Material science mystery

[Dominatus]

Last night, I rushed downstairs as my step mom freaked out. Turns out the stainless steel grill was on fire. And the funny thing was she wasn't using it to cook dinner. We had used the grill the night prior, though, seemed as if we may have forgotten to turn the gas off. At least that was our initial thought on the subject. After grabbing the extinguisher and getting close enough to put out the flames, there was an explosion, focused on the right side of the grill, where the various heat setting knobs were. The right knob was completely ejected by the explosion, and some molten metal (I presume steel since it is a stainless steel grill, but I don't really know...) was pooled below, having pooled in a two-inch diameter puddle with another inch of slag-like metal sticking out of it. The next knob (four in total; directly left of the ejected one) was partially dislodged, but still hanging on. The electronic starter, broken for years, was also almost completely ejected from the frame, the.... Plastic? Partially broken. The left two knobs were relatively okay, but had scorch marks coming from the tops of the hole they are set into.

The weirdest part was the propane. See, the knob that controls the gas release was in the off position. Yet I could hear the propane leaking from somewhere, promoting me to quickly close the valve manually.

The metal frame showed a weird bubbly effect which I'm going to infer was oxidation caused by the heat of the grill. The frame was also warped on the flat upper surface directly in front of the lid. It was dark, so I had difficulty seeing and confirming this next part, but it appeared as though severe damage was done to the metal tubing stuff underneath the grill.

My question for all of you is what is likely to have caused this accident, how long the grill would have had to be on, and what the melting and warping points of stainless steel are. I also want to know why the explosive force was triggered as soon as we used a fire extinguisher. The extinguisher sprayed a white, sandy/dusty powder, if that helps. If pictures help, I brought some of the exploded/ melted metal in for my material science teacher to look at. I can try and upload them from my phone.

I don't know much about grilling, but we had steak the previous night, meaning the right hand side of the grill was the one my dad used. So maybe that should give an idea of the heat settings. He also turned it off, as the gas release knob showed.

Thanks, all!

Edited October 3, 2014 by Dominatus

[ibeinsane]

From the description it is a little difficult to tell what happened; The best guess from in the information is as below, probably depending on how confident you are that it was turned off.

Option 1) Valve left turned on and the flame burned for a very long time. Causing heat deformation of sheet metalwork but more importantly caused melting of joints and/or the valve body.

Option 2) Leak from valve due to dirt or fault. This left a small flame burning for a long time which allowed localised braised/soldered joints of the (presumably brass) valve body to melt and cause further eventual ignition.

The explosion was probably due to rapid pressure changes and thermal shock of an already unstable situation. My guess is that the valve body had melted and the 2" pool is probably the valve and/or the switch mechanism which blew out as a result of extinguishing the flame and everything cooling down. It is also possible it may have been due to a blow back through the pipes as the flame was extinguished.

Melting point for SST varies but is generally over 1500 degrees C (2700F) which is beyond what I would expect from a propane leak (you would have to burn it efficiently in a torch to get beyond 1500 degrees C.

Warping can occur at much lower temperatures and is more dependant on the induced manufacturing stresses so could be as low as 200-300 degrees C. (I have seen cheaper steel trays warp in a standard domestic oven)

I suspect the melted material is likely to be a brass alloy which will melt at under 1000 degrees C.

Not sure how long the grill would have to be on for to cause this but I suspect an extremely long time as the temperature differences are quite small so the extent of melting and deformation you have seen would take some time.

[Dominatus]

Thank you. The melted material, my teacher confirmed, is likely some sort if alloy. The brass components (or what I would assume is brass based on it's color and the cheap function it was meant to serve) was intact with some oxidation. The melted alloy is silver in color, very shiny and the melted pieces are malleable, which may be a result of the annealing it underwent.

[ibeinsane]

If it is silver in colour and very malleable it is likely the solder material that was used although I would not have expected a huge amount of this material to be present. This is usually an indium/tin (used to be lead) based alloy with melting points between 180-300 degrees Celsius depending on the version. I believe in the USA you call it sweating rather than soldering for pipes and is just the same for water as it is for propane.

It is possible that it was braised but the materials are similar but a higher melting point.

How much of the molten material do you think you had? If the heat was on for long enough it is possible you got the solder from several connections.

[Dominatus]

I would estimate a small amount not from soldering. I will upload a picture of the melted and fragmented pieces of what I beleive is the same alloy.

[Dominatus]

Whatever this brass-like piece is made from, I would guess that the curved shape of the fragments, which represent the material (less malleable than I had thought, possibly due to heating process, as I could bend it slightly but bending it back caused it to break with little applied force) being wrapped around the brass rod. The melted pieces appear to be the same composition as the fragmented ones.

It would appear I am not allowed to attach the image. I will see about posting it online somewhere. Any site suggestions that don't require sign ups? Conversely you could send me a private message if you use the kik app, which would allow me to send you the image.

[Nuke]

my grill's regulator likes to freeze up sometimes. when this happens the valve never completely closes, and gas trickles out. for storage i usually have to unscrew the regulator from the tank and put a cap on the tank to prevent loosing all my gas and filling the shed up with a cloud of explosive death. once in a blue moon it freezes in the fully open position and will not shut down. one day i got the bright idea to dump hot water on the regulator to clear the block. somehow it made things worse, the gas started flowing at an alarming rate and the grill became a fireball (incidentally this is a good way to cook a frozen steak, assuming you want it rare in the middle). i presume i expanded the casing while the moving parts inside were still being cooled allowing more gas to flow than usual. anyway after about 30 seconds of inferno the valve snapped shut. ever since then i make damn sure that no gas is flowing before i walk away from the grill.

you probibly had a stuck valve much like mine. if your fire extinguisher was a co2, the explosion might have been cause by rapid cooling of hot metal. you may have ruptured the gas lines in the control panel. releasing more gas into the inferno. regulator freezing usually indicates overfilled tanks, because the gas doesn't have room in which to vaporize prior to reaching the regulator.

Edited October 3, 2014 by Nuke

[Dominatus]

Well, nuke... That steak... I suppose you could say you NUKED it!

But in all seriousness, we are lucky the house didn't go up. What with cedar siding... Or some sort of wood shingle siding, I don't know...

[Dominatus]

I got home, and decided to perform a little test. It took 1 minute 4 seconds for a super thin piece of this alloy to start bubbling when heated with a common household lighter. However, after quenching it subsequent attempts produced no results similar.

[Nuke]

Well, nuke... That steak... I suppose you could say you NUKED it!

But in all seriousness, we are lucky the house didn't go up. What with cedar siding... Or some sort of wood shingle siding, I don't know...

one does not simply nuke meat.

[EdFred]

Without pictures, useless this thread is.

[ibeinsane]

At this stage I would say the failure was a part which melted due to a leak or the heat being left on (most likely a leak)

To find the source I would suggest looking at the grill rather than the metal. To identify the metal becomes rather a bit more difficult, from the description though it does sound like a brass alloy.

Bubbling on the surface when heated does not mean a great deal, usually caused by impurities or oxide on or near the surface so subsequent tests may not result in the same.

Indeed very dangerous and by the sounds of it you were very lucky your mother found it when she did or it could have been a disaster.