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Metalworking and fire seems to go so naturally that we have taken the science behind it for granted. But there is so much more between the interactions of heat and metals than mere melting, casting and shaping. This relation goes deep into the molecular realm, of how the alignments of particles can make or break the structure.
And when the structure supports bridges, buildings, or vehicles, it's certainly worth it to know more. |
"The paynefull smith, with force of fervent heat,
The hardest yron soone doth mollify,
That with his heavy sledge he can it beat,
And fashion it to what he it list apply."
Edmund Spenser, Sonnet XXXII
Before modern metalworking techniques were invented, blacksmiths used heat to make metal workable. Once the metal was formed into the desired shape, the heated metal quickly was cooled. Quick cooling made the metal harder and less brittle. Modern metalworking has become much more sophisticated and precise, allowing for different techniques to be used for different purposes.
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Effects of Heat on Metal
Subjecting metal to extreme heat causes it to expand in addition to impacting its structure, electrical resistance, and magnetism. Thermal expansion is pretty self-explanatory. Metals expand when subjected to specific temperatures, which vary depending on the metal. The actual structure of metal also changes with heat. Referred to as allotropic phase transformation, heat typically makes metals softer, weaker, and more ductile. Ductility is the ability to stretch metal into a wire or something similar. Heat also can impact the electrical resistance of metal. The hotter the metal gets, the more the electrons scatter, causing the metal to be more resistant to an electrical current. Metals heated to certain temperatures also can lose their magnetism. By raising temperatures to between 626 degrees Fahrenheit and 2,012 degrees Fahrenheit, depending on the metal, magnetism will disappear. The temperature at which this happens in a specific metal is known as its Curie temperature. |
Heat Treatment
Heat treatment is the process of heating and cooling metals to change their microstructure and to bring out the physical and mechanical characteristics that make metals more desirable. The temperatures metals are heated to, and the rate of cooling after heat treatment, can significantly change metal's properties.
The most common reasons that metals undergo heat treatment are to improve their strength, hardness, toughness, ductility, and corrosion resistance. Common techniques for heat treatment include the following:
Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. It softens metal, making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.
Less expensive than annealing, quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metal's microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does.
Material is heated to the suitable temperature for hardening, then cooled rapidly by immersing the hot part is water, oil or another suitable liquid to transform the material to a fully hardened structure. Parts which are quenched usually must be aged, tempered or stress relieved to achieve the proper toughness, final hardness and dimensional stability.
Precipitation hardening is also known as age hardening. It creates uniformity in a metal's grain structure, making the material stronger. The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 500 to 600 degrees Celcius. It can take anywhere from an hour to four hours to carry out the process. The length of time typically depends on the thickness of the metal and similar factors.
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 are heated above the critical transformation temperature for the material, then cooled rapidly enough to cause the soft initial material to transform to a much harder, stronger structure. 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. Hardened materials are usually tempered or stress relieved to improve their dimensional stability and toughness.
Commonly used in steelmaking today, tempering is a heat treatment used to improve hardness and toughness in steel as well as to reduce brittleness. The process creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals, and the combination of quenching and tempering is important to make tough parts. The tempering temperature and times are generally controlled to produce the final properties required of the steel. The result is a component with the appropriate combination of hardness, strength and toughness for the intended application. Tempering is also effective in relieving the stresses induced by quenching.
Stress relieving is a heat treatment process that decreases stress in metals after they have been quenched, cast, normalized, and so on. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slowly cooled.
Normalizing is a form of heat treatment that eliminates impurities and improves strength and hardness by altering the grain size to be more uniform throughout the metal. This is achieved by cooling the metal by air after it has been heated to a precise temperature.
When a metal part is cryogenically treated, it is slowly cooled with liquid nitrogen. The slow cooling process helps prevent thermal stress of the metal. Next, the metal part is maintained at a temperature of roughly minus 190 degrees Celsius for about a day. When it is later heat tempered, the metal part undergoes an increase of temperature up to approximately 149 degrees Celsius. This helps to lower the amount of brittleness that may be caused when martensite forms during cryogenic treatment.
Heat treatment is the process of heating and cooling metals to change their microstructure and to bring out the physical and mechanical characteristics that make metals more desirable. The temperatures metals are heated to, and the rate of cooling after heat treatment, can significantly change metal's properties.
The most common reasons that metals undergo heat treatment are to improve their strength, hardness, toughness, ductility, and corrosion resistance. Common techniques for heat treatment include the following:
Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. It softens metal, making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.
Less expensive than annealing, quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metal's microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does.
Material is heated to the suitable temperature for hardening, then cooled rapidly by immersing the hot part is water, oil or another suitable liquid to transform the material to a fully hardened structure. Parts which are quenched usually must be aged, tempered or stress relieved to achieve the proper toughness, final hardness and dimensional stability.
Precipitation hardening is also known as age hardening. It creates uniformity in a metal's grain structure, making the material stronger. The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 500 to 600 degrees Celcius. It can take anywhere from an hour to four hours to carry out the process. The length of time typically depends on the thickness of the metal and similar factors.
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 are heated above the critical transformation temperature for the material, then cooled rapidly enough to cause the soft initial material to transform to a much harder, stronger structure. 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. Hardened materials are usually tempered or stress relieved to improve their dimensional stability and toughness.
Commonly used in steelmaking today, tempering is a heat treatment used to improve hardness and toughness in steel as well as to reduce brittleness. The process creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals, and the combination of quenching and tempering is important to make tough parts. The tempering temperature and times are generally controlled to produce the final properties required of the steel. The result is a component with the appropriate combination of hardness, strength and toughness for the intended application. Tempering is also effective in relieving the stresses induced by quenching.
Stress relieving is a heat treatment process that decreases stress in metals after they have been quenched, cast, normalized, and so on. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slowly cooled.
Normalizing is a form of heat treatment that eliminates impurities and improves strength and hardness by altering the grain size to be more uniform throughout the metal. This is achieved by cooling the metal by air after it has been heated to a precise temperature.
When a metal part is cryogenically treated, it is slowly cooled with liquid nitrogen. The slow cooling process helps prevent thermal stress of the metal. Next, the metal part is maintained at a temperature of roughly minus 190 degrees Celsius for about a day. When it is later heat tempered, the metal part undergoes an increase of temperature up to approximately 149 degrees Celsius. This helps to lower the amount of brittleness that may be caused when martensite forms during cryogenic treatment.
Ponder this
Does heat treatment only works on metals? What about ceramics? Or even organic materials?
Can metals that have very low melting points, such as gallium or mercury, also be heat treated?
Discuss
Why are specific temperatures must be reached for the materials treated to gain specific attributes? What are the chemical or physical mechanisms that changes a metal's strength, hardness, toughness, or ductility depending on what temperature is it treated to?
Further readings
Heat treating, from Wikipedia.
"A Quick Look at Heat Treating Processes for Metals", from Machine Design magazine.
"Heat Treatment of Plain Carbon and Low Alloy Steels", from MIT Department of Mechanical Engineering.
