Study On Microstructure And Properties Of A Low-carbon Martensitic Stainless Steel For Conical Refiner Disc Plate

Base on working conditions and failure analysis of conical refiner disc plate, a Fe-0.04C-15Cr-3Ni-0.5Mo-0.1Nb low-carbon martensitic stainless steel was designed and prepared by induction melting. Effect of heat treatment on microstructure and properties of the tested steel was investigated by means of optical microscopy, scanning electron microscopy, electron backscatter diffraction,hardness, impact and wear tests. The conclusion can be drawn as follows:The new low-carbon martensitic stainless steel becomes complete austenite when the quenching temperature reaches at 940℃. The tested steel after quenching at 940~1100 is mainly consisted of lath m℃ artensite and fine MC-type carbides containing Nb distributed in the matrix.The δ-ferrite is found in the microstructure when the quenching temperature at 1100℃. With the increasing of quenching temperature, the prior austenite grain size grows and the number density of MC-type carbides reduces gradually. The dislocation density and the hardness are all first increased and then decreased. The maximum dislocation density and the maximum hardness value of 45.2 HRC are obtained for the tested steel quenched at 1020℃.The microstructure is consisted of tempered martensite when the hot-rolled tested steel is tempered below 550 ℃ after quenching at 1020 ℃.With the increasing of tempering temperature, the lath morphology of tempered martensite gradually disappears, the number of the fine carbides gradually reduces and more and more M23C6-type carbide particles are formed in the prior austenite grain boundaries and lath boundaries when the tested steel is tempered beyond 550 ℃.New austenite grain is formed in the prior austenite grain boundaries and lath boundaries when the tested steel is tempered at 750 ℃. With the increasing of tempering temperature,the dislocation density and the hardness are all first decreased and then increased,the hardness is first increased and then decreased when the tested steel is tempered at 550~700 ℃ after quenching at 1020 ℃. The minimum dislocation density and the minimum hardness value of 33 HRC and the maximum impact absorbed energy of 102.8 J are obtained for the tested steel tempered at 700 after quenching℃ at 1020 ℃. Between 550~700 ℃,the wear resistance is decreased with increasing of tempering temperature.The microstructure is consisted of tempered martensite and strip ferrite when the hot-rolled tested steel is tempered below 550 ℃, which lath morphology of tempered martensite gradually disappears and more and more M23C6-type carbide particles are formed in the prior austenite grain boundaries and lath boundaries when the tested steel is tempered beyond 550 ℃.New austenite grain is formed in the prior austenite grain boundaries and lath boundaries when the tested steel is tempered at 700 ℃. With the increasing of tempering temperature, the hardness is first increased and then decreased when the tested steel is tempered at 550~700 ℃. The minimum hardness of 35.4 HRC value are obtained for the steel tempered at 700 ℃.