Estimate a practical starting preheat range from material condition, thickness, carbon equivalent, hydrogen level and joint restraint.
Preheating raises the temperature of the base metal around the weld before welding begins. Its main purpose is to control the cooling behavior of the weld and heat-affected zone rather than simply making the welding area warm.
For susceptible carbon and low-alloy steels, rapid cooling can produce hard microstructures in the heat-affected zone. If diffusible hydrogen is present and the joint is highly restrained, the combination can increase the risk of hydrogen-assisted cracking.
Increasing preheat generally slows the cooling rate. This can reduce excessive hardness and allow hydrogen more opportunity to diffuse from the weld area. The correct temperature, however, depends on the material, thickness, welding process, hydrogen control, restraint and applicable welding procedure.
Carbon equivalent is commonly used as an indicator of steel hardenability and weldability. Several carbon-equivalent formulas exist, and the appropriate formula depends on the material specification and welding standard.
One widely used formula is the IIW carbon equivalent:
CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
The values in this formula are elemental percentages by mass. A higher calculated carbon equivalent generally indicates increased hardenability and a greater need for control of cooling rate and hydrogen.
| Steel / Condition | Typical Situation | Approximate Starting Range | Important Considerations |
|---|---|---|---|
| Low-carbon steel | Thin section, low restraint, controlled hydrogen | Often ambient or approximately 50°C+ | Actual requirements depend on grade, thickness and code. |
| Medium-carbon steel | Moderate thickness and restraint | Approximately 100–200°C | Hydrogen control and cooling rate become increasingly important. |
| Higher carbon-equivalent steel | Thicker or restrained joint | Approximately 150–250°C+ | Use qualified welding procedures and code requirements. |
| Low-alloy steel | Thickness and hydrogen-sensitive weld | Approximately 100–250°C+ | Material-specific procedure requirements may be substantially different. |
| Martensitic stainless steel | Welding heat treatment required | Grade-specific | Preheat and interpass requirements should come directly from the qualified procedure. |
| Austenitic stainless steel | Normal fabrication | Usually little or no preheat | Controlling heat input and interpass temperature is generally more important than high preheat. |
Thicker material can remove heat from the weld area more rapidly because there is more surrounding cold metal acting as a heat sink. This can increase the cooling rate after welding.
For susceptible steels, faster cooling can increase the formation of harder microstructures in the heat-affected zone. Increasing preheat helps reduce this cooling rate.
Thickness is therefore one of several factors used when determining minimum preheat. It should not be considered independently from carbon equivalent, hydrogen level, joint restraint and welding heat input.
Hydrogen-assisted cracking is a major reason preheat is important for susceptible steels. Hydrogen can enter the weld from consumables, moisture, surface contamination and the welding environment.
Low-hydrogen electrodes and properly controlled consumables can reduce the available hydrogen. Drying and storage requirements for welding consumables should follow the manufacturer's instructions and the applicable welding procedure.
Preheat does not eliminate hydrogen. It is one part of a broader cracking-control strategy that can include low-hydrogen consumables, adequate preheat and interpass control, suitable heat input, joint design, cleaning and post-weld thermal treatment where required.
Joint restraint affects the stresses that develop as the weld and surrounding metal heat and cool. Highly restrained joints can be more susceptible to cracking because shrinkage is less able to occur freely.
| Restraint Level | Examples | Potential Effect |
|---|---|---|
| Low | Flexible fabrication, limited surrounding constraint | Lower shrinkage restraint |
| Moderate | Typical structural fabrication | Intermediate restraint and cracking considerations |
| High | Heavy sections, rigid assemblies, highly constrained joints | Greater concern for residual stress and cracking |
Preheat is the minimum temperature of the base metal around the weld area immediately before welding begins. Interpass temperature is the temperature of the weld and adjacent base metal before depositing the next weld pass.
They are related but are not interchangeable. A WPS may specify both a minimum preheat and a maximum interpass temperature.
| Term | Meaning | Purpose |
|---|---|---|
| Preheat | Minimum temperature before welding | Controls cooling rate and helps reduce cracking risk. |
| Interpass temperature | Temperature before the next weld pass | Controls accumulated heat and weld microstructure. |
| Post-weld heat treatment | Controlled heating after welding | May relieve stresses, temper microstructures or meet material requirements. |
The required preheat temperature should be measured on the workpiece in the area specified by the applicable welding procedure. The measuring method may include temperature-indicating crayons, contact thermometers, infrared instruments or other qualified temperature-measurement equipment.
Infrared measurements can be affected by surface emissivity, surface condition and viewing angle. For critical welding work, the measurement method should comply with the applicable procedure or standard.
The entire required preheat zone should be heated sufficiently rather than relying on a very small hot spot immediately beside the joint. Localized heating can create temperature gradients that do not represent the required base-metal condition.
What is welding preheat? Welding preheat is controlled heating of the base metal before welding. It is used primarily to control cooling rate and reduce cracking risk in susceptible materials.
Why is preheat required for some steels? Higher carbon equivalent, greater thickness, high restraint and increased hydrogen exposure can make some steels more susceptible to hardening and hydrogen-assisted cracking.
Does every steel weld require preheat? No. Some low-carbon steels and thin sections can be welded without significant preheat under appropriate procedures, while higher-hardenability steels may require substantial preheat.
Does thicker steel always need more preheat? Not automatically, but increasing thickness commonly increases heat loss from the weld area and can increase the required preheat for susceptible materials.
What is carbon equivalent? Carbon equivalent is a calculated representation of the effects of carbon and alloying elements on steel hardenability and weldability.
What is the IIW carbon equivalent formula? A commonly used IIW formula is CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, with elemental concentrations expressed as percentages.
Can stainless steel require preheat? Some stainless steels, particularly martensitic grades, can have specific preheat and post-weld heat-treatment requirements. Austenitic stainless steels commonly do not require high preheat, and excessive heat can create other problems.
What happens if preheat is too low? For susceptible steels, insufficient preheat can allow rapid cooling, excessive hardness and increased hydrogen-assisted cracking risk.
What happens if preheat is too high? Excessive temperature can negatively affect microstructure, productivity and dimensional control. Some stainless steels can also be adversely affected by excessive heat input or interpass temperature.
Is preheat the same as post-weld heat treatment? No. Preheat occurs before welding, while post-weld heat treatment is a controlled thermal process performed after welding when required.
Can this calculator replace a WPS? No. It is a preliminary engineering reference. The applicable welding code, material specification, qualified WPS, project specification and welding engineer's requirements take precedence.