Thermodynamic characterisation of hot work tool steels

Glavni cilj doktorske disertacije je razširitev znanj na področju orodnih jekel za delo v vročem, vpliva legirnih elementov, tehnološkega postopka izdelave in predelave ter toplotne obdelave na končne mikrostrukturne in mehanske lastnosti ter toplotno prevodnost. Prvi del naloge obsega primerjavo termodinamične karakterizacije dveh kromovih orodnih jekel za delo v vročem oznake Dievar in RS450 ter dveh jekel nove generacije oznake HTCS-130 in W600, ki so osnova za načrtovanje kemijske sestave in lastnosti novo izdelanega jekla s povečano toplotno prevodnostjo. Drugi del raziskave obsega izdelavo laboratorijske šarže novo razvitega jekla in celovito termodinamično, mikrostrukturno in mehansko karakterizacijo ter meritve toplotne prevodnosti jekla. Določena je bila kemijska sestava novega jekla, in sicer na osnovi referenčnega jekla HTCS-130, iz katerega so bile izdelane štiri šarže jekel. Referenčno jeklo je bilo v prvi šarži le pretaljeno, ostale šarže pa so bile dodatno legirane z dodatkom niobija, tantala in titana. Dokazano je bilo, da imajo dodatki legirnih elementov velik vpliv na povečanje trdote in popuščne obstojnosti jekla, na račun tvorbe in povečanja deleža novo izločenih karbidov. Mehanizem izločanja karbidov v jeklih s povečano toplotno prevodnostjo bazira na izločanju po mejah kristalnih zrn, kar negativno vpliva na poslabšanje žilavosti. V sklopu doktorske naloge je bilo dokazano, da preiskovani tip jekla dosega relativno majhne žilavosti in da velikost zrn v tem primeru nima vpliva na izboljšanje oziroma poslabšanje žilavosti jekla. Vsi dodatki so se izkazali za učinkovite pri zaviranju rasti zrn. Največji vpliv na dvig mehanskih lastnosti kot tudi na dvig toplotne prevodnosti in bistveno izboljšanje obrabne obstojnosti ima tantal. V sklopu izdelave novega jekla so bile v primerjavi z referenčnim jeklom izboljšane tako mehanske, obrabno obstojne lastnosti kot tudi toplotna prevodnost.The main goal of the doctoral dissertation is to expand the knowledge in the field of hot work tool steels, the influence of alloying elements, technological process of manufacturing, processing, and heat treatment effect on the final mechanical and microstructural properties related to thermal conductivity. The first part of the research includes a comparison of the thermodynamic characterisation of two chromium hot work tool steels Dievar and RS450, and two new generation steels with increased thermal conductivity HTCS-130 and W600. The results are the basis for planning the chemical composition and properties of the newly produced steel with increased thermal conductivity. The second part of the research includes the production of a laboratory batch of the newly developed steel and a comprehensive thermodynamic, microstructural and mechanical characterisation including measurements of the thermal conductivity of the steel. The basis of the chemical composition of the new steel was determined based on the reference steel HTCS-130, from which four batches of steel were produced. The base of the reference steel was only vacuum remelted in the first batch, while the others were additionally alloyed with niobium, tantalum and titanium. It has been proven that the additions of alloying elements have a considerable influence on the increased hardness and tempering resistance, at the expense of the formation and increase of newly formed carbides. In steels with increased thermal conductivity the formed carbides are present at grain boundaries, which has a negative effect on the decrease of toughness. As part of the doctoral dissertation, it was proven that the investigated type of steel achieves relatively low values of toughness compared to other types of hot work tools and that the grain size in this case has no effect on the improvement of steels toughness. All added alloying elements have a positive effect on preventing grain growth. The greatest impact on the increase of mechanical properties, thermal conductivity and significant improvement in wear resistance was observed in steel with addition of tantalum. In the newly developed steel, all goals of increasing mechanical properties, tempering resistance thermal conductivity while achieving maximum toughness values were achieved compared to reference steel