Chrome-moly is low alloy steel with chromium and molybdenum deliberately added. In flange work that means the ASTM A182 grades: F1, F2, F12, F11, F21, F22, F5, F9 and F91. Chromium runs from a fraction of a percent up to nine percent, molybdenum from about half a percent to one percent.
The single most useful thing to know is that neither element is there for rust resistance.
It is not stainless
Stainless steel resists corrosion because it carries enough chromium to form a passive chromium oxide film, thin, self-repairing and tenacious. The threshold for that behaviour sits at roughly 10.5 percent chromium.
Chrome-moly tops out at about nine percent, in F9 and F91. That is below the threshold. These grades do not passivate, so they corrode in wet air like any carbon steel. An F9 flange left uncoated through a wet winter looks exactly as bad as an A105 one, and needs the same coating, preservation and storage discipline.
The nine versus ten and a half percent gap is the whole story. It looks like a small difference in a chemistry table and it is the difference between a material that protects itself and one that does not.
It is not chrome plated
Chrome plating is a surface treatment: a layer of chromium deposited onto a different base metal, either decorative or as hard chrome for wear resistance. It is microns thick and it has nothing to do with the alloy content of the substrate.
Chrome-moly is alloyed all the way through. The chromium is in the steel, not on it. The two get confused on enquiries often enough that it is worth confirming which one a requisition means before quoting it.
So what is the chromium for
Three jobs, all of them at temperature.
Creep strength
Above roughly 800 degrees F, steel under sustained load deforms slowly and permanently, at stresses it would carry indefinitely when cold. Chromium and molybdenum in solution, plus stable carbides, slow that process considerably. Carbon steel sags where chrome-moly holds.
Resistance to hydrogen attack
At elevated temperature and pressure, atomic hydrogen gets into steel and reacts with carbides to form methane at grain boundaries. The methane cannot diffuse back out, so it builds pressure and fissures the metal from the inside. The damage is permanent.
Chromium forms carbides that are much more stable against that reaction. More chromium means a larger safe operating envelope, which is read off the API 941 Nelson curves. This is the main reason F11 and F22 dominate refinery hydroprocessing.
Resistance to sulfidation
Hot hydrocarbon streams containing sulfur convert steel surfaces to iron sulfide, and the wall thins steadily. Chromium-bearing scales are far more protective than iron sulfide, so chromium content directly slows the rate. That is why crude and vacuum unit service reaches for 5Cr and 9Cr.
What the molybdenum adds
Molybdenum is a strong solid solution strengthener and carbide former, and it contributes most of its value to high temperature strength and creep resistance. It also reduces susceptibility to temper embrittlement relative to chromium alone. Practically speaking, chromium and molybdenum work together, which is why the grades are named for both.
The fabrication consequence
Alloying for high temperature strength has a side effect: these steels are air hardening. They transform to hard brittle martensite as they cool from welding temperature in still air, with no quench needed.
That is why chrome-moly welding requires preheat, interpass temperature control, low hydrogen consumables and post weld heat treatment. It is not extra caution, it is what stops delayed hydrogen cracking in the heat affected zone. See preheat, interpass and PWHT for chrome-moly flanges.
Dimensionally, nothing changes
A chrome-moly flange has exactly the same dimensions as a carbon steel one. An F22 Class 300 weld neck and an A105 Class 300 weld neck are interchangeable mechanically, both taken from ASME B16.5.
What differs is the pressure-temperature rating as temperature rises, because the grades sit in different material groups in the rating tables, and the fabrication requirements. See weights and dimensions.
Where you will find it
- Refinery hydroprocessing, hydrotreaters and hydrocrackers
- Catalytic reforming
- Crude and vacuum unit hot sections
- Power boiler, main steam and hot reheat piping
- Fired heater outlet piping
- Any process where the metal spends decades hot
Grade by grade detail on chrome-moly grades.