Hardened?

[steve t]

On two recent jobs I had to straighten some twenty mm round after dipping and in both cases a hit with
the hammer resulted in the parts snapping, on inspection a really tight grain could be seen on broken surface, looked like carbon steel that had been hardened but not tempered, would this be caused by high carbon steel being in the steel somehow? or is there another reason, thanks

 

[ronan]

What sort of dipping steve ? Galvanising ?

 

[TechnicAl]

I know its obvious but its caused by brittleness

What caused it to be brittle depends on the steel grade

 

[steve t]

What sort of dipping steve ? Galvanising ?

sorry Ronan, yes galvanising

 

[ronan]

I have never heard of the usual old mild steel getting brittle after hot dip galvanising. Thats not to say it cannot happen. Can you tell us a bit more about the metal you used ? Was it mild steel or medium/high carbon steel used by mistake maybe ?

 

[steve t]

yes it was an ordinary length of 20mm round mild steel, bent cold in press and no issues, just snapped clean when hit with hammer after galvanising, a strange one

 

[Screwdriver]

Sounds like you might have hardened it during the process. What temperature is the tank and how did you cool the parts?

 

[Kent]

bend 20 mm ms cold! then whack it back again cold? frankly that is to be expected nothing to do with galv process

 

[steve t]

does manipulating steel cold make it brittle then Kent, I never new this, how does it alter the grain structure? I bend up to an inch round on press no previous issues

 

[Chris Stevens]

Maybe :https://en.wikipedia.org/wiki/Hydrogen_embrittlement

 

[Bill Edwards]

The thicker the metal the less well it will cold bend.
As it bends it's compressing on the inner part of the bend and stretching on the outer. As with everything there is a limit to how much something will give before it breaks and the thicker the metal the greater the stretching distance is on the outer.

On something 20mm bent cold I wouldn't be very surprised to see cracks. It's out of what I know, but I think galvanising can introduce hydrogen and if so would have made the metal more prone to cracking still. Putting more forces onto it (i.e hitting it) will have been enough to make it give altogether.

 

[TechnicAl]

20mm mild steel should bend to quite a tight angle

But as Kent pointed out the OP has then hit it with a hammer so Im not surprised it broke

 

[Kent]

does manipulating steel cold make it brittle then Kent, I never new this, how does it alter the grain structure? I bend up to an inch round on press no previous issues

Depends but heating allows a far less stressful bend
I think we have all bent items back and forwards to fracture them. What your doing is fracturing the grain
When you heat the molecules if that's the right word move about freely. This allows us to bend rsj etc under nothing more than its own weight and a hinge point. Planing the layout of heat for cooling speed

 

[Kent]

The thicker the metal the less well it will cold bend.
As it bends it's compressing on the inner part of the bend and stretching on the outer. As with everything there is a limit to how much something will give before it breaks and the thicker the metal the greater the stretching distance is on the outer.

On something 20mm bent cold I wouldn't be very surprised to see cracks. It's out of what I know, but I think galvanising can introduce hydrogen and if so would have made the metal more prone to cracking still. Putting more forces onto it (i.e hitting it) will have been enough to make it give altogether.

It can introduce hydrogen? I never knew that or if I did I forgot it a long time back

 

[TechnicAl]

It can introduce hydrogen? I never knew that or if I did I forgot it a long time back

Its the pickling / cleaning before they dip it that could cause Hydrogen.........Sulphuric Acid or any Acid contains Hydrogen

 

[Kent]

Its the pickling / cleaning before they dip it that could cause Hydrogen.........Sulphuric Acid or any Acid contains Hydrogen

Could this in practice actually weaken something of any note or is it just academic? In the real world

 

[TechnicAl]

Could this in practice actually weaken something of any note or is it just academic? In the real world

well if left in long enough it can cause cracking............if all the conditions are right.....which is rare

then the hot dip is 300 oC or so which would be enough (given time) to remove the hydrogen

Ive never seen it happen but Im not an experienced galvaniser

 

[Kent]

well if left in long enough it can cause cracking............if all the conditions are right.....which is rare

then the hot dip is 300 oC or so which would be enough (given time) to remove the hydrogen

Ive never seen it happen but Im not an experienced galvaniser

Me neither if there is such a thing in galvanised
Just asking because I have never seen any mention of it in structural engineer notes
100 percent sure we're the problem lies in this one
How the heck do you even go about bending such stuff cold without busting a gut I wonder anyhow

 

[Bill Edwards]

Press I would think.

 

[steve t]

Thanks for replies, yes bent on press, no gut busting involved