MARTENSITE TRANSFORMATION MICROSTRUCTURE OF 40Cr STEEL COMPLEXLY INDUCED BY LASER SHOCK

摘要

40Cr steel is laser quenched by the NEL-2500A rapidly axial flow CO_2 laser. Then the martensite induced by laser quenched is shocked by Nd:YAG laser again. Through comparing and analyzing the appearance and size of martensite, the dislocation density in microstructure between the treated zones by laser quenched and by laser quenched plus laser shock, the following results are shown: The second martensite obtained by laser compound treatment is more finer compared with those obtained by laser quenched; In the hardened zones obtained by compound treatment, a lot of slender second twin crystal martensites are induced; A lot of more high density dislocation tangles and cellular dislocations are generated. From the transmission electron microscope (TEM) micrograph after compound treatment, there are not only long lath and short nubbly martensites arranged in cross direction, but also massive nubbly and small short nubbly martensites arranged in longitudinal direction. Some martensites look like the broken blocks of quenched martensites. These new martensites are inserted transversely in the quenched martensites with large tangle. And they make quenched martensites break into pieces. Compared with the quenched martensites, the size of fresh martensites are smaller, about 0.3～0.5 μm.