Process Piping Materials – Module 2 – M06-036

Silicon (Si) is used for the deoxidizing process. Deoxidized steel is called “killed steel”.

Example

Another use of ASTM grade designators is found in pipe, tube, and forging products, where the first letter P refers to pipe, T refers to tube, TP may refer to tube or pipe, and F refers to forging.

−ASTM A 335/A335-03, Grade P22; Seamless Ferritic Alloy-Steel Pipe for High Temperature Service.

−ASTM A 213/A213M-03a, Grade T22; Seamless Ferritic and Austenitic Alloy Steel Boiler, Super-Heater and Heat-Exchanger Tubes.

−ASTM A 312/A312M-03, Grade TP304; Seamless and Welded Austenitic Stainless Steel Pipe.

−ASTM A 336/A336M-03a, Class F22-Steel Forgings, Alloy, for Pressure and High-Temperature Parts.

2.4.Carbon Steel (CS) Pipes

The most commonly used material for metal piping systems is carbon steel (CS).

Carbon is present in all steels and is the principle hardening element. It raises tensile strength, hardness, resistance to wear and abrasion. CS pipes are made of a variety of grades to meet various process requirements of the industry.

•Low Carbon Steels

−Carbon content less than 0.3%.

−Low strength and good formability.

−Good weldability and machinability.

−Not responsive to heat treatment; cold working needed to improve the strength.

−If wear is potential issue, can be carburized (diffusion hardening).

•Med Carbon Steels

−Carbon content in the range of 0.3 – 0.6%.

−Can be heat treated - austenitizing, quenching and tempering.

14

Process Piping Materials – Module 2 – M06-036

−Medium carbon steels have low hardenability.

−Addition of Cr, Ni, Mo improves the heat treating capacity ¬ Heat treated alloys are stronger but have lower ductility

−Have moderate to high strength with fairly good ductility.

•High Carbon Steels

−Carbon content 0.6 – 1.4%.

−High C content provides high hardness and strength.

−Hardest and least ductile.

−Used in hardened and tempered condition.

−Strong carbide formers like Cr, V, W are added as alloying elements to form carbides of these metals.

−Used in applications where surface is subject to abrasion.

−High percentage of carbon however lowers ductility, toughness and machinability.

Carbon Equivalent

•Material containing carbon more than 0.35 becomes brittle.

•Material containing carbon more than 0.43 are NOT weldable

The Carbon Equivalent (CE) for carbon steel intended for welding SHALL NOT exceed 0.43% based on the long formula:

CE = C + Mn/6 + (Cr + Mo + V)/5 +(Cu + Ni)/15

Characteristics

Carbon Steel losses all its stress resistance at 650°F/345°C. It is extremely difficult to select material at 1200°F/650°C satisfying the needs of pressure and corrosive properties of the fluid in the line.

•Tensile strength reduces at higher temperatures.

•Tensile strength of 60,000 psi reduces at 800°F/425°C to 10,800 psi at 900°F/480°C to 6,500psi and at 1000°F/540°C to 2,500 psi.

•At low temperature (< -18°F/-28°C), the material becomes brittle.

− Low Carbon steel (LTCS) is suited for a low temperature of -46°C.

15

Process Piping Materials – Module 2 – M06-036

•Above 800°F/425°C, the strength decreases, pitting increases and creep decreases.

−Alloy steels are recommended for temperatures > 800°F/425°C.

−Poor against most acids, alkalis and salts.

Applications

•Non-corrosive piping system (fresh water, plant air, nitrogen, fuel gas, etc)

•Temperature ranges from -18°F/-28°C to around 800°F/425°C.

Standards

The most commonly used grades for piping are ASTM A106 and A53. A106 is preferred for high temperature and high pressure services.

16