Stainless Castings

[macmet]

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.

 

[btrueblood]

Raw castings or machined? Cast by what process, sand, investment, ...? Can you post a photo?

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.

 

[eminentfoundrypro]

I assume that it is not located at the connecting point of the gating and/or risers - if so and in short this is caused by improper gating & risering removal - over entuziastic use of the grinder.
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.

 

[macmet]

Well thanks for the input. I never thought to actually try to see if I can get it off so I will give that a shot.

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.

 

[macmet]

Alright, below is a cropped picture of one of our castings. This material is ASTM A297HK but we typically use HH. Normally when I see this type of stain it is more blue.

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

 

[unclesyd]

Some years ago pump casings received from one vendor always had a blue tint on the inside surfaces. The vendor said that it came from an additive in the sand mix. They buffed the outside and left the inside alone.

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.

 

[macmet]

This material is ASTM A297HK but we typically use HH. Normally when I see this type of stain it is more blue.

 

[unclesyd]

It appears to be a pure oxide stain. If aesthetics want be problem I would leave it alone as it will be tough to remove unless you can blast it.

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.

 

[kenvlach]

The blue-maroon looks like heat tint; the brown looks like rust. Rub a white cotton clothe across; rust will stain.

Depending on end use & customer, you may want to abrasive blast or pickle, then passivate.

 

[arunmrao]

Macmet, I agree with the earlier posts that it is a problem of heat tinting.While removing the gates and risers there is a great heat input,so also during grinding if excessive effforts are put locally this tint arises.

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

 

[macmet]

Ha, we only have HK for this grate (I think we had two at the time) b/c of a mix up at the foundry, or they had some extra melt before pouring ours... or something along those lines.

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?