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.
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.
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