Austenitic stainless steel type is widely used in the engineering application
because it has excellent oxidation and corrosion resistance. However, there
are some limitation in industrial application for this type of steel where exposing
wear mechanism, for instances like camshafts, cam followers in automotive parts
and also in chemical and oil and gas industries. The introduction of nitrogen
for thermal diffusion process such as nitriding treatment is recognized where
the treatment could improve the wear resistance and surface hardness of the
steel during the operation (Triwiyanto et al., 2009).
Austenitic stainless steel wear mechanism begins when the protective oxide
layer is break and active boundary lubrication is poor (Ashby
and Jones, 2005). However, AISI 316L stainless steel become the chosen material
in the engineering industry because of its good ductility, weldability and better
corrosion resistance. Due to the inherent austenitic structure, this material
is poor due to the effect of wear and low hardness, which result in poor tribological
properties (Subbiah and Rajavel, 2005). One of the approaches
to improve wear resistance and surface hardness of the steel is by nitriding
treatment where its offer the benefits of high dimensional stability (Haruman
et al., 2006).
Nitriding is a surface hardening technique by the diffusion of nitrogen into
the surface layers and the change of chemical compositions of the steel (Elgun,
2012). The new phase compositions also known as S-phase and formed nitrided
layer (Toshkov et al., 2007) with the term of
expanded austenite γN (Subbiah and Rajavel,
2005). This phase is characterized with its good mechanical properties and
acceptable corrosion resistance (Hamdya et al., 2011).
In order to accelerate diffusion on austenitic stainless steel, the steel will
be treated at relatively high temperature, about 570°C (Bell
and Li, 2002).
However, the formation of chromium nitride/carbide could occur during the diffusion
at high temperature. As the results, chromium nitride/carbide might be precipitated
into the grain boundary and the passive layer , which is chromium oxide, Cr2O3
will unable to be produced and reduced the corrosion resistance property of
stainless steel (Triwiyanto et al., 2009). This
phenomenon is known as sensitization effect. The effort to avoid the sensitization
effect relatively with low temperature treatment, according to Zhang
and Bell (1985). They have investigated the low temperature nitriding technique
where it was found that a nitrided layer was able to form on surface of AISI
316L with plasma nitriding technique. The thickness of the layer could be formed
up to 20 μm at temperature around 400°C. The characteristics of the
layer show that it has very high hardness and excellent wear resistance, as
well as very good corrosion resistance Zhang and Bell (1985).
Time variables of treatment also could be analyzed in this low temperature
nitriding treatment. The case depths or thickness of the nitrided layer become
the relation with the time variables (Haruman et al.,
2006). There are different case depth could be identified under the nitrided
layer morphology. Moreover, the time variables may bring us to have the understanding
in the improvement of surface hardness for each different time of treatment.
For gas nitriding treatment, ammonia gas can be used as the gas resource for
nitriding. Nitrogen is introduced into the surface steel by holding the metal
at a suitable temperature in contact by ammonia. Thus, ammonia will disassociate
into gas into hydrogen and nitrogen on the surface steel (Mridha,
2006). Nitrogen then diffuses from the surface into the core of the material
at the certain temperature range.
The typical wear mechanism characteristics for austenitic stainless steels
sliding against steels are adhesive and abrasive wear mechanism (Li
and Bell, 2004). During nitriding steel, previous investigations done by
Li and Bell (2004) and Hashemi et
al. (2011) showed that the characteristics of wear mechanism through
the surface morphology analysis were shallow, narrower and superficial wear
track, as well as abrasive wear which was dominated on the surface. This is
due to the formation of layer that provide a convenient support to the protective
layer of oxides, which may introduce an increasing of wear resistance (Gallo,
This study describes characteristics of austenitic stainless steel 316L nitrided
by low temperature thermochemical treatment with different treatment durations.
MATERIALS AND METHODS
The material used is AISI 316L stainless steel and the steel supplied in the form of rod with the diameter of 50.3 mm and thickness of 40 mm. Then the samples of pin and disc are cut into 50x6 mm for disc and 6x12 mm for pin. Those four pins and discs are required for undergo the pin on-disc wear test as be standardized by ASTM G99-95a. Then all the samples surface are ground on 120, 220, 500, 800, 1000, 1200 grit SiC papers and then polished using 1 μm Al2O3 pastes to the mirror finish, followed by cleaned using ultrasonic cleaning and immersed in HCl (2 M) solution for 15 min duration to remove the native oxide film that commonly forms on austenitic stainless steel.
|| Sample classification with different treatment
Nitriding treatments were performed at 450°C in a Carbolite CTF Tube Furnace. The specimens were heated by electrical resistance heating. Prior to treating, the specimens were soaked in concentrated HCl (2 M) solution for 15 min duration with the purpose to remove the native oxide film that commonly forms on austenitic stainless steel and protects the metal matrix from corrosion. This oxide layer is believed to act as a barrier for diffusional nitrogen transport.
Furthermore, the ammonia gas will be purged together with nitrogen. The amount of ammonia and nitrogen gas is set to be 50% or 0.3 Standard liter per minute (0.3 SLPM) each. Those gases will mix in the mixing chamber before purge together into the tube furnace. After nitriding treatment, pin on-disc wear test at dry sliding condition is proceed according to the standard; ASTM G99-95a. Ducom TR-701-M6 Multi Specimen Tester machine is used. The load values for the test are 17 N with the speed of 75 rpm and sliding distance of 300 m (for 30 min time of operation). The temperature is the room temperature and the atmosphere is set to be the laboratory air. Characterization of the nitriding product were performed by using a wide range of instruments. Wear resistance required data of coefficient of friction from the wear test. Vickers hardness test instrument; Model HV-1000A Micro Hardness Tester with 10 gf load and 15 sec well time is used. The thickness of nitrided layer for each samples is measured using Field Emission Scanning Electron Microscope (FESEM) instrument; Carl Zeiss AG-SUPRA 55VP. The cross-sectional samples are required using standard metallographical technique and Marbles etching (4 g CuSO4+20 mL HCl+20 mL distilled water). Surface morphology of worn region also is investigated by FESEM instrument. Table 1 presents the classification of samples with different treatments.
RESULTS AND DISCUSSION
Wear resistance analysis: As shown in Fig. 1 which
is the comparison results of coefficient of friction vs. time profile, UN sample
yield the highest coefficient of friction, which averagely ranging from 1.4-2.0.
Then, the coefficient of friction followed by the sample of 2N with 0.8-1.4,
5N with 0.6-0.8 and 8 h 0.3-0.5 nitrided steel. Theoretically, lowest coefficient
of friction gives highest wear resistance during wear mechanism on the surface.
The presence of nitrided layer on the samples of 2, 5 and 8N could be explained
that the formation of nitrided layer provide a convenient support to the protective
layer of oxides, which may introduce an increasing of wear resistance (Gallo,
2009). According to investigation by Subbiah and Rajavel
(2005), formation of nitrided layer on steels is increased in thickness
as the time of nitriding treatment is extended. This is why the sample of 8N
achieved high wear resistance.
Microhardness measurement: From Fig. 2, surface hardness
of the nitrided steel is significantly improved from 2-8 h nitriding treatment.
The 2N sample formed nitrided layer with maximum hardness of about 480 Hv0.01,
which is much lower than the hardness of 720-754 Hv0.01 for other two nitrided samples.
|| Coefficient of friction vs. time profile for UN, 2N, 5N and
|| Depth profiles of microhardness
The increasing of surface hardness can be explained due to the extending of
time treatment where nitrogen atoms diffused in the surface become more density
in the nitrided layer.
Theoretically, formation of nitrided layer due to precipitation-free diffusion
layer by nitrogen supersaturated, which is normally, knows as S-phase (Toshkov
et al., 2007). This supersaturation of nitrogen in the austenite
will cause the expansion of the lattice of the substrate austenite (Triwiyanto
et al., 2009). Thus, a new type nitrite phase (S-phase) as a nitrided
layer provides extremely high surface hardness.
Nitrided layer morphology: From Fig. 3(a-c),
thickness of nitrided layer formed on the 2, 5 and 8 N samples were measured
by Field Emission Scanning Electron Microscopy (FESEM) instrument at 1000-5000x
||Nitrided layer morphology of (a) 8N (b) 5N and samples at
1000x magnifications and (c) 2N samples at 5000x magnifications
The 8N samples formed highest thickness of nitrided layer with 12.84 μm,
followed by 5N with 7.93 μm and 2N with 1.34 μm. Fig.
3(a) showed that nitrided layer formed on 8N samples was not uniform. However,
for 2N and 5N samples in Fig. 3(b-c), the
layers are uniform. The characteristic of gas nitriding technique more likely
to occur with irregular diffusion due to the gas accumulating near to the tempered
surface steel and diffuse instead of accelerating directly diffuse on the surface.
This might be the reason regarding non-uniform layer formed on the surface steel.
From the image of FESEM shown on the 3c, the thin nitrided layer was formed
on the surface after 2 h nitriding with the improvement of surface hardness,
which can be referred in Fig. 2 for depth profile of microhardness.
Thus, the thin nitrided layer begins to support the protective oxide layer on
this sample and resist the wear action (Triwiyanto et
al., 2009). However, the mechanism of the abrasive and adhesive wear
still took place on the sample slightly compared to untreated sample.
Surface morphology: For UN sample, the large plastic deformation is
obviously observed on the worn region which is shown in Fig. 4a-b.
The morphology of the worn region for UN sample also showed the deep plow with
plate-like wear debris. The deep plow is explains the wear severely occurred
on the sample by abrasive mechanism, while the plate-like wear debris are due
to the adhesion wear mechanism, where the material or wear debris (particles)
transferred on the surface and would then be plastically deformed and compacted
by the rubbing action between the slider and the disc (Li
and Bell, 2004). For 2 N sample, the morphology characteristic could be
obviously seen in Fig. 4c which is 100x magnification where
it appears to be less worn and the shallow plow built up on the surface. This
indicates that abrasive wear also experienced on the sample but not severely
compared to untreated sample. Figure 4d with 500x magnification,
the shallow plow and plate-like wear debris built up on the surface clearly
seen. In Fig. 4e, the worn region morphology at 40x magnification
for 5 h nitrided sample only observable with several surface digging and also
with narrower and superficial wear track (Li and Bell, 2004;
Hashemi et al., 2011). The structure seems that
the mechanism occurred is slight abrasive wear. This could be strongly support
by analyzing the morphology in 500x magnification in Fig. 4f.
The spot taken on the surface digging clearly had shown the abrasion of the
||Worn morphology of UN at (a) 100x and (b) 300x magnification,
morphology of 2N at (c) 100x and (d) 500x magnification, Worn morphology
at (e) 40x and (f) 500x magnification and Worn morphology of 8N at (g) 200x
(h) 500x magnification
This is the characteristic of nitrided steel where only abrasive mechanism
dominates the wear process (Li and Bell, 2004; Liang
et al., 2000; Li et al., 2012). The
thicker nitrided layer formed on this sample combining with protective oxide
layer prevent from intimate contact and adhesion on the surface. This is why
only abrasive mechanism took place. From the image of wear track obtained on
this sample at 200-500x magnifications in Fig. 4g-h
show that there was some crack occurred on the worn region. However, the mechanism
of the abrasive and adhesive wear still took place on the sample slightly compared
to untreated sample.
Supposedly, the wear mechanism characteristic for 8 h nitrided steel after
wear test similar with 5 h treatment, which is shallow, narrower and experiencing
and slight abrasion. However, from the result of wear track obtained on this
sample at 200-500x magnifications in Fig. 4(g-h)
show that there was some crack occurred on the worn region.
This is possibly due to the nitrided layer formed on this sample was not uniform, which can be seen on the Fig. 3 previously, in the nitrided layer morphology result. Thus, morphology of this nitrided layer leads to the crack when the slider (pin) slide on the uneven nitrided layer and hit the thicker layer.
Wear resistance and surface hardness of nitrided AISI 316L stainless steel is significantly improved through the low temperature gas nitriding treatment with 2, 5 and 8 h time variables. Eight hour treatment achieved the lowest value of coefficient of friction result, which was 0.3 and gave high wear resistance. Meanwhile, maximum surface hardness achieved was 754 Hv0.01 after 8 h treatment. These improvements are due to the formation of nitrided layer on the treated steel that successfully formed during low temperature nitriding gas treatment. The maximum thickness layer formed was 12.84 μm at 8 h treatment. Moreover, the formation of nitrided layer also supported the protective oxide layer during the wear mechanism on the surface, where the in contact surface only experienced slight abrasion with shallow and narrower wear track.
The authors would like to thank to Universiti Teknologi PETRONAS for financing these findings in the International Conference on Plant Equipment and Reliability 2012.