Or, “Keystock, we hardly knew ye. . .”

So, back in April of last year I was eulogizing Keystock; at least the longer length, higher strength, plus-tolerance kind, that for years had been a “go-to” product for a lot of people. It seemed Keystock would be going the way of the Dodo bird. But, I was apparently wrong. More correctly, it was going the way of Coke, or New Coke, or Original Coke, whatever. The point is . . . “It’s baaaack.” And, that’s a good thing.

As I mentioned then, in the steel business in most general terms, anytime you see the word “stock” attached to another word; think, “stock used to make”. Bar-stock, Bushing-stock, Brake-Die-Stock, Rifle-Barrel-Stock, Pump-Shaft-Stock, etc., would be stock that might typically be used to make those items. While it may seem to infer that there are other attributes specifically suited for the particular application specified, it may simply mean this is a material some people have chosen to make this type of part. Details should be clarified; is it closer tolerance, harder, squarer, whatever? Facetiously, a tree might be “Tooth-Pic Stock”. Works the same with the term Quality, as in; Rifle barrel quality, or Military Quality, or Drawing Quality. The explanation and caution remain the same; It may mean the steel possesses a special grade certification, but it may as likely mean that “Someone, at some time, used it for that.” So, “Trust but verify”.

Keystock is generally a low to mid-strength mild or carbon steel square or flat bar, used to make keys. Those keys are inserted into shafts. A drive motor chuck will engage with the protruding key, and rotate the shaft. The idea is that the key is of less cost and strength than the expensive shaft. If something interrupts the motion of the shaft and causes failure, it should be the cheap little key that fails not the big expensive shaft. Clear the obstruction, put a new key in, and you’re good to go, hopefully with an undamaged shaft.

The perfect piece of keystock would have less mechanical strength than the shaft, but tuned to the actual application, close enough that the shaft doesn’t fail prematurely, contributing to expensive downtime. There is the conundrum; too soft of a key and you have frequent expensive delays. Too hard of a key and you risk damaging an expensive shaft.

Ideally, commercial Keystock would be a precision cold drawn bar possessing slightly elevated strength properties, with good cross-sectional accuracy and a close oversize tolerance. The ideal oversize tolerance for many years was considered to be +.002”/ .000”. That tolerance would allow you to cut a standard keyway that would fit nice and snug. Undersize tolerance resulted in sloppy, loose keys. Too much oversize required unwelcomed machining.

Unfortunately, over the past decade or so, tolerances on commercially available Keystock gradually opened up to +/- .004 to .007” (plus or minus); even with sources that promoted “Keystock”. For several years now, it has been common to only be able to source low property mild steel, undersize Keystock, in short lengths (12” and 36”).
Longer lengths, no matter how beneficial to the end user, require expensive additional processing to eliminate camber and bow.

Running multiple draughts (passing the bars repeatedly through drawing dies) is a good practice to increase strength (strain harden) and refine tolerance. However, that process adds cost, as does additional straightening. Those expensive practices were eliminated as the marketplace shrank.

Hardness (through multi-pass cold drawing/strain hardening), tolerance, cross-sectional accuracy and straightness, are all doable. Expensive, but doable. It requires a bit of fiddling in production for an item that does not represent significant tonnage. This current economic burst has allowed “The Return of Keystock”. Check with your vendor and take advantage of a simple perk that will make your life easier.

-Howard Thomas, September 15th 2021

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

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Howard Thomas

Howard Thomas

Experience

Sr. Acct. Mgr. (US Southwest) / Metallurgical Consultant
Associated Steel Corporation
Jan 2017 – Present

Past Vice President / General Manager
Associated Steel Corporation
Apr 1998 – Jan 2017

Past Vice President / General Manager
Baldwin International
Apr 1974 – Mar 1997

Education

Cleveland State University
Kent State University
University of Denver

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