Pulsed laser surface hardening of ferrous alloys. Page: 6 of 10
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tube oriented at 150 from the surface, 450 from the horizontal and 10 mm from beam spot. The
annotated photograph of the set-up of YAG laser surface hardening is shown in Figure 2. An
infrared weld monitor, successfully used to monitor the weld quality ,[5,6] was utilized to monitor
the processing of surface hardening. The monitor was integrated into the YAG beam delivery
optics and uses oversized, off-axis optics to collect the infrared emission signal associated with
laser beam surface hardening. Monitor voltages as a function of time were collected using data
acquisition hardware and software (GW Instruments, Somerville, MA) with an Apple Macintosh
computer. The data collection rate was 2500 to 5000 Hz. The Rockwell C hardnesses along the
treated tracks were measured using a portable hardness tester. The corresponding monitor voltage
for each hardness measurement on a treated track was obtained from the monitor voltage-time plot.
After surface hardening, the treated tracks were sectioned, polished, and etched to determine
microstructure, case depth, and width. The deviation of the flatness after the treatment was
measured using a Starrett's dial indicator with a accuracy of 0.001".
RESULTS AND DISCUSSION
The microstructures of laser treated and untreated 1045 steel are shown in Figure 3. There
are a completed hardened zone and a transition zone in the laser treated case (Fig. 3 (a)). Ferrite
phase on the original pearlite boundaries remained in the transition zone because of the temperature
distribution along the case depth as shown in Figure 4. When the material temperature was larger
than Ac3, all the phases at room temperature transformed into austenite. After the self-quenching,
the austenite transformed into martensite. At temperatures in the range between Aci and Ac3, a part
of ferrite phase still remained besides the austenite phase. After self-quenching, the ferrite phase
Hardened zone Transition zone Pearlite Ferte
Figure 3 Micrographs of laser surface hardening of 1045 steel for beam power 1200 W,
pulse width 2 ins, pulse frequency 200 Hz and beam travel speed of 2 cm/s. (a) 50 X and
(b) untreated zone at 400 X.
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Xu, Z.; Reed, C. B.; Leong, K. H. & Hunter, B. V. Pulsed laser surface hardening of ferrous alloys., article, September 30, 1999; Illinois. (digital.library.unt.edu/ark:/67531/metadc706234/m1/6/: accessed February 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.