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What is the primary purpose of heat treatment of metals?
What is the primary purpose of heat treatment of metals?
What is a common manufacturing process that inadvertently involves heat treatment?
What is a common manufacturing process that inadvertently involves heat treatment?
What is a reason for softening steel?
What is a reason for softening steel?
What is a prerequisite for hardening steel?
What is a prerequisite for hardening steel?
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What is the purpose of tempering steel?
What is the purpose of tempering steel?
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Why are steels particularly suitable for heat treatment?
Why are steels particularly suitable for heat treatment?
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What is the primary purpose of heat treatment of steel?
What is the primary purpose of heat treatment of steel?
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What is the name of the diagram that serves as the base of heat treatment?
What is the name of the diagram that serves as the base of heat treatment?
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What is the result of spheroidising on the strength and hardness of steel?
What is the result of spheroidising on the strength and hardness of steel?
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What is the purpose of tempering in the heat treatment process?
What is the purpose of tempering in the heat treatment process?
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What is the difference between annealing and quenching?
What is the difference between annealing and quenching?
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What is the result of normalizing on the grain structure of steel?
What is the result of normalizing on the grain structure of steel?
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What happens to the carbon in steel when it is heated to a certain temperature?
What happens to the carbon in steel when it is heated to a certain temperature?
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What is the significance of the decalescence point in the hardening of steel?
What is the significance of the decalescence point in the hardening of steel?
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Why do different types of steel require different heating temperatures for hardening?
Why do different types of steel require different heating temperatures for hardening?
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What is the purpose of tempering after hardening?
What is the purpose of tempering after hardening?
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What is the difference between annealing and normalizing heat treatments?
What is the difference between annealing and normalizing heat treatments?
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What happens when steel cools slowly from a high heat?
What happens when steel cools slowly from a high heat?
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What occurs during tempering when the steel experiences an increase in hardness and a reduction in ductility?
What occurs during tempering when the steel experiences an increase in hardness and a reduction in ductility?
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What is the primary purpose of recrystallization in metals processing?
What is the primary purpose of recrystallization in metals processing?
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What do time-temperature transformation (TTT) diagrams measure?
What do time-temperature transformation (TTT) diagrams measure?
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What is the main difference between TTT and CCT diagrams?
What is the main difference between TTT and CCT diagrams?
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What type of structure cannot be obtained using continuous cooling?
What type of structure cannot be obtained using continuous cooling?
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What is the process by which deformed grains are replaced by a new set of defects-free grains?
What is the process by which deformed grains are replaced by a new set of defects-free grains?
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What is the primary outcome of austenitizing heat treatment if followed by quenching?
What is the primary outcome of austenitizing heat treatment if followed by quenching?
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Which heat treatment process involves heating to a high temperature, holding for a period of time, and then air cooling or faster cooling?
Which heat treatment process involves heating to a high temperature, holding for a period of time, and then air cooling or faster cooling?
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What is the primary difference between austempering and martempering?
What is the primary difference between austempering and martempering?
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What is the result of precipitation hardening?
What is the result of precipitation hardening?
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What is the purpose of stress relieving heat treatment?
What is the purpose of stress relieving heat treatment?
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What is the primary advantage of austempering over other heat treatment processes?
What is the primary advantage of austempering over other heat treatment processes?
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What is the primary reason why CCT diagrams are more practical than TTT diagrams?
What is the primary reason why CCT diagrams are more practical than TTT diagrams?
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What is the primary purpose of the Jominy end-quench test?
What is the primary purpose of the Jominy end-quench test?
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What is the primary reason why fast cooling can lead to warping and formation of cracks?
What is the primary reason why fast cooling can lead to warping and formation of cracks?
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What is the primary purpose of precipitation heat treatment?
What is the primary purpose of precipitation heat treatment?
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What is the primary reason why the martensitic content and hardness decrease from the surface to the interior of a thick piece?
What is the primary reason why the martensitic content and hardness decrease from the surface to the interior of a thick piece?
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What is the primary purpose of solution heat treatment in precipitation hardening?
What is the primary purpose of solution heat treatment in precipitation hardening?
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What is the primary effect of overaging on the hardness of a material?
What is the primary effect of overaging on the hardness of a material?
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What is the primary difference between CCT and TTT diagrams?
What is the primary difference between CCT and TTT diagrams?
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What is the primary role of heat capacity in determining the cooling rate?
What is the primary role of heat capacity in determining the cooling rate?
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What is the primary reason why oil is used as a quenching medium instead of water?
What is the primary reason why oil is used as a quenching medium instead of water?
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Study Notes
Heat Treatment of Steel
- Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.
- Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.
Reasons for Heat Treatment
- Softening: to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size, or change the electromagnetic properties of the steel.
- Hardening: to increase the strength and wear properties.
- Material Modification: to modify properties of materials in addition to hardening and softening, such as stress relieving, cryogenic treatment, or spring aging.
Heat Treatment Processes
- Annealing: heating the material above the critical temperature, holding long enough for transformation to occur, and slow cooling.
- Normalizing: heating a steel above the critical temperature, holding for a period of time long enough for transformation to occur, and air cooling.
- Quenching: heating the material to a suitable temperature and then quenching in water or oil to harden to full hardness.
- Tempering: reheating the steel at a relatively low temperature to develop the required combination of hardness, strength, and toughness or to relieve the brittleness of fully hardened steels.
Critical Temperatures
- Decalescence point: the temperature at which steel continues to absorb heat without appreciably rising in temperature.
- Critical point: the temperature at which the steel actually increases in temperature, although its surroundings may be colder.
Hardening
- The use of this treatment will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher, more durable item.
- Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material.
Austempering and Martempering
- Austempering: a technique used to form pure bainite, a transitional microstructure found between pearlite and martensite.
- Martempering: similar to austempering, but the goal is to create martensite rather than bainite.
Embrittlement
- Occurs during tempering when, through a specific temperature range, the steel experiences an increase in hardness and a reduction in ductility.
- Two types: tempered martensite embrittlement (TME) or one-step embrittlement, and temper embrittlement (TE) or two-step embrittlement.
Recrystallization
- A process by which deformed grains are replaced by a new set of defects-free grains that nucleate and grow until the original grains have been entirely consumed.
- Usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility.
Time-Temperature Transformation (TTT) and Continuous Cooling Transformation (CCT) Diagrams
- TTT diagrams: measure the rate of transformation at a constant temperature.
- CCT diagrams: measure the extent of transformation as a function of time for a continuously decreasing temperature.
- CCT diagrams are more practical than TTT diagrams as most of the processes employ continuous cooling rather than isothermal transformation.
Hardenability
- The ability of the material to be hardened by forming martensite.
- Measured by the Jominy end-quench test.
- Hardenability is then given as the dependence of hardness on distance from the quenched end.### Cooling and Thermal Gradients
- Fast cooling of a metal piece can cause warping and crack formation due to large thermal gradients.
- The shape and size of the piece, along with its heat capacity and heat conductivity, influence the cooling rate for different parts of the metal.
Heat Capacity and Heat Conductivity
- Heat capacity is the energy content of a heated mass that needs to be removed for cooling.
- Heat conductivity measures how fast energy is transported to the colder regions of the piece.
Precipitation Hardening
- Precipitation hardening, also known as age hardening, enhances hardening by forming extremely small precipitates that hinder dislocation motion.
- Precipitates form when the solubility limit is exceeded.
Steps to Achieve Precipitation Hardening
- Solution heat treatment: dissolve all solute atoms to form a single-phase solution.
- Rapid cooling across the solvus line to exceed the solubility limit, resulting in a supersaturated solid solution.
- Precipitation heat treatment: heat the supersaturated solution to an intermediate temperature to induce precipitation and keep it there for some time (aging).
Overaging
- If the precipitation hardening process is continued for a very long time, the hardness eventually decreases, a phenomenon known as overaging.
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Description
Learn about the process of heat treatment, its effects on metal properties, and its applications in manufacturing, including improving strength and machining.
