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Mechanical Engineering Design Notes

Materials Contents

Effects of Alloying Additions to Steels

Element Influence Uses
Carbon Most important alloying element. Is essential to the formation of cementite and other carbides, bainite and iron-carbon martensite. Within limits increasing the carbon content increases the strength and hardness of a steel while reducing its toughness and ductility. Added to construction steels to increase strength, hardness and hardenability.
Nickel Stabilises gamma phase by raising A4 and lowering A3. Refines grains in steels and some non-ferrous alloys. Strengthens ferrite by solid solution. Unfortunatly is a powerful graphitiser. Used up to help refine grain size. Used in large amounts in stainless and heat-resisting steels.
Manganese Deoxidises the melt. Greatly increases the hadenability of steels. Stabilises gamma phase. Forms stable carbides. High manganese (Hadfield) steel contains 12.5% Mn and is austenitic but hardens on abrasion.
Silicon De-oxidises melt. Helps casting fluidity. Improves oxidation resistance at higher temperatures. Up to 0.3% in steels for sandcasting, up to 1% in heat resisting steels.
Chromium Stabilises alpha phase by raising A3 and depressing A4. Forms hard stable carbides. Strengthens ferrite by solid solution. In amounts above 13% it imparts stainless properties. Unfortunately increases grain growth. Small amounts in constructional and tool steels. About 1.5% in ball and roller bearings. Larger amounts in Stainless and heat-resisting steels.
Molybdenum Strong carbide-stabilising influence. Raises high temperature creep strength of some alloys. Slows tempering response. Reduces 'temper brittleness' in nickel-chromium steels. Increases red-hardness of tool steels. Now used to replace some tungsten in high-speed steels.
Vanadium Strong carbide forming tendency. Stabilises martensite and increases hardenability. Restrains grain growth. Improves resistance to softening at elevated temperatures after hardening. Used to retain high temperature hardness, eg in dies for hot-forging and die casting dies. Increasingly used in high speed steels.
Tungsten Stabilises alpha phase and forms very hard carbides. renders transformations very sluggish, hence hardened steels resist tempering influences. Used mainly in high-speed steels and other tool and die steels, particularly those for use at high temperatures.
Cobalt Slows the transformation of martensite, hence increases 'red hardness'. Used in super high speed steels and maraging steels, permanent magnet steels and alloys.

'Metals Handbook', ASM, 2nd Desk Edition, 1998, ISBN: 0-87170-654-7.

David J Grieve, 10th February 2003.

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