Developing a welding procedure specification (WPS) typically involves welding a sample coupon that will be destructively tested to prove the parameters of the WPS. Different welding and construction codes may have slight differences in requirements and allowable parameter ranges, but the variety of codes are generally similar. The standard tests used universally for any material are bend and tensile tests, which prove the strength and ductility of the weld, as well as the presence of some weld defects, or hopefully, the lack thereof. These two tests can tell a lot about a weld, but generally do not provide much information about the corrosion resistance. Note that when referring to a weld, the heat affected zone (HAZ) adjacent to the weld metal is also included and is often the region of concern.
By Justin Bekker, Metallurgical & Welding Engineer, SGS Canada Inc.
Stainless steel is often used for its corrosion resistance, but welding may result in a compromise of this trait. If the chromium becomes tied up by carbon (or nitrogen) it will no longer be available to form the protective chromium oxide coating that stainless steels are known for. This process is known as sensitization, which occurs at temperatures between 500 °C and 800 °C, roughly. This temperature range is experienced during welding, but the time at this temperature is typically short.
If higher carbon percentages are present or the cooling rate is slow, more chromium carbides are formed at the grain boundaries. For this reason, welds usually involve specifically low carbon materials and/or filler metals with extra chromium. These low carbon materials are often designated with an ‘L’, i.e., type 304L. Another common welding restriction is the low interpass temperature and heat input, both meant to keep cooling rates faster and avoid prolonged dwell times within the sensitization temperature window. Although this is true when sensitization and the deleterious sigma phase is a concern, there are other factors for non-austenitic stainless steels that may require higher interpass temperatures and heat inputs.
The following is a sample of tests that may be performed on a weld:
ASTM B117 – Salt Spray Testing
Austenitic stainless steels are susceptible to chloride attack due to the nickel content. ASTM B117 is a standardized test applying a saline fog to the test specimen. This test is mostly used to determine if the material is able to maintain its ‘stainless’ aesthetics.
ASTM G123 – Chloride Stress Corrosion Cracking (SCC)
Perhaps a more important test of chloride attack on austenitic stainless steel is determining its ability to resist chloride SCC. ASTM G123 prescribes a test where a bend specimen (or other appropriately stressed sample) is exposed to a boiling 25% sodium chloride solution for approximately one week. The bend is finally examined at 20x magnification for evidence of cracking. Multiple tests may be run at various nickel concentrations and for various times to determine when cracking first occurs. This test is less commonly used on welds but may be appropriate for harsh environments.
ASTM A262 – Intergranular Corrosion Testing
Intergranular corrosion testing of austenitic stainless steels is evaluated by ASTM A262, which involves the option of several methods. Although these tests are primarily written for parent material, they may be applied to welds as well. Practice A of ASTM A262 is described as a rapid screening test, which involves preparing a cross-section of a weld, followed by electrolytic over-etching with oxalic acid. This method may require a higher degree of training and experience to recognize a negatively affected microstructure and in cases of ‘possibly affected’ microstructures, a considerable amount of time may be required to examine the cross-section at high magnification.
There is also more subjectivity involved in Practice A, which is considered a valid method to pass a test, but not to reject a test; the other practices are used as definitive methods to reject a test. Figure 1 shows an example of a significantly affected microstructure and a corrosion resistant microstructure. Practice B, C, and F involve exposing the sample to boiling corrosive solutions (choose the solution most applicable for the production application) for a period of time and then measuring the weight loss to determine if the sample has sufficient corrosion resistance. Practice E involves boiling the sample in a corrosive solution and then conducting a tight radius bend to check for any cracking.
ASTM G48 – Pitting and Crevice Corrosion Testing
Pitting and crevice corrosion resistance is measured by the tests of ASMT G48. Samples are immersed in a ferric chloride solution of moderate temperature for several days and the exposed surfaces are examined for weight loss and evidence of pits or crevices. With the necessary measuring equipment, such as a needle point micrometer or calibrated fi ne focus knob on a microscope, pits and crevices can be examined and their depths measured precisely. By iteratively testing at different temperatures, the critical temperature may be determined.
NACE MR0175/ISO 15156-3 – Materials for Use in H2S Containing Environments
For welding of duplex stainless steels, NACE MR0175 / ISO 15156 (3) Clause A.7.3 specifies that the microstructure shall be evaluated for the presence of intermetallic phases, nitrides, carbides, and sigma phase. This evaluation requires even more experience and time compared with Practice A of ASTM A262 and involves examining a weld cross-section at high magnification to identify detrimentally affected regions that may be cause for rejection. Multiple etchants are typically used for this examination. Lightly electrolytically etching with oxalic acid will reveal precipitates (carbides and nitrides, Figure 2), while a longer electrolytically etching with sodium hydroxide (NaOH) will reveal the general structure and sigma phase.
ASTM A800 – Ferrite Testing
Austenitic stainless steels are known to be susceptible to solidification cracking (during the solidification process of a weld while it is still hot). One easy to measure characteristic that provides insight into the cracking susceptibility is the ferrite content. A small amount of ferrite is desirable in most austenitic welds and can be measured using a Ferritescope and the principles of magnetism (ferrite is magnetic and austenite is not).
Another method is to perform a point count on a number of prepared metallographs. A grid of a determined size is overlayed on a metallograph and the intersections of the grid are counted as either landing on ferrite or austenite. Point counts are more commonly performed on duplex stainless steels, where the percentage is closer to 50%. Using a Schaeffler or WRC diagram, it is a relatively simple calculation to determine the predicted ferrite number of a weld based on the dilution of the weld metal with parent metal. This should be performed prior to welding if there is concern about solidification cracking.
ASTM A370 – Charpy V-Notch (CVN) Testing
Austenitic stainless steels are regarded as having good low temperature toughness and are often used in cryogenic applications. This is due to the crystal structure having a higher number of slip systems that permit the material to ‘slip’ rather than ‘cleave’. Ferritic stainless steels, on the other hand, have similar toughness to carbon steels, which is significantly reduced below the ductile- to-brittle transition temperature.
Duplex, as the name might suggest, have a toughness that lies between austenitic and ferritic. CVN tests are a measure of the material’s ability to absorb impact energy. A CVN specimen is machined to a standard size with a notch placed along one side. A hammer on a pendulum is then swung through the specimen (Figure 3), fracturing it at the notch, and the final height of the swinging hammer is recorded and the loss in energy is determined. This loss in energy is what was absorbed by the specimen. CVN testing is typically done at low temperatures, i.e., -45 °C for ferritic or duplex and -105 °C (or colder) for austenitic stainless steels.
Testing is a fundamental part of welding procedure development. In general, the more common the weld and the lower the corrosive environment severity, the fewer the tests required. A knowledge of the potential risks, such as sensitization and particular corrosive attacks will help avoid unfortunate surprises when welding stainless steels, and if the capabilities of stainless steel must be pushed, there are a variety of tests that can confirm the suitability of the material to the environment.