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CHAPTER 2
LITERATURE SURVEY
2.1 GENERAL
A major challenge in technological development has been the need
to continuously meet the stringent materials requirements for use in
progressively demanding conditions. Exposure to increasingly aggressive
environments has made modern engineering components more susceptible to
rapid degradation due to interactions between the component surface and the
environment. In the material-environment configuration, the surface of a
component serves as the first line defence against the attack by an adverse
environment. Therefore, the component surface is always of vital importance
in determining its performance and durability. This has formed the basis for
improving the material properties by adopting the surface modification
approach. This approach involves the formation of an appropriate protective
coating on the surface to impart the desired property of combating premature
degradation of the component. Generally the surface modification approach,
which is also referred to as “surface engineering”, enables the achievement of
properties that neither the bulk nor the coating is capable of imparting on its
own. Surface engineering is a multi disciplinary subject which deals with the
modification of the surface properties of engineering components in order to
improve their function and service capabilities (Kenneth G. Budinski 2001).
21
2.2 SURFACE ENGINEERING
Surfaces are the bounding faces of solids. The interaction of
component surface with the working environment results in wear and
corrosion. Surface engineering is a discipline that develops methods for
combating wear, corrosion and other surface degradation phenomena (Davis
2001; Marchione 2002). Surface engineering is a generic term now applied to
a large field with diverse technologies that can be gainfully harnessed to
achieve increased reliability and enhanced performance of degradation-prone
industrial components. The incessant quest for higher efficiency and
productivity across the entire spectrum of manufacturing and engineering
industries has ensured that most modern-day components are subjected to
increasingly harsh environments during routine operation. Critical industrial
components are, therefore, prone to more rapid degradation as the parts fail to
withstand the rigours of aggressive operating conditions and this has been
taking a heavy toll of the industry’s resources. In an overwhelmingly large
number of cases, the accelerated deterioration of parts and their eventual
failure has been traced to material damage brought about by hostile
environments like high relative motion between mating surfaces, corrosive
media, extreme temperatures and cyclic stresses. Simultaneously, research
efforts focused on the development of new materials for fabrication are
beginning to yield on the development returns and it appears unlikely that any
significant advances in terms of component performance and durability can be
made only through development of new alloys. It is vital in every product or
processes where a component or a system experiences “use” whether an
engine component, a bridge or building, a chemical plant component or power
plant components and it also refers to the control of problems originating
from the surface of engineering components (Bathelor et al 2002).
22
As a result of the above, the concept of incorporating engineered
surfaces capable of combating the accompanying degradation phenomena like
wear, corrosion and fatigue to improve component performance, reliability
and life has gained increasing acceptance in recent years. The recognition
that a vast majority of engineering components fail catastrophically in service
through surface related phenomena has further fueled this approach and led to
the development of the broad interdisciplinary subject of Surface Engineering.
Generally, the solution is special treatment of the surface to delay or even
prevent corrosion, erosion, fatigue, abrasion or adhesive wear. This special
treatment may be the application of a coating or a modification of the surface
(Alam and Ion 2002). A protective coating deposited to act as a barrier
between the surface of the component and the aggressive environment that it
is exposed during routine operation is now globally acknowledged to be an
attractive means to significantly reduce and suppress damage to the actual
component by acting as the first line of defence. The increasing utility and
industrial adoption of surface engineering is a consequence of the recent
advances in the field. Surface Engineering today is the best defined as “The
design of surface and substrate together as a system to give a cost effective
performance enhancement of which neither is capable on its own”.
2.2.1 Advantages of Surface Engineering
In brief, surface engineering is relevant to all types of products. It
increases the performance and controls the surface properties independently
of the substrate offering enormous potential for improved functionality. It also
provides solution to previously insurmountable engineering problems, the
possibility to create entirely new parts, conservation of scarce material
resources and reduction in power consumption and waste output. The success
of the surface engineering is demonstrated by the applications of surface
23
technologies like thermo chemical treatments and coatings to a variety of
engineering components.
In the past years the role of surface engineering has become a well
recognized area for ensuring enhanced materials performance particularly in
severe environments (Bhuvaneswaran et al 2003) and also because of the
growing commercial requirements of wide range of cost effective surface
technologies (Bell 1992). Surface engineering of components are subjected to
higher stress and greater fatigue, abrasion and corrosive damages than the
interior. Therefore more than 90% of the service failures of engineering
components initiate at or near the surface (Gopalakrishnan et al 2002)
2.3 SURFACE MODIFICATION / TREATMENT
By virtue of the versatility of this approach, everyone is confronted
daily, either directly or otherwise, with products that have been modified for
functional or decorative reasons. Surface engineering is now applicable to a
wide range of industry sectors and the end purpose of a surface modification
process could be manifold. There is growing realization that the surface
modification approach does not only solve component degradation problems
but can provide added value. This could be in the form of significant
improvement in productivity due to less frequent repairs and consequent
down time. The development of surface engineering has been dynamic largely
on account of the fact that it is a discipline of science and technology that is
being increasingly relied upon to meet all the key modern-day technological
requirements such as materials savings, enhanced efficiencies, environmental
friendliness etc. The overall utility of the surface engineering approach is
further augmented by the fact that modifications to the component surface can
be idealized through metallurgical, mechanical, chemical or physical ways
(Groover 2002). At the same time, the engineered surface can span at least
five orders of magnitude in thickness and three orders of magnitude in
24
hardness. The development of a suitable high-performance coating on a
component fabricated using an appropriate high mechanical strength
metal/alloy offers a promising method of meeting both the bulk and surface
property requirements of virtually all imagined applications. The newer
surfacing techniques, along with the traditional ones, are eminently suited to
modify a wide range of engineering properties. The surface properties of the
metals can be improved by changing the chemical compositions of the metal
matrix at the surface level by thermo chemical processes like carburizing,
nitriding, boriding (Wahl 1996) or by hard coatings on the surface, induction
hardening and ion-impregnation on the surface (Kamat 1995). The properties
that can be modified by adopting the surface engineering approach include
tribological, mechanical, thermo mechanical, electrochemical, optical,
electrical, electronic, magnetic / acoustic and biocompatible properties.
2.3.1 Surface Modification / Treatment Methods
Driven by technological need and fuelled by exciting possibilities,
novel methods for applying coatings, improvements in existing methods, and
new applications have proliferated in recent years. Surface modification
technologies have grown rapidly, both in terms of finding better solutions and
in the number of technology variants available, to offer a wide range of
quality and cost. Typically there are a large number of variants in each of the
coating processes available. The significant increase in the availability of
coating processes of wide ranging complexity that are capable of depositing a
plethora of coatings and handling components of diverse geometry today
ensures that components of all shapes imaginable can be coated economically.
The surface engineering options are ranging from traditional, well established
techniques to the more technologically demanding coating technologies and
surface treatments. Existing surface treatment processes fall under three broad
categories:
25
(a) Overlay coatings: This category incorporates a very wide
variety of coating processes wherein a material different
from the bulk is deposited on the substrate. The coating is
distinct from the substrate in the as-coated condition and
there exists a clear boundary at the substrate/coating
interface. The adhesion of the coating to the substrate is a
major issue. (eg. Electroplating)
(b) Diffusion coatings: Chemical interaction of the coating-
forming element(s) with the substrate by diffusion is
involved in this category. New elements are diffused into
the substrate surface. Usually at elevated temperatures, the
composition and properties of the outer layers are changed
as compared to the properties of the bulk. (eg. Nitriding)
(c) Thermal or Mechanical modification of surfaces: In this
case, the existing metallurgy of the component surface is
changed in the near-surface region either by thermal or
mechanical means, usually to increase its hardness.
(eg. Flame hardening)
A variety of surface modification techniques are currently available
and their abilities differ substantially in several aspects like (i) type of
coatings (ii) coating thickness achievable (iii) size of components coated
(iv) component geometries / accessibility of surfaces to be coated and
(v) substrate temperature during coating. Apart from the above processing
issues, the coating characteristics in terms of porosity levels, bond strength,
hardness etc., also vary considerably for each technique. As a consequence, a
single coating technology may not be the most useful, desirable or economical
for all the coating requirements that emerge. This is often dictated as (a)
function for which coating is required (b) location of surface to be coated and
26
its accessibility (c) temperature of the part in use (d) coating thickness
required (e) surface finish desired (f) dimensional tolerances and (g) the cost.
In view of the above, the advantages as well as limitations of all the available
coating options and their utility need to be fully appreciated in order to be
able to make an intelligent choice of the most suitable process to meet any
arising need for a surface coating.
With the present day availability of a wide spectrum of surface
modification technologies, the surface of a component can be virtually
“tailored” by selecting a proper surfacing method in order to meet the
required properties without compromising the mechanical characteristics of
the bulk material. Surface engineering, however, is not confined merely to
improve the component properties against surface related degradation
processes such fatigue, corrosion, wear etc. This approach can also be applied
for cases where specialized surface properties that are not necessarily related
to surface protection and /or performance enhancement of the component, are
desired as exemplified by the category of abradable coatings for engineering
applications which are sacrificial in nature (Das 2003).
Surface modification methods also prove vital in saving /
substituting valuable and scarce bulk materials and for reclaiming damaged
components. Furthermore this approach provides the designers considerable
flexibility in use of materials. All the above mentioned benefits of surface
modification methods translate into significant cost effectiveness. Numerous
surface modification techniques have been developed over the years and the
designer today has the advantage of choosing from a broad array of surfacing
technologies that offer a wide range of quality and cost. Several attempts have
been made in the past decades to modify the surface of the stainless steels to
enhance their surface hardness and tribological properties. Thermo chemical
surface engineering such as nitriding and nitrocarburizing proved to
27
significantly improve the surface hardness and wear resistance of austenitic
stainless steels (Bell 2002).
2.4 NITRIDING
Nitriding is a ferritic thermochemical treatment process which
involves diffusing atomic nitrogen in to surface of metals to produce a hard
case. This diffusion process is based on the solubility of the nitrogen in iron
as shown in the equilibrium diagram (Figure 2.1). The solubility limit of
nitrogen in iron is temperature dependant and at 450˚C the iron base alloy will
absorb upto 5.7 to 6.1% N (Pye, 1939). As the temperature is further
increased beyond the gamma prime (') phase temperature at 490˚C, the
“window” or limit of solubility begins to decrease at a temperature 650˚C.
The equilibrium diagram (Figure 2.1) shows that control of the nitrogen
diffusion is critical to process success. This process holds significant role in
the industrial applications, among the processes available for improving the
surface properties of engineering components (Kurney et al 1983).
Figure 2.1 Iron-Nitrogen equilibrium diagram (Pye 1939)
Tem
pera
ture
28
This process of case hardening was first used at the end of 1920s and since then its application has continuously spread among other things
owing to the fact that the process has been further developed and can now be applied to much larger varieties of steels than originally thought possible. The more important effect of nitriding is the ability to form hard compounds with iron. These compounds can give rise to the surface hardness as maximum as possible.
The beneficial properties imported by nitriding to steels can be summarized as follows.
High surface hardness and wear resistance together enhance
anti-galling properties.
Increased high temperature hardness / Enhanced Hot hardness
High fatigue strength and low fatigue sensitivity
Improved corrosion resistance.
High Dimensional Stability
It is well known that nitriding produces a case of moderate hardness due to the diffusion of nascent nitrogen quickly beneath the surface. The
quick diffusion forms hard complex iron-nitrides dispersed to greater depths. Since nitride forming elements have a greater affinity for nitrogen they
prevent the diffusion to a greater depth giving an extremely hard but shallow case (Ashrafizadeh 2003). Nitriding is used to confer both wear resistance and fatigue resistance on engineering components. The advantage of nitriding over the other surface hardening methods is the reduced risk of distortion
through the treatment temperature and the elimination of quenching. The notable limitation is the formation of a brittle white layer which is to be removed for many applications before a nitrided component can be put in service (Clayton and Sachs 1976).
29
Formation of the nitrided layer case begins through a series of
nucleated growth areas on the steel surface. The nucleating growth will
eventually become what is known as the compound layer (Pye 1939). This
layer is usually very hard and brittle and consists of two intermixed phases as
ε (epsilon) phase containing Fe2-3 N and γ' (gamma prime) phases containing
Fe4N (Davis 2002). This layer does not diffuse into the steel, but remains on
the immediate surface and grows thicker with time, temperature and gas
composition. The layer immediately beneath the compound zone is called as
the “diffusion zone” (Figure 2.2). This region is made up of stride nitrides
formed by the reaction of nitrogen with the nitride forming elements such as
Cr, Mo and Va. It is from this layer the fatigue and load bearing strength are
determined and the area below this zone is the core of the metal (Davis 2002).
The area below the diffusion zone is the core of the metal which comprises
formation of nucleation of γ' at the immediate steel surface interface with the
nitriding atmosphere. This nucleation process progresses and continues until
subsequent nucleation of ε at the steel surface interface.
Figure 2.2 Typical nitrided case (Pye, 1939)
30
2.5 METHODS OF NITRIDING
Nitriding is effected in a salt bath nature (liquid nitriding), in a
gaseous medium (gas nitriding) or in a plasma state atmosphere (plasma
nitriding).
2.5.1 Liquid Nitriding
Liquid nitriding (or) salt bath nitriding process as an alternative to
the gas nitriding process was sought in the mid 1930s, which would produce a
more uniform and better metallurgically formed case. A liquid would fulfill
the uniformity requirement through surface contact of the liquid to the steel.
The depth and quality of the case would be determined by the chemical
composition of the liquid. A heat source would be necessary to drive the
nitrogen into the steel surface. Salt bath nitriding utilizes the melting of salt
containing rich nitrogen source. When heat is applied from either an internal
or external source the salt melts and liberates nitrogen into the steel for
diffusion. When the steel work piece is introduced into the salt bath and
heated upto a temperature in the molten salt, controlled amounts of nitrogen
are released to diffuse into the surface. The salt bath nitriding technique
gained popularity in the early 1950s, because it required a low capital
investment than gas nitriding.
Liquid nitriding is carried out in a molten salt bath in a temperature
range of 783 – 893K. The case hardening medium in this method is molten
nitrogen bearing fused salt bath containing cyanides and cyanates (Mehrkam
et al 1991). Formation of pores which occurs during salt bath nitriding
increases with the increase in time. This restricts the nitriding time in the
liquid process to maximum of 4 hrs. Salt bath nitriding is a very active
process and more intense than the gas nitriding and plasma nitriding processes
(Funatani 2004).
31
The typical commercial bath for liquid nitriding is composed of a
mixture of sodium and potassium salts. The sodium salts which contribute
60-70 wt% of the total mixture consist of NaCN, NaCO3, and NaCNO. The
potassium salts contribute to 30-40% of the total mixture comprising KCN,
K2CO3, KCNO and KCl. The liquid bath initially aged at a selective time-
temperature for combination to increase the cyanates content and then parts
are immersed in the liquid bath for further processing.
Due to the intermetallic composition of the compound layer,
friction and the tendency to weld with the metallic counterpart are reduced.
Salt bath nitrided components exhibit excellent sliding and running in
properties as well as greater wear resistance. Salt bath nitriding improves
wear resistance, lubricity, fatigue strength and corrosion resistance as a result
of the presence of iron nitride compounds formed at the surface in addition to
a zone of diffused nitrogen in solid solution with the base metal adjacent to
the compound layer (Easterday 2001). Both of these zones are metallurgically
discernible, each providing specific engineering properties like antigalling,
antiseizing characteristics and reduced tendency for fretting corrosion. Salt
bath nitriding is an effective and economical means to enhance performance
of engineered components made up of ferrous metals.
2.5.2 Gas Nitriding
Gas nitriding is a casehardening process whereby nitrogen is
introduced at surface of a solid ferrous alloy by holding the metal at a suitable
temperature in contact with a nitrogenous gas, usually ammonia (Knerr et al
1991). The nitriding temperature for all steels is between 763K and 863K.
Because of the absence of a quenching requirement with volume changes, and
the comparatively low temperature employed in this process, nitriding of
steels produces less distortion and deformation than either carburizing or
32
conventional hardening. Some growth occurs as a result of nitriding but
volumetric changes are relatively small.
In gas nitriding, anhydrous ammonia is allowed to flow over the
parts to be hardened at 823 K. The ammonia gas dissociates into nitrogen and
hydrogen at the part surface. At the instant of dissociation, nitrogen is
liberated in atomic form and is absorbed by materials as such whereas the
hydrogen becomes the part of atmosphere in the furnace. The depth of
hardening and extent of case hardness developed due to gas nitriding depends
on several factors such as time-temperature schedule of the process, degree of
dissociation of ammonia, concentration and nature of nitride forming
elements etc. The major limitation of gas nitriding is longer process time
which gives rise to the increased cost of the process. Gas nitriding has never
been developed to its fullest potential due to the long process times, increased
material and treatment cost with lack of sufficient control over the process
(Staines 1996).
Gas nitriding becomes extremely handy, when a greater depth of
nitriding is required. For the same depth of hardening, gas nitriding causes
less dissociation of the hardened parts than the liquid nitriding. In terms of
wear resistance and toughness of hardened steels, gas nitriding is believed to
have slight edge over liquid nitriding.
2.5.2.1 Single and Double Stage Nitriding
Either a single or a double stage processes may be employed when
nitriding anhydrous ammonia. In the single stage process, a temperature in
the range of about 768 K to 798 K is used and the dissociation rate ranges
from 15 to 30%. This process produces a nitrogen rich layer known as the
white nitride layer which is extremely hard but very brittle. The double stage
process has the advantage of reducing the thickness of the white nitrided
33
layer. The first stage of the double stage process is a duplication of the single
stage process except for time. The second stage may proceed at the nitriding
temperature employed for the first stage or the temperature may be increased
from 823 K to 838 K. However, at either temperature, the rate of dissociation
in the second stage is increased to 65 to 80% (preferably 75 to 80%).
Generally an external ammonia dissociator is necessary for obtaining the
required high second stage dissociation.
The main purpose of double stage nitriding is to reduce the depth of
the white layer produced on the surface of the case. Except for a reduction in
the amount of ammonia consumed per hour, there is no advantage in using the
double-stage process unless the amount of white layer produced in single
stage nitriding cannot be tolerated on the finished part or unless the amount of
the finishing required after nitriding is substantially reduced.
2.5.3 Plasma Nitriding
The plasma nitriding process patterned by Egan (1931) is a well
demonstrated surface hardening process, which provides wear, fatigue and
corrosion resistant surfaces (Prabhudev 1998 and Baldwin et al 1998). The
plasma nitriding is a method of surface hardening using glow discharge
technology (ionized gas) to introduce nascent nitrogen into the surface of a
metal part for subsequent diffusion into the material (O’Brien and Goodman
1991). Depending on the process parameters and material composition a
diffusion zone is formed with nitrogen penetrating upto 0.7mm into the
surface. Frequently a surface compound zone is also formed on the top of the
diffusion zone with micron range thickness. In the diffusion zone the
microstructure is changed by the introduction of single intensified N atoms in
to solid solution and when solubility limit is reached, very fine coherent
nitride precipitates are formed.
34
The hardness is slightly changed by the nitrogen in solid solution,
while the hardness increases substantially when the nitrogen is in the form of
nitride. Precipitates form depending on the nitride forming alloying elements.
In the compound zone, intermetallics and nitrides of the alloying '- Fe4N, -
Fe2-3N, and Cr2N, CrN are formed. The advent of plasma nitriding (Edenhofer
1974) has led to an increased interest in the application of the process to
various alloy steels for obvious advantages offered by these materials. Such
treatment would modify the surface related properties such as resistance to
applied loads, adhesive and abrasive wear, rolling contact fatigue and
corrosion.
2.6 NITRIDNG OF AUSTENITIC STAINLESS STEELS
Nitriding is a proven technology used to improve the surface
properties, providing fatigue life, wear and corrosion resistance enhancement
of components made of austenitic stainless steels. The uptake of nitrogen onto
the surface could produce several modifications. As the nitrogen atomic
concentration grows in the region and solid interstitial solution of N exceeds
the solubility limits, precipitation of iron chromium nitrides would occur. A
more selective formation of some nitrides depends on the process parameters
like gas composition, nitrogen ion density, treatment temperature and time.
Among the most desirable nitrides to be synthesized in the compound layer,
are '-Fe4N, -Fe2-3N, N, CrN, and Cr2N. etc. These nitrides have good
tribological properties like enhanced hardness, reduced wear rate, friction and
corrosion resistance. The nitrogen ions driven by thermally activated diffusion
could penetrate far beyond the compound layer.
2.6.1 Surface Characteristics studies
Zhao et al (2005) have found that nitrogen and carbon atoms can
simultaneously be dissolved into the austenite lattice during the
nitrocarburizing process, forming a nitrogen and carbon supersaturated solid
35
solution, which has a Face Centre Tetragonal (FCT) structure (i.e. the S
phase). Glow Discharge Spectroscopy (GDS) analysis showed that the
maximum nitrogen and carbon concentrations in the nitrocarburised S-phase
layer occur at different depths from the surface. The nitrocarburised layers not
only had high hardness like the nitrided layer, but also high thickness and
gradually reduced hardness profile like the carburized layer.
Picard et al (2001) have reported that the nitrogen implantation
process induces the formation of the metastable γN phase. This γN phase
resulted from the incorporation of nitrogen into the interstitial positions of the
face centered cubic structure of stainless steels. The ion-implanted specimens
showed a significant increase in the hardness value originating from the γN
phase with compressive residual stresses and micro distortions in the high
nitrogen containing layer. From the hardness measurements and Glow
Discharge Optical Spectroscopy (GDOS) profiles, it was evident that nitrogen
diffused into steel to a depth of 3 mm. The presence of nitrogen improves the
corrosion behaviour in acid media and the stainless character was maintained.
In chloride media, the very high nitrogen content (30 at. %) modified the
repassivation process in such a way that the potential range in which
repassivating pitting occurs was broadened.
The effect of plasma nitriding time on the microstructure and phase
composition of nitrided layers on austenitic stainless steel was investigated by
Liang Wang et al (2006). The phase composition and structure of the nitrided
layer have been studied by X-Ray Diffraction (XRD) and Scanning Electron
Microscopy (SEM). The XRD analysis of samples treated at 693 K showed
the presence of γN phase in the nitrided layers for all nitriding times involved
in this study. The lattice parameters calculated based on γN (111) and γN (200)
were different and became larger with time for up to 5hrs of nitriding
treatment. The surface hardness of nitrided layer was also increased with
36
nitriding time. The maximum thickness of the nitrided layer was observed to
be 27 µm.
The microstructure of the layers produced by plasma nitriding
austenitic stainless steel at treatment temperatures 673 and 773 K were
studied by Xu et el (2000) with Scanning Electron Microscopy (SEM)
together with X-Ray Diffraction (XRD) and Transmission Electron
Microscopy (TEM). The results showed that the microstructures were
composed of ‘expanded austenite’ (γn) and α (ferrite) + CrN following plasma
nitriding at lower and higher treatment temperatures, respectively. The former
contained stacking faults and deformed twin substructures, while the latter
was made up of colonies displaying a lamellar structure.
Baranowska and Powel (2007) have investigated the mechanical
properties of thin nitrided layers produced on stainless steels and reported that
a hard nitrided layer of hardness 5 times harder than the matrix was observed
at the surface level. Brief nitriding of low carbon stainless steels makes it
possible to obtain even higher quality case with a higher hardness. The case
hardness varies with dispersity of the nitrides and the maximum hardness is
attained when the nitrides are in the precipitation stage ensuring the largest
distortion of the matrix lattice (Yakhnina and Mescherinova 1974). Plasma
ion nitrides of 316L grade stainless steel for a time of 8 hrs at a process
temperature of 723 K produced nitrogen expanded austenite (N) layer with a
thickness of 6µm and average hardness values of about 1800 Hk0.01 (Linda
et al 2006).
Priest et al (1999) have reported in their study that low pressure RF
plasma nitriding of austenitic stainless steels resulted in a surface layer of
2mm thick with a fairly constant nitrogen concentration and very thin
interface region. The X-ray diffractograms for the samples around (111) and
(200) austenite peaks were barely seen at an incidence angle of 2 theta. The
37
tribological and corrosion resistance properties of austenitic stainless steels
enhanced by the formation of austenite phase was investigated (Gontijo et al
2003). In this work, it was reported that the modified layer consists of an
austenite phase with different nitrogen content and ' - Fe4N and - Fe2-3N
phases were instrumental in enhancing the surface properties of austenitic
stainless steels.
A novel electrochemical surface modification for high nitrogen
stainless steels, which significantly lowered the wear rates under self mating
conditions, was presented (Buscher and Fischer 2003). It was also suggested
that this process might be a suitable treatment for one of the sliding partners
in all metal prostheses.
Nitriding of AISI 304 grade austenitic stainless steels using Plasma
Immersion Ion Implantation resulted in a uniform layer composed of ()
gamma expanded phase and compounds such as Fe3N. The hardness was
observed to be incremented several times compared to the untreated samples
due to the lattice expansion created by the super saturation of nitrogen
(Valencia 2004). The nitriding of austenitic stainless steels at 823K for 3 hrs
resulted in a compound layer of 44 µm thick with a hardness of 1434 Hv0.01,
consisting predominantly of γ'-Fe4N and CrN phase (Baggio-scheid et al
2006).
The effect of plasma nitriding time on the microstructure and phase
composition of nitrided layers of AISI304 stainless steels was investigated by
Liang et al (2006). The XRD analysis of the nitrided layer showed the
presence of γN phase with broad peaks of (111) and (200). An expanded
austenite layer was formed on the surface of substrate with the thickness
ranging from 2µm to 27µm. The hardness was enhanced upto a maximum of
1250Hv by the formation of nitrided layer due to the nitrogen diffusion
into substrate.
38
Plasma nitriding of stainless steels at temperature about 773K
showed nitrogen enriched layers with higher nitrogen content leading to a
significant increase in the hardness (Larisch et al 1999). The XRD patterns of
austenitic steels exhibited that a set of broad peaks associated with a
metastable phase called “expanded austenite” together with the substrate
austenite peaks. The study of sub surface modifications induced by nitrogen
ion implantation revealed that the surface hardness reached upto 600Hv
(Guemmaz et al 1998). From the XRD study it was also observed that
nitriding induced phases like ε – Fe2-3N and α – Fe besides the predominant
γN phase which remains most important.
In the study of pulsed DC glow discharge nitriding with optimal
process conditions, a compound layer of 50 m thickness with a maximum
hardness of 1400Hv0.1 was observed. From the XRD study, it was observed
that in addition to the γ'-Fe4N phases, CrN and Cr2N phases were also seen in
the compound zone of nitrided austenitic stainless steels (Berg et al 2000).
Nitrogen implantation enhanced the mechanical and surface modifications.
The hardness was improved to a maximum value upto 800Hv. The
microstructural analysis through glancing incidence of X-Ray diffraction,
showed iron-nitride (ε – Fe2-3N) formation and high level of residual stresses.
The verification of glancing angle indicated that the residual stress was non
homogeneously distributed through the depth (Pelletier, 2002). The study on
the nitriding of AISI 316 stainless steels by low pressure RF plasma nitriding
(Baldwin et al 1998) exhibited a nitrided layer containing f c c γN (111)
phase. The micro hardness of nitrogen enriched expanded austenite phase was
found to be about 700Hv with a layer thickness of 40µm.
The surface modification of austenitic stainless steel has been paid
close attention by many researchers. Xiaolei et al (2005) reported that the
peaks of phases induced by nitriding were considerably broadened and shifted
39
to lower diffraction angles when compared with the peaks of untreated
samples. This shift and broadening of phases were associated with the
expanded austenite (N) produced by nitrogen super saturation and associated
stress caused by nitrogen present in solid solution in the f c c lattice.
A combined surface modification treatment consisting of ion
nitriding at high pressure and higher temperature followed by a cycle at low
pressure using a gas mixture at H2-N2 was applied to austenitic stainless steel.
The resulting thick modified layer presented mainly the presence of expanded
austenite (N) phase and mixed iron – chromium nitrides, with a surface
hardness of 2.7 times higher of standard untreated surface value (Gomes et
al 2003). Gaseous thermo chemical treatment of austenitic stainless steels at
moderate temperature would give rise to the transformation of the surface
adjacent region into nitrogen expanded austenite (Christiansen and Somers,
2006). The hardness depth profile showed a smooth decrease near the surface
followed by a sharp drop in hardness at a position close to the surface /
substrate interface. A maximum hardness value of about 1585Hv was
obtained directly at the surface. It was evident that the nitrides of nitrided
layer are very hard and the transition from the hardened zone to the soft
substrate occurred within a few microns. The diffractograms for the nitrided
samples exhibited clearly the shift of (peaks) Bragg reflections to lower
angles, mainly due to the dissolution of nitrogen.
The influence of nitriding temperature time and gas mixture ratio of
Nitrogen to Hydrogen on the nitriding behaviour of chromium plated type
316LN stainless steels have been investigated (Kuppusami et al 2002). The
results indicated that the nitriding temperature plays a dominant role in
obtaining hardness and case depth in this material. A reasonable value of
hardness of 550Hv with a case depth of 5µm was obtained when the nitriding
temperature was about 833K. The hardness have reached a maximum value of
40
1800Hv at 1273K with a case depth of 40 m. Nitriding at an intermediate
temperature of 913K produced a peak hardness of about 900Hv with a
uniform decrease in hardness or a function of depth and hard surfaces with
excellent bonding with the matrix.
The microstructure of nitrocarburised and post oxidized 304L
austenitic stainless steels produced by Quench Polish Quench (QPQ) complex
salt bath process was investigated (Gui jiang 2008). The nitrocarburised
coating as a whole composed of 3 sub layers namely the CrN layer, the
intermediate γ'-Fe4N layer and an inner expanded austenite (N') layer.
The XRD analysis of the post oxidized sample revealed the
existence of a small concentration of ε – Fe2N sub layer. A modestly higher
sub surface hardness and an overall superior hardness profile were obtained
by post oxidizing. The measured hardness for post oxidized samples was
fairly uniform and reached a maximum value of 1200 Hv0.1 with a thickness
of 35 m and then decreased sharply with in a further depth of 20m.
The study on the crystal structure and micro hardness of
carbonitrided layers produced by RF plasma processing of austenitic stainless
steels exhibited the predominance of new phases like Fe2-3N and Cr2N (El-
Hossary et al 2001). These phases were responsible for the significant
increase in the hardness which related to the maximum value of about
1715 Hv0.01 with a case depth of 20m. Plasma processing resulted in a
uniform surface layer of hardness about 750 Hv0.05 along with an effective
hardening depth of 40µm (Suh and Lee 1997).
Recco et al (2007) investigated High Temperature Gas Nitriding
(HTGN) of austenitic stainless steel specimen and reported a hardened case of
1.2 mm thick with enhanced hardness. It was also found that the high
temperature gas nitriding reduced the wear rate of austenitic stainless steels
41
by the order of 1.5 magnitudes. The hardness, after Plasma Immersion Ion
Implantation (PIII) treatment on austenitic stainless steels reached the
maximum value of about 1200Hv with better corrosion resistance than the
base material (Mandl et al 2005). The effect of a metastable nitrogen super
saturated fcc phase, when applied as a coating using a reactive magnetron
sputtering on the corrosion wear resistance of austenitic stainless steel was
investigated by Dearnley (2002). The results showed a considerable increase
in surface hardness value to a maximum of 1600Hv. It was also reported that
corrosion wear of metastable phase reduced with the increase in nitrogen
concentration of the coating.
Usta et al (2004) studied the effect of nitriding on the properties of
surgical AISI 316L stainless steel in a fluidized – reactor at a temperature of
833K for 16 hrs. The hardness of the nitrogen rich layer of 40 m thick was
about 1200Hv and the XRD analysis confirmed the presence of nitrides such
as CrN, Fe2-3N and Fe4N on the surface layer. The hardness of nitrided
specimen decreased with distance from surface to the interior of this
specimen. Scanning Electron Microscopic study combined with energy
dispersive X- ray spectroscopy revealed the presence of nitrogen in the outer
layer and in the transition layer but not in the base metal. It was also reported
that surface design with improved hardness and corrosion resistance is
possible by sequential plasma processing process (Tsujikawa et al 2005) with
the hardness of diffused layer increased to a maximum value of about
1050Hk.
Dima and Cazacu (2006) investigated the effect of nitridation on
various grades of austenitic stainless steels. In their experiments they have
carried out the surface hardening in a fluidized bed, where a gaseous
atmosphere of N2 mixed with dissociated 30% NH3 was maintained at a
42
temperature of 823K. They found that the nitridation resulted in the strong
increase in a hardness upto 1314Hv0.05 and case thickness about 40m.
The microstructure of the plasma nitrided austenitic stainless steel
specimen was studied elaborately by Xiaolei et al (1998). The results
indicated that the surface of the modified layer consists of supersaturated
solid solution (N) based on the -Fe4N) phase. Plasma immersion ion
implantation on vacuum melted 316L grade austenitic stainless steels resulted
an increase in surface hardness with better wear resistance and increased
corrosion resistance (Saravanan et al 2007). It was also reported that the
broad XRD peaks were observed in the X-ray diffractograms due to the
presence of the mixed nitrides like FeN, ' – Fe4N.
The incorporation of nitrogen in stainless steel is widely applied to
provide major improvements in material performance with respect to fatigue,
wear, tribology and atmospheric corrosion. These improvements rely on
modification of the surface and the sub surface of the material by the
precipitation of alloying element nitrides or by the development of continuous
layer of iron based nitrides (Somers and Christiansen 2007). Nitriding at
823 K for 48 hrs in ammonia and ammonium chloride environment resulted in
a diffusion layer of 0.25-0.3mm thick with a hardness of about 820 – 850 Hv
and magnetic permeability above 1.05 G/Oe (Nikonerova and Florensova
1965).
2.6.2 Wear Studies
It was reported that ion nitriding improves the wear resistance of
austenitic stainless steel type AISI 316L. The nitrided layer had hardness
around 1200 Hv, appeared to be uniform and formed by two sub layers. These
nitrided layers reduced the subsurface plastic deformation on a rolling-sliding
test at 50 kg normal load. The applied contact stress produced a significant
43
plastic deformation on the surface and subsurface, except on the nitrided
samples tested at 50 kg load. Such deformation, observed through the micro
hardness profiles and micrographs showed that at this load level, the stresses
at the nitrided layer were lower than its yield strength because at the end of
the test, the nitrided layer still remained (Cabo et. al. 2005). The effect of
sliding wear on tribocorrosion behavior of austenitic stainless steels of grade
AISI 304L was studied and reported that the sliding wear affected the
repassivation by increasing the anodic current and greatly influenced by the
increase in normal force (Berradja et al 2006).
Singh et al (2002) processed AISI 316L grade stainless steel using
Intensified Plasma Assisted Processing (IPAP) technique and reported that
IPAP processed samples exhibited a single phase nitrided layer with a four
fold increase in the surface hardness, significantly lowered coefficient of
friction with lesser wear rate. Also it was reported IPAP showed no adverse
effect in the corrosion resistance. The compound layer formed by
corbonitriding with pulsed glow discharge process resulted in increased
hardness upto 1250Hk0.25 and exhibited the presence of -Fe3-2N, ' – Fe4N,
Fe3C and CrN phases (Gontijo et al 2003).
The wear behaviour of gas nitrided austenitic stainless steel in a
corrosive liquid environment showed that wear performance was superior.
The nitrided layers were mainly composed of γN phase. In the samples ,
nitrided at 723K, an increased quantity of nitrides mainly Cr2N and Fe2N were
found in the surface area (Baranowska et al 2007). The ion nitriding of 304L
and 316L grade austenitic stainless steel specimen resulted in a diffused layer
of thickness about 13 µm along with surface hardness of about 1060 Hv0.025
and significant improvement in wear resistance (Poirier et al., 2002). A
substantial reduction in wear by two orders of magnitude and an increase in
hardness of factor of 4 were obtained with 40-kV PIII process. It was also
44
observed from the XRD analysis that the nitrided layer of 10µm thickness
consists of γ'N expanded austenite phases (Mandl et al 1998).
A sizeable reduction of wear by more than two orders of magnitude
and an increase of hardness at the surface upto 1200Hv were obtained as a
result of plasma immersed ion nitriding of austenitic stainless steels (Richter
et al 2000). The corrosion behaviour tested by potentiodynamic measurements
in 0.1N H2SO4 solution with a sweep rate of 10mV/s, demonstrated that the
nitrided layer composed of expanded austenite was the cause for the increase
in the hardness and wear resistance without the loss of corrosion resistance at
moderate process temperature.
Lebrun et al (2002) have reported that many components used in
nuclear power stations have to meet strict requirements in terms of in-service
corrosion resistance. In their work, they plasma processed the components
made of austenitic stainless steels used in nuclear reactor assembly and
reported that the nitriding allowed a considerable extension in the wear life
time and ensured a good corrosion behavior in various media like pressurized
water, aerated water and acid etc.
Increase in the surface hardness showed higher erosion resistance
as a result of compressive stresses which were induced into the surface by a
compound layer produced on the target surface by gas nitriding (Divakar et al
2005). The ion nitriding of AISI 316L grade austenitic stainless steels
exhibited the development of a nitrided layer consisting N phase lattice
(Nosei et al 2004). It was also reported that modified surface layer could
conserve the structure at base material from distortions. The micro hardness
of this nitrided layer was observed to be about 1340Hv0.025.
45
2.6.3 Corrosion Studies
The austenitic stainless steels are widely used in many industrial
fields because of their very high general corrosion resistance; nevertheless
they suffer from pitting in specific environments and their lower hardness
with low wear resistance limit the number of possible industrial applications.
Glow discharge nitriding of AISI 316 austenitic stainless steels with varying
treatment time was investigated. The morphological analysis and surface
characterization of treated samples exhibited that the surface hardness was
much higher than the untreated specimen and varied with the treatment time
up to maximum value of about 1450Hk0.1. The electrochemical corrosion tests
in 5% NaCl aerated solution showed that the pitting corrosion resistance of
nitrided samples was higher than that of untreated samples and increased with
the increase in nitriding time up to an optimal time. Nitride samples showed a
sensibly lower damaging amount in comparison with untreated samples. As
the nitriding time increased, the dimensions and density of pits decreased and
at the end of corrosion studies, samples nitrided for optimum time appeared
almost untouched (Fossati et al 2006).
A series of nitriding experiments were conducted on the AISI 304L
grade austenitic stainless steel samples at temperature ranging from 673 –
873K (Menthe et. al 1995). The maximum Knoop hardness was found to be
1400 Hk0.01 and the maximum thickness obtained was reported as 34µm. The
XRD patterns showed the presence of the S phase at the surface layers. The
corrosion performance was tested in 0.05M H2SO4 solution at pH 3.3 and in
neutral 3.5% NaCl solution at ambient temperature. Potentiostatic and
potentiodynamic experiments yielded slightly higher passive corrosion
currents for nitrided samples. Pitting corrosion in neutral electrolytes
containing chloride was observed only for the untreated specimen. The
46
passive layers formed on the samples were similar in constitution and
thickness as determined from the XPS sputter profiles.
The effect of ion nitriding on the oxidation behaviour of 316LN
grade stainless steels compared with the as received conditions was
investigated (Rajendran Pillai et al 2007). In their study, they have reported
that the oxidation was interrupted at specific time intervals to examine the
mass change of the specimen and the mass of the spalled oxide. The analysis
of surface by GIXRD revealed the nature of different phases formed on the
surface. The corrosion resistance of stainless steels in aqueous environment
was attributed to the formation of passive film, which was extremely thin
oxide / hydroxide layer in which Cr ions are enriched as oxide or hydroxide.
The passive film was possible to be more protective by various additives such
as Mo, N. Attempts to obtain the higher corrosion resistance of metallic
materials have been tried since early 1970s.
Type 304 and 316 grade austenitic stainless steels were ion
implanted with Cr+, N+ and Mo+ in order to obtain improved corrosion
resistance in aqueous environment (Fujimoto et al 1999). The surface alloyed
with Cr+ ions improved general corrosion resistance in acid solution and never
suppressed localized corrosion in chloride environments, whereas the
implantation of N+ and Mo+ ions effectively suppressed (or) inhibited pitting
corrosion in neutral chloride solutions.
Nosei et al (2004) investigated the corrosion susceptibility of ion
nitrided AISI 316L stainless steel and compared with corrosion susceptibility
of the untreated material. Plasma nitriding resulted in the ‘S’ phase with a
thickness of 5 µm and micro hardness of 1300 – 1400Hv0.25, which was 6.5
times higher than the untreated material. Anodic potentiodynamic polarization
curves and immersion tests were performed in 1M NaCl at room temperature
to evaluate the corrosion resistance of both treated and untreated samples.
47
Nitrided samples showed a much better corrosion resistance than untreated
specimen.
In a study on the corrosion - erosion behaviour of austenitic high
nitrogen stainless steels, Lopez et al (2007) reported that, a high temperature
gas nitriding resulted in 1.5mm thick cases with nitrogen content in solid
solution upto 0.5wt%. The results of corrosion – erosion tests performed in
slurry composed of 3.5% NaCl with 10wt% quartz particles, indicated that
nitrogen addition improved the corrosion – erosion resistance owing to the
strengthening effect of nitrogen in solid solution and to the increase of
repassivation ability of the passive layer.
A series of plasma nitriding experiments have been conducted on
austenitic stainless steels at temperatures ranging from 400K to 600K using
pulsed DC plasma with various pulse durations in an N2–H2 gas mixture by
Menthe et al (1995). The maximum hardness after plasma nitriding was
observed about 1400Hk0.01 with a maximum thickness of the compound layer
as 34µm. Potentiostatic and potentiodynamic experiments yielded slightly
higher passive corrosion currents for plasma nitrided specimen.
Glow discharge nitriding treatments performed at temperatures in
the range 673-773K for 5 hrs at 10 kPa on AISI 316L grade austenitic
stainless samples, were able to produce a modified surface layer consisting of
‘S’ phase with small amount of CrN precipitates at the surface (Borgioli et al
2005). In all the nitrided samples, the micro hardness profile showed a higher
hardness values in the modified layer and a steep decrease to matrix values.
The mean hardness values in the layer reached a value of about 1450 Hk001
and a further hardness increase upto 1550Hk001 was also observed when
substantial amounts of nitrides were forming. The modified layer thickness
increased as treatment temperature increased ranging from 4µm to 47µm.
Preliminary corrosion tests performed in 5% NaCl aerated solution with
48
potentiodynamic method showed that glow-discharge nitriding was able to
increase the pitting potential of AISI 316L samples and hence their corrosion
resistance. The microstructure and corrosion properties of austenitic stainless
steels were investigated after plasma nitriding at 773K (Spies et al 2002). The
nitrided layer of austenitic stainless steels consisted of nitrogen expanded
austenitic phase. The enrichment of the surface layer with nitrogen led to a
significant improvement of the resistance against pitting corrosion compared
to the initial state.
The microstructure and phase composition of the layers produced
by the influence of gas nitriding of austenitic stainless steels, were
investigated (Baranowska and Arnold, 2006). The results revealed that during
gas nitriding it was possible to obtain a uniform protective layer composed of
expanded austenite (γN) phase. The potentiodynamic polarization test using
3% NaCl at a temperature of 292K against Ag/AgCl electrode with a scan rate
of 1 mV/s revealed that the nitrided layer exhibited a better corrosion
resistance. The corrosion potential was shifted to higher values and these
values varied with the ammonia content and temperature during the nitriding
process. The corrosion currents were much lower than the current observed in
untreated specimen.
Surface modification of AISI 304 austenitic stainless steels by
plasma nitriding using NH3 gas at substrate temperature up to 793K was
studied (Liang 2003). The modified surface was found to be containing a
compact surface nitride layer composing a γN phase with a thickness around
12µm. The micro hardness measurements showed significant increase in the
hardness from 240Hv for untreated samples up to 1700Hv for nitrided
samples. The potentiodynamic anodic polarization curves for specimen in
3.5% NaCl solution exhibited that pitting (on corrosion potential was higher)
49
and the corrosion current density were lower for treated samples than the
untreated samples.
The microstructures and corrosion properties of austenitic stainless
steels have been investigated after gas nitriding using sensor controlled
furnace at a temperature range of 523 -773K (Spies et al 2002). The corrosion
behaviour was tested with electro chemical methods and the anodic potentio
dynamic polarization curves were recorded using a 0.05M H2SO4 electrolyte
with a scan rate of at 1.8Vh-1 for general corrosion and 0.5M NaCl electrolyte
for pitting corrosion at a scan rate of 0.2 Vh-1. The enrichment of the surface
layer with nitrogen, up to a temperature of 773K resulted in significant
resistance against pitting corrosion compared to initial state. It was also
reported that the resistance against the general corrosion had been improved.
Dearnley and Aldrich-Smith (2004) reported that providing a hard
thin layer of 10 m thick with improved hardness using various surface
hardening techniques could solve the problem of surface material loss caused
by the synergistic combination of mechanical (wear) and chemical (corrosion)
processes. AISI 316 grade austenitic stainless steel as the substrate material of
specimen was plasma processed and the resulting nitrided layer was
investigated with XRD analysis for phase identification, metallography for
layer morphology and microhardness tests for hardness measurements.
Electrochemical corrosion tests have been conducted potentiostatically in a
3wt% NaCl aqueous solution to measure the anodic polarization curves of the
plasma processed surface (Sun 2005). The results exhibited that the nitrogen
enriched surface layer had a hardness of around 1500 Hv0.025 with a thickness
of 30m. The electrochemical corrosion test showed a reduction of current
density in the anodic region for nitrided sample, indicating improved
corrosion resistance. Four zones have been detected (Andreeva and Gurvich,
1959) within the thickness of nitrided layer on stainless steels with different
50
corrosion resistance and electrode potentials. The variation of corrosion
resistance of nitrided layers with the thickness of removed layer corresponded
to the change in electrode potentials.
Baranowska et al (2007) investigated the wear behaviour of gas
nitrided austenitic stainless steels in corrosive liquid conditions appropriate to
the manufacture of soft drinks. The nitrided layers obtained by the gas
nitriding were predominantly composed of n phase nitride and other nitrides
like Cr2N and Fe2N. The wear performance in the aggressive liquid was
superior mainly due to the formation of a lubricating transfer layer on the
contact surface area. Flis et al (2000) investigated the stainless steels which
were plasma nitrided in N2/H2 mixtures with 25 Vol% N2 at 858K. The
corrosion behaviour through depth the nitrided layers was examined in 0.1M
Na2SO4 at pH 3.0 and the corrosion resistance of the nitrided samples was
found to be more than that of untreated samples.
2.6.4 Summary
Several researchers investigated the effect of nitriding on the
mechanical, surface and corrosion behaviour of the austenitic stainless steels.
Some researchers reported that the nitriding of stainless steel improves wear
resistance but impairs corrosion resistance (Staines and Bell 1979; Sedriks,
1979). Others studied the relationship between the structure and the corrosion
properties of nitrided stainless steels (Zhang and Bell 1985). However little
information is available on the relationship between the mechanical and
surface properties of nitrided AISI 316LN grade austenitic stainless steels and
on the comparison of the properties obtained by the different methods of
nitriding. The present study focused in the direction of documenting the
relationship of the mechanical, surface and corrosion properties obtained by
different nitriding methods on the indigenously manufactured AISI 316LN
austenitic stainless steels.