Proceedings of SerbiaTrib '13

Tools for hot forging are used in extremelydifficult conditions: impact and very highmechanical load, variable partial and generalthermical load, friction on the surfaces of tools andmore. Each tribological system in the field of hotforging is characterized by the following elements[2]:- contact pair, which consists of tool andworkpiece-metal that is plastically deformed, withappropriate structural and mechanical properties,- lubricant with properties relevant for hotprocess,- a machine that implements processing,- micro and macro environmentThe final characteristics of the finished pieceforgings,such as size, shape, surface quality andstructure depend on the values of systemparameters. Figure 2. shows a global approach tothe modeling of machining processes, whereas thequality of lubricants is main output.Figure 2. Tribo-modelling of hot forging process [2]2. CAUSES OF DAMAGE TO THE TOOLSTools for hot forging function during cyclicmechanical and thermal loads. The complexprocess of tear occurs as a result of the two loads.Due to the high pressures and inadequatelubrication regime, the intensity of wear and teardue to friction is high. The main causes of damageto the tool, Figure 3:- Wear as a result of tribological processes- Plastic deformation,- Thermal cracks,- Mechanical fatigue crack growth,- Breakdowns (die failures).Figure 3. Failure and damaging of forging diesTool wear. This type of wear occurs as a resultof relating the material particles from the surface oftools. Consequence of the occurrence of abrasiveand adhesion processes and the formation ofwelded layers that are later destroyed.Plastic deformation occurs under the influenceof high pressure to the tool. As a result of resistmaterials, the deformation of the walls of tools andmeasuring tools changes.Thermal cracks. They are caused by cyclicthermal change and show it as a grid cracks on thesurface etching. Microcracks are connected andgrow into large thermal cracking. They can grow onthe surface and in depth.Mechanical cracks are caused by mechanicalloading tools. Consequence of fatigue, pretreatmentand initial thermal cracking. These arecalled large cracks.Failure of tools is the result of hidden defects,bad thermal processing, errors in design, irregularexploitation and so on.Generally, due to the forging temperature beingwell above 1000 ◦C, the temperature of the surfaceof the tool temporarily exceeds 500 ◦C and thus thetempering temperatures of conventional hot worktool steel. In such a case, the hardness of the tool isreduced and the mechanical impacts during forgingoperations can easily cause plastic deformation aswell as abrasion of tool material, Figure 4 [3].Life of forging tools is a complex function ofseveral parameters, the most important being:structural, method design and exploitationconditions. Figure 5. demonstrates an interactionbetween these parameters.Based on the analysis of tool wear, we can givethe following recommendations for the design oftools [4]:- The choice of tool material with high resistance toabrasion,- The application of appropriate methods forimproving surface properties of tools,262 13 th International Conference on Tribology – Serbiatrib’13