Heat treatment of welds is a critical activity which needs to be carried out before and after the welding process. Heat treatment before welding (Preheating) is necessary to bring the base weld metal at proper temperature.
- No welding shall be performed without preheating the joint to 10°C (50°F) when the ambient temperature is below 10° C.
- Preheating requirements for the various materials shall be as per the Welding Specification Chart provided by client.
- Preheating shall be performed using resistance or induction heating methods.
- Preheating by gas burners, utilizing oxy-acetylene or oxy-propane gas mixtures, with neutral flame may also be carried out when permitted by the Engineer-in-charge.
- Preheating shall extend uniformly to at least three times the thickness of the joint, but not less than 50 mm, on both sides of the weld.
- Preheating temperature shall be maintained over the whole length of the joint during welding.
- Temperature recorders shall be provided by the welding contractor to record the temperature.
Preheating is used, along with heat treatment, to minimize the detrimental effects of high temperature and severe thermal gradients inherent in welding. The necessity for preheating and the temperature to be used shall be specified in the engineering design and demonstrated by procedure qualification. The requirements and recommendations herein apply to all types of welding including tack welds, repair welds, and seal welds of threaded joints.
330.1.1 Requirements and Recommendations.
Required and recommended minimum preheat temperatures for materials of various P-Numbers are given in Table 330.1.1. If the ambient temperature is below 0°C (32°F), the recommendations in Table 330.1.1 become requirements. The thickness intended in Table 330.1.1 is that of the thicker component measured at the joint.
330.1.2 Unlisted Materials.
Preheat requirements for an unlisted material shall be specified in the WPS.
330.1.3 Temperature Verification
(a) Preheat temperature shall be checked by use of temperature indicating crayons, thermocouple pyrometers, or other suitable means to ensure that the temperature specified in the WPS is obtained prior to and maintained during welding.
(b) Thermocouples may be temporarily attached directly to pressure containing parts using the capacitor discharge method of welding without welding procedure and performance qualifications. After thermocouples are removed, the areas shall be visually examined for evidence of defects to be repaired.
330.1.4 Preheat Zone.
The preheat zone shall extend at least 25 mm (1 in.) beyond each edge of the weld.
330.2 Specific Requirements
330.2.3 Dissimilar Materials.
When materials having different preheat requirements are welded together, it is recommended that the higher temperature shown in Table 330.1.1 be used.
330.2.4 Interrupted Welding.
If welding is interrupted, the rate of cooling shall be controlled or other means shall be used to prevent detrimental effects in the piping. The preheat specified in the WPS shall be applied before welding is resumed.