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Change of ConditionCompetenciesPhase DiagramSlide 4Phase Diagram (Tin-Lead Allow system)Discuss Iron-Carbon Phase DiagramIron-Carbon PhaseSlide 8Slide 9Slide 10Slide 11Methods of Softening SteelsMethods of Hardening SteelsMethods of Hardening SteelsSlide 15Slide 16Slide 17HardnessHardenabilitySlide 20Methods Of Modifying The Properties Of SteelsMethods Of Modifying The Properties Of SteelsSlide 23Slide 24IT 208 Chapter 12 1Change of ConditionChapter 12IT 208Chapter 122CompetenciesDescribe the different methods of softening steel Describe the different methods of hardening steelDescribe the difference between Martensite and AustinsiteIT 208Chapter 123Phase DiagramIs a graphical means of representing the phases of a metal allow system as a function of composition and temperature Discuss the Water phase system (O/H)Discuss the Copper-nickel Alloy system (O/H)IT 208Chapter 124Phase DiagramDiscuss the Tin-Lead Allow system •widely used in soldering for making electrical connection.The addition of two solid phases alpha (α) and beta (β). •Alpha phase is a solid solution of tin in lead•Beta phase is solid solution of lead in tin that occurs only at elevated temperatures around 200 degrees C Between these solid solutions lies a mixture of the two solid phases, (α) + (β).Two liquidus lines that begin at the melting points of the pure metals and meet at a composition of 61.9% Sn. •Point is called the eutectic composition for the tin-lead systemIT 208Chapter 125Phase Diagram (Tin-Lead Allow system) A eutectic alloy is a particular composition in an allow system for which the solidus and liquidus are at the same temperature.The corresponding eutectic temperature, the melting point of the eutectic composition is 183 deg CEutectic temperature is always the lowest melting point for an alloy system.IT 208Chapter 126Discuss Iron-Carbon Phase DiagramSteels with less than 0.3 % carbon cannot be hardened effectively, while the maximum effect is obtained at about 0.7 % due to an increased tendency to retain austenite in high carbon steelsThe ferrous metals of engineering importance are alloys of iron and carbon. These alloys divide into two major groups; steel and cast iron.IT 208Chapter 127Iron-Carbon Phase1. Pure iron melts at 1539 degrees C (2827 deg F) during the rise in temperature from ambient, it undergoes several solid phase transformationsa. Starting at room temperature the phase is alpha iron or ferrite. With less than 0.025% carbon at temperatures below 894 deg Cb. At 912 degrees C, ferrite transforms to gamma iron, called austenite. With less than 2% carbonc. At 1394 degrees C, austenite transforms to delta iron, which remains until melting occurs at 1539 degrees CIT 208Chapter 128Iron-Carbon Phase2. Solubility limits of carbon in iron are low in the ferrite phase – only about 0.022% at 723 deg C. Austenite can dissolve up to about 2.1 % carbon at 1130 deg C. The difference in solubility between alpha and gamma leads to opportunities for strengthening by heat treatment3. The eutectoid point is the lowest temperature at which austenite can exist (722 deg C).a. Eutectoid – the temperature and composition (0.77 - 0.81% Carbon) at which a single-phase solid goes directly, on cooling, to a two-phase solid. Steels below 0.77% Carbon are considered hypoeutectoid steels those above up to 2.1% are considered hypereutectoid steels. b. The eutectoid composition of the Iron-Carbon system is called pearlite.IT 208Chapter 129Iron-Carbon Phase4. Even without head treatment, the strength of iron increases dramatically as carbon content increases, and we enter the region in which the metal is called steel. More precisely, steel is defined as an iron-carbon alloy containing from 0.02% to 2.1 % carbon.5. Another prominent phase in the iron-alloy system. Is Fe3C also known as cementite. Which is a metallic compound of iron and carbon that is hard and brittle Carbon content of about 6.7%.IT 208Chapter 1210Iron-Carbon Phase6. Above a carbon content of 2.1% up to about 4% or 5% is defined as cast iron7. If sufficient time is allowed for cooling of the austeniteit will revert completely to pearlitehowever, if the steel is cooled quickly from the austenite, martensite is formedMartensitic steel has Rockwell C hardness of about 66 and pearlite is very soft in comparison.IT 208Chapter 1211Discuss the TTT curve (12-11)Three major categories of heat treatments•Methods of softening steels•Methods of hardening steels•Methods of modifying the properties of steelsIT 208Chapter 1212Methods of Softening SteelsAnnealing is the softening of a metal to its softest possible condition. For steels, the metal must be heated into the austenitic range and cooled very slowly. Normalizing is a heat treatment used to give steel an even GRAIN size. It is used prior to machining or other heat treatments.IT 208Chapter 1213Methods of Hardening Steels Can be done by flame, induction, electron beams, and laser beamQuenching is the rapid cooling of a metal to harden it. Cryogenics, or deep freezing •done to make sure there is no retained Austenite during quenching. •When steel is at the hardening temperature, there is a solid solution of Carbon and Iron, known as Austenite.IT 208Chapter 1214Methods of Hardening Steels•The amount of Martensite formed at quenching is a function of the lowest temperature encountered. •At any given temperature of quenching there is a certain amount of Martensite and the balance is untransformed Austenite. This untransformed austenite is very brittle and can cause loss of strength or hardness, dimensional instability, or cracking.IT 208Chapter 1215Methods of Hardening Steels•Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100 % Martensite at room temperature. •High carbon and high alloy steels have retained Austenite at room temperature. To eliminate retained Austenite, the temperature has to be lowered. •In Cryogenic treatment the material is subject to deep freeze temperatures of as low as -185°C (-301°F), but usually -75°C (-103°F) is sufficient. •The Austenite is unstable at this temperature, and the whole structure becomes Martensite.IT 208Chapter 1216Methods of Hardening SteelsSurface Hardening If steel is hardened all the way through the part, it will be brittle. In parts that have wearing surfaces such as


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