Characterization of expanded austenite in stainless steels by ion carburization. Study of stability under beam irradiation composed by energetic light ions

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Published on February 15, 2014

Author: jgm_fisico

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Thesis dissertation in order to obtain the degree in Doctor in Physics. Austenitici stainless steel modification by ion carburization and study of its stability under plasma focus irradiation. Developed at Universidad Nacional de Rosario (Argentina) in March 2012

Javier García Molleja Director: Jorge Feugeas (UNR, Argentina) Advisor: Mª Dolores Calzada (UCO, Spain) Thesis presented in order to obtain the PhD degree in Physics Thesis jury: M. Magdalena Milanese (UNICEN, Argentina) Roberto R. Koropecki (UNL, Argentina) Oscar A. de Sanctis (UNR, Argentina)

   Steels Surface treatment Plasma

   Steels are alloys of iron and carbon (< 2 %) with other elements in addition. Fe-C diagram has an interesting zone when the carbon content is low. Under different temperatures there are some stable crystalline phases, and there is an eutectic point.

Carbon content develops carbide formation and steels properties are changed. Alloying elements affect final steel properties. Main steel properties are the electric, magnetic, thermal and nuclear ones.

   Stainless steels have an 11 % of chrome as minimum percentage. Passive film is formed on the surface, avoiding the oxygen influence in the structure. They have high corrosion resistance under high and low temperatures. Molybdenum improves pitting corrosion resistance.

    Austenitic steels have a fcc structure (face centered cubic). This structure is stable under high temperatures or when the steel itself has nickel (3.5-22 %) as alloying. Austenitic steels have high corrosion resistance, they are weldable, good hygienecleanliness factor, with application under an important temperature range, they are not hardened by heat treatment and they have a lot of uses. One type of austenitic stainless steel very known is the AISI 316L one, studied in this thesis.

   Surface modification can be accomplished with different techniques. One of them is ion implantation with energetic ions which penetrate in the surface as projectiles. Another one is atomic diffusion, where ions diffuse in the bulk following the Fick law.

  Surface modification of steels is obtained by ion carburization and ion nitriding techniques. These processes are based on the generation of C or N active species with plasmas. Plasmas are generated by electric discharges and the surface is under negative voltage.

     Surface modification treatments are obtained with plasmas. This figure represents the voltage-current curve of a DC discharge under low pressure conditions. We work in the bright region, called glow. In this region there are many species with different temperatures. Electron temperature is high, so this is important when we use fragile substrates, thus working with low gas temperatures. Neutral atoms are excited by collisions, emitting visible radiation.

Glow discharge has dark and bright zones. The cathode region has the major voltage fall in the discharge. In this region positive ions are accelerated to the cathode and electrons gain energy and provoke ionizations and collisions in the negative glow region. This region has high light intensity and high volumetric electron density.

  Reactors used Instruments and physical principles used to sample’s characterization

Alternatively, two reactors are used. 1. Reactor of 8.8 L of capacity with two windows. Cathode has a surface of 164 cm2. 2. Reactor of 5.1 L of capacity with an upper window. Cathode is 265 cm2 of surface, with holes to sample insertion (2 cm diameter, 6 mm thick). 

    Base pressure (rotatory pump) of ~10-2 mbar. Surface cleaning: hydrogen plasma during 15 minutes at 1.484 mbar. Working pressure was 5 mbar during carburization. Temperature between 400 and 410 ºC. Gas mixtures during carburization: ◦ C50-: 50 % Ar, 45 % H2, 5 % CH4. ◦ C80-: 80 % Ar, 15 % H2, 5 % CH4.  Process duration: 30, 60 or 120 minutes.

 In order to compare carburization with nitriding, some depositions with this technique have been done. Gas mixture of: ◦ 80 % H2, 20 % N2.  Process duration: 80 minutes.

   Optical Microscopy: OLYMPUS MG microscope (property of Laboratorio de Metalurgia, IMAE). Grazing Incidence X-Ray Diffraction (GIXRD): PHILIPS X’Pert diffractometer. Cu K anode at 40 kV and 30 mA. Incident beam has a parallel cross-section of 4x4 mm2. Incidence angles used: 2 or 10º (0.7-3 m depth). Scanning between 30 and 80º. Steps of 0.03º and 1 s.

  Vickers microindentation: SHIMAZDU HMV-2 (property of IMAE) microhardness tester. Diamond tip with rectangular base, 136º between opposite faces. Different loads between 10-15 s. Tribology: ball-on-disc method. TRM100 Dr.Ing. GEORG WAZAU tribometer. Alumina ball of 5 mm radius, linear speed of 0.1 m/s, 8842 turns and scar located at 9 mm from the sample center. Test reached 500 m of length. Load of10 N. No lubrication.

Wear is known by measurement of volume lost per unit of length: S 1 2 r ( 2 sen ) The angle is calculated with the scar diameter, D, and the ball radius r: D2 arc cos 1 2r 2

  Corrosion: During 60 days, sample immersion (with a sample of austenitic steel) in one liter of water with 5.85 wt. % NaCl. Auger Electron Spectroscopy (AES): PERKIN ELMER (property of INTEC, Santa Fe) device. Ion sputtering with 4 keV Ar and a current of 15 mA. Electron gun was used under 2 keV of energy and current of 2 A. Working pressure was 5.33·10-8 mbar.

 Focused Ion Beam (FIB): Ion gallium beam of 30 kV and 10 pA in a FEI Helios NanoLab™ 600 - (Dualbeam) device, property of Università de Roma Tre, Italy. Electrons and secondary ions are detected. SEM (Scanning Electron Microscopy) mode operation can be chosen, too. Magnification of 350000X.

   Fundamentals on expanded austenite Study of base material Surface analysis of treated steels ◦ ◦ ◦ ◦  Auger analysis Hardness analysis Wear analysis Corrosion analysis Expanded austenite stability irradiation and temperature under beam ◦ Expanded austenite stability under beam irradiation  Expanded austenite stability influence on hardness under beam irradiation: ◦ Expanded austenite stability under high temperature

  Experimental results show that C and N are lodged in the lattice by fast diffusion, reaching maximum percentages of 38 % for N and 28 % for C without any precipitation. This Colossal Supersaturation with these elements in the austenite develops a new phase called expanded austenite, which is obtained with only 12 % of C or N in the sample.

  The austenitic stainless steel has a fcc crystal structure with interstitial tetrahedral and octahedral holes. Considering the lattice composed solely with Fe and C located in the holes, this lattice will expand 33.6 pm if C accommodates in a octahedral site and 86.8 pm if it is located in a tetrahedral one.

   Expanded austenite allows a higher carbon concentration in the lattice, avoiding the maximum solubility imposed by the Fe-C diagram. C is less soluble than N (less affinity with Cr), so the concentration is lower, and the expansion too. On the other hand, film thickness will be high. The structure of the expanded austenite is the fcc one, but stressed and with defects. Some authors think that the crystal structure is triclinic or tetragonal.

   Carbon ions created by the plasma are impinged in the lattice and they will diffuse following the Fick laws. The formation of a stable solution or a compound will depend on the Gibbs free energies involved during the process. Thus, diffusion is a thermal activated process.

  Free mean path calculation for carbon ion gives that this ion will imping in the lattice with a depth of 5 Å and with an energy of 55 eV (considering the plasma properties, the size of the cathode fall and the density of the region). Because of ion low energies, they will be neutralized quickly in the lattice.

  Lattice parameter is known by GIXRD measurements and the use of Bragg’s law with the three main peaks. Using the Nelson-Riley method (graph containing lattice parameters versus cotg( )·cos( ) product and obtaining the intercept value) a= 3.584 Å is obtained.

   Under loads of 25 and 300 g, it is determined that the hardness of the base material is 264 HV, or 2.589 GPa. Under the same experimental conditions used in tribology measurements, the reached value was S= 2.23·10-8 m3/m. Corrosion attack under a solution containing chlorides, showed that there were neither pitting nor external aggression, behavior typical in AISI 316L steels.

Experimental data for C50-: Sample Time (min) Pressure (mbar) Voltage (V) Current (A) Temperature (ºC) C50-030 30 4.985 572 1.14 401 C50-060 60 5.001 545 1.08 406 C50-120 120 4.982 549 1.35 411 In the following table, experimental data for C80- were shown: Sample Time (min) Pressure (mbar) Voltage (V) Current (A) Temperature (ºC) C80-030 30 4.993 415 1.43 409 C80-060 60 4.993 455 1.39 404 C80-120 120 4.984 520 1.36 406

  For C50- samples the current density was 7.0 mA/cm2, and for C80- samples was 8.1 mA/cm2. There was a graphitic film in the surface composed by carbon atoms. They did not enter in the steel structure. Soot is avoided when methane proportion is lower than 5 %.

Cross-sections were analyzed under Optical Microscopy. Expanded austenite film is thicker when the time treatment is higher. Thickness of C50-030 was 14 m (upper image), meanwhile the thickness of C80-120 sample was 27 m (lower image). Interface is well sharp. Expanded austenite film is resistant to oxalic acid attack.

   The upper region of the cross-section was analyzed by FIB. In this expanded austenite zone there were no Hägg carbides. a) showed the different crystallographic orientations in the base material, with breadths of 2-5 m and b) showed a high amount of defects and stacking faults in C50-030, with twins of 0.5-2 m thick. The features in the upper region of the surface were provoked by the polishing process.

  Diffractograms obtained at 10º of incidence showed the typical austenitic structure, with the (111), (200) and (202) planes shifted to lower angular values, i.e. higher lattice parameters. A representation of the expansion of C50- and C80samples using the NelsonRiley method showed that the major expansion rate is reached at the first 30 minutes of treatment, obtaining a expansion about the 90 % of the value reached at 120 minutes of treatment.

Sample Lattice parameter (Å) Expansion (%) C50-030 3.660 2.12 C50-060 3.661 2.15 C50-120 3.666 2.29 Sample Lattice parameter (Å) Expansion (%) C80-030 3.654 1.95 C80-060 3.669 2.37 C80-120 3.659 2.09

  With AES, it is possible to measure the elemental profile in the first 156 nm (65 minutes of sputtering). In C50- the amount of Fe increases until reach a stationary value (62 %). C is maximum on the surface but it decreases quickly to a concentration of 15 %. O is a surface contaminant and disappears in the first layers. Cr concentration increases slowly.

  For the C80- samples, Fe increases with depth until reach a value of 60 %. C decreases with regard of its surface concentration and carbon reach a stationary value which depends on the time treatment: a) 15 %, b) 14 % and c) 28 %. Cr concentration increases with depth, but with 5 minutes of sputtering a maximum is reached (21 %), i.e. a dome-shaped concentration profile is obtained. This is a consequence of the preferential sputtering.

    With AES is possible to see the carbon chemical state inside the expanded austenite. This technique is called Factor Analysis. It is calculated the proportion of the carbon bonded chemically (carbide type) and the amount of carbon freely dissolved (graphitic type). Carbide type C is present in all samples and the amount increases with depth. It is caused by the high surface current density used (> 7.0 mA/cm2).

   In the first 2000 nm Berkovich indentation was applied using different loads on the surface. Surface hardness in C50-030 was 11.8 GPa and in C80-030 was 11.0 GPa. Both values were obtained at 200 nm depth. In the maximum depth probed, hardness decreased in both samples, until reach a value of 8 GPa.

   For higher depths than 2 m the last technique was not longer valid, so Vickers nanoindentations in the samples’ cross-section were made. The maximum hardness was reached at 10 m depth, where C50-030 reached 7.5 GPa and C80030 reached 8.8 GPa. Hardness decreased until 15 m depth, obtaining a constant value of 5.2 GPa from there to the maximum depth probed.

   For depths higher than 140 m it is mandatory to use Vickers indentations with loads of 300 g. Analyses are obtained from the expanded austenitebase material interface to 6 mm depth. C50-030 hardness was 2.84 GPa, while for C80030 sample hardness was 2.70 GPa. At depths of 1 mm hardness decreased to a value of 2.50 GPa, near to the value of the base material.

R v

  Expanded austenite is the responsible of the wear resistance improvement. More time treatment means higher wear resistance and lower friction. The free graphitic-type carbon acts like a solid lubricant. Sample S (m3/m) W% C50-030 1.34·10-8 60 C50-060 0.15·10-8 7 C50-120 5.22·10-8 234 C80-030 0.36·10-8 16 C80-060 0.34·10-8 15 C80-120 0.16·10-8 7

     At first sight, there is a graphitic film on surface. For C50- there are gray spots of 80 m diameter. They are pitting agglomeration of 5 m diameter, each one. Far away from this agglomerations, pitting have a diameter of 0.5 m. The maximum density of pitting is located in the grain borders and slip bands. They are high energy centers and provoke the precipitation of Cr, bonded with C. The high corrosion resistance of the steels is lost, but this effect is only located at the surface. In C80- samples corrosion is general and the penetration is estimated in 2 mm.

 Carburized and nitrided AISI 316L steel samples have been analyzed under beam irradiation composed by light and energetic ions. The behavior of this treated samples under long heat treatments has been analyzed, too. Their surfaces could be covered by aluminum nitride, or not.

    Samples used in this study were treated at temperatures between 400-410 ºC during 80 minutes. For carburization, the gas mixture was 50 % Ar, 45 % H2 and 5 % CH4. For nitriding, the gas mixture was 80 % H2 and 20 % N2. In nitriding, current density was 1.55 mA/cm2 and in carburization current density was 2.08 mA/cm2.

       Bombardment using light ions was developed with a plasma focus device, 2 kJ of energy and Mather design. Anode had a diameter of 40 mm. Base pressure (cleaning procedure): 0.013 mbar. Working pressure: 1.6 mbar of deuterium (D) or helium (He). Loading voltage of capacitor bank (4 F): 31 kV. Samples were placed at 82 mm from the anode, power developed was 10 MW/cm2. Half of the sample was covered with a steel foil. Number of plasma focus discharges: 1, 5 and 10.

Carburized, 1 pulse with D     Carburized, 10 pulses of He Surface morphology is analyzed with Optical Microscopy at 500X. In carburized samples there are no different effects with the change of bombardment gas (D or He). Higher number of shots, higher crater density and material sputtering. High thermal gradients (in the first layers the temperature reached 1500 ºC during the first nanoseconds) provoke slip band crosslinking.

Nitrided, 1discharge of D     Nitrided, 10 discharges of He Regarding nitrided samples, there are no differences using both types of ions. However, it is concluded that the resistance to the bombardment is lower in nitrided samples than in the carburized ones. There are craters and sputtered regions, combined with a peeling provoked by the high thermal gradients involved during the process. With the increasing number of ion discharges, the surface layers are melted.

Carburized, 1 discharge of He    Carburized, 5 discharges of D Nitrided, 5 discharges of D FIB/SEM analyses using Ga+ ions determine with better resolution the surface morphology, and their crosssection, too. Both gases used to bombard the surface provoke craters. In carburized samples there are crosslinking in slipping bands. In the nitrided sample there are cracks and bulges because of the first layers melting.

Carburized, 1 discharge of He     Carburized, 5 discharges of D Nitrided, 5 discharges of D Cross-section shows grains with strain due to the bombardment. Steel samples with several discharges show melting and amorphization located at the first layers. At 3 m depth, thermal effects do not surpass the melting point. Crystallite features can be observed, provoked by the nucleation and short growth of stressed austenitic steel grains.

    Crystallites are formed in high extent at three micrometers of depth. Temperature rising provokes the restart of diffusive processes, so this region lost all their expansive elements (C or N). Quick temperature drop halts the crystallite (of austenitic stainless steel) coarsening, so beads of 36 nm of diameter and agglomerates of them are formed. The amorphous matrix induces high residual stresses.

  GIXRD with incidence of 2 and 10º show the crystalline structure of bombarded samples and with depths between 0.7 and 3 m, respectively. The hidden region presents diffractograms similar to the ones in the sample before the plasma focus bombardment. Carburized, with discharges of D Nitrided, with discharges of D

     Only the (111) peak is analyzed because there is a similar behavior in (200) and (202) peaks. Higher number of discharges provokes peak shifting to higher angular values, that is, less lattice parameter value. There is another peak, located always in the same angular position: 43.3º. Combining with FIB/SEM, peak shifting is related to the temperature rise and the N or C diffusion to inner layers, so the fcc expansion is gradually reduced. The peak anchored at 43.3º can be created by the crystallites present in the first 3 m of material. This value represents a lattice parameter of 3.6163 Å, with a relative expansion of 0.90 %.

Carburized, with discharges of He    Nitrided, with discharges of He In both cases there is a loss of austenitic expansion with a high number of pulses. In carburized samples there is no the peak located at 43.3º, and there is an absence of crystallites when FIB/SEM analyses are developed. In nitrided samples the peak is present. Peak overlapping means that the loss of expansion will finish when the lattice parameter is the same than the lattice parameter related to the 43.3º peak.

Carburization treatment of AISI 316L steel Discharges Deuterium (Å) Expansion (%) Helium (Å) Expansion (%) 0 3.6696 2.39 3.6779 2.62 1 3.6545 1.97 3.6529 1.92 5 3.6491 1.82 3.6557 2.00 10 3.6429 1.64 3.6504 1.85 Nitriding treatment of AISI 316L steel Discharges Deuterium (Å) Expansion (%) Helium (Å) Expansion (%) 0 3.8073 6.23 3.7984 5.98 1 3.7491 4.61 3.7415 4.39 5 3.6691 2.37 3.6743 2.52 10 3.6468 1.75 3.6556 2.00

   For light ions, the irradiation effect has no effect due to the ion mass. The effect depends on (by accumulation) the number of discharges. The highest expansion (of expanded austenite) is obtained by nitriding, but the degradation of this expansion is the same for both treatments when the number of discharges is high.

 Using Vickers nanoindentations with loads of 25 g are useful in order of obtain the surface hardness. Nitrided AISI 316L steel Discharges Deuterium (HV) Helium (HV) 0 999±51 965±61 1 827±34 815±34 10 --- --- Carburized AISI 316L steel Discharges Deuterium (HV) Helium (HV) 0 461±25 443±34 1 364±34 293±6 10 366±16 367±14

    Expanded austenite hardness was measured in the zone covered by the foil. These values are the same than the ones of treated samples before the plasma focus irradiation. Nitrided samples have higher hardness than the carburized ones. In nitriding, ten discharges provoked surface melting and amorphization. Hardness could not be measured. There are not difference changing of bombarding gas. In carburization, hardness values were similar when D was used, but there were high hardness variations when He was used.

   Thermal shocks provoke the (111) peak shifting to higher values, because of the diffusion restart. In order to study the mechanisms involved in this process heat treatments of long duration have been done. Thin film aluminum nitride (AlN) deposition was developed in order to study its application as protective barrier against oxidation.

     Film coating is obtained by magnetron sputtering, in reactive mode. Magnetron has an Al target (high purity). DC voltage applied is 260 V and 154 mA of current. Power density is 5.01 W/cm2. Firstly, target cleaning with Ar and H2 during 15 minutes and a shutter is used during 20 minutes in order to obtain a stable discharge. The process is developed in a reactor of 94.03 L of capacity with a base pressure of 9.60·10-5 mbar and Ar purges. Working atmosphere is 50 % Ar and 50 % N2 at 6.65·10–3 mbar and flow of 12.0 mL/min. Target-substrate distance is 3 cm.

   Under these conditions deposition rate is 11 nm/min, so the total thickness is 330 nm after 30 minutes of deposition. The film has no crystalline structure because of the high substrate roughness (in nitrided or carburized austenitic steel). Nitrided or carburized samples with or without AlN were submitted in a furnace at 225, 325, 405 y 504 ºC of temperature during 20, 40 and 60 hours.

   With GIXRD at 2º of incidence carburized samples, with AlN and without this film, were analyzed. Both were heat treated during 60 hours at 405 ºC. It is observed that the AlN film avoids the oxygen entrance and there is a important reduction in the α-Fe2O3 peaks. The other peaks near to the oxides and austenite ones are provoked by the presence of Cr3C2, because of the AlN had no influence in the processes developed in the expanded austenite.

      Analyses were done only in the (111) peaks, because of the behavior is similar in the (200) and (202) peaks. This analysis is not dependent of the presence or absence of AlN. In carburized sample, analysis was done by GIXRD at 10º. For 225 and 325 ºC peak shifting to higher angular values (than the peak position of carburized sample not heat treated) was not observed. It is probable the ignition of reacommodation processes. Time process did not have effect in the lattice parameter reduction. At 405 and 504 ºC there was high peak shifting because of the temperature surpassed a kind of threshold in order to activate the diffusion processes.

Austenitic AISI 316L stainless steel sample was carburized and submitted to high temperature treatments during long times. Previously, lattice parameter was calculated in 3.6880 Å, so the equivalent relative expansion was 2.90 %. Temperature Time Lattice parameter Expansion (ºC) (h) (Å) (%) 225 20 3.6858 2.84 225 40 3.6841 2.79 325 20 3.6800 2.68 405 20 3.6594 2.10 405 40 3.6531 1.93 405 60 3.6521 1.90 504 20 3.6318 1.33

   Heat treatments in nitrided samples develop, according to GIXRD at 10º analyses, a similar behavior. Diffusion processes are activated after 300 ºC, but the driving force is very low. When the temperature reaches the value at which nitriding was developed (400 ºC) diffusion processes are highly triggered, so the lattice parameter reduction is greater than at low temperatures.

Nitrided AISI 316L steel sample under high temperatures and long times. Before the experiment, lattice parameter was 3.7082 Å, so the relative expansion was 3.46 %. Temperature (ºC) Time (h) Lattice parameter (Å) Expansion (%) 225 20 3.7308 4.10 225 40 3.7294 4.06 325 20 3.7173 3.72 405 20 3.6801 2.68 405 40 3.6756 2.56 405 60 3.6710 2.43 504 20 3.6398 1.56

  AISI 316L surface modification by nitriding and carburization was studied. Low pressure (5 mbar) plasma glow was used with DC power. These processes developed expanded austenite. Expanded austenite stability was analyzed under pulsed irradiation with light ion beams and high energies. Stability under high temperatures and long times was analyzed, too.

    The original austenite of steel is quickly expanded during the first 30 minutes of process. The structure obtained was expanded austenite, without macroscopic precipitates. In carburized samples (analyzed by AES), bonded C is formed in the fist half hour. After that, C is added in a graphitic way. High density currents provoked carbide formation.

   Hardness was higher than the base material until 0.7 mm depth. The maximum value was 12 GPa. Wear resistance was improved at higher time treatments because of the role of the graphitic C as solid lubricant. Corrosion resistance was reduced on the surface (156 nm depth).

    Under D and He ion irradiation there was a higher resistance of expanded austenite by carburization than by nitriding. For a particular treatment, damage effects were the same without regarding the ion beam chosen. Structure degradation was provoked by thermal shocks. They triggered the diffusion processes and the surface melting and amorphization. Crystallites were formed by austenitic steel highly stressed. This structure provoked a diffraction peak located at 43.3º.

    Crystalline structure was degraded at high temperatures and long times. AlN film coating avoided the surface oxidation. This degradation, at fixed temperature, did not change by the duration process used. After 405 ºC, which was the temperature used in order to obtain expanded austenite, the degradation process is highly enhanced.

   Expanded austenite induces on the austenitic stainless steels surfaces high hardness and high wear resistance. In ion carburization, the creation of a film composed by C non chemically bonded (graphitic) improves the wear resistance by lubrication effects. Corrosion resistance can be maintained if the first 156 nm are removed, region where carbides are located at the first stages, but they are not present in deeper regions.

          To my parents, my brother and my godmother, for their support. To Geo, for her love and help. To Jorge and Liliana for their affection and for being there every moment. To the Grupo Física del Plasma, for their friendship and interesting conversations. To electronic and mechanical workshop staffs, for their ideas during experiments and the creation of devices. To researchers and PhD-students from UNR, and all the people with whom I have worked. To my teachers and friends from Degree in Physics from UCO. To my fellows of hBirra and others, for their friendship and integration. To my friends from Spain, for theirs anecdotes and encounters. To Horacio for his excellent behavior with all the people and his help during experiments. We will never forget you.

 March 19th. ¡Viva la Pepa!

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