Éléments d'alliage

C’est le plus puissant désoxydant et par ailleurs aussi désazotant et il est très souvent employé ; de ce fait, il agit aussi très positivement sur la résistance au vieillissement. En petits ajouts, il soutient la formation de petits grains. Comme l’aluminium forme des nitrures d’azote très durs, il est le plus souvent un élément d’alliage dans les aciers nitrurés. Il augmente la résistance à l’oxydation et est de ce fait souvent allié aux aciers ferriques résistants à la chaleur. On peut augmenter la résistance à l’oxydation des aciers au carbone non alliés par « aluminiage » (ajout d’aluminium dans la surface). L’aluminium restreint la zone gamma très fortement. En raison de la forte augmentation de la force coercitive, l’aluminium est un élément des alliages pour aimants permanents en fer-nickel-cobalt-aluminium.

Il restreint également la zone gamma et il est, comme le phosphore, nuisible à l’acier en raison de sa tendance à la ségrégation. Mais l’élimination des ségrégations par le recuit d’homogénéisation est encore plus difficile que pour le phosphore. Par ailleurs, il augmente la fragilité au revenu, abaisse fortement la ténacité et influence la soudabilité.

Comme le bore présente une grande section efficace pour l’absorption de neutrons, on l’utilise souvent pour allier les aciers de régulateurs et de blindages d’installations d’énergie atomique. Les aciers 18/8 CrNi austénitiques peuvent atteindre une plus haute limite d’élasticité et une plus grande solidité à l’aide de bore vie des durcissements par précipitation, mais cela réduit leur résistance à la corrosion. Les précipitations provoquées par le bore améliorent les propriétés de résistance aux températures très élevées pour les types aciers résistant aux températures très élevées. Dans les aciers de construction, cet élément améliore le durcissement et permet ainsi d’obtenir une plus grande résistance à cœur des aciers de cémentation. Il faut cependant s’attendre à une diminution de la soudabilité d’aciers alliés au bore.

On fabrique des ressorts en spiral pour montres en alliages cuivre-béryllium, car ils ne sont presque pas magnétisables et supportent un bien plus grand nombre de variations de charges que des ressorts en acier. Les alliages nickel-béryllium sont très durs et résistent très bien à la corrosion ; utilisation dans des instruments chirurgicaux. Très forte restriction de la zone gamma. À l’aide de béryllium, on peut obtenir des durcissements par précipitation, mais on diminue la ténacité; fortement désoxydant, grande affinité avec le soufre.

Le carbone est l’élément d’alliage le plus important et le plus influent de pour l’acier. Outre le carbone, tout acier non allié contient du silicium, du manganèse, du phosphore et du soufre qui s’y ajoutent involontairement pendant la fabrication. L’acier allié est obtenu par l’ajout d’autres éléments d’alliage en vue de certains effets ainsi que l’augmentation intentionnelle des taux de manganèse et de silicium. Un taux de carbone plus élevé augmente la résistance et la trempabilité de l’acier, mais il diminue son élasticité, sa forgeabilité, sa soudabilité et son usinabilité (à l’aide d’outils enlevant des copeaux). La résistance à la corrosion vis-à-vis de l’eau, des acides et des gaz chauds n’est pratiquement pas influencée par le carbone.

Utilisé en combinaison avec le silicium, sous forme de silico-calcium, pour la désoxydation. Le calcium augmente la résistance à l’oxydation des matériaux pour conducteurs chauffants.

Has a purifying effect, as it strongly deoxidizes and promotes desulfurization; it is usually used together with lanthanum, neodymium, praseodymium and other rare precious metals as a "mixed metal". In high-alloy steels, it promotes hot workability in some cases and improves scaling resistance in heat-resistant steels. Iron-cerium alloys with about 70% cerium are pyrophoric (flint). Addition in spheroidal graphite cast iron.

Cobalt does not form carbides; it inhibits grain growth at elevated temperatures and greatly improves temper brittleness and high-temperature strength; therefore, often alloying element in high-speed steels, hot-work steels, high-temperature and high-temperature materials. Favors graphite formation. In large proportions it increases remanence, coercivity and thermal conductivity; therefore alloying base for high-quality permanent magnet steels and alloys. Under neutron irradiation, the highly radioactive isotope 60Co is formed, which is why cobalt is undesirable in steels for nuclear reactors.

Chromium makes steel oil- or air-hardenable. By reducing the critical cooling rate required for martensite formation, it increases hardenability and thus improves temperability. Notched impact strength, however, is reduced, but it lowers elongation only very slightly. Weldability decreases with increasing chromium content in pure chromium steels. The tensile strength of the steel increases around 80-100 N/mm2 per 1% chromium. Chromium is a carbide former. Its carbides increase cutting ability and wear resistance. High-temperature strength and resistance to hydrogen under pressure are favored by chromium. While increasing chromium contents increase scale resistance, a minimum content of about 13% chromium is required for corrosion resistance of steels, which must be dissolved in the matrix. The element cuts off the gamma region, thus extending the ferrite range; but stabilizes the austenite in austenitic chromium-manganese or chromium-nickel steels. Thermal conductivity and electrical conductivity are reduced. Thermal expansion is lowered (alloys for glass melting). With a simultaneously higher carbon content, chromium contents of up to 3% increase remanence and coercivity.

Copper is only added to a few steel grades because it accumulates under the scale layer and, by penetrating the grain boundary, causes great surface sensitivity during hot forming processes, which is why it is sometimes regarded as a steel pest. Yield strength and yield strength ratio are increased. Contents above 0.30% can cause hardening. Hardenability is improved. Weldability is not affected by copper. In unalloyed and low-alloy steels, a significant improvement in weather resistance is achieved by copper. In acid-resistant high-alloy steels, a copper content above 1% provides improved resistance to hydrochloric acid and sulfuric acid.

This element is a steel pest because it causes embrittlement due to a drop in elongation and necking without an increase in yield strength and tensile strength. Hydrogen is the cause of the dreaded flake formation and favors the formation of shadow streaks. Atomic hydrogen generated during pickling penetrates the steel forming bubbles. Moist hydrogen decarburizes at higher temperatures.

This element is added as a deoxidizing and desulfurizing agent. In cast iron, magnesium produces nodular graphite.

Manganese deoxidizes. It bonds sulfur as manganese sulfides and thus reduces the unfavorable influence of the iron sulfide. This is of particular importance in free-cutting steel; the risk of red fracture is reduced. Manganese reduces the critical cooling rate very considerably and thus increases hardenability. Yield strength and strength are increased by manganese addition. Furthermore, manganese has a favorable effect on forgeability and weldability and greatly increases the hardening depth. Contents above 4% lead to the formation of a brittle martensitic microstructure even during slow cooling, so that the alloying range is hardly used. Steels with manganese contents above 12% are austenitic with a simultaneously high carbon content, because manganese expands the gamma range considerably. Such steels obtain a very high work-hardening under impact stress on the surface, while the core remains tough; they are therefore highly wear-resistant under impact. Steels with manganese contents of 18% upwards remain non-magnetizable even after relatively severe cold working and are used as special steels and also as cold-hardening steels for low-temperature stressing. Manganese increases the coefficient of thermal expansion, while thermal conductivity and electrical conductivity decrease.

Molybdenum usually alloyed with other elements. By reducing the critical cooling rate, hardenability is improved. Molybdenum largely reduces tempering brittleness, for example in chromium, nickel and manganese steels, promotes fine grain formation and has a favorable effect on weldability. Increase in yield strength and strength. With higher molybdenum content, cuttability is impeded. Strong carbide former; it improves cutting properties in fast-working steels. It belongs to any element that increases corrosion resistance and is therefore widely used in high-alloy chromium steels and in austenitic chromium-nickel steels; high molybdenum contents reduce the susceptibility to pitting. Very strong narrowing of the gamma range; increase in high-temperature strength, scaling resistance is reduced.

This element can appear both as a steel pest and as an alloy component. Harmful because of reduction of toughness by precipitation processes, induction of ageing sensitivity and blue brittleness (deformation in areas of blue heat 300-350°C), and the possibility of inducing intergranular stress corrosion cracking in unalloyed and low-alloy steels. As an alloying element, nitrogen extends the gamma region and stabilizes the austenitic microstructure; in austenitic steels, it increases strength and, above all, yield strength and mechanical properties in heat. Nitrogen allows high surface quality to be achieved through nitride formation during nitriding.

These elements occur almost only together and are very difficult to separate from each other, so they are usually used together. Very strong carbide formers, therefore alloyed especially as stabilizers chemically resistant steels. Both elements are ferrite formers and thus reduce the gamma range. As a result of the increase in high-temperature strength and creep rupture strength due to niobium, it is often added to high-temperature austenitic boiler steels. Tantalum has a high absorption cross-section for neutrons; only tantalum-poor niobium can be considered for nuclear reactor steels.

Increases yield strength and impact toughness in structural steels. Nickel is also used in case-hardening and quenched and tempered steels to increase toughness. The element extends the gamma region and therefore effects the austenite structure in corrosion and scale resistant chromium-nickel steels. High nickel contents lead to steels with small temperature expansion (e.g. Invar).

This element is a steel pest, it lowers the toughness of the steel; it cuts off the gamma region. In general, little information is available about this alloying element.

Lead is actually not a "real" alloying element, since it hardly influences the given properties at certain alloying contents. This alloying element is added in contents of approx. 0.2%-0.5% to significantly improve machinability. Shorter chips and clean cutting surfaces are produced. Another application of lead is in bearings, where its excellent sliding properties are exploited.

This element is mostly a steel pest and strongly alloyed. However, phosphorus is often used in small quantities in free-cutting steels. Phosphorus is usually considered as a steel pest, as it gives strong primary segregations during solidification of the melt and the possibility of secondary segregations in the solid state due to the strong constriction of the gamma region. As a result of the relatively low diffusion rate, both in the alpha and in the gamma solid solution, given segregations can only be compensated with difficulty. Since it is hardly possible to achieve a homogeneous distribution of the phosphorus, one tries to keep the P content very low and accordingly to aim at an upper limit of 0.03%-0.05% for high-quality steels. The extent of segregation cannot be determined with certainty Phosphorus increases sensitivity to tempering embrittlement even at the lowest levels. P embrittlement increases with increase in C content, with increasing hardness temperature, with grain size and with reduction in degree of forging. The embrittlement appears as cold brittleness and sensitivity to impact stress. In low-alloy structural steels with C contents of about 0.1%, phosphorus increases strength and corrosion resistance to atmospheric effects; Cu assists in improving corrosion resistance (stainless steels). Phosphorus additions can cause yield strength increases and precipitation effects in austenitic chromium-nickel steels.

This element is a steel pest. Oxygen deteriorates the technological properties notch toughness and aging. In addition, the element produces red fracture and promotes wood fiber breakage.

This element is the most alloying. Iron sulfide, reinforced by oxygen, leads to red fracture. In addition, the toughness is deteriorated. Sulfur in free-cutting steels up to 0.3% increases machinability.

This element has a deoxidizing effect and narrows the gamma range. Silicon increases strength and wear resistance. This alloying addition also greatly increases the elastic limit, so it is added to spring steels. At high levels of silicon, the addition increases scaling resistance and acid resistance, but decreases electrical conductivity and coercivity; therefore, silicon is used in electrical sheets.

This element is a strong deoxidizer and carbide former. Therefore, titanium is often used as a stabilizer in corrosion-resistant steels.

This element is a strong carbide former. Vanadium binds nitrogen and produces a fine-grained cast structure. It increases wear resistance due to hard carbides, as well as hot strength and temper resistance. Vanadium is therefore added to high-speed steel, hot-work tool steel and high-temperature steel. It also increases the elastic limit in spring steels.

Tungsten increases tensile strength, yield strength and toughness. It is also a strong carbide former (hard carbides). Tungsten increases hot strength and wear resistance, so it is used as an additive in high speed steel and hot work tool steel.

This element is a steel pest and alloy strongly.

In special cases, this element is used as an additive element for deoxidation, dendrification and desulfurization. It is a strong carbide former. Zr additions to fully killed free-cutting steels containing sulfur exert a favorable influence on sulfide formation and thus avoidance of red fracture. It increases the service life of heatsealing band materials and causes a narrowing of the gamma range.