Endurance Limit – Fatigue Failure

Endurance limit is another way of saying fatigue strength. It may be expressed in “Cycles to failure” as opposed to “PSI”. One of the most difficult questions to answer is a question relating to endurance limit. When someone asks about endurance limit, they are trying to find out how long a finished part will last in an application involving constant/repetitive motion or vibration. Failure may be anything from a small crack to an abrupt and catastrophic event.

While it is easy to see that this is a matter of great concern, there is unfortunately no formula for arriving at an answer based on a raw piece of steel. To accurately determine the response of a particular part in a particular application, the endurance test must be performed on the finished part in a simulation that duplicates as closely as possible the motion of the actual application.

The R.R. Moor Endurance Test, is an example of a test that utilizes bending and rolling contact to test torsional fatigue. [Variable introduced may be; vibration, compression, bending, twisting, rolling, etc.]. This test is extremely expensive and the evaluation period is lengthy. Individual companies, steel mills, and independent test labs, are unable to predict failure based solely on the chemical and physical properties of a type of steel. There are general guidelines published relative to standard SAE steel grades, but those are for general reference only. Steels that have been refined or otherwise modified to enhance toughness or to resist fatigue related failure (Steel produced to Clean Steel Production Standards) would not be adequately represented on a generic chart when it comes to endurance limit.

To repeat; in order to obtain any meaningful data, relative to endurance limits, the finished part must be tested under conditions that approximate actual service conditions. This is frequently done when production run parts are involved, because the quantity offsets the cost of testing. It is generally considered cost prohibitive to test steel for maintenance replacement parts for endurance limit.

Remember.

  1. Generally there is no accurate published data to indicate a universal endurance limit for shaft material.
  2. Reference data published on steel by grade is at best general. It is not an accurate reflection of the expected service life of our material.
  3. Endurance limit relates to “Toughness.” Maintenance steel grades that have been manufactured to Clean Steel Production Refinement have enhanced toughness over their generic SAE or AISI counterparts strongly address concerns about endurance limit.

Steel that fails in service through fatigue related circumstances would have lasted longer if it was “tougher”. Toughness is achieved through an orchestrated combination of core integrity refinement during the production of the steel, combined with a specifically targeted thermal treatment and stress relief.

MAKE IT CLEANER, MAKE IT HARDER, REMOVE STRESS – INCREASE SERVICE LIFE, INCREASE ENDURANCE LIMIT.

Howard Thomas, September 8th 2020

Measurements and Tolerances

Measurement science is its own language. These few notes barely scratch the surface. If you are new to this, or will just be peripherally involved, perhaps as a sales support person, it is suggested that you learn decimal equivalents down to the sixteenths, and key metric sizes, such as 5mm, 10mm, 20mm, 50mm, etc. It will make your life a lot easier.

15/16″ is fifteen sixteenths. Not fifteen sixteens.

.010″ is ten thousandths, .003″ is three thousandths, .0005″ is; half a tenth, or five ten thousandths.

In all methods of measurement tolerances nothing is an absolute single number. Tolerances are represented in a range. It is important to both parties involved in a transaction understand what that range involves. If you are not sure, ask. It may vary from company to company, product to prodcut, or even from material to material. In the case of machine tolerances, the spread may be just a few thousandths. Bar length ranges may involve many feet.

The above is not intended to be all-inclusive but a random exposure to the topic. We will try to broaden the scope in future posts.

August 5th, 2020 – Howard Thomas

Exactly the Same, Only Different

When steel flat products are thin, they are typically referred to as sheet and strip; as opposed to flats and plate. There are specific delineations separating the two types, however in general use distinction of the terms is pretty loose.

It is customary to describe flats and plate, or strip and sheet, from the smallest dimension through to the largest dimension. Thuse, 1/4″ x 48″ x 240″ would be how a quarter inch thick piece of 4ft wide 20ft long plate would be listed. It is fairly typical for the “grain” direction of the steel to run parallel to the length of the item. There is room for caution when grain is an important consideration; such as when forming is involved, since the plate may have subsequently been cut to a smaller size. In those instances, you can not be sure about grain direction unless it has been marketed. It is good practice to mark grain direction on remnants, or “drops”, once a plate has been cut. Grain is important for several reasons. First, when forming a steel plate it is generally advisable to form against (perpendicular to) the grain. In the case of wear plate, abrasive wear due to flow, may be diminished when the plate is installed so that the flow pattern is perpendicular to the grain. Unfortunately, many times this is theoretical rather than practical due to size and shape required.

Hollow materials may be pipes and tubes, or something like structural rectangular tubing. Tubes are generally more exacting in size, shape, and quality. Pipes are “big and ugly” hollow sections. Pipes are categorized as NPS, or National Pipe Size. Sizes up to 14″ NPS are described by their I.D. Or inside Diameter. At 14″ O.D. (outside diameter), the NPS refers to the O.D. Important information required would involve; O.D. I.D., and wall thickness. Generally, you would use only two of those measurements; not three. Caution must be exercised when “telescoping” one tube inside of another. Where a closer fit is desired, the two parts must be made, or fabricated in such a way to ensure that they are completely compatible; insured compatibility.

When dealing with pipe and tube you will doubtless encounter peripherals, such as elbows, flanges, laterals, T’s, etc. Configurations and cautions here are too numerous to list. If you will be involved with tubing or pipe it is recommended that you take the time to familiarize yourself with the materials and parts you will be encountering. Complex configurations require as much artistry and experience as technical knowledge.

Solid Steel Bars (Long bar products) are referred to as “bars” or “lengths”, or each (Ea). Hydraulic Pistons or shafts are referred to as “rods”.

Typically, there is a difference between the physical properties of a grade of steel and the mechanical properties. Generally, the physical properties are those properties that will be fairly uniform (common) to the grade. Those might include; thermal conductivity, thermal expansion, % elongation (%EL), %reduction in area (%RA), etc.

When discussing properties of steel, such as flatness, straightness, or out-of-round, it is best to avoid superlatives such as “perfectly flat”, or, “perfectly straight”. Presenting your requirements in that manner may result in a “No-Quote” from a potential supplier.

While there are many methods of determining hardness, it is fairly common in carbon and alloy steel to encounter Rockwell “C” hardness testing, or, Brinell (Bhn) testing. In sheet and coil you may encounter the Olson test, which involves an even larger ball impression than the Brinell ball. The Rockwell test is more appropriate for high surface finishes or finished parts, whereas the Brinell test method is used on “Big and Ugly” materials, such as hot rolled wear plate or bar.

Remember that surface decarb must be removed completely to obtain an accurate reading.

 

-Howard Thomas, July 8th 2020

 

General Steel Terms – “Talk the Talk”

You may have noticed every activity, workplace, social group, college, etc. seems to have its own language. One sure way to expose yourself as “new to the program” is to not use the approved language customary to the endeavor you are undertaking. In those cases, the dialog/language might be said to be “esoteric” (known by a certain group) to the group or business.

I remembered vividly the first time I had to call a steel source in New York. I had a hard time getting the person to understand what steel I was interested in. Finally, after a litany of clumsy attempts, he abruptly said; “Look kid, when you know what it is you want, call me back . . . dial-tone.” You don’t want to be that guy. Most institutional dialog will come with experience. This may be a small bit of help.

Steel products grouped by general type (service centers may carry multiple groups)

  • Carbon and flat roll – Perhaps the biggest grouping of suppliers. Most general category covering commercial grades of mild steel, carbon steel, and often structural steel. Does not generally include alloy steel, stainless steel, and premium exotic grades of steel.
  • Structural Steel – Angles, Channels, Rectangular Tube, perhaps flat bar, ductile plate products. Generally used in the building, construction, and manufacturing trades.
  • Wire & Cable – Small diameter rounds, shapes and wound products, in coil form. Cold header stock used to make small production quantity parts, bolts or wire based products.
  • Pipe & Tube – Long hollow products
  • Long Bar Products – Round bars, flat bars, some shapes (Hex, Square, etc.). Solid Steel bars.
  • Non ferrous – red metals – copper, brass, bronze. Note: “Yellow Metals” may refer to a specific type of  brass, or, it may refer to an industry rather than metal type. That group involves metals used in support of heavy construction equipment; typically to that produced by Caterpillar and John Deere.

Howard Thomas, June 9th 2020

What is Keystock?

In the steel industry and in the most general terms, anytime you see the word “stock” attached to another word; think, “stock used to make”. Barstock, Bushing Stock, Rifle Barrel Stock, Pump Shaft Stock, etc. would be stock that you can use to make those items. All details need to be clarified; is it pre-drilled, pre-hardened? What is the finish allowance? The term “stock” just means “may be used for”. Facetiously, a tree might be “Home Construction Stock”. i.e. not the finished product.

Keystock is generally a square or flat shaped lower to mid strength metal, either alloy or carbon steel, that is used to make keys. Those keys are inserted into shafts. A drive motor chuck will engage with the protruding key, and rotate the shaft. The idea is that this key is of less strength than the expensive shaft, so that if something interrupts the motion and causes failure, it will be the cheap little key that fails not the bin expensive shaft. Clear the obstruction, put a new key in, and you’re good to go, hopefully with an undamaged shaft.

The perfect piece of keystock would have less mechanical strength than the shaft, tuned to the actual application, but close enough that the shaft didn’t fail prematurely, contributing to expensive downtime. It would also be a cold drawn bar with good cross-sectional accuracy and a close oversize tolerance. The ideal oversize tolerance for many years was considered to be +.002″/-.000″. That tolerance would allow you to machine a standard keyway, in which the key would fit it in nice and snug. Undersized tolerance resulted in sloppy, loose keys. Too much oversize required un-welcomed machining.

My first experience with keystock was when I was about 10 years old. My uncle took me fishing in Canada. We were in a small aluminum boat with a little outboard motor. We were trolling at a very slow speed and he let me captain the “ship” for awhile. His only caution; stay away from the shallows and weeds; the only things I seemed to have a talent for. Every few minutes we snapped a key when the prop would strike something. We went through an entire little metal pill box of keys. That experience, and his humorous disapproval, stayed with me. The solutions were; chuck me out of the boat, or get somewhat stronger keys.

There is the conundrum. Too soft of a key and you have frequent expensive delays. Too hard of a key and you risk damaging an expensive shaft. (Goldilocks and the porridge issue).

Decades ago, Moltrup Steel, Pennsylvania, was considered the benchmark for keystock tolerance and cross-sectional accuracy. Having failed to secure the quality sufficient to satisfy their own needs, they embarked on producing the finest grade of keystock available. “Moltrup Quality” actually became a legitimate descriptor. I still see reference to Moltrup Keystock today, but am unfamiliar with the exact specifications, or how closely it follows the original “Moltrup Quality” of the past. Moltrup closed in 2002 and it has been my experience that the exacting tolerances disappeared with it. (Associated Steel always maintained a substantial inventory of Moltrup Quality material. It is a diminishing commodity but worth checking). When Moltrup closed, the potential market did not seem sufficient to justify the additional die work and drawing that would insure that high degree of accuracy. The tolerance on keystock gradually opened up to +/-.004″ to .007″ (that’s plus or minus). Even with sources that promote “Keystock”, or Moltrup Quality, pay close attention to the actual tolerance that you will most likely get.

The hardness (through multi-pass cold drawing/strain hardening), tolerance, and cross-sectional accuracy are all doable. Expensive, but doable. Requires a lot of fiddling in production for an item that does not represent any significant tonnage. Until something changes, keystock that is fine-tuned to allow maximum performance of the esoteric shaft application you are dealing with, will have to be made. Your choice, either make it, or chuck the kid out of the boat.

 

-Howard Thomas, April 3rd 2020

 

Metrics. Why So Difficult?

We have been actively pursuing adopting the metric system in the US roughly since the latter part of the 1700’s. Its been dubbed “Mandatory” in 1809. The success of those efforts has only been eclipsed by Y2K and the great reveal of the contents of Al Capone’s safe. In fact, for a quick chuckle, if you can still find a veteran businessman, mention you heard we are going to be exclusively adopting the Metric System in the USA next year.

At best we can say today that we have had success adopting the metric system in some of, but not all areas of measurement. Wikipedia does a nice job delineating areas where metric measurement has successfully been adopted in the US, (science, military, medicine). The only example of metric measurement that came to mind, however, was the 2 Liter bottles of soft drinks are now illegal in New York. At best we can say Americans are not keen on jumping quickly into metric measurement and saying goodbye to “foot-long” hot dogs and jokes about the inability of men to measure correctly.

A word of caution; while we are “actively” transitioning between metrics (written in decimal notation)

and “fractions”. Make sure both parties understand the exact point of accuracy that is being discussed. If you purchase a bar of 3-1/4″ Diameter, is your supplier visualizing the same 3-1/4″ Diameter bar (in fractional context?). Or are you really expecting a 3-1/4″ Diameter bar that is 3.250″ Diameter, accurate to the third decimal? What about 3-15/16″ Dia. (3.9375″ Dia. Are you expecting accuracy to the fourth decimal?). True, one topic is diameter size, while the subsequent text refers to accuracy or diameter tolerance. Normally, that would not be an issue. The two would be understood to be separate considerations. However, when you are ping-ponging two different measurement systems as interchangeable, there is a potential for surprise. Mishaps of this nature occur more often than should be the case.

For now, in the United States, think of any changeover of our measurement system sort of like METRXIT. It will likely still take a while. Over time, you will likely see dual notation on things like automobile speedometers, etc. But when it comes to steel bar sizes, fractional annotation may be here for a while. The “fractional” system, sometimes referred to as: The English System, or Imperial Units, has served us well. Change is inevitable. We know that. We’re workin’ on it.

-Howard Thomas, March 5th 2020

Light Gage

Every once in a while, I have to engage in something that smacks of shameless sales dialog. This is one of those times. So, grit your teeth, send small children out of the room and just get through it.

If you are involved with heavy plate fabrication, and more specifically, if you occasionally handle hardened wear plate, you may want to give some thought to a product we carry that seems to be useful to fabrication and equipment repair facilities; Light Gage (1/8″) real 400bhn Q&T alloy wear plate. Forms well, readily weldable (not too rich of a chemistry to cause problems), and it provides weight reduction where installation and handling might be a problem; not to mention you can make wear resistant containers, hoppers, tanks, and that are lighter and therefore increase payload.

Perfect for emergency temporary patching of blow-outs on job-sites until heavier fabricated pieces can be delivered. An installer can get the pieces into hard to reach places where a repairman could handle the plate by hand. This is a nice alternative to 3/8″ and 1/2″ thick A36 liners. That’s a big reduction in weight! You may even find you benefit on the competitiveness of your bids. Depends on what the other guys are quoting.

We stock it in full sheets, 1/8″ x 60″ x 120″ long. We also cut pieces to order; simple rectangles and strips, but also configurations per sketch. Our sheet is cut on a Hi-Def Plasma, and our machine operator has been working with plate for 30  years. The cuts we get look almost like laser-quality with very little heat effected zone along the cut; even with alloy products.

NO SUCH THING AS WEAR RESISTANT STRUCTURALS? (Other than low-hardness A588). Think about tac-welding some clean-plasma-cut flat strips into the wear surfaces of channel and angle or beam.

CAUTIONARY NOTE; Working with hardened steel, anyone’s hardened steel, involves risks. Be sure to use appropriate safety gear (hot-mill gloves, hard had, safety glasses, etc.), Utilize persons experienced in handling hardened steels, to include certified welders, etc. Try to form against the grain, incorporate the largest bend radius the application will handle. And, COMMUNICATE, COMMUNICATE, COMMUNICATE. If there is something that you are not absolutely sure of; ask your vendor!

-Howard Thomas, February 5th, 2020

Inventory Turns – Somebody Pays

Recently we touched upon blanket orders. Today, we are going to look at them specifically as to how they might relate to inventory management.

So, my favorite cautionary snippet warns about “Not shooting pool with anyone who’s first name is the name of a major city.” To an extent, there is wisdom in that. Unless of course you’re THAT GOOD! And, most of us are not.

When it comes to the idea of keeping steel in inventory, there is another snippet that suggests; “Better that you be stuck in an elevator with 50 first graders with fevers, ice cream cones, and intestinal flu, than hold steel bars as inventory.” Doesn’t seem to leave much room for any discussion. Just don’t do it, at any time, for any reason. That used to refer to slow-moving inventory. Today, however, it seams to refer to any inventory, no matter who you are; service center, end-user, manufacturer.

In a nutshell, no one is to maintain any inventory, but everyone is expected to ship their products next day. In reality, that philosophy results in what I refer to as “kick the can”. Someone has the inventory somewhere; we just want it to be someone else’s expense. Inventory is a necessary evil then, so long as someone else is carrying it. Or, as an old and dear friend of mine would always say; “Somebody pays!” Just move the actual mass of hard steel down the line until it is not so visible. BUT, how many decision makers do you want to be between your “Just-in-Time” company and availability of the actual goods? There is no avoidance; to deliver products with the speed expected today, most of us should actually maintain some inventory (that includes end users, manufacturers, and even service operations). We just need to insure we are capable of managing that inventory with a competence that approaches an art-form level.

Heavy industry is alive and well. Globally, there is a big need for digging, busting, breaking, grinding and pushing; dirt, rocks, yogurt, whatever. Service centers are tasked with anticipating the needs of that industry months or years ahead of potential orders. No easy task. End users, however, may witness repetitive usage of materials within somewhat predictable cycles. They enjoy a level of “forecast insight” that steel service centers would love to have, no matter how slight it might be. Yet fewer than you would imagine utilize the benefits of a useful and available inventory management tool; “Blanket Orders”.

Committing “forward”, to a dedicated source of supply, for a dedicated quantity of material, may be facilitated by using a blanket order. Not a bad compromise that protects continuance of supply and improves cash flow.

-Howard Thomas, January 21st, 2020

Partnering

The “partnering” discussed here, in the steel industry, will refer to a working relationship between supplier and end-user, where benefit is derived by both parties.

One specific example that has been very effective is the blanket order, or blanket contract. Both parties need the potential advantages of a well-constructed blanket contract.

The vendor gets a bit of predictable inventory usage and potential predictable cash flow. The customer will benefit from: continuance of supply, firm pricing, cash flow (often a major concern in smaller mom and pop service centers), time and space savings, etc. When one of these “Partnering Contracts” is done well (fairly), there may even be cost savings to the customer, and increased profit to the supplier. Other benefits do not necessarily relate solely to purchase cost.

At risk for the vendor is default from the customer who’s needs may change during the tenure of the contract. A smaller service center may be concerned with exactly the same thing; a change to their customer’s needs mid-stream.

If you have never been party to a blanket contract for materials, you should be aware that there are certain predictable cautions and time proven verbiage common to most blanket contracts. The text will address standard issues like duration of contract, types of releases (fixed or “as-needed”), etc. Persons with limited familiarity, or who have never entered into a Blanket Order will likely have many concerns relative to over-committing, potential exit clauses, etc.  You should know, that similar to any type of contract there must be benefit for both parties. There is no “one-size fits all” contract.

Working with a familiar and trusted vendor and extensive COMMUNICATION will only improve the quality of any joint agreement. Formulate a rough draft of contract terms that express, and address, your concerns. There are no predetermined constraints or requirements that you will be bound to. Ensure the contract terms fit both parties.

In an earlier post, we discussed the definition of, Random Bar Lengths. Many people in this industry assume there is a fixed length spread. Not so. Bar length variances are basically determined by each supplier based on length variances that offer advantages. In appreciation for selling lengths that are the easiest and most economical to ship, a cost advantage is passed along to the customer. Look at a blanket contract in the same way. In your particular situation, what issues provide advantage? Would a good “escape clause” ease your concerns? What about an extended contract period, or releases on demand?

Blanket Contracts for Materials are an excellent form of “Partnering”. They should be viewed as opportunities, not restraints. When discussing “Partnering” be open to compromising on terms that will assist you in perhaps obtaining some “non-standard” terms that you feel strongly about.

The beginning of a new year is a great time to get dialog started.

-Howard Thomas, Jan 2nd, 2020

 

 

Are There Different Types of 4140 Alloy Steel?

Is all 4140 alloy steel the same?
It is common for 4140 alloy steel to be supplied in either the annealed (soft) condition, or Quench & Tempered (Q & T), the hardened condition. The hardened condition is used where added strength or resistance to wear is desired. The softer condition is used where considerable machining may be required, or where more ductility is required. And then of course there are different surface conditions, shapes, etc. What we will be addressing here will be something a bit less obvious.

Quality varies from mill to mill, and within even one mill, quality will depend on the specification the customer is having it made to. The number one cause of shaft failure in heavy industry is fatigue related failure. “Toughness” resists fatigue, and “cleanliness” increases toughness. Cleanliness in steel is a result of refinement process incorporated in the melting of the steel. (electric furnace melt, vacuum degassing, small grain refinement, etc.). Each process may contribute to the quality of the steel. Each has a cost to the service center requesting it. When all of the available cleanliness processes are incorporated, the collective cost of those extras could be near, or may even exceed the base cost of the steel grade.

I have always considered the steel making process to be more of an art form than merely a manufacturing effort. Very much the same as; there are certified welders, and then there are certified welders. There is often a big difference from one to another. Or, as the age old adage points out; “What do you call a doctor who graduates last in his/her class? The answer is still Doctor. It is the same with steel; who makes it and how is it made?

Often, we encounter a situation where a grade of steel has failed in service. It is common for the customer to consider changing the grade of steel. In many instances it would be more appropriate to retry the recommended steel again, but perhaps investigate purchasing from a higher quality resource.

If you believe on specific grade of steel has identical attributes to that same grade from any source, you are likely to encounter some surprises along the way. Don’t get bit by an insufficient piece of steel because you misunderstood the situation.

“A man walks up to a woman waiting at the bus stop. Next to her is a large friendly looking dog. The man asks if her dog bites. She smiles and says, “No. He is very friendly and loves people.” The man reaches to pet the dog and is immediately bitten. “I thought your dog didn’t bite!”, he exclaims. “That’s not my dog.”

An old experienced business associate of mine would often comment on something I did that did not come out as planned. He would simply say; “Well kid, you almost had it right.”

Don’t be that kid. Pay attention to the details and make your life easier.

-Howard Thomas, Nov 25th, 2019