Outokumpu’s formable duplex stainless steel grades

Outokumpu’s formable duplex stainless steel grades (FDX™) are proving to be well suited to
demanding applications such as pump casings, where the formability of the stainless steel is a
vital asset alongside strength, durability, and resistance to wear and corrosion.

Outokumpu’s latest duplex innovation, formable duplex stainless steel grades, combine the high strength of duplex stainless steel with high ductility levels. This makes them comparable to the austenitic stainless steel grades that have so far most typically been used for challenging applications where the formability of the steel is at a premium. This allows the new grades to bring the benefits of duplex stainless steel to products such as pump casings, for which existing high-strength duplex grades have previously been unsuited due to their limited formability.

Outokumpu first launched the new FDX 25™ and FDX 27™ duplex grades in May 2013. The FDX grades offer good resistance to localised and uniform corrosion. In many test environments the FDX 25 and FDX 27 show similar or better corrosion resistance to the conventional austenitic grades 304L and 316L, respectively.

Extensive tests have shown that their formability characteristics are closely comparable to those of the austenitic steel grades – those most widely used in such applications. At the same time, they have proven to be as durable as existing high-strength duplex grades. Furthermore, the FDX grades have been seen to offer all the other typical benefits of duplex stainless steels, including mechanical strength up to twice that of austenitic grades and good resistance to stress corrosion cracking.

Performing in harsh conditions

Pumps often have to function under very harsh conditions in terms of pressure, temperature, wear and corrosion risks. “It’s crucial for pump manufacturers that properties of FDX would increase the overall durability and service life of their pumps and thus reduce the need for maintenance,” says Ramin Moshfegh, Senior Specialist, Forming and Virtual Product Development at Outokumpu’s Avesta Research Centre in Sweden.

The unique combination of properties is due to the favourable microstructure and chemical composition of our FDX grades,” explains Moshfegh. The ductility of the new grades is particularly attributable to a phenomenon known as the Transformation Induced Plasticity (TRIP) effect. The TRIP effect is achieved by using a cold forming method that enhances the strength and formability of the steel.

“Before we came up with FDX™, TRIP effect had only been known to work effectively with austenitic steel grades and it had never been successfully realized in duplex grades,” points out Moshfegh.

“No one else currently produces grades like these as commercially available products,” he adds.

Opportunities for cost savings

The test findings have shown that manufacturers will be able to use the same forming tools as those used with the standard austenitic grades, after making minor alterations regarding pressing force and the use of lubricants. A stable forming process was achieved, with no damage or tendency to wear on the press forming tools being observed.

Moshfegh is convinced that positive findings will enable manufacturers to develop pumps that can reliably withstand harsher operating conditions. These include environments of high pressure, heavy wear and extreme temperature variations, as well as high salinity levels in marine applications.

“Another benefit of using high strength material like FDX is that this gives manufacturers an opportunity to down gauge the dimensions of steel components and produce light-weight pumps that can save on both material and space,” explains Moshfegh. This option to down gauge can result in considerable cost savings, as well as greater freedom for technical design.

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