Study On The Relationship Between Machinability And Microstructure Of Special Free Cutting Stainless Steel

At present,our country has already become the world's production and export country in pens manufacture,but it's not powerful.The yield of the ball-point pen that are low and middle grade are the biggest in various types of pens of our country.And the high ball-point pen has a large gap with foreign ball-point pen,the main reason is that the quality of free cutting stainless steel which makes ball-point pen is not as good as the same kind material abroad.At present,the free cutting stainless steel of high ball-point pen mainly depend on imports.Although the domestic free cutting stainless steel has been used,but its machinability is not stable,even leads to the phenomena of cutting tool breaking and cracked pen point.Therefore,it is urgent and necessary to research and develop free cutting stainless steel.In view of this situation,this article made the normal batch,the cutting tool breaking batch,the pen point cracking batch of domestic free cutting stainless steel,and SF20 T steel as study objects,researching the micro structure of free-cutting phase and elements in free-cutting by SEM and TEM,together with the result of mechanical properties and machinability test so as to study the relationship and relativity between machinability and microstructure.This article made the theory as the guide.Through a series of test,together with the analysis of the test results,the following conclusions were obtained:(1)The free cutting phase of stainless steel are mainly MnS inclusions.It is generallyconsidered that when the MnS inclusions have round or spindle shape,larger quantity,bigger size and uniform distribution,the steels have a better machinability.Among the 4 materials,the morphology of MnS inclusion in SF20 T is the best,and the normal batch of domestic free cutting stainless steel is the second.The cutting tool breaking batch has a bad machinability,due to its MnS inclusions have little size and a small quantity.The pen point cracking batch has a bad machinability either,because it has too many MnS inclusions with a high density.(2)Micro nano scale hard compounds in the Japan SF20 T free cutting stainless steel are located in the interior of the free cutting phase and avoid contacting with the cutting tool directly.The same kind compounds in domestic free cutting stainless steels are located on the edge of the free cutting phase and make a great damage to cutting tool.(3)The small size Pb regarded as a lubricant,with the big size carbides which can enhance the matrix hardness make the cutting tool breaking batch perform terrible in cutting test.The large size Pb and the small size carbides reduce the matrix hardness of pen point cracking batch.(4)Japan SF20 T free cutting stainless steel performs better than domestic free cutting stainless steel in surface roughness and tool wear,and the different batch of domestic free cutting stainless steels perform same.The domestic normal batch has a better Chip Breaking Performance than Japan SF20 T.In this paper,the effect of micro inclusions on machinability of free cutting stainless steel was emphasized in analysis by TEM,on the basis of the study of microstructure by SEM,and the relationship between machinability and microstructure was thoroughly studied.This is a growing trend of involving micro inclusions in studying the machinability of free cutting stainless steel,and more researches on the relationship between machinability and microstructure are needed in future.