Investigation Of High Speed Steel Centrifugal Composite Casting Roll

In this thesis, many measures such as color and grey metallography, XRD, SEM and EDS were utilized to systematically analyze the microstructure, heat treatment technology and properties of high speed steel (HSS) composite rolls of Fe-1.8%C-4% Mo-5% Cr-X% W (wt.%) system and HSS of Fe-1.8%C-5% Cr-5% Mo-6%V without tungsten. The effects of content of W, C, V, Nb on HSS properties were studied. The impact of modification treatment by using RE, Ti, Mg, V and their composites on the morphology, size, distribution and volumetric fraction of different carbides was studied. The effect of modification treatment on the heat treatment technology and properties of the HSS used for composite rolls was studied.The following three methods were applied:(1) Sand mold casting, (2) 12 mm ID metallic mold casting, and (3) 180 mm ID metallic mold centrifugal casting.The main results obtained were as follows:1 The Fe-1.8%C-4%V-4% Mo-5% Cr-X% W (wt.%) system was studied and the results showed:1.1 The solidification structure consisted of martensite matrix, retained austenite and all kinds of carbides such as MC, M₂C, M₆C and MC/M2C complex carbides, the most of MC carbides were distributed in the crystals and all other kinds of carbides were distributed along the boundary of crystals. As the content of W was 2%, the granular MC carbides in the structure mainly contained V (31.70% in atomic fraction), featherlike or rodlike M₂C carbides mainly contained Mo (6.47%) and W (1.57%), fishbonelike M₆C carbides mainly contained W (12.70%) and Mo (5.07%), and complex carbides MC/M₂C mainly contained Mo (6.17%) and W (1.46%).1.2 The solidification structure and the type of carbides were greatly affected by tungsten content, when tungsten contents were lower, the carbides at the grain borders were featherlike M₂C and MC/M₂C complex carbides, and the granular MC was distributed inside the crystals; with the increase in tungsten content, the amount of MC granular carbides was decreased, and more fishbonelike M₆C carbides appeared at grain borders, the reticulation of carbides being more serious.1.3 Niobium was able to promote the formation of MC type carbide in the HSS, to generate complex MC type carbides with vanadium, increasing the density of carbides and reducing centrifugal segregation. Adding 1.5% Nb into the HSS containing W would increase the amount of granular MC carbides dispersed inside grains. The alloying element of vanadium was mainly distributed in the granular MC carbides, molybdenum mainly formed M₂C carbide distributed at grain borders, while chromium was largely distributed in the reticular carbides at the borders.1.4 During the heat treatment process of HSS without tungsten, the oxidation phenomenon was more severe, the burn-out ratio of the specimen blocks was about 10%, whereas it was decreased to about 5% for specimen blocks with 2.2%W, the oxidation during heat treatment being more closely controlled.1.5 After annealing, the hardness of HSS was between 37 and 40 HRC and its cut-machining was easy. The temperature at which the quenching hardness peak was observed ranged from 1000℃ to 1100℃ and the secondary hardening temperature was 550℃. As the content of W was 2%, the reasonable heat treatment technology was as follows: (1)Annealing technology: heating the specimen to 850～870℃, sustaining the temperature for 4 hours, cooling down to 750～760℃ in stove, and after sustaining the temperature for 5 hours cooling down once again below 600℃, and then cooling in air out of stove; (2)Quenching technology: quenching temperature controlled between 1000℃ and 1050℃; (3)Tempering technology:...