Argon oxygen decarbonization (AOD) process development

Discovered in MIT Building 8 basement; Chipman's [Chapman's] student William Krivsky takes it to a small NY State steel company; AOD enables low-ppm carbon in stainless steel, drops stainless price by ~3X. Now used for essentially all stainless steel and aerospace alloys (with VOD for the latter).

One technology that was innovated — the seeds of it were right in the basement of Building 8 here. It's called argon oxygen decarburization, or AOD. The professor who worked on it was John Chipman [Chapman]. He had a student named William Krivsky. Krivsky got his PhD blowing argon bubbles through a bath of steel to look at the carbon monoxide reaction at different partial pressures of carbon monoxide. He went to a small steel company — not a big integrated steel company in the United States — and they tried to use this to decarburize high-quality steel. You can get the carbon down to extremely low values like 30 parts per million or less — you can get down to five parts per million with AOD. That's lower than you can get by just blowing air through. The equilibrium will allow you thermodynamically to get lower, and today it's the way all stainless steel in the world is made. Probably dropped the price of stainless steel by a factor of 3X and improved the quality by a tremendous amount. Here's an AOD vessel — you can get the scale of a person by the catwalk here.

"Christy" (Chipman doctoral student, 1950s) studied the carbon-monoxide reaction in steel melts using argon-diluted oxygen. The technique went from MIT thesis to Crucible Steel + Union Carbide commercial process. All 5M tons of stainless steel today made by AOD; price reduced to one-third of 1950s value.

Oxygen-blown steel — if you get down low enough in carbon, when you get down to less than a tenth of a percent carbon, the carbon will keep the oxygen low until you get very low carbon. But then if you get very low carbon, it gets really expensive to get the last part of carbon out. And for stainless steels we have to get the last part of carbon out. If you went down to where the glass slab is — the basement of Building 8 — that's the room where the original research was done by a guy named Christy in the 1950s, in his doctoral thesis with John Chipman, where he was studying just the carbon monoxide reaction. He had a melt of steel and they could bubble oxygen up through it to understand the carbon monoxide reaction, and measure the off-gases. He wanted to get to even lower pressures of oxygen, so he started bubbling argon with a little bit of oxygen — he wanted to have less than one atmosphere of oxygen, so he diluted with argon, and he was able to do his research from low carbon concentrations.

Born in basement of Building 8 from a John Chipman graduate student (Krivsky [?]) doing basic carbon-burnout chemistry; commercialized at a small steel company because the majors wouldn't risk it. Dropped stainless steel price by 5–10×, enabled <0.03% C for weldability and corrosion resistance, recovered chromium from slag.

There's a process called argon-oxygen decarburization, and it's the way we make all the stainless steel in the world today and most of the nickel-based superalloys for jet engines. Very high quality process. You think it looks like the BOF, the basic oxygen furnace, where you burn the carbon out of the steel. But in this case you take the low-carbon wrought-iron type of steel and you blow oxygen and argon through, such that the oxygen is at a lower partial pressure — you generate carbon monoxide at a lower pressure — and you can get your carbon down below 0.03 percent. You can get it down to 0.01 percent. That means you can make very good quality stainless steel.

AOD reduces carbon from 300 ppm to 30 ppm by argon blowing. Basic science done at MIT under John Chipman by Krivsky in the 1950s; process invented when Krivsky moved to a small steel company. 99% of stainless steel today made by AOD.

If you want to make a really clean steel for aerospace engines and nickel-based alloys, we do something called argon oxygen decarburization. You can only get the carbon down to about 300 parts per million in the BOF. If you want to get it down to 30 parts per million, you can blow some argon through. I can remember in the early 70s when I took thermo in this department, Tom King had us calculating the argon oxygen decarburization process. Why? Well, it had been invented right here at MIT, in the basement of Building 8.

Tom identifies AOD as an MIT-originated process (basic research in the basement of Building 8) that today accounts for ~98% of world stainless-steel production. By bubbling argon through the molten bath, the partial pressure of CO in the gas bubbles is suppressed by ~100×, allowing the equilibrium C·O activity product to drive carbon down below 300 ppm even at high chromium content. The economic consequence: cost of stainless steel roughly halved per ton since the 1960s.

Another thing is vacuum oxygen decarburization. This initially started — actually this was invented in the basement of Building 8 here, and about ninety-eight percent of all stainless steel in the world is made this way now. Stainless steel is an alloy of iron and chrome. A lot of it is iron, chrome, and nickel. But you want low carbon — ultra-low carbon. Ultra-low carbon means 300 parts per million or less.