Laser surface melting of some alloy steels

Abstract

An investigation is reported of the structures of laser surface-melted zones in a 1C- 1·4Cr steel; also some results are presented for a carburized alloy steel and a medium-carbon alloy steel. Treatment was carried out with a CW CO₂ laser at a power of 1·7kW, beams of ∼O·4-4·3mm dia., and traverse speeds of ∼8-250 mm s⁻¹ (corresponding to interaction times of 2-500 ms). Zones of smooth, flat surface profiles were produced with essentially martensitic structures and corresponding high hardness. In the high-carbon steels coated with graphite before laser melting, the hardness of the zones decreased with increasing traverse speed as a result of carbon solution, leading to a lowering of Ms and increased austenite retention. In the 1C-1·4Cr steel. zones were produced that consisted almost entirely of austenite with fine columnar cellular/dendritic structures. Refrigeration to ∼-100C was needed to form substantial amounts of martensite; stabilization effects in relation to martensite formation resulted from holding at room temperature. When laser melting was applied to specimens without any surface coating or with an MnO₂ coating, the structures were predominantly martensitic; however, they contained larger amounts of retained austenite than were present after conventional quenching, indicating a lowering of the M_s temperature.