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Heat Treat Industry News

February 18, 2010

Variable Hardness Levels for A2 Tool Steel

Original Question:
We are a small machine shop and have some A2 tool steel measuring 1/8" thick by 3" square that we have heat treated to 50 HRC. We then wire EDM these pieces but didn't perform a stress-relief temper. I need to lower the hardness. Will stress-relief tempering do this for me?

Original Answer:
A2 is an air-hardenable, deep-hardening (i.e. high-hardenability) tool steel with the ability to resist softening at fairly elevated service temperatures. It offers a combination of good wear resistance and toughness (Figs. 1 & 2) and has good resistance to chipping. Tempering, often several times, tends to convert the (relatively high percentage of) retained austenite present after hardening to tempered martensite.

A2 tool steel is typically hardened to 60-62 HRC and then tempered to final hardness. I assume the 50 HRC hardness you mention is the result of this process. A (stress-relief) temper in the range of 1100-1200°F (595-650°C) will lower the hardness, but I suspect only into the range of 44-46 HRC.

Follow-up Question:
We need three different hardness ranges (see below). Can we do this by changing the temper range?
34 – 43 HRC
44 – 46 HRC
47 – 50 HRC
Follow-up Answer:
Tempering charts indicate that you will be around 46 HRC at 1175°F (625°C) and 44 HRC at 1200°F (650°C) and perhaps as low as 42 HRC at 1250°F (675°C). Your results may vary somewhat since they are chemistry dependent. You really can't achieve less than 42 HRC by tempering. The recommended stress-relief temperature range is 1200-1250°F (650-675°C) soaking for one hour per inch of cross-sectional area.

by Daniel H. Herring - February 17, 2010

Daniel H. Herring
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

1. Tool Steel and Specialty Steel Selector, Crucible Steel


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