Resistance to appearance and propagation of cracks in hard-faced layers of hot work tool steels

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