Inspectioneering Journal

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

By Greg Garic, P.E., Senior Staff Consultant at Stress Engineering Services, Inc. This article appears in the November/December 2022 issue of Inspectioneering Journal.

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. 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.

This content is free for registered users

Register today to read this article for free.

Create your free account and you'll also get access to:

  • Unlock one premium article of your choosing per month
  • Exclusive online content, videos, and downloads
  • Insightful and actionable webinars
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.

Comments and Discussion

Posted by James Guillory on February 6, 2023
Greg - Thank you for writing this article. Very... Log in or register to read the rest of this comment.

Add a Comment

Please log in or register to participate in comments and discussions.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Training Solutions

Improve your skills in key mechanical integrity subjects.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.


Read short articles and insights authored by industry experts.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

Videos & Webinars

Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.