Page 58 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.8 CHAPTER TWO

Uses of lead pipe include existing connections to floor-mounted water closets, radioactive
wastes, and special laboratory corrosive wastes. It has very limited use in modern drainage
systems.

STAINLESS STEEL (SS)

The term stainless steel encompasses a wide variety of alloys containing 11 to 30% chromium
(Cr), 0 to 35% nickel (Ni), and 0 to 6% molybdenum (Mo) in various combinations as well
as small amounts of other elements such as titanium, manganese, niobium, and nitrogen. It
is widely used in the chemical, pharmaceutical, and food processing industries. Corrugated
stainless steel tubing (CSST) is a flexible pipe used for fuel gas piping in smaller sizes.
Pipe is available in sizes ranging from 1/8 to 48 in, and is manufactured in plain end,
prepared end for welding, and flanged. Joints can be welded, threaded, or flanged. Wall
thickness is expressed as a “schedule,” and ranges from schedule 5 (lightest) to schedule
160. The wall thickness varies with the size of the pipe. The larger the schedule number, the
thicker the pipe wall for a specific pipe size. Stainless steel pipe is also available as tubing
with wall thickness designated as a decimal.
The composition of a stainless steel alloy determines its metallurgical structure or grade,
and therefore its properties. There are five groups of stainless steel based on metallurgical
structure: ferritic, austenitic, superaustenitic, martensitic, and duplex.
1. Ferritic stainless steels contain 12 to 30% (more typically 16 to 18%) Cr, 0 to 4% Ni, and
0 to 4% Mo with a low carbon content. This material is magnetic and has good ductility
and cold formability but is not hardenable by heat treatment. This class is generally less
vulnerable to chloride-induced stress corrosion cracking. Its primary use is in transport
of strong oxidizing fluids (such as nitric acid) in process environments, machinery, and
kitchen equipment. This class is exemplified by type ASTM grade 430.
2. Austenitic stainless steels contain 17 to 27% Cr, 8 to 35% Ni, and 0 to 6% Mo. This
material is typically nonmagnetic and readily weldable, and has good ductility (even at
cryogenic temperatures) and cold formability but is not hardenable by heat treatment.
This class is more generally corrosion resistant, but is generally vulnerable to chloride-
induced stress corrosion cracking. Regular carbon grades are susceptible to corrosion
around welded joints due to migration of Cr away from the weld site. These problems
are overcome by using a low carbon grade, indicated by an L suffix, that reduces carbon
to below 0.035 percent. Grades within this class are the most commonly used stainless
steels. This class is exemplified by type ASTM grades 304 and 316 (304L and 316L).
A superaustenitic grade is also available with superior resistance to chloride pitting.
3. Superaustenitic stainless steel alloys were created to better withstand corrosion in a
more severe environment than conventional stainless steel. They are alloys of Ni, Cr,
Mo, copper, and iron typified by UNS (Unified Numbering System) alloys N08020,
N08024, and N08026.
4. Martensitic stainless steels contain 11 to 18% Cr, 0 to 6% Ni, 0 to 2% Mo, and 0.1 to
1% C. This class is magnetic, oxidation resistant, and hardenable by heat treatment.
Little used in piping applications, its primary uses are in cutlery, turbine blades, and
high-temperature parts. This class is exemplified by type ASTM grade 410.
5. Duplex stainless steel is characterized by a microstructure containing both ferritic and
austenitic types with different grades, having a mixture of 40 to 60% of each for various
alloys. Its advantages are good resistance to chloride-induced stress corrosion cracking
and high-mechanical strength properties along with good ductility and impact strength.
Disadvantages include corrosion of pipe by reducing acids and weld site corrosion by
oxidizing acids.

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