Laser Surface Processing

High-power lasers have been used to process materials to improve their wear resistance since the late 1960s. Laser surface modification techniques that are used to improve the wear resistance of cast irons include transformation hardening, melting, and alloying. The processing conditions, examples of microstructures, wear characteristics, and applications associated with each technique are discussed in greater detail in Ref 13.

Laser Transformation Hardening. The wear resistance of highly stressed ferrous alloy components, such as gears and bearings, can be improved by transformation hardening. Because ferrous materials are very good heat conductors, the high heat fluxes generated by lasers are most suitable to heat the surface layer to austenitization levels without affecting the bulk temperature of the sample. The ensuing self-quenching is rapid enough to eliminate the need for external quenching to produce the hard martensite in the heated surface. Because ferrous alloys have high reflectivity, absorptive coatings such as manganese phosphate and graphite are applied to the workpiece for efficient laser heating. Most of these coatings burn off and normally do not affect the microstructure. Processing conditions for laser transformation hardening are typically power densities that range from 5 to 100 MW/m2 (3.2 to 64.5 kW/in.2) and interaction times ranging from 0.01 to 10 s. Usually, inert gas shielding is used. A rectangular beam is often used for laser transformation hardening. The case depth depends on the hardenability of the material and rarely exceeds 2.5 mm (0.1 in.).

In cast irons, laser heat-treated surfaces appear in light contrast, as shown in Fig. 8(a) for gray iron and in Fig. 8(b) for ductile iron. The microstructure of laser-hardened gray cast iron is generally fine martensite that contains flake graphite, as shown in Fig. 9(a). Acicular bainite can sometimes occur in the martensite matrix. The microstructure of laser-hardened ductile iron is also fine martensite, but contains "bull's eye" graphite nodules, as shown in Fig. 9(b). A closer examination of Fig. 9(b) reveals a narrow region of martensite between the graphite nodule and the ferrite ring, which indicates the extent of carbon diffusion occurring during processing.

Fig. 8 Cross sections of laser heat-treated surfaces in cast irons. (a) Gray iron. (b) Ductile iron. Source: Ref 13
Fig. 9 Microstructures of laser-hardened cast irons. (a) Gray iron. (b) Ductile iron. Source: Ref 13

According to Molian and Baldwin (Ref 14), the extensive case that developed during laser hardening of gray and ductile irons was made up of sequential regions with varying degrees of microstructural modification. The region just below a very thin melt zone contained plate martensite, retained austenite, and graphite, whereas the region just above the base metal contained a mixture of refined martensite and untransformed pearlite.

From their study of pin-on-disk wear behavior of laser-hardened gray and ductile cast irons, Molian and Baldwin (Ref 14) found an improvement in scuffing and sliding wear resistance with an increase in case depth, as shown in Fig. 10. On the same samples, erosive wear test results showed that the erosion rate depended on the surface hardness and the case depth, as shown in Fig. 11, and increased as the matrix microstructure varied from ledeburite to tempered martensite to pearlite (Ref 15).

Fig. 10 Sliding wear behavior of laser-hardened cast irons as a function of case depth. (a) Gray iron. (b) Ductile iron. Source: Ref 13
Fig. 11 Erosive wear behavior of laser-hardened cast irons as a function of surface hardness and case depth. (a) Gray iron. (b) Ductile iron. Source: Ref 13

Tomlinson et al. (Ref 16) observed that besides reducing wear, laser hardening of flake graphite iron resulted in very little plastic deformation, no graphite deposition on the opposing metal, and no adhesive damage. Trafford et al. (Ref 17) used a reciprocating pad-on-plate technique to evaluate the wear resistance of laser-processed gray iron and found that fully martensitic and surface-melted ledeburite microstructures had the lowest wear rates (Fig. 12).

Fig. 12 Abrasive wear rates in gray iron as a function of microstructure arising from various laser-hardening treatments. Source: Ref 13
0 0

Post a comment