High-performance Martensite Stainless Steel Fabricated By In-situ Nitriding And Alloying Laser Cladding And Microscopic

Martensite stainless steel?MSS?has been widely used in industry owing to its high strength and excellent corrosion resistance,nevertheless,still problematic in poor ductility.It is well known that adding nitrogen and alloying can significantly improve the integrated mechanical properties and corrosion resistance of alloys.Therefore,in this paper,high-performance MSS was prepared by in-situ nitriding and alloying laser cladding.The effects of in-situ nitriding using nitrogen?N2?and alloying?Nb,Cu?on the microstructure and properties of MSS cladding layer were systematically studied by a series of characterizations,such as:XRD,EPMA,SEM,EPMA.The main research results is as follows:?1?The effect of laser in-situ nitriding on MSS laser cladding coating was studied carefully.The traditional N-bearing bulk MSS necessitates high-pressure and high-temperature treatment because of the extremely low solubility of N.In this paper,the high-nitrogen MSS coating?N content is 0.14 wt.%?is prepared in atmospheric environment bu the laser in-situ nitriding cladding technology which using nitrogen as the shield gas.The results show that the coating is composed of martensite,austenite,nano-precipitate phases(M₂₃C₆ and M₂N,M on behalf of Fe,Cr,etc),which exhibits striking mechanical properties with tensile strength of 1900 MPa and elongation of 7.3%.Moreover,the resulting coating has a excellent corrosion resistance,exceptionally superior to the reported laser cladded MSS coatings.This is due to the combination impacts of grain refinement,nano-precipitation,interstitial solid solution and dislocation strengthening.?2?The effects of Nb-alloying on the microstructure and properties of the MSS laser-cladded coatings have been carefully investigated.The obtained results show that the original Nb-free MSS coating was mainly composed of martensite,austenite,nano-M₂N and M₂₃C₆.After the addition of alloying element Nb,the lath-shaped martensite was refined,and the nano-sized Nb-enriched precipitates NbC and Nb?C,N?replace the original M₂N.The Nb addition increased the microhardness of the specimens successively(from 437 HV_0.2.2 of the Nb-free coating to 502HV_0.2.2 of the 1.0 wt.%Nb-alloyed coating).When Nb content range from0⁰.5 wt.%,the mechanical properties and corrosion resistance of cladding layer increase with the increase of Nb content;while Nb content increases to 1 wt.%,those properties decreases slightly.Significantly,a0.5 wt.%Nb addition could result in the laser-cladded MSS coating exhibiting the optimum tensile properties,i.e.,a combination of high strength?1780 MPa?and high ductility?EL:10.5%?.The striking comprehensive properties of the as-received MSS coatings can be ascribed to the combination impacts of solid solution,microstructure refinement and nano-precipitations.?3?The effect of Cu-alloying?0-2.5 wt.%?on the microstructure and properties of MSS laser cladding was studied.The results show that:For the as-cladded specimens,Cu alloying has no obvious effect on the mechanical properties of laser cladding layer.However,as Cu is the element which promotes austenitization of steel and improves its corrosion resistance,the corrosion resistance of the specimens was significantly improves cause by Cu addition lead to the present of a small amount of austenite and a Cu-rich layer.After 600?×1 h aging,Cu₄₀Fe₆₀and M₂₃C₆ precipitates exists in Cu-bearing cladding layer,and the microstructure of the cladding layer was refined,the elongation is increased 30%than original specimens,but the strength and corrosion resistance are decreased slightly.It is worth mentioning that the as-aged coating containing 2.5 wt.%Cu exhibits the optimum mechanical properties,400 MPa of UTS higher than of Cu-free specimen.