What is Brake Die Steel?

Let’s begin this post with two of my favorite words; Ubiquitous; (everywhere, like raindrops during a storm), and, Esoteric; (known by a select small group of people).

What do these mean? Brake Die, Gun Barrel or Rifle Stock, Pump Shaft Straightness, Boat Shaft, Food Service Grade Stainless, Cold Roll, “Ultrasonic Inspect to 388, (and even FDA approved). While they may have a specific meaning to someone (esoteric), they do not have a defined meaning to everyone (ubiquitous in certain industries). In reality these are generic descriptions without reference to defined requirements and properties, at best they are like answering someone’s question regarding the location of your pending vacation by responding; “Up North.”

From my experience, and from a supplier’s point of view, the only real commonality they have is the suggestion of liability. You cannot hope to avoid potential mishap if you really do not have more information on the chemical and physical requirements of the steel the person is discussing.

Brake Die Steel – Generally, a high quality carbon or alloy, appropriate for dies, that may be, or is, hardened. Often an alloy from the 4000 series. Is it pre-machined? Not necessarily. Is it pre-hardened? Not necessarily. Is it oversized square and shiny? Not necessarily.

Food Service Grade Stainless – Generally means it does not contaminate food with residue from the steel and it maintains a clean finish. Most often some grade of stainless. More information is needed.

Gun Barrel and Rifle Stock – Generally a 4000 series high integrity hardened alloy. But, not a specific grade.

Boat Shaft – You really have no information from that term. Could be anything, carbon, alloy, stainless, monel, bronze, etc. Most customers will require specific properties that conform to some sort of Marine Agency such as ABS, etc.

Cold Roll – Not a steel grade but a production method. Need more information.

FDA Approved – A misnomer. FDA does not grant approvals for metals.

Pump Shaft Straightness – The specifics are different for everyone. There are ASTM specifications but many large companies have their own “esoteric” specifications. You need to know more.

Ultrasonic Test 388 – An ASTM test method to determine the internal integrity of steel. Requires more detail such as acceptability and reject-ability levels.

Defective Material

So, for almost 50 years I have had the opportunity to investigate various material/component failures, and with all the variables, two things have been fairly consistent. The first statement that is most often heard, aside from; “Don’t look at me!” is, “It looks like defective material.” Eventually, and more often than not, the actual reason for failure involves something other than material defect; incorrect material, design and engineering misunderstandings relative to material availability and the creativeness of people when it comes to overcoming those obstacles, changes in the application or environment, or a mishap in the installation, assembly, disassembly or earlier repair, etc.

The instances of material defect are very often the least contributory to failure. Perhaps the cleanliness or toughness of the steel was not up to the task. But that is not really a defect. It is a specification problem.

There are of course defects in material. That is true with any product. The point here is, don’t make that the first assumption. Besides, you are more apt to be able to quickly fix the problems that are most often contributory, right there on location, if you can identify them early.

Not to be MOTO (Master of the Obvious) here, if you are in maintenance you already have developed a mental checklist to identify the most frequent culprits; steel grade, toughness, change in temperature, alignment, moisture, chemical environment, load, vibration, lubrication, wear, surrounding forces, surface scratches, gouges, or cuts, new employees (Don’t look at me!!), sabotage, Keyser Söze, Russian interference, inc.

When the usual suspects have been ruled out, begin planning for some failure analysis testing. You will eventually need to provide a specimen of the failed piece (preferably at least 4″ to either side of the break). Contact your supplier and request their assistance. They may contact their local independent testing facilities, may have their own lab, and/or, they may contact the producing mill. I have been reminded; “In maintenance, as in life, most of our wounds are self-inflicted”.

Routine industrial applications can destroy some of the toughest materials known. And that is just during the daily conduct of business. Be mindful of the effects of those forces and respect the potential for catastrophic failure (whether it results from material defect or not). Make use of appropriate safety gear, stay alert, and if you don’t know, ask!

-Howard Thomas, April 9th 2019

Conclusion – Ten Things to Know About “Finish Size”

REMAINING FIVE POINTS – elaborated

  1. If the finish shaft involves substantial step downs, or, if only a small portion of the bar needs to finish… share that information.
  2. Remember, your supplier cannot be responsible for a third parties work, and/or mistakes.
  3. If liability on the shaft is considerable, consider parking it in another parties driveway.
  4. Try to “Talk the Talk” Phrases like, “a stick of steel”, or, “cold roll”, or, “a length of steel”, will get you in trouble.
  5. Share important ancillary facts with your suppliers; have you received steel in the past that has not been straight enough, hasn’t cleaned up, or was inappropriate in another way?

Why share info on details of a finished part with the supplier?

Well, you don’t need to go over everything, but statements like; “It’s going to be a long small diameter shaft with a big lollipop head on one end” will tip off an astute vendor to the potential of post machining stress related problems. On the other hand, a statement such as: “There is only a very small high spot near the middle of a short shaft that needs to finish to 8″ Dia. The rest of the shaft is 7” Dia. That might make a difference on how much stock is needed for allowance on the raw material.

The caution stated in number 2, above, is just a gentle nudge to limit how much responsibility you find yourself mentally assigning to the supplier. In the final phase, except for mill defect, unless the vendor is handling the job from cradle to grave, the responsibility for success or failure is really out of their hands.

Jobbing the actual machining to an outside source is not just about saving money, mostly because it will generally cost more. But, there are times when “potential problems are probable”. Who knows the product better than the supplier? You may want to see if they felt confident making a difficult piece. If so, why not let them. That eliminates lots of “finger pointing” if the job goes south. If the job goes wobbly, who’s wallet comes out? Their product, their method, their machining, their liability. If they screw it up the burden is on them to do it over at their expense.

TALK THE TALK

Like it or not, many terms are esoteric to specific industries. With hydraulic components, a bar of steel is a rod. When dealing with freight haulers and steel coils, they use terms like; “Eye to the sky”, or, “Suicide”. If you find yourself working within the maintenance steel industry, try to use the same terms you hear the suppliers using. It will go a long way to eliminating surprises. Surprises in maintenance are generally not good.

In closing, if you know some relevant history about a specific shaft or application, by all means share it. If the bar diameter did not clean up the last time, share that. If the shaft broke or bent, share that. If you encountered hard spots or glazing, share that.

Now, if your supplier is decidedly not interested in your ancillary comments, or, if they cut you short and enjoin you to just tell them the dimensions of the steel you want, perhaps you should try a second source. No harm no foul. You can always go with the first source. It is always nice to test the waters.

-Howard Thomas, April 1st 2019

Finish Sizes & Bar Clean-Up Part 2

The following is a continuation from our last blog on ROUND bar clean up.

It is important to share both your finish diameter AND length. They would like to know if you have a short stubby shaft, where straightness is not often an issue. Or, if it is a small diameter shaft that is 18ft. long. Big difference in how you approach the raw material for the job.

We are not suggesting that you ask the supplier what size they recommend, since a savvy supplier will avoid answering that. Yes, they are there to help you, but they can’t possibly know how talented you are in making a difficult shaft. It always comes down to; When things go South, who’s wallet comes out? Sufficient stock allowance for one, might be trouble for another.

You might also share information on what grinding, or machining method you intend to use; are you grinding or turning on centers, or will it be a centerless operation; which is by far the most common. Clean-up stock and straightness requirements will be different.

What surface finished are they able to supply? Do they have pre-machined, or, rough turned stock, is it fine-turned, cold drawn, hot rolled, or as-forged? Each may have a different allowance for removal that has been left on the bar. There are of course standards that govern each type of finish, BUT, there are variances from one mill to another.

Share “end-use” information with the supplier. What is the application it is being used in. If you are making Hydraulic Rods for instance, that would be an important bit of information you might want to share.

Subsequent Operations that will be incorporated, is also important. Will you be changing the hardness, adding surface hardness or surface-coating? If they don’t seem to be interested, no harm done. They just assume you’re verbose. A good supplier, however, will recognize that your contributions are important to a successful transaction.

In the next post, we will finish up elaborating on the last of the 10 points relative to “Stock Allowance on Round Steel Bar”.

 

-Howard Thomas, March 6th 2019

 

Ten Points to Eliminate Mistakes on Finish Size – Bar Clean Up, For Steel Bars

We have talked about round bar grinding in the past. The following posts will look deeper into the communicating skills that will help you minimize disappointments in this area. Unfortunately, more people than not have purchased a bar of steel and found that it won’t make their part because there was not sufficient stock allowance to “clean-up”. That need not be the case. Most often, communication is again the culprit. We are speaking of ROUND STEEL BARS.

  1. Diameter size relative to length will be important.
  2. Subsequent machining or grinding method will be important.
  3. Mill finish and mill tolerance will be important. This often varies by mill.
  4. You should share “end use” with your supplier.
  5. Also, share any subsequent operations you will be doing on the bar.
  6. If the finished shaft involves substantial step downs, or, if only a small portion of the bar needs to finish…share that information.
  7. Remember, your supplier cannot be made responsible for a third party’s work, and/or, mistakes.
  8. If liability on the shaft is considerable, consider parking it in another party’s driveway.
  9. Try to “Talk the Talk” phrases like, “a stick of steel”, or, “cold roll”, or, “a length of steel”, will get you in trouble.
  10. Share important ancillary facts with your supplier; have you received steel in the past that has not been straight enough, hasn’t cleaned up, or was inappropriate in another way?

Each one of the above should solicit further discussion. That is going to come under the header of; TO BE DISCUSSED (TBD). I will try to do just that over the next several weeks. So, stay tuned.

-Howard Thomas, February 26th 2019

 

“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

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

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

 

 

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