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

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

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

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

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

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

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

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

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

-Howard Thomas, Nov 25th, 2019

Iron ore, found lying around near the surface of the earth, can be heated and primitively refined (a cave man could do it) to become iron. Iron is soft and pliable, great for lots of things and better than a poke in the eye, but begging for improvement. With minimal additional elements and a bit more refining “mild steel” made its debut. Mild steel is the leanest form of carbon steel. It is generally considered non-hardenable and is fairly pliable; stranger than iron and still fairly ductile. From that point you can add and subtract elements (manganese, chromium, nickel, moly, etc.) add heat and then cool and you can pretty much tweak it to be particularly well suited for an infinite variety of specific uses (carbon, alloy, stainless, and tool steel). You could make steel that holds a sharp edge; slices and dices, and could even cut someone’s shoe in half. You can make steel that gets very hard but is also very brittle, steel that gets very hard on the surface but stays softer at the core, and steel that gets very strong and will stand up to impact, shock and gouging. Fortunately for us in the present day, most any type of carbon or alloy steel you would want already has a recipe and a full set of instructions that have been worked out since people have been throwing rocks.

For most of us in the industry who will encounter grades of steel in our work-a-day-lives, we can generally categorize them into two separate camps: steel that is made into parts sold commercially to the general public and manufacturing companies, “production-steel”. And, steel that requires special properties to fix the machines that make those parts, “maintenance-steel”. When working with steel to make things, the steel used for the tooling takes quite a beating. It may have to stand up to bending, or breaking, or abrasive wear. It may be subject to twist, or corrosion.

4140 is pretty much the “Defacto-Standard” for  a good all-around general-purpose maintenance steel for replacement parts. It has been designed to possess high strength (sometimes referred to as ultra-high strength), along with noticeable resistance to gouging, bending, fatigue, wear, and to an extent even corrosion. 4140 may be supplied in several hardness ranges to deliver several levels of strength. In most cases it is considered to be fairly machinable and weldable. You can forge it and bend it with caution.

It is considered to be a “through hardening steel”, but it will also accept surface hardening to maintain a ductile core. While it may on occasion be used in production manufacturing, as the steel that becomes the parts being made, it is generally not used where speed of machining is the requirement. With 4140, you will give up a little bit of machining time to gain strength and toughness.

4140 is a great basic chemistry that is easily modified at the steel mill by use of additional elements to achieve additional, and/or, specific properties for specific industries or jobs. For instance, if you need high hardness that is deeper and more uniform, you could add Nickel to the 4140. The subsequent grade would be an alloy called 4340. Want to increase resistance to softening in higher service temperatures, like a bracket for an exhaust pipe, add Vanadium and increase the carbon a bit; there was a grade for that, 6150. Want easier welding, forging, and maybe a hardened chisel edge but soft core, lower the carbon a bit, and add a bit of Tungsten, and you have 4135 non-tempering steel. And so on. Need to make a high strength shaft with a high surface hardness for wear resistance but a very tough core, increase the carbon a bit. Remember, a rough calculation for potential surface hardness is ten points over carbon content i.e. 4150, potential surface, flame or induction hardness 60RC.

So, when do you use 4140 alloy steel?

Pretty much anywhere. It is a great all around general maintenance alloy chrome, moly, manganese steel. A great platform chemistry for many other application specific steel grades. It is fairly resilient in thermal treatment and may be quenched in water, oil, or other mediums. It is readily machinable and weldable (standard low-hydrogen process), forgeable, and with limitations; formable. It is suitable for many various hardening methods including; furnace, flame, induction, carburization, and cryogenic treatment. There are improvements to the basic grade, but, keep in mind that as steel grades get pushed toward higher and higher levels of performance, they become more and more temperamental.

-Howard Thomas, Nov 5th, 2019