Carbides segregation

P/M Tool Steels. In conventionally produced high alloy tool steels (slowly cooled cast ingots), carbide tends to segregate (48). Segregated clusters of carbide persist even after hot working, and cause undesirable effects on tool fabrication and tool performance. P/M tool steels, on the other hand, provide very fine and uniform carbides in the compact, the final bar stock, and the tools. Several tool steel suppHers consoHdate gas-atomized tool steel powder by HIP to intermediate shapes, which are then hot-worked to final mill shapes. Water-atomized tool steel powder is also available (see also T OOL materials). ... 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

The Segregation of Carbides from Stainiess Steei Containing Smaii Amounts of Carbon... 

Now to complete the solution of the problem one would need to know the solution laws for iron, and a small amount of nickel, in each of these carbide phases, since equilibrium requires that and a , in the segregated carbide must be 0-74 and 0-08 respectively as well as a., being 0-18. At present nothing is known about these laws except that the metal atoms might well be randomly distributed in the carbide phase, in which case, as an example. 

Other potential applications are ceramic powders coated with their sintering aids, zirconia coated withyttria stabilizer, tungsten carbide coated with cobalt, or nickel, alumina abrasive powders coated with a relatively brittle second phase such as MgAl204 and plasma spray powders without the segregation of alloying elements. 

The metal is most often recovered from its principal ore, zircon. The ore is mined, crushed and preliminary segregation is by gravity, electrostatic, and magnetic separation. Separated ore mixed with carbon is charged into an arc furnace and heated to about 3,500°C. This forms zirconium carbide and silicon monoxide, and the monoxide is driven off as vapor. Zirconium carbide is then placed in a chlorinator and heated with chlorine gas at high temperatures. The carbide is converted to zirconium tetrachloride, ZrCfl. Also, small amounts of hafnium that is always associated with zirconium converts to its tetrachloride, HfCfl. 

Gu, H., Shinoda, Y., and Wakai, F., Detection of boron segregation to grain boundaries in silicon carbide by spatially resolved electron energy-loss spectroscopy , J. Am. Ceram. Soc., 1999, 82, 469-72. 

A mixture of a BMI (Compimide 796) with 4,4 -bis(orf/zo-propenylphe-noxy)benzophenone (TM 23) was blended in solution with Udel 700 (polysul-fone from Union Carbide), Ultem 100 (General Electric polyether imide), and PH 10 (Bayer polyhydantoin) (Fig. 35). The thermoplastics were introduced at various concentrations up to 33%. A phase segregation did not appear with PH 10, but two phases were observed with Ultem. With both semi-IPNs the observed Glc were found to be four- to fivefold the Glc of the neat BMI [113]. 

The performance of a cast nickel-based alloy is generally based on the microstructural quality, such as the amount of interdendritic segregation, secondary carbides, and intermetallic phases. With the same overall chemical composition, the corrosion rate of the same alloy can vary by several orders of magnitude, depending on its particular microstructure. The most important metallurgical factors that need to be considered are second-phase precipitation by thermal instability and the presence of cold work. The latter is especially important in cases where SCC may be expected. (Rebak)5... 

Step cooling will not simulate embrittlement of 1 WCM Mb, though it occurs (e.g., a 100°F [38°C] increase in transition temperature was reported after 8 y at 930° F [500° C]). This is because the embrittlement in 11/iCr-1 Mo is caused by precipitation of carbides in the ferrite phase rather than segregation of impurities to the grain boundaries. Temper embrittlement can be reversed by heating at 1,150°F (620°C) for 2 h per inch of thickness. 

Titanium in steel number 405 was found both in the interdendritic austenite and in the carbides. Niobium in steel 406 segregated to the interdendritic ferrite and was also present as carbides. In steel 413, the interdendritic areas were enriched with copper. 

Although several impurities are known to segregate in these boundaries, correlations have been made with essentially continuous films of iron carbide or segregated carbon. Maximum susceptibility occurs in the range of 0.005% C it is proposed that lower carbon contents do not Table 7.9 Effect of alloying elements on stress-corrosion resistance ... 

---