ABSTRACT

When duplex stainless steel (DSS) is applied to welded components, ferrite measurement with manual point count according to ASTM(1) E562 is often used during welding procedure qualification as heat affected zone (HAZ) of the weld tends to have high ferrite content. For this measurement method, there are several testing variables and it seems that standard testing variables are not used for ferrite measurements performed by various fabricators. In this paper, the effects of three major variables described in ASTM E562, which were the grid size i.e. number of testing points in a grid, number of fields and measurement locations, on the measured ferrite contents were studied. Also, ferrite measurement with automatic image analysis according to ASTM E1245 was performed across the fusion line of the weld to compare the ferrite content measured by manual point count according to ASTM E562. In addition to the ferrite measurement, pitting corrosion test according to ASTM G48, Method A was performed on the same sample as the one used for ferrite measurement in order to see where in the weld pitting corrosion occurs. The results have suggested a necessity of establishment of standardized testing variables to be used for ferrite measurement with manual point count according to ASTM E562.

INTRODUCTION

Duplex stainless steels are the stainless steel with two-phase microstructure consisting of approximately 50% ferrite and 50% austenite and this phase balance is known to determine the corrosion resistance of this material.1 As an inherent nature of this material, the balance between ferrite and austenite in HAZ of the welds changes from that in the base metal due to welding heat input and subsequent rapid cooling and consequently HAZ tends to have high ferrite content which may increase the susceptibility to chloride pitting and chloride stress corrosion cracking. Hence, when this type of steel is applied to welded components, ferrite measurement is often required during welding procedure qualification and the typical specified ferrite content range is 30-65%,2

There are two typical methods for ferrite measurement which are manual point count according to ASTM E562 and automatic image analysis according to ASTM E1245. 3,4 For most cases in welding procedure qualification in oil and gas and refining industries, ferrite measurement is carried out by ASTM E562. Ferrite measurement using this method is conducted by placing a grid with a regular array of test points on a micrograph and counting the number of test points sitting on ferrite phase. In this measurement method, there are four major testing variables which are magnification of the micrographs, grid size i.e. number of test points within a grid, number of fields and the measurement location. Authors have been reviewing procedure qualification records (PQR) prepared by various fabricators and found that the testing variables for ferrite measurement in HAZ of DSS welds by manual point count according to ASTM E562 are always different in each PQR except for the magnification where 400X or 500X are commonly used. It seems that these differences in testing variables may have effects on the measured ferrite content. If the measured ferrite content is affected by the testing variables, it may mean that the current measurement method without having standardized testing variables would not be appropriate way for the evaluation of ferrite content in HAZ of DSS welds. Hence it is important as a first step to understand how these variables affect the measured ferrite content.

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