330 PREHEATING

330.1 General

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.