Stainless steel for heat exchangers

Materials World magazine
,
28 Oct 2019

A new grade of duplex stainless steel is helping improve plate heat exchanger performance.

Plate heat exchangers (PHEs) are used extensively to transfer and manage heat in industrial and district heating applications. They consist of a series of thin, corrugated plates, which are gasketed, welded or brazed together, depending on the device’s application. The plates are compressed in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids.

To ensure rapid fluid flows and therefore efficient heat transfer, PHEs often operate at high pressure, which can cause individual plates to warp and may result in leakage – specifically in the case of gasket-sealed exchangers (GPHEs). Heat transfer is also affected as the warped plates affect the fluid flow through the channels formed in the plates.

However, research found that a solution to prevent the deformation of plates was not in the PHE design, but in the material. Regardless of the product being of welded, brazed or gasketed design, a higher strength material has potential to reduce the risk of plate deformation and collapse.

Instead of standard, austenitic stainless steel, a high strength duplex stainless steel would most likely be required. Duplex grades are known for their higher fatigue strength, corrosion resistance, and enhanced resistance to stress corrosion cracking. Manufacturers faced one difficulty though – most duplex grades have limited formability in comparison with austenitic stainless steel. As a result, they cannot normally replicate the indentations in plates that form channels to guide the flow of fluids.

PHE designers, in turn, must accept a lower surface area for heat transfer across each plate, which impacts the unit’s overall size. In response to these challenges, Outokumpu developed a new grade of duplex stainless steel, Forta FDX 27. The product has relatively low levels of nickel (Ni) and molybdenum (Mo) compared with austenitic stainless steel. In particular, the new product has 2-4% Ni, and 0.6-1.4% Mo, while austenitic stainless steel 316L/4404 has 10.1% Ni, 2.1% Mo.

Specifically developed for forming-intensive applications, the material allows GPHE manufacturers to produce plates with the same or similar surface area as their existing plates made from austenitic stainless steel. In addition, it is possible for the heat exchange manufacturer to adopt the same tooling set when switching over to the new material.

Material improvement

Forta FDX 27’s ductility is attributable to the transformation induced plasticity (TRIP) effect, which describes changes in the crystal microstructure when plastic deformation takes place.

Microstructure, of which there are several types, defines the properties and physical attributes of stainless steel, and determines whether a stainless steel is austenitic, ferritic, martensitic or duplex.

The TRIP effect usually describes a controlled transformation of austenite into martensite at the microstructural level. The effect – which is achieved by using a cold forming method – has been known to only work efficiently for austenitic steel grades, but was possible in this new duplex.

Duplex stainless steel’s chemical composition is generally balanced to give approximately equal amounts of ferrite and austenite in solution-annealed condition. For the Forta FDX 27, the composition is balanced to give an optimal austenite stability, leading to a controlled transformation of austenite to martensite during cold forming operations so-called TRIP. Typically, the austenite content in the solution annealed condition is slightly higher for the new material than for other duplex grades. The Forta FDX alloy is not sensitive to sigma phase formation. However, like all duplex grades, it is more prone to precipitation of nitrides and carbides than the corresponding austenitic steels.

An increase in strength and formability allows designers to reduce wall thickness and bend radii, which makes it suitable for manufacturing GPHE plates and components for shell and tube heat exchangers, using stretch forming. There is also no impact on the material’s corrosion resistance.

Rigorous testing and evaluation

Researchers evaluated the duplex grade against a 316L stainless steel grade, a commonly used alloy in high pressure GPHE designs. Corrosion resistance levels are the same on both alloys and the tests compared plates of identical 0.6mm thickness.

As indicated by the true stress strain curves, Forta FDX 27 was found to have significantly higher strength similar to other duplex grades, and was verified by tensile testing in three axes compared to the rolling direction – 0˚, 45˚ and 90˚.

To confirm the suitability of Forta FDX 27 for this application, tests were carried out to use it as a replacement for the traditional grade Supra 316L/4404 to improve the high pressure capacity of an existing GPHE design. The reason for comparing Forta FDX 27 to Supra 316L/4404 is the need for similar corrosion resistance. The materials used in the comparison were Forta FDX 27 and Supra 316L/4404 with a thickness of 0.6mm. True stress strain curves for the materials showed the strength was higher in the new product, while formability was still good.

To evaluate formability, the new grade was formed into a GPHE plate design based on a 316L grade. No modifications were made to the tool design or the lubrication used on normal production, which means the test pieces were formed with the same tools as the standard 316L grade. Upon evaluation, testers focused closely on two regions on the plates that are important for forming operations. Results showed that GPHE sheets can be formed with both materials. For the design of the sheets, the test team was able to form a plate identical in design and surface area with the high strength duplex stainless steel.

Finite element analysis

Extending the study, engineers used finite element analysis (FEA) to estimate the strength of the final plate of both 316L and Forta FDX 27 grades. The objective here was to analyse the feasibility of the new stainless steel grade for the application.

For the new product, the model took account of its tested tensile strength and analysed the strain distribution during plate forming, as compared with a standard 316L plate. The study paid particularly close attention to two regions on the plate where forming operations create complex shapes that have been found to be challenging to form with other grades of duplex stainless steel.

The results found that Forta FDX 27 experiences higher strain in the two focus regions – however this is still below the limits of the material, including a 10% safety margin. In addition, the FEA showed that the overall strength of Forta FDX 27 was approximately 30% higher, which implied that GPHE designers may well reconsider their approach to high-pressure applications.

Commercial demand

Shanghai Heat Transfer Equipment Co Ltd (SHPHE), a Sino-German joint venture based in Shanghai, China, manufactures heat exchangers for customers in North America, Europe and Asia in the heating, ventilation and air-conditioning, oil and gas, bioenergy, metallurgy, chemicals, marine, food and pulp and paper industries.

In 2014, SHPHE identified a need to enhance pressure resistance in its gasketed design PHEs and resolve issues relating to external leakage. In that same year they started working with Outokompu to evaluate the new product as an alternative to austenitic stainless steel grades such as 304, 340L, 316 and 316L.

The project included material testing, design of test tools, sample pressing, prototypes, comparison analysis of tests and simulations, as well as pressure testing and thermal performance comparisons.

Pressure resistance and sealing performance is closely related to the rigidity of the metal plates, as well as the performance of the gasket. The standard approach to improve its performance is to increase the thickness of the plates, and therefore the heat transfer efficiency of each plate. The drawback, however, is the knock-on effect this has on the overall size of a PHE – it has to be larger to achieve similar performance. In contrast, the new product offered more.

Pressure performance

Accomplishing a large pressure difference between the hot and cold sides of a PHE will ensure high flow rates and improve the heat transfer efficiency. The downside is that a greater pressure difference will most likely result in the warping of the channels in the plates, thereby altering fluid flow and heat transfer performance.

The higher strength of the plates made from the new duplex grade, however, decreased the level of deformation compared with an austenitic stainless steel. Additionally, it provided greater resistance to stress corrosion cracking and more reliable sealing, as well as improved erosion and fatigue resistance.

The results of pressure resistance tests revealed a PHE manufactured with the new material achieving a maximum allowable pressure of 5.8MPa, compared with 3.8MPa for an otherwise identical unit with plates made from 316L.

District heating

The material has been used for district heating with heat exchangers, operating at a design pressure of 2.5MPa. The original PHE design for these applications was based on 316L stainless steel plate with 0.7mm thickness. After just one heating season, the PHEs were disassembled for cleaning. Upon closer inspection the technical team identified some local deformation of the plates – including the potential for leakage at points where deformation occurs at sealing positions. Pressure measurement also found that the decline in pressure on the low-pressure side was higher than the design value.

In a follow-up analysis, SHPHE concluded that the plate deformation was having an undesirable effect on performance. To resolve this, the manufacturer recommended upgrading the plates to the new material, which enabled the construction of stronger plates while at the same time reducing the plate thickness to 0.6mm.

After operating for a prolonged period, plate inspections demonstrated that deformation had been eliminated because of the introduction of more rigid plates, leading the inspection team to verify the suitability of the new duplex grade for those heat exchanger applications.

Improved heat transfer

With its resistance to the corrosive liquids that can be found inside GPHEs, austenitic stainless steel has long been a good choice of material for the plates that channel the flow and conduct heat between hot and cold sides of heat exchangers. However, until now it has not been possible to upgrade from austenitic grades to a high strength duplex grade as many duplex stainless steels are not amenable to forming operations.

As a result, it has not been possible to create plates with both high strength and formability. High strength is important to optimise flow rates and improve heat transfer performance, whereas formability is vital to creating the complex pattern of ridges that guide the flow of fluid across the plate surface and therefore optimise heat transfer in a compact space.

The new material is a new grade of duplex stainless steel that can achieve both.