Does Your Stainless Remember Things?

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