“User Friendly Steel”

User Friendly or Extended Service? Choose

Metals and various heavy industrial maintenance materials are basically dumb. There is no consciousness on their part that knows bad heat from intentional thermal processing. It’s all just heat. There is no discernment of difference between machining, grinding, and in-service abrasive wear. Remember that when you are reviewing specs on a new material that has the incredible ability to resist wear in the application, but it is listed as “Free-Machining”. Think about it a bit. If the steel can’t tell good wear from bad wear, or good heat from bad heat, how can that be? What an intelligent piece of steel! It knows exactly what you want and when you want it.

What about a dead hard piece of steel that has been flash tempered to 500bhn at 300F. We are not talking about a high grade rich chemistry tool steel here. More like a good quality 500bhn wear plate. Good quality from a fine mill. How is that going to work in a service temperature that gets up to 500F? What is going to happen to the hardness when you torch cut it or weld it?

These are the differences between “production steels”, and critical service maintenance replacement steels. If you want it to machine easily, you are not interested in the piece lasting a long time in abrasive service. If the steel requires rapid cooling and a low temperature temper to achieve hardness, know that even a very low temperature will begin to soften the material. You can’t have it both ways.

If you are working in customer service for a steel service center, or, if you are requisitioning materials for replacement parts. Remember that steels are dumb. If you want a high speed rotating or reciprocating shaft to last a long time in a very contaminated environment, with lots of particulate matter in the atmosphere, be willing to exercise some precautions and learn how to machine hardened steels like a pro.

Don’t opt for a “Free-Machining” shaft because that is not going to last long in service. Also, if it is Free-Machining, has sulfur been added to obtain that property? If it has, and you are contemplating any welding, be careful. Welding does not like sulfur.

Welding Dead Hard (500bhn) steel strips into place will require familiarity with some cautions and restrictions if you want them to remain hard in service.

Successful processing and installation of the right product for the application will most likely require some experience and talent. Getting material that is the easiest to machine and weld will probably insure poor service life.

-Howard Thomas, January 3rd 2019

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What is Welding? And, What Makes Welds Crack?

Let’s begin with a short protect your butt statement; Welding may be dangerous. it is always recommended that certified welders be utilized. Welding; alloys, tool steel and stainless may be tricky and caution is to be observed; especially if the metal has been hardened. Appropriate protective gear, and adequate ventilation is required. There are fumes, potentially harmful light rays, intense heat, and big and heavy sharp things that you should be concerned with. This is intended to be informational only, NOT INSTRUCTIONAL! It is furnished to introduce Non-welders to the topic so that they may have a frame of reference if the topic of welding arises. That statement is there to ensure that I don’t get hurt by welding.

When you weld steel, you are heating it to a temperature at which it will melt. You are adding a filler metal (weld rod or wire). The filler and base metal will melt together, and when the assembly begins to cool, the base metal and the filler should solidify and bind together.

The filler metals may be in the form of “rods” that come sealed in a container and are manually fed into the weld areas as they are consumed. Or, they may be in the form of a wire spool which allows the wire to feed automatically. Often, when you see notes that suggest rod be used instead of wire, they may simply be suggesting that to minimize heat input, since wire lends itself to heavier deposits due to the increased speed.

The following contribute to potential failure “cracking” of the weld. Dramatic and uncontrolled temperature changes are not good. Going from cold temperatures to extreme heat, then back to cold is not conducive to sound welds. Whenever possible, bring the temperature on the materials to be welded up. Ideally you will want to know the tempering temperature of the steel you are welding, and make sure you stay under that, but, it is not impossible to preheat if you don’t know the tempering temperature. If the steel is below 30RC hardness, you may generally assume a preheat of around 600F will not be detrimental to the base metal. If the steel is 40RC or higher, you may want to stay below 300F. These preheat statements are only to illustrate that some warming of the unit is better than none. Preheat also serves to evaporate atmospheric moisture present on the metal from the environment. Second to dramatic temperature change, moisture is the most troublesome condition. As the weld freezes (solidifies), hydrogen from the moisture gets trapped in the weld. Hydrogen Entrapment is contributory to cracking of the weld. Weld rods come to you in sealed containers, intended to keep them dry. Once you open the package any assurance of dryness is over. Prior to using any remaining rods from an opened container, you may want to preheat them in an oven to evaporate any residual moisture.

You do not need furnace gages to determine temperatures when you are field welding. Wax crayons or Tempil Sticks are available; they will begin to melt at, or near, indicated temperatures. You may minimize heat input by using small diameter weld rods. Further, you may “skip-and-back-step” as you are laying down the weld. Weld a little, skip forward with no weld, then weld a little more. If your parts don’t line up, it is not good to force alignment and then tac both ends of the weld seam so the pieces can’t move. As you “run” a weld bead, you ideally want the stresses to run ahead of you so they exit out the far end of the weld. If you forcibly tac them, you prohibit movement, which will eventually lead to serious warp or fracture.

Chiller bars may be used to dissipate heat. Placed adjacent to the weld, they act to absorb some of the heat (heat sink) and slow the rate of cooling. You may use several “stringer-beads” instead of laying down a weld bead that looks like a heavy hemp rope.

Moisture, heat, forced alignment that disallows movement, contaminants on the metal, such as grease, oil and rust, are all impediments to successful welds. Chemical elements contained in the steel may inhibit successful welding. Carbon levels, sulfur, phosphorus, and other elements may diminish your chances for a successful weld. In maintenance areas condition are seldom conducive to sound welding. Certified, or very experienced welders will know what cautions to observe. The “Standard Low-Hydrogen Method” is something you may want to familiarize yourself if welding discussions could be part of your occupation. 

In parting, during the nearly half of a century that I have been working in maintenance repair, qualified welders have always been in demand; great demand. Welding is very much an art-form, and a talented welder is a joy to behold. If you are of the age that you are still considering employment options, male or female, you may want to consider welding school. 

-Howard Thomas, December 14th 2018

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What is Fatigue Failure?

Let’s say you are leaving the house and you see your new credit card on the counter. You replace the expired card in your purse or wallet, but there are no scissors to cut up the old card. So, you start bending it back and fort until it breaks in half; then pitch it. You have just induced a fatigue failure.

So too with steel. A piece of steel that undergoes repeated motion (twisting, bowing, vibration, flex) will at some point fail. Conditions may encourage that failure to be earlier than expected. A nick or gouge at the surface, or an inclusion or defect (foreign element) within the steel may be the likely culprit.

In my experience, most industrial steel shaft failures are caused by fatigue. The failure may begin at the surface of the shaft (surface initiated), or, it may begin from inside (internal). Most common, will be surface initiated. Surface nicks are called “STRESS RISERS”. Think “A chip in the windshield of your car.” If not smoothed out, that nick will eventually become a crack that runs outward until the windshield fails. A Stress Riser is a break in the surface continuity of an item. Through repeated external forces, the surface-initiated nick becomes a fracture; progressing internally through the steel until a point of catastrophic failure. The shaft cracks in half, while the machine is still running. Not good!

If there are inclusions within the metal (microscopic tramp elements), they may contribute to an internally initiated fatigue failure.

What can we do to minimize fatigue type failures? First and foremost, make sure you are working with high quality materials. In the case of steel, make sure it has a high degree of cleanliness, free of internal defects. There are methods of steel production that insure your steel has excellent core integrity. Those are generally referred to as Clean-Steel-Production-Methods. Those methods, such as; Melting in an electric furnace, vacuum degassing, inclusion shape control, stirring, limiting tramp elements, etc. are available to people who require steel that has undergone refining processes. Certainly, you would want to employ those processes for materials that would be used in critical service.

Those processes address the internal portion of the steel. What about the surface? You can process the steel so as to minimize any roughness, nicks or gouges on the surface. If it is a shaft, you may want to insure you have a polished surface finish, even if the tolerance requirements of the application do not require a precision finish. Note that a highly polished surface not only resists surface-initiated fatigue failure, it provides a certain degree of corrosion resistance. Caution should be observed so that you do not get such a smooth surface that required lubricants will not adhere to the shaft. Most commercially available polished shafting will have a surface finish of about 15 micro. When you start getting into finishes much brighter than that, you may want to check into lubrication requirements.

If your finished part has contour changes (keyways, step-downs, grooves, etc), make sure the sharp corners have been radiused and if possible, even smooth out the contour.

FOOD FOR THOUGHT; “Most heavy industrial shaft failures are fatigue related. Toughness resists fatigue failures. Clean Steel Production Increases Toughness.”

-Howard Thomas, December 3rd 2018

 

 

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Surface Hardening vs. Through Hardening

In the world of heavy industrial maintenance steel, whether you call it Case Hardening, or, Surface Hardening, or, Skin Hardening, it is all the same thing. This is a localized method of hardening employed to develop a wear resistant surface while maintaining a somewhat ductile (shock resistant) core. With production items, such as gear teeth, this may be very fine tuned, sophisticated, accurately measurable. In maintenance, “one-off” items, it can be somewhat erratic and capable of surprise. If you are contemplating increasing the surface hardness of a piece of steel, please recognize that increasing the hardness, especially localized hardness, is also increasing the brittleness which subsequently increases the chances of facture. Wear appropriate safety gear. Any surprises may not be very forgiving. 

IN GENERAL, the two types of hardening are self-explanatory. A through hardened piece of steel is pretty much the same relative hardness from surface to core. Most common prehardened steels, carbon or alloy, are often shipped at a hardness of approx. 30RC. As the cross sections get larger, the hardness will “drop-off to core”. That is, as you get closer to the center of the mass, the hardness may drop a few points. Those are still considered to be Through Hardened. Surface hardened levels, typically those used in hydraulic applications, and precision automation rail applications, will be supplied with a very thin hardened surface “skin”, at about 60RC, with a great drop off in hardness toward core.

The surface hardened material provides great resistance to sliding abrasive wear while resisting bending and torque. The through hardened alloy or carbon material provides a good balance of toughness (a combination of wear, impact, and gouging resistance). The through hardened material makes no pretense to be particularly ductile. In fact, as through hardness increases, the potential for general fracture also increases.

Caution should be exercised when attempting to surface harden small cross sections. Even though your intent and processing method may be aimed at surface hardening, small cross sections cool rapidly. The rapid cooling may actually result in a through hardened condition with potentially dangerous brittle hardness.

-Howard Thomas, Nov 8th 2018

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Bearing Quality Vs. Bearing Fit

Be careful, the two are often confused between end-user and vendor. They are not interchangeable. BEARING QUALITY refers to manufacturing restrictions that are employed, when the mill makes the steel. It is commonly referred to as “Clean Steel Production”. The processing refines the steel, removing non-metallic inclusions and generally improving the quality and core integrity of the steel. This is not something that can be achieved in subsequent processing; it either is bearing quality, or it isn’t.

BEARING FIT (TOLERANCE), is achieved by subsequent processing of a steel bar or shaft. You may accomplish this at any point prior to use of the bar; do-able on-site locally, or as a specification for subsequent processing accomplished at the mill. This, as stated, refers to the tolerance alone; making no statement in regards to the integrity nor the cleanliness of the material. Bearing Tolerance is referred to as a “minus/minus” tolerance, as opposed to a plus or minus tolerance.

Typically, and depending on diameter, the tolerance would be something like minus .001″ to minus .0015″

A NOTE ON EXPRESSING BAR TOLERANCES, it is common to hear bar tolerances specified as; “Plus nothing minus .002”, or whatever the downside tolerance is. To avoid potential problems, it is better to state both plus and minus terms with a specific decimal position. Make sure that both parties know how far out that “plus” side is carried. Is it measured to the third decimal place, or the fourth? Many “Plus Nothing” bars actually may only be measured to the third (thousandths) place which allows the bar to actually be a plus tolerance. Such as plus .0005″. Note that Bearing Fit or Bearing Tolerance insures a minus tolerance.

You may further avoid potential headaches by specifying the actual diameter wanted in terms of the actual decimal, both over and under (plus and minus), such as .2500″/.2495″.

Whether it is Bearing Quality or Bearing Fit, keep in mind that there may be an additional cost to obtain that benefit.

 

-Howard Thomas, Oct 18th 2018

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Centerless vs. On-Center Grinding

Bar Grinding Centerless Vs. On-Centers – Second Part of Four Part Set

As we mentioned in our last blog; in the maintenance industry, if someone refers to grinding a steel shaft, they are most likely talking about “Centerless Grinding”. There is another method, however, and that method is called “On-Center Grinding”. A misunderstanding on which method is actually required usually results in expensive errors, and general unhappiness for all parties. Of the two types, centerless is by far the most common. So much so, that if you mention grinding a shaft, the mill or service center will assume you are discussing centerless grinding.

Centerless grinding tends to follow the outside diameter of the bar; think apple peeler. When the skin is off, you still have a recognizable apple; naked, but still looks like an apple. Grind an egg-shaped hot rolled bar, and you will have a precision finished egg. In the hands of an experienced grinding operator, many troubling issues may be corrected. Taking it to an art form, the right operator can minimize irregularities and even affect straightness; to a point. The standard in industry is centerless. So, unless specified, tolerances being discussed are taken to be based on centerless.

On-Center grinding, on the other hand, indexes on the center of both ends of the bar. The grinding head then machines the O.D. of the bar to be concentric with the I.D. (chucked up centering holes). If your bar is egg shaped, now, your ground bar will be concentric. If the bar is bent, the finished ground bar will be straight, depending on how bent it was and how much stock removal you are able to take. The roundness (concentricity) and the straightness come from the “On-Center” grinding. On center grinding requires more stock allowance “to-clean up” than centerless grinding. Where there are low spots, no stock will be removed. The on-center grinding operation will not only true up the diameter size, but, it will “machine” the bar into a true round and straight part. How do you avoid these potential problems if you are not aware of the intended grinding method? Qualify, Qualify, Qualify. If “finish size” is mentioned, ask about the grinding method. And remember; “If it doesn’t clean-up, whos wallet comes out?”

-Howard Thomas, September 5th 2018

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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

 

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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

 

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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

 

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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

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