Is Welding Heat Treated Fasteners Allowed?

  1. Abstract

    To answer the title’s question: No. Welding on heat treated fasteners would significantly alter their microstructure in the weld and heat affected zone. This microstructural disruption would adversely affect the strength of the fastener. Therefore welding on heat treated fasteners is not advised. Welding on a fastener not specifically designed to be welded will likely void any warranty on the product.

  2. Discussion

    For the purposes of this document only steel fasteners are considered. Heat treated fasteners should be considered synonymous with high strength fasteners, e.g. SAE J429 Grade 5, ISO 898-1 Class 8.8 or above. Fastener mechanical property specifications such as ISO 898-1, SAE J429, and numerous ASTM specs, closely control their chemical composition, microstructure, hardness, tensile properties, etc., in order to assure the final fastener is structurally sound and has predictable and repeatable performance. While all aspects of manufacturing and processing have an influence on the final conditions of the fastener, the foundation that establishes its properties is the chemical composition and microstructure. If the chemistry is impure, contains inclusions, tramp elements, etc., these can adversely affect the final properties by weakening the matrix or supplying a catalyst for embrittlement. Likewise, if the microstructure is uncontrolled or has incomplete conversion to, e.g. martensite, then the final mechanical properties will be affected. To create the microstructure necessary to meet many fastener specifications (i.e. ≥90% tempered martensite) we are typically heat treating the steel by annealing (~1600 °F), quenching (e.g. oil) and tempering (800-1100 °F, depends on the fastener spec).

    When steel joints are formed via welding the harshly simplified process is: joint members locally melt at the interface and re-solidify, fusing the joint members in a permanent high-strength connection. Depending on their chemical composition, steels have a range of melting point temperatures which are typically 2500-2700 °F; note, pure iron melts at 2800 °F. Melting a steel completely resets the microstructure in the weld and alters the microstructure in the heat affected zone (HAZ). If joint members were heat treated their microstructure, and therefore their strength characteristics, would have been altered/erased within the weld and severely disrupted within the HAZ.

    Manufacturers stand behind their product 100%; if they do not, they are not a reputable manufacturer. The warrantee provided by the manufacturer is something every product user does not want to void. When a fastener is welded the part has been altered by the user; no longer does the fastener have the geometry, microstructure, strength, that it did when it was first produced. After welding, effectively the user has become the manufacturer as the user has chosen to alter the part from its original design. The exception to this would be fasteners specifically designed to be welded upon such as weld studs, weld nuts, ASTM A307 Grade A and B with supplemental requirements S1 and ASTM F1554 Grade 55 with supplemental requirements S1.

  3. Conclusions

    Welding on heat treated fasteners is not advised and may void any warranty offered by the supplier/manufacturer.

  4. Solution

    Only weld on a product if it is designed to be welded, such as weld studs, weld nuts, or if parts were made to ASTM A307 Grade A and B with supplemental requirements S1, or ASTM F1554 Grade 55 with supplemental requirements S1. While welding on a fastener is formally not advised, with few exceptions, a design engineer or consultant could define permissible welding on a case by case basis, e.g. tack welding may be minimally invasive to not cause overall structural damage to joint components.

Tyler Olson
Fastener Enthusiast
Polycrew Contributor
Caution: All information in Polycrew Articles is advisory only and the use of this information is voluntary. Polycrew has made a determined effort to present the contents accurately.