meles meles
19-07-07, 10:24 PM
Maximum hardness in steels is obtained by producing a fully martensitic structure. This can be done by austenitizing the steel and then quenching it. During the austenitizing treatment all of the carbides dissolve and the ferrite transforms into austenite. Quenching this structure causes the austenite to transform via a shear mechanism (diffusionless transformation - the solute atoms do not have time to diffuse through the iron lattice if the cooling rate is great enough)into martensite. This transformation is so fast (Martensite needles grow at close to the speed of sound.) that there is no time to the carbon to diffuse out of the martensite grains or to form carbide phases. The martensite, supersaturated with carbon, is very hard and also very brittle.
Carbon, being a very effective solid solution strengthening agent, essentially determines the hardness of the martensite. Cases where a lesser degree of hardening can be attributed to the presence of other alloying elements, but these elements tend to also make it more difficult to obtain a fully martensitic microstructure. So while maximum hardness in a given steel is dependent on our ability to produce a fully martensitic microstructure, the hardness of the martensite is largely determined by its carbon content.
Hardenability
In order to form a fully martensitic structure the steel must be quenched at a rate that is equal to or greater than a critical cooling rate. If the quench is indeed fast enough and the part is thin then one can usually assume that this cooling rate can be achieved through the whole cross-section, producing a fully hardened part. However, this may not be the case for thick sections because the interior cools more slowly than the surface. But if one could modify this steel such that critical cooling rate is lower then thick pieces can be hardened throughout and even thicker pieces can be hardened to a considerable depth. This is of great practical importance not only in terms of our ability to produce a fully hardened part (which will also be fully brittle) but because subsequent tempering will be successful in producing the desired strength and ductility throughout the part. In addition, one could use less severe quenches to avoid problems with warping and cracking.
This ability of a steel to be hardened to a specified depth is called hardenability. In general, the hardenability of a steel is improved through alloying and all alloy additions except cobalt will improve the hardenability of a steel. Coarse grain size and homogeneity of the austenite also improve the hardenability. The reason this is so is not clear but is probably related to the retardation of nucleation and growth of the ferrite, carbide and bainite phases.
Effect of Grain Size and Chemical Composition on Hardenability
The two most important variables which influence hardenability are grain size and composition. The hardenability increases with increasing austenite grain size, because the grain boundary area is decreasing. This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased. Likewise, most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability. However, quantitative assessment of these effects is needed.
There are a bewildering number of steels, the compositions of which are usually complex and defined in most cases by specifications, which give ranges of concentration of the important alloying elements, together with the upper limits of impurity elements such as sulfur and phosphorus. While alloying elements are used for various reasons, the most important is the achievement of higher strength in required shapes and sizes and often in very large sections which may be up to a meter or more in diameter in the case of large shafts and rotors. Hardenability is, therefore, of the greatest importance, and one must aim for the appropriate concentrations of alloying element needed to harden fully the section of steel under consideration. Equally, there is a little point in using too high a concentration of alloying element, i.e. more than that necessary for full hardening of the required sections.
Alloying elements are usually much more expensive than iron, and in some cases are diminishing natural resources, so there is additional reason to use them effectively in heat treatment. Carbon has a marked influence on hardenability, but its use at higher levels is limited, because of the lack of toughness which results, the greater difficulties in fabrication and, most important, increased probability of distortion and cracking during heat treatment and welding.
The most economical way of increasing the hardenability of plain carbon steel is to increase the manganese content, from 0.60 wt% to 1.40 wt%, giving a substantial improvement in hardenability. Chromium and molybdenum are also very effective, and amongst the cheaper alloying additions per unit of increased hardenabilily. Boron has a particularly large effect when it’s added to fully deoxidized low carbon steel, even in concentrations of the order of 0.001%, and would be more widely used if its distribution in steel could be more easily controlled.
Quench Media
Water: Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vaporises and there is no direct contact of the water with the steel. This slows down cooling until the bubbles break and allow water contact with the hot steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots.
Water is a good rapid quenching medium, provided good agitation is done. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking.
Salt Water: Salt water is a more rapid quench medium than plain water because the bubbles are broken easily and allow for rapid cooling of the part. However, salt water is even more corrosive than plain water, and hence must be rinsed off immediately.
Oil: Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard. It's fun though: we almost set fire to Owen's shed at the spring hammer in...
Polymer quench: Polymer quenches that will produce a cooling rate in between water and oil. The cooling rate can be altered by varying the components in the mixture-as these are composed of water and some glycol polymers. Polymer quenches are capable of producing repeatable results with less corrosion than water and less of a fire hazard than oil. But, these repeatable results are possible only with constant monitoring of the chemistry.
Cryogenic Quench: Cryogenics or deep freezing is done to make sure there is no retained austenite during quenching. 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.
Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100% martensite at room temperature. However, high carbon and high alloy steels have retained austenite at room temperature. To eliminate retained austenite, the quench temperature has to be lowered. This is the reason to use cryogenic quenching.
Carbon, being a very effective solid solution strengthening agent, essentially determines the hardness of the martensite. Cases where a lesser degree of hardening can be attributed to the presence of other alloying elements, but these elements tend to also make it more difficult to obtain a fully martensitic microstructure. So while maximum hardness in a given steel is dependent on our ability to produce a fully martensitic microstructure, the hardness of the martensite is largely determined by its carbon content.
Hardenability
In order to form a fully martensitic structure the steel must be quenched at a rate that is equal to or greater than a critical cooling rate. If the quench is indeed fast enough and the part is thin then one can usually assume that this cooling rate can be achieved through the whole cross-section, producing a fully hardened part. However, this may not be the case for thick sections because the interior cools more slowly than the surface. But if one could modify this steel such that critical cooling rate is lower then thick pieces can be hardened throughout and even thicker pieces can be hardened to a considerable depth. This is of great practical importance not only in terms of our ability to produce a fully hardened part (which will also be fully brittle) but because subsequent tempering will be successful in producing the desired strength and ductility throughout the part. In addition, one could use less severe quenches to avoid problems with warping and cracking.
This ability of a steel to be hardened to a specified depth is called hardenability. In general, the hardenability of a steel is improved through alloying and all alloy additions except cobalt will improve the hardenability of a steel. Coarse grain size and homogeneity of the austenite also improve the hardenability. The reason this is so is not clear but is probably related to the retardation of nucleation and growth of the ferrite, carbide and bainite phases.
Effect of Grain Size and Chemical Composition on Hardenability
The two most important variables which influence hardenability are grain size and composition. The hardenability increases with increasing austenite grain size, because the grain boundary area is decreasing. This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased. Likewise, most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability. However, quantitative assessment of these effects is needed.
There are a bewildering number of steels, the compositions of which are usually complex and defined in most cases by specifications, which give ranges of concentration of the important alloying elements, together with the upper limits of impurity elements such as sulfur and phosphorus. While alloying elements are used for various reasons, the most important is the achievement of higher strength in required shapes and sizes and often in very large sections which may be up to a meter or more in diameter in the case of large shafts and rotors. Hardenability is, therefore, of the greatest importance, and one must aim for the appropriate concentrations of alloying element needed to harden fully the section of steel under consideration. Equally, there is a little point in using too high a concentration of alloying element, i.e. more than that necessary for full hardening of the required sections.
Alloying elements are usually much more expensive than iron, and in some cases are diminishing natural resources, so there is additional reason to use them effectively in heat treatment. Carbon has a marked influence on hardenability, but its use at higher levels is limited, because of the lack of toughness which results, the greater difficulties in fabrication and, most important, increased probability of distortion and cracking during heat treatment and welding.
The most economical way of increasing the hardenability of plain carbon steel is to increase the manganese content, from 0.60 wt% to 1.40 wt%, giving a substantial improvement in hardenability. Chromium and molybdenum are also very effective, and amongst the cheaper alloying additions per unit of increased hardenabilily. Boron has a particularly large effect when it’s added to fully deoxidized low carbon steel, even in concentrations of the order of 0.001%, and would be more widely used if its distribution in steel could be more easily controlled.
Quench Media
Water: Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vaporises and there is no direct contact of the water with the steel. This slows down cooling until the bubbles break and allow water contact with the hot steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots.
Water is a good rapid quenching medium, provided good agitation is done. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking.
Salt Water: Salt water is a more rapid quench medium than plain water because the bubbles are broken easily and allow for rapid cooling of the part. However, salt water is even more corrosive than plain water, and hence must be rinsed off immediately.
Oil: Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard. It's fun though: we almost set fire to Owen's shed at the spring hammer in...
Polymer quench: Polymer quenches that will produce a cooling rate in between water and oil. The cooling rate can be altered by varying the components in the mixture-as these are composed of water and some glycol polymers. Polymer quenches are capable of producing repeatable results with less corrosion than water and less of a fire hazard than oil. But, these repeatable results are possible only with constant monitoring of the chemistry.
Cryogenic Quench: Cryogenics or deep freezing is done to make sure there is no retained austenite during quenching. 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.
Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100% martensite at room temperature. However, high carbon and high alloy steels have retained austenite at room temperature. To eliminate retained austenite, the quench temperature has to be lowered. This is the reason to use cryogenic quenching.