Ion Nitriding of Stainless Steel: III

Transcription

1 Ion Nitriding of Stainless Steel: III INFLUENCE OF MICROSTRUCTURE ON NITRIDING PROPERTIES OF STAINLESS STEEL D. Manova, S. Heinrich, I. Eichentopf, S. Mändl, H. Neumann, B. Rauschenbach Financial Support by Europäischer Fond für regionale Entwicklung (EFRE) und Mittel des Freistaates Sachsen is gratefully acknowledged

2 Contents Motivation Experiment PIII nitriding of steel annealing mechanisms in steel Deformation of Stainless Steel Summary

3 Motivation Deformation Nitriding? Yield Strength Microstructure 2 cm Decreasing of grain size from 100 µm to 13 nm results in 1000 faster diffusion (academic exercise not suitable for large scale production) Gleichmaßdehnung ε = 850 % Bruchdehnung ε = 1025 %

4 Annealing vs.. Diffusion Plan View Cross-section Plan view Cross-section 9, ,5 900 ASTM Grain Size Number 8,0 non-annealed non-annealed 1060 C 1060 C 7,5 7,0 6,5 6,0 950 C 5,5 950 C 1120 C 1120 C 5,0 4,5 4,0 as received plane view cross-section C 1060 Annealing C Temperature 1200 C ( C) 1200 C Layer Thickness (nm)

5 Diffusion vs. Microstructure Surface Expanded Austenite Base Material Inconel 690: Fe 9 Cr 29 Ni 62 5 µm H. He, T. Czerwiec, C. Dong, H. Michel, Surf. Coat. Technol. 163/164, 331 (2003) : Fe72 Cr18 Ni10< No change in microstructure after nitrogen implantation. No grain boundary diffusion as diffusion length much smaller than grain size. Nitrogen diffusion is most probably governed by highly complex process: structure size below crystallite size: influence of 0-D & 1-D defects. Diffusion rate associated with dislocation density (?)

6 Diffusion vs. Microstructure 80 sample A Ø 15 mm B Relative Intensity (a.u) A B ,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 Depth (µm) 1000 C, 2 hours, cooled down in air. Implantation with 10 kev N 2 ions, 90 min, 350 C. Different microstructure within one sample. Almost identical nitrogen depth profiles despite of big difference in grain size.

7 Microstructure Deformation Strongly increased dislocation density. Elongation of grains along direction of deformation. Relaxation Decrease of dislocation density. Reorganisation of dislocations. Recrystallisation Competition of nucleation and grain growth. Strong decrease of defect densities (point defects & dislocations).

8 Contents Motivation Experiment Deformation of Stainless Steel local cold working (ε 5-10%) by wear experiment global cold working (ε > 25%) by mechanical deformation deformation by quenching Summary

9 Nitriding of Wear Tracks [µm] m C Layer thickness (nm) A B C 500 µm B A 5 mm 0 Wear test A B C Oscillating dry ball-on-disc geometry, WC ball (Ø 3 mm), hertzian contact pressure 1 GPa, v = 0,015 m/s, track length 40 m. Subsequent nitrogen implantation at 10 kv, 350 C for 90 min. Thicker nitride layer closer to wear track.

10 Wear vs. Microstructure 100 µm 20 µm 5 µm Preferential chemical etching of under wear track area for treated and untreated samples. Indicative of high stress and/or dislocation density in this region. Similar structure after wear test for implanted and non implanted sample. Additionally, drastic change of grain shape below wear tracks in accordance with stress simulation.

11 Simulation of von-mises Mises-Stress 10 µm 0.0 Stainless steel, load 1 N, no lateral force Stainless steel + 5 µm expanded austenite, load 1 N, no lateral force Stainless steel, load 1 N, friction Stainless steel + 5 µm expanded coefficient µ = 0.25 austenite, load 1 N, µ = GPa Pure normal loading leads to high stress, larger than yield strength, mainly just below surface region. Even higher stress during additional lateral loading (wear experiment), thus more deformation possible. But no cracking or delamination of surface layer.

12 Hardness /Wear vs. ε ,035 0,030 ε = 5% ε = 20% Hardness HV Deformation ε (%) Wear rate (µm/cyc.) 0,025 0,020 0,015 0,010 0,005 0,000 Deformation ε up to 10 % increases hardness. Almost no increase in hardness for deformation ε > 10 %: saturation level Similar wear rates for different deformation microstructure does not influence hardness

13 Microstructure vs. Deformation ε ¾ Strong change in microstructure with different aspect ratio after deformation. ε=0% ε = 20 % ¾ Elongation of grains increases with increasing deformation. ¾ Texture correlated with direction of deformation. ε = 40 % ε = 60 %

14 SIMS / XRD vs. ε ε = 5 % ε = 20 % 100 Ref % ε 5 % ε + PIII 20 % ε + PIII fcc-austenite Intensity (a.u) 10 Intensität Depth (µm) Winkel 2θ Almost identical nitrogen diffusion profiles for nitrided samples at different deformation level. Only austenitic phase together with expanded austenite seen in XRD profiles.

15 Ferrite / Martensite: Microstructure Martensitic phase Ferritic phase + Cementite inclusions 25 µm 10 µm PIII, 10 kv, 350 C, 90 min SEM viewgraphs show different microstructure, depending on annealing temperature and cooling rate: ferrite/cementite or martensite. Clearly distinguished nitrided layer with sharp interface to the base material in contrary to continuously decreased nitrogen concentration with the depth.

16 Ferrite / Martensite: Hardness + Wear base material nitrided martensite martensite ferrite ferrite 0,1 Hardness (GPa) Wear rate (µm/cyc) 0,01 1E-3 base material nitrided martensite martensite ferrite ferrite 0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Indentation Depth (µm) 1E Load (mn) Higher hardness for the base material in martensitic structure compare with ferritic structure. Relative increase of up to 4 times with nitrided martensite harder than nitrided ferrite. An increased wear resistance, by a factor of 5 20 after nitrogen implantation with about the same absolute wear resistance.

17 Ferrite / Martensite: SIMS + XRD N/Fe-Ratio (%) PIII, 10 kv, 320 C, 90 min F M F M Intensity(a.u.) F non-implanted F, PIII, 320 C, 90 min M, PIII, 320 C, 90 min Ferrite Martensite (5.5 at.% Carbon) ,0 0,5 1,0 1,5 2,0 2,5 3,0 Depth (µm) Angle 2θ (degree) Concentration independent nitrogen diffusion for martensite and ferrite structures. Similar diffusion profiles despite of completely different microstructure. No observable difference in XRD profiles, formation of expanded phase.

18 Summary & Open Questions Nitrogen diffusion in austenitic stainless is dominated by small defects, most likely point defects or dislocations (bulk diffusion dominating over grain boundary diffusion!). Annealing may lead to correlation between grain size and diffusivity. Deformation induced defect formation leads to increased hardness and wear resistance with a fast saturation. Austenite-martensite transformation with a change of diffusion mechanism not observed in Cr-Ni steel. Ferrite-martensite transformation has no significant effect on properties of nitrided layers. Difference between local and global stress levels must be included in quantification of deformation rate.

19 Thank You! J.W. Gerlach D. Hirsch Werkstatt IOM Leipzig IOM Leipzig IOM Leipzig

20 Layer Thickness: : SIMS vs. GDOS Depth (µm) 10,0 7,5 5,0 2,5 0,0-2,5-5,0-7,5-10,0-12,5-15,0-17,5 Austenite Nitrided Austenite 2m 8m 8m 20m 20m 40m 40m 160m 400m -20, ( ) Geometrically similar profiles with minimal width given by contact area. Abrasive wear for treated and untreated. Strong oxidation of rede-posited material, indicating very small crystallite size. No adhesion of removed material on WC-counterbody.

21 SIMS / XRD vs. ε

22 Steel Ion damage Metallurgy Chemistry Expanded Lattice Austenite CrN + α-fe Fe 3 N / Fe 3 C Additional reaction path: influence of deposited energy Low energy limit for expanded martensite formation! Temperature dependence: < 330 C < 400 C < Thermal activation energy, reaction enthalpy, potential barrier?