Damage Control: Brittle Fracture Mitigation

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

The previous Damage Control article in this three-part series on brittle fracture summarized modern engineering assessment methods intended to evaluate pressure equipment for protection against unstable fracture. Specifically, ASME Boiler & Pressure Vessel Code guidance for establishing component minimum design metal temperature (MDMT) was outlined, and relevant background information, including limitations, associated with the existing ASME impact test exemption curves (in both ASME Section VIII, Divisions 1 and 2) was rendered [1, 2]. Additionally, the brittle fracture screening and assessment procedures, including the use of modern fracture mechanics, the Fracture Toughness Master Curve, and commentary on ductile tearing in API-579-1/ASME FFS-1 Fitness-For-Service (API 579) were described [3].

This installment of Damage Control concludes the series on brittle fracture and provides practical insight into common brittle fracture and ductile tearing mitigation strategies. This includes mandating favorable equipment mechanical design characteristics, material selection, welding/heat treatment specifications (including understanding limitations associated with alternatives to heat treatment), and process/operating controls to limit low temperature excursions and the potential for crack-initiation due to cyclic operation or aggressive environmental service. Special considerations are generally necessary for heavy-walled hydroprocessing applications where high-pressure/high-temperature hydrogen environments and low-chrome material properties add to the complexity associated with predicting brittle fracture. Furthermore, leveraging fracture mechanics methods (described in Part 2 of this series) and coupling critical flaw sizing calculations with a logical inspection strategy for pressure equipment is an effective way to manage the risk associated with potential brittle fracture failures. This article will further explore the now-widely recognized low toughness issue affecting carbon steel flanges and fittings and will also discuss the risks associated with the removal of mandatory post-weld heat treatment (PWHT) for carbon steels of any thickness in the 2014 Edition of ASME B31.3, "Process Piping" [4]. Lastly, general good engineering practice and practical recommendations related to these topics will be offered herein.