Vpliv pogojev staranja na trdoto izločevalno utrjevalnega nerjavnega jekla 17-4 PH : diplomsko delo

Čoper, Erika (author), Nagode, Aleš (mentor)

Jeklo z oznako 17-4 PH je izločevalno utrjevalno martenzitno nerjavno jeklo, ki vsebuje okoli 17 mas. % Cr, 4 mas. % Ni, 4 mas. % Cu in 0, 3 mas. % Nb. Tovrstna jekla se zaradi odlične kombinacije trdnosti, trdote in protikorozivnih lastnostih uporabljajo v vesoljski, letalski in kemični industriji. Uporaba je razširjena tudi v določenih vejah medicine, kjer se zahtevajo višje trdnostne lastnosti za razliko od avstenitnih nerjavnih jekel. Ustrezne mehanske lastnosti jekla z oznako 17-4 PH dobimo, ko ga primerno toplotno obdelamo, kar zajema raztopno žarjenje, kaljenje in staranje. S spreminjanjem pogojev toplotne obdelave, predvsem temperature in časa staranja, vplivamo na mikrostrukturo in mehanske lastnosti jekla. V okviru diplomske naloge smo jeklo z oznako 17-4 PH toplotno obdelali po postopku izločevalnega utrjevanja in preučevali vpliv pogojev staranja, tj. temperatura in čas na trdoto ter mikrostrukturo jekla. Trdoto jekla smo merili po Rockwellu, za analizo mikrostrukture pa smo uporabili vrstični elektronski mikroskop (SEM). Toplotna obdelava je vključevala raztopno žarjenje na temperaturi 1030 °C za 45 minut, kaljenje na zraku in staranje na 482 °C, 551 °C in 621 °C, in sicer od pol do 4 ure. Najvišja trdota je bila izmerjena pri jeklih po enournem staranju, pri temperaturi 482 °C, in sicer 47,5 HRC. Njegova mikrostruktura je sestavljena iz popuščenega martenzita, v čigar osnovi se nahajajo drobni bakrovi izločki, ki povzročajo izločevalno utrjevanje. Najnižjo trdoto po staranju je imelo jeklo, starano na 621 °C / 4 ure, in sicer 38 HRC. To lahko pripišemo predvsem raztapljanju bakrovih izločkov in nastanku povratnega avstenita.

• Language: Slovenian
• Keywords: jeklo 17-4 PH, izločevalno utrjevanje, mikrostrukturna analiza, bakrovi izločki, trdota
• Work type: High school thesis
• Typology: 2.11 - Undergraduate Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [E. Čoper]
• Year: 2021
• Number of pages: XI, 33 f.
• PID: 20.500.12556/RUL-131351
• UDC: 669
• COBISS.SI-ID: 84880643
• Publication date in RUL: 25.09.2021

Secondary language

• Language: English
• Title: The effect of aging conditions on hardness of 17-4 PH precipitation hardening stainless steel : diploma work

Abstract

The steel marked as 17-4 PH is a precipitation hardening martensitic stainless steel containing about 17 wt. % Cr, 4 wt. % Ni, 4 wt. % Cu in 0. 3 wt. % Nb. Due to its excellent combinations of strength, hardness and anti-corrosion properties, it is used in the aerospace and chemical industries. The use is also widespread in certain branches of medicine, where high-strength properties are required as opposed to austenitic stainless steels. The corresponding mechanical properties of steel 17-4 PH are obtained when it is properly heat-treated, which includes solution annealing, hardening and aging. By changing the conditions of heat treatment, especially temperatures and aging times, we have an influence on the microstructure and mechanical properties of steel. As part of the diploma thesis steel marked as 17-4 PH was heat-treated according to the process of precipitation hardening and the influence of aging conditions, as temperature and time on the hardness and microstructure development of steel was studied. The hardness of the steel was measured according to Rockwell and a scanning electron microscope (SEM) was used to analyze the microstructure. The heat treatment included solution annealing at 1030 °C for 45 minutes, air quenching and aging at 482 °C, 551 °C and 621 °C for half an hour to 4 hours. The highest hardness measured in steels was after one hour of aging at a temperature of 482 °C, namely 47,5 HRC. Its microstructure consists of tempered martensite with fine copper precipitates that cause precipitation hardening. The steel had the lowest hardness after aging at 621 °C for 4 hours, namely 38 HRC. This can be attributed primarily to the dissolution of copper precipitates and the formation of reverted austenite.

• Keywords: steel 17-4 PH, precipitation hardening, microstructural analysis, copper precipitates, hardness