Damage Control: High-Temperature Creep Detection

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

High-temperature creep is a time-dependent phenomenon characterized by gradual, continuous deformation, affecting all metals and alloys, under the combined action of temperature and stress. Furthermore, stress levels below the time-independent yield strength of a given material can be sufficient to cause inelastic creep deformations and damage, especially for long loading times and elevated temperatures [1]. In general, creep damage accumulation and the propensity for rupture/failure is a function of material properties (including chemical composition, microstructure, heat treatment, strength, component geometry/constraint, and previous loading history or deformations that may be present), operating stresses (whether load or displacement-controlled), metal temperatures, and time in service. Creep damage accumulation over time can lead to void growth and crack formation, eventual rupture of pressure equipment, and failure of engineering components and structures.

Time-dependent creep deformation in metals was first observed in the nineteenth century [2]. Specifically, in the 1830s, Vicat observed time-dependent deformation (that could not be explained by typical elastic material behavior) of hardened iron wire bridge cables [3]. Later, in the late 1800s, additional observations of material behavior in steam engines and time-dependent relaxation of stress in members subject to fixed strain were documented [4]. By the early 1900s, quantitative proof of creep characteristics in a variety of materials had been established, and plastic flow dependency on temperature and loading rate in carbon steels had been described. Furthermore, the first systematic study on the topic of time-dependent material behavior, including the delineation of various stages of creep, was published in 1910 by Andrade [5]. This early experimental work involved testing lead wires under deadweight loading at room temperature. While the significance of this investigation would not be fully recognized until years later, this study laid the initial groundwork for understanding and quantifying time-dependent behavior of metals. As the twentieth century progressed, the demand for durable steels in power plants and chemical process equipment capable of withstanding higher temperatures promoted additional research related to creep behavior. Further developments in understanding creep damage in metals occurred between 1919 and 1926, where it was noted that material properties based on short-term tests are not applicable when components are exposed to elevated temperatures [6-8]. Subsequently, in 1929, Bailey and Norton published a relationship between creep rate and stress, which was intended to replace Hooke’s Law when analyzing stresses and strains at elevated temperatures [9,10]. This early creep work created the foundation for modern creep mechanics, modeling approaches, and test methods that are still in use today.

This installment of Damage Control will focus on typical damage morphology and inspection methods used to identify and characterize high-temperature creep. A high-level overview of creep behavior in carbon steels and other alloys will also be presented, and generally accepted temperature thresholds defining the lower boundary of the creep regime for varying materials will be discussed. Additionally, commonly affected pressure equipment in the oil and gas, petrochemical, power generation, and related industries will be outlined, and critical factors influencing damage susceptibility will be reviewed. Furthermore, examples of creep damage and in-service failures will be summarized to offer insight into this complex damage mechanism. Note that high-temperature hydrogen attack (HTHA) and creep-fatigue interaction will not be examined in this article [11,12].