, Hradec nad Moravicí. SURFACE HARDENING FOR LOW CARBON STEEL BY YAG:Nd PULSE LASER

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1 METAL , Hradec nad Moravicí SURFACE HARDENING FOR LOW CARBON STEEL BY YAG:Nd PULSE LASER Nelu Cazacu a Sorin Dobrovici a Elena Drugescu a Octavian Potecaşu a Adolf Bâclea b Florentina Potecaşu a a Dunărea de Jos University of Galaţi, Romania, Nelu.Cazacu@ugal.ro b S.C. COSENA S.R.L Constanţa, Romania. Abstract The paper is based by experiments for surface hardening on A3k (low carbon steel) samples using YAG:Nd pulse laser. A small x-y moving system conducted by PC was made for sample scanning, relative to fixed focused laser beam. Laser energy, frequency and defocalization influences were studied. The results of laser/sample surface interactions were investigate by macrostructure and microstructure analyses. Hardness testing was used for evaluate surface properties modifications. 1. INTRODUCTION surface treatments deposition techniques electroplating Thermal spraying PVD diffusion techniques thermal (phase hardening) thermochemical nitriding carburizing CVD flame hardening laser hardening induction hardening electron beam hardening carbonitriding ferritic nitrocarburizing boronizing... Figure 1. Principal technique for surface treatments of metals Surface treatment of metals (surface modification, surface engineering, and case hardening) is subdivided into two distinctive directions: deposition and diffusion techniques. Deposition techniques are characterized as transporting a substance from a source metal and depositing it onto the surface of another metal (electroplating, thermal flame spray (hard coating), physical vapor deposition (PVD) and chemical vapor deposition (CVD), [8]. Diffusion techniques are usual named thermochemical and thermal treatment. Thermochemical treatments (nitriding, carburizing, carbonitriding, nitrocarburizing, boronizing) are characterized as diffusing an element or many elements, into the surface of the steel by 1

2 the application of the appropriate amount of heat, time, and the steel surface catalytic reaction, [6]. Thermal techniques are those that modify the surface phases of steel containing sufficient carbon to allow the transformation from austenite to martensite when the appropriate amount of heat is applied to the immediate surface. This sometimes is known as phase hardening and is applied to processes such as flame, laser, induction, and electron beam 8. Surface hardening of steels is generally based by energy transfer and/or mass transfer (interactions) with material and exterior media Because interactions are conducted only at surface, structure modifications are generate and properties modifications will be obtained. This is thermal hardening of metals and alloys that is based on local heating of a surface under the influence of exterior heating system and subsequent fast cooling of this surface [1]. Surface hardening of steels by thermal interactions consists in formation of an austenite structure at a stage of heating and its subsequent transformation in martensite in a stage of cooling [1]. Temperature and time are more important factors and these appear for all surface treatments. For heat treatments only energy interactions was considered. Temperature influence is localized at material surfaces for high (austenitic) values and time for short values. For example at surface hardening by inductions treatments an electromagnetic field energy is localized at material surface (increasing local energy) and after heating an water spray is used for decreasing local energy. Temperature has a secondary influence on other processes like diffusion that are responsable at austenitic temperature for structure gradient and structure properties 2. EXPERIMENTAL CONDITIONS Lasers are complex systems which transform electric energy to radiation energy that having a very important property: higher control of characteristics. In these conditions surface treatments using laser radiations having a high precisions and are dedicated for small surfaces operations. For experiments was used pulse laser system KVANT 17 (C.I.S.) based on two YAG:Nd (Y 3 Al 5 O 12 ) units [3]. Basic applications of this system are ceramic cuting and microwelding of different materials. Some important technical characteristics are: active solid media: 6,3 mm diameter, 100 mm long; glass composition: Y 3 Al 5 O 12 ; wave length 1,06 µm (IR); pulse time 2 5ms; pulse freqventa 1 20 Hz; objective focalization 50 mm; trace diameter 0,3 1,3mm; pulse energy min. 8J; Maximum hardening surface 400 mm 2 (20mmx20mm) Figure 2. Pulse laser system KVANT 17 (working module, supply and command module and technological module) Figure 3. Working module of KVANT 17 system

3 system Figure 5. Hardening surfaces on A3k samples arranged by energy and focalization Figure 4. Sample and technological xy Technological module offer a x-y moving of samples with a fixed laser beam (Figure 4). Before hardening sample surfaces was prepare by succesive abrasive paper (700 matalographic paper). Hardening surface made on A3k samples is 144mm 2 (12mm x12mm) and hardening samples is showing in Figure 5. Experimental regimes is showing in Table 1 (left side). Deocalization, frequence and pulse energy (indirect measured by power condensator supply) was varried. 3. RESULTS AND DISCUSSION Sample surfaces after susscesive pulses irradiation of KVANT 17 system having an high energy traces of interaccions. In function of pulse energy, pulse frequence a focalization surface interaction appears and has a specified macroscopie aspect (Figure 6... Figure 11). Figure 6. Surface no.5 600V/7,5mm/0,3Hz). Figure 8. Surface no.9 (700V/10mm/0,3Hz). Figure 10. Surface no.3 (650V/5mm/0,3Hz). Figure 7. Surface no.7 700V/7,5mm/0,3Hz). Figure 9. Surface no V/10mm/0,3Hz). Figure 11. Surface no.4 (700V/5mm/0,3Hz)..

4 Figure 12. Surface micrograph (350x) for sample no.1 (550V/5mm/0,3Hz). Figure 14. Surface micrograph for sample no.3 (650V/5mm/0,3Hz). Figure 13. Surface macrograph (350x) for sample no.2 (600V/5mm/0,3Hz). Figure 15. Surface no.4 (700V/5mm/0,3Hz). Table 1. Hardness after laser hardening for different regimes Microscopic analyze for A3k representative steel samples are showing in (Figure Chyba! Nenalezen zdroj odkazů.). Thermale zone influence is visible in Figure 12 the energy is insufficient for local quenching, [4]. By increasing the energy the traces is largest and surface modifications is present (Figure 13). If energy increasing a local melting is present that conduced to a major modificatins on surfaces an to local quenching from melt steel (Figure Figure 15). Hardness testing (HV 5 ) was used for evaluate surface properties modifications (Table 1 ). Figure 16. Hardness variation on the sample surface after pulse laser hardening In Figure 16. is showing hardness on pulse laser working surfaces by focalization influence and pulse energy. 4. CONCLUSIONS For all samples hardness increasing that interactions phenomenon between laser beam and surfaces are in physical domain and quenching process was produced. By increasing defocalization specific energy on the interaction surface decreasing and hardness is lower. If pulse laser energy increase, the hardness increase too, but the quenching process is limiting by melting phenomenon. For all regimes a hardness increasing with 50%... 80% was measured in conditions when A3k steel has <0,1%C in chemical composition. The applications of

5 METAL , Hradec nad Moravicí YAG:Nd pulse laser is recommended to very smalls surfaces, because lower efficiency of laser (2 5%), [7]. BIBLIOGRAPHY 1. GANEEV R.E, Low power hardening of steels, Journal of Materials Processing Technology 121 (2002) ; 2. POPESCU N, etc., Tratamente termice neconvenţionale, Editura tehnică, Bucureşti 1990; pag ; 3. SAMOILA C, Ionescu M.S, Drugă L, Tehnologii şi utilaje moderne de încălzire, Editura Tehnică, Bucureşti, 1986; pag ; 4. URSU I, MIHĂILESCU I.N, PROKHOROV A.M, KONOV V.I, Interacţiunea radiaţiei laser cu materialele, Editura Academiei, Bucureşti, DONŢU O, Tehnologii de prelucrare cu laser, Editura Tehnică, Bucureşti, SAVII Gh, Laseri, Editura Facla, De KOCK J, Lasers Offer Unique Heat Treating Capabilities, Industrial Heating, oct.2001, Laser Machining Inc., Somerset, Wis. 8. PYE D, Diffusion Surface Treatment Techniques A Review, Industrial Heating, March