LASER TECHNOLOGY’S NEW HORIZON —
HEAT TREATMENT & CLADDING WITH HIGH POWER DIRECT DIODE LASER (HPDDL)
Through laser transformation hardening a material can be case hardened with negligible distortion. A comparison with flame and induction surface transformation hardening techniques clearly show that laser surface hardening is the most advantageous process. Flame hardening has poor reproducibility, poor quench and environmental issues. In induction hardening a quench is required, distortion of the part occurs and there is large thermal penetration. With laser beam hardening the applied light radiation instantaneously heats the surface. There is no radiation spillage outside the optically defined area. The bulk of the material acts as a heat sink for the extraction of heat from the surface. The major advantage of laser surface treatment is high processing speeds with precise case depths. Laser surface transformation hardening not only increases the wear resistance, but also under certain conditions the fatigue strength is also increased due to the compressive stresses induced on the surface of the component5.

The HPDDL is an ideal source for laser transformation hardening. The line of light, when moved across the work piece along the short axis [Figure 1] has high edge definition without the need for special cylindrical lenses [Nd:YAG] or water cooled components [CO2]. The wavelength - 800nm, is highly absorptive, requires no pre-coating of the work piece to get absorption. The HPDDL has a modulation bandwidth of 20KHz, making ideal for in-situ temperature control.

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, back-tempering 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.

Figure 6: This sample was produced by shifting the beam 15 mm from the initial pass to produce the second pass. The region marked A is untempered martensite. B marks the region in which the martensite is tempered. C indicates the base metal (2% Nital etch, 50X magnification).

Measurements shown in figure 7, relating back-temper to case depth, began in the middle of the first pass and extended to the center of the second pass.

Figure 7: The relationship between back-temper and case depth at a 15mm displacement.

Surface Transformation Hardening of Gray Cast Iron

Surface transformation hardening of Class 40 Gray Cast Iron was also performed without the use of an absorptive coating or inert gas shielding using a HPDDL. The beam was defocused 4 to 8 mm and moved at various speeds over the work-piece. The gray cast iron was much more sensitive than the 4140 due to a very narrow process widow between melting, producing surface carbides, and desirable hardening. It was determined that at a 6 mm defocus a case would be produced with a higher hardness value than the case produced at 8 mm defocus. The 4 mm defocus produced surface melting and carbides. At a higher degree of defocusing the case depth would increase at the expense of hardness. This will occur up to a critical amount of defocusing. This application will benefit greatly from temperature control, which is completely within the means of the HPDDL.

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