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Stainless Steel Basics #2

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

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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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Availability

Are you able to find the particular material, grade, size and shape, by conducting a simple web-search? Availability will be influenced by many factors. Let’s assume the adage; “If you got the money and the time, you can probably get it.” is still very true. But since most often general plant maintenance is the last rung on the “Exotic Metals Food Chain”, most of us will not have the money nor the time. The global state of the raw materials, usage, cost, geography, political climate, shape, size, and condition, and perhaps most importantly, the minimum required order size required. Not necessarily in that order.

During WWII, nickel (considered one of the key ingredients in stainless steel) was in short supply and was being rationed. Metallurgical engineers discovered that they could recreate stainless grades, with similar properties by substituting cheaper elements, like manganese and nitrogen for the more expensive nickel. It was a case of: “live with a little less performance from these modified grades or, have no stainless at all.”

Similarly, a screw machine shop driven by production, speed, and cost, may specify free-machining grades of stainless for which they are willing to sacrifice a bit of performance for processing speed. Those grades may have been treated by adding a mineral element that allows the tooling to pass through the steel more easily than would be possible with a non-treated grade. The addition of that mineral may not contribute to the strength or corrosion resistance or weldability of the steel. It has been added simply to promote machinability. In fact, the addition may diminish other key properties of the steel. Recent innovations have developed free-machining grades that still have high mechanical properties. Note that treated grades may only exist in the form that is used by the most screw machine shops. Such as bar only, no sheet or plate.

-Howard Thomas, October 4th 2017

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Stainless Steel

Introduction to a Series

Over the 45 years of working with materials used for heavy industrial maintenance, I’ve seen that matching stainless grades of steel with the appropriate application can be awfully confusing to the many support folks who do not have a background in the sciences. I speculate that a lot of hard working “non-metallurgical” people would be happier and more effective if they understood a bit more of what governs the decisions and requests coming from maintenance and reliability personnel. I am one of those “job taught” individuals and it is my hope that the posts that follow will provide a very general overview, i.e. the help that I would have relished many years ago. Becoming familiar with the various “personalities” of the most common available grades of stainless will hopefully encourage a familiarity within the potential supply chain and its value or limitations. Even a casual understanding of the distinct stainless personalities relative to the particular needs of the applications would help the two better get along. Think E-Harmony for stainless materials and applications.

In the coming weeks and months we will pass along some hard learned tips and stories that might help to broaden your understanding of the stainless picture. We will answer questions like; “What is Muffler Stock”, Can non-magnetic stainless attract a magnet, are there magnetic grades of stainless, Can you harden stainless, Can you buy pre-hardened stainless, why does stainless tend to “move around when you’re trying to keep it straight”, along with a host of other nagging questions.

We hope to stimulate questions that will generate future posts. If you consider yourself “non-technical” i.e. just another soul immersed in an industry that is drawn to an increasing reliance on the material, this should be right up your alley!

My approach to these articles will be a little like the differences between cooking and baking:  When cooking, you can make a darn good chili without a recipe. You can measure ingredients by handfuls and cook it until it “looks right”. Baking, on the other hand, requires keen attention to the recipe with ingredients, time and temperature. Detailed information that would be appropriate for engineering decisions is not what I’m hoping to present. So, if you are a scientist, metallurgist, or engineer who is reading this post expecting a soufflé, you will be sorely disappointed to find that it might only be even a moderately good chili.

All comments and criticisms are encouraged and welcomed however, understand that this Soufflé is going to be simmered with kidney beans and hot peppers! Hopefully, these posts will act as catalysts for readers to seek more detailed information from structured resources better equipped to deal with the on-site conundrums they may encounter.

As politicians are so fond of saying; “Let me be perfectly clear”;

What follows is not intended for the credentialed metallurgist, the engineer, or chemist. The information is not to be used for engineering purposes. This is more like an energy drink for the unsung work-a-day-support stiffs charged with keeping the supply chain flowing. They need to be incorporated into the wonders of the maintenance materials world that they have an undisputed effect on. From their caves under the catwalks, in the bowels of plants, to the mazes of cubbies in the cooler parts of the plant, they long to be a part of what it is they get blamed for anyway.

-Howard Thomas, July 31st 2017

 

 

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