Iron and iron alloy such as steel are poor materials as they
rust in air, corrode in acids and scale in furnace conditions. There is a group
of iron-base alloys, the iron-chromium-nickel alloys are called as stainless steels,
that do not rust in sea water, are resistant to concentrated acids and that do
not scale at temperatures up to 1100oC.
Good mechanical properties and manufacturing features give
the stainless steels their raison and make them vital tool for the designer.
The usage of stainless steel is nominal as compared to carbon steels that
attain a steady growth, unlike to the constructional steels.
Stainless steels as a family is certainly more non-uniform
than the constructional steels and their properties are in diverse cases
relatively unfamiliar to the designer. In some ways, the stainless steels are
an undetermined world however to take the benefit of these materials will need
a better understanding of their basic properties.
Use of stainless steel
Steel is the major industrial constructional material. Dominant
product form for stainless steels is cold rolled sheet. The applications are
dominated by major sectors: consumer products, equipment for oil and gas
plants, chemical process plants, food and beverage industry.
The commonly used stainless steel grades are SS 304 and SS
304l that form more than 50% of the global production of stainless steel. In
order to receive a perspective of the development of stainless steels, it is right
to consider the background.
Alloying elements have a specific influence on the
properties of the steel. It is the combined influence of the allying elements,
to some level, the contaminants that determine the feature of a specific steel
grade. The influence of alloying elements on the important material properties
is evaluated.
The most important feature of stainless steel is their
corrosion resistance. The reason for the good corrosion resistance of stainless
steels is that they develop a thin, invisible surface film in oxidizing
conditions.
Aqueous corrosion refers to corrosion in liquids or
moist conditions at temperatures up to 300oC, often in water-based conditions. The
corrosion process is electrochemical and needs the presence of an electrolyte
in the form of a liquid.
High temperature corrosion allows corrosion in hot
gases at temperatures up to 1300oC.
Stainless steels are significantly different from carbon
steels in some respects. There are also significant differences between the different
categories of stainless steels. Heat
conductivity for stainless steels is usually lower than for carbons steels and
reduces with increasing alloying level for every stainless steel category.
Austenitic steels are featured by supreme corrosion
resistance, good toughness and excellent weldability, they are the most common
stainless steels. The properties of Hastelloy wire grades include
resistance to general corrosion, pitting and crevice corrosion and stress
corrosion cracking. Low carbon grades attain supreme resistance to
intergranular corrosion and as a result higher alloyed steels are only
available with low carbon concentrations. Austenitic stainless steels are used
in all types of applications and industries. Common areas include piping
systems, heat exchangers, tanks and process vessels for food, chemical,
pharmaceutical, pulp and paper and other process plants.
