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Lean Duplex – Part Two: Who Can Use This Product?

The easier question to answer would be; who can’t use it?

A continuation of last month’s post:
Service temperatures should not exceed 750F. Any customer currently using stainless of the following types: 304L, 316L, 410, 416, 17-4ph should consider LDX, (ASSOCIATED STEEL’S ASC2250 LDX).

LDX is now made by several steel mills, to their own specific variances. In general, it is a great stainless grade for heavy maintenance applications where the grades listed immediately above are being used. It is more corrosion resistant, stronger, less apt to gall, better at resisting SCC, easier to machine and weld, than many of the commercial grades shown.

Lean Duplex work hardens. As shipped, it is generally about 28RC. The Austenitic portion of the grain structure contributes to strain hardening; it cold works as the size is drawn. As mentioned in part one; It is harder (hence stronger) than commercially available 304 and/or 316, but is still easier to machine. Duplex grades of stainless steel contain grain structures of equal parts Austenite and Ferrite. They are considered to be magnetic in their most common form.

It resists bending, (minimizes twisting), abrasive wear, resists failure due to SCC, resists galling, adds strength. It’s like the Ginsu knife of stainless steels. (Probably have to be my age to know what that means). Lean Duplex is not intended for use in applications currently requiring advanced alloy grades, such as; 2507, AL-6XN, Hastelloy C, 20cb, Ni625, etc.

ADVANTAGES OF ASC2250 LDX
The PRE (pitting resistance) is the accepted standard for determining a stainless grade’s comparable resistance to pitting and crevice corrosion. The lower the number, the less resistance.
304 is 18 316 is 24 Duplex grades are nearer to 40.
Associated Steel carries Lean Duplex (ASC2250 LDX) in two surface finishes; Fine-turned oversize (The size will make the nominal size), and Precision Polished Guaranteed Bearing Fit (Minus/minus tolerance). It is inventoried in long mill bars and may also be sold to specific required lengths.

ASC 2250 LDX offers advantage in:
Resistance to Stress Corrosion Cracking (SCC)
Resistance to Chloride pitting
Resistance to Crevice Corrosion Cracking
Elevated Strength Levels
Ease of machining
Ease of welding
Greater fatigue resistance
General corrosion resistance superior to 316L
Excellent resistance to “Thermal Shock” (low-cycle fatigue)
Excellent service to -30C

-Howard Thomas August 6th, 2021

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Lean Duplex Stainless: What Is It?

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

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