Who’s Wallet Comes Out

ALLOWANCE TO FINISH

Between the end-user, machine shop, and/or service center, when discussing round steel shafts, there are issues with “allowance to finish”, and even with the method of grinding that will be utilized to produce the finished polished shafts. If subsequent bar finishing or grinding will be done, always let your vendor know what method of grinding will be utilized; are you centerless grinding or grinding on centers. Remember this: “When the bar doesn’t clean up, who’s wallet comes out?”

Each method will have a unique set of requirements; we will discuss those in a future note. In a perfect world there would be one semi-finished condition for all rounds. Call it Hot Rolled, Drawn, Peeled, Rough or Fine Turned. All sizes would have a standard “stock allowance for clean-up”, no matter the mill of origin, or size of bar. All lengths would also have the same perfect straightness.

Regrettably, that is just not the case. At any given time, a service center may have stock from a half dozen various bar mills. Each one has their own description of what constitutes a “Hot Rolled” finish allowance. Some mills will only give a “peeled” or rough turned finish. Another may have hot rolled, or even forged to size with allowance, not machined.

If you are selling steel, how do you come up with a textbook answer that explains which size will make the finished size? When your customer asks what size they should order to make a given part; assume they are asking: “What is the price of a car?” As a seller, can you control the machining or grinding process? Can you insure the capabilities of the operator, or even potential “movement” of the steel? Obviously, you cannot. To even attempt to help the customer, you need much more information. Qualify, qualify, qualify.

Whether you are buying or selling, make sure both parties understand each other’s needs and abilities… When the bar does not “clean-up”, who’s wallet comes out?

-Howard Thomas, August 6th 2018

 

Does Heat Affect Steel?

This is directed to: steel novices, steel challenged, and people who might otherwise cause harm to themselves, those around them, or pieces of steel.

So, will steel be affected by temperature? That depends. What Temperature? That depends.

Let’s define temperature as “Service Temperature”. That is, the temperature the steel will encounter where it is being used. It is worth mentioning that service temperature may be “Intermittent”, or “Constant”. If the steel is exposed to intermittent temperatures, it is not exposed long enough to thoroughly take upon itself the service temperature. (It just passes in and out of a furnace but not long enough to get as hot as the furnace.) If the steel is exposed to constant temperature, it takes-on the service temperature.

When the steel mill hardens steel to obtain specific properties, it involves heating the steel and cooling it to a very specific formula. If you are now going to expose it to temperatures that approach those used in the original recipe, you increase the chances of changing the original properties (hardness, brittleness, ductility, etc.). That is reason for caution if you are intending to do anything other than drop it or throw it.

The temperature to which the steel was originally heated were specific to the elements that were in the steel. The temperature it was cooled to, as well as the rate of cooling and even the time required to move the steel from one process to another affected the properties obtained.

Heat will affect steel based on the composition of that steel and relative to the past thermal processing that steel has undergone. 

Give or take a country mile; steels will melt around 3000°F. Whereas aluminum will melt around 1200°F. Short of those temperatures, you should not have to worry about your steel leaking off the shelf. Steels will begin to soften, however, at a wide range of temperatures based on their chemical composition and the thermal processing that got them to the current hardness.

Temperatures need not be extremely high to begin to lower the properties of the steel. Some of the very hard wear plates found in industrial applications (near diamond hard) will begin to soften at 280° to 350°F. You can cook a pork butt at 280°F.

In very general terms, if you have a very hard piece of steel that will be exposed to elevated temperatures, there is a good chance it may soften. Conversely, if you have a soft steel and expose it to elevated temperatures, you may cause hardening.

In all cases, with known grades or unknown grades of steel; when heat is involved and the steel you are using may be hardened or may be hardenable, exercise caution. (safety glasses, hard hat, gloves, etc.)

-Howard Thomas, July 5th 2018

 

Straightness Is Perishable

Bananas turn brown, avocados turn mushy, cars rust. Those are things we recognize as having a shelf-life. They are not permanent. They are perishable.

When discussing steel shafting, especially in the field of maintenance, straightness is an important property. If a shaft is received at the end user’s plant bent, It is not usable. You can’t grind it. You can’t machine it. You can’t install it. In fact, unless you are cutting it into little stubs for pins, or whatever, it is pretty much useless.

So, although we can all agree that straightness is important. We must understand that even if the bar has been straightened, it will not necessarily remain straightened. Straightening, and the subsequent handling, of a steel shaft is a commitment. Think of high school kids being required to carry a raw egg around for several months without breaking it. The exercise is intended to teach responsibility. It is designed to instill a sense of appreciation of the delicate nature of that item in your care.

We should think in terms of that when discussing anything about bar straightness.

Even if you require, or purchase “Pump Shaft” straightness, or, “Pump Shaft Quality (PSQ), responsibility does not end there. From the moment that product was created it began deteriorating. The severity of the deterioration will be relative to many influences. But, probably the most influential of all will be the diameter relative to the length.

A PSQ bar of 4140 Heat Treated alloy that is 3-1/2″ Dia. x 4 ft. long will be much more likely to maintain its straightened condition than will a 1-1/2″ Dia. shaft that is 16 ft. long. Then there is movement around the plant, packaging, shipping, unloading, machining, fabrication, installation, etc. It’s like those little turtles heading for the ocean once they’ve hatched. It’s a wonder any of them actually make it to adulthood.

The point is, if you are judicious, you should be able to solve most shaft problems where straightness is the rub. But know that it is not a slam dunk, just because the invoice says “PSQ”.

-Howard Thomas, May 17th 2018

 

What is the Length of a “Random Steel Bar”?

While there may be typical answers to that question, it is still a little like asking “What is the price of a car?” It depends on a lot of variables.

The most universally accepted random bar length would be 12ft random. A close runner-up would be 20ft random. The problem that comes into play is relative to the fact that there is no literal interpretation for random bar lengths.

Further, in the steel industry, twelve foot random may imply 10ft to 12ft random; which in reality could actually be 10ft to 13ft, or even 14ft random. If the shaft you are making has a finished length of 12ft, you would not want to order a 12ft random bar without specific clarification. Communication with your vendor goes a long way. Discuss your actual needs (“Finished Length”), with the supplier.

Perhaps, if you consider the cut-to-length price as the standard, or normal, price, then random lengths would be those lengths that are advantageous for the vendor to sell. One vendor may decide to sell 3ft, 4ft, or 6ft random bars. That allows them to utilize their end cuts. By selling “random bar lengths” they can make best utilization of their stock and pass savings incentives along to their customer.

If the customer is actually cutting the bar into short pieces, it is in their best interest to share that information with the vendor. Many times we will end up shipping 26ft bars across the country for years before we finally find out that those bars are being cut into 3″ pieces. Somehow, the total footage required to yield the number of small cut pieces was taken to be the minimum bar length. Shipping shorter pieces represented many advantages to both the end-user, and the supplier, that were unfortunately never capitalized on. Most sellers will cut a long bar in half as a courtesy to facilitate shipping; sometimes they will cut it into three equal pieces, also at no additional charge.

This minimizes potential damage in transit and often results in much lower shipping charges; not to mention potential incentive savings from purchasing end-cuts.

-Howard Thomas, April 2nd 2018

It’s Not So Obvious

“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

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

 

Review of Earlier Posts (Pop Quiz)

At this point perhaps we should have a little review of the earlier posts, just to see how things are going.

 

1. 304 and 316 can be thermally hardened to

a. They can’t be thermally hardened to

b. The consistency of a ripe peach

c. A hardness that would hurt if you were struck on the head

 

2. Muffler Stock is material used to make

a. An article of women’s winter apparel

b. Similar to Woodstock

c. An economical type of stainless steel

 

3. Grades of stainless steel sometimes fall within groups based on

a. Grain Type

b. Length

c. Weight

 

4. Steel that contains more than 10-3/4% chromium is considered to be

a. Very shiny

b. Stainless Steel

c. Generally resistant to corrosion

d. both b and c

 

5. The great thing about “exotic grades of steel is that

a. They are affordable for everyone

b. They are engineered to solve difficult maintenance issues

c. They look great on the beach

 

Answers on the next post.

-Howard Thomas, December 6th 2017

 

 

Muffler Stock (Finally!)

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

 

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