As an industry consensus, API 934-A is an excellent recommended practice on the materials and fabrication requirements for Cr-Mo reactors. However, it is cautious and somewhat vague on the topic of Intermediate Stress Relief (ISR) versus Dehydrogenation Heat Treatment (DHT) for the different types of welds — which reflects the industry’s varying practices. For the advanced steels, API 934-A states that DHT should only be used with Purchaser approval, and that it should not be used on restrained welds such as nozzle welds. As a result, it is common for a DHT to be permitted on longitudinal and circumferential seams to achieve the cost and schedule savings, and ISR is used for nozzle welds. There are risks to the fabricator however, as the welds remain extremely brittle after DHT (the toughness is restored after postweld heat treatment {PWHT}, and at intermediate levels after ISR), and welding defects that are acceptable per ASME Code criterias can lead to brittle fractures during subsequent fabrication steps. The costs of the repairs and delays can then be very high, especially if the cracking is not detected until after PWHT. This paper shows the risks of acceptable defects causing brittle fractures by fracture mechanics calculations, and presents some case histories of cracking. The relative costs of ISR versus DHT, versus repairs before and after PWHT are also reviewed.

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