Definition – Interruption of welding means dropping from preheat temperature to ambient temperature prior to completion of welding or performing PWHT.
This aspects most applied on high heat-sensitive material like (P3, P4, P5A, P5C, 10H High alloy materials) with regarding to Cold cracking or HIC.
The weld can be interrupted and the weld may be allowed to cool prior to post weld heat treatment, provided the following conditions are satisfied:
1. A minimum of 3/8 in. thickness of weld is deposited or 25% of the welding groove is
filled, whichever is greater
2. The weld is allowed to cool slowly to ambient temperature.
3. The weld has been visually examined by a CWI/CSWIP or qualified equivalent to ensure that no cracks have occurred before welding is resumed, and
4. The required preheat has been applied before any welding is resumed.
Note that there’s no any requirement on CODE but the Oil & Gas project’s specification.
Additional knowledge of welding on Cr-Mo steel:
Welding Chrome Moly (Cr-Mo) Steel
This steel is mostly used in gas stations. It is also called creep resistant steel which means that it doesn’t be distorted even at high temperatures.
It should be welded with similar consumables and you should follow a very precise method. Preheat and post weld heat treatment are almost always involved. The most common consumables according to AWS have a suffix of B2, B3 (something like E8018-B2) and are designed only for welding Cr-Mo steels. Don’t use these consumables for any other steels. They are highly crack sensitive.
The photo shows a potential station steam pipe. Insulation covers heating coils were used to pre-heat the pipe to 250°C followed by PWHT of around 700-730°C.The welding consumables were TIG for the root deposit (AWS ER90S-B3, 2.4mm wire), thus MMA 3.2 and 4.00mm electrodes for the fill and cap (AWS E9018-B3). Often FCAW is used for filling and capping these large bore pipes (AWS E91T1-B3), using 1.2mm wire and Ar/CO2 gas mix.