An Introduction to Ferrous Metallurgy TSM 233 Unit 13

TSM 233 Metallurgy and Welding Processes Definitions: Cementite - iron carbide, Fe 3 C Ferrite - pure iron Pearlite - grain structure combination of ferrite and cementite in a layer formation. Austenite - heated to the transformation temperature, fine grains of ferrite and pearlite.(non-magnetic) Martensite - structure that is formed when carbon steel is rapidly cooled by quenching.

TSM 233 Metallurgy and Welding Processes Grain Structure Formation: As metals cool from a molten state, the grain (crystalline) structure of the metal forms. Metals form a lattice structure, which means that the grains develop in an ordered manner. Generally, a large grain structure is more malleable and workable. And, a smaller grain structure is harder and more brittle. The rate of cooling affects the nature of the grain structure. Also, the metal needs a sufficient time of heating in order for the heat to penetrate the metal.

TSM 233 Metallurgy and Welding Processes Allotropic Iron is one of the few allotropic metals. Allotropic means that the metal changes it’s structure as is heated and cooled. The transformation occurs at 1333 degrees F. Below the transformation temperature iron has a Body Centered Cubic lattice structure. Above the transformation temperature, iron is a Face Centered Cubic (FCC)

TSM 233 Metallurgy and Welding Processes Iron-Carbon Diagram The Iron-Carbon diagram represents the transformations that occur as the ferrous metals with increasing percentages of carbon are heated and cooled.

TSM 233 Metallurgy and Welding Processes Eutectic and Eutectoid. Eutectic point is the alloy percentage at which the lowest melting temperature occurs. For iron and carbon, the eutectic point is 2066 degrees F, and a Carbon content of 4.3 percent. Eutectoid point is the alloy composition that transforms from high temperature solid into new phases at the lowest constant temperature. For iron and carbon, the eutectoid point is at 1333 degrees F and a carbon content of 0.83 percent.

TSM 233 Metallurgy and Welding Processes Grain structures Most ferrous metals are arranged as one of two cubic systems: Body-centered cubic systems Face-centered cubic systems Carbon steels are a BCC system at room temperature. Transforms to a FCC system when heated above the transformation temperature (approx o F)

TSM 233 Metallurgy and Welding Processes BBC and FCC characteristics: Generally, BCC metals have very good strength characteristics. Examples include: Chromium, Iron, Molybdenum, Tungsten. Generally, FCC metals are more ductile and malleable: Examples include: Copper, Gold, Lead, Nickel, Platinum, Silver.

TSM 233 Metallurgy and Welding Processes Hexagonal Close-packed (HCP) Hexagonal close-packed is grain structure with 6 atoms on each end and three in the middle. Some HCP metals are: Cadmium, Cobalt, Magnesium, Titanium, Zinc These metals are not very common. They are generally considered to have low ductility and malleability.

TSM 233 Metallurgy and Welding Processes Other grain structures There are three other grain structures of metals, but these are not very common: Cubic Body-centered tetragonal Rhombohedral Note: Martensite is a body-centered tetragonal, looks like a stretched BCC.

TSM 233 Metallurgy and Welding Processes How is martensite formed? Steel heated above the critical temperature (austenite) Cooled rapidly (quenched to below 200 o F), “freezes” the structure, very hard and brittle. The grain structure is not allowed to develop into “less stressed” grains. Note : Martensite has a body-centered tetragonal cubic structure.

TSM 233 Metallurgy and Welding Processes Carbon Steel below the transformation temperature Pure iron is 100% ferrite. If a carbon steel is 0.83% Carbon it is 100% pearlite. If a carbon steel is less than 0.83% Carbon, it is a combination of ferrite and pearlite. If a carbon steel is more than 0.83% Carbon, it is a combination of cementite and pearlite.

TSM 233 Metallurgy and Welding Processes Carbon Steel above the transformation temperature If a carbon steel is 0.83% carbon transforms to austenite, eventually becoming a liquid, (melts) If a carbon steel is less than 0.83% Carbon, - transforms to a combination of ferrite and austentite, - heated further, becomes all austentite - eventually becoming a liquid. If a carbon steel is greater than 0.83% Carbon, - transforms to a combination of austentite and cementite, -heated further, becomes a combination of austenite and liquid, - eventually becoming a liquid.

TSM 233 Metallurgy and Welding Processes Transformations when welding steel. Three are three key areas of structural differences in the weld affected zone when a steel product is welded: 1.An area of metal that becomes molten. 2.An area where the steel transforms from a BCC to a FCC, does not melt, changes back to a BCC, if allowed to cool slowly. 3.And an area of metal that was heated, but did not reach the transformation point, therefore remained a BCC. Note: If a steel weld is quenched, a body-centered tetragonal may be formed which is a stressed BCC.