“If you live in a hard-partying area of the country, you may not want to buy a new car that was assembled on a Monday. And, you may not want to shoot pool with someone whose first name is the name of a major city”. Just some considerations learned from experience.
In heavy industrial maintenance, seasoned professionals have their own hard-won cautions like the above. Those may not always be obvious. Wouldn’t it be great if those tidbits of knowledge, however, were somehow automatically transferable through the generations? However, natural powers, or gremlins, seem to insure constant attrition; constant turnover of experienced maintenance personnel, and the subsequent loss of their esoteric talents.
It’s a terrible thing to know you have solved a problem only to see things go from bad to worse because of a less-than-obvious semi-related circumstance.
Let’s say you are trouble shooting a problem where there is obvious “pitting” on the surface of a stainless shaft. For a host of reasons, pitting will eventually lead to a shaft failure. Before you begin looking into the usual suspects related to corrosion, do a little forensic investigation and see if that is really the main problem you want to solve. Pitting may not be The Big Offending Kahuna.
The shaft in question may be extra long with a small diameter (Linguini). Straightness, as in the case of a vertical mixer shaft, may be a primary concern. Let’s assume the opposite configuration of a larger diameter shaft with relatively short length (fat and stubby). In either case, straightness happens to be a key element. So, in the hierarchy of concerns; pitting is subordinate to straightness.
Most stainless-steel shaft grades, by nature of their chemistry and grain structure, retain substantial amounts of stress. Those retained stresses contribute to bow, twist, or fracture. There are grades of stainless steel, however, that respond well to thermal stress relief. Most of the retained stress is able to be removed. (less retained stress, less movement in machining and in subsequent service). These grades resist pitting, but maybe not as well as other grades. Remember though, if the shaft never makes it into service, the potential life expectancy is irrelevant.
You know steel shafts may be straightened mechanically; so, just solve the pitting problem with a material change and then straighten the shaft. But, if the shaft configuration, or the final machined configuration, does not allow for conventional mechanical straightening, or that process would require equipment that is not readily available, or the straightener guy is just plain incompetent, experience may have opted for a steel chemistry that would be less susceptible to warp and bow; either in machining or in service. The luxury of post machining straightening was not considered an option. The best steel choice in this case may not be the one with the best Pitting Resistance Equivalency (PRE). (If the shaft never makes it into service, service life is irrelevant).
To be effective in the industrial maintenance field you must be intuitive and organized. Assuming you are, then pointing out the need to look at more than one contributor to material failure is obvious. Considering the relativity of an incompetent straightener to a pitting condition, is not so obvious.
-Howard Thomas, March 6th 2018
Most likely it does, regardless of the state of your memory.
When I first heard the term “Memory”, relative to stainless steel, I was anxious to find out what it referred to. An associate with one of the stainless mills responded with this little tidbit; “Memory, regarding stainless steel, generally refers to retained stress, specifically in austenitic grades. That relates to “Movement”, or “Walking”. (Bars won’t hold straightness). Those grades of stainless (304L and 316L) have memory. They are difficult to straighten in the first place. Then, after you have manhandled them into the straightness you want, they have a tendency to return to the straightness they “remember”.
If you have a bar that looks like a tapeworm and you cold straighten it to a beautiful pump shaft, then ship it across the country, expect to find a tapeworm when you open the box.
Same with people. Take an annoying coworker. Explain why you are transferring them to your sister company. Instruct them to straighten up. Ship them across the country, and bingo! Your sister company now has an annoying coworker.
If you want to look at this annoying tendency of memory in stainless steel a bit closer, you can start by noticing that we specifically mentioned austenitic grades of stainless. Those tend to be the most commonly used in industrial maintenance. And, of the austenitic grades two are by far the most common to the industry; type 304L and type 316L.
Coincidentally, it is just those two grades that seem to have the most profound memory issues.
They probably occupy over 70% of the grades used on a daily basis. Type 316L (we’ll talk about the “L” later) is a modified grade of 304L. It is an upgrade developed to better resist the damaging effects of corrosion. Both 304L and 316L are products that come under the general category of 18-8 stainless.
In that grouping, the first number represents chrome content, and the second represents nickel content; the two primary alloying elements in the austenitic grades mentioned.
Austenitic Stainless grades 304L and 316L;
Are non-magnetic; under most circumstances they will not attract a magnet.
Are not hardenable by thermal treatment
Can be hardened by cold work, strain hardening
Are generally of moderate strength as purchased
Are resistant to most common forms of general corrosion
Are resistant to the negative effects of service temperature to a little over 1000°F.
They possess some annoying attributes, however. In addition to memory issues, they tend to gall (Get stuck to mating parts, or, “cold weld” to mating parts), are a bit gummy, and tend to be of lower strength.
PARTING THOUGHT
Since 304L and 316L do not respond to thermal treatment, and since the most commonly employed stress relief for steel bars is thermal conditioning. It is understandable that austenitic stainless bars retain stresses from the manufacturing process.
Since they do retain stress, and stress will not stay in a material (it will come out as movement, warp, or fracture), it is expected that those grades would have Memory; the retained stress being released as bow, twist or warp.
Once you have made pump shaft from austenitic stainless bars, you may anticipate the stress induced in the straightening process to manifest somewhere down the road. The catalyst may be: vibration, heat, torque, whatever.
REVIEW:
MEMORY Has trouble with authority
GALLING Doesn’t play well with others
FUTURE TOPICS: “The One Handed Metallurgist”
-Howard Thomas, January 5th 2018
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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
