diplomsko delo
Leon Blatnik (Author), Blaž Karpe (Mentor), Robert Robič (Co-mentor)

Abstract

Diplomska naloga raziskuje vpliv procesa staranja na toplotne lastnosti, električno upornost in trdoto nerjavnega jekla 17-4 PH. Glavni cilj je bil ugotoviti, kdaj se prične izločanje precipitatov iz martenzitne matrice ter kako te spremembe vplivajo na toplotne in električne lastnosti materiala. Raziskava temelji na eksperimentalnih meritvah vzorcev, ki so bili toplotno obdelani pri različnih temperaturah staranja, določenih po specifikaciji AMS 5604E. V stanju jekla po raztopnem žarjenju (stanje A) je bila izmerjena najnižja toplotna prevodnost (13,47 W/(m·K)), kar je posledica močnega sipanja elektronov na defektih kristalne rešetke, ki nastanejo zaradi visoke prenasičenosti matrice z legirnimi elementi. V stanju H900 (staranje na 482 °C/ 1 uro) se je toplotna prevodnost povečala na 15,98 W/(m·K), zaradi izločanja bakrovih precipitatov (?-Cu) in urejanja kristalne rešetke, kar zmanjša število ovir za gibanje elektronov. Pri višjih temperaturah staranja ( 496, 552, 579, 593 in 621 °C) in štiri urnem staranju je toplotna prevodnost še naprej naraščala zaradi rasti precipitatov in s tem dodatnega urejanja kristalne rešetke, vendar je bil vpliv na toplotno prevodnost manj izrazit. Z meritvami električne upornosti pri segrevanju vzorca v stanju A (raztopno žarjeno in kaljeno) smo določili, da se izločanje precipitatov začne pri temperaturi 369,2 °C in konča pri temperaturi 477,7 °C. V tem območju se električna upornost znižuje zaradi urejanja matrice in posledično zmanjšanja sipanja elektronov. Končna temperatura procesa izločanja sovpada s temperaturo staranja za stanje H900. Nad to temperaturo je upornost ponovno naraščala zaradi večje amplitude nihanja atomov v kristalni rešetki. Trdota jekla je dosegla največjo vrednost (42,8 HRC) po staranju na 482 °C/1 uro (H900), ko so bili bakrovi izločki najmanjši in najbolj učinkovito ovirajo gibanje dislokacij. Pri višjih temperaturah staranja se je trdota vzorcem postopoma zmanjševala zaradi rasti izločkov in tvorbe povratnega avstenita. Najnižjo trdoto (29,2 HRC) je jeklo doseglo po staranju pri 621 °C (H1150), kar je nižja od vrednosti trdote jekla v stanju A (31,66 HRC). Rezultati meritev so v skladu s teoretičnimi pričakovanji in ponujajo vpogled v optimizacijo toplotne obdelave za želene mehanske in fizikalne lastnosti.

Keywords

jeklo 17-4 PH;toplotne lastnosti;staranje;izločevalno utrjevanje;meritve električne upornosti;metoda TPS;

Data

Language: Slovenian
Year of publishing:
Typology: 2.11 - Undergraduate Thesis
Organization: UL NTF - Faculty of Natural Sciences and Engineering
Publisher: [L. Blatnik]
UDC: 669
COBISS: 237919491 Link will open in a new window
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Downloads: 34
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Other data

Secondary language: English
Secondary title: influence of aging temperature on thermal properties of steel 17-4 PH
Secondary abstract: The diploma thesis investigates the influence of the aging process on the thermal properties, electrical resistivity, and hardness of 17-4 PH stainless steel. The main objective was to determine when precipitation from the martensitic matrix begins and how these changes affect the thermal and electrical properties of the material. The research is based on experimental measurements of samples subjected to aging heat treatments at different temperatures, as specified by the AMS 5604E standard. In the solution-annealed condition (Condition A), the steel exhibited the lowest thermal conductivity (13,47 W/(m·K)), resulting from strong electron scattering caused by lattice defects due to the high supersaturation of alloying elements in the matrix. In the H900 condition (aged at 482 °C for 1 hour), the thermal conductivity increased to 15,98 W/(m·K) due to the precipitation of copper-rich phases (ε-Cu) and lattice rearrangement, which reduced electron scattering. At higher aging temperatures (496, 552, 579, 596 and 621 °C) and four-hour aging time, thermal conductivity continued to rise as precipitates grew and the lattice further stabilized, though the effect became less pronounced. Electrical resistivity measurements during heating of the sample in Condition A (solution annealed and quenched) revealed that precipitation begins at 369,2 °C and concludes at 477,7 °C. Within this temperature range, resistivity decreases due to matrix ordering and reduced electron scattering. The final precipitation temperature coincides with the aging temperature for the H900 condition. Above this temperature, resistivity increased again due to enhanced atomic vibrations in the crystal lattice. The steel reached peak hardness (42,8 HRC) after aging at 482 °C/1 hour (H900), where the copper precipitates were finest and most effective in hindering dislocation motion. At higher aging temperatures, hardness gradually decreased due to precipitate coarsening and the formation of reverted austenite. The lowest hardness (29,2 HRC) was observed after aging at 621 °C (H1150), falling below the hardness of the solution-annealed Condition A (31,66 HRC). The experimental results align with theoretical expectations and provide insight into optimizing heat treatment to achieve desired mechanical and physical properties.
Secondary keywords: steel 17-4 PH;thermal properties;aging;precipitation hardening;electrical resistivity measurement;TPS method;
Type (COBISS): Bachelor thesis/paper
Study programme: 0
Embargo end date (OpenAIRE): 1970-01-01
Thesis comment: Univ. v Ljubljani, Naravoslovnotehniška fak., Oddelek za materiale in metalurgijo
Pages: XII, 39 f.
ID: 26471699