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I am in heat treating + forging business and we also use Ni-Cu Ratio to prevent hot cracking. The ratio is applied for alloyed steel and 0.20 max in plain carbon steels.

Stephen and co -

Please see below.

- Jim

Stephen,
It is often not simple to look up the reasoning that was applied to specification language, particularly for older specifications or even from newer ones that had material adopted from older ones.  Those sorts of details don't always make it into the archives of the Committees that write the ASTM Specifications.  I have on more than one occasion called Committee members to try to track down the reasons for this sort of question, and have not been rewarded with an answer in some instances.
The comments made about copper grain boundary liquation issues, do have basis in fact.  I did once run into a situation where someone had left surface copper from tooling or a hammer on a piece that was then heated for bending, and ended up with cracking which I identified during metallography.
Both elements do affect hardenability so to keep that characteristic within a predictable range they might be limited.  As well, I seem to remember some very old specifications that called out Ni+Cu, which I suspected were being analyzed with a method that didn't separate them well, so it was easier to just disallow any combination of them over that limit.  These specs are/were written with whatever the current industrial practice is/was for analytical methods. 
The A681 seems to be an "umbrella" specification to some extent, and I have found that broad 'good practice' type requirements are often placed in such which can be further modified by subsidiary, narrower specifications for certain end uses/product forms.  I found the same Ni+Cu statement in a 1994 version, and see that the original specification was from 1973.
Your question made me look for my old Tool Steel book, which was out of print (by the 1970s) so my professor had to get permission to photocopy it for us.  However unfortunately I cannot locate it at the moment.  I hope my comments are of value.

Stephen,
It is often not simple to look up the reasoning that was applied to specification language, particularly for older specifications or even from newer ones that had material adopted from older ones.  Those sorts of details don't always make it into the archives of the Committees that write the ASTM Specifications.  I have on more than one occasion called Committee members to try to track down the reasons for this sort of question, and have not been rewarded with an answer in some instances.
The comments made about copper grain boundary liquation issues, do have basis in fact.  I did once run into a situation where someone had left surface copper from tooling or a hammer on a piece that was then heated for bending, and ended up with cracking which I identified during metallography.
Both elements do affect hardenability so to keep that characteristic within a predictable range they might be limited.  As well, I seem to remember some very old specifications that called out Ni+Cu, which I suspected were being analyzed with a method that didn't separate them well, so it was easier to just disallow any combination of them over that limit.  These specs are/were written with whatever the current industrial practice is/was for analytical methods. 
The A681 seems to be an "umbrella" specification to some extent, and I have found that broad 'good practice' type requirements are often placed in such which can be further modified by subsidiary, narrower specifications for certain end uses/product forms.  I found the same Ni+Cu statement in a 1994 version, and see that the original specification was from 1973.
Your question made me look for my old Tool Steel book, which was out of print (by the 1970s) so my professor had to get permission to photocopy it for us.  However unfortunately I cannot locate it at the moment.  I hope my comments are of value.

Thank you for the additional comments.

Hearing that the reasoning for certain requirements in specifications is not retained or documented is concerning.  I fear that would lead to a situation where a requirement is enforced, but not understood, even if there is a sound metallurgical basis to deviate from it.  We would then be on a path of constantly adding on requirements, possibly unnecessarily increasing cost.  Much the same thing happens in government specifications, in my experience.

When specifications come up for review, are requirements ever removed?

In the case of Ni+Cu, I am in total agreement with the effect of Cu on hot workability; that is well known.  But I would think increasing the Ni content is fairly harmless, and could promote the formation of retained austenite if lower temperature toughness was desired.  Most of the users of this material in my world are tool shops that don't necessarily understand the metallurgy; they just want their material and for it to work for whatever it is they are doing.

Stephen,
It is often not simple to look up the reasoning that was applied to specification language, particularly for older specifications or even from newer ones that had material adopted from older ones.  Those sorts of details don't always make it into the archives of the Committees that write the ASTM Specifications.  I have on more than one occasion called Committee members to try to track down the reasons for this sort of question, and have not been rewarded with an answer in some instances.
The comments made about copper grain boundary liquation issues, do have basis in fact.  I did once run into a situation where someone had left surface copper from tooling or a hammer on a piece that was then heated for bending, and ended up with cracking which I identified during metallography.
Both elements do affect hardenability so to keep that characteristic within a predictable range they might be limited.  As well, I seem to remember some very old specifications that called out Ni+Cu, which I suspected were being analyzed with a method that didn't separate them well, so it was easier to just disallow any combination of them over that limit.  These specs are/were written with whatever the current industrial practice is/was for analytical methods. 
The A681 seems to be an "umbrella" specification to some extent, and I have found that broad 'good practice' type requirements are often placed in such which can be further modified by subsidiary, narrower specifications for certain end uses/product forms.  I found the same Ni+Cu statement in a 1994 version, and see that the original specification was from 1973.
Your question made me look for my old Tool Steel book, which was out of print (by the 1970s) so my professor had to get permission to photocopy it for us.  However unfortunately I cannot locate it at the moment.  I hope my comments are of value.

Thank you for the additional comments.

Hearing that the reasoning for certain requirements in specifications is not retained or documented is concerning.  I fear that would lead to a situation where a requirement is enforced, but not understood, even if there is a sound metallurgical basis to deviate from it.  We would then be on a path of constantly adding on requirements, possibly unnecessarily increasing cost.  Much the same thing happens in government specifications, in my experience.

When specifications come up for review, are requirements ever removed?

In the case of Ni+Cu, I am in total agreement with the effect of Cu on hot workability; that is well known.  But I would think increasing the Ni content is fairly harmless, and could promote the formation of retained austenite if lower temperature toughness was desired.  Most of the users of this material in my world are tool shops that don't necessarily understand the metallurgy; they just want their material and for it to work for whatever it is they are doing.