Evaluation of a New Pinch Resistance Test Method

Abstract

The existing method of pinch resistance testing before the pinch weld is made is primarily useful for recognizing electrode contact consistency. An improved test method is proposed that would provide recognition capabilities for other off-normal welding conditions. Such recognition could be used to prevent inadequate pinch welds. The new resistance testing method uses the Medar weld controller to pass a significantly higher current than that used in the existing method. Several tests were conducted to evaluate the effect of the higher test current on pinch welds and to evaluate the proposed test method’s capabilities to detect oxide film, defective electrodes, loose electrodes and conductors, internal stem defects and atypical tubing hardness. These tests showed that the new method has more pronounced indicators than the existing method. These more easily noticeable indicators make the new method attractive as a replacement for the existing method. Background The existing preweld pinch resistance test uses a low current and voltage and does not clearly indicate subtle variations in welding resistance conditions. A new resistance test method using a higher current was suggested, which has the potential to detect subtle anomalies. A series of tests were conducted using the existing and the proposed resistance tests, to determine if specific welding set up anomalies could be detected. Various electrical resistance and material changes were introduced to the weld system. These changes simulated problem-causing conditions that can occur during production. The indicators of each anomaly for the existing and proposed tests were compared. The measure of resistance is the primary test indicator for setup anomalies. Resistance is calculated from the RMS voltage measured after applying a fixed current. The existing test method applies about 2.4 Amps with a resultant voltage of about 0.008 Volts. The proposed new method uses about 1200 Amps with a resultant voltage of about 0.200 Volts. The higher current and voltage provides better indicator resolution without adversely affecting areas that will be welded later. The resistance as measured by the existing method uses probes which are inserted into the pinch weld electrodes, as shown schematically in Figure 1a. It was found that measurements at this location cannot detect resistance changes which are not between the probes. To overcome this limitation, one probe connection was moved to the power lead conductor for some of the tests. The system resistance was changed by placing stainless steel washers, which have higher resistance than the copper contacts, in series with the system current path. Figure 1b shows the probe connection location on the electrode and conductor. The changes to the shape of the Evaluation of a New Pinch Page 2 WSRC-TR-2005-00437 Resistance Test Method current and voltage signatures as a result of the probe location are shown for the new method in Figure 2.