Lincoln Electric Welder - Manual (Page 4)

2
WELDING OF STAINLESS STEELS
1.0
INTRODUCTION
Stainless steels are defined as iron
base alloys which contain at least
10.5% chromium. The thin but
dense chromium oxide film which
forms on the surface of a stainless
steel provides corrosion resistance
and prevents further oxidation. There
are five types of stainless steels
depending on the other alloying
additions present, and they range
from fully austenitic to fully ferritic.
2.0
TYPES OF
STAINLESS
STEELS
Austenitic stainless steels include
the 200 and 300 series of which
type 304 is the most common. The
primary alloying additions are
chromium and nickel. Ferritic
stainless steels are non-hardenable
Fe-Cr alloys. Types 405, 409, 430,
422 and 446 are representative of
this group. Martensitic stainless
steels are similar in composition to
the ferritic group but contain higher
carbon and lower chromium to
permit hardening by heat treatment.
Types 403, 410, 416 and 420 are
representative of this group. Duplex
stainless steels are supplied with a
microstructure of approximately equal
amounts of ferrite and austenite.
They contain roughly 24% chromium
and 5% nickel. Their numbering
system is not included in the 200,
300 or 400 groups. Precipitation
hardening stainless steels contain
alloying additions such as aluminum
which allow them to be hardened by
a solution and aging heat treatment.
They are further classified into sub
groups as martensitic, semiaustenitic
and austenitic precipitation hardening
stainless steels. They are identified
as the 600-series of stainless steels
(e.g., 630, 631, 660).
The alloying elements which appear
in stainless steels are classed as
ferrite promoters and austenite
promoters and are listed below.
2.1
FERRITE PROMOTERS
Chromium – provides basic
corrosion resistance.
Molybdenum – provides high
temperature strength and increases
corrosion resistance.
Niobium (Columbium), Titanium –
strong carbide formers.
2.2
AUSTENITE
PROMOTERS
Nickel – provides high temperature
strength and ductility.
Carbon – carbide former,
strengthener.
Nitrogen – increases strength,
reduces toughness.
2.3
NEUTRAL EFFECT
• Regarding Austenite & Ferrite
Manganese – sulfide former
Silicon – wetting agent
Sulfur and Selenium – improve
machinability, cause hot
cracking in welds.
3.0
WELDABILITY
OF STAINLESS
STEELS
Most stainless steels are considered
to have good weldability and may be
welded by several welding processes
including the arc welding processes,
resistance welding, electron and
laser beam welding, friction welding
and brazing. For any of these
processes, joint surfaces and any
filler metal must be clean.
The coefficient of thermal expansion
for the austenitic types is 50%
greater than that of carbon steel and
this must be considered to minimize
distortion. The low thermal and
electrical conductivity of austenitic
stainless steel is generally helpful in
welding. Less welding heat is
required to make a weld because the
heat is not conducted away from a
joint as rapidly as in carbon steel. In
resistance welding, lower current can
be used because resistivity is higher.
Stainless steels which require special
welding procedures are discussed in
later sections.
3.1
FERRITIC
STAINLESS STEELS
The ferritic stainless steels contain
10.5 to 30% Cr, up to 0.20% C and
sometimes ferrite promoters Al, Nb
(Cb), Ti and Mo. They are ferritic at
all temperatures, do not transform to
austenite and therefore, are not
hardenable by heat treatment. This
group includes the more common
types 405, 409, 430, 442 and 446.
Table I lists the nominal composition