Applying the rapid infrared joining technique, the authors have successfully joined steels, Inconel[sup [reg sign]]-type nickel-based alloys, titanium alloys, carbon/carbon composites, titanium matrix composites, and titanium aluminides. In this paper, a series of joining experiments with 4340 steel is reported. Joining of mild steels has been looked into rather thoroughly. However, all of the studies are for conventional long-duration brazing. Among some of the most recent investigations on the joining of mild steels, the effect of time and temperature on the bond shear strength of the joined steels was reported. In those investigations, the nickel-based brazing alloy was a popular choice to provide strong joints. It has been reported that it is possible to obtain high-temperature brazed joints which are generally free of embrittling hard phases. In the present study, they are concentrating on the application of the infrared joining technique. The objective is to investigate rapid infrared joining of 4340 steel with a nickel-based brazing alloy, AMS 4777. The effect of the bond thickness on the bond shear strength, the microstructure of the joint, and the bond shear strength as a function of overlap were emphasized.