Heat exchangers are key components for many industries, particularly in chemical and oil and gas plants where they play a vital role in process control. Industry professionals — from design engineers and fabricators to the in-plant operators who work with the equipment daily — seek new ways to improve heat transfer performance and extend the life of these units in corrosive environments. Many times, improvements are closely related to the tubing specified for use in their heat exchangers. To improve the performance of heat exchanger tubing, materials engineers have a number of options. They include specifying higher corrosion-resistant grades, selecting tube suppliers with enhanced tube manufacturing and finning processes and using state-of-the-art manufacturers that have upgraded their methods of non-destructive testing.
Making the correct decision is not an easy task. When selecting a tube material for a heat exchanger, many different options are available, depending upon the application, design and operating conditions (such as temperature, pressure and the corrosive environment). In addition, to be viable candidate materials, those tubing options need to be available and affordable. Everything from raw material costs to the availability of off-the-shelf distribution can impact the tube selection process. And, all of these factors have the potential to affect the outcome of a project.
The table above provides a summary of some of the more frequently used ASME specifications for steel tubing for pressure applications. (To view this image larger and in a new window, click here. ) It shows the wide range of alternatives that engineers and fabricators are faced with when considering candidate tubing materials for heat exchanger, condenser, boiler and feedwater heater service.
Duplex stainless steels refer to a family of stainless steels alloyed to produce a microstructure consisting of approximately equal parts ferrite and austenite.
Heat exchangers are often faced with extreme temperatures, pressures and corrosive media. Duplex stainless steels are designed to have high strengths while maintaining good toughness, have excellent resistance to chloride pitting corrosion and be more resistant to stress corrosion cracking than the 300 series austenitic stainless steels.
There are several groups of duplex stainless steels that are categorized by the level of alloying elements present. Lean duplex stainless steels have low amounts of alloying elements while duplex and super duplex stainless steels have higher amounts of alloying elements. The composition of a duplex stainless steel directly affects the corrosion resistance of the alloy. This is most often quantified by the pitting resistance equivalent number (PREn): PREn = %Cr + 3.3 x %Mo + 16 x %N).
Duplex stainless steels are useful in applications where strength is of great importance. They can help reduce the weight of components due to their increased strengths (about twice as high as typical 300 series austenitic stainless steels). The higher strength equates to thinner sections of duplex stainless steel being required to accommodate a load as opposed to lower strength materials (like carbon or austenitic stainless steels).
By comparison, many alternative stainless steel options fall short in corrosion resistance. For instance, austenitic stainless steels are readily susceptible to stress corrosion cracking under certain conditions. This limits the recommended operating temperatures of austenitic stainless steel components in stress corrosion cracking environments. Conversely, duplex stainless steels are more resistant to stress corrosion cracking and, therefore, can be used in many environments where austenitic stainless steel is not adequate. In fact, duplex stainless steels also have much improved resistance to chloride pitting corrosion as compared to austenitic stainless steels. This is of great importance when trying to find an alloy that is adequate for chloride-containing environments, where austenitic stainless steels are not suited for service.
Strength and corrosion resistance aren’t the only factors that engineers weigh in their decision-making processes. Cost and availability often come into play. Many times, manufacturers and distributors can provide duplex stainless steels at a much more stable price as compared to austenitic stainless steels due to their lower content of nickel and molybdenum. Of course, this offers some protection from the volatility of raw material pricing that other candidate materials face.
As well as it performs in many operating environments, duplex stainless steel tubing does have potential drawbacks and fabrication complexities. It requires much more precise and careful heat treatment plans as compared to austenitic stainless steels due to the tendency to form detrimental intermetallic phases. Intermetallics, which can form in duplex stainless steels, can cause the material to become brittle and impair corrosion resistance. Welding or other fabrication processes that impart high heat inputs into the material can also promote intermetallic phases to form. However, with a proper Welding Procedure Specification (WPS) and heat treating/cooling processes, detrimental intermetallic phases can be avoided.
Duplex stainless steels also have limits on the temperatures in which they can be used in service. They have the tendency to form a low-temperature intermetallic phase (alpha prime), which causes embrittlement of the material. The industry-recommended temperature range that duplex stainless steels can be used at is –22 to 617°F (-30 to 325°C). Welding duplex stainless steels can also be challenging because care must be taken to ensure that the weld has the proper phase balance. The ferrite content of the weld must fall within a specified range provided. If the phase balance of the weld does not fall within the specification, the corrosion and mechanical properties of the weld could be compromised. A proper WPS, which has been verified to yield the proper phase balance, should be used to avoid complications.
Manufacturers who are experienced at working within duplex stainless steels’ limitations are able to maximize their many potential benefits, providing end users with superior performance. Like less corrosion-resistant carbon steel or copper alloy products, duplex stainless steel tube can also be integral finned to improve heat transfer by providing an increase in its surface area.
Finning duplex stainless steels is an extremely challenging process due to the high strength of these materials. Achieving the desired number of fins and fin height without damaging them requires skill, experience and extensive research and development. Heat treatment after the finning or U-bending manufacturing steps also requires precise process control.
Utilizing corrosion-resistant duplex stainless steel tubing in place of traditional carbon steel or austenitic stainless steel can be beneficial to the performance of heat exchangers. These unique alloys have the ability to extend the lifetime of the tubing in the right applications. Yet duplex is not always the answer. Pinpointing the ideal tubing for an application — one manufactured by an experienced supplier that can meet tight turnaround requirements — can be a challenge. But finding that knowledgeable source is well worth the effort because getting the materials-selection process right or wrong impacts everything from budgets to production schedules.