Question

What is the process of acquiring white cast Fe, Gray cast Fe, ductile cast Fe and malleable cast Fe, as well as their carefully sketch microstructures?

Answer 1

There are four basic types of cast iron

• White iron
• Gray iron
• Ductile iron
• Malleable iron

White Cast Iron:

Characterized by the prevalence of carbides, impacting

• high compressive strength
• hardness
• good resistance to wear
• The typical microstructure of white cast iron, consisting of dendrites of transformed austenite (pearlite) in a white interdendritic network of cementite, Higher magnification of the same sample reveals that the dark areas are pearlite.

• Malleable Cast Iron

  White cast iron heat-treated to improve higher ductility

• MALLEABLE CAST IRON MANUFACTURING:

White irons suitable for conversion to malleable iron are of the following range of composition:

Carbon =2.00-2.65%

Silicon =0.90-1.40%

Manganese =0.25-0.55%

Phosphorous Less than 0.18%

Sulphur = 0.05%

Commercially, It can be prepared by two steps: (1) First stage annealing. (2) Second stage annealing. In the first-stage annealing, the white-iron casting is slowly reheated to a temperature between 1650 ˚F and 1750 ˚F. This graphitization starts at the malleableizing temperature.

1. GRAY CAST IRON :It is the cast iron in which most or all of the carbon is uncombined in the form of graphite flakes. Most gray cast irons are hypoeutectic alloys containing between 2.5 and 4 percent carbon. The initial appearance of combined carbon is in the cementite resulting from the eutectic reaction at 2065F<. There is experimental evidence that, with proper control of the alloying element, the alloy will follow the stable iron-graphite equilibrium diagram, ), forming austenite and graphite at the eutectic temperature of 2075F.

GRAY CAST IRON

1. At any rate, any cementite which is formed will graphitize rapidly. The graphite appears as many irregular, generally elongated and curved plates which give gray cast iron its characteristic grayish or blackish fracture.

Gray Cast Iron

Characterized with graphite in the microstructure, giving

• good machinability
• good resistance to wear and galling.

Ductile Cast Iron

Gray iron with small amounts of magnesium and cesium which nodulates the graphite, resulting

• high strength
• high ductility

PRODUCTION :

Cast iron is made by re-melting pig iron, often along with substantial quantities of scrap iron, scrap steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively. Other elements are then added to the melt before the final form is produced by casting. Iron is more often melted in electric induction furnaces or electric arc furnaces. After melting is complete, the molten iron is poured into a holding furnace or ladle.

1. HEAT TREATMENT

a. SUB-CRITICAL ANEALING: The sub-critical annealing process typically produces a microstructure where existing pearlite dissolves into the ferritic matrix which lowers hardness and strength while improving machinability.

b FULL ANNEALING: The full annealing process involves taking a casting to a temperature above the critical eutectoid temperature in the 1475F-1650F range for a length of time commiserate with the cross section size and then slow cooled to promote a ferritic microstructure

. c SOLUTION ANNEALING HEAT TREATMENT PROCESS: Solution Annealing stainless steel castings is a process which takes the carbides that have precipitated in the grain boundaries and dissolves then into the surrounding matrix.

d. WATER QUENCHING STEEL: Many steel foundries cast low carbon or low alloy steels that require a severe water quench to adequately achieve the proper martensitic microstructure and hardness requirements.

ANNEALING QUENCHING

a. HEAT TREATMENT POLYMER QUENCH HARDNING: Polymer quench hardening of steel castings like 4130, 15B41, 1045, and 5130 is the proper way to balance the necessity for rapid quench rates while minimizing the risk of quench cracking the castings.

b.OIL QUENCH HARDNING: Steel castings made from alloys in excess of 0.40% carbon content can typically be oil quench hardened to achieve their desired mechanical or hardness requirements. c.NORMALIZED HEAT TREATMENT: Gray iron and ductile iron castings are typically normalized to move the carbon from being locked in carbides or graphite into more of a pearlitic microstructure. d.AUSTEMPERED DUCTILE IRON: In addition to austempering ductile iron , gray irons and several alloys of steel castings can also be austempered.

e.CARBON RESTORATION: To insure that the steel castings are capable of achieving full surface hardness during subsequent hardening processing the castings may require a carbon restoration of their surfaces which is done through several furnaces.