In this tutorial, the effects of selected elements in the steels are discussed.
Aluminium (AI)
Aluminum is the most powerful, very frequently used deoxidizing and also detriding agent. It also has an extremely favorable effect on resistance to ageing. Small additions assist fine-grained structure. In nitriding steels, it is usually an alloying element and forms very hard nitrides with nitrogen. Since it increases scaling resistance, it is frequently added to alloy ferritic heat resistant steels. With unalloyed carbon steels, scaling resistance can be promoted by introduction of aluminum into the surface. Aluminum very sharply restricts the gamma phase. Aluminum is also an alloying element in iron-nickel-cobalt-aluminum permanent magnet alloys, since it greatly increases coercive field intensity. Fig. 1 below shows Solubility of AI-N in austenite at temperatures below 1260C.
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Cobalt (Co)
Cobalt is frequently used as alloying element in high speed steels, hot forming tool steels, creep-resistant and high temperature materials. It inhibits grain growth at elevated temperatures and improves retention of temper and high temperature strength. Cobalt does not form any carbides. Instead, it promotes graphite formation. In large quantities, it increases remanence, coercive field intensity and thermal conductivity, and therefore alloying base for super high quality permanent magnet steels and alloys.
Chromium (Cr)
Chromium is a primary contributor for corrosion resistance of steels; normally 13% chromium is necessary for such a good feature, and this must be dissolved in the matrix. Scaling resistance of steels increases with increasing Cr contents. Chromium reduces critical rate of cooling necessary for martensite formation, and thus increases steels hardenability and and its susceptibility to hardening and tempering. It also renders steels oil and air-hardenable. The tensile strength of the steel increases by 80-100 N/mm per 1% Cr. High temperature strength and highpressure hydrogenation properties are also promoted by chromium. As a disadvantage, it reduce notch toughness, and weldability decreases in pure chromium steels. Thermal and electrical conductivity, as well as thermal expansion are reduced. Cr is a carbide former. Its carbides increase the edge-holding quality and wear resistance. The element restricts the gamma phase and thus extends the ferrite range. It does however stabilize the austenite in austenitic Cr-Mn and Cr-Ni steels. With simultaneously increased carbon content, Cr contents up to 3% increase remanence and coercive field intensity. For low carbon content, e.g., 0.10% C and 12% Cr, the hardness obtained on hardening is very modest.
Copper (Cu)
Copper is regarded as a steel parasite in most cases, since it concentrates under the layer of scale and through penetrating into the grain boundary, causes high surface sensitivity in hot forming processes. The yield point and the yield point/strength ratio are increased. However, in alloy and low alloy steels, Cu produces significant improvement in weathering resistance. In acid resistant high alloy steels, a Cu content above 1% produces improvement in resistance to hydrochloric acid and sulphuric acid. Hardenability is improved. Weldability is not affected by copper. Contents above 0.30% can cause precipitation hardening.
Lead (Pb)
Lead improves machinability. It is added to free-cutting steels in contents of about 0.2-0.5%. Due to its extremely fine suspension-like distribution, it forms shorter chips and clean faces of cut. The lead contents stated hardly affect the mechanical properties of the steels at all.
Manganese (Mn)
Manganese increases hardenability, yield point and strength. In addition, it improves forgeability and weldability and increases hardness penetration depth. Manganese deoxidizes. It also compounds with sulphur to form Mn sulphide, thus reducing the undesirable effect of the iron sulphide. Besides, it reduces the risk of red shortness. Contents > 4% also lead with slow cooling to formation of brittle martenstic structure, so that the alloying range is hardly used. Steels with Mn contents > 12% are austenitic if the C content is also high, because Mn considerably extends the gamma phase. Such steels are prone to very high degree of strain hardening and thus are highly resistant to wear under the influence of impact. Steels with Mn contents of at least 18% remain unmagnetizable even after relatively large cold forming. Therefore they are used as special steels and steels working at subzero low temperature stress. The coefficient of thermal expansion increases with increasing Mn, while thermal and electrical conductivity decrease.
Molybdenum (Mo)
Molybdenum is usually alloyed together with other elements and it favorably impacts steel quality. It increases yield point, strength and weldability. It significantly reduces temper brittleness, such as in the case of Cr, Ni and Mn steels, promotes fine grain formation. By reducing the critical cooling rate, it improves hardenability. Mo is a remarkable carbide former; and thus it improve cutting properties with high speed steel. It increases scaling and corrosion resistance and is used frequently with austenitic Cr-Ni steels and highCr steels. The only drawback is that in some cases forgeability is reduced with addition of Mo.
Titanium (Ti)
Titanium has a remarkable deoxidizing, denitriding, sulphur bonding and carbide forming effect. It is used widely in stainless steels as carbide former for stabilization against intercrystalline corrosion. Ti also restricts the gamma phase very remarkably and possesses grain refining effect. In high concentration, it leads to precipitation processes and is added to permanent magnet alloys for high coercive field intensity. Ti increases creep rupture strength through formation of special nitrides. Finally, Ti tends to segregation and banding.