Универзитет у Крагујевцу, Факултет инжењерских наука
Abstract
Uspešno projektovanje alata u savremenoj industriji zahteva detaljno poznavanje karakteristika materijala i procesa u kojima će dati alat (ili konstrukcija) biti eksploatisani. Kada je reč o alatima za kovanje u toplom stanju, s obzirom na njihove radne uslove i stalno prisutna udarna opterećenja i visoke temeprature, podrazumeva se da se oni izrađuju od čelika koje odlikuje visoka jačina i tvrdoća, koje su postojane i na povišenim temperaturama. Pored toga, veoma važna karakteristika jeste i
otpornost materijala na pojavu i rast prslina koje dovode do otkaza alata. To dalje vodi ka zastoju u proizvodnji, povećanju troškova i vremena izrade proizvoda, padu
produktivnosti i dr. Međutim, budući da se alati neminovno oštećuju, kada do toga
dođe, postavlja se pitanje da li alat zameniti novim ili reparirati postojeći?
Današnja težnja industrije je u velikoj meri zasnovana i na samoodrživosti što
podrazumeva težnju ka sposobnosti kompanije da problemi koji nastanu budu rešeni u
okviru postojećih kapaciteta. S obzirom na to, reparatura alata zavarivanjem ili
navarivanjem se ističe kao jedan od najefikasnijih načina. Naravno, podrazumeva se
da navarivanje nije idealan proces, jer se njegovom primenom umnogome utiče na
svojstva pojedinih zona materijala koji se regeneriše, ali niz prednosti tog procesa
ga nameće kao nezaobilaznog u ovoj grani industrije. Zbog toga je neophodno što bolje
poznavati proces i ponašanje materijala pri navarivanju i biti u mogućnosti da se
predvide karakteristike materijala u zoni navarivanja, tj. otpornost materijala u
radnim uslovima alata i, ako je to moguće, predvideti radni vek regenerisanog alata.
Cilj ove doktorske disertacije je da, na jedan sistematičan način, analizom niza
karakteristika materijala dobijenih opsežnim eksperimentalnim ispitivanjem,
ukaže na uspešnost primene navarivanja za reparaturu oštećenih alata izrađenih od
termopostojanih čelika. U radu su analizirana dva čelika koja se najčešće koriste za
izradu alata za toplo kovanje i shodno njima, dva dodatna materijala. Radom je
obuhvaćeno i propisivanje tehnologije navarivanja ploča određene debljine iz kojih
su pripremani uzorci za različita ispitivanja. Ona su obuhvatila ispitivanje na
zatezanje na sobnim i povišenim temperaturama, ispitivanje tvrdoće i određivanje
mikrostrukture različitih zona navara, ispitivanje udarne žilavosti i određivanje
trajne dinamičke izdržljivosti za osnovne materijale. Kao najvažnija ispitivanja,
čiji parametri se mogu koristiti u proceni radnog veka navarenog alata, izdvajaju se
ispitivanja rasta zamorne prsline (da/dN) u različitim zonama navarenih ploča. Ta
ispitivanja su praćena optičkom i SEM metalografijom koje su poslužile za potvrdu
zaključaka donetih na osnovu eksperimentalno dobijenih rezultata. Na kraju je
izvedena i numerička analiza odgovarajućih modela u softverskom paketu ANSYS a
dobijeni rezultati su upoređivani sa eksperimentalnim.
Predložena tehnologija navarivanja, kao i koraci sprovedeni pri ispitivanju i
na kraju dobijeni rezultati, mogu veoma korisno da posluže firmama koje se bave
kovanjem i u svojim pogonima imaju alate izrađene od ovih i sličnih čelika.Successful design of tools in contemporary industry requires detailed knowledge of materials' characteristics and processes in which the given tool (or a structure) would be exploited. When the subject matter are forging tools for hot working, taking into account their operating conditions - constantly present impact loading and high temperatures, one assumes
that they are being manufactured from steels characterized by high strength and hardness,
which remain stable at elevated temperatures, as well. Besides that, their very important
characteristics should be resistance to appearance and growth of cracks that might lead to tool
failure, which would then lead to production down-times, an increase in costs and time of
manufacturing, productivity decay etc. However, considering that tools are ineviatably being
damaged during the exploitation, the question arises whether to replace the damaged tool by
the new one or to repair the existing.
Nowadays, aspiration in the industry is largely based on self-sustainability, which
means striving for the company's ability to solve the emerging problems within existing
capacity. Considering that, the tool reparation by welding or hard-facing becomes prominent
as one of the most attractive ways for solving those problems. Hard-facing, of course, is not
an ideal process, since its application strongly affects properties of certain zones of the
material being repaired, but a whole series of this process' advantages are imposing it as
unavoidable in the tool manufacturing industry. Due to those reasons, one must know, as best
as possible, the hard/facing process, material behaviour during it and has to be able to predict
the material properties in the hard-faced zone, i.e. material strength in the tool operating
conditions and, if possible, to predict the repaired tool's service life.
The objective of this doctoral dissertation is to show, in a systematic way, by
analyzing a number of material characteristics, obtained through extensive experimental
testing, the success of application of hard-facing for repair of the damaged tools made of
thermally stable steels. Two types of steels were analyzed, which are used for hot forging and,
accordingly, the two filler metals. That also included prescribing the hard-facing technology
of plates of a certain thickness, from which the samples for experiments were prepared.
Investigations included the tensile test at room and elevated temperatures, hardness testing
and determination of microstructure of different zones of the hard-faced layers, impact
toughness tests, as well as determination of the permanent dynamic durability of the two base
metals. As the most important tests, the parameters of which can be used for estimates of the
hard-faced tool's service life, were recognized as the fatigue crack growth investigation
(dа/dN) in different zones of the hard-faced layers. Those tests were accompanied by the
optical and SEM metallography, which served for verification of experimentally obtained
results. In the end, the numerical investigation was performed of the corresponding models in
the software package ANSYS and obtained results were compared to the experimental ones.
The proposed hard-facing technology, the steps that were taken during the
investigation, as well as obtained results, can be of great use to the forging companies that
have, in their plants, tools made of these or similar steels