Laser Heat Treating and Cladding
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.
Click
here for a video demonstration
Hardening
of 4140 Steel Sample
