Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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In a recent thread about the HT for a high alloy stainless steel, the advantage/disadvantage of oil quench was discussed. Since I had a "free" hour this morning ( daylight savings time ), I spent it doing some reading of old information. I re-read Roman Landes' theory and application of austenitzation and quenching high alloy and stainless steels and how to tweak the HT to get the most from these steels. For those who don't know Roman Landes , he wrote the book ( literally and figuratively) on HT for knife steels.....Messer und Stahl.
Anyway, I decided to condense some of his posts on another forum into a fairly compact regimen with a little explanation, but not getting too technical.
The steels that will benefit from this process are CPM 3V, CPM 154, the CPM S__V series, and other complex alloy steels.
All information given assumes the user will have a good understanding of the HT of stainless steel, and the various methods of oxygen exclusion, quenching, etc.
The Crucible charts for these steels are based on a commercial operation and the HT done industrially. As knifemakers, we have the ability to fine tune the HT to gain a little here and there. An industry doesn't have that luxury. This doesn't make the Crucible charts wrong, it just makes the possibility of getting a bit more from the steel a reality.
The main stages of the HT for complex steels are:
Pre-heat
Austenitization
Quench/cooling
Sub-zero and Cryo - completion of the cooling process
Temper
Let's take these individually:
Pre-heat - Complex steels need time to change from one phase to another. You need to allow these changes to occur and stabilize/equalize before moving the steel into another change. Most complex steels should be evenly heated to above 1200F/650C and then taken to 1500F/815C and allowed to equalize for 15-30 minutes.
Austenitization - This is the main area that gets played with in complex steels. The transformation into the austenite phase and the dissolving of the alloy elements is what happens here. The chromium and molybdenum dissolve, and other carbide formers, like W and V, will stay as carbides to varying degrees. This allows the carbide grains to be somewhat refined, but is mainly a function of the alloy blend. The process for these things to happen takes time......a relatively long time. One must hold the steel accurately at the target temperature for a period of 30 minutes to give these elements time to change their arrangement and get evenly distributed. Rushing the process will greatly affect the HT in the most negative way. This time period starts when the blade reaches the target temperature.
The austenitization range for these steels is from 1800F/980C to 2100F/1150C, with a mid range area around 1950F/1065C. One should test an assure that their ovens are reading the correct temperature. A little off and the results can vary a lot.
The selection of the target temperature will affect the outcome in two ways. The higher the temperature the harder ( wear resistance) the steel gets...and the lower the temperature, the tougher the steel gets. The trade off is a balance of the desired qualities the knife is expected to have. Choppers need more toughness, and slicers need more hardness. For most complex steel knife blades, the middle temperature of the charts is the place to start. Testing of the outcome with your equipment and procedures will tell you if you need to move up or down from there. While a Rockwell test will be of great use in testing the hardness, it won't tell you the whole story, and the knife edge should be tested by other tests designed to show toughness and wear resistance.
Cooling - The steel must be cooled from the austenitized state to a point where it can convert into martensite. The rate that this happens at ( quench speed) and the method will be the second greatest area that affects the final outcome. Most complex steel are classified as air quench steels. That means that the cooling rate of air is fast enough to allow the transformation. All well and good, but we don't just want the blade cooled, we want to control the outcome. This is where a variety of cooling and quenching procedures are applied to guide the steel along to our desired end result. First, the steel needs to drop below the pearlite formation nose...around 900-1000F/500-540C. This drop needs to be done in some way to allow an orderly cooling and avoid warp. There are three ways to do this - 1) cool it in still or moving air. 2) cool it in warm oil. 3) cool it between two large metal plates ( usually aluminum).
Air is going to incur the least stress to the cooling rate, but also may loose some hardness.
Oil is going to assure the fastest cooling, but needs to be an interrupted quench to avoid overly stressing the steel. Done right, it is the most effective method, but there is no way to accurately adjust the rate. It is trial and error, and can change due to many factors.
Plates offer a controlled and repeatable cooling rate as well as help control warping. The entire foil packet can be placed in between the plates and pressed together with moderate pressure to allow transfer of the heat to the aluminum plates, while still excluding oxygen from the hot steel. For most knifemaker purposes , the plates are used at ambient temperature. Chilling the plates in a tub of ice water or a freezer will help, but is unnecessary except for the most difficult steels. If you are having a problem reaching your target hardness, try chilling/freezing the plates and see if that increases the outcome.
Sub-zero and Cryo - The quench does not stop at 900/1000F !!!!. The steel will continue to cool, usually in the quench plates, to below 400F/200C where it starts to convert into martensite. This is called the martensitic conversion start point ,or Ms. On simple steels, this ends somewhere between 80F/25C and 200F/95C . On complex steels it does not end until around -80F/-60C , called the martensitic conversion finish point, or Mf. The austenite to martensite conversion is extremely fast, and there is no need to hold the blade at the sub-zero temperature for any long time. If using liquid nitrogen and doing Cryo, it takes a few hours for the carbides to convert, but the conversion of austenite into martensite happens at the speed of sound. The treatment of the steel during the period between Ms and Mf is the third place that we can greatly affect the outcome. The steel is a mix of retained austenite and martensite at room temperature. Even your home freezer will only slightly affect the transformation. It requires a sub-zero cooling to complete the process. This can be done in a dry ice bath, or done to the most effect in liquid nitrogen. The cooling of the steel from austenite to martensite must be a fairly continuous process without long holds at any point. If you stop the process at any place, some of the austenite will stay there and become stabilized...or retained...austenite. Ra is not harmful in itself, but can affect the hardness of the blade.
Tempering - Upon cooling to the Mf, at -80F/-60C, most of the austenite is converted to martensite, and is a brittle mixture. It is then tempered where we coax it into a more stable and less brittle state. During the temper, the retained austenite is converted into new martensite.
Tempering looses a little hardness, but gains toughness. This is an entirely controllable process, where we get to pick the desired results. Complex steels have two ranges to temper in. The low temperature range assures the high hardness being retained, but does not yield the maximum toughness. Hardness loss is very slow in this range as the temperature increases. The higher temperature range gives maximum toughness, but can reduce the hardness rapidly with only a small variation upward. The temperatures between the two ranges are to be avoided at all costs, as this is where embrittlement occurs. If the tempering is done in the upper range, it should be cooled with a water or oil quench to assure the steel drops through the embrittlement range as fast as possible. Quenching from temper is beneficial in all cases, regardless of steel type. If allowed to cool slowly, the Ra will become stabilized. Since there is no change of state involved, cracking and warp are not an issue in this cooling quench from temper. Once the blade drops to room temperature from the first temper, there is now a mix of tempered martensite, some remaining retained austenite ( less that before), and the newly converted martensite that is still brittle. The New martensite must be tempered, and if we leave the Ra there for even a short time period, it will become permanent. To deal with both these situations, we do a second sub-zero treatment.....immediately after the first temper. Don't wait until tomorrow. The entire Ht process from pre-heat to final temper should be done as one continuous session. This will convert most of the remaining Ra ( all that will convert) and then we can follow this with a second temper cycle. The steel at this point should be mainly tempered martensite and a small amount of retained austenite. If the steel is very complex, a third temper may be beneficial, but additional sub-zero treatments won't gain anything. All tempering cycles are usually two hours periods.
Anyway, I decided to condense some of his posts on another forum into a fairly compact regimen with a little explanation, but not getting too technical.
The steels that will benefit from this process are CPM 3V, CPM 154, the CPM S__V series, and other complex alloy steels.
All information given assumes the user will have a good understanding of the HT of stainless steel, and the various methods of oxygen exclusion, quenching, etc.
The Crucible charts for these steels are based on a commercial operation and the HT done industrially. As knifemakers, we have the ability to fine tune the HT to gain a little here and there. An industry doesn't have that luxury. This doesn't make the Crucible charts wrong, it just makes the possibility of getting a bit more from the steel a reality.
The main stages of the HT for complex steels are:
Pre-heat
Austenitization
Quench/cooling
Sub-zero and Cryo - completion of the cooling process
Temper
Let's take these individually:
Pre-heat - Complex steels need time to change from one phase to another. You need to allow these changes to occur and stabilize/equalize before moving the steel into another change. Most complex steels should be evenly heated to above 1200F/650C and then taken to 1500F/815C and allowed to equalize for 15-30 minutes.
Austenitization - This is the main area that gets played with in complex steels. The transformation into the austenite phase and the dissolving of the alloy elements is what happens here. The chromium and molybdenum dissolve, and other carbide formers, like W and V, will stay as carbides to varying degrees. This allows the carbide grains to be somewhat refined, but is mainly a function of the alloy blend. The process for these things to happen takes time......a relatively long time. One must hold the steel accurately at the target temperature for a period of 30 minutes to give these elements time to change their arrangement and get evenly distributed. Rushing the process will greatly affect the HT in the most negative way. This time period starts when the blade reaches the target temperature.
The austenitization range for these steels is from 1800F/980C to 2100F/1150C, with a mid range area around 1950F/1065C. One should test an assure that their ovens are reading the correct temperature. A little off and the results can vary a lot.
The selection of the target temperature will affect the outcome in two ways. The higher the temperature the harder ( wear resistance) the steel gets...and the lower the temperature, the tougher the steel gets. The trade off is a balance of the desired qualities the knife is expected to have. Choppers need more toughness, and slicers need more hardness. For most complex steel knife blades, the middle temperature of the charts is the place to start. Testing of the outcome with your equipment and procedures will tell you if you need to move up or down from there. While a Rockwell test will be of great use in testing the hardness, it won't tell you the whole story, and the knife edge should be tested by other tests designed to show toughness and wear resistance.
Cooling - The steel must be cooled from the austenitized state to a point where it can convert into martensite. The rate that this happens at ( quench speed) and the method will be the second greatest area that affects the final outcome. Most complex steel are classified as air quench steels. That means that the cooling rate of air is fast enough to allow the transformation. All well and good, but we don't just want the blade cooled, we want to control the outcome. This is where a variety of cooling and quenching procedures are applied to guide the steel along to our desired end result. First, the steel needs to drop below the pearlite formation nose...around 900-1000F/500-540C. This drop needs to be done in some way to allow an orderly cooling and avoid warp. There are three ways to do this - 1) cool it in still or moving air. 2) cool it in warm oil. 3) cool it between two large metal plates ( usually aluminum).
Air is going to incur the least stress to the cooling rate, but also may loose some hardness.
Oil is going to assure the fastest cooling, but needs to be an interrupted quench to avoid overly stressing the steel. Done right, it is the most effective method, but there is no way to accurately adjust the rate. It is trial and error, and can change due to many factors.
Plates offer a controlled and repeatable cooling rate as well as help control warping. The entire foil packet can be placed in between the plates and pressed together with moderate pressure to allow transfer of the heat to the aluminum plates, while still excluding oxygen from the hot steel. For most knifemaker purposes , the plates are used at ambient temperature. Chilling the plates in a tub of ice water or a freezer will help, but is unnecessary except for the most difficult steels. If you are having a problem reaching your target hardness, try chilling/freezing the plates and see if that increases the outcome.
Sub-zero and Cryo - The quench does not stop at 900/1000F !!!!. The steel will continue to cool, usually in the quench plates, to below 400F/200C where it starts to convert into martensite. This is called the martensitic conversion start point ,or Ms. On simple steels, this ends somewhere between 80F/25C and 200F/95C . On complex steels it does not end until around -80F/-60C , called the martensitic conversion finish point, or Mf. The austenite to martensite conversion is extremely fast, and there is no need to hold the blade at the sub-zero temperature for any long time. If using liquid nitrogen and doing Cryo, it takes a few hours for the carbides to convert, but the conversion of austenite into martensite happens at the speed of sound. The treatment of the steel during the period between Ms and Mf is the third place that we can greatly affect the outcome. The steel is a mix of retained austenite and martensite at room temperature. Even your home freezer will only slightly affect the transformation. It requires a sub-zero cooling to complete the process. This can be done in a dry ice bath, or done to the most effect in liquid nitrogen. The cooling of the steel from austenite to martensite must be a fairly continuous process without long holds at any point. If you stop the process at any place, some of the austenite will stay there and become stabilized...or retained...austenite. Ra is not harmful in itself, but can affect the hardness of the blade.
Tempering - Upon cooling to the Mf, at -80F/-60C, most of the austenite is converted to martensite, and is a brittle mixture. It is then tempered where we coax it into a more stable and less brittle state. During the temper, the retained austenite is converted into new martensite.
Tempering looses a little hardness, but gains toughness. This is an entirely controllable process, where we get to pick the desired results. Complex steels have two ranges to temper in. The low temperature range assures the high hardness being retained, but does not yield the maximum toughness. Hardness loss is very slow in this range as the temperature increases. The higher temperature range gives maximum toughness, but can reduce the hardness rapidly with only a small variation upward. The temperatures between the two ranges are to be avoided at all costs, as this is where embrittlement occurs. If the tempering is done in the upper range, it should be cooled with a water or oil quench to assure the steel drops through the embrittlement range as fast as possible. Quenching from temper is beneficial in all cases, regardless of steel type. If allowed to cool slowly, the Ra will become stabilized. Since there is no change of state involved, cracking and warp are not an issue in this cooling quench from temper. Once the blade drops to room temperature from the first temper, there is now a mix of tempered martensite, some remaining retained austenite ( less that before), and the newly converted martensite that is still brittle. The New martensite must be tempered, and if we leave the Ra there for even a short time period, it will become permanent. To deal with both these situations, we do a second sub-zero treatment.....immediately after the first temper. Don't wait until tomorrow. The entire Ht process from pre-heat to final temper should be done as one continuous session. This will convert most of the remaining Ra ( all that will convert) and then we can follow this with a second temper cycle. The steel at this point should be mainly tempered martensite and a small amount of retained austenite. If the steel is very complex, a third temper may be beneficial, but additional sub-zero treatments won't gain anything. All tempering cycles are usually two hours periods.
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