Welding Rebar: A706 vs A615, AWS D1.4, and When You Shouldn't

Q: "Can you weld rebar?"

"It depends entirely on the grade. ASTM A706 is the weldable reinforcing bar grade, with a controlled chemistry and a maximum carbon equivalent that AWS D1.4 is written around. ASTM A615, the most common rebar in the yard, has no carbon equivalent limit in its standard chemistry, so a given bar can be anything from easily weldable to crack-prone, and you cannot tell by looking. For any structural or load-bearing application, you weld A706, you follow AWS D1.4, and you do it under the engineer of record. For a garden trellis or a sculpture armature where nothing depends on the joint, the stakes are different, but you still control your fume and heat."

Q: "What is the difference between A706 and A615 rebar?"

"A615 (ASTM A615) is plain carbon-steel rebar made primarily to meet strength requirements, and its standard chemistry does not cap the carbon equivalent. A706 (ASTM A706) is low-alloy rebar made to be welded, with tighter limits on carbon, manganese, and phosphorus and a maximum carbon equivalent of 0.55 percent for bars No. 6 and smaller (and 0.55 for larger bars under the supplementary requirement, verified on the mill certificate). A706 also has more controlled mechanical properties, which matters for seismic detailing. If a drawing calls for welded splices, it almost always calls for A706."

Q: "Does A615 rebar always need preheat to weld?"

"When A615 is welded at all under AWS D1.4, preheat is mandatory and the amount is driven by the carbon equivalent calculated from the actual mill certificate, not a guess. Because A615 chemistry is not controlled for weldability, the carbon equivalent can be high enough to require substantial preheat to avoid hydrogen cracking. This is exactly why A615 is avoided for welded structural work and A706 is specified instead. Do not weld A615 splices in a structure without the mill cert, a D1.4 procedure, and the engineer's sign-off."

Published June 17, 2026 · Updated June 18, 2026 · 8 min read

Most rebar in the yard is ASTM A615, and most A615 should not be welded in structural work. Its standard chemistry has no carbon equivalent limit, so one bar can weld fine and the next can crack, and you cannot tell which is which by eye. The weldable grade is ASTM A706, governed by AWS D1.4 (Structural Welding Code, Reinforcing Steel). If a drawing calls for welded rebar splices, it calls for A706, mill certs, controlled preheat, and the engineer of record. Tying with wire is the default for a reason.

Can You Weld Rebar? The Grade Decides

The honest answer to “can you weld rebar” is: only if you know the grade, and for structural work that grade is A706.

Reinforcing bar exists to be tied into a cage and embedded in concrete. The standard way to join bars is mechanical: lap splices, wire ties, or mechanical couplers. Welding is the exception, not the rule, and it carries a specific set of code requirements because the consequences of a brittle, cracked splice in a column or a footing are not cosmetic.

The trap is that A615 and A706 look identical. Both are deformed bars in the same diameters. The grade is rolled into the bar markings and stated on the mill certificate, and that paperwork is the only reliable way to know what you are holding.

Property	ASTM A615 (carbon steel)	ASTM A706 (low-alloy, weldable)
Made for welding?	No	Yes
Carbon equivalent limit	None in standard chemistry	Capped (typically 0.55% max)
Chemistry control	Strength-driven, loose limits	Tight limits on C, Mn, P, S, V
Mechanical properties	Less controlled	Controlled tensile-to-yield ratio
Common use	General concrete reinforcement	Welded splices, seismic detailing

A706 is the bar you reach for when the connection has to be welded. It is a low-alloy steel with a controlled carbon equivalent and a controlled ratio of tensile to yield strength, which is why seismic codes lean on it. A615 is the general-purpose bar that fills most footings, slabs, and walls where the splices are lapped, not welded.

What AWS D1.4 Actually Requires

AWS D1.4, the Structural Welding Code for Reinforcing Steel, is the document that governs welded rebar in structures. It is a separate code from AWS D1.1 (which covers structural steel shapes and plate), and it exists because rebar chemistry and joint geometry behave differently from rolled structural sections.

D1.4 covers the same kinds of things you would expect from any AWS welding code, applied to bar:

Acceptable base metals and the carbon equivalent calculation that drives preheat
Qualified welding procedure specifications (WPSs) for direct and indirect butt splices, lap joints, and rebar-to-steel connections
Welder qualification on the joint type and position used in production
Preheat and interpass temperature requirements tied to bar size and carbon equivalent
Inspection and acceptance criteria

The carbon equivalent is the center of gravity in D1.4. The code gives a formula based on the bar chemistry from the mill certificate, and the result sets the minimum preheat. Higher carbon equivalent means higher preheat to slow the cooling rate and keep hydrogen-induced cracking out of the heat-affected zone. With A706 the chemistry is capped, so the preheat stays in a workable range. With A615 the carbon equivalent can land high enough that the required preheat becomes impractical, which is one more reason A615 is the wrong starting point for welded structural splices.

You do not assume the carbon equivalent. You calculate it from the actual mill cert for the actual heat of steel you are welding. No mill cert, no carbon equivalent, no procedure.

Direct vs Indirect Butt Splices

D1.4 recognizes more than one way to join bar end to end:

A direct butt splice joins the two bar ends to each other, typically a single-V or double-V groove on larger bars, often with a backing.
An indirect butt splice uses a splice member (a piece of steel) that both bars are welded to, transferring load through the splice member rather than a direct bar-to-bar weld.

The choice is the engineer’s, made on the drawings, based on bar size, position, and how the load has to transfer. It is not a field decision.

When You Should Not Weld Rebar

This is the part that matters most, so it gets stated plainly. Do not weld A615 rebar splices in a structure. Do not weld any rebar splice in a structure without the mill certificate, a qualified D1.4 procedure, the required preheat, and the engineer of record’s approval. Tack welding rebar to “hold the cage together” is a common shortcut on jobsites and it is not harmless: a tack at a crossing point can create a stress riser and a brittle spot in the bar, and many specifications prohibit it outright unless the bar is weldable and the welding is part of an approved procedure.

A few specific traps:

Welding A615 because it was what the yard had. The grade has to match the drawing and the procedure.
Welding to galvanized or epoxy-coated bar. The coating produces fume and disrupts the arc, and welding through it is its own controlled situation. The zinc on galvanized bar releases fume that can cause metal fume fever, the same concern covered in our galvanized welding coverage.
Field-welding splices in seismic systems without the seismic detailing the engineer specified. A706 exists partly for this reason, and the code requirements are stricter, not looser.

If the drawing shows welded splices, that work belongs to a qualified welder following a written procedure, witnessed and inspected, exactly the way it does for structural welding procedures on rolled shapes. The acceptance call on a welded splice is made against the governing code and by the engineer of record, not by the person running the bead.

Non-Structural Rebar: Trellises, Sculpture, and Shop Fixtures

There is a whole other world of rebar welding where nothing structural depends on the joint: garden trellises, plant cages, abstract sculpture armatures, shop carts, and rough jigs. Here you are not under D1.4, because nothing is carrying a calculated load into concrete or holding up a building.

That does not mean chemistry stops mattering. A615 of unknown carbon equivalent can still crack at the weld, especially on heavier bar that cools fast. For light decorative work a few practical habits keep you out of trouble:

Grind off the mill scale and any rust at the joint so the arc has clean metal to fuse.
Run a hot enough setting to get real fusion. Cold, sticking-on-top welds are the usual failure on rebar.
On heavier bar (No. 5 and up) a little preheat with a torch slows the cool-down and reduces cracking risk, even on a trellis.
Expect spatter and a rougher bead than you get on clean plate. The deformations and scale make rebar a dirty thing to weld.

A615 of unknown chemistry is fine for a trellis because the trellis does not care. The same bar in a column splice is a problem because the column does care. That is the whole distinction.

How to Weld A706 Rebar Splices (Under a Procedure)

When the work is structural, A706, and covered by a D1.4 procedure, the mechanics look familiar to anyone who has run groove welds. The procedure document tells you the specifics. In general terms:

Confirm the grade and chemistry. Read the bar markings, match them to the mill certificate, and confirm the carbon equivalent the procedure was written for.
Prep the joint. Bevel and fit per the WPS. Clean the bar of scale, rust, oil, and any coating in and around the joint.
Preheat to the procedure temperature. This is set by the carbon equivalent and bar size, not by feel. Verify with temperature-indicating crayons or a contact pyrometer.
Weld per the WPS, holding interpass temperature, using the specified filler (commonly a low-hydrogen electrode such as E7018 or an equivalent low-hydrogen process), and depositing the passes the procedure calls for.
Let it cool in still air. You never water-quench a structural weld to speed things up. Fast cooling is exactly what drives hydrogen cracking.
Inspect. Visual inspection at minimum, with additional NDT if the spec requires it. Surface methods like dye penetrant testing or magnetic particle testing catch surface cracks the eye can miss.

Low-hydrogen practice carries straight over from structural plate welding: dry electrodes, clean joints, and preheat are how you keep hydrogen out of the weld and the heat-affected zone. A cracked rebar splice is a rejectable defect under the code, and the disposition of a rejected splice is the engineer’s call following the code’s repair provisions, not a grind-and-cap fix.

Common Problems Welding Rebar

Problem	Likely Cause	What to Check
Cracks in the weld or HAZ	High carbon equivalent, no/low preheat, hydrogen	Mill cert chemistry, preheat temp, electrode dryness
Lack of fusion / cold weld	Heat too low, mill scale not removed	Amperage per the WPS, joint cleanliness
Heavy porosity	Coating, rust, or contamination at the joint	Grind to bright metal, confirm bar is not coated
Splice rejected by inspector	Wrong grade, no procedure, profile or size out of spec	Grade vs drawing, WPS, D1.4 acceptance criteria

The recurring theme is that almost every serious rebar welding problem traces back to one of two things: the wrong grade for the job, or skipping the carbon-equivalent-driven preheat. Get the grade right and respect the preheat, and the welding itself is ordinary low-hydrogen work.

This page is general reference information, not engineering direction for a specific structure. Welded reinforcing steel is governed by AWS D1.4 and the engineer of record, who has final authority on grade, joint type, procedure, and acceptance. For the broader code picture on structural welding, see our AWS D1.1 structural welding code overview, and for the rest of the silo, head back to structural steel welding.

Frequently Asked Questions

Can you weld rebar?

It depends entirely on the grade. ASTM A706 is the weldable reinforcing bar grade, with a controlled chemistry and a maximum carbon equivalent that AWS D1.4 is written around. ASTM A615, the most common rebar in the yard, has no carbon equivalent limit in its standard chemistry, so a given bar can be anything from easily weldable to crack-prone, and you cannot tell by looking. For any structural or load-bearing application, you weld A706, you follow AWS D1.4, and you do it under the engineer of record. For a garden trellis or a sculpture armature where nothing depends on the joint, the stakes are different, but you still control your fume and heat.

What is the difference between A706 and A615 rebar?

A615 (ASTM A615) is plain carbon-steel rebar made primarily to meet strength requirements, and its standard chemistry does not cap the carbon equivalent. A706 (ASTM A706) is low-alloy rebar made to be welded, with tighter limits on carbon, manganese, and phosphorus and a maximum carbon equivalent of 0.55 percent for bars No. 6 and smaller (and 0.55 for larger bars under the supplementary requirement, verified on the mill certificate). A706 also has more controlled mechanical properties, which matters for seismic detailing. If a drawing calls for welded splices, it almost always calls for A706.

Does A615 rebar always need preheat to weld?

When A615 is welded at all under AWS D1.4, preheat is mandatory and the amount is driven by the carbon equivalent calculated from the actual mill certificate, not a guess. Because A615 chemistry is not controlled for weldability, the carbon equivalent can be high enough to require substantial preheat to avoid hydrogen cracking. This is exactly why A615 is avoided for welded structural work and A706 is specified instead. Do not weld A615 splices in a structure without the mill cert, a D1.4 procedure, and the engineer's sign-off.