ASTM A487 CA6NM Cl B and Age Hardening

Residual austenite. Might not have been properly hardened and tempered. Check the hoding times and the micro structure after the tempering about the %RA (retained Austenite)

The errors on the observations may also be measurement errors.

Both were done by the same instrumentss, same operator etc ?

Surface conditions same ?

The possibility exist that the first reading being on the higher side of the tolerance, the second one going out was highlighted.

You are likely to have the micro results too, why not compare them (before and after for the RA?)

The material was austenized at 1875F and hold for 3 hrs. Air cooled.

then 1st tempered at 1255F. Hold for 5 hrs. Air cooled

2nd tempered at 1125F. Hold for 5 hrs. Air cooled

I also suspect the Cu. Form the analysis reulst, 0.32% of Cu. Though it is still in spec per ASTM A487. It is an austenizing element. Did it cause the residuel austenite?

The Copper although a gamma stabiliser, may not have that effect since it is well within the max (0.50%)

The Quenching in the first step is done in Air. Please check the Critical Cooling rate is maintained or not (I don't have the data on hand for A 487 though it does permit Air and Liquid quench media). This could have easily come out any way by the Retained Austenite.

The double tempering is also in line with the recommendation and I am not finding any abnormality in the cycle.

SB,

Thank you for the info! I will do more investigation at the vendor.

In the meanwhile you can also check up

1. The measurement method and apparatus at both measurements (The first one is likely the vendor instrument and the second one your own). I was reading some paper on the hardness measurement and the gage R&R may become very poor if proper precautions are not taken.

2. Normally it is not-mentionable but were the surfaces well prepared before measuring ?

4. The hardness mapping of the casting at various points will give some idea. i don't know the size of the casting, but as per our in house standards, the hardness checking points are defined (in the report sketch is given) and each point are reported on the sketch. This ensures proper treatment across various wall thicknesses.

5. In the age hardening the decomposition of the RA will not complete in the small duration 7-9 days. Is the hardness still changing?

6. What was the previous experience with the same vendor/ other vendors.

7. A387 standard says in table 3 note B "The conversion is based on CA6NM test coupons"

The casting as per spec is supposed to be almost purly martensitic. You may still check for the presence of Austenites.

The part was checked twice by us at the same places within 10 days, and we found this increasing hardness issue. Surface was well prepared. I don't have much history with this vendor. Will conduct more serach on this.

Thank you very much!