Answers to questions regarding the GB/T 4334—2020 standard

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In the standard GB/T 4334—2020 \"Test methods for intergranular corrosion of metals and alloys – Austenitic and ferritic-austenitic (duplex) stainless steels\", methods such as E, F, and G all use the bending method to determine whether intergranular corrosion has occurred in the specimens. However, in actual production, it is often difficult to determine whether bending cracks have appeared. Section 7.4.5 of this standard stipulates that when it is not possible to assess bending of the specimen or when it is difficult to determine bending cracks, a metallographic method shall be used for assessment. The metallographic specimens should be taken from areas of the specimen that are not bent (with the exception of weld joints and welded pipes). After corrosion (but not excessive corrosion), the specimens are examined under an optical microscope; the allowable depth of intergranular corrosion is determined through consultation between the supplier and the buyer. This regulation is ambiguous: one interpretation is that the sampling locations for the metallographic specimens of welded joints and welded pipes cannot be at non-bent areas, yet no clear specification is given regarding these sampling locations – whether they should be horizontal or vertical. Put differently, metallographic methods cannot be used to evaluate welded joints and welded pipes. Section 46.1.1 of the ASTM A262—2015 standard specifies that when assessment is in doubt, the longitudinal cross-section of the tensile face of the bent specimen shall be examined under an optical microscope at 100–250x magnification to determine the presence of intergranular corrosion. When comparing the two, it is clear that the specifications in ASTM A262—2015 are more reasonable; after tensile testing of the microstructural examination surface, it becomes easier to determine whether intergranular corrosion is present in the specimen. Answer: In the GB/T 4334—2020 standard, methods E, F, G, etc., all involve placing austenitic stainless steel and duplex stainless steel specimens in specific acidic solutions; after a boiling test, the tendency for intergranular corrosion is determined using bending or metallographic methods. If intergranular corrosion occurs in the specimen, it leads to the weakening of the grain boundaries, causing the specimen to crack under bending stress, with cracks being visible to the naked eye or under a magnifier. At this point, attention should be paid to cracks arising at the edges and corners of the bent parts of the specimen, as well as slip lines, wrinkling, and surface roughness that do not come with cracks – such defects cannot be considered to be caused by intergranular corrosion. (The same bending test can be conducted on specimens that have not undergone corrosion testing to determine whether the cracks observed in the specimens tested under corrosion conditions are indeed caused by intergranular corrosion.) Section 7.4.5 of the GB/T 4334—2020 standard stipulates that when it is not possible to evaluate bending of the specimen (for example, if the specimen is too small to be bent) or when it is difficult to determine cracks resulting from bending, the metallographic specimen should be taken from a part of the specimen that does not bend. This is because, after corrosion, the degree of grain boundary and intergranular corrosion can be observed under an optical microscope with a magnification of 100–250 times. By agreeing on the allowable depth of intergranular corrosion between the supplier and the buyer, it is possible to determine whether there is a tendency for intergranular corrosion, as well as the depth of such corrosion. If the metallographic specimen is taken from the bent area, under an optical microscope magnification of 100–250 times, it becomes necessary to identify and remove cracks resulting from the sharp edges at those bent areas; at the same time, it is difficult to distinguish cracks caused by intergranular corrosion, such as slip lines, wrinkling, and surface roughness, which do not come with cracks. For welded joints and welded pipe specimens, since the weld area is often the center of bending, metallographic specimens used to determine the intergranular corrosion tendency in the welded area cannot be restricted to being taken from areas that are not bent. Section 461.1.1 of ASTM A262—2015 specifies that when assessment is in doubt, intergranular corrosion should be determined by observing at an optical microscope magnification of 100–250 times the \"outer radius of a longitudinal section of the bend specimen near the tensile face\", rather than the \"longitudinal section of the tensile face\" (since the tensile face of the bend specimen is perpendicular to its longitudinal section); this is because such a section facilitates the identification of intergranular corrosion cracks and the measurement of their depth (the same requirement applies to the cross-sections of metallographic specimens in Japanese Standard JIS G 0575:1999/AMD1—2012). The American standard ASTM A262—2015 does not specify directly the method for sampling weld specimens; rather, Section 45.1.5.4 states that the types of face bend, back bend, and side bend shall be agreed upon by the supplier and the purchaser, but the weld shall be at the center of bending (see Section 45.1.5.3). In summary, metallographic methods can be used to evaluate welded joints and welded pipes. The sampling location for metallographic specimens must be at the weld area, in order to facilitate the identification of intergranular corrosion cracks and the measurement of their depth; it is not necessary that the sampling take place in areas that are not subject to bending. Section 8.2.2 of Method E in Q2 GB/T 4334—2020 specifies that a heating device shall be provided to maintain the test solution in a state of gentle boiling, while Section 8.3.4 requires that the test solution be heated to keep it in a state of gentle boiling during the test. Method E in the GB/T 4334—2020 standard modifies Method A from ISO 3651—2:1998 \"Determination of intergranular corrosion resistance of stainless steels – Part 2: Corrosion testing in sulfuric acid medium of ferritic, austenitic and ferritic-austenitic (duplex) stainless steels\" by using a redrafting approach, whereas ISO 3651-2:1998 does not contain a similar provision. Why is it required that the test solution be at a ‘gentle boil’, and how is such a state of ‘gentle boiling’ achieved for the test solution? Answer: “Just below boiling point” is the state of a liquid just before it begins to boil, at which point the temperature is also 100°C. Since the temperature, concentration, and boiling state of the sulfuric acid-copper sulfate solution directly affect the corrosion rate of the stainless steel specimens, it is difficult to monitor the temperature of the test solution before boiling. Moreover, after boiling, the degree of convection in the test solution, as well as the acidity and concentration of the solution after water evaporates, change continuously. Therefore, maintaining the test solution in a state of ‘gentle boiling’ is a cost-effective way to ensure consistent experimental conditions. ASTM A262—2015 and JIS G 0575:1999/AMD1—2012 also require that the test solution be in a state of ‘gentle boiling’. Note 19 in ASTM A262—2015 states that ‘measures should be taken to minimize bumping of the solution’, while clause 3.b of JIS G 0575:1999/AMD1—2012 (in the English version) specifies that ‘a heater capable of keeping the test solution gently boiling throughout the test period’ is required. To control the test solution from \"micro-boiling,\" the temperature of the heater can be fine-tuned so that the solution does not bubble continuously, with an interval between bubbles of at least 3–5 seconds.