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So, in our last post we briefly introduced Stainless Steel (SS) in general overview; We introduced the grades most common to general industry (304 and 316), and we looked at some of the properties of those grades.

There are, however, many grades of Stainless, each with its own strengths and weaknesses and economies. It is my feeling that due to overall education of the pros and cons of many of these grades, and lack of familiarity, many grades of stainless are used inappropriately in the maintenance arena. So many grades, so many mechanical forces, so many types of corrosion. (Variables relating to corrosive conditions alone are nearly infinite). One grade of stainless may be working in a particular application, but that may be only relative to other materials that have worked less effectively. So, let’s begin by getting acquainted with the benefits various grades may afford.

Two important things to keep in mind are; A specific grade, perfect for your application, may exist but may not be available in the form needed (tube, sheet, plate, bar). The grade may exist and be available, but, due to limitations in fabrication, may not readily lend itself to the configuration of your part.

Most often used, Type 304 and 316, fairly economical, good general corrosion resistance, elevated temperature resistant, and somewhat user ambivalent to a bit combative.
(Consider grade 303, if machining speed is a serious consideration, which is generally not the case in maintenance situations). These grades are austenitic 18-8 type, non-magnetic, will work-harden, but do not respond to thermal hardening.

Commonly used when increased strength is required; 410 and 416. Note that 416 is 410 modified for ease of machining. Superior to 304 and 316 in Sour Service Applications. (Martensitic, magnetic, responds to heat treating.)

Persistent problems, not addressed by either of the two groups listed above, will require consideration of modified, or enhanced grades. Those grades will involve modifications to element content (increasing content of elements, such as Nickel, Moly, Nitrogen, etc). Expect increases in cost. Among these are non-galling grades (such as Nitronic grades, Mir50), elevated temperature grades (such as 309 and 330), and a host of trade named products that are relatively economical and effective. Then there are, what I like to refer to as “The Exotics”; the next level up. Exotic in chemistry, and exotic in cost. Cost is of course relative to benefit; however, too often exotic materials are selected in an express leap to the ultimate solution. Generally, once that selection is made, the user is committed to a cycle of selecting grades equal or superior to that grade. Assumptions being, the grade in use is the minimum required grade. Going forward, service-life needs to be improved; Current service-life never being assumed to be the best-case scenario.

Modified grades may already be numerically identified (AISI SAE, UNS, etc.). Often, those targeting specific maintenance conditions, will be identified by a Trade-Name, identifying the producer. That producer will issue guidance on their trade named grades, to insure they are correctly applied in the field.

IMPORTANT CONSIDERATIONS:
As you add to the chemistry and increase corrosion resistance, strength, and toughness, etc.,
you are creating a product that fights and resists forces that attack the part in service. Read that as a product that “Does not play well with others.” Keep in mind that when you need to work with it, it may not play well with you either. It may solve the problem in service, but you will have to get it into service first.

Pay close attention to “User Friendly” stainless. How does it machine? How weldable is it? Does it tend to gall? Does it move? Then take a second look at your part. User friendly stainless may be a bit of an oxymoron, but it can be as important as the ultimate in-service performance.

-Howard Thomas, November 18th, 2022

THE 300 SERIES OF STAINLESS STEEL

If a steel has more than 10 ¾% Chromium content, it is classified as a type of Stainless Steel. There are many types (grades) of Stainless Steel, just as there are many types of corrosion. One grade of stainless steel may work well with one type of corrosion and then be terrible for service in another type of corrosion. The most common stainless grade is 304. Modify it a bit (improve) and you get 316. Enhance that a little, and you get 317. If you add Sulphur to type 304 stainless, you get 303 stainless, which is a “Free-machining grade”. Note: Generally, as you add elements to stainless steel to improve machinability, or strength, you may lessen strength and service-life.

Three hundred series grades are often referred to as; 18-8 stainless; where the 18 stands for percent of chromium content and 8 refers to the percent Nickel content. Those grades have some commonalities; grain structure of Austenite, non-magnetic, not generally responsive to thermal hardening, ability to work-harden.

Steel that has an Austenitic grain structure is not hardenable by heat treatment, but it is hardenable by cold working. Austenitic stainless grades are non-magnetic; i.e. they won’t, under general conditions, attract a magnet.
Strain-hardening, Cold-drawing, Cold-Finished, Cold-working, and Work-hardening, are all pretty much synonyms. Bang it around, Squash it, or keep bending it back and forth and it’s going to get harder; sometimes near 50RC hard.

18-8 Types of stainless steel are popular (common) for good reason. They are excellent for many applications exposed to corrosion, and they are readily available from suppliers. There are of course limitations imposed by various specific types of corrosion as well as limitations imposed by physical characteristics of the actual parts required (strength, straightness, etc). We will explore those in later posts.

 

-Howard Thomas, October 17th, 2022

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

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