Austenitic stainless steel having high properties

The invention provides a steel containing by weight 23% .ltoreq.Cr.ltoreq.28%; 15%.ltoreq.Ni.ltoreq.28%; 0.5%.ltoreq.Mn.ltoreq.6%; 3%.ltoreq.Mo.ltoreq.8%; 0.35%.ltoreq.N.ltoreq.0.8%; and 1%.ltoreq.W.ltoreq.5%. The steel has a high resistance to corrosion and may be used for manufacturing massive parts for any application, in particular for manufacturing equipment for oil platforms and chemical works, containers for transporting corrosive products, ship hulls and ply sheets.

1. Field of the Invention 
The present invention relates to an austenitic stainless steel having high 
strength, high resistance to corrosion and a very good structural 
stability. 
2. Background of the Invention 
For the manufacture of equipment intended in particular for smoke 
depolluting installations, oil well platforms, the chemical industry, the 
paper pulp industry, there are employed austenitic or superaustenitic 
stainless steels having high strength and a high resistance to corrosion. 
These stainless steels generally contain high proportions of nitrogen and 
molybdenum. Such steels have been disclosed in particular in two European 
patents: EP-A-0,438,992 and EP-A-0,342,574 and in the French patent 
application FR-93-06468. But these steels have the drawback of a certain 
incompatibility between a good behaviour with respect to corrosion and a 
good structural stability. Consequently, there is for example a certain 
difficulty in conciliating the operations for manufacturing equipment, 
such as welding or hot forming, and a very high resistance to corrosion of 
all of the parts of this equipment. 
Austenitic stainless steels having high strength and high resistance to 
corrosion known in the art have another drawback in that they cannot be 
used in the form of massive parts. Indeed, in the course of the cooling of 
the parts, the instability of the structure causes intermetallic 
precipitations which very markedly adversely affect the resistance to 
corrosion and the mechanical properties of the steel. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an austenitic stainless 
steel having high mechanical properties, a very high resistance to 
corrosion in a chlorinated medium, and a very good structural stability. 
For this purpose, the invention provides an austenitic stainless steel 
having high mechanical properties, high resistance to corrosion, and a 
high structural stability, the chemical composition of which comprises, by 
weight: 
0.35%.ltoreq.N.ltoreq.0.8% 
1%.ltoreq.W.ltoreq.5% 
and at the most 6% Mn. 
Preferably, the chemical composition of the steel comprises by weight: 
23%.ltoreq.Cr.ltoreq.28% 
15%.ltoreq.Ni.ltoreq.28% 
0.5%.ltoreq.Mn.ltoreq.6% 
0%.ltoreq.Cu.ltoreq.5% 
0%.ltoreq.C.ltoreq.0.06% 
0%.ltoreq.Si.ltoreq.1% 
0%.ltoreq.Nb.ltoreq.0.5% 
0%.ltoreq.V.ltoreq.0.5% 
0%.ltoreq.Al.ltoreq.0.1% 
3%.ltoreq.Mo.ltoreq.8% 
0.35%.ltoreq.N.ltoreq.0.8% 
1%.ltoreq.W.ltoreq.5% the remainder consisting of iron and impurities 
related to the preparation. Still more preferentially, the chemical 
composition of the steel comprises by weight: 
23%.ltoreq.Cr.ltoreq.26% 
21%.ltoreq.Ni.ltoreq.23% 
2%.ltoreq.Mn.ltoreq.3.5% 
1%.ltoreq.Cu.ltoreq.2% 
0%.ltoreq.C.ltoreq.0.03% 
0%.ltoreq.Si.ltoreq.0.4% 
0%.ltoreq.Nb.ltoreq.0.5% 
0%.ltoreq.Al.ltoreq.0.1% 
4.5%.ltoreq.Mo.ltoreq.6.5% 
0.4%.ltoreq.N.ltoreq.0.55% 
2%.ltoreq.W.ltoreq.3.5% 
the remainder consisting of iron and impurities related to the preparation. 
Preferably, the chemical composition of the steel according to the 
invention satisfies the following formula: 
EQU CP=20.times.%CR+0.3.times.%Ni+30.times.%Si+40.times.%Mo+5.times.%W+10.times 
.%W+10.times.%Mn+50.times.%C-200.times.%N&lt;710 
which ensures that the kinetics of the precipitation of the intermetallic 
phases will be as slow as possible. 
Moreover, in order to obtain the best possible resistance to corrosion, the 
chemical composition of the steel must correspond to the following: 
EQU PRENW=%Cr+3.3.times.%Mo+16.times.%N+1.7%W&gt;47. 
Lastly, in order to obtain very high mechanical properties, the chemical 
composition of the steel must preferably satisfy the relation: 113+16 (% 
Mo+0.7% W)+525% N&gt;420. 
According to the invention, this steel may be used for manufacturing 
massive parts. It may also be used for manufacturing equipment for massive 
oil platforms or for manufacturing equipment for chemical works, paper 
pulp works, depolluting installations, or for manufacturing containers for 
transporting corrosive products, or lastly for manufacturing ship hulls. 
This steel may also be employed for manufacturing ply or claded sheets.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention will now be described in detail in a non-limitative manner. 
Those skilled in the art know austenitic stainless steels which are 
iron-base alloys, have a high chromium and nickel content and have a 
naturally austenitic structure in the solid state substantially at any 
temperature. For most of these steels, the structure is not 100% 
austenitic at around the solidification point but becomes so as soon as 
the temperature drops. For some of these steels, termed superaustenitic 
steels, the structure is 100% austenitic upon solidification. These steels 
are considered to be known. 
The inventors have noticed that, surprisingly, by simultaneously adding to 
these steels high contents of nitrogen: 0.35% by weight to 0.8% by weight, 
and preferably 0.4% to 0.55%, and of tungsten: 1% by weight to by weight, 
and preferably 2% to 3.5%, there were obtained at once high mechanical 
properties, a very high resistance to corrosion in a chlorinated medium 
and a very good structural stability, i.e. a very slow kinetics of 
precipitation of intermetallic phases at elevated temperature. 
A very good structural stability permits manufacturing massive parts, for 
example: thick sheets, thick tubes, forged parts, moulded parts or welded 
assemblies of which the mechanical properties and the behaviour with 
respect to corrosion are at every point excellent including in the 
vicinity of welds. 
It is preferable that these steels contain by weight, in addition to the 
nitrogen and the tungsten in the aforementioned contents, the elements 
indicated below. 
Chromium: more than 23% so as to ensure a good resistance to localized 
corrosion and a good solubility of the nitrogen, but less than 28% and 
preferably less than 26% so as to limit the risks of precipitation of 
chromium carbides. 
Nickel: more than 15% and preferably more than 21% so as to ensure an 
austenitic solidification which guarantees a good solubility of the 
nitrogen so as to obtain a good resistance to corrosion in a sulphuric 
medium and limit the propagation of the localized corrosion, less than 28% 
and preferably less than 23% so as to avoid excessively reducing the 
solubility of the nitrogen and because nickel is an expensive metal. 
Manganese: more than 0.5% and preferably more than 2% so as to obtain 
sufficient solubility of the nitrogen and limit the susceptibility to 
cracking in the hot state, less than 6% and preferably less than 3.5% so 
as to limit the risks of precipitation of intermetallic phases and limit 
the wear of the refractories in the course of the preparation of the 
steel. 
Copper: 0% to 5% and preferably 1% to 2% so as to improve the resistance to 
corrosion in a sulphuric and an acid chlorinated medium. 
Molybdenum: the content by weight of molybdenum of the steel must be more 
than 3% and preferably more than 4.5% so as to improve the resistance to 
localized corrosion, the solubility of the nitrogen, the mechanical 
properties at room temperature and at elevated temperature and limit the 
risks of cracking in the hot state upon welding; but this content must be 
less than 8% and preferably less than 6.5% so as to avoid formation of 
segregations and precipitation of intermetallic phases. 
The functions of the nitrogen and tungsten are the following: 
The nitrogen permits obtaining high mechanical properties, a good behaviour 
as concerns localized corrosion, a good structural stability; however, in 
excess it deteriorates the resilience. 
The tungsten permits obtaining a good resistance to corrosion in acid and 
reducing chlorinated media, a good resistance to corrosion in splits when 
it is associated with molybdenum and nitrogen, reinforcing the mechanical 
properties at room temperature and elevated temperature; however, in 
excess, it causes precipitations which have an adverse effect on the 
properties of use. 
Such steels always contain a little carbon, silicon and aluminium. The 
carbon content must be less than 0.06% and preferably less than 0.03% in 
order to avoid precipitation of carbide in the grain joints. 
The silicon and aluminium which acted as deoxidizers in the course of the 
preparation of the steel are limited to 1% for the silicon and 0.1% for 
the aluminium. 
Other elements such as magnesium, cerium or calcium may be added as 
deoxidizing agents. It is also possible to add up to 0.5% niobium and/or 
vanadium to improve the mechanical properties. In order to ensure that the 
properties of the stainless steel according to the invention are optimum, 
the chemical composition must be adjusted to be within such composition 
ranges that: 
the mechanical properties are high, for which the following relation is 
required: 
113+16 (% Mo+0.7% W)+525% N&gt;420 
the resistance to localized corrosion is maximum, for which the following 
is required: 
PRENW=% Cr+3.3(% Mo)+16 (% N)+1.7 (% W)&gt;47 
the kinematics of the precipitation of the intermetallic phases is very 
slow, which is obtained if: 
CP=20% Cr+0.3% 
Ni+30.times.%Si+40.times.%Mo+5.times.%W+10.times.%Mn+50.times.%C-200.times 
.%N&lt;710 
With this chemical composition an austenitic stainless steel is obtained 
whose yield point Rp 0.2 % at room temperature is higher than 420 MPa and 
whose structural stability characterized by the kinematics of the 
precipitation of intermetallic phases at 850.degree. C. is higher than 
those of grades which are otherwise equivalent. Consequently, the 
resistance to corrosion is unaffected by a thermal cycle employed in the 
use of the metal such as welding, which is not the case of steels of the 
prior art. 
As an example, a steel having the following chemical composition was 
prepared: 
______________________________________ 
Cr = 23.7% C = 0.015% 
Ni = 21.5% Mn = 2% 
Mo = 5% Si = 0.2% 
N = 0.45% Nb = 0.02% 
W = 2% V = 0.15% 
Cu = 1.5% Al = 0.02% 
______________________________________ 
This steel has a yield point of 452 MPa, a coefficient of sensitivity to 
corrosion by PRENW prick=50.8 and a coefficient of sensitivity to 
precipitations CP=627 so that the incubation time for the precipitation of 
intermetallic compounds at 850.degree. C. is 180 sec. 
After hyperquenching, the rate of corrosion in a hydrochloric medium is 100 
MDD (mg/dm.sup.2 /day); after hyperquenching followed by a sensitization 
treatment by maintaining the steel at 800.degree. C. for 15 min, the rate 
of corrosion under the same conditions is 200 MDD. 
In comparison, a steel of the prior art having the following composition: 
______________________________________ 
Cr = 24 Mn = 3 Si = 0.4 
Ni = 22 C = 0.01 Al = 0.02 
Mo = 7 Nb = 0.2 
N = 0.45 V = 0.15 
______________________________________ 
has a yield point of 461 MPa, a PRENW=54.3, a CP=716, an incubation time 
for the precipitation of 60 sec, a rate of corrosion after hyperquenching 
of 99 MDD and a rate of corrosion after a sensitization treatment at 
850.degree. C. for 15 min of 980 MDD. 
The steel according to the invention is much less sensitive to the 
sensitization thermal cycles. Consequently, it is possible to produce ply 
or claded sheets constituted by a layer of steel according to the 
invention and a layer of structural steel whose stainless plating 
properties are comparable to the properties of a massive stainless steel 
sheet produced from the same grade. 
As the steel according to the invention has a high structural stability it 
can be used for manufacturing in particular moulded parts, forged parts, 
rolled bars, rolled sheets, shapes, weld-less tubes and welded tubes, in 
particular when these objects are massive, i.e. when they are employed in 
the form of thick parts, i.e. parts having a minimum thickness greater 
than 4 mm, in particular when it concerns parts having a thickness 
exceeding 4 mm and less than 40 mm; they then have a good homogeneity of 
the properties within the thickness; as concerns thicknesses exceeding 40 
mm, the very good structural stability permits retaining high levels of 
resilience and ductility within the thickness. 
Owing to its mechanical properties, its resistance to corrosion, its 
aptitude for welding and the manufacture of thick parts, the steel 
according to the invention may be used advantageously in particular for 
manufacturing: 
tubes, flanges, collectors, oil pipelines, gas pipelines, separators, 
pumps, compressors, exchangers adapted to be used in contact with sea 
water or fluids containing chlorides and H.sub.2 S, in particular for any 
equipment for firefighting processes or circuits conducting sea water on 
marine oil platforms, 
tubes, flanges, tanks, reactors, pumps, compressors and more generally any 
part or wall of equipment for the chemical industry, for manufacturing 
paper pulp, hydrometallurgy, depollution operating in contact with fluids 
or corrosive effluents and in particular when it concerns corrosion by 
acid chlorinated media; concerned in the paper pulp industry are in 
particular chlorination filters, whitening towers, in particular, 
whitening towers with oxygen peroxide and ozone, mixers, washers, 
impregnators, 
tanks for road, rail or maritime transport of corrosive products, 
hulls of ships, 
equipment operating at elevated temperature and in particular equipment for 
the petrochemical industry, cement industry, incineration of garbage, 
smoke flues, chimney stacks. 
These applications are not intended to be exhaustive and more generally 
this steel permits obtaining an improved behaviour in service over steels 
of the prior art and at a lower cost than with nickel-base alloys for all 
the applications: 
in an oxidizing chlorinated medium, 
in a medium containing chlorides and H.sub.2 S, 
in an acid chlorinated medium, 
in particular when the parts produced must be thick or massive or when the 
temperature of utilization is elevated.