Advanced Complex Analysis of the Thermal Softening of Nitrided Layers in Tools during Hot Die Forging

Abstract

This article is devoted to the issues of thermal softening of materials in the surface layer of forging tools. The research covers numerical modeling of the forging process, laboratory tests of tempering of nitrided layers, and the analysis of tempering of the surface layer of tools in the actual forging process. Numerical modeling was supported by measuring the temperature inside the tools with a thermocouple inserted into the tool to measure the temperature as close to the surface as possible. The modeling results confirmed the possibility of tempering the die material. The results of laboratory tests made it possible to determine the influence of temperature on tempering at different surface layer depths. Numerical analysis and measurement of surface layer microhardness of tools revealed the destructive effect of temperature during forging on the tempering of the nitrided layer and on the material layers located deeper below the nitrided layer. The results have shown that in the hot forging processes carried out in accordance with the adopted technology, the surface layer of working tools is overheated locally to a temperature above 600 °C and tempering occurs. Moreover, overheating effects are visible, because the surface layer is tempered to a depth of 0.3 mm. Finally, such tempering processes lead to a decrease in the die hardness, which causes accelerated wear because of the abrasion and plastic deformation. The nitriding does not protect against the tempering phenomenon, but only delays the material softening process, because tempering occurs in the nitrided layer and in the layers deeper under the nitrided layer. Below the nitrided layer, tempering occurs relatively quickly and a soft layer is formed with a hardness below 400 HV.

Hardness Assessment Considering Nitrided Layers Based on Tempering Tests for Numerical Wear Prediction for Forging Processes

“…Despite the fact that a direct comparison with other studies is difficult in detail due to partly unknown process boundary conditions and nitrided layer parameters, the observations from the tempering and forging experiments are examined in the following with regard to plausibility on the basis of related studies in the literature. In fact, in analogy to a study by Krawcyk et al [ 18 ], significant reductions in hardness were observed in the nitrided surface microstructure as a result of laboratory isothermal tempering tests. This study also comes to the conclusion that the start of tempering of the nitrided layer investigated can be observed from a minimum temperature of 600 °C.…”

“…Due to this, an interface has already been created in the preliminary work in order to not only reference dynamic hardness curves of the base material, but also to implement a dynamic hardness model for the nitrided layer based on the calculated wear depth [ 16 ]. Krawcyk et al already investigated the thermal softening behaviour of nitrided layers of forging tools using isothermal tempering tests and laboratory forging tests [ 18 ]. By varying the temperature between 500 °C and 700 °C for 2 h and 4 h, significant annealing effects (hardness reductions of the nitrided layers) could be observed.…”

“…By varying the temperature between 500 °C and 700 °C for 2 h and 4 h, significant annealing effects (hardness reductions of the nitrided layers) could be observed. In further laboratory forging tests, Krawcyk et al [ 18 ], Widomski et al [ 19 ] and Gronostajski et al [ 20 ] were able to show that the thermo-mechanical load during die forging also leads to annealing effects in the nitrided surface layer. However, since the thermal load on the surface layer is superimposed by abrasive wear and plastic deformation in these tests, a comparison of the two tests (tempering tests and forging tests) is only possible to a limited extent.…”

Perspectives on data-driven models and its potentials in metal forming and blanking technologies

“…This model enables a comprehensible calculation of wear locations by evaluating local contact stresses, material sliding velocity and local tool hardness [77]. While the conventional Archard model considers the tool hardness as constant, current literature agrees that cyclic thermomechanical loading to the surface leads to extensive changes of microstructure and hardness of the layer during the running production [78,79]. With regard to a data-driven modelling of this phenomenon, a test methodology was developed at the Institute of Metal Forming and Forming Machines (IFUM) to realistically replicate this load accumulation using a forming dilatometer to later analyze the resulting changes in the hardness of forging tool material [80,81].…”

Reduction of Die Wear and Structural Defects of Railway Screw Spike Heads Estimated by FEM

“…After forming 500 forged gearwheel specimens by each method, they determined that the slightest geometrical deviation was achieved with the milled die, while the most significant deviation occurred in the turned die [19]. Likewise, Krawczyk et al [20] studied surface thermal softening in forging dies. They measured surface temperatures in the order of 600 • C, which produced a softening up to a depth of 0.3 mm.…”