Conversations involving Endurance limit (fatigue strength) are generally concerned with discovering the life-expectancy of a part. “If I use steel type “A” instead of steel type “B”, how much longer may I expect the part to last?

A normal concern for sure, however, there is no test or formula for arriving at an even remotely accurate answer, based solely on the chemical and physical properties of a type of steel. Data posted showing “Endurance Limit” relative to standard SAE grade steels, is intended for general guidance. In order to obtain any meaningful data, relative to endurance limits, the finished part must be tested under conditions that simulate actual service conditions. You want to know about your part? You got to run tests on your part. There are tests that will provide relatively accurate estimates of predictable service life, when the actual part is in simulated service conditions.

The R.R. Moor Endurance Test, is an example of a test that utilizes bending and rolling contact to test torsional fatigue. [Variables introduced may be; vibration, compression, bending, twisting, rolling, etc.]. This test is expensive and the evaluation period is lengthy. Results may be expressed in “Cycles to failure” as opposed to “PSI.”

Specialty alloys and stainless shafts furnished by Associated Steel are made to requirements
intended to extend service-life over what is typically experienced with their commercial equivalent alloy. General metallurgical data available for public reference is typical to standard grade SAE steels and is at best general in nature. It is not intended to be an accurate reflection of the potential extended service life of refined or modified material.

Endurance limit relates to “Toughness.” Toughness in metals considers the durability (Ductility) of the metal at elevated strength levels. You can enhance “toughness” by modifying the metals chemistry, minimizing defects, refining grain and improving thermal conditioning during manufacturing production.

Associated Steel is a supplier of value added materials intended for critical service and intended to extend service-life.

 

-Howard Thomas, December 8th, 2022

So, in our last post we briefly introduced Stainless Steel (SS) in general overview; We introduced the grades most common to general industry (304 and 316), and we looked at some of the properties of those grades.

There are, however, many grades of Stainless, each with its own strengths and weaknesses and economies. It is my feeling that due to overall education of the pros and cons of many of these grades, and lack of familiarity, many grades of stainless are used inappropriately in the maintenance arena. So many grades, so many mechanical forces, so many types of corrosion. (Variables relating to corrosive conditions alone are nearly infinite). One grade of stainless may be working in a particular application, but that may be only relative to other materials that have worked less effectively. So, let’s begin by getting acquainted with the benefits various grades may afford.

Two important things to keep in mind are; A specific grade, perfect for your application, may exist but may not be available in the form needed (tube, sheet, plate, bar). The grade may exist and be available, but, due to limitations in fabrication, may not readily lend itself to the configuration of your part.

Most often used, Type 304 and 316, fairly economical, good general corrosion resistance, elevated temperature resistant, and somewhat user ambivalent to a bit combative.
(Consider grade 303, if machining speed is a serious consideration, which is generally not the case in maintenance situations). These grades are austenitic 18-8 type, non-magnetic, will work-harden, but do not respond to thermal hardening.

Commonly used when increased strength is required; 410 and 416. Note that 416 is 410 modified for ease of machining. Superior to 304 and 316 in Sour Service Applications. (Martensitic, magnetic, responds to heat treating.)

Persistent problems, not addressed by either of the two groups listed above, will require consideration of modified, or enhanced grades. Those grades will involve modifications to element content (increasing content of elements, such as Nickel, Moly, Nitrogen, etc). Expect increases in cost. Among these are non-galling grades (such as Nitronic grades, Mir50), elevated temperature grades (such as 309 and 330), and a host of trade named products that are relatively economical and effective. Then there are, what I like to refer to as “The Exotics”; the next level up. Exotic in chemistry, and exotic in cost. Cost is of course relative to benefit; however, too often exotic materials are selected in an express leap to the ultimate solution. Generally, once that selection is made, the user is committed to a cycle of selecting grades equal or superior to that grade. Assumptions being, the grade in use is the minimum required grade. Going forward, service-life needs to be improved; Current service-life never being assumed to be the best-case scenario.

Modified grades may already be numerically identified (AISI SAE, UNS, etc.). Often, those targeting specific maintenance conditions, will be identified by a Trade-Name, identifying the producer. That producer will issue guidance on their trade named grades, to insure they are correctly applied in the field.

IMPORTANT CONSIDERATIONS:
As you add to the chemistry and increase corrosion resistance, strength, and toughness, etc.,
you are creating a product that fights and resists forces that attack the part in service. Read that as a product that “Does not play well with others.” Keep in mind that when you need to work with it, it may not play well with you either. It may solve the problem in service, but you will have to get it into service first.

Pay close attention to “User Friendly” stainless. How does it machine? How weldable is it? Does it tend to gall? Does it move? Then take a second look at your part. User friendly stainless may be a bit of an oxymoron, but it can be as important as the ultimate in-service performance.

-Howard Thomas, November 18th, 2022

THE 300 SERIES OF STAINLESS STEEL

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

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

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

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

 

-Howard Thomas, October 17th, 2022

Did you know you can buy precision finished high-strength shafting that already has a keyway machined in it? You can. You can purchase sizes from ½” Diameter through
2 15/16” Diameter. This product is available to ship from stock from Associated Steel.

The product name is Mirraloy FM (Free-Machining) Pre-Keyed. It is an elevated strength material that is also precision finished and polished, In most sizes accurate within a couple of thousandths. The keyway runs the entire length of the shaft.

For some, that configuration may work just as received; just cut it to the length you need. For others, it will be sufficient to get you back up and running quickly while you wait for an OEM part or custom machined part.

The main thing is, be aware that it is available, and the standard length (10/12ft rdm) can be cut-in-half as a courtesy, and rushed out via an expedited package service.

Cutting keyways can be a fun and surprising endeavor. Some materials, particularly stainless grades and hardened alloy, can move and bow, and not just a little. Re-straightening is often required; sometimes even an additional stress-relief is needed. When you purchase the shaft pre-keyed, Associated Steel has already insured you get a prime, accurate, usable shaft. Just install it and carry on.

-Howard Thomas, August 24th, 2022

IT IS A HUGE BENEFIT TO THOSE CUSTOMERS WHO PURCHASE ASSOCIATED STEEL KROMITE CD60 FINISHED RAIL TO BE ABLE TO JUST BOLT IT IN PLACE AND GET ON WITH THEIR LIFE. The manufacturer assumes the risk and accepts the liabilities associated with the production of the product.

ASSOCIATED STEEL;
“If we supply it, we stand by it”. We cannot guarantee the workmanship of our customer or their agents. Making finished hardened and machined custom wear rail is not for the faint of heart. Anyone who has done so knows there is truth in that statement.

Associated Steel has a Special Services Department (SSD) dedicated to custom fabrication. They have developed a specific fabricated rail from Kromite Cold Drawn Stock. Because of unique properties and characteristics, that product has been trade named; “Redi-Rail / Kromite CD60”.

Kromite CD60 is a semi-finished or finished rail that has been fabricated, to the customer’s specifications. Much like the unique processing technique used in the production of I.D. hardened wear pipe, the processing sequence and technique for Kromite CD60 is an inseparable component of the finished product. It is only CD60 when Associated Steel supplies it complete. Associated Steel has dedicated inventory, and devoted countless hours of engineering and refinement, toward making the highest quality product. CD60 is the product of an esoteric process. It has been field tested and proven to work exceptionally well. We do not share, nor do we release esoteric production technique or production sequencing information.

There is an “art” to making exceptional surface hardened rails, and that is why we trade-name finished and semi-finished rails. The actual process is an open-air process that is accomplished as much by sense and feel as by instructions and manuals. We have been supplying custom automation rails for over 50 years.

Occasionally, we will get a call from a customer who has made their own rails, advising they are getting low hardness readings. There are times where the actual hardness testing is the culprit. However, the majority of the calls could be avoided with a little bit of education. During the flame hardening process, De-carb is created on the surface of the bar. De-carb is that grey powdery coating that is visible on the surface that has been heat treated.
During the hardening process, carbon in the steel, at the surface, is burned off and disburses into the atmosphere. (No carbon, no hardness). Carbon near the surface can also burn off, although not completely. Testing into the De-carb will result in hardness readings that are not accurate, therefore a grind spot is recommended. A refined grain structure ensures the higher hardness range that we achieve after processing.

Surface hardening, when done correctly, allows the core of the bar to remain somewhat softer in order to maintain limited ductility. That is necessary for subsequent straightening and various other installation operations.

Heavier cross sections of flat bar retain more ductility. They are easier to work with during production and installation. Thinner flat bar will be more brittle and subsequently more difficult to manufacture and install. So, a 1” x 4” bar will generally be less troublesome than a piece of ¼” x 4”. On thinner sections, “crowning” across the flat becomes an issue and brittleness can come into play. At some point, if the ratio of thickness to width to length becomes unreasonable, Associated Steel will decline to quote. There are no charts to determine the exact point of concern. Experience and familiarity is required to make those determinations.

OUR COMMITMENT
To insure we provide the best possible hardened rails, we have developed unique jigs, fixtures, custom quenching mediums and procedures, at great expense to our company. There are times when we recognize that costlier additional processing procedures may be required. In rare instances, we may decline to quote certain configurations.

Associated Steel has the ability to provide custom surface hardened railroad type rails and custom hardened rail configurations specific to beef processing and other automation industries.

 

-Howard Thomas, August 6th, 2022

Part 2 of 3

MAKING RAILS
a. if you start with cold rolled or cold drawn material remember that the manufacturing process for that product generally contributes to a noticeable amount of “retained stress”. That stress will tend to be released by temperature, and/or, vibration; meaning, while you are making the rail, or when the rail is in service.

b. if you start with a hot rolled bar, you must allow more stock removal, not only for potential decarb on the surface, but also distortion in shape (out-of-square, etc.). The mill allowance for out of square with hot rolled bars is significantly more than it is for cold drawn material. Often, a final grind is needed after processing.

So, there will be limits of what you can do to that piece of steel. Many variables will have to be taken into consideration. When we say, “Our steels machine better, weld better, or wear longer”, you must understand that is within a certain context. The statements we make are relative to other high strength steels used for maintenance applications (apples to apples). We are not saying they machine better than free-machining steel, or mild steel, or copper, or plastic.

A round bar that has been supplied as hardened from the mill, may not want to be straightened to a very tight T.I.R. To achieve a specific T.I.R. the straightener will have to overcome many factors. Consideration will have to be given to size vs length, the bar’s propensity to retain stress, etc. The operator will have to know something about the specific material’s “memory” properties and be familiar with all the potential conditions that might prohibit the required end result.

Conditions that might prohibit achieving that, are present in different forms for all fabrication jobs. They may relate to; number of pieces, grade, hardness, length, configuration, tolerance, surface finish, service temperature, etc. In addition, there are a limited number of companies offering value added services who are comfortable dealing with maintenance steels.

Metals fabrication shops only work with “serious” experienced companies, or the people they trust at those companies. They can easily lose faith in a job if you don’t have all the information needed. They can back away from a job if you keep feeding them additional or changing information every couple of days. Maintenance jobs cannot afford delays.

Metals fabrication is serious business. It is not a training ground for eager novices. Some words of wisdom you might want to tattoo somewhere on your person are: “Fabrication may be late, wrong, and dangerous.”, “Wear Plate is big and ugly!” and,” If you can’t stand the heat, stay out of that kitchen.”

Rule #1
Steels for maintenance and tooling need to be hard and tough. They need to resist abrasive wear, or gouging, or bending, or fatigue failure, or all of those issues.

Rule #2
Once you have made the base metal hard and tough, you have made a material that does not want to be messed with.

Let’s review
Making finished hardened and machined custom wear rail is not for the faint of heart. Anyone who has done so knows there is truth in that statement. If you are familiar with Murphy’s Law, when rail is being discussed, think of it as “Murphy’s Rail”.

 

-Howard Thomas, June 21st 2022

Cold Drawn – a Process Not a Material 

Cold Roll or Cold Drawn bar is often requested from a supplier as if the term indicated a grade of steel. Those terms refer to production methods not chemistry or grade. I point this out not to be stuffy, but to try to eliminate potential safety problems in the field.

Cold Roll, Cold Drawn, or Cold Finished bars may be made from 1018, 1020, 1045, 8620, 4140, etc. While most often they would be annealed (soft), there is the chance a cold drawn bar could be supplied in a hardened condition. The real problem is somewhat related to geography. In certain parts of the country, Cold Roll (Cole-roll), is most often produced as 1018 or 1020. Elsewhere, in other mills, the preponderance of production might lean toward 1045. The 1045 grade is hardenable; 1018 and 1020 are not. Expecting a material to be ductile in all circumstances could lead to significant problems should that steel turn out to be brittle.

Cold Drawn or Hot Rolled – When?
Cold Drawn steel materials may be used for various types of rails in heavy industrial applications. Hot Rolled Hardened wear bars may also be used for types of rails. If smooth and accurate travel along a rail is a concern, or if a finished cam/wheel is riding on it, you will probably want Cold Drawn, Cold Rolled, or Cold Finished (all the same) bars. If the application is “Big & Ugly”, let’s say a large heavy tank car is rolling over the rails and the rails eventually fail due to gouging, deforming, weight and wear, think Hot Rolled and Hardened. (Q&T).

Basically, a flat bar (cold drawn or hot rolled) is referred to as a rail when it has undergone some machining, and/or, hardening, or is used in an application where something is conveyed along it, or guided by it. A roller cam or trolley may ride along any surface of the flat bar, whether the bar is lying down flat, or is positioned up on end. Product may simply be slid along one surface of a flat, with no cam to assist movement. In the case of sliding, you will want to review “galling”.

If the application is “Big and Ugly”, such as moving bins of molten metal in a steel mill, the rail may be just hardened hot rolled bar stock. Pay close attention to the “out-of-square” allowable tolerance on hot rolled hardened mill bars. It is much more open than the tolerance of cold drawn bar.

WEAR BARS OR HOT ROLLED FLATS AS RAIL
Where the configuration is not sensitive to smooth uninterrupted movement (such as is required in an automation facility), hot rolled hardened bars may be used. These could be sizes like 3/8” x 2 ½”, or 5” x 8” depending on the weight of what is travelling the rail.

Generally, these types of rails will be welded or bolted to a substrate. Even Big and Ugly rails may require precise hole locations on the bars for alignment to pre-drilled holes in the substrate. When this is a requirement it is important to measure the holes centered on the length of the rail. So, you would not measure from the outside length dimension of the flat bar. You would find the center of the bar length and index the holes from that point radiating out in both directions. Otherwise, if the length was a bit oversize or undersize, the holes would not line up. The same goes for the width of the flat bar.

Our next post will focus on fabricating finished “Rails”; specifically impediments to fabrication and subsequent service-life. The third, and final post on CD bars will be an introduction to the trade-name rails produced and marketed by Associated Steel.

-Howard Thomas, May 9th, 2022

ASSOCIATED STEEL PRODUCTS: KROMITE® #3 (Hot Roll) & MIRRALOY® (TG&P)
The following tips assume you have an existing piece of shafting in 4140 or 4150 alloy steel, that is presently at a through hardness of approx. 30RC. And, that you require an increased hardness of approximately high 40’s to low 50’s RC.

PREFERRED METHOD: Long cycle anneal
Anneal at 1525F, one hour per inch of greatest cross section.
Cool in a furnace to 800F, at a rate of about 20F per/hour.
Re-heat to 1525F, one hour per inch of greatest cross section.
NOTE: In the event you have to move the material a significant distance between the heat temp and the oil quench, take precautions not to let the temperature drop below 1525F between the heat and quench. Cool in oil until smoking but no flame, approximately 250F to 300F.

NOTE: Alternate is a “sub-critical anneal”. Heat the bar to 1350F – 1400F, hold for one hour per inch of greatest cross section, atmosphere cool (in a protected atmosphere) to 350F. If the carbon is closer to .50, temper at approx. 700F to 800F for three hours per inch of greatest cross section (even for small diameters) should yield about 50 to 54RC. If carbon is closer to .40, lower the temp to approximately 400F to 600F.

NOTE: These are intended to be, and should be taken to be, suggestions for consideration, not instructions. This is not exact. If the material is too hard, repeat the cycle and try a higher tempering temperature. Repeated thermal cycling is not detrimental to the material, provided excessive temperatures are not encountered. If you do have to try several times, you may develop significant surface de-carb (powdery surface coat), which you will have to remove if you are not getting the results expected. CAUTION: Do not overlook removing all decarb in the area you will be checking hardness, no matter if using Rockwell “RC” or Brinell (bhn) equipment. Just grinding to “bright metal” does not mean you have removed all decarb. If your tests indicate you are only getting 18RC or lower, you are probably still into decarb. Even in an annealed condition those alloys would likely register a higher hardness than that.

Simply reaching for a higher hardness is not always the answer. If you try to go too high, brittleness may become a factor. In shaft applications where 28 to 32RC are deemed to be insufficient, you may want to try 32RC to 38Rc. Years ago, I was advised by an old friend; “With 4140 and 4150, “funny things begin to occur when you exceed 40RC (they tend to spit carbides). Don’t ask me what that means. I just got a strong negative visual and have been happy to follow that little gem for many years. (Real hard things can shatter like glass).

The above “tips” are just that; TIPS, as in “Hey, my brother in law’s uncle’s wife tried this once.” Plus, they are reduced to the simplest form. These basically relate to shaft stock and do not take into account a host of variables that may affect your safety and results. Part configuration, cross sectional differences, experience, etc. In all instances it is our recommendation that you utilize experienced thermal treatment personnel familiar with thermal conditioning alloys, who make use of all appropriate safety gear.

 

-Howard Thomas, April 8th, 2022

4140 and 4150 – KROMITE #3 AND MIRRALOY – modified grades of high strength alloy.

Last month, we talked about the word “modification” appearing on a Mill Test Report (MTR). This month I have been asked to clarify just how modifications have been incorporated into two key shaft materials provided by Associated Steel. Those items are Kromite #3 and Mirraloy. Both of those products incorporate changes (metallurgical and non-metallurgical), that address requirements of Heavy Industrial Maintenance.

Many times, in fact in most cases, modifications incorporated into the production of a grade of steel fall within the parameters of an established grade. They are usually adjustments to processing requirements and are specific to a particular customer. Modifications will appear on the MTRS. Some will change the allowable range of a chemical element but still carry the original numeric grade identifier, such as in the case of “H” Band Steel. H-Band steels have altered chemical ranges of certain elements to assist in thermal treatment. The change may allow slightly higher Carbon and Chromium content. The grade, however, remains basically the same. It retains the base metal grade numeric designation, plus a suffix addition. In this case “H”.

There are times when the modifications, intentional or not, may change the grade, i.e., prohibit the intended grade from being certified without an exception noted. Even a very small change in the content of an element may be sufficient to change the grade.

Many years ago, a manufacturer required a slight increase in resistance to service temperature for a part made of 4150. Over time, fatigue from repetitive motion, in elevated temperature, contributed to failure. One of the effects of vanadium is that it increases temperature resistance. The addition of approximately .15 vanadium content, to the 4150 chemistry resolved the customer’s problem and introduced a new grade of steel, 6150. In applications involving stainless steel, lowering the carbon content to .030, in type 304 stainless, created 304L, a modified grade. An “H” suffix in grades of stainless steel indicates elevated carbon content; again, a modification to the base grade.

The point is modifications are neither good nor bad. You just need to know how they will affect your application. The alloy steel grades Kromite #3 and Mirraloy, supplied by Associated Steel, are chemically modified to improve cleanliness of the melt. They incorporate bracket restrictions, (high-side for beneficial elements, low-side for detrimental or tramp elements). Improved cleanliness results in improved “toughness”, the key property that resists fatigue failure in shaft applications.

Associated Steel has been in continuous service to heavy industrial maintenance customers for nearly a century. Their products have been field proven in critical service.
The round bar shafts Kromite #3 and Mirraloy are both the same chemistry and follow the same thermal processing and grain refinement. Mirraloy is the precision finished product produced from Hot rolled Kromite #3.

 

-Howard Thomas, March 7th, 2022

Part One of Two

Steel grades in the U.S. are generally referred to by the materials standards and grading systems presented by various entities. The most common being: SAE (Society of Automotive Engineers), ASTM (American Society of Testing Materials), AISI (American Institute of Iron and Steel), and ASME (American Society of Mechanical Engineers). Those are certainly not all of them. Add to those an entire host of international agencies and it becomes a very large index. Grading systems are often common within a particular industry. Automotive may primarily use one numbering system, while pressure vessel operations would typically use another. As your time in a particular job builds up, it will serve you well to become familiar with the “esoteric” agencies and specifications typically used in that job. Be cautious, however, that your familiarity does not lead to assuming that grading systems used by your suppliers are the same because you recognize similar numbering. Be certain that the agency matches the specification referenced. Then pay special attention to any prefix, suffix, or reference to nomenclature, such as, “Modified”. The presence of that word does not have any specific connotation or purpose, other than to alert you to the fact that something relative to the specification is noteworthy. Nor does it mean that the original specification is no longer valid. While it may well mean an improved grade for one application, it may not affect your application at all.

The scope of potential modifications is broad. For instance, the content brackets for a certain element that the grade allows may have been tightened (restricted) for a particular heat-lot (melt) of steel, the bulk of which may have been destined for a particular industry. Let’s say the standard grade allows a carbon content of .38 to .42. If the melt content simply happens to fall within that grade, no modifier will be shown. However, if the ladle metallurgist has intentionally restricted the low-end content to be no less than .40, or a new restricted range of .40 to .42, the words “modified analysis” should be shown. For your needs, there may be no concern, since the content still falls within the range for the original grade. But, for the customer whose parts must hit a minimum hardness, it is very important that the carbon content is higher. For that privilege, that end-user will have paid an up-charge. Modifications may be made to element content to facilitate temperature resistance in service, strength, cleanliness, etc.

Prefix and suffix indications are more specific. An “E” prefix may mean Electric Furnace, which is a specific type of furnace treatment. An “H” suffix (H-band) relates to a chemistry adjusted to insure specific hardening results. An “H” modifier does not change the grade of the steel, it does change the hardenability potential. There may be “S” for sulfur additions, “L” for lead additions in steel, or low-carbon for stainless.

If you are not charged with the actual engineering and safety of the job, but are in an affiliated support position, you are not required to understand all of the specifics that relate to the metallurgy involved and the certification of the grade of steel. The thing to understand is that steel grades commonly bantered about in the industrial marketplace are a means of dialog. But, by the time the pen hits the paper on a requisition or purchase order, it is important that all parties are on the same page. Your general familiarity with numbering and grading systems will help you to effectively accomplish your tasks from a non-engineering position. The action you take will differ with each situation. It may be as simple as pointing out that a modifier appears. Expect a response like; “Thanks for the heads-up dimwit,” whether they were aware of it or not. Be forensically functional in whatever you do. Pay attention wherever you see the word “modified”, whether on your lunch order or marriage license.

 

-Howard Thomas, February 7th, 2022