Corrosion with 440C SS Knife Blade

K

kaag

Joined
Dec 6, 2006
Messages
11
Hi,

I need some answers to a situation that i'm now facing.
We are a young company intending to produce fillet knives using 440C S/S.
After only few days of use of our knives, it seemed like the material has been attacked leaving black spots (small black holes) on different part of the blades.
I'll mention that our blades are laser-cut, heat and cryogienically treated to hardeness 57 RC. Then, the blades are ground "flat grinding", sharpened and polished.

My question is : why these small holes appearing on the blades ?

I made another experience just to see if this has been caused by chlorines by putting a knife in a hypoclorine solution for few hours. The knife came out with rust all over it. the blade was more damaged and there were a few more holes.

I look forward to hearing from you.

Thanks,

Arnold.
 
In the highly acidic realm of food processing a high carbon knife must be cleaned and maintained meticulously, religiously even. The best I have found for durability is the Forschner granton 6". I don't know if that helps but I would start there.
 
I don't know what the demands are in the industry, but I'd say you are underhardening your 440C. You will see better corrosion resistance if you raise the hardness, but you can also drop to a steel with a better chrom/carbon ratio if 440C at that hardness is uneconomical or find it too brittle (a lower carbide volume helps in this case).
 
Most knife makers generally keep their blades between 56 and 58 HRC. Ours are at 57 HRC. Does it make a difference to grind the blades before heat treating or after ? could this have an impact on the corrosion resistance of the material ? At this moment, the heat and cryo treatment are done on the laser-cut blades and the grinding process occur later on.

Thanks 4 replying.

Arnold.
 
I guess a question that should be asked is why was 440C chosen (what were the design criterions)? I am not being interrogative, it is just a question that would be helpful and informative if you can provide an answer to. Grinding the blade after heat treating is fairly normal. Many do this to prevent warpage during the heat treatment that is possible if the blade is already ground. When you say polished, do you mean the whole blade is polished or just the edge? A finer surface finish generally enhances corrosion resistance.
 
Criteria : high corrosion resistance (salt & chlorines), good edge retention, easy to get (SS 440C is available here in Montreal).
Yes, the whole blade is mirror-polished.
 
There are two possibilities .Improper heat treatment . Contamination - if the blade is ground or polished with with things that are also used for carbon steels ,the grinder or polishing wheel will pick up bits of carbon steel . These bits of carbon steel will transfer to the stainless, rust and the rusting will continue into the stainless steel !!
 
mete said:
There are two possibilities .Improper heat treatment . Contamination - if the blade is ground or polished with with things that are also used for carbon steels ,the grinder or polishing wheel will pick up bits of carbon steel . These bits of carbon steel will transfer to the stainless, rust and the rusting will continue into the stainless steel !!
Click to expand...

That's just what I was going to say. Contamination is a major problem when working with stainless.
 
Hi Mete,

I'll make some investigation on the grinding process...If the grinder wheel is also the one they use for our 440C S/S, can this wheel be cleaned so as to prevent this contamination or the wheel should be changed ?

thanks again for your help.

Arnold.
 
Welcome to Bladeforums!

moving-van.jpg
 
mete said:
There are two possibilities .Improper heat treatment . Contamination - if the blade is ground or polished with with things that are also used for carbon steels ,the grinder or polishing wheel will pick up bits of carbon steel . These bits of carbon steel will transfer to the stainless, rust and the rusting will continue into the stainless steel !!
Click to expand...


I suspect contamination. Are you pasivating the blades after they are polished? I believe that pasivating (pickleing in an acid bath) is a common practice when working stainless steel. The acid bath removes any traces of carbon steel.
 
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 !
 
Do not overlook the possibiility that the steel itself is contaminated. Pitting is not unknown with steels like 440C and ATS34, especially from offshore sources. Among custom makers, it is a well known phenomenon on some steels to have a perfectly good looking blade at 600 grit, only to find there are small pits when it is advanced to a mirror polish.
 
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 the spots appear on the flats of the blade and not on the bevels the problem is improper polishing (in short they are pits from the hot rolling process that were not taken out at the first step of polishing.)
 
Any stainless such as 440C and ATS34 will corrode very fast if the blade isn't cleaned and oiled immediately after finishing and sharpening. If the steel is completely clean, it may rust within an hour.

Citrus juice and blood will corrode the steel badly if left on the blade. :eek:
 
The corrosion resistance of 440C decreases if the hardening temperature is too low,or other elements of the HT are not observed (soak time,etc.).If the person doing the HT is skimping on time and temperature,due to the low target hardness,then the blades will corrode much more easily.I would look closely at the HT log on your blades.
Stacy
 
george tichbourne said:
If the spots appear on the flats of the blade and not on the bevels the problem is improper polishing (in short they are pits from the hot rolling process that were not taken out at the first step of polishing.)
Click to expand...
Hi Georges,

I read on your web site that you suggest hardness at 57 HRC for 440C SS.
You also make fillet knives using this material, have you ever faced this kind of problem. Do you always use grinding wheels made for stainless steel ?

Our blades are vacuum quenched and then cryo treated.

As many of you have suggested, I'll ask for the material certificate to learn more about who makes this product and how it has been made.

Some of you have mentionned that if the treated material could have been contaminated during the grinding process...I'll recognized that this sample blades have not been passivated ? Do you think this may have caused this early corrosion ?

Thanks for replying.
 
george tichbourne said:
If the spots appear on the flats of the blade and not on the bevels the problem is improper polishing (in short they are pits from the hot rolling process that were not taken out at the first step of polishing.)
Click to expand...
How do you make sure that the pits appearing on the material disappear during the polishing process ?
 
Don Robinson said:
Any stainless such as 440C and ATS34 will corrode very fast if the blade isn't cleaned and oiled immediately after finishing and sharpening. If the steel is completely clean, it may rust within an hour.

Citrus juice and blood will corrode the steel badly if left on the blade. :eek:
Click to expand...
Hi Don,

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