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Stainless Steel – The Very Basics

THE 300 SERIES OF STAINLESS STEEL

If a steel has more than 10 ¾% Chromium content, it is classified as a type of Stainless Steel. There are many types (grades) of Stainless Steel, just as there are many types of corrosion. One grade of stainless steel may work well with one type of corrosion and then be terrible for service in another type of corrosion. The most common stainless grade is 304. Modify it a bit (improve) and you get 316. Enhance that a little, and you get 317. If you add Sulphur to type 304 stainless, you get 303 stainless, which is a “Free-machining grade”. Note: Generally, as you add elements to stainless steel to improve machinability, or strength, you may lessen strength and service-life.

Three hundred series grades are often referred to as; 18-8 stainless; where the 18 stands for percent of chromium content and 8 refers to the percent Nickel content. Those grades have some commonalities; grain structure of Austenite, non-magnetic, not generally responsive to thermal hardening, ability to work-harden.

Steel that has an Austenitic grain structure is not hardenable by heat treatment, but it is hardenable by cold working. Austenitic stainless grades are non-magnetic; i.e. they won’t, under general conditions, attract a magnet.
Strain-hardening, Cold-drawing, Cold-Finished, Cold-working, and Work-hardening, are all pretty much synonyms. Bang it around, Squash it, or keep bending it back and forth and it’s going to get harder; sometimes near 50RC hard.

18-8 Types of stainless steel are popular (common) for good reason. They are excellent for many applications exposed to corrosion, and they are readily available from suppliers. There are of course limitations imposed by various specific types of corrosion as well as limitations imposed by physical characteristics of the actual parts required (strength, straightness, etc). We will explore those in later posts.

 

-Howard Thomas, October 17th, 2022

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Availability

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.

-Howard Thomas, October 4th 2017

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Stainless Steel

Introduction to a Series

Over the 45 years of working with materials used for heavy industrial maintenance, I’ve seen that matching stainless grades of steel with the appropriate application can be awfully confusing to the many support folks who do not have a background in the sciences. I speculate that a lot of hard working “non-metallurgical” people would be happier and more effective if they understood a bit more of what governs the decisions and requests coming from maintenance and reliability personnel. I am one of those “job taught” individuals and it is my hope that the posts that follow will provide a very general overview, i.e. the help that I would have relished many years ago. Becoming familiar with the various “personalities” of the most common available grades of stainless will hopefully encourage a familiarity within the potential supply chain and its value or limitations. Even a casual understanding of the distinct stainless personalities relative to the particular needs of the applications would help the two better get along. Think E-Harmony for stainless materials and applications.

In the coming weeks and months we will pass along some hard learned tips and stories that might help to broaden your understanding of the stainless picture. We will answer questions like; “What is Muffler Stock”, Can non-magnetic stainless attract a magnet, are there magnetic grades of stainless, Can you harden stainless, Can you buy pre-hardened stainless, why does stainless tend to “move around when you’re trying to keep it straight”, along with a host of other nagging questions.

We hope to stimulate questions that will generate future posts. If you consider yourself “non-technical” i.e. just another soul immersed in an industry that is drawn to an increasing reliance on the material, this should be right up your alley!

My approach to these articles will be a little like the differences between cooking and baking:  When cooking, you can make a darn good chili without a recipe. You can measure ingredients by handfuls and cook it until it “looks right”. Baking, on the other hand, requires keen attention to the recipe with ingredients, time and temperature. Detailed information that would be appropriate for engineering decisions is not what I’m hoping to present. So, if you are a scientist, metallurgist, or engineer who is reading this post expecting a soufflé, you will be sorely disappointed to find that it might only be even a moderately good chili.

All comments and criticisms are encouraged and welcomed however, understand that this Soufflé is going to be simmered with kidney beans and hot peppers! Hopefully, these posts will act as catalysts for readers to seek more detailed information from structured resources better equipped to deal with the on-site conundrums they may encounter.

As politicians are so fond of saying; “Let me be perfectly clear”;

What follows is not intended for the credentialed metallurgist, the engineer, or chemist. The information is not to be used for engineering purposes. This is more like an energy drink for the unsung work-a-day-support stiffs charged with keeping the supply chain flowing. They need to be incorporated into the wonders of the maintenance materials world that they have an undisputed effect on. From their caves under the catwalks, in the bowels of plants, to the mazes of cubbies in the cooler parts of the plant, they long to be a part of what it is they get blamed for anyway.

-Howard Thomas, July 31st 2017

 

 

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