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Hi,

I am seeking advice on macroetching coupons of forged 15-5 PH to improve the visibility of grain flow.

For the initial attempts (pictured), the process was not tightly controlled. The specimens were milled and lapped to get a smooth surface. I used a room-temperature bath of Fry's Reagent, which did not produce any visible results.

Following ASTM E381 guidance, I used a 50% HCl solution in a large beaker heated on a hot plate (targeting 70-80C) for several hours, with a single stir bar positioned to the side. However, the solution may not have reached or maintained the desired temperature due to its volume. I also attempted gradual additions of 3% hydrogen peroxide, although not at room temperature as described in E381. Some grain flow is visible but not to the desired contrast.

In subsequent (unpictured) trials, I used smaller samples (without forge scale on the edges), prepared in the same manner (milled and lapped). These were etched in a lower-volume 50% HCl solution at an estimated 80 deg-C, with a higher amount of gradually added hydrogen peroxide. Immediately prior to etching, the surfaces were lightly rubbed with Scotch-Brite, and surface residue (scum) was removed midway through the process. Despite these adjustments, there was no significant improvement in revealing grain flow.

As a next step, I plan to use the slightly more-detailed procedures in ASTM E340 and/or repeat with fresh, clean solution. I would prefer to avoid HF, but I am open to alternative etchants or process adjustments. Thanks for any information that the ASM community can provide.

Good morning Tiffany,

Etching very clean materials like this is always an uphill battle, for a couple reasons. The first thing to understand is that the term "grain flow" is not a strictly precise description of what you're looking at. In a carbon steel context, a lot of the "grain flow" that you see in a macroetch slice like this is actually banding (segregation) of impurities in the steel like sulfur and phosphorous, or elements affecting hardenability (carbon, manganese, etc.) which will then produce bands heavier in pearlite or bainite/martensite and in both cases that contrast is visible. You're not literally seeing the grain boundary curvature of the grains themselves. When it comes to something like 15-5, however, you have none of this going on - the carbon is very low, the microstructure is very uniform because the hardenability is so high (100% martensite, maybe a tiny bit of retained austenite or delta ferrite), and the impurities are very low (often 15-5 is VAR melted, although I'm not sure about your specific sample). Basically, the material is too homogenous to produce any contrast from etching. As for the grains themselves, if they're a fine equiax like they should be, you'll never see them with the naked eye - to literally see the flow of grains in a macro, they have to be huge, and perhaps have some sort of grain boundary embrittlement going on to precipitate higher-contrasting species at the grain boundaries.

You have a few options then. When I worked at a forge shop where we made 15-5 forgings and we had to produce a grain flow for the qualification piece, we would forge a carbon steel dummy in the same dies out of something medium carbon, medium hardenability like 4130, which produced a heterogeneous enough microstructure to produce a visible "grain flow". The main point of grain flow is to ensure that the forging process isn't producing laps or re-entrant flow. Since this is more a function of the part geometry more than its material, the substitution is acceptable.

However, given that you work for Dow, I assume that you're not forging this piece yourself, but rather you're probably doing a failure analysis on a component that you pulled from service. In this case, I would just ask what it is you're trying to accomplish with this test. If you're trying to verify that there were no quality issues with the material, really the macroetch is just going to reveal gross segregation issues if present, and not much more (at least microstructurally; obviously if there are soundness issues like porosity or cracks, you'd see those). But the more useful tests would be microetching to make sure you don't have stuff like excessive delta ferrite, and then mechanical testing to ensure that you're meeting the desired properties.

Happy to discuss more if you're interested.

Hi Sean,

Thank you very much for your input.

The goal of this work is to compare grain flow between the same component type from a previous forge supplier (now out of business) and a current vendor to improve the service life. Specifically, I am evaluating how grain flow may affect performance, with respect to crack initiation and propagation, and I'm planning to macroetch ones that failed in service but in an expected manner. I'm also hoping to sort of "reverse-engineer" how it was forged by the previous forge shop. With the current vendor, we've required more back-and-forth to meet mechanical requirements. Following additional macroetching, the below shows the desired "grain flow." Would the curves be indicative of hammer strike history during forging? If a semi-closed die process was used, the left side would be on the outer diameter.

Good morning Tiffany,

I have found Picric acid containing etchants work best with high Chrome containing steels. Also best practice is to minimize the time between final polish and etching to seconds if possible. This is to avoid the Chrome Oxide layer from developing and inhibiting good etching. If this is difficult to do you can immerse the sample in Acetone to prevent oxidation.

Hope that helps,

-Rob

The forged 15-5 PH block shows clear, directional flow lines on the surface, consistent with plastic deformation during forging and the part geometry. The flow pattern appears continuous, with no obvious macroscopic defects such as laps, folds, or severe re-entrant flow at the visible scale. This supports that the material was properly worked rather than simply showing segregation banding.

Because 15-5 PH is fairly uniform as mentioned in the chat, macroetch contrast may be limited. microetching as suggested is your next step to ensure there is not banding or alloy segregation and check for hidden defects linked to your issue. If banding occurs it would be obvious at least as I was instructed and learnt (see attached files 17-4PH round stock the etched micrograph is a long. cross sectional mount micrograph), so the next steps should be:

Microetching with Vilella's or Marble's reagent to reveal grain structure and any discontinuities

20% NaOH electrolytic etching to show delta ferrite distribution

SEM/EDS to evaluate inclusions, stringers, or elemental segregation along the flow path

Cross sections in both longitudinal and transverse directions.

Microhardness testing on the flow lines to and away from them to ensure hardness is consistent

Note: if you have access to NDT that would help ID defects and imperfections: here are some snips from an internal procedure: