Evaluation of Milling Cutter Weld Processing Failures and Other Selected Projects from TAPThomas R. Kanaby - Purdue Graduate Student
The Purdue TAP (Technical Assistance Program) is a state funded outreach of Purdue University to provide Indiana businesses access to University expertise and facilities for the solution of engineering problems otherwises beyond their reach. The program has been successful in saving such businesses considerable production related expenditures and at the same time saving jobs for their workers.
The Materials Engineering Dept. at Purdue participates in TAP in three ways.
1) Material Selection Assistance
2) Corrosion Study and Failure Analysis
3) Process Analysis
One particular project of this third type is the topic of this presentation. It involved a company that had been for years successfully resistance butt welding M4 tool steel mill cutters to 4140 shafts. Upon retirement of the person performing this weld, the company found that it could not train new operators to achieve a reliable weld using the same equipment. Detrimental cracks were usually present. The task of TAP was to make the process "more flexible" and less operator dependent.
Initially the process was reviewed. It was found that a fixed amount of current was applied through the abutting metals, and at a given amount of collapse (1/4") the operator released a foot pedal stopping the current. There was little desire by the company to spend money on changing equipment or the basic process which had been successful for over 30 years.
Next the weldments were examined. The defective ones were found to have a continuous crack in the M4 steel adjacent to the weldment. Hardness and microscopic work indicated the M4 near the weld interface was in a metastable state, while the 4140 was relatively unchanged from parent metal.
Three suggestions were sequentially made to deal with this problem.
1) use "slow cooling" (this was not pleasing to the company as production efficiency would be adversely effected, and it was found it didn't work anyway).
2) Offset the weld such that the M4 would be heated less. This was accomplished by making the cross section of the M4 large (relative to the 4140), shifting resistive heat away from the M4.. This added "bloom" of material would later be machined away. Like the first solution, this did not work.
3) Do the reverse of method 2 -- that is force higher heating of the M4 in hope that the crack would be "expelled" from the weld joint since the highly heated material would be displaced to the side. This was accomplished by adding a "projection" to the M4 of smaller diameter than the 4140 abutting it. This method proved to be successful. An interesting observation is that the crack which occurs in the excessively heated M4 actually "turns away (U-turn style)" from the central portion of the weldment and remains at the outer extremities of the weldment diameter. Acceptable parts are created by this method, saving the company considerable investment in new technology.