“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

For a long time, austenitic stainless grades enjoyed the lion’s share of business in the world. Austenitic refers to the grain type, Austenite.

Very simply described, these were non-magnetic (would not attract a magnet) grades of stainless, that were not hardenable by heat treatment.

They were very good for general corrosion and had some strength. These grades contained significant amounts of Chromium and Nickel. They were often referred to as Type 18-8. (Where 18 was roughly the percent of chromium contained, and 8 was nickel content). More on that later. We will also look at alternate ways to harden austenitic stainless steels.

There were needs for harder and stronger stainless materials, and usage supported the manufacturing. Modifications were made. Necessity may be the mother of invention, but don’t forget that “Pay to play” governs availability in the marketplace.

Stainless grade 410 was developed based on the need for a less expensive stainless grade that could be thermally hardened. As mentioned above, most commercially available stainless at the time was lower hardness lower strength. Differences in chemical make-up of Type 410, from the Austenitic types (410 had a different grain structure called Martensite) among other things was the lower content of Nickel and chrome. The new grade was affordable, hardenable by heat treatment, and immediately gained acceptance in many industries. Because of this, it was generally available in many shapes and sizes. Specifically, it was found to be a prime candidate for car and truck mufflers, although the cost was still a consideration.

Similarly, a common 4000 series alloy (not a stainless) was being used for the brackets that held the mufflers to the car. That grade was not holding up to the temperatures developed at the muffler and tail pipe.

The brackets would eventually weaken an fail, causing the related parts to be deposited on the street. That grade was modified by adding a small content of Vanadium, which raised its ability to resist the higher temperatures, and the grade 6150 was born.

It was discovered that Type 410 could be modified by reducing the content of some of the more expensive chemical elements, thus reducing the cost while still maintaining an effective material for the task at hand. The resultant grade was 409, or “muffler stock”.

Years later, another major modification to the grade was prompted by the needs of an ailing coal industry. Certain types of coal caused applications already subject of abrasion, to now be subject to higher degrees of corrosion. The combination caused hardened steel wear plates to wear out faster than ever. Standard grades of stainless that would resist the corrosion, would fail quickly due to corrosion. Plus they were too expensive to be practical. So, an industry that had even less disposable income to play with than the automotive industry required a material that could address this difficult climate that was also affordable. Once again, 409 was modified, and an affordable corrosion /abrasion resistant wear plate was developed and used to process high sulfur coal.

Like the story of the little girl; “When she was good, she was very very good. However, when she was bad she was horrid.”

There are always trade-offs when responding to the esoteric needs of specific industries. While this new corrosion/abrasion resistant wear plate served the needs of the coal industry, it was neither a stellar wear plate nor a stellar corrosive resistant material for many other maintenance applications.

But back to “muffler stock”. When I was young, shortly following the end of WWII, I lived on one of the small cookie cutter city blocks, in a blue collar neighborhood in Cleveland, Ohio. Notable in these post WWII neighborhoods were three things that littered the red brick streets, kids, rusted car mufflers and tail pipes. Kids were everywhere mostly playing baseball in the street. Those ubiquitous children were the “Baby Boomers”, prodigy of “The Greatest Celebration.”

Rusted Mufflers and tailpipes, on the other hand, were the result of them being made from steel that simply rusted. Soon, the milk truck, the ice truck, the coal delivery truck, and even your father’s car sounded like freight trains; God’s way of warning the kids playing in the street that something was coming.

Mufflers eventually commanded so much of the usage of 409 stainless, it was easy to find in sheet form but almost impossible to find as bar; unlike 410. Usage determines availability. That is even more true today than it was when Baby Boomers were knocking out windows and cursing “nickel smashers” (cheap baseballs had the consistency of a ripe peach).

Types 410 or 409 stainless represented only a small portion of stainless usage. Types 304 and 316 were the defacto standard for decades for commercial stainless usage. They were great for general corrosion, they were readily available, and somewhat affordable. Even those grades, however, have seen modifications over the years, as costs of raw materials fluctuate and the needs of manufacturing change.

In the coming posts we hope to present other materials topics related to the maintenance world of industry.

We may look at; why stainless walks and moves around

                             Is it possible for stainless to attract a magnet

                             Why are some grades of stainless gummy

                             What steels have memory

Along with a host of other topics that command our attention.

-Howard Thomas, November 7th 2017

Stainless steel is defined as any steel containing a minimum of 10-3/4% chromium, or 11%, depending on what your reference is.

Stainless steel generally resists corrosive damage better than carbon steel and medium-alloyed carbon steel. Corrosion might be atmospheric (airborne particles), spray, gaseous, or liquid. How bad the corrosion is will be subject to an infinite number of variables, such as:

• Variations in temperature (service temperature)
• movement
• solution strength
• stress
• torque
• air bubbles (aeriation)
• vibration,

There is not just one type of corrosion. That would be like saying that Mankind defines just one type of human being.

In the steel community stainless steel is defined by grades that may fall within different groups, just as tool steels are  broken into logical groups. With stainless steel, grain type has the most influence on the grades. If I try to talk from a molecular level here, it will not be good. I will stay on more familiar ground and use baking again. When you think of grain composition of steel, think of the differences between bagels and angel food cake; different consistency, different texture, very different personalities. You have probably tasted a bagel. But, if you haven’t had angel food cake you need to have a piece so you get the point on grain structure. Do it for research sake.

In this post, however, stainless groups will be defined by the business-type they most likely serve. In the final analysis, global usage determines price and most importantly, availability. Your definition of “availability” of the steel will be dependent on whether or not you can not only get the grade, but get it in the right shape, size, length, condition, and quantity that you need.

Stainless for general use and production -generally those grades have been engineered to be economical, serviceable, and subject to ease of manufacturing. They are also WIDELY AVAILABLE, so long as somebody continues to use them in sufficient quantity. Generally, they will be available in the form most often utilized by the biggest end user. An example would be types 303, 304L and 316L. For decades these have been accepted as “standard grades” because they continue to enjoy a robust usage. Other steel grades (not just stainless) have pretty much gone the way of the Dodo bird; to a large extent 4330, 4135, 6150, and others come to mind. They may be available to certain groups for specific needs, but, in general they are no longer considered to be standard grades.

Economical grades that have been slightly modified for specific service in specific applications. – Such as “muffler stock” (which we really are going to discuss here in a bit), stainless material for appliances, etc.

Very refined stainless materials, expensive and somewhat less available. These grades have been engineered to solve particular problems in critical service conditions. I have always referred to these grades as “Exotics”. In context of any posts written here, they will include the “high nickel” grades. You can purchase them, but they are generally cost prohibitive for resolution of all but the most serious of problems in a plant. They will be available in somewhat of a limited availability of shape, and size, etc. To repeat: these high end grades are most appropriate for the industries that can afford them. They are generally cost prohibitive for all but military, aerospace, medicine, and grant projects.

-Howard Thomas, September 5th 2017