ABSTRACT

Recent experiences with duplex 2205 stainless steel welds have shown that Sulfide Stress Cracking (SSC) can occur in air-fan cooler header-boxes over years leading to catastrophic failures. Known microstructural parameters such as ferrite phase fraction seems to play a significant role in crack propagation. Therefore, quantifying ferrite phase fraction is an important step in ensuring weld quality. Several methods such as ASTM E562 point count, electro-etching, color-etching, and optical image analysis as well as Electron Backscattered Diffraction (EBSD) are utilized to quantify the ferrite fraction in duplex welds that have failed in service. Results of quantification by these methods are compared in terms of accuracy, versatility, and ability to apply in-field. Quantification results in cross-sectional samples indicate that local ferrite phase fraction is more meaningful than average fraction as cracks tend to propagate along high ferrite regions such as heat affected zone. Investigations on a number of cracked welds reveal that initiated cracks from the root pass reach the high ferrite region in Coarse-Grained Heat Affected Zone (CGHAZ) and propagate along this region into filling passes. Additionally, it was observed that high ferrite repair passes could also facilitate crack initiation. Further detailed microstructural results are presented and discussed.

INTRODUCTION

Duplex stainless steels are used in refinery and petrochemical plants due to the combination of strength and resistance to corrosion. However, the tailored microstructure of duplex is altered by post-processing (i.e. welding) which affects mechanical or corrosion properties. Thus, standards and recommended practices have been developed to assess post-processing microstructure and ensure an acceptable quality.1-3 Duplex steels have shown susceptibility to H2S cracking.4-6

Recent incidents in Air Fan Coolers (AFC) of hydroprocessing units where plates with thicknesses of 19 mm or greater are welded to create header-boxes indicate that existing criteria may be inadequate. Welds in these units are known to suffer from Sulfide Stress Cracking (SSC).7-8 Two main criteria are often considered for duplex weld quality assessment which are ferrite fraction and hardness. An allowable ferrite fraction range of 35-65% and average hardness of 310 HV10 with no point higher than 320 HV10 are commonly considered as acceptance criteria.1-3

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