Analiza vzrokov in preprečevanje pojava razpok na glavi vijakov pri nakrčevanju : diplomsko delo

Jančič, Luka (author), Bombač, David (mentor), Zagajšek, Aljaž (comentor), Nagode, Aleš (member of the commission for defense), Knap, Matjaž (member of the commission for defense)

Pri postopku masivnega preoblikovanja z vročim nakrčevanjem jekla 42CrMo4 se pojavljajo poškodbe odkovkov. Poškodbe se kažejo v obliki razpok na zgornjem delu odkovka, predvsem na območju izteka glave izdelka, kjer med preoblikovanjem nastajajo največje natezne napetosti in lokalne deformacije. Takšne poškodbe negativno vplivajo na kakovost izdelka, povečujejo delež izmeta ter povzročajo dodatne stroške proizvodnje. Ena izmed glavnih omejitev pri odpravljanju nastalih poškodb so predpisane geometrijske tolerance odkovka, ki omejujejo možnost spreminjanja dimenzije radija prehodov, ter uporabo predoblike pri procesu masivnega preoblikovanja. Zaradi teh omejitev je potrebno poiskati takšne tehnološke procesne rešitve, ki bodo zmanjšale nastanek razpok, ne da bi pri tem odstopali od predpisanih dimenzijskih zahtev izdelka. V okviru diplomskega dela je bila izvedena analiza celotnega proizvodnega procesa, s poudarkom na stabilnosti postopka utopnega nakrčevanja v vročem, ter nadaljnje toplotne obdelave odkovkov. Analiza je obsegala spremljanje in optimizacijo ključnih tehnoloških procesnih parametrov, ki so vplivali na tok materiala, porazdelitev deformacij in na nastanek poškodb. Poleg tega je bila preučena možnost prilagoditve geometrije utopa za nakrčevanje, predvsem povečanja radija prehoda na kritičnem območju, kjer so bile spremembe omejene z dovoljenimi tolerančnimi območji izdelka. Za ugotavljanje vzrokov nastanka razpok so bile uporabljene eksperimentalne raziskovalne metode. Mikrostrukturna preiskava je bila izvedene z uporabo svetlobne optične mikroskopije in vrstične elektronske mikroskopije. Analiza mikrostrukture je omogočila identifikacijo morfologije nastalih razpok ter določitev mehanizma nastanka. Dobljeni rezultati so bili povezani s tehnološkimi procesnimi parametri preoblikovanja, z namenom ugotovitve vplivov na pojav razpok. Na podlagi izvedenih raziskav je bila podana ocena vpliva posameznih procesnih parametrov na kakovost odkovkov ter predlog optimalnih tehnoloških parametrov vročega nakrčevanja in možnih konstrukcijskih sprememb utopa. Namen raziskave je bil zmanjšati pojav razpok na izteku glave odkovka in s tem povečati stabilnost proizvodnega procesa ter izboljšati kakovost končnega izdelka.

• Language: Slovenian
• Keywords: nakrčevanje, glava odkovka, razpoka
• Work type: Bachelor thesis/paper
• Typology: 2.11 - Undergraduate Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: L. Jančič
• Year: 2026
• Number of pages: XIII, 45 f.
• PID: 20.500.12556/RUL-188931
• UDC: 669
• COBISS.SI-ID: 293957123
• Publication date in RUL: 30.09.2026

Secondary language

• Language: English
• Title: Analysis of causes and prevention of cracking on bolt heads during forging : diploma work

Abstract

During the bulk metal forming process by hot upsetting, defects have been observed in forgings manufactured from 42CrMo4 steel. The defects appear as cracks on the upper part of the forging head, particularly in the head transition region, where the highest tensile stresses and localized plastic deformations occur during the forming process. Such defects adversely affect product quality, increase the reject rate, and lead to higher production costs. One of the main limitations in eliminating these defects is the prescribed dimensional tolerances of the forging, which restrict modifications to the transition radii as well as the use of a preform during the bulk forming process. Consequently, it is necessary to identify process improvements that reduce crack formation while maintaining compliance with the specified dimensional requirements of the component. This thesis will include an analysis of the entire manufacturing process, with particular emphasis on the stability of the hot die upsetting process and the subsequent heat treatment of the forgings. The study will focus on monitoring and optimizing the key process parameters that influence material flow, strain distribution, and defect formation. In addition, the possibility of modifying the die geometry, particularly by increasing the transition radius in the critical region, will be investigated, provided that the modifications remain within the specified dimensional tolerances. Experimental investigations will be carried out to determine the causes of crack initiation and propagation. The microstructural characterization will be performed using light optical microscopy and scanning electron microscopy, enabling a detailed analysis of the crack morphology and the mechanisms responsible for their formation. The results of the metallographic examinations will be correlated with the analysis of the forging process parameters to determine their influence on defect formation. Based on the results obtained, the influence of individual process parameters on forging quality will be evaluated, and recommendations for the optimization of the manufacturing process and potential die design modifications will be proposed. The primary objective of this research is to reduce crack formation in the head transition region of the forging, thereby improving process stability and enhancing the quality of the final product.

• Keywords: forging, forging head, crack