Studies On Powder-pack Alumino-chromizing Enhanced By Alternating Current Field

Compared with the powder pack aluminizing and chromizing layers,the powder pack alumino-chromizing layers of the steel have better comprehensive properties,which can effectively improve the surface hardness,wear resistance,corrosion resistance and high temperature oxidation resistance.The conventional pack alumino-chromizing(CPAC)has some problems,such as high treating temperature,long soaking time and low utilization rate of agent.Q195 steel and 45 steel were used for the surface modification in the present study.An alternating current field(ACF)was applied to the pack Alumino-chromizing during the soaking to overcome those problems.Investigating,testing and analyzing were made on the treated layer's structures,phases,growth,microhardness distribution and high temperature oxidation resistance to study the characteristics of the ACF enhanced pack alumino-chromizing(ACFPAC).The results show that compared with the ACFPA,adding an appropriate amount of chromium powder or high carbon ferrochrome powder to the aluminizing agent can significantly promote the growth of the layer in which the aluminizing dominated.The layer's thickness can be increased by about one time at most.The promoting effect of high carbon ferrochrome powder is stronger than that by chromium powder.If the amount of chromium powder or high carbon ferrochrome powder in the pack agent is too high,the growth of the ACFPAC layer will be inhibited that the thickness of the ACFPAC layer is thinner than that of the ACF pack aluminizing layer,or even close to the thickness of the layer by conventional pack aluminizing.Increasing the ACF intensity can promote the growth of the ACFPAC layer.When the ACF current is 4A,the Q195steel's ACFPAC layer increases by at least 4 times at 800? for 4 hours ACFPAC.When the ACFPAC is carried out at 600? for 4 hours,a 36?m thick layer and a 24?m thick layer are respectively formed on the surface of Q195 steel and 45 steel.Adding a little aluminum powder into chromizing agent has a weak promoting effect on the growth of the ACFPAC layer in which chromizing dominated.Compared with the ACFPC,by adding 0.05% aluminum powder to chromizing agent,the ACFPAC layer's thickness increases slightly.If increasing the amount of aluminum powder,the layer's thickness will decrease slightly.The thicker and denser the layer dominating aluminizing is,the better its high temperature oxidation resistance is.Compared with the ACFPC layer,adding 0.05% aluminum powder to the chromizing agent can improve the layer's high temperature oxidation resistance.The high temperature oxidation resistance of the aluminizing dominated ACFPAC layer is better than that of the chromizing dominated ACFPAC layer.It is proposed that the ACF has a thermal effect and an electromagnetic stirring effect in the ACFPAC,which not only quickly produces more active atoms,but also increases the diffusion channel for active atoms' diffusion.The diffusion activation energy of active atoms is thus lowered.The mechanism for the promoting effect by the small amount of chromium/aluminium is similar to that by rare earth,which increases the lattice distortion of surface layer and provides more diffusion channels for active atoms.Al and Cr in the Alumino-chromizing surface layer react with oxygen at high temperature to form dense and stable protective composite oxide film,which hinders the contact between the matrix and oxygen.The oxidation rate of the matrix is thus reduced.