Mill scale on steel

[Natlek]

How deep they are ? How much I need to grind to get rid of them ? How I would know that's enough ? I grind only left side on this blade so right side is just little hand sanded to get clear finish .I see that mill scale on right side but have no idea how deep to grind ? 0.1mm is enough?
This is what I found on Wikipedia............

Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are being produced by rolling red hot iron or steel billets in rolling mills. Mill scale is bluish-black in color. It is usually less than 0.1 mm (0.0039 in) thick, and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating.

 

[mete]

In the lower center hole , is that a crack to the next hole ? It's better to have a few large holes rather than many small ones. More efficient and holes should be 1/8" to the next hole.
Very nice tapers to that blade! . What steel is used ?

 

[weo]

How deep they are ? How much I need to grind to get rid of them ? How I would know that's enough ?

You'll see a difference in how the steel looks. Like your wikipedia article says, it's bluish-black, where as fresh ground steel is shiny-silver.

 

[J.Marc]

Mill scale isn’t too hard to grind through, as long as it is mill scale and not something else. Like everyone else has said, once you are able to see clean bright shiny steel with no dark spots or blemish,it’s safe to assume you got through it all. Just grind slowly until you reach that point. Don’t worry about numbers too much, I don’t know if you now this, but 0.0039” is a pretty small window.

 

[Natlek]

In the lower center hole , is that a crack to the next hole ? It's better to have a few large holes rather than many small ones. More efficient and holes should be 1/8" to the next hole.
Very nice tapers to that blade! . What steel is used ?

Thanks @mete It is just little deeper scratch from handling around in my shop ...I drill more and smaller hole because I m afraid of wrap/bend when quench ? I finish with grinding this blade maybe two months ago and it is time to HT it. It is 5mm 1.2519 steel .
Thanks for answering @weo
Thanks for help @J.Marc
I need to do little more sanding on right side , I still see some dark spot ...

 

[Wowbagger]

I see that mill scale on right side but have no idea how deep to grind ? 0.1mm is enough?

Sorry if I don't understand what you are after but chemically removing mill scale is the way to go.
>>>>>Link click on this.

 

[Stacy E. Apelt - Bladesmith]

Best way is to remove it before you grind. An overnight soak in sodium bisulfate or a 30 minute soak in Muriatic acid ( HCl) will remove it. Sodium BiSulfate ( Ph Down) is much less dangerous.

 

[Wowbagger]

Sodium BiSulfate ( Ph Down) is much less dangerous.

 

[Natlek]

Best way is to remove it before you grind. An overnight soak in sodium bisulfate or a 30 minute soak in Muriatic acid ( HCl) will remove it. Sodium BiSulfate ( Ph Down) is much less dangerous.

Ok , thanks ! If I try now , acid will eat good steel from finished side too , right ?

 

[Atlas Knife Company]

I was going to recommend vinegar, but that works too.

 

[Stacy E. Apelt - Bladesmith]

Ok , thanks ! If I try now , acid will eat good steel from finished side too , right ?

Not in a normal concentration unless you leave it way too long. The Sodium Bisulfate won't eat much steel at all unless you leabe it for days.

 

[joedhiggins]

Definately +1 for Stacy's comment. HCL is the thing. 30 min will do thick forge scale. I generally find I can deal with mil scale with a 5 min soak and a wire brush when I rinse off the acid. YMMV.

 

[john april]

if you leave things slightly thicker before heat treat, post heat treat sanding will surely get rid of it. my edges are usually about .025-.030" preheat treat, and around .010 after post heat treat sanding. that insures about .012 is removed from reach bevel side and the scale and decarb will be gone.

 

[Natlek]

Thanks guys Next time I will first surface grind that side .I like grinding like this , grinding only one side is much easier . HT went great , no wrap no bend ....hard as glass Now I take break from hand sanding and I m working on this beautiful piece of walnut for scale ......
And this one is deserved the name ..............SCAR He found a way to burn my finger pretty , pretty good .I keep my gloves new

I slightly moistened the wood..............