Cryogenic Treatment
Cryogenic treatment is performed at the boiling point of liquid nitrogen or -196 ° C, through the fog of 'nitrogen.
The cryogenic has a 'greater effectiveness in increasing the wear resistance and hardness of steel with respect to subcooling. This is also where a proper subcooling allows the complete transformation dell'austenite residual martensite.
The reason is still under study, but it seems that there is a fine precipitation of carbides during the cryogenic treatment.
What happens with the cryogenic treatment itself, or run at -197 degrees?
The structure becomes very similar to that of martensite, a structure that is formed in the high-alloy T which is the secondary hardening.
This structure, and 'consists of a ferrite matrix within which are dispersed particles (spherical) of carbides. Major will be 'the number of carbons, the lower' their volume (for each carbide) and the more 'the total area of separation between the two phases.
More surface area and, the greater saa hardness, wear resistance and mechanical strength.
The substantive differences between the two types of martensite (the first with the subcooling, the second with the cryogenic treatment) are that, in the second, you will have a better distribution of carbides and particlelle ques'utlime will be more purposes.
The cryogenic treatment, however, although you make a better relaxation of stress with respect to undercooling, does not totally eliminate the stress between the grains, which instead can be obtained with the triple tempering.
In addition, among the particles of martensite obtained by cryogenic, there cohesion less than that obtained with a triple tempering.
Bos is running a "strong subcooling", then:
use (-)130 grades because, as there is written in the linked document, already at -80 degrees you can completely transform the austenite into martensite (tetragonal, body-centered, not found).
To lower horizons Ms and Mf also contribute to different elements of the alloy.
Cobalt, for example, and one of the few that raises the horizon Ms.
We should also say that the residual austenite is formed also 'cause the phase range is not' a homogeneous phase but 'heterogeneity and the austenite formed in areas rich in defects or alloying elements is very stable and can persist even after off.
As to changes in the structure, however, I wrote that the structure formed was very similar to that of martese found, formed by ferrite and carbides. The latter, however, are much finer and uniformly dispersed in the steel compared to those obtained through discovery.
In fact, the structure that is formed with cryogenic consists of tetragonal martensite and fine carbides. The changes in the structure are not yet very clear and under investigation but it seems that, during quenching, while austenite-martensite transformation occurs, some C atoms occupied interstitial posizoni come out and go to fomare carbons for increasing pressure (due , I suppose, to the formation of martensite).
The cryogenic treatment the size of these carbides decrease because, as low T are inhibited covalent bonds between atoms C (or between these atoms and those of other elements that form carbides).
An other theory is a micronucleated carbides precipitated in the vicinity of lattice defects, also formed due to the contraction of steel at such low temperatures.
The thing I think that is discharged, with the cryogenic, there is a precipitation of carbides and the consequent impoverishment of the C martensite, which reduces distortion and then, but nevertheless always tetragonal.
In short, it seems has not yet made a full investigation of what happens inside the steel Effective when you run that kind of treatment as well as the results are erratic and not always very encouraging, even for the high risk of fractures treated piece .
Subcooling
Introduction
The subcooling is performed at temperatures up to between -40 ° C to -120 ° C.
Generally a temperature of -80 ° C is enough for most tool steels, to transform austenite into martensite throughout the TCC, if the subcooling is done immediately after shutdown.
Otherwise, the retained austenite tends to stabilize too.
Of course immediately after treatment or after treatment of subcooling crogenico, when the steel is slowly returned to room temperature, it must make the discovery for detensionare steel.
However, the subcooling can be run in the traditional way, in the freezer, for example, although the efficacy is not comparable to that of an industrial subcooling, performed with specific equipment.
Practical application of the treatment of undercooling during forging of a cutting tool.
Take for example the 52100 steel.
Usually it applies on the break is one hour at 204 degrees to ensure that homogenize the temperature between the heart and superficie.In general to stop these steels are used at such temperatures and when it was still 'under the formation of Martensite Start.
MS stands for Martensite start and is the temperature at which martensitic transformation begins. The martensite structure of steel is hardened to the state. Or rather, the tempering is done to obtain a martensitic structure.
At a temperature of quenching the steel takes on a structure called austenite. When cooled with sufficient speed (Vs, higher critical speed), the austenite, the martensite get (transformation adiffusionale).
Now, if I stopped off at a temperature slightly higher than Ms and keep the steel at that temperature for a long time, the austenite will transform into bainite .*
If I stopped off at a temperature between ** Ms and Mf (martensite finish) I will have to stabilize and then residual dell'austenite been transformed martensite start later.
In my opinion, if you want to make a stop to homogenize the temperature, or make a thermal quenching, it is worth it just above Ms, for a time sufficient to ensure improved ** equalizing the temperature, but not enough to Start pouring bainitic transformation.
Time and temperature, of course, depend on the type of steel.
Also, please note that while the pearlitic transformation, and bainitic ferrite advance, at a given temperature, with the advance of time, the martensitic transformation progresses only with decreasing temperature (obviously in the range between Ms and Mf) .
As already 'written undercooling austenite transforms into martensite tetragonal body-centered.
This technique therefore allows a greater wear resistance than a single discovery.
This is because the discovery of single-austenite is transformed, according to the T for the year, following the changes indicated in the TTT curve (isothermal transformation), and then will change' in perlite or bainite.
The sottoraffredamento but not from the point of view of performance, no benefit compared to the technique of multiple discovery.
With this technique, and 'can not just turn all the residual austenite into martensite but optimally eliminate all residual stresses.
As with the multiple discovery? This must be executed immediately after power off when the steel is more in T than 80Gradi.
As 'doing the amount' of residual austenite and 'better but this way you can also decrease the C content of the austenite (rush it' in the form of carbides as Fe3C, Fe2, 4C or other hard carbides, depending on the T tempering used).
Triple tempering technique.
This technique, described in the book of John Verhoeven, allows you to refine the austenitic grain and therefore the martensite, on the principle that affirms "the finer will be the starting structure, the finer will be the final structure (which forms Departure from the hotel). "
Explanation of Triple Tempra.
In practice, this technique is nell'austenizzare steel at a temperature as close as possible to Ac1 (obviously above Ac1) or Ac3 for steels ipoeutettoidi, and maintain it for as briefly as possible (just enough to ensure the heart of Austenitizing the piece).
Without this you do a shutdown, high oil usually drastic.
Repeat the cycle three times. (On consecutive days).
In this way, with the first austeinizzazione you get a fine austenitic grain. row martensite that forms from the austenite will be very fine (martensite crystals break up the previous austenitic grains).
Similarly, the austenite to martensite will be formed by the fine will also be very fine, and then, off to the next, will result in even finer martensite and so on.
Subcooling, if it takes place immediately after shutdown, or after the final quenching, the real and propane, helps transform dell'austeite remaining part (or all of the retained austenite) in martensite. If you do that after three findings, 24 hours apart, is of little use to me, or because the residual is austere and 'already' transformed into martensite (or intermediate structures such as pearlite or bainite) or you and 'stabilized so as to render ineffective the subcooling.
The rate of "subcooling" must be low, because the thermal shock could cause the appearance of cracks. Keep in mind that both the thermal shock that the further transformation of martensite due to subcooling create internal tensions that come in addition to those shut down.
The duration of subcooling may be even a few hours, but over time has the temperature raggiunta.Dopo subcooling you return the steel to room temperature and then you make the discovery.
Indeed, in light of the foregoing, I think that in practice is to be preferred a sottraffreddamento followed by a double or triple tempering short black to cryogenic treatment.
The increase in hardness and wear resistance that makes the subcooling is normal. There is nothing that bringing the steel at temperatures nearer the horizon Mf, which for most of the high-alloy steel (almost all) and for low alloy steels with more than 0.7% of C is located below the 0 ° C.
So, for the same T subcooling and holding time, the effectiveness of treatment depends on the composition of the steel because it determines the temperature Mf.
In fact, the residual austenite is not homogeneous and therefore that in this area with the biggest concentration of alloying elements or where there is an enrichment of local imperfections is more stable and thus might be required to lower operating temperatures. At the end of the double or triple tempering should fix everything (when run with the appropriate temperature), as well as to relax the internal stresses, promotes the precipitation of carbides and decreases the distortion of the tetragonal lattice (which in the area of secondary hardening returns to CCC ).
Another consideration is interesting to note that increasing the temperature of Austenitizing, with the same half-off, increase the amount of retained austenite. This should be because in this way, the steel remains for longer in the area where the austenite is stable (above Ac1 and / or AC3) and metastable (below Ac1 and / or AC3). The same is true within certain limits and except in special cases, if the cooling rate decreases (again according to the above you). Only in some cases, due to shutdowns too drastic, there is an increase of residual austenite (such as in the case of K340 and some HSS).
For hardening of high-alloy steel the best thing would be, in my opinion, use the higher tempering temperatures, in order to facilitate the solubilization of hydrocarbons, and maintain, adequate off after a series of discoveries in a row (if possible at temperatures in those of secondary hardening), possibly preceded by adequate subcooling.
Endnotes
Both these methods (cryogenics and subcooling) increase internal tensions, due to the martensitic transformation and stresstermico dell'austenite residual (voltages that are additive to those of hardening).
In some cases the subcooling or cryogenic treatment can be performed between the first and second discovery, but as I wrote there is a risk of an excessive stabilization of martensite.
The cryogenic can not be done in a home (soak the hardened steel in liquid nitrogen provocorebbe heat stress too high).
The amount of retained austenite of a steel depends on the content of alloying elements. All in solution in the iron range, increase the amount of residual austere after shutdown (with the exception of Co and Al).
Bainite *: The lower bainite is a structure with a hardness of not very high (about 50 to 55 HRC), but with a 'high impact resistance and flexibility (in general).
** Off-EQ: Do not confuse the stops to heating with those of equalization is turned off.
At the heating can be done one or two stops equalizers (for low alloy steels-practically the most part), one at 400-450 ° C and at 600-650 ° C (otherwise only at 600 ° C). The residence time depends on the thickness of the pieces. Let's say 120 seconds for 4-5 mm thick.
The rest of equalization, a temperature higher than Ms, to be implemented for a period not long enough, to enter the field of bainitic transformation (CCT diagrams and we need to see TTT, also a function of temperature austeizzazione, at least for steels hypereutectoid) .
Further reading:
Personally, if you want to Start pouring the study of metallurgy, steel, and heat treatments on the board to begin to study the diagram Fe-C (with their main characteristics of the various phases and various constituents of the steel) and the TTT and CCT diagrams.
Then the effects of tempering. Then you can explore the various topics (in particular the influence of various alloying elements on various characteristics of the steel, especially in the structure and function of heat treatment).
As for the council to start with reading books in Italian. "Metallurgy, general principles" of Walter Nicodemi, published by Zanichelli. "The Fe-C diagram and TTT curves" of Leno Matteoli, published by AIM.
Then you can explore the various topics with readings in English. First and foremost, "Metallurgy of steel and other who bladesmith for heat treat and forge steel" John Verhoeven.
Companies that perform cryogenic treatments:
Induction Heat Treating Corporation
One Cryo
Cryogenic treatment is performed at the boiling point of liquid nitrogen or -196 ° C, through the fog of 'nitrogen.
The cryogenic has a 'greater effectiveness in increasing the wear resistance and hardness of steel with respect to subcooling. This is also where a proper subcooling allows the complete transformation dell'austenite residual martensite.
The reason is still under study, but it seems that there is a fine precipitation of carbides during the cryogenic treatment.
What happens with the cryogenic treatment itself, or run at -197 degrees?
The structure becomes very similar to that of martensite, a structure that is formed in the high-alloy T which is the secondary hardening.
This structure, and 'consists of a ferrite matrix within which are dispersed particles (spherical) of carbides. Major will be 'the number of carbons, the lower' their volume (for each carbide) and the more 'the total area of separation between the two phases.
More surface area and, the greater saa hardness, wear resistance and mechanical strength.
The substantive differences between the two types of martensite (the first with the subcooling, the second with the cryogenic treatment) are that, in the second, you will have a better distribution of carbides and particlelle ques'utlime will be more purposes.
The cryogenic treatment, however, although you make a better relaxation of stress with respect to undercooling, does not totally eliminate the stress between the grains, which instead can be obtained with the triple tempering.
In addition, among the particles of martensite obtained by cryogenic, there cohesion less than that obtained with a triple tempering.
Bos is running a "strong subcooling", then:
use (-)130 grades because, as there is written in the linked document, already at -80 degrees you can completely transform the austenite into martensite (tetragonal, body-centered, not found).
To lower horizons Ms and Mf also contribute to different elements of the alloy.
Cobalt, for example, and one of the few that raises the horizon Ms.
We should also say that the residual austenite is formed also 'cause the phase range is not' a homogeneous phase but 'heterogeneity and the austenite formed in areas rich in defects or alloying elements is very stable and can persist even after off.
As to changes in the structure, however, I wrote that the structure formed was very similar to that of martese found, formed by ferrite and carbides. The latter, however, are much finer and uniformly dispersed in the steel compared to those obtained through discovery.
In fact, the structure that is formed with cryogenic consists of tetragonal martensite and fine carbides. The changes in the structure are not yet very clear and under investigation but it seems that, during quenching, while austenite-martensite transformation occurs, some C atoms occupied interstitial posizoni come out and go to fomare carbons for increasing pressure (due , I suppose, to the formation of martensite).
The cryogenic treatment the size of these carbides decrease because, as low T are inhibited covalent bonds between atoms C (or between these atoms and those of other elements that form carbides).
An other theory is a micronucleated carbides precipitated in the vicinity of lattice defects, also formed due to the contraction of steel at such low temperatures.
The thing I think that is discharged, with the cryogenic, there is a precipitation of carbides and the consequent impoverishment of the C martensite, which reduces distortion and then, but nevertheless always tetragonal.
In short, it seems has not yet made a full investigation of what happens inside the steel Effective when you run that kind of treatment as well as the results are erratic and not always very encouraging, even for the high risk of fractures treated piece .
Subcooling
Introduction
The subcooling is performed at temperatures up to between -40 ° C to -120 ° C.
Generally a temperature of -80 ° C is enough for most tool steels, to transform austenite into martensite throughout the TCC, if the subcooling is done immediately after shutdown.
Otherwise, the retained austenite tends to stabilize too.
Of course immediately after treatment or after treatment of subcooling crogenico, when the steel is slowly returned to room temperature, it must make the discovery for detensionare steel.
However, the subcooling can be run in the traditional way, in the freezer, for example, although the efficacy is not comparable to that of an industrial subcooling, performed with specific equipment.
Practical application of the treatment of undercooling during forging of a cutting tool.
Take for example the 52100 steel.
Usually it applies on the break is one hour at 204 degrees to ensure that homogenize the temperature between the heart and superficie.In general to stop these steels are used at such temperatures and when it was still 'under the formation of Martensite Start.
MS stands for Martensite start and is the temperature at which martensitic transformation begins. The martensite structure of steel is hardened to the state. Or rather, the tempering is done to obtain a martensitic structure.
At a temperature of quenching the steel takes on a structure called austenite. When cooled with sufficient speed (Vs, higher critical speed), the austenite, the martensite get (transformation adiffusionale).
Now, if I stopped off at a temperature slightly higher than Ms and keep the steel at that temperature for a long time, the austenite will transform into bainite .*
If I stopped off at a temperature between ** Ms and Mf (martensite finish) I will have to stabilize and then residual dell'austenite been transformed martensite start later.
In my opinion, if you want to make a stop to homogenize the temperature, or make a thermal quenching, it is worth it just above Ms, for a time sufficient to ensure improved ** equalizing the temperature, but not enough to Start pouring bainitic transformation.
Time and temperature, of course, depend on the type of steel.
Also, please note that while the pearlitic transformation, and bainitic ferrite advance, at a given temperature, with the advance of time, the martensitic transformation progresses only with decreasing temperature (obviously in the range between Ms and Mf) .
As already 'written undercooling austenite transforms into martensite tetragonal body-centered.
This technique therefore allows a greater wear resistance than a single discovery.
This is because the discovery of single-austenite is transformed, according to the T for the year, following the changes indicated in the TTT curve (isothermal transformation), and then will change' in perlite or bainite.
The sottoraffredamento but not from the point of view of performance, no benefit compared to the technique of multiple discovery.
With this technique, and 'can not just turn all the residual austenite into martensite but optimally eliminate all residual stresses.
As with the multiple discovery? This must be executed immediately after power off when the steel is more in T than 80Gradi.
As 'doing the amount' of residual austenite and 'better but this way you can also decrease the C content of the austenite (rush it' in the form of carbides as Fe3C, Fe2, 4C or other hard carbides, depending on the T tempering used).
Triple tempering technique.
This technique, described in the book of John Verhoeven, allows you to refine the austenitic grain and therefore the martensite, on the principle that affirms "the finer will be the starting structure, the finer will be the final structure (which forms Departure from the hotel). "
Explanation of Triple Tempra.
In practice, this technique is nell'austenizzare steel at a temperature as close as possible to Ac1 (obviously above Ac1) or Ac3 for steels ipoeutettoidi, and maintain it for as briefly as possible (just enough to ensure the heart of Austenitizing the piece).
Without this you do a shutdown, high oil usually drastic.
Repeat the cycle three times. (On consecutive days).
In this way, with the first austeinizzazione you get a fine austenitic grain. row martensite that forms from the austenite will be very fine (martensite crystals break up the previous austenitic grains).
Similarly, the austenite to martensite will be formed by the fine will also be very fine, and then, off to the next, will result in even finer martensite and so on.
Subcooling, if it takes place immediately after shutdown, or after the final quenching, the real and propane, helps transform dell'austeite remaining part (or all of the retained austenite) in martensite. If you do that after three findings, 24 hours apart, is of little use to me, or because the residual is austere and 'already' transformed into martensite (or intermediate structures such as pearlite or bainite) or you and 'stabilized so as to render ineffective the subcooling.
The rate of "subcooling" must be low, because the thermal shock could cause the appearance of cracks. Keep in mind that both the thermal shock that the further transformation of martensite due to subcooling create internal tensions that come in addition to those shut down.
The duration of subcooling may be even a few hours, but over time has the temperature raggiunta.Dopo subcooling you return the steel to room temperature and then you make the discovery.
Indeed, in light of the foregoing, I think that in practice is to be preferred a sottraffreddamento followed by a double or triple tempering short black to cryogenic treatment.
The increase in hardness and wear resistance that makes the subcooling is normal. There is nothing that bringing the steel at temperatures nearer the horizon Mf, which for most of the high-alloy steel (almost all) and for low alloy steels with more than 0.7% of C is located below the 0 ° C.
So, for the same T subcooling and holding time, the effectiveness of treatment depends on the composition of the steel because it determines the temperature Mf.
In fact, the residual austenite is not homogeneous and therefore that in this area with the biggest concentration of alloying elements or where there is an enrichment of local imperfections is more stable and thus might be required to lower operating temperatures. At the end of the double or triple tempering should fix everything (when run with the appropriate temperature), as well as to relax the internal stresses, promotes the precipitation of carbides and decreases the distortion of the tetragonal lattice (which in the area of secondary hardening returns to CCC ).
Another consideration is interesting to note that increasing the temperature of Austenitizing, with the same half-off, increase the amount of retained austenite. This should be because in this way, the steel remains for longer in the area where the austenite is stable (above Ac1 and / or AC3) and metastable (below Ac1 and / or AC3). The same is true within certain limits and except in special cases, if the cooling rate decreases (again according to the above you). Only in some cases, due to shutdowns too drastic, there is an increase of residual austenite (such as in the case of K340 and some HSS).
For hardening of high-alloy steel the best thing would be, in my opinion, use the higher tempering temperatures, in order to facilitate the solubilization of hydrocarbons, and maintain, adequate off after a series of discoveries in a row (if possible at temperatures in those of secondary hardening), possibly preceded by adequate subcooling.
Endnotes
Both these methods (cryogenics and subcooling) increase internal tensions, due to the martensitic transformation and stresstermico dell'austenite residual (voltages that are additive to those of hardening).
In some cases the subcooling or cryogenic treatment can be performed between the first and second discovery, but as I wrote there is a risk of an excessive stabilization of martensite.
The cryogenic can not be done in a home (soak the hardened steel in liquid nitrogen provocorebbe heat stress too high).
The amount of retained austenite of a steel depends on the content of alloying elements. All in solution in the iron range, increase the amount of residual austere after shutdown (with the exception of Co and Al).
Bainite *: The lower bainite is a structure with a hardness of not very high (about 50 to 55 HRC), but with a 'high impact resistance and flexibility (in general).
** Off-EQ: Do not confuse the stops to heating with those of equalization is turned off.
At the heating can be done one or two stops equalizers (for low alloy steels-practically the most part), one at 400-450 ° C and at 600-650 ° C (otherwise only at 600 ° C). The residence time depends on the thickness of the pieces. Let's say 120 seconds for 4-5 mm thick.
The rest of equalization, a temperature higher than Ms, to be implemented for a period not long enough, to enter the field of bainitic transformation (CCT diagrams and we need to see TTT, also a function of temperature austeizzazione, at least for steels hypereutectoid) .
Further reading:
Personally, if you want to Start pouring the study of metallurgy, steel, and heat treatments on the board to begin to study the diagram Fe-C (with their main characteristics of the various phases and various constituents of the steel) and the TTT and CCT diagrams.
Then the effects of tempering. Then you can explore the various topics (in particular the influence of various alloying elements on various characteristics of the steel, especially in the structure and function of heat treatment).
As for the council to start with reading books in Italian. "Metallurgy, general principles" of Walter Nicodemi, published by Zanichelli. "The Fe-C diagram and TTT curves" of Leno Matteoli, published by AIM.
Then you can explore the various topics with readings in English. First and foremost, "Metallurgy of steel and other who bladesmith for heat treat and forge steel" John Verhoeven.
Companies that perform cryogenic treatments:
Induction Heat Treating Corporation
One Cryo
