General introduction about Duplex Stainless Steel [611797]

Chapter One
General introduction about Duplex Stainless Steel

1.1 Introduction
Stainless steels are extremely useful as engineering materials because of their wide range of
properties and general resistance to environmental conditions. For example, their proof stress values
range from 200 MPa to 2000 MPa, they have high ductility and toughness over a wide range of
temperatures, exhibit excellent corrosion and high temperature oxidation resistance. They can be used not only for structural and containment applications but also as creep resistant materials in power
plant engineering. The temperature range over which they are used extends from the boiling point of
nitrogen to temperatures well above the melting point of aluminum. They are readily formed and welded, exhibit no ductile -brittle transition during fracture in the austenitic con dition, and can be
produced with a wide range of magnetic and thermal properties. Their main disadvantage is, of course, their cost although in the context of other advanced materials, the cost is negligible. The most
important property of stainless steels is undoubtedly their corrosion resistance, without which they
would find little use because their mechanical properties and formability can be equaled or exceeded
by much cheaper but non -stainless steel (Pickering, 1984).
One of the first industrial development of a stainless steel was done by H. Brealey some seventy five
years ago (Truman, 1980) after establishing the corrosion resistance of chromium containing steels.
The 13 wt. % Cr, 0.25 wt. % C martensitic stainless steel thus commercially launch ed as a cutlery
material is still almost identical with the current steel that is used for stainless knives. Actually, some
of the earliest work carried out almost 100 years earlier noted that chromium improved the corrosion
resistance of steel. These observations did not receive the attention they deserved, largely because the
detrimental effect of carbon was not then appreciated and also because an extensive program of
research by Hadfield in 1892 had shown that chromium decreased the corrosion resistance of steel in
sulphuric acid. Between 1904 and 1914, several workers
(For example Guillet, 1904; Geisen, 1909) studied the metallurgy and properties of Cr and Cr -Ni
steels, some of which had austenitic, ferritic and martensitic structures. The martensitic steels, first
exploited commercially, were not suitable for sheet manufacture, but the development of the
austenitic steels just prior to the first World War formed the basis for the now ubiquitous T8 Cr – 8
Ni’ alloys which are ideal for the production of fiat rolled products and for subsequent fabrication.
Since the early 1920’s the development of the various types of stainless steel has proceeded with ever increasing rapidity. It was quickly recognised that more than 10 wt.% Cr imparted good corrosion

resistance and that somewhat higher Cr contents conferred high temperature oxidation resistance as
well. Thus the stainless steels came to be used as heat resisting materials, and when suitably alloyed,
as creep resisting steels. The use of the stainless steels for these special applications was perhaps the
first indication that, whilst corrosion resistance was paramount, it had to be combined with other
properties and optimum com binations of properties are obviously highly desirable. Such properties
are:
• Corrosion/oxidation resistance in the operating environment
• Good fabric ability, both during hot and cold processing
• Weld ability without impairment of either corrosion resistance mechanical properties
1.2 Types of Stainless Steels
• Transformable or Martensi tic Steels
• Ferritic stainless steels
• Austenitic stainless steels
• Duplex stainless steels
1.3 Duplex stainless steels
Duplex stainless steels are a family of grades combining good corrosion resistance with high strength
and ease of fabrication. Their physical properties are between those of the austenitic and ferritic
stainless steels but tend to be closer to those of the Ferritics and to carbon steel. The chloride pitting
and crevice corrosion resistance of the duplex stainless steels are a function of chromium, molybdenum, tungsten, and nitrogen content. They may be similar to those of Type 316 or range
above that of the sea water stainless steels such as the 6% Mo austenitic stainless steels. All duplex
stainless steels have chloride stress corrosion cracking resistance significantly greater than that of the 300-series austenitic stainless steels. They all provide signif icantly greater strength than the austenitic
grades while exhibiting good ductility and toughness. There are many similarities in the fabrication of austenitic and duplex stainless steels but there are important differences. The high alloy content
and the high strength of the duplex grades require some changes in fabrication practice. This manual
is for fabricators and for end users with fabrication responsibility. It presents, in a single source,
practical information for the successful fabrication of dupl ex stainless steels. This publication
assumes the reader already has experience with the fabrication of stainless steels ; therefore, it
provides data comparing the properties and fabrication practices of duplex stainless steels to those of
the 300- series austenitic stainless steels and of carbon steel.
The first international duplex conferences ( St LOUIS/USA/82 and Den HAGUE/NL/86 ) were mainly focused on “scientific” aspects since they appeared very attractive for the metallurgical

aspects including phase precipitations (structures, kinetics’,.), corrosion resistance, mechanical
properties. The worldwide industrial production in that time was almost marginal and standardization
has still to come. BEAUNE ’91/France conference was the first duplex conference with a mix
between scientific and market applications. The new grades with increased nitrogen additions were presented. Duplex grades gained in structure stability, weld ability and corrosion resistance
properties! New standards were proposed. The duplex family included the popular 2205 grades with
increased nitrogen additions (0.16/0.18%N instead of 0.12/0.15%N) and optimized Mo contents.
PREN values were proposed between 33 and 36 with a most common value 34/35. Sigma free grades
were recommended as well as a minimum Mo level of 3%. Grade 2304 was already developed but
for marginal applications. Several super duplex grades were also promoted – SAF 2507, UR 52N+,
Zeron 100… – For the most severe applications including off shore. Those super duplex grades were
also redesigned before the BEAUNE’91 conference in order to have a PREN value minimum of 40 and a nitrogen level minimum of 0.25 %.The grades were mainly produced with EF + VOD or AOD
+ continuous casting devices i.e. the most efficient stainless ste els tools. First extensive applications
were reported as well as new areas of developments. Confidence in duplex grades gained the end-users and the cost savings aspects partially obtained through the high mechanical properties were
expected. New codes for duplex grades had to come. The potential growth of the grades was clearly
emphasized. All this partially has explained the extremely wide audience of the BEAUNE’91 conference.
York’94, Maestricht’97 and Venice’2000 conferences were also successful conferences with
increasing return of information on practical experiences. Most of the applications still concerned the quartz
plates and tubing. The first lean duplexes appeared. More concerning the technical content of those conferences will be discussed
Although Schaeffler diagram is mainly used for welded structures, it is very useful to illustrate the
different areas of stability for the stainless steel microstructures. The classical austenitic grades – the
so called 300 series – contains a minimum of 9/ 10%Ni while the more alloyed grades in Cr and/or Mo
needs even more Ni to stabilize the austenitic phase. The most popular stainless steel – the 304 – is
the lowest alloying grade of the austenitic area. Ferritic stainless may be produced when Ni content
is almost 0% and Cr minimum content 12% Most of the ferritic grades have a Cr content included in
the 12/18% range. More recently >20%Cr grades with Ti+Nb or Cu+Nb are developed to extend the
Mo-free ferritic family. Between the austenitic and the ferritic areas, we obtain a mixed ferrite +
austenite microstructure. Most of the duplex industrial grades have a typical 50%ferrite/50%austenite microstructure. The three main families of duplex grades are plotted in the Schaeffler diagram. As
observed it is almos t impossible to develop duplex grades having less than 19%Cr without formation
of martensite.With the extreme volatility of alloying element costs – Ni for example has increased of
more than 500% this latest 3 years – new grades have been introduced recentl y in the market. This

concerns austenitic grades with partial replacements of Ni by combined Mn and N additions. Such
grades were already developed on a marginal production since more than 50 years. The newly
developed grades have Cu additions and often lower Cr contents in order to reduce the interstitial
elements like C and N. Those grades are softer than the former 200 grades and make it possible to
utilize the same tools to manufacture equipment – drawings for ex… – Than those used for 304 grades
(Tab le 1.1 and Fig.1.1).

Typical mechanical properties of the different stainless steel families are presented Figure
1.2. Mechanical properties of the different stainless steels are directly linked to the nitrogen + carbon
contents and microstructure. Ferritics have slightly higher yield strength than austenitic at room
temperature but cannot be strengthen by interstitial add itions. (Lack of solubility) The duplex grades
have the highest mechanical properties due to a grain refining effect obtained by the two phase microstructure. This will be very useful for the design of pressure vessels where weight reductions
can be expect ed when properly designed.
1.4 Composition of Duplex Stainless Steel s
The table lists the duplex stainless steel covered in ASTM specification for plate, sheet, and bar
products. As showed in table 1.2.
1.5 DUPLEX STAINLESS STEELS IN THE FUTURE
Some new duplex grades have been introduced in the market or are concerned by R&D activities.
Nowadays the main target is the development of so- called lean duplex having much less alloying
elements than the standard 2205 duplex grade. There targets is the replacement of 316 and even 304 grades. Some of those chemistries are presented Table 1.3. Chromium contents are in the range 20/22
while Ni additions are reduced by further increase of Mn contents.
For some more corrosion resistance applications Mo contents are considered. Table II presents also
some R&D or ongoing work on new grades. Alloying with copper and tungsten are also considered.
Those developments may boost the developments of duplex grades in the future. The real huge boost
in duplex production is still pending upon the successful industrialization of thinner gauges! Duplex
grades, particularly the lower alloying grades are known to have poor hot workability properties.
Production of wide hot coils free of defects remains a technical challenge! I n case of success, taking
in to account their corrosion resistance properties and alloying costs they have promising future even we cannot expect for thin gauges weight savings due to their improved mechanical properties. Typical
corrosion resistance prope rties and PREN ranking are presented in Figs. 1.3 and 1.4.

1.6 Benefits
a) High strength,
b) High resistance to pitting, crevice corrosion resistance.
c) High resistance to stress corrosion cracking, corrosion fatigue and erosion,
d) Excellent resistance to chloride stress- corrosion cracking
e) High thermal conductivity
f) Low coefficient of thermal expansion
g) Good sulfide stress corrosion resistance,
h) Low thermal expansion and higher heat conductivity than austenitic steels.
i) Good workability and weld ability
j) high energy absorption [7].
1.7 Applications
a) Heat exchangers, tubes and pipes for production and handling of gas and oil,
b) Heat exchangers and pipes in desalination plants,
c) Mechanical and structural components,
d) Power industry FGD systems,
e) Pipes in process industries handling solutions containing chlorides,
f) Utility and industrial systems, rotors, fans, shafts and press rolls where the high corrosion
fatigue strength can be utilized,
g) Cargo tanks, vessels, piping and welding consumables for chemical tankers.
h) High strength highly resistant wiring [17] .

1.8 The main objective of the dissertation
The main objective of this dissertation is to study the changes that occur in the microstructure
evolution during the different temperatures of the thermo- mechanical pr ocesses in the Super Duplex
Stainless Steel F55 (SDSS F55) alloy.
1.9 The structure of the d issertation
The present dissertation is organized in 6 chapters. Chapter 1 is consisting of general information
about duplex stainless steels ; in addition to the objectives and structures of the dissertation. Chapter
2 includes Thermo mechanical processing of duplex stainless steels . Besides, chapter 3 includes
information about Super Duplex Stainless Steels F55 alloy. Chapter 4 includes materials and
methods . Chapter 5 comprised the outcomes of the study along with the discussion part. The last
chapter provides the conclusion of the