Grain Size Requirements

Grain size requirements should be based on what is actually needed for a given application and manufacturing process.

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For gear material supplied in the intermediate heat treat condition, the capability grain size would be appropriate. The goal is to show (usually on a test coupon) that the steel is capable of meeting the grain size requirement after further heat treatment that involves changing the grain size (heating to the austenitic temperature range). Since this grain size is checking the characteristic of the heat, it is sufficient to perform this only once per heat. Some examples are:

McQuaid-Ehn and mock carburizing methods on carburizing gear grades supplied in the normalized, normalized and tempered, or annealed condition. For the results to be most meaningful to the end use, the austenitizing temperatures during specimen preparation should mimic the temperatures in the actual carburizing cycle.

Flame and induction hardened gear materials supplied in the quenched and tempered or normalized condition. These grades generally contain 0.30% carbon or more, which is ideal for oxidation grain size method.

So what are the misuses? One example is requiring McQuaid-Ehn or oxidation grain size on a per heat and per heat treat lot basis. Let’s say the gear material’s actual grains are coarsened as a result of a furnace malfunction. The grain size failure will be missed because the specimen preparation will refine the grain size again, thus defeating the purpose of checking the effectiveness of the heat treatment. Another example is to require fine grain size in the intermediate heat treat condition when the subsequent heat treatment will refine the grain sizes again. For the most part, the goal is for the gears to have fine grain sizes when placed into service. Whether the grains are coarse or fine before the final heat treatment that refines the grain is irrelevant. Perhaps the biggest issue is not specifying the method to reveal grain sizes at all, e.g. report grain size per ASTM E112. Doing so leaves the test method open for suppliers to use whatever method they find convenient to obtain a fine grain size result, which may or may not be a meaningful result for the given manufacturing process.

At Scot Forge, we often find ourselves working to incomplete grain size requirement as discussed earlier, and it is not always practical or even possible to get the complete requirement clarified with the customer. In such cases, we would exercise the best engineering judgment for a given situation and use the most appropriate grain size sample preparation method to provide the customer with the most meaningful data, while remaining strictly adherent to the purchase order and specification requirements. In instances where a given steel alloy in a heat treat condition is difficult to reveal the as-heat treated grain size, we would seek customer involvement to come up with a mutually agreeable sample preparation method.

Normally a grain size of ASTM 5 or finer is considered “fine,” but I have also seen grain size requirements of 6, 7, 8, or finer. When I have asked our customer the reason for the higher grain size requirement, the answer I get is usually “because past results have shown it is not a problem meeting the tighter requirement.” This suggests to me that the requirement is results-driven, rather than what the application actually requires. I find this to be a concerning trend, because this could mean more unnecessary rejections if, for example, the grain size comes in at 6 and the requirement is 7 or finer, whereas the actual application may require a grain size of 5 or 6. Whenever possible, we should always let end use dictate the design requirement, not the statistical scatter of actual test results.

Grain size measurement was initially used as a crude method to assess the mechanical properties of materials where very coarse grain size would result in brittle steels. The importance of grain size result becomes of secondary importance when more quantitative material characterization methods are used, such as hardness, tensile, impact, and fracture toughness testing. If all tests except the grain size are tested to be acceptable, then consideration should be given to full acceptance of the material. Often the reason for a grain size “failure” is due to difficulty in revealing all the grain boundaries in the heat treated condition. This may surprise some people, because textbooks state that finer grains are necessary for higher yield strength and better toughness. However, if direct tensile and toughness results show that the material met the design requirement, what is the problem if the grain size does not meet the requirement?

In summary, grain size requirements should be based on what is actually needed for a given application and manufacturing process. Additionally, the specific grain size test requirement should include both the appropriate method to establish grain boundaries and the appropriate testing frequency. Hopefully these columns have served as a starting point toward accomplishing that goal.