Proceedings of SerbiaTrib '13

- Reliable exploitation tools, primarily because ofthe importance of lubrication,- Use of an active role of friction,- New design tool that allows the hydrostatic orhydrodynamic lubrication.Figure 4. Microstructure of the convex radius ofdifferent hot work tool steels after 1000 forging cycles.Tool temperature, 200 ◦C; forging material, C45; forgingtemperature, 1100–1150 ◦C; lubricated contact; cycletime, 13 s; hardness of tools, 47 HRC.[3]Figure 5 . The interaction of construction, technologyand the industrial conditions of working tools [4]3. METHODS OF INCREASING THE LIFEOF TOOLSThe predominant causes of damage are the resultof tool wear and thermal and thermal-mechanicaldamage. Forging dies are exposed to highmechanical loads, which are accompanied byextreme thermal and tribological load for a verynarrow surface layer. As a result of preheating steelparts at temperatures above 1000 ° C, the size of thehard oxide particles come in contact zone causingvery strong abrasive wear. At elevated temperaturestribological conditions are very favorable for theprocesses of adhesion and a transfer of material toand from the surface of the tool. During executionof cyclic forging process heat is transferred due tocontact with the heated work pieces followed bycooling lubricant tools in the form of spray, whichis performed at room temperature. In this way,thermal shocks occurring within the tool material,resulting in high internal stresses initiate cracking.Further development of the cracks formed inparallel to the contact surface may lead to itspeeling, when these cracks meet with cracks normalto the contact surface. In case of insufficientcooling of tools, it is possible to slightly releasetool steel [5].To increase the life of hot forging tools,different surface modification techniques are usedsuch as welding, thermal application,electrodeposition, diffusion method and othercombined methods. These special methods ofsurface modification increase the hardness, wearresistance and corrosion resistance of the tool athigh temperatures. One possible way to satisfy allthe conditions of hot forging process is acombination of thermo-chemical surfacemodification process (ie, carbonization andnitriding) with coating processes (ie, PVD, CVD, orPACVD), which is known in the literature asduplex modification process surface [6].Figure 6 shows the results of testing the wearalloy tool steel to functioning in a warm envirnment(WNL-55NiCrMoV6) using various surfacetreatments: nitriding, sulphurizing, diffusionchroming, Cr plating, plasma spraying withmetallic coatings of Cr, WFe, WC-types,burnishing [4].Using spray-metal coatings leads to triplereduction of wear compared to conventionaltreatment tools. It also shows that the wear of thetool depends directly on the oxide that is generatedon the surface of pieces and to a lesser extent on theoxide surface tools. Cr and WC plasma sprayedcoatings should be applies to hot working tools.These coatings are characterized by considerablewear ressistance and thermal and imact fatiqueresistance.According to the investigations [7] the bestresults for die service life were obtained for weldoverlay coated dies, Table 1. Compared to receiveddies, the results showed an increase of 892%. Theresults were 206% better than TOKTEK Coatings,which held the second place.13 th International Conference on Tribology – Serbiatrib’13 263