Grinding steel, what does it mean if it turns blue?

[VinceFox]

Long story short, i'm detabbing a bike frame ready for spraying and when taking a bit of metal off on one of the rear tubes, the metal has gone blue. Have i ground too far through? Is it an indicator of the wall of the steel tubing ive ground into becoming too thin, or is it purely temperature ie holding the grinder on too long?

Bit out of my depth here, any genuine knowledge greatfully received.

[MarkP1973]

blue after it's cooled down? I thought that was just an indication of the temperature the metal had reached previously?

[rigga]

You got it hot.

[VinceFox]

Still a tiny trace of it. It's a tube that had another tube welded to it at an angle. I cut the other tube off and ground down the remains but i'm a bit concerned the blueing means ive thinned the tube wall too much.

[shirt]

it means the steel has become too hot when you've worked it, and has lost it's temper [heat treatment, not thrown the teddy's out]

http://en.wikipedia.org/wiki/Tempering_(metallurgy...

you've weakened the material in this area.

[VinceFox]

rigga said:

You got it hot.

Not necessarily an indicator of the tube thickness being compromised then?

Sorry to ask a stupid question, but it's a motorbike i'm about to have blasted and painted and i want to be sure it's right before i do.

[Dr Interceptor]

It means the temper has been drawn - have a google, lots of info out there.

[VinceFox]

For context, here's the frame shape...



Rear shot...



And a close up of the bit that discoloured...



It doesnt take any load directly, apart from my fat arse, the swingarm pivot is forward of it, and the shocks join right at the top end obviously. Is this a significant stress risk when riding?

[Magic919]

shirt said:

it means the steel has become too hot when you've worked it, and has lost it's temper [heat treatment, not thrown the teddy's out]

http://en.wikipedia.org/wiki/Tempering_(metallurgy...

you've weakened the material in this area.

Its temper, not it is temper. Teddies.

[McWigglebum4th]

VinceFox said:

Not necessarily an indicator of the tube thickness being compromised then?

Sorry to ask a stupid question, but it's a motorbike i'm about to have blasted and painted and i want to be sure it's right before i do.

Yes, No, maybe

If i take a foot square lump of inch thick steel and polish it and then heat it to the correct temperature (540C) in a heat treating oven it will go blue

If i take the same lump of steel and run a flame over it that is at 1000C for a minute it won't go blue as the mass of the steel will absorb the heat

Now if i take a inch square piece of 0.5mm sheet steel and polish it and stick it into the same oven then it will also go blue

Now if i run a flame over it then it will go blue as it won't have enough mass to absorb the heat


So if i was grinding something and it turned blue i would suspect that i have either been grinding way too hard or i have removed far too much material and it has got thin.

[mat205125]

It'll be fine, however use a wire wheel rather than a grinder for cleaning the rest of the paint off (unless it's going to be dipped). Cutting wheels followed by sanding wheels are a more effective and cleaner way of losing brackets than b@ll@cking them off with angle grinders.

[davidexige]

The colour change is due to the heat generated in the area you were grinding, you have actually tempered the steel in that area, the blue colour would indicate a local temperature of about 300 centigrade, the colours when steel is heated start at light straw at about 200C upto bright yellow at 1100C with a whole range of colours in between.

[McWigglebum4th]

I'd hunt down your nearest NDT firm and throw a wee bit of cash round the back and see if they can do a UT thickness measurement

[99t]

Stab the blue bit a few times with the tip of a fine screwdriver (good and hard).

If it barely marks the metal you're likely fine, if it goes through then you've seriously thinned it and probably need to replace the section anyway.

[VinceFox]

Thanks for all the advice cheps.

I just took a mini hand file to it and sanded it a bit, it's silver underneath. Was just a bit concerned as i have taken some meat off the wall.

The frame's being blasted before painting so all i need to do once i finish this stage is get a rough sand shape on the ground areas.

Thanks again all.

VinceFox, fking stuff up with powertools since 1972.

[MarkP1973]

more importantly, when it's rebuilt and back on the road, you're out riding it and nailing it down some quiet dual carriageway, will there be a niggle at the back of your mind asking if that section of frame is about to fail and what the consequences could be?

[VinceFox]

MarkP1973 said:

more importantly, when it's rebuilt and back on the road, you're out riding it and nailing it down some quiet dual carriageway, will there be a niggle at the back of your mind asking if that section of frame is about to fail and what the consequences could be?

There is now, you massive s**t!

[pad58]

V.P. is this the first time you've done something like this?
Strikes me that it is.
As a poster above has said if you can get it checked, do it.
Personaly I liked it the way it was....different.

[BliarOut]

VinceFox said:

MarkP1973 said:

more importantly, when it's rebuilt and back on the road, you're out riding it and nailing it down some quiet dual carriageway, will there be a niggle at the back of your mind asking if that section of frame is about to fail and what the consequences could be?

There is now, you massive s**t!

Get a pair of calipers and measure the thickness where you have been grinding so you can calculate how much wall thickness you have left. I'd be far more concerned about that personally.

[Dr Doofenshmirtz]

Use a 'flap disk' on an angle grinder for doing stuff like that. Don't use (as I suspect you have?) a solid grinding disk.