

• The metal’s chemistry and composition (Alloying elements in a metal affect its microstructure)
• The thickness of material (Fast cooling causes cracking)
• The joint geometry, shape, and complexity (Thin sections cool faster than thick sections)
• The restraint and rigidity of the members (Parts stressed by loads need careful control of the microstructure development)
• The preheating process (Each preheat method has different rates of heat input into the metal)
• The welding process heat input (Lower preheat temperatures are needed if the welding process introduces a lot of heat)

The amount of preheat is indicated by measuring the temperature of the metal before welding.

Temperature can be determined by using:

• indicating crayons that melt at prescribed temperatures (use high and low temperature indicating crayons so both the upper and lower limit can be monitored)
• electronic pyrometers and contact thermometers
• thermocouples attached to the part and connected to a display read-out.

It is important to control the cooling rate of each weld pass.

Usually, a metal that requires preheating must be kept at that temperature between weld passes.

Most times a weld pass inputs sufficient heat but on big welds or on long welds additional heating from torches may be necessary.

The rule is to “keep it hot”.

Controlling the cooling rate

Molten metal cooled off fast freezes the microstructure in the wrong form and is hard, brittle, and full of micro-cracks.

Molten metal cooled down at a slow, controlled rate consists of well-shaped crystals with alloy elements well distributed throughout the microstructure and few micro-cracks.

Once a part is welded the rate of cooling in the weld and HAZ must be slowed to acceptable limits. Tables and charts specifying cooling rates are available from welding material suppliers.

The usual practice is to wrap the area around the weld in insulation.

If the section is thick and the cooling is done in a draft free area it may be unnecessary to use insulation.

What if preheat cannot be applied?

If situations arise where preheat cannot be applied there are several options which can be adopted.

• Use low hydrogen electrodes to reduce the risk of cracking.
• Peen the weld as it cools off with a blunt pointed hammer. The hammer blows from peening the weld vibrate the microstructure and tend to break it up into finer crystals.
• Use multiple weld passes to seal the join. Each subsequent pass tends to heat treat the preceding welds and provides a cover to reduce the rate of heat loss.

Mike Sondalini – Maintenance Engineer

If you found this interesting, you may like the ebook Centrifugal Pump Problems & Answers.