Page 196 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 181

Stainless steels. Because of its expense, stainless steel is rarely used
for structural applications. The unique characteristics of stainless
steel, however, make it ideally suited for applications where the
structural material is subjected to corrosive environments, high or
low service temperatures, and for applications where the material is
to be used in its uncoated state. Certain grades of stainless steel are
readily weldable, whereas others are welded with great difficulty.
When stainless steel is used as a structural material, particularly
when it is joined to carbon steel elements, it is important to recognize
the difference in thermal expansion between the two materials. With
stainless expansion rates being 1.5 times that of carbon steel, the dif-
ferential expansion can cause problems in structures that are subjected
to variations in temperature.
The American Welding Society is currently developing a welding
code to govern the fabrication of stainless-steel structures. It will be
known as AWS D1.6 and, although not complete at the time of the
writing of this chapter, it should be available in the near future. This
code will provide welding requirements similar to those contained in
AWS D1.1, but will deal specifically with stainless steel as the base
material.
Metallurgy of stainless steel. Stainless steels are iron-chromium
alloys, usually with a low carbon content, containing at least 11.5%
chromium, which is the level at which effective resistance to atmos-
pheric corrosion begins.
The American Iron and Steel Institute (AISI) classifies stainless
steels by their metallurgical structures. This system is useful because
the structure (austenitic, ferritic, or martensitic) indicates the general
range of mechanical and physical properties, formability, weldability,
and hardenability. The austenitic type generally have good weldability
characteristics, high ductility, low yield strength, and high ultimate
strength characteristics that make them suitable for forming and
deep drawing operations; they also have the highest corrosion resis-
tance of all the stainless steels. Austenitic grades account for the
highest tonnage of weldable stainless steels produced.
Ferritic stainless steels are characterized by high levels of chromium
and low carbon, plus additions of titanium and columbium. Since
little or no austenite is present, these grades do not transform to
martensite upon cooling, but remain ferritic throughout their nor-
mal operating temperature range. Principal applications of the
ferritic types are automotive and appliance trim, chemical process-
ing equipment, and products requiring resistance to corrosion and
scaling at elevated temperatures, rather than high strength. Ferritic
stainless steels are not easily welded in structural applications. If welding

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