Damage Control: Metallurgical Embrittlement Mitigation

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Introduction

This article concludes this three-part Damage Control series on metallurgical embrittlement. The first two articles in this series outlined critical considerations relating to metallurgical embrittlement identification/susceptibility and engineering assessment methods, respectively. Specifically, hydrogen embrittlement, temper embrittlement, liquid metal embrittlement (LME), 885°F (475°C) embrittlement, and sigma phase embrittlement were discussed in detail, including notable parameters that increase damage proclivity and photomicrographs of accompanying cracking damage morphologies were rendered. For each type of embrittlement, quantification of associated reductions in low-temperature fracture toughness and degradation of high-temperature creep resistance (as applicable) was provided. Additional commentary on strain age embrittlement and the effects of carburization was offered, and the importance of leveraging modern fracture mechanics principles to establish minimum pressurization temperature (MPT) envelopes for heavy-walled, hydroprocessing equipment was highlighted.

This article will build upon the metallurgical embrittlement fundamentals previously discussed and will offer practical perspective on mitigating in-service material property degradation. Moreover, as described herein, methodical pressure equipment design, material selection, and welding/fabrication (and heat treatment) techniques that reflect good engineering practice play a consequential role in maximizing long-term equipment reliability, particularly in process environments where in-service embrittlement is a potential concern. To this end, understanding the key contributing factors that increase the risk for in-service material embrittlement, and ultimately crack initiation/propagation is imperative to mitigate potential catastrophic unstable fracture events. Moreover, pragmatic steps to improve material fracture properties and alleviate in-service cracking concerns will be offered in this concluding Damage Control article on metallurgical embrittlement.

Mitigating Hydrogen Embrittlement

Mechanistically, when hydrogen is present in steel, it acts to ease dislocation motion by reducing repulsive stresses between initially isolated material dislocations. These dislocations themselves generate localized stress concentrations and local plasticity in the steel and usually occur more frequently in regions of high stress gradients such as an existing flaw or defect [1-3]. The increased mobility of these dislocations results in a greater tendency for numerous dislocations to accumulate at material grain boundaries. Furthermore, dislocations moving toward the grain boundary transport atomic hydrogen with them and deposit it at this interface. This build-up of hydrogen reduces the binding energy between grains, leading to a lower barrier to failure at these interfaces, which results in the underlying hydrogen embrittlement phenomenon (e.g., a reduction in fracture toughness or an upward shift in the ductile-to-brittle transition temperature) [4,5].

The source of hydrogen includes welding electrodes, other impurities from manufacturing, high-temperature (e.g., greater than 400°F [205°C]) gaseous hydrogen environments, or lower temperature wet H₂S and hydrofluoric (HF) acid service environments [6-8]. Following welding, hydrogen cracking can be delayed and manifest in hours or even days after fabrication or repair procedures. Weld residual stress represents a significant crack driving force, in general, and is a known contributor to delayed hydrogen cold cracking. An example of delayed hydrogen cracking, initiating at the toe of a multi-pass fillet weld, is shown in Figure 1. Characteristically, hydrogen embrittlement-related cracking often occurs near areas of elevated applied and residual stress, with thicker-walled components being distinctly prone to damage because it is more difficult for hydrogen to diffuse out of the steel (that is, the outgassing process requires more time).