Description. Alloy 20 is a nickel-iron-chromium based, austenitic alloy with excellent corrosion resistance in chemical environments containing sulfuric acid and many other aggressive media. This alloy is stabilized with niobium to resist intergranular corrosion.
Stainless Steel SA-351 CF8 is the cast equivalent of Type 304 Stainless Steel, part of a range of austenitic chromium-nickel alloys that provide good strength and ductility. They are also known as 18/8 stainless due to their chemical composition which includes approximately 18% chromium and 8% nickel by weight.
First, we'll look at the cast versions of 304 and 316 stainless steel, known as CF8 and CF8M respectively. The compositional makeup of these two alloys are similar: chromium and nickel contents. However, CF8M contains 2% to 3% molybdenum that CF8 does not have, so we might expect CF8M to cost more.
CF3M has similar corrosion resistance and mechanical properties to CF8M. The difference is that the tensile yield strength of CF3M is somewhat less than that of CF8M at ambient temperatures. CF3M alloy is limited to a maximum temperature of 800F.
WCB stands for “Wrought Carbon” with Grade B. Sometimes you can think “C” as casting. there are 3 kind of Grade A,B & C which define the ductility & tensile strength. Grade B is having very good ductility & tensile strength as compare to Grade A & C.
AISI grade 316 and ASTM grade CF8M are called austenitic Stainless steels, while grades 430 and CB-30 are called ferritic Stainless steels. Grade CF8M is slightly magnetic because, while predominantly austenite, it contains normally 5 – 20 percent ferrite.
Type 316 steel is an austenitic chromium-nickel stainless steel that contains between two and 3% molybdenum. The molybdenum content increases corrosion resistance, improves resistance to pitting in chloride ion solutions, and increases strength at high temperatures.
WCB stands for "Wrought Carbon" with Grade B. Sometimes you can think “C” as casting.
Four Different Types of Stainless Steel
- Ferritic Stainless Steel. Based on Chromium with small quantities of Carbon, ferritic stainless steel has a similar microstructure to both carbon and low alloy steels.
- Austenitic Stainless Steel.
- Martensitic Stainless Steel.
- Duplex Stainless Steel.
Type 304: The best known grade is Type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8%/10% nickel, respectively. Type 316: The second most common austenitic stainless steel is Type 316.
As with all corrosion-resistant alloys, duplex stainless steels can suffer corrosion when exposed to suitably corrosive conditions. As they contain a higher chromium content than standard 3xx grades, their Pitting Resistance Equivalent Number (PREN) is higher, indicating increased resistance to pitting corrosion.
They are used for their good mechanical properties in the as-cast (and therefore as-welded condition) and/or their excellent corrosion resistance properties (particularly to stress corrosion cracking).
SS (Stainless Steel) can be Austenitic SS, Ferritic SS or Martensitic SS. On the other hand, Duplex SS are Austenitic - ferritic SS. They are called “duplex” because they have a two-phase microstructure consisting grains of ferritic and austenitic stainless steel.
Duplex stainless steels are based around a composition of 22% chromium, whereas super duplex stainless steels are based around a composition of 25% chromium. By increasing the chromium content, the level of pitting corrosion resistance is also increased.
Martensitic stainless steels are used when corrosion resistance and/or oxidation resistance are required in combination with either high strength at low temperatures or creep resistance at elevated temperatures.
Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element.
Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels.
The most basic difference between grade 304 and grade 316 stainless steels is that 316 tends to have more nickel and a bit of molybdenum in the mix. Though the stainless steel 304 alloy has a higher melting point, grade 316 has a better resistance to chemicals and chlorides (like salt) than grade 304 stainless steel.
Duplex stainless steel. Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure.
In short, duplex' are welded with relatively high heat input and low interpass temperatures. This does not make the welding process more difficult. The reason these parameters are important is because duplex stainless rely on a nearly equal balance of ferrite and austenite .
Ferritic, martensitic and duplex stainless steels
The ferritic and martensitic stainless grades ('400' series) are generally free from austenite, have high permeabilities and so will attract a magnet. They are classed as ferromagnetic.Duplex stainless steels are typically magnetic because they contain a mixture of austenite and ferrite. The substantial amount of ferrite (which is magnetic) contributes to Duplex steels being magnetic.
They are all fully magnetic all the time. The best known duplex grade, 2205, resists corrosion even better than 316 because it contains 22% of chromium and 3% of molybdenum. Both are magnetic, because both are ferritic.
Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. Strength: Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.
Duplex Stainless Steel Piping. The manufacturing of chemicals, petrochemicals, and oil & gas products, exposes process equipment to highly corrosive fluids. Analyses of fluids, pressures, and manufacturing temperatures used in these processes have shown Duplex Stainless Steel Pipe to be very corrosion resistant.
Pitting resistance equivalent number (PREN) is a predictive measurement of a stainless steel's resistance to localized pitting corrosion based on its chemical composition. In general: the higher PREN-value, the more resistant is the stainless steel to localized pitting corrosion by chloride.
