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FFS Forum: New Annex 9J – Using Fracture Mechanics to Determine the Minimum Allowable Temperature (MAT)

By Greg Garic, Principal at Stress Engineering Services, Inc. This article appears in the November/December 2022 issue of Inspectioneering Journal.
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What is the MAT?

The minimum allowable temperature, or “MAT” is a term defined in API 579 Part 3, Brittle Fracture. It is defined as:

“…the lowest (coldest) permissible metal temperature for a given material and thickness based on its resistance to brittle fracture.”

The MAT can be represented by a single temperature, or if desired, it can be cast as a function of load (e.g., pressure) to give a set of MATs specific to different load levels. The latter is often referred to as a “minimum pressurization temperature” (MPT) curve.

Part 3 MAT Determination

The brittle fracture rules of Part 3 define the MAT in terms of the “Exemption Curve.” The exemption curve is used in the ASME Boiler & Pressure Vessel Code (BPVC), API 579 Part 3 (brittle fracture), and other codes, to screen for brittle fracture without having to perform actual Charpy testing. The requirement for brittle fracture screening entered most pressure system codes around 1987.

Before discussing the new Annex 9J procedure, let’s briefly review the original Part 3 procedure.

The exemption curve procedure requires only material “governing thickness” and the material specification to establish the MAT. For the exemption curves, materials are lumped into four groups: Group A – Group D. Group A are the worst performing materials (from a brittle fracture standpoint); Group D are the best.

The “governing thickness” is defined in the BVPC, Section VIII, Division 1 (VIII-1), paragraph UCS-66, or API-579-1:2021 (API 579) para. 3.4.2.1(e). For most of the full penetration butt welds in pressure vessel construction, the governing thickness is just the nominal thickness of the components at the weld joint.

A reproduction of the exemption curves is included in Figure 1. To use the exemption curve, first identify the governing thickness of the material on the X-axis. Then follow that thickness value up to where it intersects the applicable material curve (Curves A – D). This allows the corresponding MAT to be read directly off the Y-axis.

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