Larrin
Knifemaker / Craftsman / Service Provider
- Joined
- Jan 17, 2004
- Messages
- 4,982
We have already discussed the effect of heat treatment variables on simple tool steel toughness: https://www.bladeforums.com/threads/cru-forge-v-toughness-testing.1503654/
the effect of chemistry and carbide volume on toughness: https://www.bladeforums.com/threads/no-your-pm-stainless-steel-isnt-tough.1535555/
and the difference between freezer, dry ice, and liquid nitrogen on retained austenite content: https://bladeforums.com/threads/liquid-nitrogen-vs-dry-ice.1540810/
Now I am ready for edge retention. How can we predict edge retention? Composition and hardness? Well just like toughness the key to edge retention is carbide volume. Carbides are hard particles formed between carbon and different elements such as iron, vanadium, chromium, molybdenum, etc. Lots of these hard particles lead to poor toughness. Because these particles are all much harder than the steel itself, they all basically equally contribute to a reduction in toughness. However, unlike toughness the type of carbide also matters when it comes to edge retention, as there is a wide range in carbide hardnesses, as can be seen in the chart on page 2: https://www5.kau.se/sites/default/files/Dokument/subpage/2010/02/21_269_287_pdf_18759.pdf
Cementite (Fe3C), the main carbide type in simple carbon steels, is around 1000 Vickers, where vanadium carbides (VC) is around 2500. Therefore, we would expect vanadium carbides to have a stronger contribution to wear resistance than cementite. This has been observed, of course, in practice, as 10V steel has much higher wear resistance to 1095, even when heat treated to the same hardness. Is there a way we can use this information to make predictions about edge retention?
The best source of quantitative edge retention testing is CATRA, where a testing knife is used to cut through silica-impregnated cardstock and the amount of cardstock cut in a fixed number of cuts is compared between different blades. There has been some discussion about how practical the CATRA test is in comparison with actual cutting of everyday materials, but it is the best information that is typically available. One downside of this test is that it is affected by the geometry of the edge and how well it is sharpened, but it still better correlated with knife performance than a simple wear resistance test.
I gathered CATRA results from a variety of sources, the best collection of them I have found is here: http://www.cliffstamp.com/knives/reviews/CATRA.html There are potential complications from using CATRA results from different sources as they are likely to have differences in the test knives and sharpening, but it allows the use of a bigger database of materials. However, calculations are similar if I use only the Bohler-generated numbers: http://www.bucorp.com/media/CATRA_Test2.pdf In some cases I had to assume a hardness to add it into the table which is another limitation of the larger database. The Bohler table all adds a + sign in front of many hardness values, i.e. 61+ Rc, in those cases I added 0.5, such as 61.5 Rc in the table.
I used two sources in calculating the effect of wear resistance: experimentally reported carbide volumes and types as well as thermodynamic software calculated carbide volumes. The R2 values for either of the calculations was approximately the same, around 0.77, indicating that both give a pretty good fit. The equations found were the following:
JMatPro: -1884 + 37.5*Rc + 50.6*MC + 10.1*Cr23C6 + 18.9*Cr7C3 + 5.4*M6C
Experimental: -4948 + 86.5*Rc + 47.7*MC + 19.6*CrXC + 44.2*M6C
In either case there aren't very many steels with M6C carbides (typically in high speed steel) so I would take those values with a grain of salt. In both cases the MC (vanadium carbides) have a significantly stronger effect on edge retention than chromium carbides which was expected. In the case of experimentally reported values the chromium carbides were not differentiated between the M23C6 and M7C3 varieties.
Several years ago my father and I performed CATRA testing with 154CM and AEB-L. Both were heat treated to 60 Rc. We got 440 on 154CM and 190 on AEB-L. AEB-L has a very small volume of chromium carbides while 154CM has a relatively high volume of chromium carbides. The 190 for AEB-L is a little on the low side for the calculations I gave above, perhaps indicating that the carbide volume was even lower than predicted, or that AEB-L carbide predictions are closer to experimental than 154CM is, as experimentally reported carbide volume is significantly higher than predicted for 154CM. Using higher hardness AEB-L would give higher values, as can be seen by the 64 Rc O1 in the table with much higher CATRA result, as O1 has a low volume of relatively soft cementite.
Anyway, edge retention is confirmed to be controlled by hardness, carbide volume, and carbide type/hardness and estimates of edge retention can be made using a combination of regression equations and thermodynamic software-calculated carbides.
the effect of chemistry and carbide volume on toughness: https://www.bladeforums.com/threads/no-your-pm-stainless-steel-isnt-tough.1535555/
and the difference between freezer, dry ice, and liquid nitrogen on retained austenite content: https://bladeforums.com/threads/liquid-nitrogen-vs-dry-ice.1540810/
Now I am ready for edge retention. How can we predict edge retention? Composition and hardness? Well just like toughness the key to edge retention is carbide volume. Carbides are hard particles formed between carbon and different elements such as iron, vanadium, chromium, molybdenum, etc. Lots of these hard particles lead to poor toughness. Because these particles are all much harder than the steel itself, they all basically equally contribute to a reduction in toughness. However, unlike toughness the type of carbide also matters when it comes to edge retention, as there is a wide range in carbide hardnesses, as can be seen in the chart on page 2: https://www5.kau.se/sites/default/files/Dokument/subpage/2010/02/21_269_287_pdf_18759.pdf
Cementite (Fe3C), the main carbide type in simple carbon steels, is around 1000 Vickers, where vanadium carbides (VC) is around 2500. Therefore, we would expect vanadium carbides to have a stronger contribution to wear resistance than cementite. This has been observed, of course, in practice, as 10V steel has much higher wear resistance to 1095, even when heat treated to the same hardness. Is there a way we can use this information to make predictions about edge retention?
The best source of quantitative edge retention testing is CATRA, where a testing knife is used to cut through silica-impregnated cardstock and the amount of cardstock cut in a fixed number of cuts is compared between different blades. There has been some discussion about how practical the CATRA test is in comparison with actual cutting of everyday materials, but it is the best information that is typically available. One downside of this test is that it is affected by the geometry of the edge and how well it is sharpened, but it still better correlated with knife performance than a simple wear resistance test.
I gathered CATRA results from a variety of sources, the best collection of them I have found is here: http://www.cliffstamp.com/knives/reviews/CATRA.html There are potential complications from using CATRA results from different sources as they are likely to have differences in the test knives and sharpening, but it allows the use of a bigger database of materials. However, calculations are similar if I use only the Bohler-generated numbers: http://www.bucorp.com/media/CATRA_Test2.pdf In some cases I had to assume a hardness to add it into the table which is another limitation of the larger database. The Bohler table all adds a + sign in front of many hardness values, i.e. 61+ Rc, in those cases I added 0.5, such as 61.5 Rc in the table.
I used two sources in calculating the effect of wear resistance: experimentally reported carbide volumes and types as well as thermodynamic software calculated carbide volumes. The R2 values for either of the calculations was approximately the same, around 0.77, indicating that both give a pretty good fit. The equations found were the following:
JMatPro: -1884 + 37.5*Rc + 50.6*MC + 10.1*Cr23C6 + 18.9*Cr7C3 + 5.4*M6C
Experimental: -4948 + 86.5*Rc + 47.7*MC + 19.6*CrXC + 44.2*M6C
In either case there aren't very many steels with M6C carbides (typically in high speed steel) so I would take those values with a grain of salt. In both cases the MC (vanadium carbides) have a significantly stronger effect on edge retention than chromium carbides which was expected. In the case of experimentally reported values the chromium carbides were not differentiated between the M23C6 and M7C3 varieties.
Several years ago my father and I performed CATRA testing with 154CM and AEB-L. Both were heat treated to 60 Rc. We got 440 on 154CM and 190 on AEB-L. AEB-L has a very small volume of chromium carbides while 154CM has a relatively high volume of chromium carbides. The 190 for AEB-L is a little on the low side for the calculations I gave above, perhaps indicating that the carbide volume was even lower than predicted, or that AEB-L carbide predictions are closer to experimental than 154CM is, as experimentally reported carbide volume is significantly higher than predicted for 154CM. Using higher hardness AEB-L would give higher values, as can be seen by the 64 Rc O1 in the table with much higher CATRA result, as O1 has a low volume of relatively soft cementite.
Anyway, edge retention is confirmed to be controlled by hardness, carbide volume, and carbide type/hardness and estimates of edge retention can be made using a combination of regression equations and thermodynamic software-calculated carbides.
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. I'm planning on working with AEB-L soon and have been doing some google searching on this steel. There seems to be a range of differing information on the attributes, for instance I recently read this concerning AEB-L.
