Scratch Marks Over Stainless Steel

Exactly, this problem is causing rejection of blades with high amount of rejection PPM and we haven't faced this earlier.
I am having doubt over the material as the only thing which has been changed is material supplier. But even if the problem is with material, how can I confirm it? I mean, is there any quantifiable measure/property?

as the only thing which has been changed is material supplier

Could the new material be impure somehow? Perhaps due to damaged press heads or similar transferring carbon steel into the stainless stock? Go back to the supplier for clarification (perhaps there is a batch problem).

Alternatively switch to a better (more reliable supplier), last thing you need is getting a name for blunt or rusty stainless blades (bad publicity).

But the material complies the SS420 grade. We have checked the material composition at testing lab with help of spectro chemical analysis.

Wow,... that's taking it to a whole new and next level... it would be great if you had the spec's from your previous supplier.

my next question would be why did your company switch suppliers?

Successful attempt to find a cheaper supplier? (In both senses.)

I didn't want to assume,... but yes.

The reason behind switching the supplier was totally different commercial matter regarding supply to one of our another divisions and we are still paying almost the same rate which we were used to pay to earlier supplier.

When your received the material, did you get the heat, spec. or mill sheet with the material? It states composite, batch number and mill specifications.

it would be great if you could match it up with the original suppliers mill sheet.

Yes, I have mill test certificates from both, when compared I can conclude that

• the new SS420 material have Cr % more (by 1%) than that was used to be in the old material but both of them comply the range of Cr % of standard chemical composition of SUS 420J2 grade (i.e. 12%-14%)
• Hardness of old raw material was used to be approx 91 HRB and it was used to be approx 85 HRB in case of old one.
• Other mechanical properties (except hardness) are not mentioned in the mill test certificates of old supplier.

Mill Certs can lie , even though it would be a criminal offence to do so ......

i would be sending a few new samples to a lab for spectroscopic analysis and compare to the mill report .

not sure about where you are but my cost is usually $110 - $130 a visit for 2 - 3 shots

many foundries have their own Spectro gear and charge less than a certified lab

( because of the cost of white lab coats these days ;)n

** edited ** sorry I just read above that you already did spectro **

Yeah, agree. That's why we have also got tested the material at certified lab at our end. But chemical composition is found satisfactory during spectro-chemical analysis.

good point!

I forgot to mention, but I have checked the same and didn't found the scratches in raw material.
Even the blades are being checked after each subsequent grinding operations, and scratches are found to be generated at each stage.

if the scratches are being added at each step during grinding then your new supplier is sending you material with hard inclusions in it. it is easy to miss these during chemical analysis.

Well, in that case, how can I check for the same. Any suggestions?

Well I would be saving the scratched pieces to run tests on, first a scratch test for hardness, maybe the raw material is not uniform in composition....then for impurities at the lab....That some are being scratched and some not seems like a clear signal that the material is not uniform in hardness....of course there could be other forces at work, but that is how it looks from here....

"Material not uniform in hardness"

Two possible causes:

1) Because of non uniform raw material

2) Improper heat treatment of knife blades

Have to check both....But when inspected with Rockwell tester by indentation, after heat treating the blades, the hardness is found to be approx 52 HRC uniformly along the length of the blade.

Your hardness measure is borderline for this application....

Hardness of knives steel

"The table below gives an overall overview of the characteristics of knives steel with different hardness levels.

• Up to 52 HRC: Too soft for making knives.
• 52-54 HRC: Quite soft steel, reasonable quality.
• 54-56 HRC: The hardness of many French chefs’ knives. The steel is hard enough for kitchen use, but regular use of a sharpening steel is required to keep the knife sharp. Knives of this hardness are usually easy to sharpen.
• 56-58 HRC: Hardness applied for professional German kitchen knives. Knives of this hardness remain sharp long enough for kitchen use, can be sharpened on a sharpening steel and are reasonably easy to sharpen.
• 58-60 HRC: Hardness you usually find in better quality pocket knives like Spyderco, Cold Steel and Buck, and kitchen knives from Japan, like Global. Those knives remain sharp considerably longer than cheaper knives, but are a lot more difficult to sharpen.
• 60-62 HRC: Knives of this hardness remain sharp for a long time, but they are at risk of becoming brittle and the knives are often difficult to sharpen. These disadvantages are quite easy to suppress with modern steel types, but the quality depends on the quality of the whole production process.
• 63-66 HRC: Currently knives of hardness up to 66 HRC (Twin Cermax of Zwilling J.A. Henckels) are available. These are no knives suitable for the majority of users, but rather a specific group of amateurs. Knives of such hardness have the disadvantage that they become brittle, resulting in breakage of blade parts when used carelessly and often a low resistance against corrosion. In other words: clean immediately after use."

https://www.knivesandtools.com/en/ct/knife-steel-types-and-information.htm

I agree with Solar Eagle, that is far too soft to easily avoid scratches and too soft to hold a decent edge.

If there are inclusions you should notice such when you grind the blades. Inclusions should leave visible evidence when ground, especially if polished. I doubt Inclusions are scratching the material.

If scratches are occurring at each stage, then you can remedy the situation by improving cleanliness and handing at each stage, heat treating to a higher hardness at the beginning, or both.

Depending on how deep the scratches are, a final clean and polish and perhaps additional scratch resistant surface treatment might remedy the situation and save some would-be rejects.,,,maybe even add value to the product.

For you to detect this by indentation hardness, you would need to be exactly on the inclusion and that inclusion would need to be "supported" by matrix behind it.

I have run into this many times. If you suspect subsurface inclusions the simple method would be a polish then etch with around a 10% nital etch. Any subsurface inclusions would be readily observant at < 100x magnification.

My bet too.

The grinding process would be a good place to look. Has there been a change in process, equipment, consumables, personel?

The blades may be picking up fine hard duct/particles from breakdown of material used in grinding. This fine hard material may be causing the scratches later as the piece is moved/worked.

I'd check all the blades along the procesa and see if you can locate where scratches occur or where cleanliness is lost.

No change has been made to grinding process.

Grinding dust/burr getting stuck over the blade may cause scratch marks over blade during handling. And most probably it is one of the cause for scratch marks.

But my concern is, we were following the same grinding and cleaning process since long and hadn't faced such major rejections due to scratch marks.

Maybe your polishing wheels are worn out, or your polishing wheels got mixed up, or the polishing wax is the wrong grit, or your polisher is an alcoholic and you are just finding out....there are many things that could be going wrong, you seem to have eliminated only one, the knife raw material is the same.....keep looking

Yes there are many more possibilities, but what we are facing currently is somewhat weird.
As far as, wheels are concerned, we do have different wheels for different grinding operations and scratches are found after almost each stage.
If the blades are checked after each subsequent grinding and cleaning operations, scratch marks are found to be generated at each stages. I mean, not all the blades have scratches at each stage, but few blades having scratches are found after each operations. This leads me to doubt over raw material.
One more possibility could be magnetic property of material. SS420J2, being a magnetic steel, there are chances of grinding dust/burrs getting stuck more firmly with the blade causing scratches over them.

Is it wet or dry grinding ?

if dry grinding do you have a vacuum extraction system ?

if there is a different magnetic induction occurring can you reverse it with an opposite pole electromagnet to make the dust drop off ?

could it be a static charge build up. ,and if so can you ground the parts with a static strap ?

instinct tells me you have dust clinging to your parts which are then handled by the traditional method and carry their untraditional dust with them ?

No, it is wet grinding.
Whatever the case may be, the problem which we are facing currently was not there at such extent, even though the manufacturing set up/processes are followed in the same way which were followed since very long. And I am more interested in identifying that root cause.

Do you have any of the material from the old supplier ? Insert a few labelled pieces through at the same time as the new product and see if they come through unscratched.

that will confirm straight away if it's the material or a change / contamination in the process .

Unfortunately, we do not have raw material from old supplier.

Well I recommend that you get some.

TNTC nailed it, or so I believe. The only material around that will "dig" into the stock is the grinding media. AP (the OP AP) needs to look at his people, and if they maintaining clean work areas, and not sliding new blades over flat surfaces that are contaminated with shed particles of grit.

Other option is protection of the polished sides of blades with plastic sheet, but that will create a waste material issue.

How does your process differ from this...?

In the video, the knives are forged knives whereas we manufacture them by stamping/blanking.
After blanking, we do heat treatment with the help of conveyor belt type furnace in an inert atmosphere of nitrogen created by cracking ammonia, followed by tempering process.
In grinding, we do have 3-4 stage grinding operations.

just because we have changed the supplier of SS420-J2 coil.

And there is your answer.

Clearly you have not properly completed a root cause analysis as you say you have 2 sources where the scratches could be generated. So before seeking speculative advice from here, is it not wiser to narrow it down to the actual root cause: grinding, handling, then asking here how to solve the issue, once you have provided better details!

You have 4 basic process's that may affect the problem and a materials spec change. And you have not found the root cause? Shot in the dark answers which may all be quite right.

is there a cleaning process done to the blanks. there may be a different type of oil/lubrication being used by the new supplier that is not completely being removed. this may be holding grit in your other processes.

Just because the supplier says that the material is the proper grade, that doesn't mean it truly is. I know an electropolishing company that was having a terrible time with pitting while trying to EP some supposed 304 ss a customer sent to them, which, after analysis was found to contain over 10% copper. Definitely not 304ss. The customer was the one who was duped by his supplier - it happens these days.

You say your grinding process is unchanged. However sometimes small variances could be causing overheating of the blade during grinding. Coolant line not adjusted properly, different coolant, different grinding wheel, grinding wheel loaded up with debris.

Can operators change any of these: feed rates, depths of cut?

Just a few things to check that might affect hardness down line.

Have you done a hardness check after each grinding process?

Though, operators can't change grinding parameters, we have already cross verified the same and found to be OK.
And the main problem is we are getting scratches generated at almost each stages of subsequent grinding operations to few blades.

I am going to assume you switched suppliers in order to get a cheaper material, which if you understand English and the various meanings of "Cheaper" you will see the problem and switch back to the supplier that provided you with reliable material.

It is difficult for most manager /bean counters to understand that a profit made by shorting the customer is a temporary one, as is everyone's job at that point.

The reason behind switching the supplier was totally different commercial matter regarding supply to one of our another divisions and we are still paying almost the same rate which we were used to pay to earlier supplier. So it is not the case of making profit by affecting the product quality.

The fact you are paying almost the same rate doesn't exclude the possibility it's inferior material being delivered to you.

You said it's "totally different commercial matter regarding supply to one of our another divisions "

That seems to imply this new material is being forced on you because another division wants to use this supplier?

That could indicate corruption in the buying process.

Can you override the other division and go back to the original supplier?

You have all the arguments on your side - tell the buyers that the knife dept is losing material/sales/time/money because of this problem.

If they say there's no way that they're changing back to the old supplier then it would implicitly confirm corruption in the buying process.

What country are you in btw?

You raise a legitimate concern, though care should be taken what accusations are made or implied.

A refusal to return to the original supplier doesn't confirm corruption, but neither does it do anything to disprove concerns about corruption either.

Can you add to the process and return your product to the quality of the original material? Yes. Will this add cost to the process? Probably. Will your product be as good? Possibly. Will it really end up being a "good for the business" move? Now that is the question isn't it.

Now we are getting progress.

You are confient that it's happening at "grinding".

You are confident that it's not isolated to one grinding stage. This probably rules out issues with your liquid filtering system and operators unless it's diamonds from your dressing method embeded into the wheels.

Sounds like the hard inclusions I suggested back in post 12. Get a reasonable microscope and examine the blades, good and bad and stages in between. Examine the scratches and see if you can see residue in the scratches and "grains" dragged inside the surface.

We need high magnification of the scratches....

I would suggest that you inspect blades at each stage of your process to identify where/when the scratches are appearing, then carefully look at the preceding process to identify what has changed in that process or machine recently ..... perhaps something has broken or been damaged internally in one of your machines and is causing the scratches...

Have you read the previous posts?

Comparative crystallography of before and after supplier batches - of samples of each before and after supplier batch at each stage of processing.

Why not just use a specular reflectance scan using a coherent light source (laser)? All of the phase information is still contained as long as the focus is tight enough.

Good point. The changes in the tools available to crystallographers since my single encounter (a scanium alloy) in the early 1990's are largely pass my pay grade, but the laser reflectance sounds like a good one. A lot was learned about the alloying variations with my crystallography experience.