©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
V 5.4.2.2
The amount of ferrite in austenitic
stainless steel weld metal may be measured by magnetic
devices because the ferrite is magnetic. A small amount of ferrite in austenitic
stainless weld metal is good, because
it prevents weld cracking. If the weldment is to be in
very low temperature service, however,
large amounts of ferrite should be avoided because ferrite
is not tough at low temperatures. Also, if the weldment is to be used in
high temperature (higher
than 1000°F) service, the ferrite should be maintained at low levels because
the ferrite becomes
brittle at those temperatures. 5.5 CALCULATION
OF FERRITE CONTENT IN
STAINLESS STEEL
Several
simple, yet accurate, methods have been developed for determining the balance
between the austenite and ferrite forming
elements in iron. When the chemical composition of
the weld metal is known, the Schaeffler
or WRC-1992 diagrams can be used. See Figures 9 and
10. 5.5.0.1
The purpose of these diagrams is to
calculate the nickel and chromium equivalent of the
weld metal in question and plot the point on the appropriate diagram. The
nickel equiva- lent
is the sum of the nickel content and all other austenite formers, multiplied by
coefficients representing
their austenite forming effect as compared to that of nickel. The chromium
equivalent is calculated in the same
manner. In both diagrams, the nickel equivalent is the
vertical axis, and the chromium equivalent
is the horizontal axis. The WRC-1992 diagram has an
advantage since it also takes the nitrogen content into consideration. Nitrogen
is a power- ful
austenite forming element. If the nitrogen content is not known, we assume
0.06% for GTAW
and SMAW electrodes and, 0.08% for GMAW and FCAW filler metals.
5.5.0.2 When
chemical composition is not available, two common instruments can also be
used to determine ferrite content.
Since ferrite at room temperature is magnetic and austenite
is not, a relationship between magnetic
response and ferrite content can be established. The
more magnetic response to the instrument,
the more ferrite present in the metal. The two commercially
available instruments that use this principal to measure ferrite content are the
Magne gage and the Severn gage. The
Magne gage is a laboratory instrument, while the Severn
gage is a pocket-size instrument designed for on-site readings.
5.5.0.3 In
the past, ferrite was expressed as a volume percent of the metal. However,
because of non-standard calibration,
conflicting and inaccurate results often occurred. To
eliminate this problem, the ferrite
volume percent was changed to a standardized expression known
as the ferrite number (FN) and has been adopted by the Welding Research Council
(WRC), the American Welding Society
(AWS), and other agencies. Ferrite numbers (FN) are
