A 4340 shaft (Fig. 1) weighing 93 kg (205 pounds) and measuring 740 mm (29 inches) was hardened in an integral-quench furnace. The journal areas on the shaft were 100 mm (4 inches) in diameter and the pinion area was approximately 200 mm (8 inches) in diameter. The part was run alone, and as one would expect in this type of furnace, there were no load thermocouples.
The heat-treatment cycle was as follows: austenitize at 830°C (1525°F) for 3.5 hours, quench in 60°C (140°F) oil, wash and temper at 565°C (1050°F) for 4 hours followed by air cooling. Specification requirement was 321-363 BHN (34.5-41.0 HRC), and the as-tempered hardness was 34.6 HRC as measured on the surface of the major diameter.
After machining (Fig. 2), the customer returned the part indicating that the journal reading was 290-300 BHN and the hardness was 32-34 HRC in the area of the teeth machined on the large diameter. The machined part weighed 69 kg (154 pounds).
What went wrong?
While the austenitizing time was marginal, the hardness requested was achieved, albeit on the low side of the specification. The heart of the issue lies in the effect of mass in the as-received form versus the final-machined state. The failure by the designer to understand that neither the hardness nor the mechanical properties would remain the same throughout the cross section (Tables 1-2) and the failure to specify hardness values at the depth of interest (i.e., the “finished” part condition) contributed to the problem. Also, Brinell rather than Rockwell testing may have revealed additional insights.
This is an invaluable lesson for both the customer and the heat-treat shop in communication. It is vital to understand how the raw material will be manufactured into the finished product so that appropriate steps can be taken and cautions can be expressed.
1. Modern Steels and Their Properties: Carbon and Alloy Steel Bars, Handbook 268, Bethlehem Steel Company, 1949.