Bizarre glass bead blasting effect

[Twincam]

I've been glass bead blasting (#7 size) a whole lot of parts recently. Everything has gone very well.  Yesterday, I changed out my chamber with a load of new beads and picked up an AEG163 head and started blasting a combustion chamber, lightly.  Something seemed off.  I stopped and saw that the blasting had torn into the cast iron like it was made of styrofoam!  What the????  I double checked the bead load and verified it was what I thought it was. Then grabbed a piece of cold rolled 16GA steel used for welding practice and it blasted just fine.  Also took a junk 850 block and blasted some machined surfaces and it was fine. Both those items had the normal satin bead blast look without any damage.

I've heard the cast iron on the 163 heads was substandard but I couldn’t believe it would be this bad.  Anyone have a thought as to what happened here.  Could the head have degraded from past overheating?  I had soaked the head in a molasses/water solution for a couple weeks prior to this to remove some surface rust.  It did a decent job with no outward ill effect.

E8ED6984-0F15-4FD5-8105-B162AE3D2FE1.jpeg

[wantafaster1]

Yes soft material.

I've never blasted iron but I have done silver soldering on pipes for an aircraft application, I'm only allowed to use 20psi on it. If it's not a good brazed joint you can end up with something looking like an aero bar.

If you want to continue try the already used blast media, it gets blunt for want of a better description.

[Twincam]

Wow, extraordinarily soft it is.

I remember immediately before the forum “crash” (someone sent everyone a PM), there was a post from someone who asked what would cause his bead blasting setup to suddenly one morning produce a really coarse surface finish. I thought it was from someone at a US Mini parts suppliers workshop but I’m not sure.

I remember making a suggestion that maybe someone had used his equipment overnight, because in my experience, when beads get broken down, they break into small sharpe pieces that don’t have much mass, but being sharpe, they can give a slightly rougher finish sort of flat, not satin.

Unfortunately the Mk1 site went down on the same day and I believe that post wasn’t restored (I can’t find it). If anyone remembers that post, or knows if it still exists, I’d love to contact the poster to see if the part they were blasting was a 163 head!

[Dearg1275]

Could this be anything to do with hydrogen embrittlement? If you soak iron and steel in any form of acid it takes up hydrogen from the solution. This interferes with the metallic structure of the metal causing it to be brittle. If you acid soak ferrous materials to remove rust you should heat soak them to drive off the hydrogen to restore the integrity of the metal.

It’s not a good idea to acid soak safety critical components such as brake callipers and suspension parts.

D

[ricardo]

I have blasted (glass bead) AEG163 heads and that never happened to me.

I've seen in the past similar results of soaked parts in acids. Try to soak your 850 block for the same time the head was and then blast it to see what happens.

The head was modified, I get the pain

[Hipwell]

That looks like the person who blasted it got bored and gave the chambers a good grit blasting first. This happened to some aluminium sheet work I had blasted which was like this in some areas.

Edit: if you have done it yourself clearly not what I just put above! lol

Water damage then? Is the head sound.

[Twincam]

I will perform a controlled experiment using an 850 head instead of a block in case the casting process or material is different for heads vs blocks. Unfortunately there’s too many unknowns to cover all variables so this will be a pretty simplified test.

I’ll blast one chamber and then immerse It in the molasses solution for 3 weeks, then blast an adjacent chamber at the same angle and for the same time. Will post results.

[Twincam]

This isn’t very comforting.

https://metalfinishingsltd.co.uk/articl ... ittlement/


I’ll have to check the pH of the molasses solution.

[Dearg1275]

About 5.4 to 5.6. (50% solution by volume). Phosphoric acid is 1.6. (85% solution). It doesn’t seem very strong but it has plenty of time to do damage in 3 weeks.

D

[Twincam]

About 5.4 to 5.6. (50% solution by volume). Phosphoric acid is 1.6. (85% solution). It doesn’t seem very strong but it has plenty of time to do damage in 3 weeks.

D

The solution started as 1 part Molasses to 8 parts water, then further diluted to about 1:10 to replenish drag out.

[mk1]

That is most likely simply because the 163 was corroded & the blasting removed the rotted material. Doesn't look like "overblasting" to me. You often get this happen on a single chamber as when the engine is stored one cylinder will be open to the elements & the other 3 won't.

[Twincam]

Got a full range litmus paper selection and measured the pH. Best I can tell is it's at around 5 as D predicted, which doesn't sound good....

....and yes Mark, the head was pretty rust aged.

A good friend and I pulled an early Mk3 998, that had resided in the wet Pacific Northwest woods for over 30 years without a bonnet or intake manifold. It has a couple of combustion chambers that gave a new meaning to the word RUST from exactly the situation you mentioned. I'll use that head to see the effects of Molasses in a long term test.

As for the 163 head I'll assume the combination of the molasses soak, rusted combustion chamber and my lack of care has turned it into a big beautiful paper weight. Aargh!