welding of rebar

huangyhg

welding of rebar
can anyone tell me why welding rebar results in a brittle weld?  what does aws d1.4 say about welding rebar?
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the alloying elements present in rebar (primarily carbon) makes the welding operation somewhat more critical than that of plate/shape steel products. the relatively high amount of carbon requires preheat and interpass temperatures higher than that of "mild steels".
preheat/interpass temps. and electrode selection are typically determined by the carbon equivalent value determined from the chemical composition of the material noted in the mill test report (mtr).
factoids:
1.) there are no prequalified joints or welds in this code. there are code approved weld and joint details. all weldments and personnel require qualification by testing.
2.) workmanship and technique are critical to welding of reinforcing steel.
3.) welding of intersecting bars is almost always prohibited.
4.) bend radii of these bars is also critical to the materials performance.
5.) essential variables are more restrictive than those of the d1.1 code.  
6.) the d1.4 code does not encompass impact testing of welds.
others can provide more insight.
i believe that there is special rebar designed for welding, or at least i re  
it is common practice to weld rebar in the uk. it is particularly useful when producing beam cages for ground beams. the cages are delivered pre-formed to site and the only manual fixing required is for the splices.
however, the cages only require spot welds to hold them together and i have never heard of any problems. if you need to join long runs of bar, it is always best to use a mechanical splice.
regards
ajuk
cwic is essentially correct.  there is weldable rebar, and if rebar is spec'd to be weldable, then preheat/interpass temps should be spec'c.
tack welding of cages can be a real problem.  a crack can initiate at one of the tack welds.
generally the steel used for rebar can be a mishmash of volkswagons or amtrack rail...
i saw a building site in europe where a man walked around with a clamp (spot) welder and used this in place of metal wire. he was moving right along so it was probably more a question of time. i believe that the reinforcement mesh is made this way (spotwelded). i would be concerned to depend on a weld in the rebar to absorb any significant loads. the steel is really low grade (recycled metal cans and cars) however spotwelds to hold it together and make the installation easier may not be a problem. how strong is a metal wire? the only potential problem could be if you weaken or embriddle the rod while welding but if all you are doing is spotwelding then i don't think that this is a problem.
astm a706 is considered "weldable" rebar. there are many types of reinforcing steel that are also weldable, but the 706 has a distinct marking of "w" (weldable) stamped into the deformations for identification. or a "w" and "s" denoting compliance with multi-grade bars (a706/a615). many think this is the only type that is weldable and will not spec anything else.
the mesh used for reinforcement is spot welded. but, depending where it is being used may require it to be inspected regularly during manufacturing by the building departments (via icbo es or equivelant watch-dog organization).
there is a rebar mill here in so cal that manufactures rebar from old cars (as noted by dik), and lots of confiscated firearms.
i have samples of rebar with cracks in the tack welds and plain old cracked welds. all were taken from jobs where the welder(s) said they knew how to weld rebar. have also been involved in an investigation regarding faulty rebar welding.
while in taiwan a few years ago i watched a tack welder run around tacking #57 bars without any preheat. he was trying to stay ahead of the pump pouring grade beams for a high rise. we walked down to the site and noticed about 1/2 of the tack welds had cracked and/or completly broke in two. he did give it his best effort though... i took numerous photos after the big one there a couple of years ago where rebar had failed cleanly through the welds. this was noted all about the structures where the epicenter of the quake occurred.
rebar is not handled and welded in many countries the way we do right here in the us.
the grade of rebar is controlled in the uk otherwise how could you design consistently with rebar that is variable grade? however, paperpete is correct that the spot welds are used to keep the rebar in position until the concrete is placed, after that the welds are irrelevant.
thanks all for the input.  my questions about welding rebar comes from this situation-
a precast double t parking garage has flange clip connections every 8' o.c. max.  these connections are all coming "unzipped".  the connection is 2 lengths of rebar embedded into the concrete t flange, then a vert. stainless steel plate is welded to the ends of the embedded rebars.  the t's are erected in the field, then the stainless plates are connected by welding a piece of rebar inbetween them.  there have been fractures of the welds from the embedded rebar to the stainless plates, and the welds from the plate-to-plate rebar connection.
my first thought was "no wonder they're coming unzipped.  a)  never weld rebar, and b) welding dissimlar materials is never a good idea, especially when rebar is one of the materials".
am i on the right track with thinking that was a terrible idea from the start, or is that generally industy practice?
if the rebar is weldable, then with care, there shouldn't be a problem.
it's common to weld clip plates to rebar for precast.  the pci manual used to show this (i don't have a copy here to confirm that this is current).  my preference for connectors is using 'headed studs'.
with a proper procedure it is possible to weld many dissimilar materials, including stainless, cast, etc.
you may want to check the rebar (might be metallurgical testing) to determine if it is weldable.  it appears that the weld itself is failing and the problem may be with the procedure used.  find out from the erector what method was used for welding and qualifications of welder.  also if you can post a note on what electrodes or type of welding was used.  if inert gas and exposed to outside breeze, etc. then may be problems with the welding equipment.
pylco:
you have shed new light on this subject. an assumption was made on my part that your application was rebar to rebar.
rebar to ss is another situation alltogether. as noted by dik, the welding process, electrodes/filler metals, shielding and workmanship/technique are paramount. it is feasible, but much is dependant on the tradesman. the metallurgical considerations increase with joining a relatively high c steel to a low c/high cr/ni material.
dissimilar metals are commonly joined. this may not be the case with regards to structural-field welding. i assume the fabricator/erector submitted a procedure (wps) and test results (pqr) prior to the field/shop welding? it really sounds like it may be a procedural issue to me. (or lack thereof.)
ajuk:
here in the us we have an ongoing trend of permitting multi-designation of materials. the other day i saw a piece of a36/a572 gr.50/a992/a709 material (there were a few more specs., but this is what i re