International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017
ISSN 2229-5518
Low Nickel Austenitic Stainless Steel – A Review
of its Physical Metallurgy
Prof.Dr.A.Kanni Raj
Abstract— The present review elaborately discusses the Physical Metallurgy of Low Nickel Austenitic Stainless
Steels. Costlier nickel is replaced by cheaper manganese and nitrogen. Yield and tensile strengths of high-nitrogen alloys can exceed those of conventional AISI 200 and 300 series stainless steels by 200-350% without any sacrificial loss
in toughness. A sufficiently high content of austenite-forming elements such as nickel, nitrogen, carbon and copper
are required in the steel to allow the desired austenite structure to form at annealing temperatures, and to persist to
room temperature. In most austenitic stainless steels, the austenite structure is not stable at room temperature, and
tends to transform to martensite when the steel is cold worked; the amount of transformation to martensite upon
heavy cold working should be limited to 10% or less, to avoid excessive wear or cracking of cold-working tools such
as dies.
Index Terms— Nickel austenitic stainless steel, tensile strength, yield strength, ductility, austenite, martensite.
——————————  ——————————
1 INTRODUCTION
T
HE Ni is alloying element traditionally used in stainless
steels to stabilize the face-centered-cubic (FCC) crystal
structure at room temperature. The substitution of Ni by
Mn and N is an interesting proposition, both from an economical as well as an engineering point of view. Although Mn can
be regarded as an austenite stabilizer, the addition of Mn alone
is not sufficient to stabilize the austenite phase at room temperature, especially in the presence of Cr which is a strong
ferrite stabilizer. The addition of Mn, nevertheless, is effective
in increasing the solubility of N in the liquid steel and the fraction of austenite formed during solidification. N is a strong
austenite stabilizer and solid solution strengthener that also
has a positive effect on the pitting corrosion resistance. Unlike
traditional FCC stainless steels, Mn and N alloyed low stacking fault energy steels exhibit a clear ductile-to-brittle transition in impact toughness below room temperature [1, 2].
Alloying with nitrogen has several advantages over alloying with carbon: (i) nitrogen is a more effective solid-solution
strengthener than carbon and also enhances grain size (HallPetch) strengthening; (ii) nitrogen is a strong austenite stabilizer thereby reducing the amount of nickel required for stabilization; (iii) nitrogen reduces the tendency to form ferrite and
deformation induced martensites; (iv) nitrogen has greater
solid-solubility than carbon, thus decreasing the tendency for
precipitation at a given level of strengthening; and (v) nitrogen
is beneficial for pitting corrosion resistance. Yield and tensile
strengths of high-nitrogen alloys can exceed those of conventional AISI 200 and 300 series stainless steels by 200-350% in
the annealed condition, without sacrificing toughness. Cold
deformation can produce further increases in strength resulting in materials with yield strengths above 2GPa [2, 3].
————————————————
 Prof. Dr. A. Kanni Raj is currently working as Professor in Faculty of
Mechanical Engineering, PSN College of Engineering and Technology
(Autonomous), Tirunelveli-627152, Tamil Nadu, INDIA, Telephone –
(91)4634279068, Cell–(91)9489692606, Email ncmde17@psncet.ac.in
2 PHYSICAL METALLURGY
2.1 What is Low Nickel Austenitic Stainless Steel
Nickel (Ni) used in AISI 304 is costly and it does not occur
abundantly in our country. Therefore, an obvious way to produce cheaper austenitic stainless steels is to replace the Ni
with other austenite forming elements. Over two thirds of all
stainless steel manufactured falls within the AISI 300 series,
and of these alloys, Type 304, 316 and their low carbon (“L”)
grades make up more than 80% of the volume sold. The most
commonly made alloys, 304 and 304L, are metastable austenitic grades, and are considered general purpose stainless steels
because of their good corrosion resistance, formability and
weldability (304L). However, they are not commonly considered for use as structural materials except in situations where
their corrosion resistance or toughness is an advantage. Additionally, AISI 300 series alloys especially 304 and 316 grades
have suffered from high production cost due to the high and
volatile price of nickel. Nickel contributes 50% of the total operating costs in the manufacture of cold-rolled 304 [1].
Elements such as N, Mn and Cu are very small. Lownickel austenitic stainless steels (LNiASSs) (Cr- Mn-Ni-N
stainless steels) are a modification of the popular austenitic
stainless steel grade AISI 304 (Table 1). The most successful
development has been the 200-series steels, dating from the
1950s. The 200-series of austenitic stainless steels uses a combination of Mn and N to replace part of the Ni. These types of
alloys have a higher strength than Type 304; Type 201, which
is the most commonly made of the 200-series, has a minimum
proof stress of 330MPa. The higher strength is due to the higher nitrogen contents of these steels. Nitrogen is a strong interstitial solution strengthening element, adding ∼65MPa to the
yield strength for every 0.1% compared to 2MPa for 1% of Cr
or Mn. However, apart from the 200-series, there are other
varities of LNiASS grades have been developed in last two
decades where the nickel content was minimized upto 0.1 wt%
[4-7].
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International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017
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2.2 Why Low Nickel – Effect of Cost
The cost of the common stainless steels is substantially determined by the cost of ingredients. The cost of the chromium
that is the essential "stainless ingredient" is not high, but additions of elements that improve the corrosion resistance (especially molybdenum) or that modify the fabrication properties
(especially nickel) add very much to the cost. Costs for nickel
have fluctuated from US$5,000 or US$6,000 in 2001 to
US$15,000 per tonne in 2004. Similarly, molybdenum has dramatically increased from approximately US$8,000 per tonne in
2001 to around US$50,000 per tonne in 2004. These costs impact directly on the two most common grades: 304 (18%Cr,
8%Ni) and 316 (17%Cr, 10%Ni, 2%Mo). The impact is most
keenly felt in grade 316, which has suffered an increase to its
cost premium above 304. Other grades such as the duplex 2205
(22%Cr, 5%Ni, 3%Mo) and all more highly alloyed stainless
steels are also affected.
Figure 1 Relative cost of few alloying elements.
Relative costs of the ingredients are shown in Figure 1, but
these do vary widely and sometimes rapidly over time. Therefore, with the recent price increases and the price volatility of
nickel, effect of nitrogen and manganese are ever more important as an alloying element for a number of reasons. First,
nitrogen is easily available everywhere and thus is not subject
to speculation at the Metal Exchange. Second, in addition to
making stainless steels austenitic, nitrogen can also make them
stronger and more corrosion resistant. It is also a well and
clearly established fact since many years, that nitrogen in solid
solution makes austenitic stainless steels more wear resistant
and more fatigue resistant. Third, Mn increases the solubility
of nitrogen at high temperature and also acts as an austenite
stabilizer [8].
2.3 Effect of Alloy Addition – Mn and Ni
The substitution of Ni by Mn and N is an interesting proposition, both from an economical as well as an engineering point
of view. Although Mn can be regarded as an austenite stabilizer, the addition of Mn alone is not sufficient to stabilize the
austenite phase at room temperature, especially in the pres-
ence of Cr which is a strong ferrite stabilizer. The addition of
Mn, nevertheless, is effective in increasing the solubility of N
in the liquid steel and the fraction of austenite formed during
solidification. N is a strong austenite stabilizer and solid solution strengthener that also has a positive effect on the pitting
corrosion resistance. Unlike traditional stainless steels, Mn
and N alloyed low stacking fault energy steels exhibit a clear
ductile-to-brittle transition in impact toughness below room
temperature. Substitutional elements which stabilize the ferrite structure (W, Mo, V, Si and Cr) have a mild positive effect
on the yield stress, while austenite stabilizing elements (Cu,
Ni, Co and Mn) have little, or in the case of Ni, a negative effect on the yield stress of the austenite phase. Interstitial elements (N, C and B) increase the strength of austenitic stainless
steels more significantly than solid solution elements, and N
has the greatest effect [3, 8-11].
2.4 Nitrogen Solubility in Liquid Fe-based Alloys
The low solubility of nitrogen in liquid Fe, which is only 0.045
wt.% at 1600 °C and atmospheric pressure, is a major obstacle
to the production of high-nitrogen steels. However, nitrogen
solubility in liquid Fe-based alloys generally follows Sievert's
law and is proportional to the square root of the N2 gas pressure over the melt. At higher pressures and/or nitrogen levels,
there is deviation from Sievert’s law and increases in nitrogen
concentrations with pressure occur to a power of less than 1/2.
Additions of Cr and Mn increase, and Ni reduces, nitrogen
solubility. Therefore, the solubility of N in Fe-Cr-Ni alloys is
much lower than in Fe-Cr-Mn alloys with comparable Cr concentrations. These characteristics of nitrogen solubility are
illustrated in Figure 2. The effect of alloy additions on nitrogen
solubility in iron at 1600°C, normalized to the effect of Cr, are
shown in Figure 3. Some of the elements which have the
greatest positive influence on nitrogen solubility in the liquid,
such as Ti, Zr, V, and Nb, also have a strong tendency to form
nitrides. Chromium is not only the most important alloying
element in stainless steels, it also significantly increases nitrogen solubility in liquid iron alloys with a lesser tendency,
compared to Ti, Zr, V, and Nb, for nitride formation in the solid-state. Although Ni is an important alloying element in
stainless steels, its negative influence on nitrogen solubility
has led to reductions in its levels in most LNiASSs. Manganese
is used extensively in many low nickel austenitic stainless
steels to increase nitrogen solubility.
2.5 Austenite Formation and Stability
A sufficiently high content of austenite-forming elements such
as nickel, nitrogen, carbon and copper are required in the steel
to allow the desired austenite structure to form at annealing
temperatures, and to persist to room temperature. Expressions
giving the chromium and nickel equivalents for annealing at
1075°C are given below:
Creq = Cr + 1.5Mo + 0.48Si
…(1)
Nieq = Ni + 18N + 30C + 0.33Cu + 0.1Mn-0.01(Mn)2
…(2)
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Where, the alloy contents are in mass percentages. The minimum required nickel equivalent to ensure a fully austenitic
structure at 1075°C is then given by:
Nieq (required) = 1.2×Creq – 13
…(3)
summarized in Figure 4, which gives the nickel content which
is required to give a fully austenitic structure, for different
manganese contents. The figure confirms the weak austenite
forming ability of manganese (the nickel content required for
austenite formation changes little if manganese is added), and
that the nickel replacement effect of manganese is zero around
10% Mn, and is negative at higher manganese contents. The
strong nickel replacement effect of nitrogen is evident from
the figure, as is the moderate effect of copper.
Copper can be seen (from Equation 2 and Figure 4) to have
roughly one-third the austenite-forming (or "gammagenic")
ability of nickel. Since the price of nickel is historically about
three times that of copper, there is no cost advantage to using
copper instead of nickel to form austenite. Also, the extent of
copper addition has to be limited to avoid surface quality
problems such as hot shortness during rolling [13].
Figure 2 Nitrogen solubility in liquid Fe-based alloys at
1600°C as a function of N2 gas pressure.
Figure 4 Calculated nickel contents in 17%Cr-0.05%C-0.1%N
stainless steel, and with added Cu and higher N, required to
obtain a fully austenitic structure at 1075°C.
Figure 3 Effects of various alloying elements on nitrogen
solubility in liquid Fe at 1600°C, normalized to the effect of
chromium (interaction coefficient for element divided by Cr
coefficient).
As demonstrated in this document, the numbering for sections
upper case Arabic numerals, then upper case Arabic numerals,
separated by periods. Initial paragraphs after the section title
are not indented. Only the initial, introductory paragraph has
a drop cap. The predicted effect of manganese was tested for a
temperature of 1075°C by using equilibrium phase calculations with FactSage and the steel database [12]. The results are
Figure 5 Calculated effect of manganese alloying on the solubility of nitrogen in molten stainless steel containing 17%
Cr (and no additional alloying elements other than Mn and
N);results are for the correlations of Montagnon and Moraux
[14], Speidel [3], and FactSage calculations for 1600°C.
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Evidently manganese alloying additions of 6-10% are not
primarily used to form austenite. Rather, manganese serves to
increase the solubility of nitrogen in liquid and solid steel.
Figure 5 shows the predicted effect of manganese alloying on
the solubility of nitrogen in liquid stainless steel, for two of the
literature correlations and from FactSage predictions. Solidstate solubility of nitrogen is also an important consideration,
especially the substantial decrease in solubility that occurs
when the steel transforms to ferrite, generally upon solidification [13].
In most austenitic stainless steels, the austenite structure is
not stable at room temperature, and tends to transform to
martensite when the steel is cold worked; the amount of transformation to martensite upon heavy cold working should be
limited to 10% or less, to avoid excessive wear or cracking of
cold-working tools such as dies [15]. The tendency to form
martensite can be related to the Md temperature, which is the
temperature at which deformation will result in a defined
martensite content in the steels (so steels which are more stable have lower Md temperatures). A correlation for the Md30
temperature (where 50% martensite forms upon deformation
in tension to a true strain of 0.30) is as follows[16]:
Md30(°C) = 497–462(%C+%N)–9.2%Si–8.1%Mn
–13.7%Cr–20%Ni–18.5%Mo
trated in Figure 6, where YS and UTS are plotted as a function
of increasing nitrogen concentration for a series of Fe-17Cr10Mn-5Ni-N alloys. The total contribution of nitrogen to the
flow stress of austenitic stainless steels is made up of two
components, a strongly thermal one, primarily due to solidsolution strengthening and proportional to N1/2, and an athermal component, generally attributed to nitrogen-enhanced
grain size strengthening, which is proportional to N. The
thermal component is weak at temperatures above 200 °C, and
very strong at temperatures below ambient. It is not surprising, therefore, that the total effect of nitrogen on YS is often
observed to be fairly linear at room temperature and above.
Nitrogen also has a significant effect on grain size hardening,
which can be described using the familiar Hall-Petch equation.
The grain size hardening effect (Hall-Petch slope) increases
proportionally to the nitrogen concentration. This is important
since a disadvantage of carbon-alloyed austenitic stainless
steels is their low potential for grain size hardening [3,18,19].
…(4)
Angel [16] fitted this equation to measured Md30 temperatures for a range of austenitic stainless steel compositions, containing up to 9.1% Mn. Although Equation 4 was developed
for steels with lower nitrogen contents than typically encountered in low-nickel stainless steels, it does give a general indication of the relative effects of the elements. For example, the
ratio of the coefficients of nitrogen to nickel for austenite stabilization in Equation 4 (a ratio of 23) is greater than for austenite formation in Equation 2 (a ratio of 18). This implies that
low-nickel steels which substitute nitrogen for nickel, and are
sufficiently highly alloyed with nitrogen to obtain 100% austenite during annealing, should have a smaller tendency to
form martensite during deformation than regular austenitic
Cr-Ni stainless steels. This prediction is supported by application data [17]. Therefore, the strong austenite forming effect of
nitrogen contributes to the extensive use of nitrogen in lownickel stainless steels. Indeed, it appears not to be possible to
produce low nickel austenitic stainless steels without nitrogen
alloying. Nitrogen, as an interstitially dissolved element, does
have a significant strengthening effect in these steels [3,13],
which can be an advantage in some applications.
2.6 Solid Solution Strengthening
Substitutional elements which stabilize the ferrite structure
(W, Mo, V, Si, and Cr) have a small positive effect on yield
strength (YS), while austenite stabilizing elements (Cu, Co,
Mn, and Ni) have little, or in the case of Ni, a negative effect.
Interstitial elements (N, C, and B) increase the strength of austenitic stainless steels much more than substitutional elements,
and nitrogen is more effective than any other element. The
powerful strengthening effect of interstitial nitrogen is illus-
Figure 6 Strengthening of austenitic stainless steel by interstitial nitrogen.
2.7 Mechanical Properties – Ductility & Cracking
The high strength of these steels at hot-working temperatures
often requires the use of high rolling temperatures to avoid
excessive mill loading. In some cases their inherently low hotductility may lead to edge cracking and other defects. There
are many factors that can affect the hot ductility of steels, such
as: (i) temperature, strain rate, (ii) composition, (iii) grain size,
precipitates, (iv) non-metallic inclusions, and (v) prior thermal
and mechanical treatments. Grain refinement increases the
rate of recrystallization and decreases the recrystallized grain
size [20]. This applies to static recrystallization between rolling
passes and to dynamic recrystallization at the high strains.
Carbides and nitrides taken into solution at soaking temperatures may, during multi-pass operation, precipitate on dislocations and in grain boundaries, strengthening the matrix and
reducing the rate of recrystallization. Also the nonmetallic
inclusions have a detrimental effect on the hot-working behaviour and the non-deformable inclusions have more serious
effects than deformable ones. The presence of second phase
particles in general enhances recrystallization. According to
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International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017
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literature, however, the presence of some δ ferrite is highly
detrimental to the hot ductility of low nickel austenitic stainless steels and leads to cracking of the austenite/ferrite interface [1]. Therfore, LNiASSs still present several limitations.
Edge and surface cracks as well as forging cracks can appear
during hot deformation. These types of cracks can be generated by secondary phases, segregations, dynamical recovery,
strain induced precipitations, and technical forming parameters [21]. It has been reported that the surface and edge cracks
of blocks are caused by the precipitation of carbide/nitride and
intermetallic phases. The precipitation of Cr2N has a detrimental effect on the formation of these cracks [1].
[2]
[3]
[4]
[5]
3 APPLICATIONS
Industrial applications where Low Ni austenitic stainless
steels (LNiASSs) may be utilized include the power-generating
industry, ship building, railways, cryogenic processes, chemical equipment, pressure vessels, and the petroleum and nuclear industries. There are several attributes which makes the use
of LNiASSs favorable compared to the more conventional alloys. Some of these are (i) high yield and tensile strength and
ductility, (ii) high strengthfracture toughness combination, (iii)
high strain hardening potential, (iv) resistance to deformation
induced martensite formation, (v) low magnetic permeability,
and (vi) favorable corrosion properties (increased pitting corrosion resistance) [1].
A few of the specific applications where LNiASSs are either being applied or considered for use are: (i) bolt materials
for high-strength and high-temperature applications in which
YS values in excess of 900 MPa are required; (ii) superconducting magnet housings which require structural alloys that can
withstand the large magnetic forces of superconducting magnets, have low potential for martensite formation, high elastic
moduli, low thermal and electrical conductivities, and excellent fracture toughness at cryogenic temperatures; and (iii)
wire ropes, springs, ski edges, and railroad wheels [18].
At present, the primary commercial application LNiASSs
is for retaining (end) rings which are shrink fitted into position
over each end of the generator rotor to hold the end windings
in-place on electrical generators. The power generator retaining rings are required to have high YSs (> 1000 MPa), adequate
ductility, high strain hardening potential, low magnetic permeability, and favorable stress-corrosion and pitting resistance. The high strength required for the end rings is generally achieved by cold forming, which can accelerate carbide
nitride precipitation. Thus, it is necessary to use a material
with the lowest susceptibility to precipitation as possible [19].
LNiASS annual production crossed 25,00,000 tonnes and
more than 5,00,000 tonnes are exported. This grade is primarily used today as material for utensil production in India and
countries like South Africa. But, it is used for manufacturing
kettle (animal feeding vessels) in developed countries like UK
and USA.
[6]
[7]
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[9]
[10]
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[13]
[14]
[15]
[16]
[17]
[18]
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