ISO 17660 Welding of Reinforcing Steel: Standard Scope, Joint Types, and Qualification Requirements

What Is ISO 17660 and Why Does It Apply to Rebar?

ISO 17660 — formally DIN EN ISO 17660 in Germany — is the international standard for the welding of reinforcing steel. It was developed to address a specific challenge: reinforcing steel bars are not structural steel in the conventional sense. Their chemical composition is optimised for ductility and bond performance in concrete, not for welding. When rebar is welded without proper control, the heat-affected zone can become brittle, eliminating the ductility that is critical to a structure’s seismic and overload performance.

ISO 17660 is a two-part standard:

• ISO 17660-1: Load-bearing welded joints — covers welding that forms structural connections carrying direct stress (e.g. butt welds, T-joints in cages, cross-welded joints that must resist tension or shear forces)
• ISO 17660-2: Non-load-bearing welded joints — covers assembly welds used only to hold bars in position during concrete placement (e.g. tack welds on stirrups and links), which do not carry design loads

The distinction matters for qualification: Part 1 demands full Welding Procedure Specification (WPS) qualification and welder certification; Part 2 allows simpler qualification routes for assembly tacking.

Weldability of Reinforcing Steel: The Carbon Equivalent (Ceq)

Not all rebar is weldable. The key parameter is the carbon equivalent (Ceq), calculated from the chemical composition. EN 10080 and DIN 488 define weldability as a property of the declared grade, not as an inherent guarantee for every heat. The standard formula used is:

Ceq = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

For B500B (hot-rolled, high-ductility) reinforcing bars, a maximum Ceq of 0.50% is the widely accepted threshold for weldability under standard conditions (no preheat). Actual Ceq values for B500B typically fall in the range 0.40–0.50%, making the grade weldable — but this must be confirmed from the MTC chemical analysis for the specific heat, not assumed.

B500A (cold-rolled) and B500C (seismic) grades require specific attention: cold-worked bars can lose ductility properties if welded, and B500C’s tighter k-ratio window means the metallurgy must be verified before any welding is specified.

ISO 17660-1: Load-Bearing Welds — Qualification Requirements

For load-bearing welded joints, ISO 17660-1 requires:

• A qualified Welding Procedure Specification (WPS) based on a Welding Procedure Qualification Record (WPQR) — destructive testing of qualification welds demonstrates joint ductility and strength
• Welder qualification per ISO 9606-1 (fusion welding of steel), specific to the joint type, position, and process (MAG, MMA, etc.)
• A Responsible Welding Coordinator (RWC) on site with appropriate qualifications per ISO 14731
• Verification that the steel grade and Ceq are within the range qualified by the WPS
• Post-weld inspection: visual inspection is mandatory; NDT (e.g. magnetic particle or radiographic inspection) may be specified by the engineer

Mechanically spliced joints using threaded rebar couplers are often preferred over welded butt joints for load-bearing connections precisely because they avoid the heat-affected zone issue entirely and offer guaranteed performance up to the bar’s full yield capacity.

ISO 17660-2: Non-Load-Bearing Assembly Welds

Part 2 covers tack welds and assembly welds that position bars in the cage but do not carry structural loads. The qualification requirements are less demanding:

• A documented welding procedure is still required, but full WPQR testing is not mandatory
• Welders must be trained and competent but formal ISO 9606-1 certification is not compulsory
• Visual inspection is the primary quality check

Important caveat: even assembly tack welds can damage rebar if they are too large, improperly placed at bend points, or applied to cold-rolled bars. The weld should never be at a location where the bar is expected to deform plastically under seismic or impact loading. Project specifications often restrict tack welding on B500C seismic grade for exactly this reason.

Welding Processes Covered by ISO 17660

Flash butt welding (process 24) is commonly used in mills and prefabrication shops to produce extended-length bars or to join coil ends. It is a high-energy, short-cycle process that produces a forged joint; when qualified per ISO 17660-1, it can achieve full-strength connections. Flash-butt welded joints on rebar can be supplied as part of a cut-and-bend service — for details see Rebar Stirrups & Cut-and-Bend.

Implications for International Buyers

If your project specification requires welding of rebar on site or in a prefabrication yard outside Germany, confirm that the local welding contractor holds qualifications under ISO 17660 (or the local equivalent). The steel itself — supplied with DIN 488 / EN 10080 documentation and a Ceq on the MTC — gives you the material foundation. The fabricator’s qualification provides the process assurance. Both must be in place before welded joints are incorporated into a structure. For more on our full standards framework, or to request a quote for weldable B500B, please get in touch.