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Group 3 Steels: Eutectoid Composition Steels. Steels with carbon contents just below the eutectoid to the eutectoid composition ( 0.6 – 0.8 C ) are used when high strength and toughness is required throughout the thickness of a part . Examples include:
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Group 3 Steels: Eutectoid Composition Steels Steels with carbon contents just below the eutectoid to the eutectoid composition (0.6 – 0.8 C) are used when high strength and toughness is required throughoutthe thickness of a part. Examples include: Heavy steel forgings such as marine crank shafts. Small forgings for highly stressed parts such as connecting rods for internal combustion engines and rails for locomotive transport The massive components cannot be heat treated so their strength must come from a fine pearlite structure which is developed by finishing the hot working of the steel at a low enough temperature to permit recrystallization of the austenite, while preventing austenite grain growth.
These are made from 1060 Steel, which contains 0.55 – 0.65 %C and 0.8 %Mn. It is essential that the composition be kept on the ferrite side of the eutectoid because the presence of any proeutectoid cementite would make forging very difficult and make the product brittle on cooling to room temperature. The forging process has two stages: The first stage is “upset forging* ”, which breaks up the cast structure of the billet and gives a fine austenite grain size. The second stage shapes the component, eg., crankshaft, and is also performed at temperatures above the eutectoid. Steel Forgings How would upset forging create finer grain size? It increases the dislocation density, which act as nucleation sites, especially at grain boundaries to enhance recrystallization during austenizing, which enhances the formation of small grains.
Steel Forgings For smaller components such as connecting rods for the Internal Combustion engine, it may also have a final cold forming forging treatment such as stamping. Stamping may be possible to put the part into its final shape but care must be taken to avoid the formation of cracks that could lead to catastrophic failure of the part. The smaller components are often heat treated by quenching to form 100% martensite followed by a tempering at 200 oC.
Tempered martensite formed in AISI 1060 forging quenched from 925 oC and tempered at 200 oC. Pearlite-ferrite structure of hot rolled 0.75 %C rail steel. The ferrite in the grain boundaries is due to the depression of the eutectoid temperaturecaused by rapid cooling. The microstructure should be 100% pearlite.
Mechanical Properties of Eutectoid Composition Steels We will look at some of the details of these alloys.
Steel Rails Steel rails are also made from ASTM A1 steel. This was the first alloy specified by ASTM. This is similar to AISI 1080 and contains 0.78-0.80 %C, 0.70-1.0 %Mn and 0.1-0.50 %Si. A microalloyed version, A1+ has with superior mechanical propertiescontains 1.25 %Mn and 0.25 %Cr. 14 %Mn is added to rails intended to be used for switching, which creates pockets of retained austenite. Under the heavy load of a train, the retained austenite at the surface transforms to martensite to give an extremely hard wearing surface layer, which is progressively replenished by successive transits. This is a good example of “stress-induced transformation” of g to martensite.
Steel Rails The cast structure of the steel is broken down by hot cross rolling after which the billet is straight rolled to a size just greater than the finished size. The semi-finished rail is then moved over to finishing rolls, which are machined with grooves that match the profile of the finished rail. During this final rolling stage the temperature falls to just above the eutectoid and after rolling is completed, the rail is cooled relatively rapidly in flowing air to give a fine pearlite structure. Rails for “Bullet Trains” have this process down to a fine art.
