Effect Of Etching On Cr Alloying And Zr/Cr Composite Alloying Of AISI440B Stainless Steel And Oxidation Behavior Of The Modified Steel

AISI440B martensitic stainless steel with high strength,hardness,wear resistance and corrosion resistance is widely used in industrial manufacturing,petrochemical and medical equipment.This kind of stainless steel has certain high temperature oxidation resistance in medium and low temperature conditions,but its high temperature oxidation resistance becomes worse when the temperature is higher than 650 ?.Firstly AISI440B steel was etched with hydrogen and hydrogen+argon gas by plasma etching technology,and then the gradient chromizing layer of and the Zr/Cr composite layer were prepared on the etched AISI440B surface by double glow plasma surface alloying technique.The influence of etching processes on the formation mechanism and bonding strength of chromized layer and Zr/Cr composite layer were studied.The oxidation experiments were carried out at 900 ?,950 ? and 1000 ?,respectively.The mechanism of improving oxidation resistance of chromizing layer and Zr/Cr composite layer at high temperatures was studied according to the morphology of oxide scale and oxidation kinetics,.The results of plasma etching show that there are a large number of precipitates in etched AISI440B surface.The surface roughness increases and the carbon in the surface almost nothing after etching.The etching intensity is obviously increased with the addition of argon.The surface roughness increases with increase of etching strength,and the surface roughness is the largest after hydrogen etching for 1 h,but as the etching strength continue to increases,the surface roughness reduces.The results of chromizing show that the main phases of the chromium-modified layer contain Cr,Cr₂₃C₆,and Cr₇C₃,and the modified layer is composed of chromium-rich layer/Cr_xC_y diffusion layer/substrate.The etching process leads to increases of diffusion thickness.The bonding region between the chromzing layer and substrate is more compact and the distribution of elements in the region is more continuous after hydrogen+argon etching for 1 h.Plasma etching enhances the pinning effect and mechanical lock cooperation between the Cr-modified layer and substrate,resulting in an improved bonding strength.The results of Zr/Cr alloying show that surface Cr rich layer/Cr₃C₂-ZrC rich layer/Zr-Cr-Fe-C diffusion layer/substrate composite layer is formed on AISI440B surface after plasma zirconizing followed by chromizing.The Zr/Cr composite layer consists of Cr,Cr₃C₂,Cr₇C₃,ZrC,and Zr.The etching increases the thickness of the composite modified layer,and the bonding strength between the modified layer and the substrate is improved as well.The results of high temperature oxidation show that?1?the untreated AISI440B stainless steel indicates a worse oxidation resisitance due to the formation of Fe₂O₃ scale after exposure at 900?,950 ? and 1000 ? for 100;?2?A stable Cr₂O₃ protective scale is formed in chromizing-treated AISI440B surface after oxidation at 900 ? and 950 ? for 100 h,but the Cr₂O₃ stability decreases when the temperature rises to 1000 ?.The continous Cr₂O₃ scale makes much lower weight gains as well as excellent oxidation resistance of chromizing surfaces.The internal oxidation occurs with the growth of Cr₂O₃ scale;?3?The weight gain of Zr/Cr composite alloyed steel decreases compared with untreated AISI440B steel,and the oxidation resistance is improved at high temperatures.The volume of inner ZrO₂ changes with oxidation temperature,causing cracks or spalling of the top Cr₂O₃ layer,accelerating oxidation process as well as reduced high temperature oxidation resistance.The oxided surface of Zr/Cr composite layer is composed of Cr₂O₃ + ZrO₂ mixed scale after oxidation 100 h.