I don’t know what the actual status is, but there used to be a special clause allowing a maximum 23HRC instead of 22HRC for cast and wrought CA-6NM. The conversion from HRC to HBN was another source of debate. Documented conversions: 22HRC->256HBN and 23HRC->262HBN specially for CA-6NM.
From what I found even 262HBN (23HRC) is very difficult to attain. C should be below 0.03% and NACE specifies the following heat treatment:
austenitise at 1010°C min, air cool or oil quench at room temperature
temper at 675°C, air cool to room temp.
temper at 600°C, air cool
This treatment does not guarantee attaining a hardness of less then 23HRC (or 262HBN) for C<0.03%!
All this is based on old data (1990’s). Possibly things have changed in the mean time.
The material specified for your casting is a
corrosion resistant martensitic type with about
0.6%C 13%Cr 4%Ni and 0.4%Mo
General recommendations for heat treatment of this
type of stainless include:
1) - Homogenization above 2000 F (1090 C) to promote
uniformity of chemical composition and of microstructure:
this process is used to diminish the presence of alloy
segregation and of dendritic structure, especially in heavy
sections.
2) - Austenitizing at 1800 to 1850 F (980 to 1010 C)
and air cooling for hardening.
3) - Tempering to take place immediately, before parts reach
room temperature, to suit hardness requirement,
possibly 1150 F (620 C).
Tempering range from 700 to 1100 F (370 to 590 C) is to be
avoided because it produces low impact values.
Double tempering is recommended.
As "gieter" indicates - even after following the recommended cycle in NACE MR 01 75 odd high hardness readings can sometimes occur - some trials have shown that slowly bringing down the tempering temperature in three or four steps can be beneficial, 675, 650, 625, 600. This however adds cost and if you want to be really picky doesn't actually conform to the NACE cycle.
Depending on your customer you might want to ask whether they want NACE hardness or NACE cylce?
2 stage tempering is beneficial,but slowly cool in the furnace till you reach 100C (do not open the furnace door). Sometimes due to cold blast of air at higher temperatures higher hardness result.
It is very essential to hold carbon under 0.03% the lower the better.
Austenatize at 1900F and Air cool to Room temperature. Double temper is recommended; 1250F , air cool to 60F min (important) and 1150F, air cool to room temperature. Longer holding times at both temperatures yeild lower hardness values.
The % carbon coupled with the above heat treatment will typically develop a hardness in the range of 228 - 248 BHN @ 3000kgf - 15 sec. Measure the hardness in BHN.Conversion from BHN to Rc is not recommended since there is very little known about the relation netween BHN to Rc with respect to CA6NM. Per NACE, the maximum hardness desired for sour environments is 255BHN. The above heat treatment follows NACE, ASTM A487 and A352.
