Subject: Re[2]: Stainless steel forging

From: [email-masked]
Date: Wed, 31 Jan 1996 11:12 -0500 (EST)

Pete,
     
     That's an interesting contribution.  I always assumed that passivation 
     removed the iron from the surface layer of SS, leaving the Ni and Cr 
     behind.  Apparently there's more to it than that (tho maybe that is 
     going on as well).  
     
     For scientific instruments we passivate SS by soaking in 50% nitric 
     acid - an oxidizing acid.  Your explanation might account for the use 
     of nitric instead of another strong acid, like sulfuric.  
     (Hydrochloric=muriatic is a no-no!)  I'm interested whether there are 
     other passivation techniques in use.
     
     Bruce

______________________________ Reply Separator _________________________________
Subject: Re: Stainless steel forging
Author:  [email-masked] at GATEWAY
Date:    96/01/31 9:48 AM

On Wed, 31 Jan 1996, David Hoopes wrote:
     
> >Stainless steel that has been forged must be heat treated, passivated and 
> >polished to maintain its stain resistance and strength.  See Carpenter
> >Steel Handbook on stainless steels. 
> >
> What does passivated mean?
> ------------------------------------------------------------------------ 
> David Hoopes
> [email-masked]
> Lenexa, Kansas (Near Kansas City, Mo) 
>
>
David,
Passivation means there is something "blocking" further reaction i.e., a 
film coating on the metal that stops oxidation. "The corrosion-resisting 
property is due to a thin, adherent, stable chromium oxide or nickel 
oxide film that effectively protects the steel against many corroding 
media." (Avner,1974)  Stainless steel will corrode rather dramatically 
when in contact with chlorine.
     
FWIW
Pete Goering
Flamingo Forge
Polvadera NM 87828

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From: [email-masked]
Date: Wed, 31 Jan 1996 18:47:49 -0500

To All,
Stainless is stainless because of the formation of a thin layer of chromium
oxide on the surface. Passivation is the process by which this layer is
formed artifically. Typical passivation solutions include dilute nitric acid
and solutions of potassium dichromate and water. Passivation is necessary if
you remove the normal protective oxide or scale from the forging process by
chemical means, usually hydrocloric acid. HCl is a reducing acid whereas HNO3
is oxidizing. The oxide layer reformes very rapidly if the scale is removed
by shotblasting or grinding. If you etch off the scale with HCl, stainless
will rust with amazing speed.

Regards,

Bill Hochella

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From: [email-masked]
Date: Wed, 31 Jan 1996 18:58:42 -0500

To All,

Removing the nickel or iron from the surface of stainless steel has nothing
to do with passivation. It is the formation of the protective chromium (rich)
oxide layer. Also, if you slow cool unstabilized  versions of 300 series SS,
you will precipitate chromium carbide at the grain boundaries leaving
chromium poor zones. These zones will corrode very rapidly. You should
actually quench 300 series SS from an orange heat to prevent this. There is a
process for making SS powder by exploiting this characteristic. Stabilized
SS's are 316, 347, etc. Stabilized SS's have small amounts of titanium,
columbium or tantalum to soak up the carbon so the precipitation does not
occur. 300L series are low in carbon to take care of this too.

Regards,

Bill Hochella

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From: [email-masked] (Clay Spencer)
Date: Thu, 1 Feb 1996 22:58:19 -0600

At the Alabama Forge Council Blacksmithing Conference at Tannehill State
Park, September 6 -7, 1996, one of our demonstrators, Al Fouche', of
Jackson, GA is going to show us how he forges stainless forks or ladles.
Tannehill Home page is at
>                http://www.the-matrix.com/tannehill/park.html
Tannehill is SW of Birmingham, just off I 20, at Exit 100.
See you there.
Clay

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From: [email-masked] (James Batson)
Date: Fri, 2 Feb 1996 07:27:55 -0500

>What does passivated mean?

According to Webster: passivate- Metallurgy, to treat a metal so as to form
a protective coating on its surface and reduce its chemical activity.

When SS is heated and forged the chrome and nickle separate from the iron
leaving areas of iron which will show as rust spots on the surface.  To
break up these areas into smaller areas with less stain, one raises the
temperature to about 1900 degrees F. and quenches in water.  After this to
get the most stain resistance, one must polish the surface and passivate in
a solution of about 5% nitric acid for a period of time.  This is for
Ornamental Iron.  Some SS's are age hardening and others can be hardened
and tempered for cutting instuments.

Jim

Bladesmith & Bowie Historian

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From: [email-masked] (MR WILLIAM A HOCHELLA)
Date: Sat, 03 Feb 1996 17:07:53 EST

To All,
300 series stainless steel, for the most part, is a solid solution of 
nickel, iron and chromium. 400 series is just iron and chromium. No 
seperation occurs duing heat treatment or forging. You can deplete 
the chromium from the areas around the grain boundaries if you allow 
chromium carbide to form during slow cooling. The stabilized grades, 
316, 347 and the L series either have elements in them to tie up the 
carbon or have so little carbon that it does not make any diference. 
Passivation reestablishes the protective layer of chromium oxide. 
Letting a freshly ground surface in  air  at room temperarure will 
passivate the piece too. Passivation is really necessary when 
hydrocloric (muriatic) acid is used to descale the work.

Regards,

Bill Hochella