Advanced Methods for More Accurate Brittle Fracture Screening Assessments

Most pressure equipment utilized in the refining and petrochemical industry is constructed of carbon or low alloy steel where metal temperature highly influences the fracture toughness. At low temperatures, the material tends to behave in a brittle manner, making it much more susceptible to fracture. Pressure vessels, piping, and tanks may experience low temperatures from the ambient environment or from operating and upset conditions. An engineering evaluation is typically required to determine permissible pressure-temperature limits. Unfortunately, significant inconsistencies and inadequacies exist in the current brittle fracture screening procedures in the various ASME and API design codes and standards as well as the 2016 edition of API 579-1/ASME FFS-1, Fitness-For-Service (API 579).^[1] This article provides an overview and discussion on brittle fracture (including details regarding several industry failures) along with a brief summary of deficiencies and concerns with current published methods for screening susceptibility of equipment to potential brittle fracture failures. Lastly, this article will provide details and a case study example demonstrating the benefits of implementing a fracture mechanics approach to establish safe operating limits.

Introduction

Brittle fracture is the sudden, rapid propagation of a crack-like flaw under stress (residual or applied) where the material exhibits little or no evidence of ductility or plastic deformation.^[2] This definition outlines the three key components that drive susceptibility to brittle fracture failures in carbon and low alloy steels:

• Crack-like defect: Brittle fracture typically initiates at a crack-like defect. Defects can result from environmental damage (such as wet H₂S or caustic exposure), mechanical damage (such as gouges, dents, or fatigue), or from original fabrication (such as laminations, lack of fusion, lack of penetration, slag inclusions, porosity, etc.). Performing a detailed inspection including both surface examination techniques (such as dye penetrant examination or magnetic particle examination) and volumetric examination techniques (such as angle beam ultrasonic examination methods) can be utilized to detect and characterize any crack-like defects present in pressure equipment.
• Stress (residual and/or applied): Stress provides the energy necessary to drive a defect to fracture. Typical sources for stress include pressure, weight, and thermal loads, in addition to residual stress from the welding processes. A properly designed and executed post weld heat treatment (PWHT) stress relief operation will significantly reduce weld residual stress.
• Material fracture toughness: Fracture toughness is the ability of a material to absorb energy and plastically deform without fracturing. It is a function of both the material strength and ductility. As previously noted, for carbon and low alloy steel, toughness is also a function of metal temperature. In Figure 1, absorbed energy is plotted against test temperature to demonstrate the transition from brittle-to-ductile behavior. Absorbed energy is typically measured using a Charpy V-notch (CVN) impact test.