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Title:Razvoj modela za napovedovanje zdržljivosti preoblikovalnih orodij za vroče kovanje s trdimi prevlekami
Authors:ID Podgrajšek, Martin (Author)
ID Ren, Zoran (Mentor) More about this mentor... New window
ID Glodež, Srečko (Comentor)
Files:.pdf EDOK_Podgrajsek_Martin_2016.pdf (19,64 MB)
MD5: FE73E87FEA0EBCE1C34BDA87DBA9B359
 
Language:Slovenian
Work type:Dissertation
Typology:2.08 - Doctoral Dissertation
Organization:FS - Faculty of Mechanical Engineering
Abstract:V priloženi doktorski disertaciji je obravnavana tematika razvoja modela za določevanje zdržljivosti orodja za vroče kovanje, zaščitenega s trdo prevleko. Kovaško orodje za delo v vročem je med obratovanjem izpostavljeno visokim cikličnim termomehanskim, kemičnim in tribološkim obremenitvam, ki po določenem času na tribološkem stiku povzročajo nastanek različnih mehanizmov obrabe, poškodb, plastične deformacije in termomehanskega utrujanja materiala. Zdržljivost orodja lahko izboljšamo z oplemenitenjem efektivnih površin gravure na osnovi ustrezne difuzijske plasti in trde prevleke. Pri oplemenitenju površin je vedno bolj v uporabi tehnologija zaščite gravure s postopkom »dupleks«, ki ga sestavlja kemo-termična obdelava ter nanos trde obrabno obstojne prevleke. V okviru računalniških simulacij je bil izdelan numerični model vročega kovanja, na podlagi katerega je bila izdelana analiza glavnih parametrov vročega kovanja. Na osnovi numeričnega modela smo v gravuri kovaškega orodja določili temperaturno in napetostno – deformacijsko polje, ki predstavljata osnovo za izvajanje eksperimentalnega dela ter določitve dobe trajanja kovaškega orodja. Pri eksperimentalnem delu je bila izvedena podrobna metalografska analiza poškodb in obrabe kovaškega orodja, zaščitenega s trdo prevleko. S pomočjo elektronskega mikroskopa smo določili karakteristična področja gravure in analizirali glavne mehanizme poškodb površine orodja. Z uporabo trdomera smo analizirali potek trdote v odvisnosti od globine delovne plasti gravure. Metalografska analiza je na izpostavljenih mestih gravure pokazala izrazite poškodbe površine v obliki luščenja materiala zaradi prisotnosti razpok v difuzijski coni. Kritične poškodbe delovne plasti gravure so nastale že po 6.500 udarcih kladiva. Nanos trde prevleke na kovaškem orodju ni prispeval k izboljšanju zdržljivosti kovaškega orodja. Zdržljivost orodja je bila pri izbrani konstrukciji gravure in pri danih termomehanskih obremenitvah manjša od pričakovane. Za izboljšanje zdržljivosti orodja je potrebno optimizirati konstrukcijo orodja in karakteristike difuzijske plasti. Izvedeni sta bili tudi analizi statične trdnosti in analiza parametrov nizkocikličnega utrujanja orodnega jekla pri povišani temperaturi 150 °C in 500 °C na osnovi testiranj preizkušancev z in brez trde prevleke. Eksperimentalni preizkusi so pokazali, da je v režimu nizkocikličnega utrujanja pri napetostih okoli meje tečenja vpliv PVD površinske prevleke nezanemarljiv. V režimu visokocikličnega utrujanja pri napetostih v elastičnem področju pa ima PVD prevleka pozitiven učinek na utrujenostno obnašanje materiala, kar prispeva k daljši obratovalni dobi v primerjavi z materiali brez prevleke. Ugotovljene utrujenostne parametre je mogoče uporabiti za napovedovanje obratovalne dobe podobnih orodij za vroče kovanje v pogojih nizkocikličnega utrujanja.
Keywords:vroče kovanje, obraba, prevleke, doba trajanja, računalniško modeliranje
Place of publishing:Maribor
Publisher:[M. Podgrajšek]
Year of publishing:2016
PID:20.500.12556/DKUM-61631 New window
UDC:004.942:539.435:621.73.07(043.3)
COBISS.SI-ID:286603008 New window
NUK URN:URN:SI:UM:DK:YJYUCO9L
Publication date in DKUM:20.09.2016
Views:2021
Downloads:306
Metadata:XML DC-XML DC-RDF
Categories:KTFMB - FS
:
PODGRAJŠEK, Martin, 2016, Razvoj modela za napovedovanje zdržljivosti preoblikovalnih orodij za vroče kovanje s trdimi prevlekami [online]. Doctoral dissertation. Maribor : M. Podgrajšek. [Accessed 23 April 2025]. Retrieved from: https://dk.um.si/IzpisGradiva.php?lang=eng&id=61631
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Secondary language

Language:English
Title:Die wear model development for durability prediction of hot forgings dies protected with hard coatings
Abstract:Die wear model development for durability prediction of hot forging dies protected with hard coating is presented in this doctoral thesis. The forging dies are subjected to complex loading conditions at tribological contact surface during hot forging process, which are characterized by fast changing rate of mechanical, thermal, chemical and tribological loading. Different combinations of these loadings lead to appearance of different dominant types of die damage, like wear, plastic deformation, mechanical and thermal fatigue and surface cracking, which are all time dependent. The effective way to enhance durability of forging dies is to modify the die surface properties by proper constitution of die surface diffusion layer, overlaid with hard coating. The most widely used surface treatment technology is called a duplex treatment, a combination of plasma nitriding of die surface layer with subsequent deposition of hard antiwear coating. Computational modelling and simulations of the forging process were performed to estimate the temperature, stress and strain distribution in the investigated critical regions of the forming tool during the forging process. The experimental testing and comprehensive failure analysis of the hot forging die insert, made of hot working steel and additionally surface improved with plasma nitriding and hard coating, used in a hot forming operation on a hydraulic drop hammer in real industrial environment was conducted. Failure mechanics of the die surface, diffusion layer and hard coating was investigated by metallographic analysis using the SEM microscopy. The hardness distribution in the surface layer of the die insert before and after testing has been evaluated with the microhardness measurements. The metallographic investigation of the hot forging die showed significant surface spalling, caused by a number of cracks appearing in the surface coating and underlying diffusion layer at some critical regions after only 6.500 forging strokes. Surface coating of hot forging die insert contributed to significant decrease of its fatigue resistance due to combination of die design and high local thermomechanical loading caused by rapid energy transfer of forging force to the workpiece in the penultimate forging operation on a hydraulic drop hammer. Optimized die shape design with combination of improved plasma nitriding process is required to extend the service life of hot forging die. An additional experimental investigation was conducted to determine the low cycle fatigue behavior of the hot working steel. The influence of hard coating on the low cycle fatigue behavior of treated material was additionally studied with consideration of typical hot forging die surface operating temperatures of 150 C and 500 C. The experimental results have shown that in a low cycle fatigue regime at stress ranges around yield strength the influence of PVD surface coating on the fatigue behavior is negligible. However, in a high cycle fatigue regime at stress ranges in elastic domain the PVD surface coating has a positive effect on the material fatigue behavior, which results in a longer fatigue life in comparison to the uncoated specimens. The determined fatigue parameters can be used to predict the fatigue life of similar hot forging tools under low cycle fatigue conditions.
Keywords:hot forging, wear, coatings, service life, computational modeling


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