I was curious about stainless options for a scuba knife, and I run a little experiment to see how workable a "regular" stainless was on rough environments.
Here're the params:
- 154cm (not cpm)
- Sub zero quench.
- Austenized at the top of the range (maximum as quench hardness after sub-zero treatment)
- Citric acid passivated: 40C for 12 hs.
- Matte finish to 1000 grit and then back to 600 grit.
The theory behind the test:
- The cpm variant may have a marginally better corrosion resistance due to the more homogeneous structure. Whether this effect is large enough to be noticed I don't know.
- Corrosion resistance is highly dependent on the austenizing temperature due to less carbon taking up chromium. The higher the temperature, the better the corrosion resistance.
- The workable maximum for austenizing temperature is limited by retained austenite. Lowering the minimum quench temperature allows for a higher austenizing temperatures before retained aus. becomes a problem.
- The oxide layer providing the corrosion resistance is at equilibrium with the redox potential of the environment. Passivation does not provide any long term advantage here. It does, however, take care of small non-stainless inclusions that can happen during the grinding process, reducing thus the chances of a false negative during the test.
- A mirror polish enhances the corrosion resistance via two mechanisms: Reduction of the surface area (probably not an issue with stainless unless in a confined space with limited oxidative potential) and by removing any deep pits and scratches that may start pitting corrosion.
Some practical limitations.
- I used the non-cpm variant since I had it at hand.
- I don't have a setup for liquid nitrogen, so I went with dry ice. I doubt the difference is big, but it would be worth testing.
- The optimum austenizing temperature was done with several coupons at different temperatures. The temperature providing the maximum as-quench hardness after sub-zero was used. The hardness vs. temperature curve peaks shallowly, and then drops sharply. This makes easier to identify the drop and dial it down a few degrees.
- I was time pressed, so I couldn't go for a full mirror polish.
The test:
I took two 5-day scuba trips to the Channel Islands in a period of 1 month. The routine was to have 5 to 10 dives a day, and no fresh water for cleanup. The equipment was left wet on deck between dives and through the night. Rinsing only happened after getting back home after each trip. Daytime temperatures were in the mid 80s, dropping to the low 70s at night.
The results:
After the second trip, the two knives had 3 or 4 small superficial rust spots. All of them cleaned easily with a kitchen scotch-brite in less than a minute. Close inspection (visual, unassisted) showed no signs of pitting at the center of the spots. There was no noticeable edge degradation due to corrosion (there were not used).
The conclusions:
I feel confident than this material and treatment will work perfectly for me, and the added sharpness I'm getting from this steel is a welcomed change in contrast to the cheap 440a Chinese crap that all scuba stores sell. They do require maintenance, but this is minimal in contrast to all the rest of my scuba equipment.
I would also recommend this steel/treatment to any knowledgable client capable of understanding this results. I would be a little uneasy to market it to a broader audience, though.
Regards,
Here're the params:
- 154cm (not cpm)
- Sub zero quench.
- Austenized at the top of the range (maximum as quench hardness after sub-zero treatment)
- Citric acid passivated: 40C for 12 hs.
- Matte finish to 1000 grit and then back to 600 grit.
The theory behind the test:
- The cpm variant may have a marginally better corrosion resistance due to the more homogeneous structure. Whether this effect is large enough to be noticed I don't know.
- Corrosion resistance is highly dependent on the austenizing temperature due to less carbon taking up chromium. The higher the temperature, the better the corrosion resistance.
- The workable maximum for austenizing temperature is limited by retained austenite. Lowering the minimum quench temperature allows for a higher austenizing temperatures before retained aus. becomes a problem.
- The oxide layer providing the corrosion resistance is at equilibrium with the redox potential of the environment. Passivation does not provide any long term advantage here. It does, however, take care of small non-stainless inclusions that can happen during the grinding process, reducing thus the chances of a false negative during the test.
- A mirror polish enhances the corrosion resistance via two mechanisms: Reduction of the surface area (probably not an issue with stainless unless in a confined space with limited oxidative potential) and by removing any deep pits and scratches that may start pitting corrosion.
Some practical limitations.
- I used the non-cpm variant since I had it at hand.
- I don't have a setup for liquid nitrogen, so I went with dry ice. I doubt the difference is big, but it would be worth testing.
- The optimum austenizing temperature was done with several coupons at different temperatures. The temperature providing the maximum as-quench hardness after sub-zero was used. The hardness vs. temperature curve peaks shallowly, and then drops sharply. This makes easier to identify the drop and dial it down a few degrees.
- I was time pressed, so I couldn't go for a full mirror polish.
The test:
I took two 5-day scuba trips to the Channel Islands in a period of 1 month. The routine was to have 5 to 10 dives a day, and no fresh water for cleanup. The equipment was left wet on deck between dives and through the night. Rinsing only happened after getting back home after each trip. Daytime temperatures were in the mid 80s, dropping to the low 70s at night.
The results:
After the second trip, the two knives had 3 or 4 small superficial rust spots. All of them cleaned easily with a kitchen scotch-brite in less than a minute. Close inspection (visual, unassisted) showed no signs of pitting at the center of the spots. There was no noticeable edge degradation due to corrosion (there were not used).
The conclusions:
I feel confident than this material and treatment will work perfectly for me, and the added sharpness I'm getting from this steel is a welcomed change in contrast to the cheap 440a Chinese crap that all scuba stores sell. They do require maintenance, but this is minimal in contrast to all the rest of my scuba equipment.
I would also recommend this steel/treatment to any knowledgable client capable of understanding this results. I would be a little uneasy to market it to a broader audience, though.
Regards,
