AFRICAN FUSION—MARCH2014
4
SASSDA's duplex stainless steel seminar
S
ASSDA, in its role as a promoter
of stainless steel industries in
SouthernAfrica,hosted thisseries
of duplex stainless steel seminars to
showcase the expertise and capability
of the South African manufacturing
industry and to support the increased
adoptionandgrowthof duplexstainless
steelswithin local and regionalmarkets.
Opening the seminar, SASSDA’s
John Tarboton, a metallurgist who
has spent his entire working life in the
stainless steel industry, presented an
The Southern Africa Stainless Steel Development Association (SASSDA) recently hosted a
series of duplex stainless steel seminars in Cape Town, Durban and Johannesburg.
African
Fusion
presents a summary the opening sessions.
Duplex stainless steel andwelding
overview paper entitled: ‘
The history,
development and properties of duplex
stainless steels
’.
Showing the typical microstructure
of duplex stainless steel, Tarboton says
that the material has a two-phase
microstructure consisting of ferrite and
austenite in roughly equal proportions.
The chemical composition has to be
carefully balanced to achieve this, and
this is done by looking at the austenite
formers in thealloy composition– such
as nickel (Ni), carbon (C), nitrogen
(N) and copper (Cu) – and carefully
balancing these with ferrite formers:
chromium (Cr),molybdenum (Mo) and
niobium (Nb). “As we increase the Cr
andMo to improvecorrosion resistance,
weneed to increase the compositionof
austenite formers, suchasNi andN, to
maintain the50% ferrite50%austenite
balance.” Tarboton explains.
Using the Shaeffler diagram, he
demonstrates that with high levels of
Cr and low levels of Ni, a ferritic stain-
less steel will form, while increasing
the composition of Ni will take a stain-
less steel into the austenitic range to
produce 304 and316 stainless steels.
“There is a narrow band between the
austenitic and ferritic regions of this
diagram that representsduplexstainless
steel varieties,” he points out.
Onanewconstitutiondiagramdevel-
opedby theUS-basedWeldingResearch
Council (WRC) toassist inpredicting the
ferrite content of duplex stainless steel
weldmetals from the chemical compo-
sition of a stainless steel alloy, Tarboton
outlines the different duplex stainless
steel material grades, starting from the
lean2001,2101,2202,2304andLDX
2404 grades. In the centre of theWRC
diagramare the standard grades – such
as2205– followedby the super-duplex
grades –2507– and further to the right,
with high levels of both austenite and
ferrite formers are hyper-duplex grades,
such as 2707.
The first duplex stainless was cast
inSweden in1929andcommercialised
the followingyear, toeventuallybecome
type329. “The first patentwasgranted
in France in 1935 and this became
known as the Uranus 50 duplex stain-
lesssteel.Thiswas followedbyaduplex
developed to resist stress corrosion
cracking (SCC), 3RE60 developed in
Sweden in the 1950s.
But the developments that enabled
duplex tobecomemorewidelyadopted
took place in the 1970s. “The dif-
ficulty with first generation duplexes
waswelding, because theheat affected
zone (HAZ) became toobrittle. Thekey
breakthrough came with the introduc-
tion of nitrogen as an alloying element,
which resulted in better toughness of
duplex welds,” says Tarboton.
It was in the 1970s that the com-
monlyused2205gradewasdeveloped,
and with the increased levels of N ad-
ditions, pitting corrosion resistance, as
measured by PRE (pitting resistance
equivalence) numbers steadily im-
proved. Super duplexes, such as 2507
with PREs of between 40 and 45,
emerged in the 1980s, as did the first
lean duplex, 2304. Hyper duplexes,
withPREnumbersgreater than45 then
began to appear in the 2000s.
Properties of duplex stainless
“The ferritic stainless steels have a
fairly low proof strength, around 170
to 240 MPa, with an elongation of
around18 to22%.Utility ferritics offer
improvedproof strength,withalloys like
3CR12 or 1.4003 having a minimum
proof strength of between 280 and
320MPa, butwith similar elongation,”
Tarboton says.
The advantages of duplex’s higher strength are particularly useful in the oil refinery and
offshore industries, according to Frederick Lobinger, Lincoln Electric’sMetrode global
welding consumable specialist.
SASSDA’s John Tarboton presents the
history development and properties of
duplex stainless steel in Johannesburg.