When reviewing and working to customer steel specifications, it seems to me that one of the most misused and misunderstood gear material requirements is the grain size requirement. A common grain size requirement I see is, “The grain size shall be 5 or finer when tested in accordance with (grain size standard)” with no additional information included. The grain size standard is typically ASTM E112, or perhaps another industry standard such as ISO 643. There are a multitude of problems with such a requirement as stated, and I will attempt to specifically address each one. But before I do that, let’s review the two main types of grain size test methods.
• The first method provides grain size in the as-found/as-received condition without subjecting the specimen to a reaustenitization cycle that would result in recrystallization of the microstructure, which could alter the as-found/as-received condition grain size. This may be simply to metallographically prepare the specimen and etch it according to its chemical composition and heat treat condition. Sometimes re-heating the specimen to a temperature below the lower critical temperature (Ac1) is necessary to aid in outlining the grain boundaries, but doing so does not alter the specimen’s grain size.
• The second method is to observe the grain size after the specimen has been subjected to some kind of additional furnace cycle that involves reaustenitization, such as McQuaid-Ehn, mock carburizing, or oxidation, in order to establish the prior austenite grain boundaries (PAGBs). These methods are used to demonstrate the steel’s ability to meet certain grain size requirements after it is further heat treated; essentially this is a capability test.
Back to the problem with grain size requirement, questions remain as to which type of grain size is truly required, as-supplied forgings’ grain size or the capability grain size, and what is the associated testing frequency? Seeking customer clarification of these questions sometimes does not yield the required information due to not understanding the metallurgy of grain size formation and subsequent testing. Often we are instructed to “just follow ASTM E112.”
Contrary to common belief, ASTM E112 does not specify which type of grain size (or the method to establish grain boundaries) for a given material grade and heat treat condition. Therefore, only stating a requirement of “report grain size per ASTM E112” does not provide guidance to gear material suppliers on which grain size test method to use. Someone may ask, “why doesn’t ASTM E112 specify the specific method for testing?” When I asked ASTM E04.08 Subcommittee (which has jurisdiction over E112) during a recent meeting on whether E112 should do so, the committee’s response was that the material specifications should specify which E112 method to use for a given application. This makes sense, since ASTM E112 could not possibly cover every scenario, nor is Committee E04 interested in assuming the associated liability.
Therefore, it makes sense to me that when the grain size is desired on any gear forging material supplied in the final heat treated condition (excluding any further thermal treatment that does not change grain size such as stress relieve, temper, nitride, etc.), the grain size testing should be specified on a per heat treat lot basis on specimens taken from the actual gear or representative test block. The specimens should not be subjected to any further thermal heat treatments that cause partial or full reaustenitization. Here are some examples:
• Through-hardened gear grades in the quenched and tempered condition, e.g. AISI 4340, 4350.
• Nitriding grades in the quenched and tempered condition. This is because the subsequent nitriding is usually performed at a temperature below the Ac1 temperature, such as 925-975°F (495-525°C), where the grain size is not affected.
It should be pointed out that revealing PAGBs in the heat treated state with a martensitic and/or bainitic structure can be very challenging at times. Fortunately, for the common carbon and low-alloy steel grades used for gear applications, some successful etchants have been developed for such a purpose. One of the most successful etchants I found for steels is heating aqueous saturated picric acid, HCl (3-5 ml per 100 ml picric acid), and a wetting agent to 80-95°C. The etching technique is to immersion etch the polished specimen until a haze develops, followed by back polishing on the final cloth. Sometimes it may be necessary to repeat the etch/back polish several times to get the desired results. The photomicrograph (Figure 1) is an example of a specimen removed from heat treated AISI 4340 that has been etched with this method in which the grain boundaries are clearly delineated.
In next month’s article I will discuss grain size testing of gear material supplied in the intermediate heat treat condition, and I will also discuss possible unintended consequences of not specifying the desired grain size test method.