If you utilize UT, and there is a great enough transition in metallurgical conditions in the transition areas you may see a signal return coming out of the transition zone.
The technician will need to be watching his or her RF window for phase transitions to help them determine if this is a flaw or an interface signal. The probability of that can be checked by running a few transmission coefficient calcs. DM weld inspection via UT requires an experienced technician that knows what they are doing. Without having that particular set up and combination of materials and their specific velocities I can't tell you from here if it will or won't, but I did run a few calcs that suggest it is probable they will see a transition signal around 10-15 percent worth of DAC. If that is the case, knowing the constituent material velocities will be critical.
http://www.ndt.net/article/wcndt2004/pdf/materials_characterization/138_gur.pdfHere is a study performed on 1040 and 4140 that has some application to yours. Take note on page 5 the change in velocities depending on the heat treat.
The conditions your describing have apparently been enough to create a permeability change local to the transition zone that is creating an MT indication. I believe the conditions in the linked study will be less severe than your conditions if that is the case, so keep that in mind when your performing that exam.
Overall I don't think you have another option other than UT to get any meaningful NDE results from that weld if those conditions are as stated.