How Are Additively Manufactured Duplex Stainless Steel Tubes Achieving Optimized Wall Thickness Profiles?

According to the worst-case stress point, a heat exchanger tube is typically drawn with the same wall thickness from end to end. This limitation is eliminated by additive manufacturing, which allows for the printing of a duplex stainless steel tube with a thicker wall close to the tubesheet, where stresses are concentrated, and a razor-thin wall in the center, where only pressure containment is important, squeezing out every last bit of thermal performance. The design of small, high-efficiency heat exchangers for offshore platforms, chemical plants, and aerospace applications is changing due to the ability to adjust wall thickness throughout the tube length.

The Traditional Limitation: Consistent Wall Thickness Throughout
Traditional duplex stainless steel tubes (such as 2205 and 2507) are made by rolling, drawing, or extrusion. These procedures result in tubes with walls that are the same thickness throughout. The location of the maximum stress-typically the tube ends, where the tube is rolled or welded into the tubesheet, or where bending stresses are strongest-is used to determine the wall thickness. The center section of the tube is therefore over-designed for pure pressure containment. Weight, expense, and thermal resistance are all increased by excess metal. A uniformly thick tube reduces efficiency because heat transmission across a tube wall is inversely related to thickness (for a given material and fluid conditions).

Variable Wall Thickness Profiles: The Additive Manufacturing Solution
Laser powder bed fusion (LPBF) is used in additive manufacturing to create optimum designs for duplex stainless steel tubes. Layer by layer, fine powder of duplex stainless steel (such as UNS S31803 or S32750) is selectively melted by a high-power laser. The procedure is based on a three-dimensional computer-aided design (CAD) model that specifies the tube's inner diameter, outer diameter, and axis variations. The wall thickness can be continually changed, in contrast to traditional techniques.

Common optimization techniques consist of:

Thicker ends: More layers of metal are applied to the first 50–100 mm of the tube near each end, where it comes into contact with the tubesheet, increasing the wall thickness. High local stresses from rolling, expansion, vibration, and bending moments are managed by this reinforced zone.

Thinner midsection: The minimal thickness needed to comply with the pressure design code (such as ASME Section VIII, Division 1 or 2) is printed in the middle area, which solely suffers hoop stress from internal pressure. Compared to a traditional drawn tube of the same material and pressure rating, this middle is sometimes 30–50% thinner.

Gradual transition: To prevent stress concentration at a quick step, the change in wall thickness is tapered over a brief distance (e.g., 20–30 mm).

The end product is a functionally graded component, with metal carefully positioned where it enhances mechanical integrity and eliminated where it only adds weight and heat resistance.

Enhanced Heat Transmission and Diminished Mass
There are two measurable advantages to the variable-wall approach:

Improved total heat transfer coefficient (U): A thinner tube wall lowers conductive resistance under the same internal and external fluid conditions. Duplex stainless steel has a thermal conductivity of roughly 15–17 W/m·K. The conductive thermal resistance is reduced by approximately 40% when the wall thickness is reduced from 2.0 mm to 1.2 mm (a 40% reduction), increasing the overall U-value by a noticeable margin (usually 10–15%, depending on the film coefficients).

Weight reduction: The thin-walled center of a long tube (e.g., 6 m) makes up the majority of its length. An comparable conventionally drawn tube with homogeneous thickness made for the same end loads may weigh 25–40% more overall than an additively built tube. For offshore platforms and aerospace applications where every kilogram counts, the cumulative weight savings in a heat exchanger bundle with hundreds of tubes is significant.

Additive Manufacturing's Metallurgical Benefits for Duplex
The microstructure produced by LPBF of duplex stainless steel differs significantly from that of wrought material. A highly fine, uniform combination of austenite and ferrite, frequently with grain sizes in the micrometer or submicrometer range, is produced by the extraordinarily quick solidification (cooling rates up to 10 °C/s). This exquisite structure can offer:

greater yield strength (typically 20–30% higher than wrought equivalents in the as-printed state).

strong impact resistance with appropriate thermal treatment.

minimal alloying element segregation, which lowers the possibility of intermetallic phase development during printing.

However, most additively built duplex components need a post-process solution annealing and quenching to get the best phase balance and corrosion resistance. The normal cycle of heat treatment is:

Heat to 1050–1100°C, depending on the grade.

Hold for a brief period of time (for example, 30 minutes for every 25 mm of section thickness).

To prevent the precipitation of sigma phase and other intermetallics, quickly quench with water or gas.

Additively produced duplex stainless steel matches or surpasses the pitting resistance (PREN > 40 for super duplex) of its wrought counterpart following proper heat treatment.

The Effect of Surface Finish on Performance
There are two conflicting effects of this roughness: