In Part 1, we looked at why preheat matters in welding. We discussed hydrogen assisted cold cracking, heat affected zone hardness, cooling rate, residual stress, carbon equivalent, and the relationship between preheat and weld integrity.

In Part 2, the focus moves from theory to practice.

This section explains how preheat should be measured, how it can be applied, why interpass temperature control is important, what happens when the weld becomes too hot, and which standards are commonly used for preheat and interpass temperature control.

Preheat does not make a bad weld good, but without it, a good weld can still crack.

1. Measurement and Application of Preheat

Preheat is only effective when it is applied correctly and verified properly.

It is not enough to wave a flame over the joint and assume the steel is ready for welding. The required preheat temperature must be achieved in the weld area and maintained for the period required by the Welding Procedure Specification, project specification, or applicable code.

As a general fabrication practice, preheat is commonly applied over a band extending at least 75 mm on each side of the joint. For thick sections, highly restrained joints, critical welds, or components with high heat sink, a wider heated area may be necessary.

The actual heating width should always follow the approved Welding Procedure Specification, project specification, or applicable fabrication code.

The preheat temperature should be measured before welding starts. For multipass welding, the interpass temperature should be checked immediately before depositing the next weld run.

Temperature should not be measured directly in the flame path or immediately beside the arc, because this may give a misleading reading.

Where possible, temperature should be checked on the opposite face of the joint, especially for thick sections. This helps confirm that heat has penetrated through the material thickness. Where opposite face access is not possible, the measurement location should be clearly defined in the Welding Procedure Specification or inspection plan.

A specified preheat temperature that is not verified is only an assumption.

2. Tools Used for Measuring Preheat

Common tools for measuring preheat include:

1. Contact thermometers
2. Thermocouples
3. Temperature indicating crayons, such as Tempilstik
4. Infrared thermometers

Infrared thermometers are convenient, but care is needed. Surface condition, oxidation, curvature, stainless steel reflectivity, and emissivity settings can affect the reading.

For critical work, contact thermometers or thermocouples normally provide better temperature control. The important point is simple: the required temperature must be measured, not assumed.

3. Common Methods of Applying Preheat

The method used to apply preheat depends on material thickness, component size, joint geometry, access, heating rate, production requirement, and the level of temperature control needed.

Figures below are common methods of welding preheat.

3.1 Open Flame Heating

Open flame heating is one of the simplest and most common preheat methods. It normally uses propane, natural gas, or oxy fuel torches to heat the joint area. It is practical for site work, small components, repair welding, tack welding, and local preheating.

The main limitation of open flame heating is control. Uneven flame movement can create hot spots, and the joint can cool quickly if the heat source is removed too early. For critical welds, the temperature should be checked frequently and maintained until welding begins.

Open flame heating can be effective, but it requires discipline. The operator must avoid overheating one area while leaving another area below the required temperature.

3.2 Resistance Heating Pads

Resistance heating uses ceramic pads containing electrical heating elements. The pads are placed around or near the weld area and controlled using thermocouples and a temperature controller.

This method provides better temperature control than open flame heating. It is useful for pipe welds, pressure components, thick sections, and controlled fabrication work.

Resistance heating is also useful where a more uniform heating band is required.

The disadvantage is that it requires setup time. The pads and thermocouples must be placed correctly, and the heating arrangement must be suitable for the component geometry.

For controlled welding applications, this additional setup time is often justified by improved temperature control.

3.3 Induction Heating

Induction heating uses an electromagnetic field to generate heat directly within the component.

It is fast, efficient, clean, and highly controllable when properly set up. It is particularly useful for pipe welds, heavy sections, repetitive production work, and applications where uniform heating is important.

Induction heating can also reduce the need for open flames, which may improve safety and control in some working environments.

The main limitations are equipment cost, coil access, operator training, and suitability for complex joint geometry.

Whatever preheating method is used, the key point remains the same: preheat must be measured, controlled, and maintained.

4. Interpass Temperature Control

Interpass temperature is the temperature of the weld area immediately before the next weld pass is deposited. In multipass welding, it is just as important as the initial preheat temperature.

Interpass temperature is the traffic light between weld passes. Too cold, stop and reheat. Too hot, stop and wait.

If the weld area cools below the specified minimum interpass temperature or minimum preheat temperature between passes, welding should not resume until the joint has been reheated to the required temperature.

The important point is that each weld pass must be deposited under the thermal conditions specified in the Welding Procedure Specification.

If the interpass temperature becomes too high, welding should be paused until the joint cools to within the permitted range.

Excessive interpass temperature can lead to:

1. Excessive heat build up
2. Wider heat affected zones
3. Grain coarsening
4. Reduced toughness
5. Higher distortion
6. Loss of control over final weld properties

For steels, interpass temperature affects the cooling time between 800°C and 500°C, commonly known as the t8/5 cooling time. This cooling period has a major influence on the final microstructure formed in the weld and heat affected zone.

If cooling is too fast, hard and brittle microstructures may form. If cooling is too slow, toughness and strength may be affected, depending on the steel grade, welding process, heat input, and procedure requirements.

Interpass control is therefore not just a temperature check between passes. It is a key part of controlling cooling rate, hardness, toughness, hydrogen cracking risk, and overall weld quality.

The practical rule is simple: do not continue welding unless the joint temperature is within the range specified by the approved Welding Procedure Specification.

5. Maximum Interpass Temperature

Too cold can crack the weld. Too hot can damage the steel.

Maximum interpass temperature is the highest temperature permitted in the weld area immediately before the next weld pass is deposited. It is a critical welding variable because excessive heat accumulation can damage the properties of the weld and heat affected zone.

If the interpass temperature is too high, the joint may remain hot for too long. This can lead to excessive grain growth in the heat affected zone, reduced toughness, higher distortion, and loss of control over the final weld properties.

In some materials, especially quenched and tempered steels, TMCP steels, and high strength steels, excessive interpass temperature can cause softening in the heat affected zone. This may reduce strength and compromise the intended performance of the welded joint.

High interpass temperature can also increase the total thermal exposure of the joint. This may be harmful where impact toughness, hardness control, corrosion resistance, dimensional control, or service performance is important.

For many carbon and low alloy steels, maximum interpass temperatures may commonly fall within the range of about 230°C to 300°C. However, this range is not universal. Some materials require much lower limits, while others may allow higher temperatures depending on the welding procedure, material grade, thickness, heat input, service requirement, and applicable code. For example, preheat for stainless steel could be about 150°C. However, the correct limit is the one specified in the approved Welding Procedure Specification.

Interpass temperature should never be guessed. It should be measured, recorded where required, and controlled throughout welding.

6. Practical Control Using Temperature Indicating Crayons

A practical method of controlling interpass temperature is to use two temperature indicating crayons, such as Tempilstik. One crayon represents the minimum required temperature. The other represents the maximum permitted temperature.

Before depositing the next weld pass, the minimum temperature crayon should melt. This confirms that the joint is hot enough to continue welding. The maximum temperature crayon should not melt. This confirms that the joint has not exceeded the permitted limit.

For more controlled work, thermocouples, contact thermometers, or calibrated digital instruments may be used. The measurement location should be clearly stated in the Welding Procedure Specification or inspection plan.

A sketch may also be used to show where the temperature is to be checked, especially on critical welds or complex joint geometries.

7. Standards and Codes for Preheat and Interpass Temperature

Preheat and interpass temperature should not be selected by guesswork. They should be determined from the applicable code, material standard, welding procedure qualification, project specification, and approved Welding Procedure Specification.

Several standards and codes provide guidance on preheat, interpass temperature, hydrogen cracking control, and temperature measurement. The standard to be used depends on the structure, material, location, client specification, design code, and contractual requirement.

Therefore, welders, supervisors, inspectors, and welding engineers should not rely on general rules alone. The approved Welding Procedure Specification remains the controlling document at the point of welding.

8. Common Standards Used for Preheat and Interpass Temperature

The following standards and codes are commonly used or referenced for preheat, interpass temperature, welding procedure specification, welding procedure qualification, and hydrogen cracking control.

8.1 BS EN ISO 13916:2025: Welding. Measurement of preheating temperature, interpass temperature and preheat maintenance temperature.

This is one of the key standards for measuring preheat temperature, interpass temperature, and preheat maintenance temperature for fusion welding. It is particularly useful for defining how and where these temperatures should be measured.

8.2 BS EN ISO 15609-1:2019: Specification and qualification of welding procedures for metallic materials. Welding procedure specification. Part 1: Arc welding.

This standard covers the content of Welding Procedure Specifications for arc welding. Where applicable, the Welding Procedure Specification should state the preheat temperature, interpass temperature, preheat maintenance temperature, and any postheating requirement for hydrogen release.

8.3 ISO 15614-1:2017: Specification and qualification of welding procedures for metallic materials. Welding procedure test. Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys.

This standard covers welding procedure qualification for steels and nickel alloys. Preheat and interpass temperature are important because changes in these temperatures can affect hardness, microstructure, toughness, and cracking risk.

8.4 ISO/TR 17671-1:2002: Welding. Recommendations for welding of metallic materials. Part 1: General guidance for arc welding.

This technical report gives general guidance for arc welding of metallic materials. It provides useful background guidance on welding practice and welding related controls.

8.5 BS EN 1011-2:2001: Welding. Recommendations for welding of metallic materials. Arc welding of ferritic steels.

This standard is widely used for guidance on arc welding of ferritic steels. It includes methods for estimating preheat temperature to reduce the risk of hydrogen cracking.

8.6 AWS D1.1/D1.1M:2025: Structural Welding Code. Steel.

This code provides structural welding requirements for steel. It includes minimum preheat and interpass temperature requirements for different steel groups and thickness ranges.

8.7 CSA W59: Welded Steel Construction (Metal Arc Welding).

This standard provides requirements for welded steel construction and includes guidance on minimum preheat and interpass temperature.

9. Important Note on Standards

The standards listed above should always be checked against the project specification and client requirement before use.

A project may require a specific edition, a national adoption, a contract specific revision, or a different design code.

For site welding, shop fabrication, inspection, and procedure qualification, the approved Welding Procedure Specification remains the controlling document.

10. Conclusion

Preheat is not just a welding formality. It is one of the most important controls against hydrogen assisted cold cracking in steels.

By raising the starting temperature of the joint, preheat slows the cooling rate, reduces heat affected zone hardness, supports hydrogen diffusion, lowers thermal severity, and improves the chance of producing a sound weld.

However, preheat must be controlled properly. Too little heat can allow cracking. Too much heat can damage mechanical properties, increase distortion, and reduce toughness.

This is why preheat and interpass temperatures must be specified, measured, and controlled in accordance with the approved Welding Procedure Specification and applicable standards.

In critical welding, preheat is not a matter of convenience. It is a matter of weld integrity.

A weld can fail before it is even finished if the steel is too cold, too hard, too restrained, and too full of hydrogen. Preheat helps break that dangerous chain before cracking begins.

Frequently Asked Questions

What is preheat temperature in welding?

Preheat temperature is the minimum temperature to which the parent metal is heated before welding starts. It helps slow the cooling rate, reduce heat affected zone hardness, support hydrogen diffusion, and lower the risk of cracking.

Why must preheat be measured before welding?

Preheat must be measured because the required temperature must be achieved in the weld area before welding starts. If the temperature is assumed rather than checked, the weld may cool too quickly and become more susceptible to cracking.

Where should preheat temperature be measured?

Preheat temperature should be measured in the area specified by the Welding Procedure Specification or inspection plan. It should not be measured directly in the flame path or immediately beside the arc. Where possible, checking the opposite face of thick material helps confirm heat penetration through the thickness.

How far from the weld should preheat be applied?

As a general fabrication practice, preheat is often applied over a band extending at least 75 mm on each side of the joint. However, thicker sections, restrained joints, critical welds, and specific codes may require a wider heated area.

What tools are used to measure preheat?

Common tools include contact thermometers, thermocouples, temperature indicating crayons, and infrared thermometers. For critical work, contact thermometers and thermocouples usually provide better temperature control.

Are infrared thermometers reliable for preheat measurement?

Infrared thermometers can be useful, but they must be used carefully. Surface condition, oxidation, curvature, stainless steel reflectivity, and emissivity settings can affect the reading. For critical welds, contact measurement is often preferred.

What are the common methods of applying welding preheat?

Common methods include open flame heating, resistance heating pads, and induction heating. The best method depends on material thickness, component size, joint geometry, access, heating rate, and required temperature control.

What is interpass temperature in welding?

Interpass temperature is the temperature of the weld area immediately before the next weld pass is deposited. It is important in multipass welding because it controls the thermal condition between weld runs.

What happens if interpass temperature is too low?

If interpass temperature is too low, the weld may cool too quickly. This can increase heat affected zone hardness, reduce hydrogen diffusion time, and increase the risk of hydrogen assisted cold cracking.

What happens if interpass temperature is too high?

If interpass temperature is too high, excessive heat accumulation can occur. This may cause grain coarsening, reduced toughness, higher distortion, softening in some steels, and loss of control over weld properties.

What is maximum interpass temperature?

Maximum interpass temperature is the highest permitted temperature in the weld area immediately before the next weld pass is deposited. It is specified to prevent overheating and protect weld and heat affected zone properties.

Why are two temperature crayons sometimes used for interpass control?

Two temperature crayons can be used to check both minimum and maximum temperature limits. The minimum temperature crayon should melt to confirm the joint is hot enough. The maximum temperature crayon should not melt, confirming that the limit has not been exceeded.

What is t8/5 cooling time?

t8/5 cooling time is the time taken for a weld or heat affected zone to cool from 800°C to 500°C. In steels, this cooling period strongly influences microstructure, hardness, toughness, and cracking risk.

Which standard covers measurement of preheat and interpass temperature?

BS EN ISO 13916 covers measurement of preheating temperature, interpass temperature, and preheat maintenance temperature for fusion welding.

Which standard covers WPS requirements for preheat?

BS EN ISO 15609-1 covers the content of Welding Procedure Specifications for arc welding. Where applicable, the WPS should include preheat temperature, interpass temperature, preheat maintenance temperature, and any postheating requirement.

Can welders choose preheat temperature by experience alone?

No. Experience is useful, but preheat temperature should be specified in the approved Welding Procedure Specification and based on material type, thickness, carbon equivalent, hydrogen level, restraint, heat input, service requirement, and applicable standard.

Is preheat always beneficial?

No. Preheat is beneficial when required, but excessive heat can damage weld properties, increase distortion, reduce toughness, or cause softening in some steels. The correct temperature range must be controlled.

What is the most important rule for preheat and interpass control?

The most important rule is simple: weld only when the joint temperature is within the range specified by the approved Welding Procedure Specification.

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