Lasersko zavarivanje čelika s povećanim sadržajem ugljika

Bauer, Branko (2002) Lasersko zavarivanje čelika s povećanim sadržajem ugljika. = Laser welding of steels with higher carbon content. Scientific master's thesis , Sveučilište u Zagrebu, Fakultet strojarstva i brodogradnje, UNSPECIFIED. Mentor: Kralj, Slobodan.

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Abstract (Croatian)

Ispitivanjem laserom zavarenih spojeva čelika s povećanim sadržajem ugljika otkriveno je da veliki porast tvrdoće u području zavarenog spoja ne djeluje bezuvjetno negativno na mehanička svojstva zavarenog spoja. U ovom radu obrađeno je istraživanje utjecaja parametara laserskog zavarivanja, unešene energije, položaja žarišta u odnosu na površinu materijala, te vrste i protoka zaštitnog plina na geometriju, mehanička svojstva, strukturu zavara i zone utjecaja topline i pojavu pukotina. U uvodnom dijelu rada, na temelju literaturnih podataka obrađene su fizikalne osnove i tehnologija laserskog zavarivanja. Obrađeni su utjecajni čimbenici na proces zavarivanja odnosno kvalitetu zavarenog spoja. Navedene su metode kontrole kvalitete kod laserskog zavarivanja. Obrađena je prikladnost čelika za zavarivanje laserom. U eksperimentalnom dijelu, zavarivani su limovi od čelika za poboljšavanje 25 CrMo 4 i 42 CrMo 4 (DIN EN 10083-1) koji prema kemijskom sastavu odgovaraju čelicima C 4730 i C 4732 (oznaka prema hrvatskoj normi). Debljina limova iznosila je 2 mm. Zavarivan je sučeljeni spoj. Zavarivanje je izvedeno pomoću cw Nd:YAG lasera srednje snage 2000 W, s vođenjem snopa pomoću optičkog vlakna promjera 600 pm i fokusne optike 120/120 mm koja omogućuje fokusiranje snopa na točku promjera 0,6 mm. Zaštitni plinovi argon i helij dovođeni su kroz koaksijalnu sapnicu promjera 5 mm. Relativno gibanje izvođeno je pomoću robota IGM limat RT 280-6. Namještena snaga iznosila je 1800 W, a gustoća snage 6,4.105 W/cm2. Pokusi su provedeni prema centralno kompozitnom planu. Zavari su ispitivani vizualnom i radiografskom metodom, izmjerene su geometrijske karakteristike zavarenog spoja, ispitivana je tvrdoća, analizirana je mikrostruktura, te je napravljen vlačni pokus na epruvetama sa zarezom. Provedena je statistička obrada rezultata mjerenja i matematičko modeliranje metodom odzivnih površina. Provedeno je ocjenjivanje kvalitete zavarenih spojeva prema HRN EN ISO 13919-1. Dobiveni su zavari visoke kvalitete, bez pojave pukotina. Dobivene maksimalne tvrdoće odgovaraju tvrdoćama koje se postižu kod potpunog rastvaranja sadržaja ugljika u austenitu te koreliraju s dobivenom mikrostrukturom. Dobivene čvrstoće zavarenih spojeva vise su od čvrstoće osnovnog materijala. Statistička obrada rezultata pokazala je da u promatranom intervalu parametara postoji značajan utjecaj brzine zavarivanja na geometrijske karakteristike zavarenog spoja, značajan utjecaj položaja žarišta na maksimalnu tvrdoću u zavarenom spoju te značajan utjecaj brzine zavarivanja na čvrstoću dobivenu kidanjem epruveta sa zarezom. Utjecaj vrste zaštitnog plina na sve promatrane karakteristike zavarenog spoja u ovom istraživanju nije se pokazao značajnim.

Abstract

Testing of joints made by laser welding in steels with increased carbon content revealed that high increase in hardness in the heat affected zone does not necessarily produce deterioration of mechanical properties of a joint. Research of effects of parameters of laser welding, heat input, location of focal plane related to the plate surface, kind and flow rate of shielding gas upon the geometry, mechanical properties, structure of weld metal and heat affected zone and occurrence of cracks is presented. In the introductory chapter, based on the reference data, physical principles and technology of laser welding are considered. Parameters affecting the process of welding and quality of welded joint are discussed. Also methods used in testing of quality of welded joints are mentioned. Weldability of steel when laser process is applied is considered too. In the section presenting research work, welding of heat treatable steel grades 25 CrMo4 and 42 CrMo4 – DIN EN 10083-1 (equivalent to C 4730 and 4732 according to Croatian national standards) is described. Thickness of base material has been 2 mm, and joint was of butt design. Welding was made applying Nd:YAG laser unit rated at 2 kW mean power in cw mode. Laser beam was guided through the optic fibre of 600 pm diameter and 120/120 focusing optics that provided focusing of beam into a spot of 0,6 mm diameter. Shielding gases (argon and helium) were supplied through the coaxial nozzle of 5 mm dia. Relative motion was performed by the IGM LIMAT RT 280-6 robot. Nominal power at welding was 1800 W and power density was at 6,4.105 W/cm2 level. Experimental work was conducted applying model of central composite plan. Welded joints were tested applying visual and x-ray methods, and geometry of joints has been measured. Also, hardness of joints has been tested, as well as microstructure. Tensile test applying notched samples was performed. Results of testing have been processed applying statistical methods and mathematical modelling using method of response surfaces is made. Quality of the produced welds has been evaluated applying HRN EN ISO 1319-1 standard. Produced joints are of high quality, without cracking. Values of maximum hardness are at the levels that may be expected for full dissolving of carbon content in austenite, and are well correlated to the revealed microstructure. Produced welds are of higher strength than that of base material. Statistical analysis revealed that in the tested range there are significant effects of welding speed upon the welded joint geometry, position of focal plane upon the maximum hardness in the welded joint and welding speed upon the strength registered at tensile test applying notched samples . Effect of type of shielding gas upon any of the weld characteristic proved to be insignificant in this research.

Item Type: Thesis (Scientific master's thesis)
Uncontrolled Keywords: lasersko zavarivanje; čelik; zavarljivost; kvaliteta zavarenog spoja
Keywords (Croatian): laser beam welding; carbon steel; weldability; quality of welding joint
Divisions: 1100 Department of Welded Structures > 1110 Chair of Welded Structures
Date Deposited: 22 Sep 2014 18:00
Last Modified: 16 Oct 2015 13:19
URI: http://repozitorij.fsb.hr/id/eprint/170

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