Tom,
Not sure what they're getting at with that either. I'm guessin its an old spec that nobody has bothered to take a look at in a long time and it just keeps gettin handed down.
And its actually more perplexing than your comment would indicate, in my opinion.
If there was a concern for toughness you would think that A/SA 106 would not be the choice since this is a material intended for high temp service where toughness is not an issue and the material is generally furnished in the hot finish condition, which means large grain size.
The grain size of the HAZ will not grow much, if at all, its already pretty big, and higher heat inputs will actually tend to expand the width of the HAZ and contribute to a reduction in grain size over a larger area upon cooling. Used this trick in Section III impact testing. Go get big grain materials to beat the UBM with impacts.
And there isn't even a supplementary impact test for this material.
Controlling weave, and perhaps intended heat input, or whatever, seems to demonstrate a lack of understanding of the material of choice.
Now, having said that you will still most likely get decent impacts at moderate temps with this stuff (possibly even fairly low) simply because steel makers are better at their business than they used to be and the stuff is cleaner.
Also, seems to me that if toughness was a concern a better control would be actual heat input.
I'm thinkin the weave limitation is a dinosaur.