The short answer is that in most cases, welding high strength bolts is not allowed. In the fastener industry, the term “high strength” typically refers to any medium carbon or alloy steel which undergoes a heat-treating process to develop the strength properties necessary to meet the requirements of a given specification. These ASTM specifications include A449, A325, A193 GRADE B7, F1554 GRADE 105, A354 GRADES BC AND BD, AND A490 among others. When heat is reapplied to a bolt that has been heat-treated, it is likely that the physical properties (strengths) of the bolt may be altered. When heat is applied in an uncontrolled environment, it is impossible to determine what effect this application of heat has had on the fastener. Therefore, welding to high strength bolts is not recommended.
The issue of altering the high strength bolt’s mechanical properties when welding can potentially be avoided by performing the welding operation prior to the fastener undergoing the heat-treating process. In other words, a plate, a nut or other component could potentially be welded to a bolt prior to the fastener being heat-treated. The problem is, the base medium carbon or alloy steel used to make high strength bolts is technically not weldable due to high levels of carbon and manganese. This could perhaps be overcome with special welding procedures, but the bottom line is that it would be best to avoid welding high strength bolts altogether.
For anchor bolt applications, instead of welding a nut and/or plate to the bottom of a high-strength anchor rod, consider using an anchor bolt with a forged hex head, peening the thread to prevent the nut from backing off, jamming two nuts together to lock them in place, or sandwiching a square plate between two nuts on the embedded end of the anchor rod.