420A SS heat treat and passivation question 2

[J.Godby]

BACK STORY:

The blade has been traditionally produced using 420B (.3% carbon). Passivated to ASTM A9767-13, and HEAT TREAT AND DOUBLE TEMPER TO 50-56 Rc. AND CRYOGENIC TREAT TO -90 F OR LOWER
FOR 2 HOURS (MIN.)

420A is readily available (.2% carbon). 420B is next to impossible to find currently: how much of a variance can I expect from a .1% difference in carbon content? The customer print only says 420, so we are good there. What type of a time difference if any will there be required in heat treat?

 

[EdStainless]

You will need a lower temper temperature to reach that hardness, and it might be a big ask for 0.2C.
If I look at the ATI datasheet I don't think that you can reach HRC50 with 0.2C.
They show 420 with 0.25C only making 48HRC with a 400F temper.
What is your process sequence?
Aust and quench, temper, cryo, temper, passivate?

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

lower hardening temperature by 50F (from 1900F to 1850F, temper at 400F yield a min of 50HRC, temper at 300F yields a min 53HRC. Lower carbon in 420A asks for a lower hardening temperature because it
decreases the α → γ transformation temp, lower temp also limits austenite grain growth. In hte meanwhile, the martensite transformation temp will increase, you may be able to eliminate the subzero cool.