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ACKNOWLEDGEMENTS
The authors would like to thank General Motors R&D Center, Warren. Michigan, USA for funding the work. One of the author, P. L. Menezes, would also like to thank GM R&D group for the scholarship during the initial stages of the work. The authors would also like to thank Mr. H. S. Shamasundar, Department of Mechanical Engineering, Mr. K. R. Kannan, Senior Scientific Officer, and Mr. K. Sathyanarayana, Technical Assistant, Materials Research Centre and Mr. Gurulinga, Technical Assistant, Department of Metallurgy, Indian Institute of Science, Bangalore, for their help.
REFERENCES
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32.Wang, Y. and Hsu, S.M. (1998). Wear217, 104.
Scratch resistance of high nitrogen austenitic stainless .\·teeIs |
281 |
INTRODUCTION
Stainless steels (SS) have been widely used in different components working under corrosive effects. Investigations have shown that nitrogen is an important alloying addition to stainless steel improving simultaneously localized corrosion resistance, fatigue life, creep resistance, tensile strength and wear resistance [1-6]. These characteristics may be important in many applications, particularly where wear and corrosion mechanisms acting together in mechanical parts such as valve seats and artificial hip joints [7]. Some reasons for these favorable effects are related to the increase of the metallic component of interatomic bonds and the promotion of a short range ordering of atoms in contrast to the clustering of carbon atoms. One of the practical consequences of that is a better corrosion response [8].
The production of such kind of steels is not trivial in view of the limited solubility of nitrogen in liquid metal. Production routes of High Nitrogen Stainless Steels (HNSS) by alloying, pressure metallurgy, powder metallurgy and solid-state diffusion have been studied [9-11]. A low cost thermochemical treatment, which allows alloying the surface and near surface regions of a conventional SS with nitrogen, was developed recently [6, 11-17]. This High Temperature Gas Nitriding treatment (HTGN) is performed exposing steel parts to still N2 gas atmospheres in the range 12731473 K.
Microstructures without nitride precipitation and nitrogen contents at the surface ranging from c.a. 0.2 to 1.0 wt-% can be obtained if adequate nitriding temperatures and N2 partial pressures are used [17-19]. Typical case depths obtained vary between c.a. 0.5 to 2.0 mm after 5 to 12 hrs nitriding-treatments. Thus, this HTGN treatment is quite different from conventional nitriding, usually performed between - 750 and 850 K, in which intense chromium nitride precipitation occurs, greatly increasing the hardness, but impairing the corrosion resistance ofnitrided parts.
Improvements of wear resistance (cavitation, erosion, sliding and fretting) and localized corrosion resistance (pitting and crevice) of martensitic, austenitic and duplex SS after HTGN have been reported [5-6, 11-15, 18-19]. One of the problems in optimizing the HTGN treatment conditions is the considerable amount of performance tests needed to evaluate the processing vs properties relationship. Therefore, in the present work, the possibility of using a single pass pendulum scratch test to assess the tribological properties of high temperature gas nitrided SS is discussed.
Single scratch sclerometric technique is a cheap and quick test, which has been mainly used for calculating dynamic hardness and assessing abrasion resistance of different bulk and coating materials [20-23].
The single pass pendulum scratch test has also been used to evaluate the machinability of materials without performing time consuming and expensive in-field machinability tests [24].
The aim of this work is to study the effect of nitrogen addition - through high temperature gas nitriding - on the scratch resistance of an UNS S30403 austenitic stainless steel, by using a single-pass pendulum scratch test. The scratch resistance is evaluated through the specific energy and the results of scratch tests are analyzed taking into account the stress - strain behavior during depth sensing indentation tests and values of energy absorbed during Charpy impact tests. In addition, a comparison between scratch resistance and cavitation-erosion resistance, measured in previous work [19], is made.
282 Scratching of materials and applications
EXPE~ENTALPROCEDURE
High Temperature Gas Nitriding and Solution-Annealing Treatments
Samples for hardness, scratch and impact tests were cut and machined from a 6 mm thick sheet ofUNS S30403 hot-rolled austenitic SS. The chemical composition is given in Table l.
Table I: Chemical composition of the UNS S30403 austenitic stainless steel (wt- %)
Cr |
Ni |
Mn |
Si |
Mo |
C |
Ti |
I8.1 |
8.5 |
1.3 |
0.5 |
0.04 |
0.025 |
0.0056 |
|
|
|
|
|
|
|
Four sets of samples were high temperature gas nitrided aiming to obtain different nitrogen contents at the near surface region. One set of samples was solution-annealed aiming to obtain specimens without nitrogen additions.
The HTGN treatments were carried out in a tubular furnace. The specimens were heated up to I473 K, under 0.13 Pa, and then exposed to a high purity Ar + N2 atmosphere during I0 hrs, at 0.02, 0.05, 0. I and 0. I 7 MPa N2 partial pressures. After nitriding the samples were direct quenched in water. The solution-annealing treatments were performed in an Ar atmosphere using the same thermal cycle reported for the HTGN treatments.
Scratch Test
Rectangular prismatic tests samples (6 mm X 9 mm x 55 mm) were metallographically polished up to I ~m diamond paste and submitted to the single scratch test in a modified Charpy impact pendulum, with 50 J of maximum capacity. The sintered tungsten carbide scratching stylus, was a truncated square-base pyramid of 40° apex angle, with a 0.5 x 0.5 mm flat top. The initial pendulum height was set to accumulate a potential energy of 35 J, resulting in a velocity ofthe scratching stylus, at the beginning of stylus-sample contact, of 3.16 m/s. The depth of the scratches, controlled by vertical adjustment of the specimen holder, was set at I 05 ~m. Four scratches were made in each of two specimens obtained for each treatment condition.
The absorbed energy (E) was measured with a readability ofO.Ol J. The removed mass (W) was measured using a Scientech SAI 20 scale with a readability of 0. I mg.
The specific absorbed energy (e)- which represents the energy consumed during the removal of I g of material - was calculated from the measured absorbed energy (E) and the removed mass
(W) using equation (1) [20].
E
e= - |
(I) |
|
W |
||
|
The system was equipped with strain gages to measure tangential and normal forces developed during the test.
Charpy Impact Tests
Notched bar (5 x IO x 50 mm) sub-sized specimens were tested at 300 K in a pendulum machine with 300 J of maximum capacity, according to the ASTM E23 standard. The depth of the notch was 2 mm. Three samples for each treatment condition were tested.
Scratch resistance of high nitrogen austenitic stainless steels |
283 |
Specimens Characterization
Optical microscopy and scanning electron microscopy were used to analyze the microstructure and topography of the scratched specimens and of the turns. SEM was performed in a Philips XL30 microscope. An Oxford WDX600 X-ray spectrometer was used to analyze the nitrogen contents at the surface of the nitrided samples.
Depth-sensing indentation tests on top of rectangular (6 x 10 x IO mm) samples were carried out in aFisherscope HJOO apparatus, using a Vickers indenter tip. The maximum load was 250 mN, the loading and unloading times were 90 s and the dwell time at peak load was 20 s. The indentation data were analyzed using the method proposed by Oliver and Pharr [25]. Each data point is an average of II measurements. The indentation data analyzed were: hardness (H), total
indentation work (W1), irreversible indentation work (Wir), reversible indentation work (We), loading slope (SI), unloading slope (S) and strain-hardening coefficient during indentation (n). The n coefficient was calculated introducing a correction function, f(n) [26], to the Oliver and Pharr procedure, expressed by equation 2 for a Vickers indenter:
f(n) = I.202- 0.857n + 0.302n |
2 =~W, |
(2) |
|
sw. |
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RESULTS
Characterization ofthe As Treated Samples
Both the solution-annealed and the nitrided samples had austemt1c microstructure free of precipitates. The hardened case depth, determined by hardness measurements in the transverse section of the nitrided samples, was between 1.2 and I.6 mm.
Table 2 gives the results of depth-sensing indentation tests, WDS chemical analysis and Charpy impact tests.
The nitrogen content at the surface of the samples increases with increasing N2 partial pressure. One can see a strong hardening effect of nitrogen alloying, associated to a tenuous decrease of
both the strain-hardening coefficient (n) and the Wi-/Wt ratio (ductility index). Increasing the nitrogen content up to 0.5 wt-% increases hardness from 2.05 to 3.2 GPa and decreases n from O.I55 to 0.13, W1 from I85.1 to 156.4 nJ and Wi-/Wt from 0.899 to 0.854.
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Scratching of materials and applications |
Table 2: Results ofWDS chemical analysis, depth-sensing indentation tests and Charpy impact tests.
|
Nitrogen |
Depth-sensing indentation tests |
Cbarpy |
||||
PN2 |
|
|
|
|
|||
Content |
|
|
|
|
impact energy |
||
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|
|
|
||||
(MPa) |
|
|
|
|
|||
|
(wt-%) |
H |
|
w. |
W;/W1 |
(J) |
|
|
|
n |
|||||
|
|
|
(GPa) |
(nJ) |
(nJ/nJ) |
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|
0.0 |
0.0 |
2.05 ±0.10 |
0.155 |
185.1 ±J |
0.899 |
171.5±IO |
|
|
|
|
|
|
|
|
|
0.02 |
0.05 ±0.0 5 |
2.25 ±O.I3 |
0.145 |
181.5 ± 4 |
0.895 |
171.5 ± 15 |
|
0.05 |
0.11 ±0.07 |
2.35 ±OI 5 |
0.135 |
178.2 ± 4 |
0.892 |
165.0 ± 10 |
|
0.10 |
0.31 ±0.0 5 |
2.85 ±0.10 |
0.125 |
165.5 ± 25 |
0.885 |
159.0 ± 15 |
|
0.17 |
0.5 ±0.07 |
3.20 ±OIO |
0.130 |
156.4 ± 2 5 |
0.854 |
|
|
|
|
|
|
|
|
|
|
The Charpy samples suffered large macroscopic plastic strain before fracturing in a fully ductile manner. Table 2 shows that the HTGN treatments almost did not affect the impact resistance of the UNS S30403 steel. Increasing the N2 partial pressure decreases slightly the impact energy
from 171.5 to 159 J. The tenuous decrease in the absorbed impact energy is due to a slight decrease of both the work hardening ability and the ductility index.
Scratch Resistance
Figure 1 shows mean values of the parameters analyzed during the scratch experiments: absorbed energy (E), removed mass (W), specific absorbed energy (e), maximum normal force (NF) and the ratio of the tangential force to the normal force (TFINF). Fig. 2 shows a representative force-distance scratch curve, as well as the ratio TF/NF during the scratching event, for each studied treatment condition.
The specific absorbed energy increases with nitrogen content whereas both the absorbed energy and the removed mass decrease with increasing nitrogen contents. When the nitrogen content of the specimens increases the mass removed during the scratch test decreases more rapidly than the absorbed energy, leading to an increase in the specific absorbed energy.
