Surface Morphology And Properties Of Mold Steel SKD11 After High Current Pulsed Electron Beam Treatment

In this study,one kind of mold steel,SKD1 lwas treated by High Current Pulsed Electron Beam(HCPEB) with different treating regimes on a 'Nedazhda-2' type equipment.The main task of this work is to investigate the fundamental regularities of surface morphology and microstructure evolutions induced by HCEPB treatment and their effect on the surface property and service performance of SK.D11 mold steel.Under the HCPEB treatment,the difference of thermal physical properties between Cr-rich carbide(Cr,Fe)₇C₃ located in subsurface and material matrix will cause the eruption of carbide melt droplet and leave the typical crater forms on the modified surface.The distribution of craters on the treated samples exhibits the following regularity:for the same accelerating voltage,the area density of craters decreases with the increasing number of pulses;while the crater size increases to its maximum,then decreases with the increasing number of pulses.As for the same pulses,the higher voltage gives fewer craters and entered the stable stage earlier than the case of lower.The surface roughness decreases with the increasing number of pulses.The carbide in the initial sample is large sized and irregular shaped.It exhibits an inhomogeneous distribution.When treated by HCPEB,the carbide in the surface layer could be erupted and dissolved.The carbide in the treated sample become refined,spherical and exhibits a homogeneous distribution.The distribution of Cr and C becomes homogeneous after HCPEB treatment.Ferrite is transformed to austenite instantly in the fast heating process during HCPEB irradiation.During the fast cooling process,the austenite is suppressed until room temperature and carbide is included in the austenite.Remelted layer with high volume austenite is formed.The content of ferrite and carbide decreases with the increasing number of pulses.Contrarily,the content of austenite increase gradually.After 15 pulses treatment, ferrite has already transformed to austenite and the content of austenite gets its maximum. The decomposition of austenite would occur again when using too many numbers of pulses because the accumulation of heat in the matrix would lead to the decrease of cooling rate,as a result the process of transformation from austenite to ferrite could not be suppressed.Because of the surface morphology and microstructure evolution after HCPEB treatment, the surface properties exhibit the following evolution.The microhardness values measured on the treated surface are all smaller than initial sample;it would decrease with the increasing number of pulses firstly,and then decrease.Wear and corrosion resistance would increase firstly and then decrease with the increasing number of pulses;the HCEPB treated sample at 15 pulses treatment exhibits the extremum of the resistance values.