The field of Material Science and Heat Treating is far too vast to give a comprehensive treatment on this single webpage, so instead I mean to give some basic theory and application examples that would be relevant to one of my clients endeavoring a typical product development. Heat treating primarily refers to processing of metal parts after their manufacture process (machining, casting, forging, etc) . For most metals, exposure to specific temperatures and durations, as well as cooling and heating rates, can greatly effect the microstructure and properties of the base metal. The reason we do this is primarily for increasing the strength of the material, but heat treating can also be necessary for relieving internal stresses resulting from manufacturing processes that might cause premature failure of the part.
Theory
Microsturcture
All metals are crystalline, which means that when solid, the atoms line up in repeating patterns and forms a very uniform network…much like how ice crystals form when water freezes. Bulk metal is comprised of millions of these very tiny crystals that are all oriented differently, so that the interface between two neighboring crystals have an irregularity in the atomic pattern. When material is under stress, and starts to take permanent shape change (called plastic deformation) whether by stretching, bending, etc…the atoms in the crystals have to move around each other and continuously re-align themselves.
Dislocations
All throughout the crystals are millions of interruptions in the atomic arrangement called ‘dislocations’. It is the actual movement of dislocations through the crystals, and from one crystal to the next that actually results in plastic deformation. The purpose of heat treating is to change the microstructure of the material to make barriers that prevent or at least get in the way of dislocation motion. The more ‘road blocks’ you put in front of dislocations, the stronger the material gets.
Grain Size Refinement
One of the best ways to block dislocations is to increase the amount of interface between crystals, or ‘grains’. Because neighboring crystals don’t line up in atomic pattern, its difficult for dislocations to pass from one grain to the next. Reducing the size of grains will increase the total grain surface area, and thus increase strength. Grain size is related to how quickly the material is cooled during the solidification process.
Impurities
Pure metals have very clean crystals. If you introduce a few atoms of another material, and stick them in the middle of a crystal, it will create a disruption in the atomic pattern that will impede dislocation motion. It doesn’t take very much impurity, sometimes less than 1%, to dramatically increase the strength of a pure metal. This is the fundamental reason why steel is so much stronger than plain iron.
Phases
When you combine two or more different metals, called alloying, sometimes different phases are formed. A phase is a specific metallic compound that has its own crystal structure. In a basic steel alloy (combination of iron and carbon), there are many different phases that form at different temperatures. The main phases at room temperature are Ferrite (BCC: Body-Centered-Cubic iron) and Iron Carbide, or Cementite (Fe3C). When heated beyond a certain temperature a different phase becomes stable (FCC: Face-Centered-Cubic iron). By manipulating time, temperature, cooling rate, and alloy composition, you can get many other phases to form at room temperature that can result in hard and/or tough steel. Some of these other phases are called Austenite, Pearlite, Bainite, and Martensite.
More Dislocations
Lastly, dislocations themselves can be barriers for movement of other dislocations. The more you deform a material, the more dislocations you create, and the stronger the material apparently gets. This is the phenomena of ‘work hardening’. Although it isn’t strictly a heat treating process, it should be considered as means of improving material properties.
Application
Steel
There are countless ways of heat treating steel. The most common method is quenching from a high temperature (>727°C). Typically it is quenched in various mediums: air quench, oil quench, and water quench. Different phases are formed with different cooling rates, but generally the hardest phases are formed with the fastest cooling. The very hardest and strongest phase of steel is call Martensite. However in plain steel, it is so hard that it is too brittle to use in many applications. Often a process must be chosen to balance hardness and toughness of the steel.
The other major type of heat treating in steel is Carburizing, which is the act of increasing the carbon content on the surface of the part. Generally, the higher the carbon content, the more carbides are formed (which is a ceramic) and the harder the material is. Carburizing is achieved by heating the part up in furnace with a carbon-rich atmosphere; over time the carbon will diffuse a few ten-thousandths of an inch, and increase the surface hardness without making the whole part too brittle.
Aluminum
The methods of heat treating aluminum are very different then that of steel. The principle method of heat treating is Precipitation Hardening, which falls under the ‘impurities’ category discussed above. The process generally involves an aluminum alloy that would have an impurity content that would be supersaturated at room temperature when cooled sufficiently rapidly. Then, the material is re-heated at a low temperature (a few 100°’s C) and tiny particles of the saturated phase form or ‘precipitate’ out of solution. Some alloys will even precipitate without further heating. These AlCu alloys will ‘Age Harden’ after a few days or weeks…they simply get stronger the longer you wait! The most common extrusion grade alloy used in most products is 6061, and it is often sold pre-heat treated, to the ‘T6’ specification.
HEAT TREATING
The field of Material Science and Heat Treating is far too vast to give a comprehensive treatment on this single webpage, so instead I mean to give some basic theory and application examples that would be relevant to one of my clients endeavoring a typical product development. Heat treating primarily refers to processing of metal parts after their manufacture process (machining, casting, forging, etc) . For most metals, exposure to specific temperatures and durations, as well as cooling and heating rates, can greatly effect the microstructure and properties of the base metal. The reason we do this is primarily for increasing the strength of the material, but heat treating can also be necessary for relieving internal stresses resulting from manufacturing processes that might cause premature failure of the part.
Theory
Microsturcture
All metals are crystalline, which means that when solid, the atoms line up in repeating patterns and forms a very uniform network…much like how ice crystals form when water freezes. Bulk metal is comprised of millions of these very tiny crystals that are all oriented differently, so that the interface between two neighboring crystals have an irregularity in the atomic pattern. When material is under stress, and starts to take permanent shape change (called plastic deformation) whether by stretching, bending, etc…the atoms in the crystals have to move around each other and continuously re-align themselves.
Dislocations
All throughout the crystals are millions of interruptions in the atomic arrangement called ‘dislocations’. It is the actual movement of dislocations through the crystals, and from one crystal to the next that actually results in plastic deformation. The purpose of heat treating is to change the microstructure of the material to make barriers that prevent or at least get in the way of dislocation motion. The more ‘road blocks’ you put in front of dislocations, the stronger the material gets.
Grain Size Refinement
One of the best ways to block dislocations is to increase the amount of interface between crystals, or ‘grains’. Because neighboring crystals don’t line up in atomic pattern, its difficult for dislocations to pass from one grain to the next. Reducing the size of grains will increase the total grain surface area, and thus increase strength. Grain size is related to how quickly the material is cooled during the solidification process.
Impurities
Pure metals have very clean crystals. If you introduce a few atoms of another material, and stick them in the middle of a crystal, it will create a disruption in the atomic pattern that will impede dislocation motion. It doesn’t take very much impurity, sometimes less than 1%, to dramatically increase the strength of a pure metal. This is the fundamental reason why steel is so much stronger than plain iron.
Phases
When you combine two or more different metals, called alloying, sometimes different phases are formed. A phase is a specific metallic compound that has its own crystal structure. In a basic steel alloy (combination of iron and carbon), there are many different phases that form at different temperatures. The main phases at room temperature are Ferrite (BCC: Body-Centered-Cubic iron) and Iron Carbide, or Cementite (Fe3C). When heated beyond a certain temperature a different phase becomes stable (FCC: Face-Centered-Cubic iron). By manipulating time, temperature, cooling rate, and alloy composition, you can get many other phases to form at room temperature that can result in hard and/or tough steel. Some of these other phases are called Austenite, Pearlite, Bainite, and Martensite.
More Dislocations
Lastly, dislocations themselves can be barriers for movement of other dislocations. The more you deform a material, the more dislocations you create, and the stronger the material apparently gets. This is the phenomena of ‘work hardening’. Although it isn’t strictly a heat treating process, it should be considered as means of improving material properties.
Application
Steel
There are countless ways of heat treating steel. The most common method is quenching from a high temperature (>727°C). Typically it is quenched in various mediums: air quench, oil quench, and water quench. Different phases are formed with different cooling rates, but generally the hardest phases are formed with the fastest cooling. The very hardest and strongest phase of steel is call Martensite. However in plain steel, it is so hard that it is too brittle to use in many applications. Often a process must be chosen to balance hardness and toughness of the steel.
The other major type of heat treating in steel is Carburizing, which is the act of increasing the carbon content on the surface of the part. Generally, the higher the carbon content, the more carbides are formed (which is a ceramic) and the harder the material is. Carburizing is achieved by heating the part up in furnace with a carbon-rich atmosphere; over time the carbon will diffuse a few ten-thousandths of an inch, and increase the surface hardness without making the whole part too brittle.
Aluminum
The methods of heat treating aluminum are very different then that of steel. The principle method of heat treating is Precipitation Hardening, which falls under the ‘impurities’ category discussed above. The process generally involves an aluminum alloy that would have an impurity content that would be supersaturated at room temperature when cooled sufficiently rapidly. Then, the material is re-heated at a low temperature (a few 100°’s C) and tiny particles of the saturated phase form or ‘precipitate’ out of solution. Some alloys will even precipitate without further heating. These AlCu alloys will ‘Age Harden’ after a few days or weeks…they simply get stronger the longer you wait! The most common extrusion grade alloy used in most products is 6061, and it is often sold pre-heat treated, to the ‘T6’ specification.