Fastener hardness, threads vs shank

I am hoping to get a little real world experience on fastener heat treating (12-18mm diameter range). I have a family of custom bolt and the manufacturer claims (and data supports) that the threaded section is always softer than the shank. I have been unable to find anything in my searches supporting that this is always the case but it certainly is for them. Has anyone spent significant time investigating bolt hardness that can substantiate this claim? I can reason in my head why it could go both ways but don't have the data to support it. I'm going to begin some investigation on micro-structure to understand what's happening but thought I'd see what others have experienced.

The difference we are seeing is small, on the order of 0.5 to 1 HRc.

Pete

I'm not aware of any standard that allows for a significant difference in hardness between the threads and grip (shank). Fasteners are normally cut in an un-heat treated condition and hardened and heat treated as a final condition.

Ask your supplier for the applicable industry standard.

Thanks for the response Kelly. ASTM F606 Section 3.2.5 states that “the difference between the mid-radius and core shall not be more than 3 points on a Rockwell C scale”. Section 3.1.3 defines the “Arbitration Test Location” which is in the threads. The supplier believes this is due to the inherent hardness difference, I suspect it’s more likely due to the fact that this is the highest stressed region of a fastener. I’m mostly interested in if the suppliers claim that threads will always be softer can be substantiated, there is no mention of this in the spec.

Pete

I would tend to agree with your statement that it has to do with the higher stress in the threaded region. Is the "supplier" giving you this information a manufacturer or a distributor?