Did you ever wonder if the test coupon run with the load is truly representative of the results obtained on the parts themselves? The answer we tell ourselves is yes, it is. However, at least a shadow of doubt creeps in as the part size becomes larger and the test coupon size remains the same. Conducting tests on actual parts, while often both time-consuming and expensive, is extremely rewarding. Let’s learn more.
The cooling transformation characteristics of large SAE/AISI 4320 mill pinions were investigated by instrumenting the teeth of the pinion with thermocouples (Fig. 1) so that the quench rate and resultant metallurgical properties could be investigated and compared to that of the standard 2.5-inch (63-mm) test coupon run with every load. The pinion weighed approximately 18.5 tons (37,000 pounds) and was some 18 feet (5.5 meters) long. The largest diameter was 35 inches (890 mm) with a 4-inch (100-mm) gun-drilled hole through to the core.
The pinions are designed to meet AGMA 6014/AGMA 321 specification requirements. They are carburized in an atmosphere pit-style furnace for 80 hours at 1750-1775˚F (955-970˚C) in an endothermic gas atmosphere with natural gas additions holding a carburizing potential of 1.1% for the first 75% of the cycle, followed by a 0.95% carbon potential for the balance of the cycle time. The targeted effective case depth is 0.190-0.240 inch (4.83-6.10 mm).
The part is then cooled in the furnace to 1550˚F (845˚C), stabilized and quenched into well-agitated polymer (UCON E), followed by tempering. The polymer concentration is held in the range of 19%. Resultant surface hardness (of the part) is in the range of 58-59 HRC, while the test coupon measures 62 HRC with a surface carbon of 0.80±0.20%.
In production, a load of two pinions is charged into a hot furnace, and the heating and drop temperature are performed in steps (Fig. 2). This same type of procedure was used for this test with the exception that the furnace was brought up from ambient temperature with a single pinion in place (Fig. 3).
Test data of this type is rarely published, but it is invaluable. The test provided actual cooling transformation characteristics of the part (Fig. 4) as well as valuable data on the speed and uniformity of cooling (Table 1). The surface hardness (62 HRC) and microstructure of the test coupon were deemed acceptable as a quality-control measure once the test data was collected (i.e., microstructure reviewed and the tensile and yield strength, applied torsion stress and applied bending stress determined on the actual part). The materials and heat treatment were found to meet or exceed case and core strength requirements.