Effect of Deposition Thickness on the Microstructure of Laser Solid Forming 34crnimo6 Steel

In this study, the microstructure evolution of laser solid forming (LSF) 34CrNiMo6 steel with different deposition thickness was investigated. The microstructure of 10 layers (1.35 mm) sample from bottom to top presented the martensite with a large amount of upper bainite, the mixture of martensite and lower bainite, and the mixture of martensite and retained austenite. When deposition thickness reached 20 layers (3.12 mm), the microstructure of the deposition zone consisted of a large amount of bainite (needle-like lower bainite and feather-like upper bainite) and a small amount of martensite. Besides, the number and size of the feather-like upper bainite gradually increased from the top to the bottom of the deposition, and the morphology of martensite changed from lath-shaped to island-shaped. Two types of carbides morphology existed in lower bainite at the top of 20 layers (3.12 mm) sample: a short rod-shaped with an angle of 55-60° from the main axis and a strip-shaped approximately parallel to the main axis. The carbides in lower bainite can be identified as Fe3C.