Študij razvoja vključkov pri izdelavi obraboodpornih jekel

Soklič, Nina (author), Knap, Matjaž (mentor), Klančnik, Grega (comentor)

V diplomski nalogi je predstavljen razvoj vključkov pri izdelavi obrabo odpornih jekel ter vpliv le-teh na končne mehanske lastnosti materiala. Obrabo odporna jekla se najpogosteje uporabljajo pri vitalnih delih strojev za predelavo in transport zemljine in rude, agrikulturi asfaltnih bazah ipd. Zato je potrebna dobra abrazijska odpornost in dobra varivost. Izbrana kemijska sestava, plastična predelava in toplotna obdelava omogočajo funkcionalnost jekla z dobro žilavostjo pri nizkih temperaturah in dosežen razpon trdot od 250 do 600 HB. Tako morajo zaradi svoje namembnosti združevati med seboj izključujoči se lastnosti – visoko trdoto in dobro upogibno sposobnost. Prvo zagotavljamo s primerno stopnjo legirnih elementov in pravilno vodeno vročo predelavo (vroče valjanje) in toplotno obdelavo. Druga pa je v večji meri odvisna od mikročistosti jekla, na katero vplivajo postopki na področju sekundarne metalurgije. V eksperimentalnem delu je opisan postopek odvzema in priprave vzorcev, analiza na optičnem in svetlobnem mikroskopu ter uporaba termodinamskih izračunov. Opisana sta tudi postopka merjenja trdote po Brinellu in udarnega preizkusa po Charpy-ju. Pri metalografski analizi smo ugotovili, da je mikrostruktura jekla HB 400 sestavljena večinsko iz martenzita, v manjših količinah pa tudi bainita, ki se je transformiral iz avstenita. Na pozitivnih izcejah se pojavljajo grobi TiN, ki so lahko potencialna mesta za nastanek mikrorazpoke. S kontrolo topnostnega produkta [Ti][N] in temperaturo solidus je izločanje grobih TiN mogoče omejiti. Na koncu so predstavljen ternarni diagrami Ti-N-B in Nb-Ti-C z vrisanimi sestavami nitridnih nekovinskih vključkov za doseženi topnostni produkt [Ti][N]. V primeru preblagega mešanja taline z argonom in/ali previsokega doseženega topnostnega produkta [Ti][N], je obstoj primarnih nitridov (TiN) potrjen že v talini nad temperaturo likvidus.

• Language: Slovenian
• Keywords: jeklo HB 400, nekovinski vključki, mikrostruktura, titanov nitrid, CALPHAD
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2019
• PID: 20.500.12556/RUL-107805
• COBISS.SI-ID: 1814367
• Publication date in RUL: 26.05.2019

Secondary language

• Language: English
• Title: Study of inclusions development in the production of wear resistant steels

Abstract

In this diploma thesis a development of inclusions in manufacture of wear-resistant steels is presented, as well as their influence on the final mechanical properties of a material. Wear-resistant steels are mainly used in the vital parts of machines for processing and transport of soil and ore, agriculture, asphalt production etc. Therefore, a good abrasion resistance and welding properties are required. The chosen chemical composition, plastic rolling sequence and heat treatment enable a full functionality of steel with good toughness at low temperatures and a range of hardness from 250 to 600 HB. Thus, due to their purpose, they must combine the exclusive properties of each other - high hardness and good bending ability - bendability. The first can be provided with a suitable level of alloying elements and properly guided hot processing (hot rolling) and heat treatment. The second is largely dependent on the micro-purity of steel, which is influenced by processes in the field of secondary metallurgy. The experimental part describes the process of sampling and sample preparation, analysis with the optical and light microscope. Procedures regarding the Brinell hardness test and the Charpy Impact test are described. In the metallographic analysis, it was found that the microstructure of the HB 400 steel is composed mainly of martensite and, in small quantities, bainite, which has been transformed from austenite. On positive bands, coarse TiN0,0 appear, which may be potential sites for the formation of a microcracks. By controlling the solubility product [Ti][N] and the solidus temperature, the elimination of coarse TiNs can be limited. Finally, the ternary diagrams Ti-N-B and Nb-Ti-C are presented along with the composition oft he present nitrides / carbo-nitrides within achieved solubility product [Ti][N]. In the case of insufficient argon stirring of the melt with argon and / or the excess of solubility product obtained [Ti][N], the existence of primary nitrides (TiN) is confirmed already above the liquidus temperature.

• Keywords: steel HB 400, non-metallic inclusions, microstructure, Titanium nitride CALPHAD