Abstract
The effect of laser surface heat treatment on the microstructure of AISI 4340 steel was investigated and compared with conventional furnace heat treatments. The microstructural variables, i.e. prior austenite grain size, substructure of martensite, amount of retained austenite and its distribution and carbide precipitations, were carefully studied using transmission electron microscopy. The two significant observations were the presence of fine cementite particles of 200 Å size and large amounts of thin films of retained austenite in the as-quenched structures. The microstructure exhibited was fully martensitic, the substructure of which contained extensive twins. The average prior austenite grain size was measured to be 35 μm. A comparison with conventional furnace heat treatment (870°C treatment) showed that there was an enhancement in the amount of retained austenite and precipitation of fine cementite in the laser-heat-treated specimens. However, the prior austenite grain size and martensitic substructure remained essentially the same as those of conventional heat treatment.
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