Effects of Heat Treatment on Microstructure and Tensile Properties of Laser Melting Deposited AISI 431 Martensitic Stainless Steel

- Dec 13, 2016-

AISI 431 steel is a ferrite and martensite duplex stainless steel with high strength and ductility, and it is widely used as one of the key materials in submarine motor shaft and steam turbine applications. In the present work, a plate of AISI 431 steel was fabricated by the laser melting deposition technique. The microstructure of the as-deposited steel post heat treatment was investigated to understand the influence on the mechanical properties of the steel.

(1) The laser deposited AISI 431 steel has a fine well-aligned directional structure with dendritic phases and inter-dendritic phases. During the solidification process, the δ-ferrite forms first and gradually transforms into austenite and then turns into martensite during air cooling. When tempered at 680, (Cr, Fe)23C6 carbides precipitate along the boundaries of δ-ferrite.

(2) During the crack formation, the cracks first initiate at the interface between ferrite and (Cr, Fe)23C6 carbides, then propagate along the interface between the dendritic phases and the inter-dendritic ferrite phases.

(3) After solution heat treatment, the interlayer heat affected zone was eliminated. The increase of heat treatment temperature from 1050 to 1100 °C, the gradually dissolved carbides particles and an increase in volume content of ferrite were observed.

(4) The tensile strength of the quenched-and-tempered steel was higher than the as-deposited ones and the elongation and reduction in area of the quenched-and-tempered AISI 341 steel were slightly lower than those of the as-deposited steel due to the formation of martensite and dissolution of brittle carbides. The optimal tensile properties were achieved for solution heat treatment at 1050 °C.