Impact of alloying nitrogen in stainless steel.

1. * Student, Dept. of Mechanical Engineering, PSIT Kanpur
* Student, Dept. of Mechanical Engineering, PSIT Kanpur
# Assistant Professor, Dept. of Mechanical Engineering, PSIT Kanpur
CHARACTERIZATION OF NICKEL FREE
STAINLESS STEEL
Himanshu Bajpai*
, Rohit Raj*
, Bhola Chaurasia#
Abstract---
Stainless steel is defined as a steel alloy with a minimum of
10.5-11% chromium content. It is produced, basically
having three types of structures-ferritic, martensitic and
austenitic. Generally, steels contain nickel as one of the
alloy that increases its hardenability. But due to high cost
and its reactivity with organic matter an alternative i.e.
nickel free stainless steels are developed. High-strength
austenitic stainless steels can be produced by replacing
carbon with nitrogen. Nitrogen has greater solid-solubility
than carbon, is a strong austenite stabilizer, potent
interstitial solid-solution strengthener, and improves pitting
corrosion resistance. The introduction of nitrogen causes
linear increase in yield and tensile strength at room
temperature. The increase in N2 concentration strengthens
the austenitic steels further with the greater planar slip and
higher density of deformation twin. The cold deformation of
these steels later increases their hardness but at the same
time decreases the ductility of these steels. So, these steels
are properly heat treated to regain ductility.
Keywords— Austenite, Martensite, Nitrogen Solubility, Strength,
Hardness, Microstructure, SEM, Biological Implants, Cold
Deformation
I. INTRODUCTION
Stainless steel does not stain, corrode, or rust as easily as
ordinary steel. The Schaeffler diagram does evaluate the
presence of austenite, ferrite, bainite and martensite depending
on chemical composition and ‘proper cooling’ after pouring.
The reality will always be somewhat less austenite because in
the case of austenitic stainless steel cooling will be slower than
required. This diagram is intersecting because, by quantifying
the amount of types of structures, it does give an indication that
the material will comply with the standard [2]. The
development of nitrogen stainless steel with nickel as major
content being replaced by nitrogen and manganese is a matter
of high importance due to restricted availability and high cost
of nickel.
Fig.1-Schauffler Diagram [2]
Alloying with nitrogen has several advantages over
alloying with carbon : (1) Nitrogen is more effective solid
solution strengthener than carbon and enhances grain size
[8,9]; (2)Nitrogen is a strong austenitic stabilizer thus
reduces amount of Ni required for stabilization;(3)
Nitrogen reduces the tendency to form ferrite and
martensites; (4) Nitrogen has greater solid-solubility than
carbon, thus decreasing the tendency for precipitation at a
given level of strengthening [10]; and (5) Nitrogen is
beneficial for pitting corrosion resistance[10]; (6) 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 [11]; (7) Cold deformation can
produce further increases in strength resulting in materials
with yield strengths above 2 GPa [9].
II. NITROGEN SOLUBILITY IN LIQUID IRON BASED
ALLOY
At room temperature, the solubility of nitrogen in liquid
iron is 0.045 wt% at 1600 deg. C. This is a major obstacle
in production [10]. It follows Sievert’s Law which states
that the solubility of nitrogen is directly proportional to
square root of N2 gas pressure over the melt [10, 11].
However, at high pressure there is a deviation from this

2. law because addition of Cr, Mn causes decrease in N2
solubility. Hence, nitrogen solubility in Fe-Cr-Mn steel is
greater than Fe-Cr-Ni steel. Elements helping N2 solubility are
Zr, Ti, V, Nb, Mn as they have strong tendency to form
nitrides. Chromium significantly increases N2 solubility in
liquid iron alloys with a lesser tendency for nitride formation in
solid state [2].
Fig.2-Sievert’s Law [4]
III. EFFECT OF NITROGEN ON STAINLESS STEEL
A. Strength
Interstitial element like N increase the strength of austenitic
steels much more than substitutional elements like W,Mo,V,etc.
Nitrogen increases the strength of austenitic steels but lowers
the ductility and fracture toughness [2]. Fig shows the
strengthening effect of interstitial nitrogen for a series of Fe-
18Cr-13Mn-N alloys. The ultimate tensile strength and yield
strength increase linearly at room temperature with increasing
interstitial N content of the steel. The YS of steel at room
temperature is due to solid solution hardening (Sievert’s Law).
Theoretical tensile strength can be calculated by Pickering
Model [2]
• 0.2 pct YS (MPa) = 15.4[4.4+2 3(C) +1.3(Si) +0.24(Cr)
+32(N) +0.16(δ-ferrite) +0.46d-1/2
]
•UTS=15.4[29+35(C) +5 5(N) + 2.4(Si) +0.11(Ni) +0.14(δ-
ferrite)]
(δ-ferrite and d) neglected
Fig.3-Stress strain diagram for 0.05% N stainless steel [2]
Fig.4-Stress strain diagram for 0.2% N stainless steel [2]
B.) Microstructure
The noticeable micro structural features of annealed and
30% cold-worked sample are as follows:
- Austenite matrix
- Annealing twins in the annealed plate and deformation
twins in the cold-worked ones
- Smaller grain size in cold-worked ones
- Small amount of scattered precipitated particles, possibly
chromiumnitride Cr2N and/or sigma phase
- Absence of deformation-induced martensites
Reportedly, N assists the twinning of austenite on {111}
planes under deformation. Therefore, an increase in N
concentration strengthens the austenitic steels further with
the greater planar slip and higher density of deformation
twin. Fig.2 depicts the microstructures of specimens
subjected to different aging treatments. In specimens aged

3. at 700 C, 800 C, and 900 C, the precipitates were found along
grain boundaries. In specimens aged at 800 'C, the precipitates
start to grow inward austenitic grains. It is obvious that 800 C is
the most sensitive temperature to precipitation. The
microstructure of specimen aged at 950 "C for 96 hour is as
similar to that of the fully austenitic microstructure, which
suggests that the ceiling temperature for precipitation is 950 C,
and no precipitation will occur above this temperature [5].
Fig.5- Microstructure of specimen subjected to different aging treatments-(a)
Annealed Condition (0.05%N, 100X), (b) 30% reduction (0.05%N, 100X), -(c)
Annealed Condition (0.2%N, 100X), (d) 30% reduction (0.2%N, 100X)
C.)Hardness
Hardness essentially depends on the interstitial content as it was
shown by Hsiao and Dulis that the effect of chromium and
Manganese on hardness was minimal. The variation of hardness
(Hv) with cell size (d) of discontinuous precipitation was found
to follow the Hall–Petch relationship:
Hv =¼ Hov + kH.d-1/2
Where Hov and kH are constants
Fig.6 (a)-Effect of Nitrogen on Hardness
0
100
200
300
400
500
600
700
Annealed
C ondition
15%
R eduction
30%
R eduction
Hardness
0.05%
N
0.2% N
Fig.6 (b)-Effect of cold deformation on Hardness
D.) SEM-Scanning Electron Microscopy
SEM examination is carried out on fractured specimens of
the three batches. Fractographs are taken on fracture
surface from different area at various magnifications
ranging from 50X The fractures of the tensile specimens
are observed with the help of SEM. The fracture surface
essentially consisted of dimples indicating that fracture is
due to micro void coalescence which is a characteristic
feature of ductile fracture [2].
Fig.7-SEM fracture of specimen–(a) Annealed Condition (0.05%N,
1000X), (b) 30% reduction (0.05%N, 5000X), -(c) Annealed Condition
(0.2%N, 1000X), (d) 30% reduction (0.2%N, 5000X)

4. IV. EFFECT OF COLD DEFORMATION ON NITROGEN
STEEL
A.)Yield Strength
Cold working is an effective method for increasing the strength
of austenitic steel over solid solution strengthening, since these
single phase austenitic materials are not strengthened by heat
treatment. It is found that increasing nitrogen concentration in a
series of Fe-18Cr-13Mn-N alloys systematically increased the
strength coefficient (K1), thereby increasing the ability of the
austenite to be strengthened by deformation [2].
Fig.8-Yield strength vs. Cold deformation curve
B.)Hardness
Hardness is lowest for the sample which is at annealed
condition and increases as sample is cold rolled. It is highest for
the sample that is 30% cold rolled. The reason for this is the
work hardening which takes place in the sample during cold
rolling. The disadvantage of this is the decrease in ductility.
Thus after cold rolling samples have to be given proper heat
treatment in order to restore ductility of samples.
Fig.9-Curve showing decrease in ductility of specimen i.e. increase in hardness
V.RESULTS AND DISCUSSION
Nitrogen is a very powerful solid-solution strengthening
element, and high-nitrogen austenitic stainless steels can
have yield strengths exceeding those of the standard,
carbon-based, austenitic grades by 200-350%.Yield
strengths can be increased an additional 300% through cold
working.
1) High yield and tensile strength and ductility,
2) High strength-fracture toughness combination,
3) Resistance to deformation- induced martensites
formation,
4) These stainless steels have high hardness
However, high-nitrogen stainless steels are thermally
metastable, and in the absence of strong nitride formers,
such as Ti, V, and Nb, are prone to the rapid precipitation
of Cr2N within a broad temperature regime. Due to their
high contents of chromium, molybdenum, and nitrogen,
these steels are highly corrosion resistant. Therefore, they
do dissolve neither in human organic fluids nor in sweat
and thus do not emit any ions to the human body to any
measurable degree. [1]
Fig.10-Influence of nitrogen on the temperature sensitivity of flow stress
in austenitic stainless steel
VI. APPLICATIONS
Due to non magnetic property of austenite steels it is used
in making household utensils. Industrial applications where
high-nitrogen austenitic stainless steels may be utilized
include the power-generating industry, ship building,
railways, cryogenic processes, chemical equipment,
pressure vessels, and the petroleumand nuclear industries.

5. A few of the specific applications where high-nitrogen stainless
steels are either being applied or considered for use are: (1) bolt
materials for high-strength and high-temperature applications in
which YS values in excess of 900 MPa are required; (2)
superconducting magnet housings which require structural
alloys that can withstand the large magnetic forces of
superconducting magnets, have low potential for martensites
formation, high elastic moduli, low thermal and electrical
conductivities, and excellent fracture toughness at cryogenic
temperatures; and (3) wire ropes, springs, and railroad wheels.
In medical fields, it is used for biological implants due to low
reactivity with the organic fluids [1].
VII.FUTURE SCOPE
HNS find applications in chemical industry, transportation, ship
yard biomaterials, sporting equipment, environment-friendly
technology. Hard and wear resistant HNS are likely to be used
for cutting tools or for erosion-wear resistant parts. Many
applications for HNS are likely to emerge such as, high
temperature-resistant alloys (in case the Ti Zr, V, Nb, Cr, Mo
content is high), nanostructured HNS, composite nitricermet,
parts of actuators, some shape-memory alloys, nitrides with
very high saturation magnetization , gradient materials based on
the diffusion of N2 under chemical gradient or stable stress
field, wear resistant coating, thermal barrier coatings, creep
resistant NDS (nitride dispersion steel) and NDA (nitride
dispersion alloy) based on similar principle to ODS (oxide
dispersion steel) and superplastic HNS and nitrogen-enriched
alloys for which the grain size has to be kept at a nanometric
size. When nitrogen alloying does not lead to a clear
improvement or to cost reduction as compared to well-known
classical steel grades, the motivation for developing new grades
should be limited. It is reasonable to foresee applications of
HNS and HNA (high nitrogen alloys) to take advantage of a
specific property improvement resulting from nitrogen
addition.[12]
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