[0001] The present invention relates to stainless steel alloys.
[0002] Duplex stainless steel alloys of the ferritic austenitic type have been developed
over recent years for their combination of mechanical properties, high resistance
to localized corrosion and their resistance to stress corrosion cracking in a seawater
environment. Such steels are described for example in U.K. Patent No. 1,158,614 and
U.S. Patent No. 3,065,119 and are available under the designations CD4MCu and Ferralium.
[0003] A typical composition of such an alloy is as follows:

[0004] The compositional levels of the elements promoting either austenite or ferrite in
the microstructure are controlled to produce a structure which is balanced and which
contains a minimum of 65% austenite. This microstructural control provides an alloy
which has a good combination of toughness and corrosion resistance in thick section
castings when adopting air cooling from solution heat treatment temperatures.
[0005] However austenite levels of 65% and above result in an overall reduction in tensile
strength since the latter is sensitive to changes in the austenite/ferrite balance.
[0006] There is a need for stainless steel alloys which have an improved tensile strength
as compared to the above alloys and which still have a good combination of toughness
and corrosion resistance.
[0007] It is known that the addition of carbon and nitrogen as interstitial solid solution
strengthening elements is the most effective way of strengthening in both ferrite
and austenite (see for example K.J. Irvine, et al, J. Iron Steel Inst. 1961, Vol 199,
p 153 and K.J. Irvine et al, ibid, 1963, Vol 201, p 944).
[0008] However increased carbon in a stahless steel is undesirable as it has an adverse
effect on corrosion resistance.
[0009] Nitrogen is used in amounts up to 0.5% in austenitic stainless steels of the A.I.S.I.
200 series to impart high strength levels. However the solubility of nitrogen in the
conventional duplex 25 Cr/5Ni steels reaches a maximum of approximately 0.2% (J.F.
Elliot et al Thermochemistry for Steelmaking, Vol II, Adison and Wesley Publishers,
1963). The inclusion of amounts of nitrogen in excess of 0.2% in the conventional
duplex 25Cr/5Ni steels results in severe gassing problems in castings.
[0010] It is known that manganese additions are effective in several ferrous alloys in promoting
nitrogen solubility. However manganese has been shown adversly to effect localised
corrosion resistance in austenitic stainless steels (R.J. Brigham and E.W. Tozer,
Corrosion N.A.C.E., Vol 30, No. 5, May 1974, p 161) and also to promote formation
of the embrittling sigma phase.
[0011] It is also clear from the second K.J. Irvine et al reference quoted above that an
increase in the austenite content in the microstructure will increase nitrogen solubility.
However this results in a net decrease in mechanical properties arising from a loss
of dispersion strengthening reflected in the sensitivity of the proof stress value
with ferrite volume fraction at higher austenite contents.
[0012] It is an object of the present invention to provide a stainless steel alloy which
has an improved combination of mechanical properties, resistance to localised corrosion,
and resistance to stress corrosion cracking in a sea water environment than the conventional
duplex alloys.
[0013] According to the present invention there is provided a stainless steel alloy having
the following composition

[0014] said alloy having a duplex microstructure of austenite and ferrite. Preferably the
alloy is of the following composition

[0015] Preferably the alloy has a microstructure comprising at least 60% and more preferably
at least 65% austenite
[0016] The inclusion of 0.15 - 0.3 % N
2 in the alloy provides an increase in tensile and proof stresses as well as in localised
corrosion performance. Amounts of nitrogen less than 0.15% do not give the required
properties for the alloy whereas amounts greater than 0.3% cannot be obtained due
to its limited solubility. Amounts of manganese less than 3.5% do not give the required
N
2 solubility whereas amounts greater than 5.0 % lead to excessive formation of sigma
phase.
[0017] The alloy may be prepared from a melt containing the various components in the required
amount by casting of the melt and subsequent heat treatment in the manner known for
the production of duplex alloys of the austenite ferrite type. The nitrogen for the
alloy may be provided by high nitrogen ferrochrome.
[0018] The invention will be further described by way of example only with reference to
the accompanying drawings, in which;
Fig. 1 is a graph showing the effect on Ultimate Tensile Stress, 0.2% Proof Stress
and Elongation of increasing the austenite content of a conventional duplex stainless
steel alloy of the ferritic, austenitic type;
Fig. 2 is a TTT curve illustrating the rate of formation of sigma phase in various
alloys of differing manganese content;
Fig. 3 is a graph showing the effect on corrosion properties of increasing the manganese
content of a stainless steel alloy at various fixed nitrogen contents; and
Fig. 4 shows the effect on corrosion properties of increasing the nitrogen content
in a stainless steel alloy of fixed manganese content.
Fig. 1 illustrates the variation in Ultimate Tensile Stress (UTS), 0.2% Proof Stress
(PS) and Elongation with increasing austenite content in a duplex alloy of nominally
25% Cr obtained by heat treatment at 1120°C with quenching. The variation in austenite
content is obtained by varying the nickel content of the alloy.
[0019] It will be seen from Fig. 1 that the alloy with above 60% austenite content has a
tensile strength of 650 MPa or less and a 0.2% Proof Strength of 490 MPa or less.
[0020] In order to improve the properties of the known alloy it was decided to investigate
whether a level of manganese addition could be found which would aid nitrogen solubility
without loss of toughness or corrosion performance thereby resulting in improved mechanical
properties.
[0021] Initially a series of ferritic austenitic alloys were cast which contained nominally
25% Cr, 6-7% Ni, 2% Mo, 0.05% N
29 amounts of manganese from 1 to 15%, and a balance of iron plus unavoidable impurities.
[0022] A study was then made on the effects on microstructure, and tensile and hardness
properties.
[0023] Additions of manganese up to 8% were found not to effect the austenite/ferrite phase
balance. Proof stress and ultimate tensile strengths were found not to be affected
up to a level of 12% Mn.
[0024] A progressive increase in hardness was recorded at manganese levels of 5% and above
in the solution treated and air cooled condition and was attributable to hard, brittle
sigma phase.
[0025] Fig. 2 illustrates this formation of sigma phase and is a Time Temperature Transformation
curve showing the rate of sigma phase formation in a conventional duplex base alloy
(nominally 25Cr/5Ni) (curve A), an alloy containing 3% Mn (curve B), and one containing
5% Mn (curve C). Superimposed on Fig. 2 is a continuous cooling path typical of the
cooling rate 'seen' at the centre of a 6" diameter billet.
[0026] It will be seen from Fig. 2 that rate of formation of sigma phase increases with
increasing manganese cortent, and above 5% Mn the ferritic microstructure at the centre
of a 6" billet will transform partially or wholly to sigma phase upon cooling from
the solution heat treatment temperature of 1100°C.
[0027] By way of example, Fig. 2 also shows the TTT curve showing rate of formation of sigma
phase in Uranus 50, a typical duplex stainless steel alloy with a 50/50 austenite/ferrite
microstructure.
[0028] From the above discussion it will be appreciated that amounts of manganese of less
than 5% should. be used in the alloys of the invention to ensure that undesirable
amounts of sigma phase are not formed.
[0029] In order to study the effect of manganese addition on nitrogen solubility, two series
of duplex alloys (nominally 25Cr/5Ni) were prepared. One series of alloys contained
a nominal 1% Mn and the other contained 4.5% Mn. The alloys of each series were prepared
with nitrogen levels of between 0.2 and 0.35%.
[0030] On visual examination it was evident that the alloys containing 1% Mn had gassed
severely once a level of 0.15 N
2 was exceeded, whereas the higher manganese alloys showed good soundness up to 0.3%
N2.
[0031] Tests were conducted to determine the effect of manganese and nitrogen additions
on the critical crevice and pitting temperatures of the alloys.
[0032] Fig. 3 is a graph showing the relationship of increasing manganese content on the
critical crevice and pitting temperatures in artificial seawater of two nominally
25Cr/5Ni duplex alloys, one containing 0.1% N
2 and the other 0.23% N
2. In Fig. 3, the solid curves represent pitting temperatures and the dashed lines
represent crevice temperatures.
[0033] It will be seen from Fig. 3 that the alloy with the higher nitrogen content has the
higher values of critical pitting tmmperature and critical crevice temperature; the
alloy with 0.23% nitrogen maintains a critical pitting temperature of over 60°C up
to a manganese addition of 5% and also shows a rise in critical crevice temperature
for manganese additions of 3 to 4%.
[0034] Fig. 4 is a graph showing the relationship of increasing levels of nitrogen on the
pitting and crevice temperatures of a series of duplex alloys (nominally 25Cr/5Ni)
containing 4.5% Mn. The alloys were subjected to cyclic anodic polarisation measurement
in artificial seawater to determine their localised corrosion performance.
[0035] It will be seen from Fig. 4 that increasing the nitrogen content to 0.3% improves
the critical pitting and crevicing temperatures by more than 20°C as compared to an
alloy containing only 0.1% N
2.
[0036] The inclusion of amounts of manganese of from 3.5 to 5% and amounts of nitrogen of
from 0.15 to 0.3% in the alloys of the invention results in numerous advantageous
tensile properties for the alloys of the invention as will be appreciated from the
following.
[0037] The tensile properties of an alloy of the invention and containing 0.28% N
2 and 4.5% Mn were compared with those of a conventional duplex alloy of the type set
out in Table I above. The comparison for the two alloys for the solution heat treated
and water quenched conditions is given in Table II below.

[0038] It will be seen from Table II that the properties of the alloy are much improved
as compared to the conventional alloy. In particular the increase in proof stress,
in excess of 100 MPa is totally due to the increased nitrogen addition held in solid
solution in the austenite.
[0039] It should be appreciated that a number of modifications may be made to the composition
of the alloy of the invention.
[0040] The addition of copper will have a beneficial effect on corrosion resistance. Thus,
for example, the alloy may include copper in an amount up to 1% by weight. If greater
than 0.5% by weight of copper is added, the active dissolution rate of a duplex alloy
in boiling hydrochloric acid and the crevice corrosion loss in a chloride solution
is also decreased.
[0041] Tungsten upto 1% by weight increases the immunity potentials to crevice corrosion
of a duplex alloy in high temperature chloride solutions above which crevice corrosive
starts.
[0042] Tin and antimony in amounts upto 0.10% and 0.2% by weight respectively improve the
stress corrosion cracking and pitting corrosion in environments containing significant
concentrations of H
2S.
[0043] Such modified alloys are also within the scope of the invention.
1. A stainless steel alloy having the following composition

said alloy having a duplex microstructure of austenite and ferrite.
2. An alloy as claimed in claim 1 and having the following composition
3. An alloy as claimed in claim 1 or 2 and having a microstructure comprising at least
60% austenite.
4. An alloy as claimed in claim 3 and having a microstructure comprising at least
65% austenite.
5. A stainless steel alloy as claimed in any one of claims 1 to 4 including copper
as an alloying addition in an amount of greater than 0.5% by weight.
6. A stainless steel alloy as claimed in any one of claims 1 to 4 including tungsten
as an alloying addition in an amount of up to 1% by weight.
7. A stainless steel alloy as claimed in any one of claims 1 to 4 including tin as
an alloying addition in an amount of up to 0.10% by weight.
8. A stainless steel alloy as claimed in any one of claims 1 to 4 including antimony
as an alloying additions in an amount of up to 0.2% by weight.