| In this paper,the process of preparing ultrahigh carbon(2.6wt.%C)ultrahigh chromium(26wt.%Cr)tool steel by vacuum hot-pressed with pre-alloy powder has been explored.At the same time,research on the microstructure and mechanical properties of pre-alloy powder and bulk steel has also been conducted.The composition of the powder is determined by chemical analysis.The melting and solidification curves of the powder are obtained by a differential thermal analyzer.The onset,peak,and end temperatures of powder melting are 1246.7°C?1269.1°C?1275.0°C,.When the onset,peak and end temperatures of solidification are 1260.2°C?1254.1°C?1248.1°C?1233.3?,and the cooling rate is 20°C?mm-1.The distribution range of powder size is obtained with a particle size analyzer between 5-400?m,and 85%of them are 45-300?m.X-ray diffraction analysis is performed,the matrix structures are martensite and residual austenite,and the carbide is Cr7C3.The powders are divided into four categories according to size:58-75?m,106-120?m,160-180?m and 270-300?m.With the four powder samples are corroded,observing the structure.The carbide sizes of the four powder samples are:205nm?223nm?272nm and 319nm.By the microhardness test,the hardness of the powder is:HV601?HV593?HV581 and HV568.The vacuum hot-pressing experiment are performed as four processes:1100?,40MPa,holding time for 2h,3h and 6h;1150?,40 MPa,holding time for 2h,3h and 6h;1190?,10 MPa,holding time for 2.5 h;1240?,10 MPa,holding time is 15s.The theoretical density of block steel is 7.44-7.47 g·cm-3.By X-ray analysis,the matrix structure of the bulk material is martensite and residual austenite,and the carbides include Cr7C3 and Nb C.The average carbides sizes of the hot-pressed bulk steel at 1100?,(holding time 2h),1150?,(holding time 2h),1190?,and 1240?are 2.0?m,2.3?m,3.0?m and 3.4?m.After the above block steels are quenched at 1150?,and tempered at 500?,the average size of carbides are 2.2?m,2.6?m,3.2?m and 3.6?m.When the tempering temperature is550?,the average size of carbides are 2.3?m,2.7?m,3.3?m and 3.6?m.When temperatures of hot pressing are 1100?and 1150?.Under the premise of compactness,the increase of holding time has little effect on the hardness of tempered steel.When the HP temperature are 1100?(6h),1150?(6h),1190?,and 1240?,quenched at 1150?,and tempered at 500?,the hardness is HRC61.9±0.5,HRC60.9±0.4,HRC59.0±0.2 and HRC58.7±0.2.When the tempering temperature is 550?,,the hardness is HRC58.4±0.6,HRC 57.8±0.6,HRC57.7±0.3 and HRC57.6±0.5.These results show that under the same quenching and tempering conditions,when the hot-pressing temperature is lower,the number of carbides is huger and the size is smaller.The dispersion strengthening effect on martensite is higher,so the hardness is higher.When the holding time is 6h,the metallurgical combination of block steels in 1100?and 1150?are most sufficient,the three-point bending strength of tempered steels is highest.When the HP temperature are 1100?(6h),1150?(6h),1190?and 1240?,quenched at 1150?and tempered at 500?.the bending strengths are:2551±231MPa,2385±121MPa,2248±80MPa and 2167±323MPa.When the tempering temperature is 550?,the three-point bending strengths are:2694±65MPa?2634±108 MPa?2544±319 MPa and 2408±309 MPa.The reason why the bulk steel has higher bending strength is that the carbide size in the low-temperature hot-pressed bulk steel after quenching and tempering is smaller,and the effect of the increase in carbide size on the three-point bending strength exceeds the effect of the hardness on reducing the three-point bending strength. |
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