Corrosion-resistant steel tubing is a key component in heat exchangers used in the chemical, oil-and-gas processing and power-generation industries. In addition, upstream oil-and-gas operations depend on coiled tubing for many corrosive applications. Depending upon the operating environment, engineers in these fields specify seam-welded tubing in materials ranging from carbon steels to corrosion-resistant alloys (CRAs).
Many corrosion studies have been performed and reported for corrosion-resistant alloys in the wrought condition. The corrosion performance of autogenously welded tube seams, however, is not well understood. Even without this knowledge, cold working the weld is often specified as the only requirement for manufacturers — with the goal of ensuring maximum corrosion performance. For welded corrosion-resistant alloy tubing — austenitic, ferritic, duplex stainless and nickel alloys, for example — maintaining good corrosion characteristics on seam-welded sections is as critical as the integrity of the weld itself.
This article addresses available methods of seam corrosion testing and summarizes variables that can impact the corrosion performance of the weld seam. Manufacturing process variables include:
- Welding technique.
- Heat treatment.
- Post-anneal microstructures of the weld.
- Heat-affected zones.
- Presence of intermetallics.
Standard Corrosion Testing Method. The American Society for Testing and Materials (ASTM) recommends standard practices for corrosion testing based on steel types and purposes. Susceptibility to intergranular attack associated with the precipitation of chromium carbides for austenitic stainless steel is covered by ASTM A262, and ferritic stainless steel is covered by ASTM A763. Both practices use oxalic-acid etch testing for acceptance of material by microscopic examination on etched surfaces. After a rapid-screening test, further evaluation practices may be applied and reviewed. Also, the corrosion rate may be determined by weight loss or metallographic examination after a bend test.
Duplex stainless steels are susceptible to the formation of intermetallic compounds during exposures in the temperature range of approximately 600 to 1750°F (315 to 954°C). The presence of detrimental intermetallic phases can be tested using ASTM A923. The etched cross-sectional areas should be examined on a metallurgical microscope across all zones such as seam welds, weld-affected zones and base metals. Further evaluation methods may be applied and reviewed along with the corrosion rate, which can be determined by weight loss or a Charpy impact test.
The corrosion properties of the weld areas also can be tested using the S7 weld-decay test method described in the specification ASTM A249: Welded Austenitic Steel Boiler, Superheater, Heat-Exchanger and Condenser Tubes.This test method includes submerging samples in a boiling solution of equal parts hydrochloric acid and water long enough to reduce the base-metal thickness by approximately 50 percent. The wall thickness along the same straight line — away from the seam weld — is measured before and after testing. This same technique is used on the seam weld. A ratio of the wall loss as compared to the seam weld loss is calculated. A corrosion ratio of 1.25 or less for the thinnest section of the seam weld is permissible. Other criteria such as a ratio of 1.00 or less may be specified depending upon the application.
For testing of welded tubing in sour conditions, the National Association of Corrosion Engineers (NACE) MR 0103 describes the standard practice for materials resistant to sulfide stress cracking in corrosive petroleum-refining environments. Sulfide stress cracking (SSC) is cracking of a metal under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide (H2S). Because sulfide stress cracking is affected by complex interactions of variables such as chemical composition, heat-treatment condition and environmental parameters, maximum hardness (including weld area) is specified based on the material types and conditions.
Types of Welding Techniques
Typically, the majority of seam-welded stainless steel (austenitic, ferritic and duplex) and nickel-alloy tubing is welded with autogenous laser welding or gas tungsten arc-welding methods.
Laser welding is a keyhole welding technique that heats the workpiece above the vaporization temperature and forms a keyhole. Due to the high temperature and short time period of the welding process, segregation of the alloys is minimal, resulting in oxide-free full-fusion welds; thus, good corrosion resistance can be maintained.
Gas tungsten arc welding (GTAW) — also known as tungsten inert gas (TIG) welding — is an arc-welding process that uses a nonconsumable tungsten electrode. Gas tungsten arc welding requires a longer time to generate heat to melt and bond the material together.
Both of these welding methods use automatic welding processes without the addition of filler metal, and both can produce acceptable welding results. However, depending upon the material grade, thickness and welding speed, laser welding may have more consistent control than gas tungsten arc welding.
The major differences between these two weld types are heat input and solidification (or cooling) rate. In general, gas tungsten arc welding requires a higher heat input than laser welding for a given grade. Also, it results in a slower cooling rate as well as a much larger heat-affected zone (HAZ). Figure 1 shows stainless steel examples.
With the same solution-annealing treatment, the corrosion performance of these two welding methods’ HAZ sections can be different. As an example, laser and gas-tungsten arc-welded S30403 stainless steel tubes were annealed at the same temperature; however, ASTM A249 S7 weld-decay test results showed very different corrosion performance between the two samples (figure 2).
Heat Treatment and Post-Anneal Microstructure
Solution annealing of stainless steel is more than a softening process. Conducted properly, solution annealing will remove alloy segregation, and grains will recrystallize as a development of becoming strain-free. Recrystallization is the second part of a three-step process (recovery, recrystallization, grain growth) that occurs during full annealing. Cold work introduces dislocations, which increase the energy state of the grains, and creates the driving force to lower the free energy of the grains during the annealing process.
Some applications require the weld microstructure to be 100 percent recrystallized for optimal corrosion resistance with a seam weld that has the same corrosion properties as the base material. However, we should acknowledge that measuring the amount of recrystallization is not a simple process. It often will have different results depending on the test method or the laboratory that conducts the testing. As an example, recrystallization can be evaluated as a reference value using microstructural appearance because the mechanical properties can change without recrystallization having occurred (only by recovery). Figure 3 shows examples of laboratory-created samples from approximately 0 percent, 50 percent and 100 percent recrystallized weld area microstructure, which were taken from S30403 stainless steel that was laser seam welded.
Sometimes, cold working before solution-anneal heat-treatment is specified. Figure 4 shows laboratory-created samples from solution-annealed tubing with 5 percent, 15 percent and 25 percent cold work of the weld area microstructure using S30403-grade stainless steel laser-welded tubing. The results did not show any significant differences.
Presence of Intermetallic
When austenitic stainless steels are exposed to temperatures in the range 900 to 1500°F (482 to 815°C) for extended periods, there is a risk that the chromium will form chromium carbides (a compound formed with carbon). This condition reduces the chromium available to provide the corrosion resistance. This is referred to as sensitization. It results in the precipitation of carbides at grain boundaries, making it susceptible to intergranular corrosion.
When duplex stainless steels (a ferritic and austenitic microstructure) are exposed to temperatures in the range between 600 and 1750°F (315 to 954°C), some phase transformations can occur such as chromium nitrides precipitation, chromium carbides precipitation and sigma-phase formation. The formation of such secondary intermetallic compounds leads to loss in both corrosion resistance and fracture toughness due to chromium depletion in the matrix. Figure 5 shows laboratory-created samples of S30403 stainless (sensitized at 1250°F [676°C] for one hour) and S32750 super-duplex stainless steel tubes.
Corrosion Performance of the Seam Weld
A corrosion-performance test should be the main indicator of the corrosion characteristics of the seam weld — not the status of the cold work or amounts of recrystallization. The corrosion resistance of the autogenously welded tube seam of austenitic stainless steels is influenced by welding methods and the amounts of recrystallization. Pre-anneal weld microstructure, annealing temperature, time at temperature, and the amount of cold reduction are interdependently affected by the amount of recrystallization that occurs in the weld.
A full recrystallization of the weld microstructure itself is not required to restore the corrosion resistance of the seam weld. For gas tungsten arc-welded tubing, this condition is not as apparent as in the laser weld. Even full recrystallization of the weld may not be enough for optimum corrosion resistance. Promoting chemical homogenization in the weld to reduce the differences in electronegativity between the base metal and the weld area also is the key to restoring the corrosion resistance of the weld.
The corrosion resistance of the autogenously welded tube seam for duplex stainless steels is most influenced by the final microstructure, especially the phase balance between ferrite and austenite and the exclusion of intermetallic precipitates.
Maintaining acceptable corrosion performance of the weld seam section is not solely dependent on the solution-annealing process. Often, it requires careful monitoring of the welding and heat-treating process and a good understanding of the material.
In summary, performing the corrosion test methods described provides the best means for ensuring the quality of welded tubing, whether it is joined by gas tungsten arc welding or laser equipment. In fact, laser welding may offer more consistent process control than gas tungsten arc welding — depending upon the material grade, thickness and welding speed — and provide additional benefits. Those requiring seam-welded tubing should consider both options when specifying corrosion-resistant alloy tubular products and base material selection on sound data produced by laboratory testing.
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