Leskovšek, Vojteh (1999) Optimiranje postupka toplinske obradbe brzoreznih čelika u vakuumu. = Doctoral thesis , Sveučilište u Zagrebu, Fakultet strojarstva i brodogradnje, UNSPECIFIED. Mentor: Liščić, Božidar.
Full text not available from this repository.Abstract (Croatian)
Opisano je optimiranje vakuumske toplinske obradbe brzoreznog čelika S 6-5-2. Nakon vakuumske toplinske obradbe takav čelik sadrži veći udjel zaostalog austenita, austenitna zrna su veća, njihove kristalne granice jače su označene precipitatima karbida nego u slučaju konvencionalne toplinske obradbe u solnoj kupki. Zapremninski udjel neotopljenih eutektičkih karbida u takvom čeliku ovisi samo o temperaturi austenitizacije. Mikrofraktografske analize pokazale su, da se iniciranje loma u relativno tvrdom i krhkom brzoreznom čeliku odvija djelomično i koalescencijom mikropora. Lomna žilavost takvog čelika opisana je modificiranim oblikom Hahn-Rosenfieldove korelacije, koja uzima u obzir i utjecaj mikrostrukturnih parametara: zapremninskog udjela neotopljenih eutektičkih karbida i srednje razmake medu njima te zapremninski udjel zaostalog austenita. Udjel tog austenita numerički je skoro jednak eksponentu deformacijskog očvršćavanja čelika. Utjecaj naprezanja tečenja i lomne duktilnosti izražen je tvrdoćom čelika. Budući da je veličina plastične zone pri mjerenju lomne žilavosti toga čelika u pravilu manja od veličine kristalnih zrna, onda ona ne utječe na lomnu žilavost. Pokazalo se da se vrijednosti lomne žilavosti, koje su izračunate pomoću ovdje razvijene jednadžbe, dobro slažu s rezultatima provedenih eksperimenata kao i s eksperimentalnim podacima iz literature. Analiza dimenzijskih promjena pri vakuumskoj toplinskoj obradbi epruveta Navy C-ring , pokazala je da se te promjene mogu kontrolirati i da na veličinu promjena dimenzija utječe udjel zaostalog austenita u mikrostrukturi i postignuta tvrdoća. Eksperimentalno je potvrđeno da pri eksploataciji alata za rad u hladnom koji je izrađen od brzoreznog čelika ne dolazi do pojave deformacijski inducirane transformacije zaostalog austenita u martenzit.
Abstract
The optimisation of vacuum heat treatment of high speed steel S 6-5-2 is described. After vacuum heat treatment of this steel the quantity of retained austenite in microstructure is larger, austenite grains are coarser, and their boundaries are more densely covered with precipitates than after heat treatment in salt bath. The volume fraction of undissolved eutectic carbides in high speed steel depends on austenitisation temperature. The microfractography showed that the initiation of the fracture in relatively hard and brittle high speed steel occurs also with a local coalescence of microvoids. For the calculation of fracture toughness a modified form of Hahn-Rosenfield correlation is developed. It considers also the influence of microstructural parameters such as: the volume fraction of undissolved eutectic carbides, their average mutual distance, and the volume fraction of retained austenite. This fraction is numerically appr. equal to the strain hardening exponent of the steel. Both the yield stress and the fracture ductility are considered in relation to steel hardness. Since the size of plastic zone is, as a rule, smaller than the austenite grains size, it does not influence the fracture toughness. It was proved that the values of fracture toughness calculated with the developed equation correspond with our experimental results, and also with already published experimental results. The analysis of vacuum heat treated Navy C-ring specimens confirmed that the dimensional changes could be controlled. The main factors that influence the dimensional changes are the volume fraction of retained austenite and steel hardness. It was confirmed experimentally that during exploitation of cold working tools from high speed steel no strain induced transformation of retained austenite to martensite occurs.
| Item Type: | Thesis (Doctoral thesis) |
|---|---|
| Uncontrolled Keywords: | vakuumska toplinska obradba; brzorezni čelik; mikrostruktura; tvrdoća; eksponent deformacijskog učvršćavanja; lomna žilavost; dimenzijske promjene |
| Keywords (Croatian): | The optimisation of vacuum heat treatment of high speed steel S 6-5-2 is described. After vacuum heat treatment of this steel the quantity of retained austenite in microstructure is larger; austenite grains are coarser; and their boundaries are more dense |
| Divisions: | 1000 Department of Materials |
| Date Deposited: | 22 Sep 2014 18:00 |
| Last Modified: | 16 Oct 2015 13:20 |
| URI: | http://repozitorij.fsb.hr/id/eprint/75 |
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