I had to cut a badly rusted steel end sill into three pieces today to put it in the scrap dumpster. I had a hell of a time cutting it. The torch would blow through some places, but mostly, squeezing the oxygen lever actually made the cut less effective and look less hot. It wasn't my torch settings, because cutting a piece of pipe with the same settings went perfectly normally.

The only thing I can figure is that the metal was so rusty, it was already saturated with oxygen. I got the cuts made by using just the acetalyne to melt through. The metal acted more like slag than metal doing this, sort of bubbling up when it got bright orange, then just dropping away exactly like slag from a normal cut. Any idea what was going on?

What size tip? Did you chip off loose material first? Heating flakes of rust and paint will not help you cut steel. Doesn't sound like you have a future as a burner in a scrap yard.

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For cutting scrap, I always had a much easier time with a scrapper's torch.

Sounds like you were trying to cut mostly rust without much actual steel left. When cutting steel with a torch, you’re actually melting it with the acetylene and blowing the melt out with the oxygen, thus rapidly oxidizing the melt. Rust is already oxidized and doesn’t really melt with acetylene and it deflects your flame so the oxygen is going to blow hot rust crumbs around at best. When you do hit unoxidized steel, it’s likely to blow back at you since you don’t have a hot enough edge that’s melting. If you knock the heavy rust off first, it’s easier to torch but still can be a pain. I might not have the above details exactly right but that’s the gist. It might have been better to use a cutoff disc and saved yourself some splatter. I learned about that the hard way.

Jayrod beat me to it.

Oxygen cutting is an oxidation reaction, like the combustion of flammable gas but with metal as 'fuel'. Once the iron in a piece of steel has oxidized, why would you expect it to react further with oxygen to produce enough exotherm to keep the reaction going?

T'would be as silly as the Meyerites thinking you can catalytically combust water to use as a fuel...

Depending on the situation, sometimes you just drew a real shit job. Agree sometimes prepping the cut area with an air hammer/needle scaler will help. I have found smoke boxes and any steel from an old refer where they used salt or truck scales where lots of corrosion can be, some of the most frustrating areas to cut. Bottom of a rotted out tender tank another. About all you can do is hammer anything loose you can. But again, some cutting jobs are just plain rotten. About the only thing you can do is decide if what you are doing, where your cutting is worth the effort. Maybe a bigger patch would be better where you can reach better steel? All depends on exactly what your doing. Cut once and put in scrap bucket? To me one of the worse jobs is cutting saddle bolts out of a smoke box. Ideally one might drill a hole to start the cut so the slag can exit downward. Cut the head end off as well as bottom off so you only have a couple inches to cut through. Using a torch as a fire drill is about as much fun as cutting flaking cruddy steel. The job gets worse if your working in an awkward position so you ad physical discomfort to an already difficult assignment. Grinning through a chunk of hot slag that went into your boot is a dream. Short of dunking your foot in a tub of water you can seldom get your boot off fast enough to save the day. Oh I miss some of this like I miss the plague. Regards, John.

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This is misleading. The flame preheats the metal, and the oxygen jet 'burns' the steel. In ideal conditions you can cut the fuel gas off entirely (scrap yards use propane around here) and you can continue to cut the steel with only heat being the burning steel. That heat from the steel oxidizing is pretty critical because that is a lot more heat then the preheat flame creates.

Crappy metal makes a crappy cut, but also the tip can get dirty or damaged pretty quick and make things even harder yet. When that 'jet' of oxygen turns into a spray the cut goes to hell.

I hacked up the other sill today. The second one wasn't quite as bad as the first, but it still had lots of loose crap inside that turned into white foam when the flame hit it. And of course the O2 ran out in the middle of it. Ain't nothin' easy!

Both end sills on the car were replaced a couple months ago (not like they needed replacing or anything!), we're just now getting rid of the old ones. Wet toilet paper has more structural integrity than some of the steel in these sills! I was cutting them into more manageable pieces to get into the scrap dumpster.

Thanks for the replies. I suspected the problem was the amount of rust, y'all have confirmed that suspicion!