Mikrostrukturne promjene pri izradi cijevi iz lima od austenitnog nehrđajućeg čelika

master's thesis

• Title (english): Microstructural changes during fabrication of pipe from austenitic stainless steel sheet
• Author: Martin Petek
• Mentor: Vera Rede (mentor)
• Granter: University of Zagreb; Faculty of Mechanical Engineering and Naval Architecture; Zagreb
• Defense date and country: 2018-07-12, Croatia
• Scientific / art field, discipline and subdiscipline: TECHNICAL SCIENCES; Mechanical Engineering; General Mechanical Engineering (Construction)

Abstract

Ovaj rad je napravljen s ciljem određivanja mikrostrukturnih promjena koje nastaju tijekom izrade šavnih cijevi iz lima od austenitnog nehrđajućeg čelika X2CrNi18-9, debljine 0,5 mm. U teorijskom dijelu rada navedena je podjela nehrđajućih čelika, opisana su njihova mehanička svojstva, korozijska postojanost i područja primjene za svaku skupinu. Posebna pozornost je posvećena ponašanju navedenih čelika pri zavarivanju. U eksperimentalnom dijelu rada opisan je proces proizvodnje šavne cijevi iz lima, koji se kroz faze savijanja, zavarivanja, provlačenja i toplinske obrade dovodi u oblik cijevi vanjskog promjera 8 mm i debljine stijenke 0,5 mm. Nakon svake faze, na pripremljenim metalografskim uzorcima provedena je kvalitativna i kvantitativna analiza mikrostrukture. Mikrostruktura polaznog materijala lima u potpunosti je austenitna, s nešto višom mikrotvrdoćom u poprečnom nego u uzdužnom presjeku. Nakon savijanja lima u oblik cijevi, mikrostruktura nije promijenjena. Metalografskom analizom utvrđena je zadovoljavajuća kvaliteta zavarenog spoja. Oblik zavara je pravilan, linija staljivanja je jasno izražena, a u tipičnoj ljevačkoj mikrostrukturi metala zavara nisu uočene nepravilnosti. U ZUT-u su mjestimice izlučene nepoželjne faze po granicama austenitnih zrna. Unesena toplina pri zavarivanju smanjila je mikrotvrdoću osnovnog materijala, a mikrotvrdoća zavarenog spoja je viša od mikrotvrdoće polaznog lima. Nakon provlačenja u svrhu postizanja traženog oblika i dimenzija cijevi, austenitna mikrostruktura osnovnog materijala je dosta ugnječena, a deformacije je pretrpio i zavareni spoj. Sve to utjecalo je na povišenje mikrotvrdoće. Rastvornim žarenjem na 1050 °C homogenizira se mikrostruktura u zoni utjecaja topline, zaostala naprezanja nastala tijekom zavarivanja i provlačenja se smanjuju, što je rezultiralo smanjenjem mikrotvrdoće materijala.

Abstract (english)

The goal of this paper was to determine microstructural changes which occur during the production of a seam pipe from sheet metal 0.5 mm thick, made of austenitic stainless steel grade X2CrNi18-9. First part of the paper contains the classification of stainless steels, their mechanical properties, chemical properties and areas of application. Behavior of various stainless steels during welding was discussed in detail. Second part of the paper describes the production process of a seam pipe, which is made by forming sheet metal into a tube, welding, pulling and heat treating. The final product is a pipe with the outside diameter of 8 mm and pipe wall thickness of 0.5 mm. Qualitative and quantitative analysis of the microstructure was conducted on metallographic samples after each phase of production. The microstructure of starting material is completely austenitic. Microhardness is a bit lower in the rolling direction than in the perpendicular direction. After the sheet metal is formed into a pipe, no microstructural changes were observed. The metallographic analysis determined that the quality of the welded joint is adequate. The weld shape is correct and the solidification line is clearly visible. Typical cast microstructure of the weld metal does not contain any anomalies. The heat affected zone contains a small quantity of unwanted phases which precipitated along the austenitic grain boundaries. Heat ingress during welding lowered the base material microhardness. Microhardness of the welded joint is higher than the starting material. Pulling is done to achieve the desired pipe shape and dimensions of the pipe. Squashing of the base material and deformation of the welded joint were observed after pulling. This caused the microhardness to rise. Annealing at 1050 °C caused the heat affected zone microstructure to become homogenous and it reduced all residual stresses, which resulted in reduction of microhardness.

• Keywords: austenitni nehrđajući čelici; šavna cijev; zavarivanje; mikrostruktura
• Keywords (english): austenitic stainless steels; seam pipe; welding; microstructure
• Language: croatian
• URN:NBN: urn:nbn:hr:235:677865
• Study programme: Title: Mechanical Engineering; Study programme type: university; Study level: graduate; Academic / professional title: magistar/magistra inženjer/inženjerka strojarstva (magistar/magistra inženjer/inženjerka strojarstva)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2019-05-22 22:29:56