Cru Forge V was developed by Crucible for those who forge their steel for knives [1]. It was developed shortly before Crucible’s bankruptcy and is reported to have been tested with the help of knifemakers Howard Clark and Dan Farr and that the code name prior to its official name was 1086V [2]. The steel is not listed anywhere on Crucible’s website and does not appear to be in production any longer, but as of March 2018 is still available from some third party steel sellers [3][4][5]. The steel has the following composition [1]:
The Mn and Cr were added for hardenability, the steel likely has somewhat lower hardenability than O1, but is still classified as oil hardening. The vanadium was added to form hard VC carbides to contribute to wear resistance. Most forging steels have relatively little wear resistance, as most have a small volume of relatively soft cementite (iron carbides). Crucible provided a micrograph in the original datasheet, why the scale bar is in inches I have no idea but 0.0001 inches is equivalent to 2.54 microns [1]:
It appears the carbides are relatively small and evenly distributed, with some scattered carbides of somewhat larger size, likely the vanadium carbides. JMatPro calculations show a similar amount of carbon in solution as O1, and a relatively close carbide volume, but Cru Forge V has just under 1% vanadium carbide replacing some of the cementite found in O1:
Therefore CruForgeV would be expected to have similar hardness and toughness to O1 but superior wear resistance due to the small amount of vanadium carbides. Because getting carbon in solution is reliant on the dissolution of cementite rather than vanadium carbides, heat treatment should be similar to O1 rather than a more complex tool steel.
My father and I previously worked on a processing and toughness testing experiment with Cru Forge V in an attempt to optimize its properties as well as to test some reported processing variables. We tested forging temperatures from 1550-2000°F, austenitizing temperatures from 1450-1550°F, tempering temperatures from 300-400°F, three different anneals, and a multiple quench process. All were normalized after forging at 1600°F, and all were given a liquid nitrogen treatment for 30 minutes after the final quench prior to tempering. All were quenched in Parks 50 oil. The annealing processes were as follows:
- Subcritical – 1250°F for 2.5hrs after normalizing
- Divorced Eutectoid Transformation (DET) [6]: an anneal studied by Verhoeven that was designed to shorten the time required for annealing, whereby a ferrite plus spheroidize carbide microstructure is formed directly rather than forming ferrite-pearlite that must then be spheroidized. A full explanation of the process cannot be covered in this article. The steel was held at 1460°F for 30 minutes followed by cooling at 670°F/h to 1260°F and finally cooling to room temperature in air.
- Temper annealing – similar to a subcritical anneal but starting from a martensitic microstructure. The steel was austenitized at 1450°F for 10 minutes followed by oil quenching, then performed again, followed by 1250°F for 2.5hrs.
The triple quench was performed by doing two austenitize and quench treatments at 1450°F for 10 minutes with an oil quench followed by 1500°F for 10 minutes followed by an oil quench. Rockwell hardness was performed by testing in 3 locations and averaging. The toughness was measured either in the longitudinal or transverse directions with 1/4 thickness subsize unnotched charpy specimens, and is the average of 3 samples.
The results of the experiments are as follows:
All of those numbers are provided for whoever wants to scrutinize them, but I have provided the following plots and analysis for everyone else:
It is thought that lower forging temperatures can lead to a reduction in grain size and therefore an improvement in properties since grain growth occurs more rapidly at higher temperatures. However, the 1550, 1850, and 2000°F forging temperatures led to very similar hardness and toughness, while 1700°F led to higher hardness with a small decrease in toughness. I cannot say whether this was an outlier, or an indication of a true “peak” forging temperature, but I lean toward outlier. This seems to show that the recommended forging temperatures of 1700-2000°F [1] are good and will not lead to excessive grain growth, at least with the use of a power hammer and rolling mill from 1-3/16″ round bar. Slower hand forging may behave somewhat differently. The datasheet recommends against forging below 1600°F but no drop off in behavior was observed. Low temperature forging may be more problematic if larger forging equipment were used. The reason that the forging temperature did not greatly affect the properties may be for one of the following possibilities:
- The vanadium carbides are effective for pinning the grains and therefore excessive grain growth did not take place regardless of the forging temperature.
- The normalizing treatment was effective for “resetting” the grain size and forming a consistent cementite structure prior to annealing.
Austenitizing, however, did see a drop off in toughness with excessive temperature. The datasheet recommends a range of 1500-1550°F, but we used 1450-1550°F. See my previous article for information on the effect of austenitizing temperature on properties: Austenitizing Part 2 – Effects on Properties The datasheet also recommends to equalize at temperature and directly quench but we used a short soak of 10 minutes, as a soak is typically recommended for steels like O1 and 52100. The 1550°F austenitize led to very brittle behavior with no corresponding increase in hardness. This is somewhat troubling since this is part of the recommended rage of the datasheet. Perhaps if we had not used any soak the toughness would be improved. However, it appears that going lower than the recommended 1500°F is viable and may lead to increased toughness. The toughness likely dropped either due to excessive carbon in solution or excessive grain growth with the higher temperature. The cause for the slight reduction in hardness between 1500 and 1550°F austenitizing temperature is likely due to experimental variability, as the retained austenite should have been largely eliminated during the liquid nitrogen treatment.
Performing the triple quench led to increased hardness and a corresponding decrease in toughness; grain refinement from multiple quenching is generally expected to increase both strength and toughness, as described in my article previously published: Austenitizing Part 3 – Multi-Step Austenitizing. The hardness was likely increased due to further dissolution of carbides with repeated cycling for more carbon in solution. Because no metallography was performed as part of this study it is unknown if the triple quench successfully led to grain refinement, or if the grain refinement simply did not lead to increased toughness. There are several possibilities for why the triple quench may not have led to grain refinement:
- The vanadium is effective for maintaining a fine grain size that is little effected by multiple quenching.
- The soak times were too long for achieving grain refinement.
- The initial or final soak temperatures needed to be lower because of the further dissolution of carbides and to ensure fine grain.
Higher tempering temperatures led to decreased hardness and increased toughness, as was expected. The datasheet recommends 400-500°F but no major reduction in toughness was detected with even a 300°F temper. The hardness values achieved were somewhat higher than reported in the datasheet [1], but this may be explained by the use of cryogenic processing in this study, which should have largely eliminated the soft retained austenite.
The final hardness was greatest with the temper anneal followed by the subscritial and finally the DET. This indicates that the hardening response was best for the temper anneal. However, the temper anneal was given the 1700°F forging treatment relative to the DET and subscritial anneal on the plot which are shown with the 1850°F forging temperature. The DET anneal also had higher hardness when done in conjunction with the 1700°F, perhaps lending evidence to the peak hardness with the 1700°F forging temperature not being an outlier. The temper anneal also appears to have led to the highest combination of hardness and toughness especially when compared to the subcritical anneal. The temper anneal may have been helped by the fact that it received a double austenitize and quench prior to the temper anneal, which may have helped refine the grain size.
When all plotted together it becomes apparent that hardness was the greatest controlling factor for toughness in this study. It can also be seen that the transverse toughness is 49-76% of the longitudinal toughness, despite the steel having a relatively small and even carbide distribution. This is likely due to elongated impurities and other directionally oriented features, or perhaps carbide banding is present that was not shown in the micrograph provided by Crucible. The orientation relative to the rolling direction certainly matters. The strongest parameters for controlling properties were the austenitizing and tempering temperatures, and was relatively insensitive to the forging temperature, anneal employed, and multiple quenching cycles. A 1550°F austenitize with a 10 minute soak is certainly not recommended, and the lower end of the austenitizing temperature range is probably best. Lower tempering temperatures than recommended may be used where very high hardness is necessary. While there are a couple data points that appear to be slightly above the “trendline” they may be within the noise of the testing, and probably wouldn’t be detectable in any actual use, therefore we cannot conclude that any of the specialized anneals or multiple quenching treatment will lead to superior properties. If more tests were conducted I would probably want to try the following tests:
- Slower oil than Parks 50. Parks 50 is nearly as fast as water quenching and is likely faster than necessary for this steel which could lead to extra stresses during quenching.
- Use lower austenitizing temperatures and shorter soak times for multiple quenching.
- Try higher tempering temperatures to ensure the recommended tempering is good (400-500°F), and avoids any tempered martensite embrittlement in that range.
To show where Cru Forge V fits, toughness wise, with some other steels that we have tested with the same subsize charpy specimens, here is a plot showing its longitudinal toughness along with a few comparisons:
The toughness of Cru Forge V is good though unspectacular, considering it is only marginally better than the stainless steels 19C27 and CPM-154 which have considerably higher carbide volume. This is consistent with the reported toughness of other water and oil hardening steels, however, such as when comparing O1 and A2 with Latrobe toughness data, where A2 is approximately 80 ft-lbs and O1 is approximately 50 ft-lbs at 60 Rc with unnotched izod impact tests [7][8]. This may be due to the difficulty in controlling grain size in low alloy steels along with the relatively high carbon in solution when heat treating them relative to many air hardening steels.
[1] https://www.alphaknifesupply.com/Pictures/Info/Steel/CruForgeV-DS.pdf
[2] https://www.bladeforums.com/threads/1086v-steel.576264/
[3] https://www.alphaknifesupply.com/shop/cruforgev-carbon-steel
[4] http://www.hightemptools.com/steel.html
[5] https://usaknifemaker.com/cru-forge-v-by-crucible-225-see-length-note.html
[6] Verhoeven, J. D. “The role of the divorced eutectoid transformation in the spheroidization of 52100 steel.” Metallurgical and Materials Transactions A 31.10 (2000): 2431-2438.
[7] https://www.alphaknifesupply.com/Pictures/Info/Steel/A2-DS-Latrobe.pdf
[8] https://www.alphaknifesupply.com/Pictures/Info/Steel/O1-DS-Latrobe.pdf
