Surface Transformation Hardening of 4140 Steel

One of the primary uses the HPDDL is large area surface transformation hardening, where it is desirable to achieve a 100% hardened surface. However, backtempering occurs due to the overlapping passes heating a portion of the subsequent pass into the tempering temperature range. This results in a portion of the interpass zone having a lower hardness than the hardened zone. Experiments were done holding the energy density as a constant and relating the amount of back-temper in a pass to the beam displacement along the long axis. The minimum back-temper reading for two passes at a given displacement was found to be 15 mm. The hardness within the case was found to be in the range of 55 to 65 Rc, while the case depth was generally between 0.7 and 1.5 mm, an acceptable case was assumed to be at 0.5 mm.

At a displacement of 15 mm along the long axis between passes the back tempered region has a width of only 1.5 mm [3- 5%], demonstrating very high laser beam edge definition. Figure 6 shows the interpass zone for this sample. The hardness in the back-tempered region is generally 30 to 40 Rockwell C.