©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
V 5.3.0.2
When certain alloy steels are cooled
rapidly from above the transformation tempera- ture,
a very hard brittle phase occurs. This phase is called martensite. Steels
that contain 5- 15%
chromium have this special characteristic. Unless special care is used in
welding such steels,
they become crack sensitive. These are the martensitic stainless steel alloys.
5.3.0.3 When
more than 16% chromium is added to the steel, the room temperature crystal-
line structure, ferrite, is stabilized
and the steel remains in the ferritic condition at all tempera-
tures. Hence the name, ferritic
stainless steel is applied to this alloy base. 5.4 AUSTENITIC
STAINLESS STEELS Austenitic
Stainless Steels are designated by a series of 300 numbers according to the Ameri-
can Iron & Steel Institute (AISI).
Nominal compositions of some of the more important types
are shown in Figure 7. About
80% of the stainless steel welded is of the austenitic type.
AISI No. Chromium
% Nickel %
Molybdenum % Columbium
% 301
17 7
302 18
9 304
19 10
309 23
13 310
25 20
316 17
12 2.5
317 19
13 3.5
347 18
11 1
MOST COMMON TYPES OF AUSTENITIC STAINLESS
STEELS FIGURE
7 5.4.1
Carbide
Precipitation - Many of the
austenitic stainless steels are subject to the phenomenon
of carbide precipitation. At elevated temperatures in the range of 800-1600°F,
the carbon content in excess of 0.02%
migrates to the grain boundaries of the austenitic structure
where it reacts with chromium to form chromium carbide. If the chromium
is tied up with
the carbon, it is not available for corrosion resistance. Thus, when the
steel with carbide precipitation
is exposed to a corrosive environment, intergranular corrosion results, allowing
the grain boundaries to be eaten away.
Figure 8 shows how intergranular corrosion may take
place in a tank holding a corrosive
liquid. Notice that the corrosion takes place only in the heat
affected zone on the inside where the
corrosive media is located, and there is no evidence of failure
on the outside. 5.4.1.1
Carbide precipitation has no other
effect on the steel, however, other than loss of corrosion
resistance in the heat affected zone. During welding, the heat-affected
zones along the
sides of the weld in austenitic stainless steel are exposed to the temperatures
that cause carbide
precipitation.
