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I am reviewing parts that were heat treated by a new heat treat vendor. I am seeing a thin (generally less than 0.001"), irregular layer on the surface of many of the parts. The layer appears to contain porosity and oxide particles. I have attached some images showing the surface layer. I am wondering if anyone has an idea as to what could cause this? Could it be a scale layer that was partially deoxidized in the heat treat atmosphere?

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Kenneth Kirby
Senior Project Engineer
Snap-on, Inc.
Kenosha WI
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Hi Kenneth,

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Sanjay Kulkarni
Materials Engineer
MSSC, NA
1965 Research Drive
Troy, MI 48083
sanjay.kulkarni@msscna.com
Cell: 248-840-1056
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The thin dark boundary at the surface is very likely residual oxide/scale or a heat-affected surface reaction layer, HTO scaling from prior processing etc. can you share some more information about the process and Heat treatment, air, solution, HT, air quench etc. thanks for sharing!! good day!

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Miguel Diaz
Sr. Technician Materials
Woodward Inc.
Los Angeles CA
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Original Message:
Sent: 06-26-2026 09:29
From: Sanjay Kulkarni
Subject: Irregular layer at surface of Q+T steel

Hi Kenneth,

After reviewing all the microstructures, I do not see any evidence of structural changes at the surface. The only feature present is a thin oxide layer, which is not a concern. If necessary, it can be removed using standard surface cleaning methods. These are my observations based on the information you provided.

Thanks,

------------------------------
Sanjay Kulkarni
Materials Engineer
MSSC, NA
1965 Research Drive
Troy, MI 48083
sanjay.kulkarni@msscna.com
Cell: 248-840-1056
------------------------------

The material is a proprietary steel similar to S2 tool steel. The part is made from cold-finished hex stock. The surface layer had similar appearance on all surfaces regardless if original bar surface or machined. The machined surfaces were clean and free of scale before heat treat. Original bar surfaces may have had some mill scale.

The heat treat process is:

-Stress relieve at 1100°F for 2 hours, furnace cool to 800°F then air cool (I don't know if there is a protective atmosphere. I would guess not.)

-Preheat at 900°F for one hour

-Harden at 1595-1655°F for 1 hour in neutral carbon potential (I believe endothermic atmosphere)

-Quench in oil at 200°F

-Wash

-Temper for two hours minimum. I'd prefer not to give the exact temperature, but it is a relatively low tempering temperature.

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Kenneth Kirby
Senior Project Engineer
Snap-on, Inc.
Kenosha WI
------------------------------

Original Message:
Sent: 06-29-2026 10:40
From: Miguel Diaz
Subject: Irregular layer at surface of Q+T steel

The thin dark boundary at the surface is very likely residual oxide/scale or a heat-affected surface reaction layer, HTO scaling from prior processing etc. can you share some more information about the process and Heat treatment, air, solution, HT, air quench etc. thanks for sharing!! good day!

------------------------------
Miguel Diaz
Sr. Technician Materials
Woodward Inc.
Los Angeles CA
------------------------------


Original Message:
Sent: 06-26-2026 09:29
From: Sanjay Kulkarni
Subject: Irregular layer at surface of Q+T steel

Hi Kenneth,

After reviewing all the microstructures, I do not see any evidence of structural changes at the surface. The only feature present is a thin oxide layer, which is not a concern. If necessary, it can be removed using standard surface cleaning methods. These are my observations based on the information you provided.

Thanks,

------------------------------
Sanjay Kulkarni
Materials Engineer
MSSC, NA
1965 Research Drive
Troy, MI 48083
sanjay.kulkarni@msscna.com
Cell: 248-840-1056
------------------------------

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