If memory serves, I did a post on Lean Duplex some time ago. It is an important grade of stainless and worth a revisit.

Basically, Lean Duplex is a leaner chemistry derivative of Duplex Stainless Steel. Duplex Stainless Steel is recognized as having a unique shared grain structure; Austenite and Ferrite. Each of those grain types contributes to the characteristics and performance of the steel grade. The grade was developed to provide resistance to Stress Corrosion Cracking (SCC), a type of corrosive failure prevalent in SOUR SERVICE applications in operations such as Refineries and Pulp & Paper plants. Sour Service applications involve acidic or base (alkaline) exposure. 2205 Duplex is probably the most common grade of Duplex that the industry is familiar with; although there are several others. I refer to 2205 as the “Original Grade”.

The short and sweet history of steel usage to combat SCC in sour service maintenance applications is this: 304 and 316 (Austenitic grain) worked tolerably well, but lacked strength. 410 and 416 (Martensitic Grain) provided the needed strength, but offered less general corrosion resistance. Turns out the catalyst was nickel content, but that’s another topic. The thought was, develop a grade that was half Austenite and half Martensite (Duplex), and enjoy the best of both worlds. The development of the original chemistry 2205 Duplex did just that. It fit the bill, but it was expensive and somewhat user unfriendly.

Years later, when the cost of elements used in the chemistry of the Duplex grades became prohibitive, those grades were pruned to the bone (reduced the expensive elements) to develop a new “More Economical” grade, Lean Duplex. Engineers were content to live with a much less effective product, in order to come up with a more affordable product. Nickel prices alone, had seen a ghastly increase at that time. The new LEAN DUPLEX, however, displayed an unexpected phenomenon; resistance to general corrosion and SCC was very near that of the original grade. Strength was also maintained; and, machinability was increased dramatically.

Today, LDX is most commonly used globally in tube and sheet form in construction of container vessels and conveyance items.

Lean Duplex (LDX)
Resists pitting and crevice corrosion similar to 316L
Resists Intergranular attack better than 304L or 316L
Resists Stress Corrosion Cracking better than 304L, 316L, 410, 416
Resists General Corrosion better than 304L, and 316L
Weldability – less restrictive than 2205

-Howard Thomas, July 6th 2021

So, what is Lean Duplex Stainless Steel? When and where would you use it? A big topic for a short article. This is merely a brief introduction. Perhaps it will encourage further investigation. Also, we are referring here only to solid round bar, sheet and plate.

For many decades, Austenitic Stainless Grades dominated the industrial and commercial marketplace. They provided excellent general corrosion resistant properties. Then, hardenable martensitic grades began to see usage where increased strength was required. The successful combination of the grain types (Austenite and Ferrite) into one steel introduced Duplex Stainless Steel. Duplex grain structures allowed the broadest utilization of the properties of each. The initial concept was not terribly new; having been introduced about 80 years ago. LEAN DUPLEX stainless steels were the offspring of that product. Early improvements to the Duplex grade involved the enrichment of chemistry of the initial Duplex grade creating upgrades called; Super Duplex and Hyper Duplex; elevated property products with elevated price tags. A significant nickel shortage in the late 1960’s sent engineers scrambling to reverse the “richer is better” trajectory. They embarked upon a project that would minimize the expensive elements in the steel to lower the cost. Minimizing the content of key elements was expected to likewise minimize effectiveness. That was acceptable provided any new grade was still effective in combatting corrosion (specifically Stress Corrosion Cracking, or SCC) and increasing strength over the levels provided by the Austenitic and Martensitic grades used prior to the inception of Duplex.

The resultant steel was Lean Duplex (LDX). It was leaner and cheaper by a long shot. The great surprise was that the resultant loss in corrosion resistance and strength was not as significant as was anticipated. In fact, it was minimal for most intended service applications.

Fairly quickly, the LDX grades enjoyed overwhelming acceptance in the global manufacturing of various tanks, vessels and tubing. So much so, that it is rare these days to find any of those items that do not contain some percentage of Lean Duplex Stainless Steel. What was not readily apparent was the huge potential for daily use as upgraded replacements for the myriad of mundane, and unheralded daily maintenance wear parts and widgets that represent the lowest rung on the maintenance metals food chain; the gremlin maintenance parts that bend and pit and wear. The parts that won’t disassemble because of galling.

My opinion: Lean Duplex shafting, sheet and plate is under-used as a maintenance material. It needs a publicist, promoter or talent agent. Some gnarly champion that might say; “You can use that sh..t on anything!”

It’s that good. Why, it’s the Ginsu knife, the Veg-O-Matic of stainless shafts.

the lean duplex grades are stronger than the austenitic grades of 304 and 316. In my opinion they are generally better grades than 410 and 416. But wait, there’s more, Lean Duplex resists Stress Corrosion Cracking in Sour Service Applications. Since it will most likely be non-similar to your current stainless grades and because it will most likely have a different hardness, it is not disposed to gall. Plus, it is stronger and easier to machine.

Conclusion & Sales Pitch: Associated Steel is one of the few service centers that carries Lean Duplex (ASC2250® LDX) in long shafts they will cut to size. They also carry IN STOCK two different shaft finishes; Fine Turned Oversize “the size will make the size” resulting in less machining and less wasted material, and a Precision polished finished guaranteed bearing fit. Try it! Inquire on hi-def plasma cut parts from plate. Make maintenance life a little easier. Note: Your particular maintenance application has unique characteristics. Always refer to published material information sheets for qualification. 

-Howard Thomas, May 6th 2019