Zirconium
Atomic number 40
Density, 20° C (68° F) 6.4 g/cm3
Atomic weight 91.22
Melting point 1860° C (3380° F)
Boiling point 4377° C (7911° F)

GENERAL

Zirconium is highly reactive, and forms stable compounds with oxygen, sulfur, nitrogen and carbon. Its affinity for the first three of these elements accounts for its principal uses in steelmaking: the control of nonmetallic (sulfide and oxysulfide) inclusions and the fixation of nitrogen, primarily in boron steels. Zirconium will also inhibit grain growth and prevent strain aging, although its use for either of these functions is quite limited.

Although zirconium is reasonably plentiful, elaborate extractive metallurgical processes make the pure metal very expensive. Fortunately, these operations are not required for the zirconium addition agents used in steelmaking. Nonetheless, its relatively high price and the availability of cheaper replacements has restricted its general acceptance as an alloying agent in steels.


AVAILABLE FORMS

Among the more common zirconium addition agents are iron-silicon-zirconium (35-40% Zr, 46-52% Si), nickel-zirconium (67-74% Zr, 24-30% Ni), ferrozirconium (75% min. Zr), zirconium alloy scrap and pure zirconium sponge. Tin levels are typically 1.5% max. High purity ferrozirconium grades (0.30% max. tin) are produced from zirconium sponge. Complex proprietary alloys, sometimes, but not always used as carriers for boron, may also contain manganese, aluminum, silicon and titanium. These will usually contain well less than 10% Zr. Ferrozirconium, produced by the remelting of zirconium (zircalloy) and steel scrap may contain between 40 and 90% Zr, depending on grade. Nickel zirconium (30 or 70% Zr) is used as an addition agent in superalloys.

Choice of the addition agent used depends mainly on the intended application. Equally important is the level of silicon allowed in the steel composition. This in turn is related to the product properties (e.g., weldability of plates for line pipe), steelmaking method, deoxidation and desulfurization practices and special treatment, such as vacuum degassing, if any.


ADDITION PRACTICE

Zirconium is almost always added in the ladle, occasionally in the ingot mold. Furnace additions are not possible since high oxygen levels, in addition to the ever-present potential for reoxidation before solidification, will invariably lead to vanishingly small zirconium recoveries.

Because zirconium is so easily oxidized, it should only be added to fully (aluminum) killed steels. Thermodynamic data show that ZrO
2 is more stable at steelmaking temperatures, 1600 C (2912 F), than SiO2 or even Al2O3. However, its stability relative to that of alumina reverses at lower temperatures.

Precautions should be taken against reoxidation of the heat after zirconium addition. The use of shrouded nozzles, multiport nozzles, non-turbulent teeming streams and synthetic slags have all proved helpful in improving recoveries. Stable, basic ladle linings (such as magnesia) should be used since zirconium can reduce the SiO
2 or aluminosilicates found in acid refractories. Ladle spraying with magnesia or burned-dolomite base mix can also help. Further, the use of steel ladle desulfurization may be partially negated if acid ladle linings are used. The basic slags resulting from desulfurization can react with acid refractories to produce a sulfur-rich ladle glaze. This glaze can actually pump sulfur (and oxygen) back into the steel if powerful desulfurizers/deoxidizers such as zirconium are subsequently added.

When used as a microalloying agent, zirconium recoveries will invariably be quite low. One must bear in mind, however, that the function of zirconium in this respect is not to remain in solution in steel but to scavenge impurities (oxygen, sulfur, nitrogen) or modify inclusions through the formation of complex sulfides and oxysulfides. The efficacy of zirconium additions will therefore be measured not by the amount of residual "acid soluble" metal which remains, but by the extent to which inclusions are beneficially modified or, when so used, by the potency of desired boron additions.


ROLLING/FORGING

With its strong ability to fix sulfur, zirconium can be used as a partial replacement for manganese to prevent hot shortness. It has alternatively been proposed that the amount of sulfur fixed is equal to (%Zr-0.15)/10 and that sulfur will be entirely combined when the (unoxidized or otherwise combined) zirconium/sulfur ratio exceeds 1.41, the stoichiometric value for ZrS
2. Further, it has been observed that whereas a Mn/S ratio of 7.5 must ordinarily be maintained to eliminate hot shortness, the addition of zirconium reduces the limiting value of (Mn + Zr)/S to about 5.0.

Of course, the fixation of sulfur has a beneficial effect on transverse ductility and impact properties as well. Levels of zirconium between 0.03 and 0.30% are known to prevent the formation of detrimental Type II grain boundary film sulfides.


HEAT TREATMENT

The hardenability factor for zirconium probably lies between those for vanadium and titanium: moreover, unlike the case with these strong carbon scavengers, there is no drop-off in hardenability factor as zirconium levels are raised. Nonetheless, for practical and economic reasons, zirconium is not used for the sake of promoting deep hardening.

The presence of zirconium compounds does, however, reduce grain coarsening, permitting the use of higher hardening or carburizing temperatures. Zirconium produces only slight changes in the mechanical properties of quenched and tempered steels, though these changes are generally beneficial. It produces a more uniform distortion during heat treatment than, for example, vanadium. Possibly because of its modification of nonmetallic inclusions, zirconium improves ductility and impact strength with most significant changes occurring in the transverse direction. It also raises the yield/tensile ratio and improves weldability through the reduction of underbead cracking and the elimination of porosity. In high alloy steels, zirconium increases hardness but decreases ductility. In stainless steels, zirconium retards the formation of sigma phase.


APPLICATIONS

Several quenched and tempered HSLA steels, produced domestically and in Europe, contain 0.10-0.15% Zr, mainly for sulfide shape control. These steels will have yield strengths of 550 or 690 MPa (80 or 100 ksi) depending on grade and have general structural applications. Among alloy and HSLA steels listed under ASTM/ASME specifications, grades A514 (YS = 620 MPa, 90 ksi and A588-D (YS = 290 MPa, 42 ksi) will contain 0.05-0.15% Zr.

Zirconium is also a constituent of several nonferrous alloys, the most notable being the zircaloy series, one grade of which is used extensively as a fuel cladding material in light-water nuclear reactors.

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