Corrosion with 440C SS Knife Blade

[kaag]

I would suggest to start by going through the hardening process. It seems to me somewhat underhardened at 57 HRC with cryo treatment. If that process seems ok I'd check the material itself. Make a claim with a few pieces and demand a materials certificate for your batch of steel. Chech composition ond level of contaminations.

If you still have a problem there is always a possiblility to change steel.

In my opinion, no offence intended, high carbon grades are a bit risky for marine environments. I'd look for something with ~0.5 %C and minimum 14% Cr.

//Jay

If 440C is underhardened at 57 HRC what is the suggested level for knives ? 59 -60 ?

Concerning the 440C hardness at 57 HRC i'd like to read M. tichbourne on that since all his knives are hardned to this level.

 

[kaag]

The entire process should be carefully examined for contamination.Machining,grinding ,polishing ,bead blasting ,etc. I think you should be able to save the grinding wheel if it is dressed.And yes, it's the proper thing to passivate the stainless even when everything is clean. This is done with nitric or citric acid and it removes contaminants and produces a thicker protective oxide coating. Much of the 'stainless ' properties are due to the oxide coating rather than just the steel itself !

Does the passivation have to take place after the grinding process or the polishing ?

 

[mete]

The passivation is the final process , otherwise you would grind or polish off the oxide from the passivation !

 

[Bruce Bump]

I used to use allot of 440C and when it arrived it was rusted. The corrosion resistance increases with hardness but there is always exposed carbon on the surface unless it recieves the passivation. Paul Bos always arrived at 58 on 440C and I dont think he did a passivation treatment. Ive never had one rust. I have seen ats-34 rust in salt water.

When you are grinding after heat treatment, are you keeping the blades cool? 440C has a low temper around 350 degrees. You cant overheat it when regrinding and/or buffing like most other air hardening steels because it will reduce the hardness and therefore rust.

Are you sure its really 440C?

 

[Don Robinson]

Hi Don,

what do you mean by cleaning the blades ? Passivating ?
Is necessary to always oil the 440C blades after sharpening or finishing ?

Passivating isn't necessary. The steel must be oiled after it's finished. Also, if the blades are left for hours or days between processes while and after the blades are ground, the same thing applies.

It isn't necessary to clean it, it's already clean if it's been ground. That's the problem.

 

[kaag]

I used to use allot of 440C and when it arrived it was rusted. The corrosion resistance increases with hardness but there is always exposed carbon on the surface unless it recieves the passivation. Paul Bos always arrived at 58 on 440C and I dont think he did a passivation treatment. Ive never had one rust. I have seen ats-34 rust in salt water.

When you are grinding after heat treatment, are you keeping the blades cool? 440C has a low temper around 350 degrees. You cant overheat it when regrinding and/or buffing like most other air hardening steels because it will reduce the hardness and therefore rust.

Are you sure its really 440C?

Of course Bruce, it's 440C SS. If you say that there is always exposed carbon on the surface, from what you said passivation should be done.
Those who are in charge of grinding and polishing our blades generally oiled the blades but do not passivate.

Do we have to passivate and oil the blades to prevent corrosion ? If yes, which operation should be done first, passivating or oiling ?

If we want to passivate our blades, is the passivation solution available on the market ? Any product or brand you can suggest ?

Thanks

 

[kaag]

I used to use allot of 440C and when it arrived it was rusted. The corrosion resistance increases with hardness but there is always exposed carbon on the surface unless it recieves the passivation. Paul Bos always arrived at 58 on 440C and I dont think he did a passivation treatment. Ive never had one rust. I have seen ats-34 rust in salt water.

When you are grinding after heat treatment, are you keeping the blades cool? 440C has a low temper around 350 degrees. You cant overheat it when regrinding and/or buffing like most other air hardening steels because it will reduce the hardness and therefore rust.

Are you sure its really 440C?

Yes, there is flow of water coming over the blades during the grinding process to prevent overheating.

 

[mete]

Google 'passivation' They use nitric acid or cirtric but citric is becoming mor e common since disposing of waste is not such a big problem. You could oil after passivation but it's probably not necessary.

 

[george tichbourne]

Even good looking 440C or ATS 34 can have pitting from the mill. It is not uncommon for untrained grinders to miss seeing the minute pits during the initial grinding operation. These show up in final polishing as the surface gets smoother.

The only way to get them out is to go back a couple of steps and regrind the flats again until the pits dissappear.

I dry grind on belts only. My wet belt grinder is never used on Carbon steel for fear that the coolant will become contaminated with carbon steel particles. Contamination shows up on polished stainless as tiny rusty spots not black spots.

The blades are first hardened to RC 62-63 in state of the art vacuum heat treating ovens, cryo treated, then tempered to RC 57 for flexibility.

My job for the past 20 years has been polishing metals, primarily stainless steel for the architectural and sanitary industries, passivation is seldom called for except in the drug industry.