An example of metal spalling

Hi All ;  Wanted to show an example of how only medium high heat can spall metal.  The "peter " channel for my J tube is on its 3rd season.  This was brand new 3/16 carbon steel.  By the end of last season it was looking warped... A few months into this season and it looks BAD.  Temperature on the plate just as it end is rite around 1000 F + , temp in the burning bottom of firebox was over 1500 F . Inside the burn tunnel and coming up the riser, I can easily see how the temps are getting in the 1800 F + range .  If 1000F did this to my air channel what would 1800 F + due to a metal core or riser ...  well my guess is the same thing it did to my plate steel only twice as fast...  Yes metal will hold up for a season .... maybe even two, but ultimately it will fail. Build your core out of fire bricks or clay/perlite but do not build it out of steel or it will not last.  The last photo's show just wood just sitting in the hot J tube and hitting 456 F and spontaneously igniting !    

i made a video to show spalling steel

This is what you get when the temperature of the steel is getting high enough in an oxygen-rich environment. Anything above 1500 ºF and the steel is corroding away in a very spectacular way. As such, the damage is done waaay lower than melting temperature. What you see is a channel that's being blocked on purpose or accidentally during quite a lot of burns.

Since it is a p-channel shown in the video I feel I need to comment on this.
It's all about temperature, so when one would be able to keep the temperature of the steel part low enough it wouldn't spall away. That's the idea of the p-channel and floor channel arrangements. By streaming colder air through such a duct it is possible to keep the temp down. An overhead p-channel, like this one probably, does need to have an overhang over the top of the port in order to suck air in with a sufficient velocity. The lowest pressure and thereby the greatest suction is just behind the narrowest point in the venturi. In this case right at the p-channel opening which is helping immensily, making a difference like between day and night.

The floor channel is different in this respect: the horizontal part is sporting a csa that's about 1.5 to 2 times larger as the csa of the vertical part. Which means the vertical part is a temporarily restriction, resulting in a higher velocity according to Benoulli's law, keeping the most vulnerable part cooler. Not cool enough to ensure the steel would last forever, by the way. My heater is consuming about 1.6 cords of soft wood, scraps and so on, per season is going into it's fourth season now while the floor channel is still in one piece but visibly battered.

But then, car tires won't last forever for example and almost everybody thinks it's a fact of life. So a secondary air channel out of normal steel is actually a replacable part. An acceptable lifespan is required but other than that there's nothing wrong with the philosophy.

Judging by the video, you've lost a lot of weight Paul!