Stainless Castings
Stainless Castings
(OP)
I have just received a few sets of stainless castings that came from the shop with a blue 'stain' on the surface. It looks like an oil mark.
I have seen this on our stainless castings in the past and I am just wondering what causes this? Is it only present on stainless castings.
I have seen this on our stainless castings in the past and I am just wondering what causes this? Is it only present on stainless castings.





RE: Stainless Castings
Does the stain wipe/rinse off with acetone or toluene? Likely it's a marking ink or transfer ink.
If the stain is discoloration that's tightly adhered, surrounded by a zone of tan/straw color, then it's an oxide layer, i.e. the part was exposed to air at a fairly high temperature.
RE: Stainless Castings
Another potential root cause is the inmold atmosphere exposure - air entrapment, combined w. improper directional cooling - again something that is related to improper gating & risering procedure.
In reality this "blue " effect is a negative effect as far as the performance of the casting is concerned - unless you are machining that area of the surface off.
RE: Stainless Castings
These are sand castings and I have seen the blue stain all over the castings. These castings are not really machined but sometimes the foundry does grind down some rough spots.
We haven't noticed a negative effect on performance on grates with this marking. But, we have always just assumed this was a superficial effect.
I'll try to get a good pic of the staining.
RE: Stainless Castings
Like I said, this does not seem to affect performance but I am often asked why they look blue and I don't have an answer.
http://tinypic.com/view.php?pic=53pudxz
RE: Stainless Castings
What was your material?
We also had some 321 SS castings that would get a very light blue tint at different times while undergoing our thermal cleaning process.
RE: Stainless Castings
RE: Stainless Castings
We use a lot of SS precision parts in one of our processes and deliberately impart a Blue Oxide using steam air oxidation plus a smidgen of organics, polymer or oil.
RE: Stainless Castings
Depending on end use & customer, you may want to abrasive blast or pickle, then passivate.
RE: Stainless Castings
In the case of machined CF 8 or CF 8M castings you might note this tinting while thread cutting or if you perform a weld repair on the machined surface. This is unacceptable mostly in such applications. Try to buff grind the area locally with a cloth emery.
But in your application this will not pose any problem in service. On the lighter side you seem to have got rich by upgrading to HK from HH(or are you celebrating the drop in Ni price ( 14.2$/lb)
" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke
RE: Stainless Castings
So the heat tint is just leaving an oxide layer. Is this oxide layer made up of iron, nickel or chromium? or somethign else entirely?