Application notes

AISI 446 is a ferritic, heat resistant, high-chromium stainless steel, characterised by: extremely good resistance to reducing sulphurous gases, very good resistance to oxidation in air, a good resistance to oil-ash corrosion, as well as to molten copper, lead and tin. Its composition complies with the following standards:

ASTM: 446-1
ASTM: S44600
EN: 1.4749
W Nr.: 1.4749
DIN: X 18 CrN 28
SS: 2322Sandvik 4C54

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More info on AISI 446;
Corrosion resistance
Precautions regarding AISI 446

AISI 446 is highly resistant to oxidation, both at constant and at cyclically varying temperatures. AISI 446 has a scaling temperature in air of about 1070°C. The service temperature in air should not exceed about 1100°C.

HOT CORROSION / SULPHIDATION

Owing to its high chromium content and the absence of nickel, AISI 446 has relatively good resistance to slags containing vanadium pentoxide and sodium sulphate. It has very good resistance in sulphidising gases / heavy oil combustion gases and salts. In sulphurous flue gases , especially reducing atmosphere (low oxygen pressure), it possesses considerably better resistance than the austenitic steels.

HYDROGEN

Dissolved hydrogen increases tendency of ferritic steels to cold shortness. Presence of sigma phase and carbide precipitations increases risk of hydrogen corrosion of the ferritic steel (see next slide).

NITROGEN

Nitrogen pick-up can occur in reducing gas mixtures with high concentrations of nitrogen, cracked ammonia or mixtures of nitrogen and hydrogen. Nitrogen pick-up leads to embrittlement and reduced oxidation resistance. AISI 446 steel is no more sensitive than austenitic steels in environments where nitrogen pick-up can occur.

CARBURISING ATHMOSPHERE

Carburisation can occur in contact with hot gases containing hydrocarbons and carbon monoxide. The extent of carburisation depends on the composition of the material and of the gas. The relatively high chromium content provides some protection against carburisation. However, carburisation occurs quickly due to ferritic structure, especially when protective layer of chromium oxide on the surface is destroyed.

METAL and SALT BATHS

The ferritic structure of 4C54 gives it good resistance in baths of molten copper. It also possesses good resistance in other molten metals, such as lead, tin, bearing metals, brass and magnesium. AISI 446 is not recommended for salt baths.

Precautions regarding AISI 446 sheath

Due to very good resistance of AISI 446 to oxidation and sulphur containing reducing atmospheres cable with this sheath could be recommended for application at the temperatures between 800°C and 1100°C in air, reducing sulphurous gases, oil-ash containing atmospheres, molten copper and tin.

Cable could also be used at the temperatures between 550°C and 800°C, however some precautions, connected to the nature of the AISI 446, need to be taken into account:

1. AISI 446 has low creep strength at the highest temperatures. Therefore allowance should be made in order to avoid distortion due to the mass of the cable.
   
   
2. As other ferritic chromium steels, AISI 446 is less tough than austenitic stainless steels. AISI 446 also has relatively high ductile-brittle transition temperature (DBTT) and after a period of operation at room temperature, toughness of the sheath can decrease further. Therefore, careful attention must be given to the way in which the cables are supported. Large impact stresses and the like should be avoided during repairs. Ductility of the sheath can be restores to its original condition by heating at a temperature above 100-150°C.
   
   
3. Temperatures of about 350-550°C should be avoided for even short periods of time, whether the steel is in service or merely being held at that temperature. Severe embrittlement, known as 475°C embrittlement, can take place in this temperature interval. It becomes noticeable after the cables have cooled to room temperature. Ductility can be restored by short term heating at a temperature above 600°C with rapid cooling in the air or quenching.
   
   
4. Formation of sigma phase as well as precipitation of carbides on the grain boundaries after prolonged exposure at temperatures 550-750°C can cause embrittlement in air, decrease local corrosion resistance and increase risk of hydrogen embrittlement of the steel. Steel can be restored to its original condition by heating at a temperature above 800°C. Increased temperatures intensify diffusion, although time need to be allowed in order to dissolve precipitations.