Proposed Methodology Changes to Determine Minimum Design Metal Temperature of ASME/API Impact Tested Materials Based on Fracture Mechanics

For equipment designed to ASME or API standards, it is common practice to perform impact testing of base material and/or weldments to establish the Minimum Design Metal Temperature (MDMT). The impact test is typically a Charpy V-Notch (CVN) test and the test temperature is set equal to the MDMT. The required Charpy energy at MDMT can vary anywhere from 10 ft-lbs to 40 ft-lbs depending on material specification, thickness, and the ASME/API standard. The detailed historical background behind the Charpy energy requirements of different ASME/API standards is not well documented. Additionally, no credit is given for post weld heat treatment (PWHT) of impact tested materials. The CVN tests are used because they are quick and economical for quality control, but the tests only provide a relative indication of material toughness. Consequently, the current impact test requirements lead to inconsistent results in brittle fracture assessments, conducted through explicit fracture mechanics. In this paper, two examples are presented to highlight the inconsistencies of the current impact test requirements. A methodology of estimating MDMT for impact tested materials based on fracture mechanics, consistent with Welding Research Council (WRC) Bulletin 562 [1] is also presented. Furthermore, this methodology explicitly accounts for the effects of PWHT (and the influence of weld residual stress on crack driving force) for impact tested materials. A methodology of adjusting MDMT for in-service impact tested materials is also presented. In the interest of moving towards harmonizing the impact test requirements, an alternative procedure for establishing impact test requirements is presented for ASME/API consideration.