Sour Testing of Corrosion Resistant Alloys: Comparison of Four-Point Bend and Uniaxial Tensile Test Methods

ABSTRACT

In order to resolve reported discrepancies between the results of four-point bend tests and uniaxial constant load tests on UNS S31603 in sour environments, a series of systematic tests has been conducted to compare the two test methodologies under carefully controlled conditions. It was demonstrated conclusively that tests performed in uniaxial tension at constant load are inherently more severe than tests performed in constant displacement four-point bending due to the presence of low- temperature creep in the former. The enhanced susceptibility to stress corrosion cracking is attributed to the presence of additional tensile plastic strain in specimens that have undergone low-temperature creep. Tests performed in uniaxial tension were also found to be more severe than those performed in four-point bend even when dynamic strain was eliminated by performing them at constant displacement. A key observation is that no load relaxation due to low-temperature creep occurs during the four-point test. The results emphasize the need to take into consideration factors such as the level of tensile plastic strain in service when selecting the appropriate test method for a given application.

INTRODUCTION

In the oil and gas industry, 316L stainless steel (UNS⁽¹⁾ S31603) is a popular choice of corrosion resistant alloy (CRA) for use in sour service owing to its low cost when compared with duplex steels and nickel alloys, and the relative ease with which it can be welded and supplied. In aqueous sour environments, the use of 316L stainless steel (SS) is restricted by the onset of stress corrosion cracking (SCC), which is primarily governed by a combination of temperature, pH and the concentration of chloride and H₂S in the aqueous phase. NACE MR0175/ISO⁽²⁾ 15156¹ specifies the acceptable limits for the use of 316L SS in terms of these variables, with sufficient conservatism to minimize the risk of SCC in service.

NACE MR0175/ISO 15156¹ also details requirements for experimental validation of the acceptable limits for materials with regard to SCC. Typically these qualification tests are performed in uniaxial tension (UT) under constant load. The drawback of this testing methodology is that it can be expensive and time consuming, given that usually only a single specimen can be tested at a time using a specialist rig. Testing in four-point bend (FPB) offers a convenient alternative, whereby numerous specimens can be tested in a single vessel using a relatively simple jig to apply the load. The caveat is that FPB tests performed in this way are loaded in a state of constant deflection rather than constant load. A concern that arises from this subtle difference is that, under the action of creep, a test loaded to a constant deflection may suffer from load relaxation. To address this concern, it is standard practice to load FPB tests at a constant displacement to 100% s_0.2, rather than the 90% s_0.2 used in UT testing^1,2 It is also noted that for alloys that are known to creep, it is advised to test in UT rather than FPB¹.