Guide Handbook of Ferroalloys: Chapter 16. Technology of Zirconium Ferroalloys

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Table of contents

• Foundry Practice for Ferrous Alloys | Metals Handbook Desk Edition | Handbooks | ASM International
• Account Options
• Handbook of ferroalloys : theory and technology
• Handbook of Ferroalloys
• Non-ferrous metal

The type of recarburizer chosen depends on the cast iron being produced. Crystalline recarburizers, i. If the tap sulfur content is below this value, it may be more efficient to perform primary recarburization with high sulfur petroleum coke and trim with graphite just before casting. When inoculation is to be avoided, as in white irons, coke is the preferred addition agent. However, if nitrogen is known to produce porosity defects, it can either be avoided by using graphite or a low-nitrogen coke or controlled through the addition of titanium, zirconium or similar stabilizing elements.

It should be noted that in adding carbon, a reduction in temperature of 2.

In general, such processes become more difficult as carbon content increases. The effect of carbon is first felt in the soaking pit or re-heat furnace. High carbon steels are more sensitive to thermal shock and must be heated slowly to avoid cracking. Step heating - allowing the ingot to equalize in temperature at several plateaus before the rolling or forging temperature is reached - may be necessary, especially for large cross sections. Steels with more than 0. This leads to cracking or, at best, unacceptable surface conditions in the final product and almost always requires that the burned ingot be reverted to scrap.

High carbon steels should therefore be heated slowly and evenly, avoiding hot spots due to direct flame impingement. Both hot- and cold-rolling forces increase with carbon content. In hot rolling, the effect becomes more pronounced as the finishing temperature is approached. An additional 0.

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The energy required for cold working is strongly dependent on carbon content, a consequence of the proportion of pearlite in the microstructure. The need for intermediate anneals therefore increases with carbon content, all other factors being equal. It should be noted that the carbon has a relatively strong tendency to segregate in thick sections e.

This can lead to nonuniform carbon distribution in the final product, such as the familiar "banding" caused by phosphorus segregation: high P areas rejecting C seen in hot rolled sheet and plate. This is not necessarily detrimental, however. For steels utilizing microalloying additions, the ratio of atomic percent microalloying element MAE to carbon percent determines the amount of MAE precipitate formed at a given low temperature.

Here, both cold rolled and annealed sheet steels rely on carbon content being under 0. Carbon increases the strength of hot rolled steels but decreases the notch toughness, ductility and weldability. Reference Vanadium, Columbium, and Titanium for details on carbon's use in continuously cast and hot rolled steels. Essentially carbon-free steels such as the maraging grades are heat treated as well, but these are special cases, which will not be considered here.

Unless added for specific reasons, e.

Carbon dissolves in iron, but the solubility limits depend on crystal structure. High temperature d-ferrite can contain up to 0. Less than 0.

The iron-carbon equilibrium diagram Fig. At C F , d-ferrite containing more than 0. Iron containing more than 2. At C F , austenite decomposes eutectoidally into the lamellar composite, pearlite. Carbon lowers the allotropic g - a transformation temperatures from C F for pure iron to the eutectoid temperature at 0. Below the eutectoid C, F , carbon has a strong influence on the kinetics rate of pearlite formation and reacts with iron to form the nonequilibrium phases bainite and martensite.

Pearlite forms at higher temperatures, between about C F and the eutectoid temperature, becoming increasingly finer in structure as the transformation temperature is lowered. Between roughly C F and the lower limit of the pearlite formation range, austenite transforms to bainite, of which there are two main types: Upper bainite is formed at higher temperatures.

It has an acicular structure containing cementite particles oriented along the boundaries of the ferrite regions. Lower bainite is also acicular, though much finer in structure. In it, the carbide particles are oriented across the ferrite regions, a fact that contributes to its higher toughness. The temperature dividing upper and lower bainite is a function of composition, especially carbon content. Both bainites grow at rates determined largely by the diffusion of carbon in iron. The diffusionless, or shear, transformation of austenite to martensite at temperatures below about C F is the most important reaction in commercial heat treating.

The martensite start, or MC as the eutectoid composition is approached. If the product application requires a hard, wear resistant surface but a tougher, more ductile core, the steel may be carburized. In this process, carbon is intentionally diffused into the surface layer of a low carbon steel, normally to a depth not exceeding a few thousandths of an inch. Carburizing is carried out at about C F , and it is important that the steel's composition allows it to remain fine grained at this temperature.

After carburizing, the steel will be heat treated as usual. A full listing of their applications is obviously impossible. Carbon steels are used as castings and forgings, pipe and tubes, sheet and plate, wire, rod, rails and structural shapes. Carbon steels are, of course, the least expensive ferrous alloys and designers will endeavor to specify them unless specific properties necessitate the use of more expensive alloy grades.

Carbon steels may be classified in several ways. Many of these standards identify the same steels by their own individual specifications. The individual user may choose to add such requirements to any general specification to suit his needs. Many large users, such as automotive and construction equipment manufacturers, prefer to establish their own standards, which may be more restrictive than those published by national organizations.

Sheet steels tend to have the lowest carbon levels under 0. In general, ultra-low carbon steels include high formability sheet steels with carbon under 0. Low carbon steels include most hot rolled strip, plate and pipe with 0. Medium carbon steels include the forging grades with 0. The high carbon steels include rail steels with over 0. In decreasing order or abundance, other important members of this series include lanthanum, neodymium, praseodymium and yttrium.

While several rare earth metals REM have commercial applications in the electronics and glass industries, they are supplied to steelmakers cost effectively only in the form of mixtures of the metals or their compounds. The following discussions will therefore deal with the REMs as a group.

Handbook of Ferroalloys

The REMs are, in fact, plentiful enough that several thousand tons of metal and compounds are used annually. The leading supplier country today is China. Mischmetal is an alloy of REMs with a composition corresponding roughly to rare earth concentrations in the ore, from which they are electrolytically extracted. It is sold as precast canisters suitable for a plunging addition or as small piglets or pellets.

Rare earth silicide contains approximately equal portions of REMs, silicon and iron. It is less reactive than mischmetal, and is not intended for plunging additions. Silicide is sold in lump form, generally not finer than 1 in. Rather, their function is to control the shape of inclusions remaining in a steel which has already been deoxidized and desulfurized, and their efficient use depends strongly on prior treatments the steel has received. When REMs are added to steel, no matter in which form, they combine with oxygen and sulfur, and with oxides, sulfides and silicates already present, in many cases reducing them completely.

Recommended practice for highest recovery of REM additions includes the following: 1 establishing a tap sulfur content in the range 0. For a well deoxidized steel, REM additions should be calculated on the basis of 0. For oxide inclusion control, total oxygen must be considered. In practice, therefore, REM additions will range 0. REM silicides are added to the second pouring ladle when reladling a heat, or to the ingot mold. Addition is made to the pouring stream to insure adequate dispersion. Fines should be avoided as these have a tendency to clump.

Tap silicon levels should not be near specified maximum values, due to the full recovery of silicon from this addition agent. REM oxide fumes are generally believed to be non-toxic. Mischmetal additions are made by plunging precast canisters of the REM alloy into the ladle. Canisters of appropriate weight, depending on residual sulfur level, are attached to the end of a billet to provide deep immersion in the ladle. The above recommendations regarding desulfurization, deoxidation, slag treatment and ladle temperatures apply. Further precautions against reoxidation are strongly advised.

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The fact that some reoxidation will invariably occur reduces recovery from plunging below that obtainable with ingot mold practice. If reoxidation is severe enough, sulfur will revert to the melt from REM oxysulfides. Mischmetal additions may also be made to the molds, lbs.