THE INFLUENCE OF THE TYPE OF LASER WORK UPON THE CONSTRUCTION OF TOOLING HIGH-ALLOY STEEL S SURFACE LAYER.

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1 THE INFLUENCE OF THE TYPE OF LASER WORK UPON THE CONSTRUCTION OF TOOLING HIGH-ALLOY STEEL S SURFACE LAYER. a) Józef Jasiński b) Jacek Selejdak a) Wanda Jeziorska a) Aneta Markuszewska b) Robert Ulewicz a) Technical University of Częstochowa, Department of Engineering material`s Al. Armii Krajowej 19B, PL Częstochowa b) Technical University of Częstochowa, Department of Management, Division of Production Engineering. Al. Armii Krajowej 19B, PL Częstochowa Abstract The work analysed the impact of continuous laser with varied beam velocity from to W/cm 2 upon the structure and properties of the surface layer. Stratified construction of the surface layer was evaluated based on light and electron microscopy tests, as well as micro-hardness tests. Phase types and distribution were asserted on the basis of roentgenostructural tests. In order to evaluate the hardening depending on beam power, functional interdependences were asserted. 1. INTRODUCTION Laser applications in the metallurgical industry are quite universal. Laser beams are used for cutting, welding, quenching, alloying, and the like. All these operations, in principle, are based upon the heat effect of radiation impact upon metals; but as compared to the traditional heat processing methods, laser treatment has a number of advantages. Some of the most significant ones are: the operation s contact-less nature, local character of heating, a possibility to obtain big temperature gradients, and high speeds of heating and cooling. Due to these specific advantages, laser processing is used for hardening high-load surfaces of machines and tools (surfaces of bearings, toothed wheels, blades, etc.). Effects of such processing are dependable upon a number of factors, such as: type of steel, type and density of radiation power, processing time. Controlling these factors, we can obtain various structures, and what follows various mechanical properties of steel [1, 2]. 2. RESEARCH MATERIAL In the analyses, WCL hot-work tooling steel, and SW7M high-speed steel were used. Tables 1 and 2 present the chemical constitutions of the analysed types of steel. Table 1. Chemical constitutions of WCL steel Chemical constitutions C Mn Si P S Cr Mo V max max 0,42 0,57 0,78 0,02 0,026 5,1 1,35 0,41-1 -

2 Tabela 2 Chemical constitutions of SW7M steel Chemical constitutions C Mn Si Cr W Mo V 0,81 0,45 0,51 4,45 6,23 4,58 1,79 WCL steel analysis input material was a sample in the shape of a plate sized 40 x 15 x 5 mm. The sample was cut off from a bar, and subjected to the soft annealing process.. The soft annealing process was conducted with the following parameters: - softening temperature C (1073 K) - time - 3h (10,8 ks) - cooling -with the furnace SW7M high-speed tooling steel sample was enriched by diffusion in carbon and nitrogen inside a fluid bed furnace. The process of carbon-nitriding was conducted with the following parameters: - carbon-nitriding temperature C (1153 K) - time - 1h (3,6 ks) - excess air coefficient - α p =0,22 - and ammonia addition into the atmosphere - 2,5 % Fig. 1 presents input microstructures of the materials used in the analyses: a) WCL, b) SW7M. Fig. 1. Input material s microstructure: a) WCL steel in the condition after softening, area x 500, etched with nital b) SW7M steel in the condition after carbon-nitriding, area x 500, etched with natal The samples were treated with continuous laser beams 4 paths were made upon a sample. The paths were arranged at the sample s width, perpendicular to its length. Distances for steel between the subsequent paths were 8 mm. Fig. 2 presents sample schemes with paths plotted upon them

3 8mm 15 mm 5mm 40 mm Fig. 2. Sample schemes with paths plotted upon them. Diameters of laser bundles for the particular re-focusings were calculated according to formula (1), and power density according to formula (2). Laser treatment parameters are presented in Table 2. d = fb f (1) q = 4Q Πd 2 (2) Table 2. Parameters of laser treatment Variant processing changes of focusing f, mm heat flux Q, W Speed travelling v, mm/s Laser beam diameter d, mm Power density 10 3 W/cm ,80 13, ,73 7, ,60 3, ,53 2,40 3. RESEARCH RESULTS As a result of the analyses conducted, it was found that for WCL steel, with a decrease in power of a laser bundle, the thickness of a hardened layer increases. Analysing the distribution of the hardened zone for SW7M steel, it was also found that laser bundle s power density exerts some influence upon the increase in thickness of the hardened zone; this zone, however, is thicker. Fig. 3 compares the influence of laser bundle power density upon its interaction with analysed sample materials

4 WCL SW7M Thick of hardened layer, mm 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 13*10E3 7,3*10E3 3,3*10E3 2,4*10E3 Power density W/cm 2 Fig. 3. The influence of laser bundle s power density upon the thickness of a hardened layer of the analysed material. Micro-hardness analyses were carried out using the Vickers method with the load of 0.5 N. Analysed for micro-hardness were samples after laser treatment perpendicular to their surfaces, in the middle of a laser bundle s focus. Fig. 4 presents micro-hardness measurements results Sample no 1, WCL, power density 13*10 3 W/cm 2 Sample no 2, WCL, power density 2,4*10 3 W/cm 2 Sample no 1, SW7M, power density 13*10 3 W/cm 2 Sample no 2, SW7M, power density 2,4*10 3 W/cm 2 Hardness HV ,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Distance from the surface, mm Fig. 4. Micro-hardness distribution in the zone hardened with laser beams perpendicular to the sample s surface in the focus of the bundle for such materials as: WCL, SW7M. As a result of SW7M steel metallographic analysis conducted, cracks were observed in the zone of heat impact, which arose due to the occurrence of heat stresses, at the border of - 4 -

5 carbide releases. Carbide releases have a striped morphology[3]. Fig. 5 presents characteristic cracks at the border of carbides striped releases. Analysing the structure of WCL steel heat influence after laser treatment, no occurrences of cracks were observed. Fig. 5. SW7M steel structure in the heat influence zone with heat cracks shown: a) area x 100 etched with nital, b) area x 1000 etched with nital Fig. 6 shows the SW7M steel surface after passage of a laser bundle with varying power density. Together with an increase in power density, the paths become wider and wider. c) Fig. 6. Sample surface at spots of a laser bundle impact: a) sample no 1, area 35 x, power density 13 x 10 3 W/cm 2, b) sample no 2, area 35 x, power density 7.3 x 10 3 W/cm 2, c) sample no 2, area 35 x, power density 2.4 x 10 3 W/cm 2-5 -

6 At Fig. 6 one can see, in the central part of the path, some arches that emerged as a result of material shift caused by the movement of the laser radiation bundle. This effect can be compared to the effect of a padding weld. SW7M material s stratified structure can be described using control model 4: - the first stratum arose as a result of re-melting of the material, - the second one is the stratum of partial melting of the material, - the third one hardened material, - the fourth one tempered material. In effect of WCL steel s surface treatment by means of continuous laser beams, a 3- stratum structure of the top layer was observed: - a bright white stratum, - having arisen from the re-melting of the material; - a hardened warp stratum; - and an intermediary stratum. Fig. 7 presents the stratified structure of WCL and SW7M steels. Fig. 7. Stratified structure of the top layer of steel: a) WCL after treatment with continuous laser beams, b) SW7M after treatment with continuous laser beams 4. RECAPITULATION In the course of the analyses conducted, it was noted that re-melting zones occurred in both of the analysed materials. Fig. 8 illustrates the structure of the analysed steels. Fig. 8. Scanning micro-structure etched with nital area x a) WCL, b) SW7M - 6 -

7 Fig. 8a shows partial meltings upon surfaces of the treated materials. As a result of a very fast disposal of heat, fine-acicular martensite and residual austenite arose. Also, there appeared a process of coagulation of carbides, and a process of increasing their size. Looking at Fig.6b, one can notice some very small releases in the acicular form. Due to the carbonnitriding process carried out earlier, these could be needle-shaped releases of carbide nitrides. Carbides that can be seen at this photo are very fine and rather uniformly arranged. Fig. 9 a, b depict the occurrence of a dendritic structure in the stratum of SW7M steel after laser processing. Micro-hardness analyses results suggest that the heat impact zone was limited by striped releases of carbides that did not allow the heat impact field to extend in steel SW7M, which was not observed in the case of steel WCL. Based upon the results as listed at Fig 4, it is easy to see the increase in micro-hardness inside the hardened zone, at a certain distance from the surface for steel SW7M, whereas in the case of steel WCL, hardness decreases together with an increasing distance away from the surface. Fig. 9. Dendritic micro-structure for steel SW7M etched with nital, hardened with laser beams with bundle density 2.4*10W/cm 2. The structure as shown at Fig. 9b was achieved as a result of very strong etching of remeltings, using nital. One can easily see dark grains isolated from one another with white phases. Dark areas are most probably martensite, and the white phase in between these grains may be some releases of cementite eutectic (carbides) at the borders of martensite grains. Surface areas in other samples also posses the same cellular structure. Looking at the areas located deeper away from the surface, a layer was obtained having a dendritic structure, typical for a quick-cooled material. The micro-structure of this zone is illustrated at Fig. 9a. LITERATURE 1. Zenker R., Reisse G., Zenker U.: Niektóre aspekty laserowej obróbki cieplnej stali, II Konferencja Obróbki Cieplnej, Karl-Marx-Stadt 1985, tłum. Strauss J., Materiałoznawstwo i obróbka cieplna, nr 83-84, Serżysko J., Sobusiak T., Sokołow K. N.: Wpływ obróbki cieplnej laserowej na strukturę i własności stali szybkotnących, Metaloznawstwo i obróbka cieplna, Inżynieria powierzchni, nr , Kusiński J. Naprężenia własne w konstrukcyjnych stalach chromowych poddanych laserowej obróbce cieplnej, Materiały konferencyjne Obróbka Powierzchniowa II Ogólnopolska Konferencja Naukowa Kule