The tension force is M/3.6" +/-, and this is divided by the length of your weld, to get a shear flow of lbs./inch on the weld. The tensile strength of the weld is {.6*F(EXX)}{the weld throat} and the weld throat for a fillet is {.707(D/16)}. Draw a full scale detail of your weld in that groove btwn. the plate and the HSS, and you can measure the throat, it is the shortest cross sectional dimension through the weld. Keep in mind that your welder will not likely get all the way down in that groove with good sound weld, and he has trouble getting good penetration into either piece of parent metal down there. The difficulty or uncertainty is that you tend to get a very poor weld quality at the root of the weld in that detail. A second poor condition is at the start and stop points in the length of the weld where weld quality can be inferior, and you may have some prying (stretching, twisting action) of the weld at this high stress location with the stress raisers caused by the inferior weld. It’s one thing if this is a static problem and quite another if it is a fatigue sensitive detail, but you must pay particular attention to the quality of the weld root in that detail, which leads to your effective weld throat, and account for it in your design. You can get cracking starting from the poor root condition.