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Alloy Steels

Describes the propertises of the major alloyng elements used in Steel
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Alloy Steels

  1. Usually in Alloy Steels the Pearlite Structure requires less Carbon for it's formation than in ordinary Carbon Steels.
  2. Alloying elements change the temperatures at which the structural cahangesd take place. This means that Alloy Steels require different heat treatments to Carbon Steels
  3. Alloy additions delay Structural Changes.

Silicon Steel

  1. Forms a Silicon Ferrite in which the Silicon is in solid solution within the Iron. Silicon Steel has a very low Carbon content.
  2. Silicon Steel is magnetic but posses a very low MAGNETIC HYSTERESIS It is therefore universally used in the Iron cores of Transformers; Electric Motors etc.
  3. The steel is tough and stands up well to fatique but it must be very carefully heat treated to prevent the growth of large grains
  4. Silicon Steel is used in Springs when alloyed with Manganese. For Automotive valve springs the Manganese is replaced by Chromium . This is because both elements raise the Critical Points and there is less likely hood of successive hardening and tempering during heating and cooling.

Manganese Steel

  1. Lowers the Critical Temperatures. This allows a lower hardening temperature, particularly for small sections.
  2. Above 2% the Critical Points merge so that only one change point occurs, This allows air hardening for small sections to a Martenistic Structure. Unfortunately this is very brittle. This type of Steel also allows relatively large sections to acquire a high degree of hardness.
  3. High Manganese Steels ( 11% - 14%Magnesium. and 1%Carbon) have the feature that they are usually in an Austenitic state. Quenching from 1000 deg. C results in total retension of Austenite.
  4. High Manganese Steels work harden and this produces a resistance to abrasion.
  5. These Steels can be forged or cast and under special processes, machined.
  6. Annealing makes the Steel brittle. Tempering in the quenched condition changes the structure to Pearlite; Banite etc.
  7. A similar effect to Tempering is achieved by cold working. This accounts for the resistance to abrasion. Cold worked Steel is magnetic.

Nickel

  1. A very important alloying element although it is very expensive.
  2. Lowers all the Critical Points and increases the strength and toughness of the Steel.
  3. With 4% Ni theA_3 and A_2 points merge and with still higher percentages produce only one Critical Point.
  4. Also increases "Thermal Lag".
  5. An inclusion of 30% Nickel reduces the Critical Point to zero and the Steel remains Austenitic.
  6. Improves the forgability of Steel and is used in Drop Forged Case Hardened Parts and for General Structural Parts.
  7. Produces a fine grained Steel but promotes the freeing of Carbon. Therefore some other element must be added to stabilise the Carbon. This is usually 0.6% - 0.9% Manganese or Chromium.
  8. Nickel is added in amounts up to 3% to Manganese Steel to ensure that the Austenitic state is retained after Air Cooling from 1000 deg.C without the need for Water quenching.

Chromium.

  1. The key element in a wide range of Stainless and Heat resisting Steels.
  2. Combines freely with Iron but is usually found as Chrome Carbide.
  3. In the presence of Carbon some, at least, of the Chromium is found in the Carbide that separates from Austenite on cooling.
  4. Alters the Eutectoid Carbon content and raises the Critical points.
  5. Stabilises the Carbide due to the solid solution of Chrome Carbide in Iron Carbide. This dissolves much more sluggishly than pure Iron Carbide when the Steel is heated above it's Critical Point.
  6. Up to 1% increases the strength of Steel without reducing ductility. It increases th ability to withstand wear. Particularly that involving abrasion as opposed to impact damage.
  7. Great care must be taken during heat treating. Over heating causes brittleness due to the overgrowth of the grains.
  8. Even small amounts produce a resistance to corrosion.
  9. Carbon free Iron Alloys with over 12% Chromium do not undergo allotropic changes and remain as Ferritic solid solutions at all temperatures.

Vanadium

  1. The element goes into solid solution with both alpha and gamma Iron.
  2. Above about 1.1% the Iron Vanadium Alloys do not undergo allotropic changes and consist of Ferrite solutions at all temperatures.
  3. Vanadium Carbide behaves like Chrome Carbide only it is even more sluggish in going into solution in gamma Iron. This means that the Steel is not as readily overheated as other Steels

Molybdenum

  1. Goes into solid solution with both alpha and gamma iron.
  2. In the presence of Carbon, goes into complex Carbides.
  3. Refines the grain size in Steel.
  4. Added to Nickel - Chrome Steel to avoid temper brittleness.
  5. Sometimes used as a part substitute for Tungsten in High Speed Steels.

Tungsten

  1. Goes into solid solution with both alpha and gamma Iron.
  2. In the presence of Carbon goes into complex Carbides.
  3. In High Speed Steels from 14% - 22% Tungsten is used. In this case a very hard Iron Tungstide is formed.
  4. Increases the temperature at which the structural changes take place. The cooling changes are, however, greatly retarded so that the Steels are strongly air hardening.
  5. The Steel will stand up to much higher temperatures without the Martenistic structure breaking down. They are therefore used for Cutting Tools.
  6. When hardened, Steels of this type have a much smaller grain size than Steels of corresponding Carbon content.

Cobalt

  1. Goes into solid solution
  2. Introduced into Magnet Steel to improve their remanence and coercivity in proportions from 3% - 35%.
  3. The Steel is expensive and difficult to forge.
  4. Added to High Speed Steels to improve "red hardness". i.e. The Steel will remain hard even when red hot and the cutting efficiency will not be impaired.

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Properties of the main alloy forming elements