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
MECHANICAL PROPERTIES: Generally, density, thermal and electrical conductivity are considered to be PHYSICAL PROPERTIES. The following represent MECHANICAL PROPERTIES.
If you expect a piece of steel to make a certain part, or provide certain benefits, you should know something about the nature of the steel you are purchasing. Understanding the mechanical properties of the steel will give you a better understanding as to how hard it will be to fabricate (cut, form, drill, tap), as well as an idea about how the steel might perform in your intended application (wear resistance, twist or bow, gouging, etc.).
HARDNESS How hard is it? This relates directly to strength, and/or brittleness.
Note: Hardness could well be an entire study i.e., variables affecting results, and interpretation.
YIELD At what point will it bend? (Plastic Deformation)
TENSILE At what point will it break? (Ultimate Tensile)
% REDUCTION IN AREA Pull it from both ends to the point of fracture. This measures the ratio of the reduced diameter at the break, to the original diameter. FUN WITH MECHANICAL PROPERTIES; roll a piece of Playdough to a pencil shape. Then pull it from each end until it breaks. The longer you can pull it and the smaller the diameter at the break, the more desirable.
% ELONGATION When pulled from both ends to the point of fracture, this measures the ratio of the length, at fracture, to the original length once it has been pulled apart from both ends. FOR MORE FUN SEE ABOVE. Once again, the longer you can pull it, the more desirable it is. There are always exceptions to your particular needs, but greater Reduction and Elongation are most often desired.
LCVN (Longitudinal Charpy “V” Notch) How much impact will it withstand?
Stick a short test specimen vertically protruding from a vice.
Notch it, then strike it above the notch with a weighted pendulum. Measure how far the pendulum travels after the specimen breaks
This is the closest indicator of “Toughness.” (Generally accepted as the standard for measuring impact strength.). “Toughness” is the main deterrent to Fatigue Failure, one of the greatest causes of shaft failure in heavy industrial applications.
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RULES OF THUMB – NOT FOR ENGINEERING PURPOSES!
Two of the most common hardness testing methods are The Rockwell Test and The Brinell Test.
30 RC Hardness, in Rockwell “C” scale equals roughly 300BHN in The Brinell Test, 40RC = 400bhn, and so on…
Tensile (Breaking Point) equals about one half of The Brinell (BHN) reading.
EXAMPLE: Where BHN is 300, the Tensile will be approximately 150,000psi.
Yield (Bend Point, or point of plastic deformation) = is approximately 70% of Tensile
Generally, you want the yield strength to be somewhat lower (by 20 to 30%) than
the tensile. “Gives you a chance to get out of the room before the shaft breaks.”
THOUGHTS TO REFLECT ON: The test reports (TRs) for steel mill heat lots (batches of steel), are simply random checks. For 5000pcs of ½” Diameter steel bars, only a sampling will be tested. Even within those tests, results will vary. Recorded readings will usually show some variation which may be due to interpretation of results, test preparation, instrumentation, location in furnace, bar ends, etc. The surface of a wear plate may display several different hardness readings when tested in different places on the plate, by different facilities or by different people. This is not to suggest that readings recorded on Test Reports (TRs) are invalid. It is to encourage perspective within the realm of any mechanical testing. In fact, in situations where criteria are highly critical, the best method is to statistically check the actual finished part. Endurance limit testing is an excellent example of this. Many times, customers will request endurance limits on raw steel. Unless the actual finished part is tested, endurance limits posted on any raw steel product are useful only for the broadest suggestion of potential performance.
There is an excellent movie that illustrates this perspective; I believe it is “No Highway in the Sky”, starring Jimmy Stewart. It is an old movie (1950). Very popular among metallurgists and engineers. Enjoy.
-Howard Thomas, January 7th, 2022
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Howard Thomas
Experience
Sr. Acct. Mgr. (US Southwest) / Metallurgical Consultant
Associated Steel Corporation
Jan 2017 – Present
Past Vice President / General Manager
Associated Steel Corporation
Apr 1998 – Jan 2017
Past Vice President / General Manager
Baldwin International
Apr 1974 – Mar 1997
Education
Cleveland State University
Kent State University
University of Denver
