Boosting the performance of plate heat exchangers with a new grade of formable duplex stainless steel

Until a few years ago, duplex stainless steel was a term that applied to a handful of alloys, but this branch of the stainless steel family tree has since developed into a family in its own right. Outokompu Project Manager, Rodrigo Signorelli, talks about a new grade of duplex stainless steel that offers a combination of high strength and formability, making it ideal for manufacturing plate heat exchangers. A manufacturer in Shanghai has already put it to the test.

Plate Heat Exchangers (PHEs) are used extensively to transfer and manage heat in industry and district heating applications. To ensure efficient heat transfer they often operate at high pressure to allow rapid fluid flow. The downside is that such high pressures often lead to plate deformation, which causes leakage. Gasket-sealed plate heat exchangers (GPHEs) are particularly prone to this. Plate deformation can also affect the fluid flow through the channels formed in the plates, thus resulting in heat loss.

To eliminate this drawback, engineers introduced several alternative design options, but with mixed results. No matter whether heat exchangers are based on a welded, brazed or gasket design, it became evident that they would all benefit from a higher strength material to reduce the risk of plate deformation and collapse.

The solution was to adopt a high strength duplex stainless steel for the plates instead of standard austenitic stainless steel. Apart from having high strength, duplex grades have higher fatigue strength, erosion resistance and enhanced resistance to stress corrosion cracking.

There was one snag though: formability. Most duplex grades have limited formability compared with standard austenitic stainless steel. As a result, manufacturers found it difficult to replicate the indentations in plates that form channels to guide the flow of fluids. This also impacted PHE designers who had to contend with a lower surface area for heat transfer across each plate, which affects the size of the unit.

Having taken note of these challenges, Outokumpu developed a new grade of duplex stainless steel for applications where the formability of other duplex grades is insufficient or limits the design capability. Forta FDX 27 has been created specifically for GPHE manufacturers to overcome these limitations. Manufacturers can produce plates in the new material with the same or similar surface area as their existing plates made from traditional austenitic stainless steel. An added benefit is that it is often possible to adopt the same tooling set when switching to the new material.

The differentiating factor

What sets Forta FDX 27 apart from its counterparts in terms of formability, is a phenomenon known as Transformation Induced Plasticity (TRIP). This denotes changes in the crystal microstructure that take place during forming when plastic deformation takes place.

The properties and physical attributes of a stainless steel is defined by its microstructure, of which there are several variations. These variations result in the formation of austenitic, ferritic, martensitic, and duplex stainless steels. For Forta FDX 27, the composition is balanced to lead to a controlled transformation of austenite to martensite during forming at the microstructural level.

While the TRIP effect has in the past only worked for austenitic steel grades, Outokumpu has successfully transferred it to Forta FDX 27 as a duplex grade. The result is higher strength and greater formability, enabling designers to reduce wall thickness and bend radii. There is also no impact on corrosion resistance. These features make the duplex grade highly suitable for stretch forming used in the manufacture of heat exchanger components such as GPHE plates and parts for shell and tube heat exchangers.

An added boon is that, similar to other duplex grades, Forta FDX 27 has relatively low levels of nickel and molybdenum, which is beneficial when forecasting financial performance. The price of both metals can fluctuate considerably – so their low-level presence ensures price stability.

Proof of concept

To validate its suitability in a PHE manufacturing environment, Outokumpu undertook a rigorous onsite test and trial programme. Forta FDX 27 was evaluated against a 316L stainless steel grade, which is commonly used in high pressure GPHE designs. Both alloys have a similar level of corrosion resistance and the tests compared plates of identical 0.6 mm thickness.

Like other duplex grades, Forta FDX 27 was found to have superior strength, as shown by the true stress strain curves.

Comparison of true stress strain curves (Transversal direction) for the two materials

The real test, however, came when formability and strength were evaluated. Using the exact same tools and lubrication as normally used for the standard 316L grade, the new duplex grade was formed into a well-established GPHE design. No modifications were made to the tool design when the test plates were formed.

Researchers then evaluated the duplex stainless steel plates, paying close attention to two regions on the plates that are particularly important for forming operations. The test results revealed that GPHE sheets can be formed successfully with both materials. For this particular design, the team was able to form a plate in high-strength duplex stainless steel with an identical design and surface area as the 316L grade.

To estimate the strength of the final Forta FDX 27 and 316L plates, engineers utilised Finite Element Analysis (FEA). They found that the overall strength was approximately 30 percent higher for Forta FDX 27, which suggests a rethink of GPHE designers’ approach to high pressure applications.

Engineers paid close attention to formability of the new steel in two specific regions on the test plate
FEA analysis shows that plates manufactured from Forta FDX 27 (right) have 30 percent greater strength, with the scale being blue to red as low to high

New duplex stainless steel provides 50 percent greater pressure performance for Shanghai manufacturer

One manufacturer is already offering its customers the option of the new material in its production of stainless steel heat exchanger plates. Shanghai Heat Transfer Equipment Co., Ltd. (SHPHE in short) is a Sino-German joint venture that specialises in the design, manufacturing, installation and service of plate heat exchangers across the globe. 

Having identified the need to both enhance pressure resistance in its GPHE and reduce leakage, the group has been collaborating with Outokumpu since 2014 to evaluate Forta FDX 27 as an alternative to austenitic stainless steels. Outokumpu provided support throughout the project, which included material testing, design of test tools, sample pressing, manufacturing of prototypes, comparison analysis of tests and simulations, as well as pressure testing and thermal performance comparisons.

SHPHE soon realised that Forta FDX 27 could enable them to create high-strength plates that would withstand deformation, allowing them to operate PHEs with a large pressure difference between the hot and cold sides. This meant a greater transfer of heat and less likelihood of leakage.

During testing, plates made of the new duplex grade showed less deformation compared with standard austenitic stainless steel, as well as greater resistance to stress corrosion cracking, erosion and fatigue. The result was more reliable sealing performance.

SHPHE’s pressure resistance testing found that a PHE with plates manufactured from the new material achieved a maximum allowable pressure of 5.8 MPa, whereas a standard PHE with plates made from 316L grade achieved 3.8 MPa. This proved that Forta FDX 27 can offer 50 percent greater pressure performance than a comparable austenitic grade of stainless steel in GPHE designs.

Inspired by this outcome, SHPHE has subsequently utilised Forta FDX 27 at two district heating installations with great success. Plans are also afoot to introduce the material as an option in its welded PHEs.