Carbide hardening alloys

Carbide hardening alloys, including cobalt-base/carbide (WC-Co) and some iron-base superaUoys... 

Decarburisation Carbon removal causes the dissolution of carbides degrading the creep properties of carbide-hardened alloys. Occurring at high temperature, this damage is fast and deep. Shindo and Kondo [14] show that the creep rupture life of Hastelloy XR shortens greatly under decarburising helium. 

Prevention of abrasive wear is possible through proper material selection and the use of surface engineering treatments. A number of material families have demonstrated good resistance to abrasive wear. They are typically hard materials that resist scratching, and include ceramics, carbide materials, alloyed white cast irons containing hard chromium carbides (see Fig. 8), and hardened alloy steels. 

When a steel is cooled sufficiendy rapidly from the austenite region to a low (eg, 25°C) temperature, the austenite decomposes into a nonequilihrium phase not shown on the phase diagram. This phase, called martensite, is body-centered tetragonal. It is the hardest form of steel, and its formation is critical in hardening. To form martensite, the austenite must be cooled sufficiently rapidly to prevent the austenite from first decomposing to the softer stmeture of a mixture of ferrite and carbide. Martensite begins to form upon reaching a temperature called the martensite start, Af, and is completed at a lower temperature, the martensite finish, Mj, These temperatures depend on the carbon and alloy content of the particular steel. 

Carbides of the Iron Group Metals. The carbides of iron, nickel, cobalt, and manganese have lower melting points, lower hardness, and different stmctures than the hard metallic materials. Nonetheless, these carbides, particularly iron carbide and the double carbides with other transition metals, are of great technical importance as hardening components of alloy steels and cast iron. 

Austenitic stainless steels are the most corrosion-resistant of the three groups. These steels contain 16 to 26 percent chromium and 6 to 22 percent nickel. Carbon is kept low (0.08 percent maximum) to minimize carbide precipitation. These alloys can be work-hardened, but heat treatment will not cause hardening. Tensile strength in the annealed condition is about 585 MPa (85,000 Ibf/in"), but workhardening can increase this to 2,000 MPa (300,000 Ibf/in"). Austenitic stainless steels are tough and ducdile. 

Hardenable stainless steels usually contain up to 0.6% carbon. This is added in order to change the Fe-Cr phase diagram. As Fig. 12.7 shows, carbon expands the y field so that an alloy of Fe-15% Cr, 0.6% C lies inside the y field at 1000°C. This steel can be quenched to give martensite and the martensite can be tempered to give a fine dispersion of alloy carbides. 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

Ni,Fe)4>ased superalloys. Ni,Fe-based siqieralloys, such as 718, can behave in a complex fashion, which is associated with the formation of various carbides and the interplay between three major precipitated phases S based on NisNb, 7 based on Ni3Al and a metastable phase 7" which is related to the 6 phase. Inconel 625 (IN625) was the prototype for the Nb-hardened NiFe-type superalloys and it is instructive to look at the complex precipitation phenomena which occur in this alloy which has the composition Ni-21.5Cr-9Mo-3.6Nb-5Fe-0.2Al-0.2Ti-0.05C (in wt%). 

Cobalt in small amounts is an essential element associated with vitamin B12, but at high levels can be toxic. There are no daily recommended intake levels for cobalt. Intestinal bacteria use cobalt to produce cobalamin, which in turn is an essential component of vitamin B12. Industrially, cobalt is used in pigments, permanent magnets, and as an alloy to harden metals as in tungsten carbide blades or drills. 

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