Introduction

Temperature control is an important part of welding quality. In many welded joints, especially carbon steels, low alloy steels, thick sections and restrained joints, the correct preheat and interpass temperature can reduce the risk of cracking, control cooling rate and support consistent weld quality.

BS EN ISO 13916:2025 gives requirements for measuring three important welding temperatures:

1. preheating temperature;
2. interpass temperature;
3. preheat maintenance temperature.

The standard applies mainly to fusion welding, but it can also be used for other welding processes where appropriate. It does not apply to the measurement of post-weld heat treatment temperature (PWHT).

1. What is preheating temperature?

Preheating temperature, written as Tp, is the temperature of the workpiece in the weld zone immediately before welding starts.

In simple terms, it is the temperature the joint must reach before the first weld run is deposited.

Preheat is normally specified as a minimum temperature. For example, a WPS may state:

Minimum preheat temperature: 100°C

This means welding should not start until the weld zone has reached at least 100°C.

Preheat is commonly used to slow the cooling rate of the weld and heat-affected zone. It can also help reduce the risk of hydrogen-assisted cold cracking in susceptible materials.

2. What is interpass temperature?

Interpass temperature, written as Ti, is the temperature of the weld area immediately before the next weld run is deposited in a multi-run weld.

In simple terms, it is the temperature of the weld before the welder deposits the next pass.

Interpass temperature is normally specified as a maximum temperature. For example, a WPS may state: Maximum interpass temperature: 250°C. This means the next weld run should not be deposited if the weld area is hotter than 250°C.

Controlling interpass temperature is important because excessive heat build-up can affect weld metal properties, corrosion resistance, toughness, distortion and microstructure.

3. What is preheat maintenance temperature?

Preheat maintenance temperature, written as Tm, is the minimum temperature that must be maintained in the weld zone if welding is interrupted.

This is important where the joint must not be allowed to cool below a specified temperature before welding continues. For example, if welding stops for lunch, shift change, inspection or repair, the weld area may need to be kept warm until welding restarts.

4. Difference between preheat, interpass and preheat maintenance temperature

Temperature type	Symbol	When measured	Usually specified as
Preheating temperature	Tp	Before welding starts	Minimum
Interpass temperature	Ti	Before the next weld run	Maximum
Preheat maintenance temperature	Tm	During welding interruption	Minimum

Also read: The Role of Preheat in Welding: Part 1

5. Where should preheat temperature be measured?

For workpiece thickness up to and including 50 mm, the temperature is normally measured on the surface facing the welder.

Where the heat source is centred on the groove, the measurement point is normally taken at: A = 4 × t

where:

• A is the distance from the longitudinal edge of the groove (see the included drawings below);
• t is the material thickness.

However, this distance should not be more than 50 mm from the edge of the groove.

Example

If the material thickness is 10 mm: A = 4 × 10 = 40 mm. Therefore, the preheat temperature should normally be measured about 40 mm from the edge of the groove.

If the material thickness is 20 mm: A = 4 × 20 = 80 mm,. But because the maximum distance is 50 mm, the temperature should normally be measured 50 mm from the edge of the groove.

6. What if the material is thicker than 50 mm?

For workpieces thicker than 50 mm, the required temperature should exist in the parent metal for at least 75 mm from the joint preparation, unless otherwise agreed.

Where practical, the temperature should be measured on the face opposite the heated side. This helps confirm that the heat has penetrated through the thickness. If it is not practical to measure the opposite face, measurement can be taken on the heated face after allowing time for temperature equalisation.

A useful guide is to allow approximately: 2 minutes for every 25 mm of parent metal thickness

before confirming the temperature.

7. Where should interpass temperature be measured?

Interpass temperature should be measured on the weld metal or on the parent metal immediately adjacent to the weld.

The key point is that the measurement must represent the actual temperature of the weld area immediately before the next run is deposited. The distance between points of measurement are shown below in accordance with BS EN ISO 13916:2025.

8. When should interpass temperature be measured?

Interpass temperature should be measured immediately before applying the next weld run. This is important because the temperature of the weld area can change quickly, especially on thick materials, high heat input welds or welds made with local preheating.

If the temperature is too high, welding should stop until the weld area cools to within the WPS limit. If the temperature is too low and a minimum temperature is required, the area should be reheated before welding continues.

9. What equipment can be used to measure welding temperature?

The equipment used for temperature measurement should be specified in the Welding Procedure Specification.

Common temperature measurement methods include:

Method	Abbreviation	Example
Temperature-sensitive material	TS	Temperature crayons, temperature paints
Contact thermometer	CT	Digital probe thermometer
Thermocouple	TE	Attached thermocouple for monitoring
Contactless device	TB	Infrared or optical temperature device

The selected equipment should be suitable for the welding environment, the material, the temperature range and the required accuracy.

10. Temperature-sensitive crayons and paints

Temperature-sensitive crayons or paints are simple and widely used in fabrication shops. They are useful for checking whether a surface has reached a specified temperature. However, they are usually better for spot checks than continuous monitoring. They should be applied correctly and used within their intended temperature range.

11. Contact thermometers

Contact thermometers use a probe to measure the surface temperature.

They are useful where a direct reading is needed. The probe must make proper contact with the workpiece, and the surface condition should allow reliable measurement. Poor contact, scale, surface contamination or incorrect positioning can give unreliable readings.

12. Thermocouples

Thermocouples are useful where temperature needs to be monitored continuously or recorded during welding.

They are commonly used for critical welding applications, thick sections, controlled preheating and temperature monitoring during interruptions.

13. Contactless temperature measurement

Contactless devices, such as infrared thermometers, can be useful where access is difficult or where rapid checks are needed.

However, care is required because surface condition, distance, angle, emissivity, reflection and scale can affect the reading.

For critical work, the suitability of contactless measurement should be confirmed before use.

Also read: The Role of Preheat in Welding: Part 2

14. What should be recorded?

Where temperature records are required, the welding temperature record should include, as applicable:

• measured preheating temperature, in °C;
• measured interpass temperature, in °C;
• measured preheat maintenance temperature, in °C;
• the measuring method used;
• any deviation from the specified method;
• reference to the applicable WPS;
• reference to BS EN ISO 13916:2025.

15. Example of temperature designation

A measured preheat temperature of 120°C using a contact thermometer may be recorded as:

Temperature ISO 13916:2025 Tp 120 – CT

An interpass temperature measured several times, with the lowest reading of 135°C and highest reading of 160°C using a thermocouple, may be recorded as:

Temperature ISO 13916:2025 Ti 135/160 – TE

This type of designation makes the record clear, traceable and easy to understand.

16. Why preheat and interpass temperature control matters

Preheat and interpass temperature control can affect:

• weld cooling rate;
• risk of hydrogen-assisted cracking;
• hardness in the heat-affected zone;
• weld metal toughness;
• residual stress;
• distortion;
• microstructure;
• corrosion resistance in some materials;
• repeatability of welding quality.

For this reason, temperature measurement should not be treated as a casual activity. It is a welding control variable and should be managed through the WPS, inspection plan and quality records.

17. Practical welding inspection points

Before welding starts, confirm that the required preheat temperature has been reached.

During multi-run welding, measure the interpass temperature before the next run is deposited.

If welding is interrupted, maintain the required preheat maintenance temperature where specified.

Always measure at the correct location.

Use suitable and calibrated or verified equipment where required.

Record the result when the WPS, inspection plan, contract or quality system requires a record.

Conclusion

BS EN ISO 13916:2025 provides a structured approach for measuring preheating temperature, interpass temperature and preheat maintenance temperature in welding.

The main purpose is to ensure that the temperature stated in the WPS is measured at the correct time, at the correct location and with suitable equipment.

Good temperature control helps improve weld quality, reduce cracking risk and support consistent welding procedure compliance.

Frequently Asked Questions

What is BS EN ISO 13916:2025?

BS EN ISO 13916:2025 is the British adopted version of the international standard for measuring preheating temperature, interpass temperature and preheat maintenance temperature in welding.

Does BS EN ISO 13916:2025 apply to post-weld heat treatment?

No. The standard does not apply to the measurement of post-weld heat treatment temperatures.

What is preheat temperature in welding?

Preheat temperature is the temperature of the workpiece in the weld zone immediately before welding starts.

What is interpass temperature in welding?

Interpass temperature is the temperature of the weld and adjacent parent metal immediately before the next weld run is deposited.

Is preheat temperature a minimum or maximum value?

Preheat temperature is normally specified as a minimum value.

Is interpass temperature a minimum or maximum value?

Interpass temperature is normally specified as a maximum value.

Where should preheat temperature be measured?

For thickness up to 50 mm, it is normally measured on the surface facing the welder at a distance of 4 × t from the groove edge, but not more than 50 mm.

Where should interpass temperature be measured?

Interpass temperature should be measured on the weld metal or immediately adjacent parent metal before the next weld pass is deposited.

What is preheat maintenance temperature?

Preheat maintenance temperature is the minimum temperature that must be maintained in the weld zone if welding is interrupted.

What equipment can be used to measure welding temperature?

Common equipment includes temperature crayons, temperature paints, contact thermometers, thermocouples and contactless temperature measuring devices.

Should temperature measuring equipment be listed in the WPS?

Yes. The equipment or method used for temperature measurement should be specified in the Welding Procedure Specification.

Why is interpass temperature important?

Interpass temperature is important because excessive heat build-up can affect weld quality, mechanical properties, corrosion resistance, distortion and metallurgical structure.

Also read:

The Role of Preheat in Welding: Part 1

The Role of Preheat in Welding: Part 2