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