>>...but there is a consideration that makes me think
>>replacing with Grade 8 is sound>>
  The thing I'd still be concerned about is a sudden, sharp blow.
  While the Grade 8 ratings may be good, remember that it is more "brittle".
Loads put on smoothly and gradually may be plenty survivable, but how about
that sudden, sharp blow?
  You can put a fair amount of weight on an arch made of glass, but do not
try tapping it with a hammer.
  Machine tool steel is -very- hard, but I've heard it can shatter when
dropped on a concrete shop floor.
    Ed in NC
  Ever hear of a cat with insomnia?
 |