1IFIR - CONICET - Universidad Nacional de Rosario, Argentina - IFAS-CONICET Universidad Nacional del Centro de la Provincia de Buenos Aires, Argentina

3 4FCEIA-Universidad Nacional de Rosario, Argentina - INTEC-CONICET-UNL, Argentina

2

An experimental study is made in order of knowing the behaviour of austenitic stainless steel, which we previously nitrided or carburized, under the impact of energetic pulses of ions and plasma. Austenitic stainless steel has good mechanical properties and excellent corrosion resistance. This type of steel (AISI 316L in our case) can improve its hardness and wear resistance by using ion nitriding or ion carburizing. The crystalline structure is modified by a cold plasma treatment which causes the formation of a new phase called expanded austenite, formed by means of the diffusional processes triggered on the steel surface. These processes are well known but a complete characterization was never reached. In this work we nitrided austenitic steel AISI316L samples using a 80 % hydrogen - 20 % nitrogen

oatmosphere at 5 mbar and a temperature of 400 C. Ion carburization has been made, too. In this case we used the same working parameters and a mixture of 50 % Ar - 45 % H - 5 % CH . After that, we studied this expanded austenitic phase under energetic ion 2 4

bombardment with a dense plasma focus device, with an energy of 2 kJ. The vacuum chamber was filled with low mass gases, 1.6 mbar deuterium or helium or 2.4 mbar of hydrogen. The respective sample was located at 82 mm from the anode. The number of pulses (1, 5, and 10) gradually modifies the crystalline structure of nitrided or carburized samples. A characterization of the samples was made with a Reichert optical metallographic microscope. Crystalline structure was determined by means of GIXRD (Grazing Incidence X-Ray Diffraction), comparing structure modification with different gas and number of pulses. Vickers microhardness was used to know surface hardness before and after the treatment.

ABSTRACT

1 2 2 2 3 4 1 2 2 1García Molleja J ,Milanese M ,Moroso R ,Niedbalski J ,Nosei L , Ferrón J ,Burgi J , Supán J , Guichón S , Feugeas J

STUDY ON A PLASMA FOCUS APPLICATION: EFFECT ON NITRIDED OR CARBURIZEDAUSTENITIC STAINLESS STEEL OF ENERGETIC LIGHT IONS BOMBARDMENT ifas

Instituto de Física Arroyo Seco

LAWPP 2011 - November 20-25, 2011 - Mar del Plata, Argentina

Tandil, Buenos AiresARGENTINA

1 INTRODUCTION

2 EXPERIMENTAL PROCEDURE

Current technology requires materials which can be mechanized and offer good properties. The stainless steel has good anti-corrosive properties, low cost and can be applied to many fields, but has got low hardness and resistance to the erosion. With surface treatments physical and mechanical properties of the first micrometers of the material can be altered in order to get better properties.

Austenitic steel (AISI 316L in our case) is a type of steel which has got a fcc cristaline structure and good properties. In the last years ia new kind of steel has appeared, the superaustenitic steel (AISI 904L as the major representative), characterized by a very high proportion of Ni (26 %) with 44% of Fe and 21 % of Cr). This proportion of Ni, together with the addition of Mo and Cu makes it one of the more non-corrosible steels. Surface properties of these steels can be improved keeping the inherent ones.

Plasma treatments are very effective in steel surface treatment. Treatment of this kind is made and presented in this work. These processes are well known but a complete characterization was never reached. In order to know something of the behavior of this material, particularly under energetic plasma and ion beams environment, we studied this expanded austenitic phase with a dense plasma focus device.

PLASMA NITRIDING AND CARBURIZING

Cold plasma treatment modifies the surface properties of a piece of material that acts as cathode in a glow discharge. Ions reach the steel piece and diffuse into the lattice. They will locate in interstitial voids. The process induces the lattice expansion; a high hardness and good erosion resistance layer is formed: the so called expanded austenite. The more employed techniques are carburizing and nitriding (if C or N are, respectively, the atoms inserted in the crystal lattice). This is a simple, fast, cheap and non-contaminating process.

-3The steel samples are polished. The chamber is evacuated up to is 1.3 x 10 mbar. Several washes with hydrogen are made, including a discharge in this gas.

oA glow discharge heats the cathode by ion bombardment in an Ar - H mixture, which reaches a temperature of 400 C. A mix of 80% H and 2 2

20% H (5 mbar) during 80 minutes is employed for nitriding. A mix of 50% Ar, 45% H and 5% CH (5 mbar) during 80 minutes is employed for 2 2 42carburizing. The current density is lower than 2 mA/cm .- The rectified DC voltage is, respectively, 630 – 490 V for nitriding and carburizing.

BOMBARDMENT BY PLASMA FOCUS

Plasma treatments, even when they are nowadays widely used, have not been fully characterized yet. In order to study the effect of radiation and plasma shocks on the expanded austenite, the samples were bombarded with high energy light ions and plasma. With this purpose the Mather type dense plasma focus device called PACO operating in the GPDM at Tandil is employed. The coaxial gun into the discharge chamber is mainly composed by an OFHC anode 40 mm in diameter, 40 mm free length, a Pyrex insulator 15 mm long; a cathode made of rods arranged in a circumference of 110 mm in diameter. The capacitor bank has 4 μF, the charging voltage is 31 kV, the parasitic inductance is 47 nH. The operating energy is 2 kJ.

-6The discharge chamber is evacuated up to 5x10 mbar. Then it is filled at a pressure of 1.6 mb with Deuterium or Helium. The number of shots to bombard the respective sample are 1, 5 and 10. Each sample consists in a expanded austenite disk (20 mm in diameter, 5 mm thick) previously nitrided or carburized, which is located at a distance of 82 mm from the anode end.

A half of each sample is covered to verify the structural difference between the bombarded and the occult part. After that, the samples are treated in the plasma focus discharges, they are studied by several techniques.

4 OPTICAL CHARACTERIZATION

The surface morphology seems to depend only on the number of plasma focus discharges but not on the kind of gas used. The desquimated and the crossed sliding bands arise from the rapid heating and cooling by thermal shock. AISI 316L steel is more resistant to bombardment. There is a better resistance to the bombardment in the carburized samples than in the nitrided ones.

Reichert Metallographic Microscope

Optical Microscope

A reichert metallographic microscope and other optical microscopes are used in the optical charaterization of the samples. The magnification used in the Reichert is 200x and 500x for the other one. The thickness of the nitrided layer is 11 μm and the thickness of the carburized one is 15 μm.

4 X-RAY DIFFRACTION AT GRAZING INCIDENCE

The crystal structure of the samples was analyzed by X-rays. GIXRD technique was used to analyze the first superficial microns of the 2samples. GIXRD technique uses a parallel beam of Cu K radiation (40 kV, 30 mA) with an aperture of 4x4 mm . The incidence is at 2˚ and 10˚ α

sweeping the scintillation detector between 30˚ and 80˚. The step size is 0.03˚ and permanence of 1 s.

A growing number of shots on expanded austenite decreases its lattice parameter to the value of the base steel. A double peak at the (111) planes appears (except for carburized steel with 10 shots of helium). The fisrt one can be attributed to the expanded austenite which undergoes a strong thermal shock at high temperature. The second, to 43.3°, is typical of plasma focus nitriding processes, so it may be due to the structural disorder created in the first layers by collisions of deuterium or helium ions. It is registered more expansion in nitriding than in carburizing (the lattice parameter of AISI 316L steel is 3.5978 Å).

Change of lattice parameter for cementation (in Å) Shots Deuterium Helium Expansion (%) 0 3.6696 3.6779 2.00-2.22 1 3.6545 3.6529 1.57-1.53 5 3.6491 3.6557 1.43-1.61 10 3.6429 4.6504 1.25-1.46

5 VICKERS HARDNESS

AISI 316L (nitriding during 80 minutes)Shots Deuterium Helium 0 999 96 1 827 815

10 --- ---

AISI 316L (carburized during 80 minutes) Shots Deuterium Helium 0 461 443 1 364 293 10 366 367

The hardness of nitrided layers is higher than in the carburized. The hidden part of the specimens show a hardness similar to those which were not treated under plasma focus. There is not much difference in hardness for different types of gases to the same number of shots. For nitriding, ten shots are enough to cause amorphization and destruction of the outer layer, making it impossible to measure the hardness.

For carburizing, deuterium does not produce a big change in hardness with an increasing number of shots. With helium hardness is higher for higher number of shots, perhaps by the effect of the thermal shock.

6 CONCLUSIONS

Steel samples AISI 316L (austenitic) and 904L (superaustenitic) are nitrided or carburized by cold plasma treatment . The samples are then bombarded using a plasma focus device with Deuterium and Helium as filling gas to test their resistance to the impact of energetic light ions and plasma.

Crossed slip bands and craters appears on the surface. In case of nitriding, the thermal shock removes the surface and with a higher number of shots amorphization is initiated. AISI 904L steel is less resistant to the bombardment.

Energetic light ion bombardment gradually reduces the lattice parameter of expanded austenite when the number of shots is increased. The change of expansion by bombardment depends on the treatment but not on the kind of gas used.

The thermal shock reduces the expansion. There is a tendency to stabilization to a situation where the diffraction peak corresponding to (111) planes is located at 43.3˚, where another peak is locatedwhich can be attibuted to the expansion caused by ion bombardment.

Change of lattice parameter for the nitriding (in Å) Shots Deuterium Helium Expansion (%) 0 3.8073 3.7984 5.82-5.58 1 3.7491 3.7415 4.20-3.99 5 3.6691 3.6743 1.98-2.12 10 3.6468 3.6556 1.36-1.60

After five shots, the expansion degree is similar for both treatments. For a given treatment and a fixed number of shots, the expansion is not altered greatly by changing gas.

Lattice parameter change (in Å) Nitriding Nitriding Cementation Expansion

+ Deuterium + Helium + Helium (%)Shoting 3.7824 3.7824 3.7623 5.07-5.07-3.51No shots 3.6662 3.6356 3.6361 1.84-0.99-1.00

In AISI 904L steel also undergoes more expansion in nitriding that in cementation. A large number of shots decreases greatly the lattice parameter.

Bombarding with helium eventually leads to the same lattice parameter regardless of the steel treatment.

Deuterium bombardment causes a lesser loss of expansion. The peak originated by the Plasma Focus can be observed at 43.3˚ superposed on the peak (111) .

7 IMAGES TAKEN WITH A SCANNING ION MICROSCOPE

Nitrided AISI 904L bombarded with Deuterium (1 shot). The surface appears

cracked and shows craters.

Nitrided AISI 904L . The typical structure in grains with sliding bands can be

observed.

Carburized AISI 316L, with one shot in Deuterium. Deep craters can be

observed. Sliding crossed bands, due to high thermal gradients, can be observed.

Carburized AISI 316L, 10 plasma focus shots in He. Sliding cross bands and

ejection points of the material can be observed.

Nitrided AISI 316L, one shot in Deuterium. Craters and peeling dominate

the surface.

Nitrided AISI 316L, 10 shots in He. Evidence of amorphization can be

observed.

Deuterium – A shift of the peaks to the base material value can be seen.

Deuterium – Apart from the displacement due to the growing amount of shots, a peak at 43.3˚

Helium - With a single shot, a peak at 45.08 ° appears, perhaps due to Fe C and 3

Cr C .3 2

Helium - Plane (111) peaks are superposed.

AISI 904L nitrided bombarded with 15 pulses of deuterium. (111) peak is quite contracted.

The peak at 43.3 ° is observed.

The expansion in the 904L nitrided steel (20 pulses of He) is high. The nitrided sample and its covering show almost identical diffractograms.

904L steel and cemented with 18 shots of helium shows an overlay of peaks: the expanded austenite

one and the one created by plasma focus.cathode

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