An industrial laser cutting machine tool was used to harden 1117 and 1144 steel to examine effects on fatigue life. Transformation hardening leaves residual compressive stresses at the surface, which greatly enhances fatigue life in addition to increasing the hardness. The laser, being localized heat, allows faster cooling rates than conventional heat treatment methods. As a result, hardening with laser facilitates hardening of low carbon steel which is not hardenable by conventional methods.
All specimens received discrete hardened lines from the same laser treatment, 200 Watts of continuous wave passing over at 0.381 meters per minute. Fatigue life was measured for 160 specimens to generate S-N curves above the endurance limit for both steels, one S-N curve for each line spacing. Laser hardened line spacing was shown to be inversely proportional to fatigue life in low carbon steel and somewhat inversely proportional in medium carbon steel.
Continuous wave low power laser was used to demonstrate the potential of a relatively low cost laser heat treating machine which uses a sealed resonator instead of a gas flow type. Because spot size is not as critical in heat treatment, real time control of defocusing may not be necessary as with cutting or welding.
This research demonstrates the possibility of low cost laser heat treating in various line spacing and line widths to obtain improved fatigue life from low carbon and low alloy steels which are less costly. This is especially applicable to shafts and axles due to their fatigue life requirements. Shafts and axles are usually made from high alloy steels, induction hardened, then straightened. Shafts hardened by laser could be made of less costly steel, and due to the laser’s low heat input, would not need to be straightened. Eliminating the need to straighten and less material cost makes this process a good candidate for manufacturing shafts and axles at lower cost without a reduction in quality.