34 JOB KNOWLEDGE NACE TM0177 –
1 NACE TM0177 – “Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments
NACE TM0177 is probably one of the most referenced sour testing standards, and it is the source of the original ‘NACE solution,’ which is now NACE TM0177 Solution A. It covers four SSC/SCC test methods, namely A; Tensile, B; Bent-Beam, C; C-ring, and D; Double Cantilever Beam, and it details environments and procedures. It does not give acceptance levels or pass:fail criteria. It covers solution chemistries, solution volume: surface area ratio, test duration, specimen geometry and loading, amongst other testing details for the four different methods, see below.
UNIAXIAL TENSILE TESTING The smooth uniaxial tensile test is Method A in NACE TM0177. Dead weight, proof ring or hydraulic loading may be applied. The specimens are fully machined and, because of the waisted geometry, cannot sample weld surface details or near surface microstructures. There is a standard and a subsize geometry in TM0177 ( 0.25” (6.35mm) diameter, 1” (25.4mm) gauge length and 0.15”
(3.81mm) diameter, 0.6” (15mm) gauge length respectively). The standard warns that subsize specimens can result in shorter failure times than full size specimens, but does not suggest that the threshold stress will be lower. The shoulder radius is specified as 15mm minimum, but 20mm minimum is advised for CRAs in EFC 17, to avoid “unwanted preferential cracking at these locations”. Loading has to be by ‘sustained load’ or ‘constant load’ devices, where ‘sustained load’ refers to a spring loaded device such as a proof ring, and ‘sustained load’ refers to dead weight or hydraulic loading. The standard warns that slight load relaxation in a sustained load device may mean that the specimen cracks, without failing, and therefore stipulates visual examination of the specimens after test. Although the standard requires load to be ‘applied carefully to avoid exceeding the desired value’, there is no warning against inadvertent torsion or bending (non-axial loading) which can affect results. In order to generate a threshold stress, specimens
WELDING WORLD MAGAZINE | ISSUE 02 | APRIL 2018
are tested at a range of loads for up to 720 hours each.
This test can result in SOHIC (stress- oriented hydrogen-induced cracking) in susceptible materials, and thresholds of around 50% yield stress have been recorded in materials which had survived up to around 100% yield stress in a bend test on a welded sample at TWI (Pargeter 1986).
BENT BEAM TESTING
The bent beam tests are NACE TM0177 Method B. These specimens are very small – 4.5mm wide, 1.5mm thick, with two 0.7mm diameter drilled holes, and the results are difficult to interpret. This test method is not often used.
C-RING TESTING
The C-ring test is NACE TM0177 method C, and there is also guidance given in EFC 16 and EFC 17. It is effectively a bend test, suitable for relatively small tubular material, producing stress in the hoop direction. If the ring is compressed, a
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