Study On Chromizing Of High Alloying Austenite Stainless Steel In Salt Bath

310S is one of the Ni-Cr austenitic stainless steel,Because it contains high content of chromium and nickel,it has good oxidation resistance,corrosion resistance and high temperature resistance,it has been widely used in aerospace,shipbuilding,daily life,energy,transportation and coastal operations and other fields.But in the process of using the sediment pump and the mud pump,its surface is susceptible to wear and erosion,which affects its application range.Chromizing in salt bath can obtain high hardness Chromizing Layer on the surface of the material.It is one of the economical and easy-to-use surface technologies.Therefore,it is of great significance to enhance the surface hardness and wear resistance of 310S through salt bath chromizing.In this paper,B₂O₃ is used as the base salt,and the salt bath chromizing test is conducted with 310S stainless steel.The work carried out and the important results obtained are as follows:The effect of three reductants（SiC,B₄C and Al）on the chromizing effect of310S stainless steel was studied.The study found that the surface of the SiC type specimen was not formed with a penetrating layer,and the sample surface was seriously eroded,because SiC did not participate in the reduction reaction,salt corrosion of specimen surface in high temperature environment,at the same time,the Gibbs free energy was calculated as△G=170 KJ/mol.On the surface of B₄C and Al type specimen form chromized layer.By Cr₇C₃,Cr₃C₂,CrC and Fe（Cr,Ni）solid solution phases,the thickness of the layer was 18.46μm and 14.23μm,Gibbs free energy was calculated as△G=-314.5KJ/mol and△G=480.99 KJ/mol,respectively,and the positive reaction tendency is strong.After comparison,the effect of B4C as reducing agent is the best.The effect of Chromizing agent ratio on 310S stainless steel chromizing was studied,and the influence of the ratio of NaF/B₄C on the effect of Chromizing was emphatically studied.Research shows that 6 groups of samples formed on the surface of the diffusion layer,mainly composed of Cr_xC_y phase,when the mass ratio of NaF/B₄C is less than 2.2,the obtained chromizing effect is good,No.1-4 layer thickness of around 30μm,No.5 and No.6 of the thickness of the layer are respectively 17.28μm and 16.6μm.After the hardness test showed that the 1-4surface hardness is above 2000HV,the surface hardness of No.5 and No.6respectively is 1782HV and 1522HV,chromizing increases the surface hardness of310S stainless steel 7-11 times,the friction coefficient decreased by 38%,contrast analysis No.2 chromizing had the best effect;NaF/B₄C will affect the ratio is too high sound chromizing effect,through the experiment of NaF/B₄C has the best ratio between 1.6-2.2 were determined.The influence of temperature and time of Chromizing in salt bath on the permeation layer was studied.The results showed that in different salt bath treatment temperature and time,were formed on the specimen surface chromized layer;in chromium infiltration under the same temperature,layer thickness and surface hardness of samples increased with increasing time.Chromium,chromium in the same time permeability,thickness of layer and surface hardness of samples as the temperature increases,the chromizing treatment,the surface hardness is increased by7-11 times,and the diffusion activation of chromium atoms is calculated,The energy Q is 223.9KJ/mol.The effect of adding La₂O₃ on chromizing was studied.4 groups of experiments were designed according to the amount of different La₂O₃ additions.The results show that the addition of La₂O₃ salt bath chromizing,surface hardness is very high,at more than 2200HV,and the experimental groups without La₂O₃ addition,the tissue sample surface is smaller,mainly composed of irregular shaped particles,distribution more uniform,but excessive La₂O₃ may hinder the growth of oxide layer,corrosion sample surface;analysis of lanthanum in the whole process of Chromizing showed that lanthanum is involved in the entire process of chromizing.Including chromizing agent decomposition,diffusion and adsorption of chromium atoms,the chromium atoms in infiltration;chromizing process accelerates the decomposition of chromium infiltration agent,as chromium atoms"collector,accelerating chromium atoms penetrate into the substrate surface of lanthanum will cause surface distortion,increase the diffusion channel,enhance the chromizing speed,and can increase the nucleation rate,surface refinement.However,it is necessary to pay attention to the addition of lanthanum elements,otherwise it will act as a hindrance.In this paper,a new type of salt bath chromizing agent has been developed,and a high hardness,high interfacial bonding force and high wear-resistant Chromizing Layer have been obtained on austenitic stainless steel.It provides scientific basis and basic technical support for the application of austenitic stainless steel in more extensive fields.