BDCS Notes - Steel

Steel

Classifications of steels:

1. Structural Steel: for use in plates, bars, pipes, structural shapes, etc.
2. Fastening Products: used for structural connections, including bolts, nuts and washers.
3. Reinforcing Steel: for use in concrete reinforcement.
4. Miscellaneous Products: forms and pans.

Steel Production

1. Reducing iron ore to pig iron
2. Refining pig iron to steel
3. Forming the steel into products

Materials used to produce pig iron – coal, limestone, and iron ore.

Coal – Supplies carbon used to reduce iron oxides in the ore.

Limestone – Helps remove impurities.

Iron – Magnetically extracted from the waste, and extra

cted material is formed into pellets and fired.

Blast Furnace – used to reduce the ore to pig iron. Ore is heated in presence of carbon.

Three types of furnaces used for refining pig iron to steel:

1. Open Hearth
2. Basic Oxygen
3. Electric Arc

Open Hearth / Basic Oxygen – Remove excess carbon by reacting the carbon with oxygen to form gases. Lances circulate oxygen through the molten material.

Electric Furnaces – Use an electric arc between carbon electrodes to melt and refine the steel. Require a tremendous amount of energy.

During the steel production process, oxygen becomes dissolved in the liquid metal. As steel solidifies, oxygen combines with carbon to form carbon monoxide bubbles that are trapped in the steel and act as points for failure. Deoxidizing agents, such as aluminum, ferrosilicon and manganese, eliminate the formation of the carbon monoxide bubbles.

Killed Steels –

- Carbon content greater than .25%

- All forging grades of steels

- Structural steels with carbon content between 0.15 and 0.25 %

- Some special steel in the lower carbon ranges

Molten steel with desired chemical composition is cast into ingots (large blocks of steel).

Iron-Carbon Phase Diagram

In refining steel from iron ore, quantity of carbon used must be carefully controlled in order for steel to have desired properties.

Figure below represents the iron-iron carbide phase diagram:

Abscissa extends to 6.67% out of convention. The left side of the figure demonstrates that pure iron goes through two transformations as temperature increases. Below 912 C there’s a BCC crystalline structure called ferrite. At 912 C there’s a polymorphic change to a FCC structure called austenite. At 1394 C another polymorphic change occurs, returning the iron to a BCC structure. At 1539 C the iron melts into a liquid. The high and low temperature ferrites are identified as (delta) and (alpha) ferrite, respectively.

At 0.77% carbon and 727 C a eutectoid reaction occurs. A eutectoid reaction is a solid phase change that occurs when the temperature or carbon content changes. Below 727 C ferrite and iron carbide form thin plates, a lamellae structure. This eutectoid material is called pearlite.

At carbon contents less than 0.77% carbon, hypoeutectoid alloys are formed.

Heat Treatment of Steel

Annealing: refines the grain, softens the steel, removes internal stresses and gases, increases ductility and toughness, and changes electrical and magnetic properties.

Full Annealing: (1) Heating the steel to about 50 C above the austenitic temperature line and holding the temperature until all the steel transforms into either austenite or austenite-cementite. (2) Cooling the steel at a rate of about 20 C per hour in a furnace to a temperature of about 680 C.

Process Annealing: Used to treat work-hardened parts made with low carbon steel (less than 0.25 percent carbon). The material is heated to 700 C and held long enough to allow recrystallization of the ferrite phase.

Stress Relief Annealing: Used to reduce residual stresses in cast, welded, and cold-worked parts and cold-formed parts. The material is heated to 600 to 650 C, held at temperature for about one hour, and then slowly cooled in still air.

Spheroidization: An annealing process used to improve the ability of high carbon steel to be machined or cold worked. Improves abrasion resistance.

Normalizing: similar to annealing, with difference in the temperature and the rate of cooling. Steel normalized by heating to about 60 C above the austenite line and then cooling under natural convection. The material is then air-cooled. Provides a uniform, fine-grained microstructure.

Hardening: Steel is hardened by heating it to a temperature above the transformation range and holding it until austenite is formed. The steel is then quenched by plunging it into, or spraying it with, water, brine, or oil.

Tempering: The predominance of martensite in quench-hardened steel results in an undesirable brittleness. Tempering improves ductility and toughness.