As-cast carbides in high-speed steels
- PDF / 2,559,765 Bytes
- 8 Pages / 597.28 x 785 pts Page_size
- 19 Downloads / 227 Views
I.
INTRODUCTION
T H E basic feature o f the microstructure in high-speed steels is that it contains a lot o f carbides. F o r a given alloy composition, carbide content, size, shape, and spatial distribution are determined in the course o f solidification. It has been known that the cutting performance of high-speed steels is determined by the amount o f primary carbide and the degree o f secondary hardening conferred upon the matrix during tempering. Analyses of high-speed steels indicate that the contribution o f matrix hardening and the blocky carbides can be roughly equal, ll-41 The distribution o f the primary carbide also has an influence on the toughness o f the tools because large carbide clusters nucleate microcracks w h i c h eventually cause rupture. Is'6] Thus, the structure, morphology, transformation, solution, and precipitation behavior o f cast carbide have been the most important considerations in the research o f high-speed steels. Most o f the early studies were focused on the solidification process,17'sl carbide composition, 19'~°j and morphology o f cast carbides in several general high-speed steels, lll,12,t31 and they remain the main gap in the understanding o f the influence o f alloy elements on the cast primary carbide. The present article is a systematic investigation o f the effects o f alloying elements W , Mo, and V on the morphology and amount o f primary carbide in different highspeed steels. The alloys investigated represent the whole range o f customary high-speed steel compositions. II.
EXPERIMENTAL
Experimental materials include five groups according to the tungsten and molybdenum percent: W18Cr4, Wl2Mo3Cr4, W9Mo4.5Cr4, W6Mo5Cr4, and Mo9Cr4. Vanadium content was changed from 0 to 4 pct in each group. Zero, 1, 2, 3, and 4 pct were used, respectively. All the steels investigated contained 0.8 pct C and 4 pct Cr. The tungsten equivalent is 18. Cast specimens were prepared by remelting in a hightemperature furnace. Specimens were melted in small crucibles under an argon atmosphere. The cooling rate PEIDAO DING, Professor, GONGQI SHI, Associate Professor, and S H O U Z E Z H O U , Professor, are with the Department of Metallurgy and Materials Engineering, Chongqing University, Chongqing 630044, People's Republic of China. Manuscript submitted March 1 9 , 1992. METALLURGICAL TRANSACTIONS A
used was 5 °C/min, w h i c h is similar to the cooling rate used in the practical solidification o f industrial ingots. The temperature profile was recorded by Pt-PtPh thermocouples embedded in representative samples. By using this method, all o f the samples were prepared using the same conditions. The errors caused by the differences, o f course, can be avoided. Carbide powder was extracted from prepared as-cast specimens with methanol electrolyte at a current density o f 0.05 A / c m2. The electrolytic temperature was - 5 °C. The p o w d e r was dispersed by an ultrasonic oscillator in absolute alcohol, which was used as a dispersing agent, and then was placed on aluminum foil for examination
Data Loading...
