Study On The Formation And Property Of Carburized Or Borided Layer On Stainless Steel

Carburizing or boronizing heat treatment can effectively improve the surface properties of metal-based materials.Exploring the formation and property of carburized or borided layer are of vital importance to the development and application of austenite stainless steel.The content of this thesis includes two parts:In part I,the effect of plasma carburizing heat treatment on hydrogen embrittlement susceptibility of metastable austenitic stainless is studied and the main conclusions are as follow:?1?Electrochemical hydrogen charging process results in significant decrease in the mechanical properties of austenitic stainless steel such as yielding strength and elongation.The formation of carburized layer can effectively suppress the hydrogen induced mechanical properties loss.?2?The entry of hydrogen into metal can cause intergranular fracture in the near surface.On the existence of carburized layer the hydrogen diffusion process is partly suppressed.?3?Electrochemical hydrogen charging process can induce martensitic transformation on the surface of austenitic steel.Plasma carburizing treatment can stabilize the crystal structure of austenite preventing the lattice distortion caused by hydrogen.The carburized layer is very stable even during tensile test.In part II,the formation and properties of borided layer are studied and the main conclusions are as follow:?1?The use of traditional boronizing media leads to poor surface state as media tends to stick to samples surface tightly.The addition of rare earth element into boronizing media has little effect on reducing the boronizing temperature.Addition of carbon powder as filler into boronizing media can improve the surface quality.The chemical compositions of optimized boronizing media are 10wt%B₄C+10wt%NaBF₄+50wt%SiC+28.5wt%carbon+0.3wt%Na₂CO₃+1.2wt%BaCO₃??2?The thickness of borided layer increases as boronizing temperature increases and the optimized boronizing temperature is 950?.The thickness of borided layer increases as boronizing time increases.The dryness of boronizing media,airtightness of boronizing heat treatment facility and the tight pack of samples with boronizing media are of vital importance to the quality of borided layer.?3?Borided layer consists of three sub-layer structures.They are FeB layer,Fe₂B layer and transition layer from outside to inside.There is a Ni-rich area between Fe₂B layer and transition layer and the transition layer is Cr-rich area because of the precipitation of chromium boride.?4?FeB layer and Fe₂B layer have extremely high hardness which lead to great improvement of surface mechanical property of 304 austenite stainless steel.The friction and wear result indicates that the wearing property is great improved by the borided layer and the hydrogen permeation result shows that borided layer can effectively reduce the diffusion rate of hydrogen into the austenite.