ABSTRACT

In the refining industry, Incoloy 800H (UNS N08810) has been employed for components operating in the intermediate temperature regime (500~700 °C). However, it is known to suffer from relaxation cracking in the HAZ after short duration in service. In the present study, detailed microstructure of the crack-tip region of a failed tube was examined using SEM, TEM and EBSD to clarify the relaxation crack mechanism. The current findings suggest that dislocation enhanced M23C6 precipitation near grain boundary promoted oxidation near grain boundary, and then a crack propagated along the oxide layer or interface between the oxide layer and base metal. Details of the microstructural findings and a proposed mechanism of stress relieve cracking will be discussed.

INTRODUCTION

In the refining industries, austenitic steels (UNS S30400, S31909, S34700, N08810, N 06617) are often employed in the intermediate temperature range (500~700°C). However, these austenitic steels suffer from stress relaxation cracking, which is referred to as Reheat Cracking, Stress Relief Cracking or Strain Oxidation Cracking.1-5 Characteristics of stress relaxation crack are well summarized in the past work.1,2,4 Some important features of the stress relaxation cracking listed here are collected from pastliterature.1-5 First, the typical location of cracking is HAZ (Heat Affected Zone), weld metal or cold deformed area where hardness value exceeds 200Hv. Second, the stress relaxation cracking is normally intergranular, and cavities are often observed along grain boundary ahead of the crack tip. Third, a Ni-rich metallic filament exists and is bounded by a Cr-rich oxide layer near the crack tip.

However, some researchers have not observed metallic filament.1,2 Finally, the critical temperature range for cracking has been known to be 550~750°C. Although characteristics of stress relaxation cracking and mitigation method (e.g., heat treatment at 900~950°C) are well established in the industry practice, crack propagation mechanisms are not still understood.4 Hence, in the current paper, a failed 800H (UNS N08810) tube was examined to improve the understanding of crack propagation mechanisms using state-the art of tools.

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