yes.. check the codes...
The ACI 318-99 Building Code Requirements for StructuralConcrete (which is referenced by all
building codes in the US) states:
"7.5.4 -- Welding of crossing bars shall not be
permitted for assembly of reinforcement unless
authorized by the engineer."
"R 7.5.4 -- 'Tack' welding (welding crossing bars) can
seriously weaken a bar at the point welded by creating
metallurgical notch effect. This operation can be
performed safely only when the material welded and
welding operations are under continuous competent
control, as in the manufacture of welded wire fabric."
If you use ASTM A 706 reinforcing bar then no preheat is required for welding.
most standard rod(ASTM A 615) requires 300 degrees F for a minimum of 20 minutes prior to welding.
However note that the intent of welding reinforcing
steel is focused on direct splices and connections to
other metal devices to be embedded in the concrete, not
for rebar cage assembly or tackng in lieu of wire tie..
but then again I dont think you are getting a permit for this anyway .. :-)http://www.k1ttt.net/technote/welding.html