Microstructure and its effect on toughness and wear resistance of laser surface melted and post heat treated high speed
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I.
INTRODUCTION
T H E technique of laser surface melting of steel has, since it was developed in the mid-70's, been used in a number of tribological applications. The obtained microstructures have been studied and characterized, but the detailed understanding of their nature and how to control their formation is yet to be learned. The creation of completely new structures is not to be expected, l but beneficial effects on the mechanical properties due to refinement and other alterations of the conventionally obtained structures may well occur. Bee and Wood 2 have shown that the rapid solidification causes a refinement in grain size, suppressed M, temperatures, and a dramatic decrease in the proportions of 6-ferrite and retained austenite in 12 pct Cr steels. Other researchers 3'4 have reported increased amounts of 6-ferrite and austenite after laser melting and rapid quenching of alloy steels and tool steels. The results seem to contradict each other, but may be explained by the different process parameters. Strutt et al.~5 have characterized the microstructure obtained after laser surface melting of M2 high speed steel. However, the effect of the laser treatment on the mechanical properties and wear resistance of the HSS were not considered. The grain refinement and increased homogeneity resuiting from the rapid solidification can be expected to yield improved toughness. This is of great interest in the optimization of the microstructure of high speed steel tools for metal cutting. Recent studies on the wear of bi-metal hacksaw and band-saw blades 6'7 have shown that good toughness combined with high hardness is desirable to withstand the different wear mechanisms. With conventional heat treatment it is difficult to increase toughness without a corresponding decrease in hardness. The present paper deals with laser melting as a method of increasing the toughness of HSS hacksaw blade teeth. Due to the problem of rounding of sharp edges at melting, blanks were laser treated and subsequently annealed in order to allow processing of teeth by milling. The blades were then hardened and tempered using conventional methods. The optimized treatments and the microstructures after each heat
treatment are described in detail, and the results of sawing in quenched and tempered steel and austenitic stainless steel are discussed.
II.
EXPERIMENTAL
A. Laser Treatment
Soft annealed hacksaw blade blanks of M2 high speed steel were used, the chemical composition and hardness of which are given in Table I. One edge of each blank was laser melted from two sides (see Figure 1) one at a time, in a protective argorl atmosphere using a 2.5 kW CO2 laser. At a power density of approximately 10 6 W / c m 2 the blanks were translated at a velocity of 0.8 m per second relative to the laser beam. The reflected, useless energy was roughly estimated to be 10 to 30 pct of the energy incident on the steel surface. Table I. Chemical Composition (Wt Pct) and Hardness (HV0.5) of Annealed M2 High Speed Steel C 0.82
Cr 4.1
Mo 4.9
W 6.0
METALLUR
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