Influence of nitriding and coatings on the properties of hot work tool steels

Alatni čelici za topli rad namijenjeni su za izradu alata za oblikovanje metalnih i drugih materijala koji se u radu zagrijavaju na temperaturu iznad 200°C poput kalupa za tlačni lijev, ukovnja, alata za prešanje, ekstruziju, valjanje i duboko vučenje te kalupa u staklarskoj industriji. Navedeni alati u radu su izloženi, osim visokim toplinskim opterećenjima uslijed kontakta površine alata s obrađivanim materijalom, značajnim mehaničkim, tribološkim i kemijskim opterećenjima. Stoga, alatni čelici za topli rad moraju zadovoljiti cijelu skalu vrlo specifičnih svojstava. Za tribološku zaštitu i poboljšanje svojstava reznih alata, u cilju produljenja njihova vijeka trajanja i povećanja produktivnosti, najčešće se koriste tvrde prevlake. Prednosti primjene tvrdih prevlaka, koje su dobro poznate za rezne alate, u znatno su manjoj mjeri istražene kod alata za oblikovanje. Uzroci su složena geometrija, velika masa i niža tvrdoća osnovnog materijala ovih alata što negativno utječe na prionjivost prevlake, a time i na svojstva površinskog sloja. U okviru doktorskog rada razvijeni su novi površinski slojevi postupcima plazmatskog nitriranja i plazmom potpomognutog prevlačenja iz parne faze (PACVD) s ciljem poboljšanja svojstava alatnih čelika za topli rad i produljenja vijeka trajanja alata. U tu su svrhu razvijeni plazmatski nitrirani sloj i dvije višeslojne gradijentne prevlake TiCN i TiBN. U teorijskom dijelu rada detaljno su opisani mehanizmi dotrajavanja alata od alatnih čelika za topli rad, postavljeni su najvažniji zahtjevi vezani uz njihova svojstva te je dan pregled postupaka modificiranja i prevlačenja iz perspektive primjene na alatnim čelicima za topli rad. U eksperimentalnom dijelu rada analizirana su postignuta svojstva novorazvijenih površinskih slojeva, od karakterizacije slojeva do svojstava koje bi alatni čelici za topli rad trebali imati, s posebnim naglaskom na otpornost prema toplinskom umoru kao glavnom uzroku dotrajavanja ovih čelika. Ispitivanje površinskih slojeva i usporedba s konvencionalnom toplinskom obradom kaljenja i popuštanja koja se koristi u praksima provedena je na alatnom čeliku za topli rad X38CrMoV5-3. Detaljna karakterizacija dobivenih površinskih slojeva provedena je analizom kemijskog sastava slojeva (GDOES i EDS) i njihove mikrostrukture (svjetlosnim i skenirajućim elektronskim mikroskopom), određivanjem kristalne strukture (XRD) i hrapavosti površine, ispitivanjem prionjivosti prevlaka metodom brazdanja i metodom utiskivanja Rockwell C indentora, debljine prevlaka metodom utiskivanja kuglice i mikrotvrdoće dobivenih slojeva. U cilju kvantificiranja utjecaja novorazvijenih površinskih slojeva na svojstva i trajnost alatnog čelika za topli rad provedeno je ispitivanje otpornosti na trošenje u uvjetima abrazijskog (suhi pijesak/gumeni kotač), adhezijskog (eng. load scanning test) i erozijskog trošenja (erozija česticama), određivanje faktora trenja i kliznog trošenja (kuglica na ploči) te ispitivanje otpornosti na koroziju. Ispitivanje otpornosti na toplinski umor provedeno je na vlastito konstruiranom i izrađenom uređaju. U cilju potvrđivanja rezultata laboratorijskih ispitivanja novorazvijenih površinskih slojeva i istraživanja njihovog utjecaja na trajnost kalupa za tlačni lijev provedeno je ispitivanje u eksploatacijskim uvjetima na kalupu za tlačni lijev radijatora od aluminijske legure. Provedena analiza dobivenih rezultata ukazala je na bitno poboljšanje svojstava i produljenje vijeka trajanja alatnog čelika za topli rad primjenom novorazvijenih površinskih slojeva što je i potvrđeno ispitivanjem u eksploatacijskim uvjetima.Hot-work tool steels are intended for the fabrication of tools used for the shaping and forming of metals and other materials, such as die casting dies, drop hammer dies, tools for pressing, extrusion, rolling, deep drawing, and moulds used in glass industry. In service, these tools are heated to temperatures above 200oC because the tool surface and the material being formed are in contact. Besides being subjected to high temperatures, these tools are also subjected to mechanical loads and there are tribological issues and chemical actions involved in the process. Therefore, hot-work tool steels must have a wide range of specific properties. Hard coatings are generally used for tribological protection of cutting tools in order to improve their properties and thus to prolong their life and to increase productivity. Advantages of the application of hard coatings that are well known in the field of cutting tools have been studied less extensively in the field of tools for shaping and forming. This fact can be accounted for by a complex geometry and great weight of the tool together with a lower hardness value of the tool base material, which have a negative effect on the coating-substrate adhesion, and thus on the surface layer properties. In this doctoral thesis, new surface layers have been developed by using plasma nitriding and plasma assisted chemical vapour deposition (PACVD) with the aim of improving the properties of hot-work tool steels and of prolonging the tool life. For that purpose, a plasma nitrided layer and two gradient multilayer coatings, TiCN and TiBN, have been developed. The theoretical part of the thesis gives a detailed description of wear mechanisms in tools made of hot-work tool steels together with key requirements related to their properties. In addition, an overview of modification and coating processes is given regarding their application to the hot-work tool steels. In the experimental part of the thesis, the properties achieved in the newly-developed surface layers are analysed in terms of layer characterisation and expected properties of hotwork tool steels, with special focus on resistance to thermal fatigue as the main cause of failure in hot-work tool steels. The developed surface layers on the X38CrMoV5-3 hot-work tool steel were tested and results were compared with the results of conventional heat treatment processes of hardening and tempering commonly used in practice. Detailed characterisation of the surface layers was carried out by the analysis of their chemical composition (using Glow Discharge Optical Emission Spectrometry – GDOES and Energy Dispersive Spectrometry – EDS) and microstructure (using a light microscope and a scanning electron microscope), by determining their crystal structure (using X-Ray Diffraction – XRD) and surface roughness, by testing the adhesion of coatings (using the scratch test method and the Rockwell hardness test method - Rockwell C indenter) and the thickness of layers (using the ball indentation test) and by testing the micro hardness of nitrided layers and PACVD coatings. In order to quantify the effect of newly-developed surface layers on the properties and durability of hot-work tool steels, the following properties were tested: abrasive wear resistance (using the dry sand/rubber wheel test), adhesive wear resistance (using the load scanning test), erosive wear resistance (using the solid particle erosion test), and corrosion resistance. Resistance to thermal fatigue was tested using a device designed and manufactured by the doctoral candidate. In addition, the coefficients of friction and sliding wear were determined using the ball on flat sliding test. A die for the casting of aluminium alloy radiators was tested in real conditions in order to confirm the results of laboratory testing of the newly-developed surface layers and of their influence on the durability of the die. The analysis of results showed that the properties of the tool steel were significantly improved and the tool life prolonged by the newly developed surface layers, which was confirmed in real conditions