Effects Of C And Nb Addition On Microstructure And Mechanical Properties Of P/M M2 And 420 Steels

M2 and 420 with high hardness,strength and wear resistance,are the most widely used high-speed steel and stainless steel at present,used in knives,tools,mechanical parts,medical equipment.The sintering rate of these steels during the solid-state sintering is slow,and the density is low;under the liquid state sintering condition,the faster the sintering rate obtained,associated with the massive grain size and poor performance.Therefore,the best sintering zone is where the austenite+liquid phase coexists with carbides,but this zone is extremely narrow that is only 2-4degrees.In this paper,we aim at the sintering difficulty and the narrow sintering window of M2 and 420,study the sintering mechanism by adding C and Nb elements and find the ideal process parameters;and achieve higher densification and excellent mechanical properties.In the sintering experiment of M2 high-speed steel,studies have shown that with the increase of C,the solidus temperature drops,and the sintering window expands by10 to 40?.The types of carbides change from small white block M₆C and off-white spherical MC to large block M₆C and film-like MC,and finally into net-like eutectic M₆C,fishbone eutectic MC and feather-like eutectic M₂C.At 1240?,the maximum hardness of the M2+0.22%C sample is 699.3Hv,and the tensile strength can reach1069.4MPa.The addition of Nb increases the Solid phase line temperature and samples achieve sintering densification at 1280??1300?.The carbide type changed from white small block M₆C and off-white spherical MC to networked eutectic M₆C,and a new type of chrysanthemum-shaped eutectic M₇C₃carbide was produced.At 1280?,the hardness of M2+2%Nb samples can reach up to 570.2Hv and the tensile strength is 894.8MPa.When the C content of M2 high speed steel is0.22%?0.42%,the best sintering temperature can be determined as 1200??1240?.And when the Nb content is 1%?2%,the sintering temperature can be determined as1260??1280?.In the sintering experiment of 420 stainless steel,the study showed that the relative density of 420+0.2%C sample increased significantly as the sintering temperature increased.Because the addition of C lowered the solidus temperature which caused the liquid phase sintering and make a final relative density at 98.5%.420+Nb and 420+Nb C samples can also reach a final relative density more than 98%,which is better than 420 stainless steel.Because Nb C has a certain pinning effect on the grain boundary,which hinders the movement of the grain boundary and provide enough time of eliminating pores by grain boundary movement,resulting a better densification.The 420+0.2%C sample showed a obvious grain boundary-pore separation behavior with a reaching of grain size of 315?m at 1410?,of which the pore in intracrystalline become obviously spherical closed and cannot continue to pin the grain boundaries,resulting in a dramatic increase in grain size.The SEM analysis shows that Nb C is pinned at the grain boundaries and distributed in the grains of420+Nb and 420+Nb C samples,which are needle-like,continuous granular and circular.420+0.2%C have a maximum tensile strength of 990.6Mpa,and a small elongation.420+Nb and 420+Nb C both exhibit elongation,but the tensile strength is basically 400?500MPa.And Because of the generation of a portion of ferrite,the420+1%Nb C specimen has a large number of tough nests in the fracture causing a ultra-high plasticity and a ductility up at 39.1%.The best sintering temperature for420 stainless steel is 1390?,and the best sintering composition is 420+0.2%C and420+1%Nb C.