Uncovering the Mysteries of Duplex Stainless Steels

Duplex Stainless Steel Atlas of Microstructures

Uncovering the Mysteries of Duplex Stainless Steels

Post by:
  • 1:50PM Jul 26, 2014
  • Comments off

As early as the 1930s, companies were researching the development of duplex grades of steel that were more resistant to cracking and corrosion, but it wasn’t until the 1960s and 1970s when improvement in refining techniques resulted in better product quality, that major progress was made. Since that time, the use of duplex stainless steels has grown and new grades have entered the market. How these grades react under various conditions is not precisely known since many of these steels are produced by individual companies whose formulations differ. The Duplex Stainless Steel Atlas of Microstructures published by Materials Technology Institute, Inc. (MTI) helps uncover these mysteries with an atlas of instructive, high quality images of the microstructures of duplex stainless steels. Written by Jim Pellegrino (RJ Lee Group), Heather Stine (Materials Technology Institute), Jim Fritz (TMR Stainless/Outokumpu High Performance Stainless), and Hira Ahluwalia (Materials Selection Resources/Nickel Institute), the atlas discloses seldom-seen images of duplex-alloy grade product forms that include flat rolled and tubular products with and without welds, and cast, forged, and HIP materials that have been exposed to various thermal treatments, and more.

Super Duplex Stainless Steel
Cross section perpendicular to the axis of the weld showing narrow heat affected zone (HAZ). The microstructure in the HAZ consists of larger grains of ferrite and a relatively small volume fraction of austenite.

Simplifying the Duplex . . . through Images

Duplex stainless steels have a two-phase microstructure containing both ferrite and austenite–a combination that results in a variety of superior properties such as high corrosion resistance and mechanical strength. However, despite their advantages, duplex stainless steels are also known for a complicated microstructure that makes them more difficult to produce and fabricate. They are often subject to precipitation of phases that are detrimental and may adversely impact both strength and corrosion resistance. It is important to reveal and evaluate these properties and their microstructure to understand their relationships to each other.

To better understand these relationships, MTI member companies prepared and donated samples of various stainless steel grades and products to RJ Lee Group’s laboratory for metallographic analysis. More than 100 samples, which included specimens as-produced, annealed, and those artificially aged in used materials, were metallographically prepared and examined using optical microscopy, scanning electron microscopy (SEM), and scanning electronmicroscopy with energy dispersive x-ray spectroscopy (EDS). Some of the samples were heat treated and all were photo-documented at RJ Lee Group’s headquarters laboratory to show how the microstructure was impacted.

Super duplex stainless steel
Cross section perpendicular to the axis of a weld showing as-solidified weld metal. The microstructure consists of acicular austenite (light colored phase) in a ferrite matrix.

Jim Pellegrino, recognized steel expert, shot thousands of optical and electron images, analyzed samples, and contributed to portions of the text. A variety of etchants have historically been used to selectively reveal matrix phases and second-phase constituents, but to better understand the complicated microstructure of the samples, he developed a special etchant (Pellegrino’s Etchant) which he used to examine the samples. Using this etchant, he was able to control the etch depth, so that, when necessary and depending on the alloy content, the etchant could be modified so that it was slow acting allowing him to investigate secondary phases and reveal not only the general microstructure, but also carbides, nitrides, and the intermetallics in the samples. Anyone who needs to understand the microstructure and behavior of available duplex stainless steels will find this atlas to be an expert handbook.

The 159-page atlas is available in hardcover to MTI members.

MTI Membership

MTI members can utilize MTI’s books, reports, software and video training programs immediately as needed. To inquire about membership in MTI, please contact MTI at www.mti-global.org or at mtiadmin at mti-global.org or call 314-576-7712.