Svetsaren 1/2007 - Esab

particular alloying elements on the pitting
corrosion resistance is taken into consideration.
There are different formulae but the one
commonly used to classify duplex steels is:
PRE = % Cr + 3.3 x %Mo + 16 x %N
Austenitic-ferritic steels, characterised by PRE 40,
are classified as duplex steels, whilst those steels
with a PRE value greater than 40, are designated
as super-duplex grades. Figure 1 shows the
simplified correlation between the pitting corrosion
resis-tance (critical pitting temperature, CPT) of
different grades of austenitic and ferritic-austentic
steels and a PRE formula with the factor 30 x %N.
In the meantime, a further class of duplex steel
grades became available – lean duplex steels.
They have a slightly lower content of chromium,
whilst nickel content has been considerably
decreased and partially replaced by manganese.
This type of duplex steel provides better corrosion
resistance and mechanical properties than a
standard 1.4301/304 - type stainless steel and its
cost is significantly lower than a standard duplex
steel due to its lower nickel content.
Ideally, the microstucture of a duplex steel
consists of approximately 50% austenite and
50% ferrite, Figure 2. Such conditions can be
obtained after annealing at temperatures of
1020°C and 1100°C for about 5 minutes and
subsequent quenching in water. In the Schaeffler
diagram, duplex steels are located in the middle
of the “austenite + delta ferrite” area. In Figure 3,
the Schaeffler diagram indicates the position of a
duplex steel grade 1.4462 (AISI 2205). For the
calculation of the corresponding Ni equ
value, the
nitrogen content has also been taken into
account (30 x %N 2
) /3/.
Initially, any duplex steel solidifies, from the liquid
state, into delta ferrite, which is then partially
transformed into austenite on further cooling. In
the state of equilibrium, the transformation
temperature is approximately at 1250°C. The
amount of austenite that will be present in the
microstructure at ambient temperature depends
on the content of alloying elements and cooling
conditions, ie, the cooling rate.
Figure 1. Comparison of CPTs of different materials in dependence of
the PRE [acc. to GRÄFEN/KURON]
Clearly, maintaining a well-balanced ferrite/
austenite ratio in the weld metal is very important
for the properties of a duplex steel weldment.
There are, however, different methods of determining
the ferrite content in a material. Originally,
parent metals and weldments were often
specified to have a certain percentage of ferrite.
The ferrite percentage is usually determined
using metallographic methods, which means
that a cross-section of the weld or the parent
material is prepared and then examined.
Examination of the prepared sample is by
manual or computerised planimetry, the result
being a direct ferrite percentage.
Using metallographic methods inevitably means
destroying the workpiece which, in many cases,
is undesirable. Non-destructive methods of
determining the ferrite content are usually based
on the ferromagnetic properties of ferrite. Early
techniques measured the amount of force to
remove a permanent magnet probe. Since the
correlation between the amount of ferrite and the
magnetic force is not exactly linear, the amount
of ferrite was indicated in FN (ferrite number).
For duplex steels, rough conversion from FN to
volume percentage is 70%. For example, 100FN
is approximately 70% ferrite /4/. Also, in most of
the diagrams used for ferrite prediction - such as
the DeLong- or WRC1992-diagram - ferrite
content is indicated in FN. Further information on
ferrite measurement and FN can be found in
ISO 8249 /5/. Today, most ferrite measuring
devices make use of electric fields rather than
Figure 2. Microstructure of a duplex
steel (1.4462), as delivered. Etching
Beraha II.
Austenite grains appear white, ferrite
grains appear dark. Magnification
750:1
Figure 3. SCHAEFFLER diagram. Indicated is the range
of chemical composition of a 1.4462 duplex steel.
The nitrogen content was taken into account by adding
the factor 30 x %N 2
when calculating the Ni eq
.
42 - Svetsaren no. 1 - 2007