Heat treatment defect??

[limct]

Hi all,
I'm facing this problem, can anyone help?
Required specs....
material: JIS SNCM220 (AISI 8620)
carburised to achieve:
surface hardness 600~700 HV
EHT 550 HV at 0.3~0.5mm
core hardness 390~450 HV

Obtained hadness readngs....
at 0.05mm : 646 HV
at 0.1mm : 585 HV
at 0.2mm : 470 HV
at 0.3mm : 384 HV
at 0.5mm : 336 HV
at 0.6mm : 298 HV
at core : 426 HV

I can tell that the case depth is out of specs, and that the core hardness is reasonably good, but I do not understand why it's so "soft" at the depth of 0.3~0.6mm.

Thank you.

Best regards,
ct

 

[metengr]

limct;
From the looks of your hardness profile, it would appear that during carburizing you had either reduced or lost your carbon potential on the surface of the charge during the process of diffusing the carbon into the substrate.