Influence of the nitrided layer thickness of dies made of two types of tool steel used in hot extrusion of valve forgings made of nickel–chromium steel on the durability of these tools

Abstract

The study constitutes an analysis of the durability of dies used in the first operation of producing valve forgings from chromium–nickel steel (NCF 3015) for motor trucks. The average durability of the dies (subjected to standard thermal treatment and nitriding) in this operation equals about 800 forgings. To perform an in-depth analysis of the effect of the nitrided layer thickness (0.1 mm and 0.2 mm) and the tool material (W360 and QRO90) on the possibilities of increasing the die durability, complex studies were carried out, which included: a macroscopic analysis combined with 3D scanning, microstructural examinations using a scanning microscope and a metallographic microscope, as well as hardness measurements. A minimum of three tools were tested for different variants, and for each of them, one representative die was selected for detailed examinations. The research showed the presence of abrasive wear, thermo-mechanical fatigue and traces of adhesive wear as well as plastic deformation on the surface of the working impressions. Also observed was the effect of the extruded material sticking to the tools (high friction and the presence of intermetallic phases in the extruded material) and the forging being blocked in the smallest section of the die, which is a critical factor causing a production shutdown and the necessity of tool replacement. The highest mean durability equalling 2600 forgings was obtained for the dies with a lower carbon content and a higher content of vanadium and the nitrided layer thickness at the level of 0.2 mm. The lowest mean durability (after one forging item) was recorded for the dies made of steel with a higher carbon content and a higher chromium content, forming less stable compound carbides and the thickness layer at the level of 0.1 mm.

Wear Analysis of Forging Tools Used in an Industrial Production Process—Hot Forging in Closed Dies of the “Head-Disk” of an Engine Valve Forging

“…For this reason, it was decided to collect forgings from the forging process in a cyclic distance of 100 items. By the application of the reverse scanning method (developed by the authors), it is possible to determine the change in the calotte's height based on the measurements made by means of the 3D scanning method [34][35][36]. Figure 9 shows an exemplary diagram of the changes in the calotte's height for the cyclically collected forgings from the punch that produced 3000 forgings.…”

Methods for Improving the Quality of Forgings and Blanks Obtained by Forging through Intensifying Shear or Alternating Strain in the Bulk of Deformable Metal

“…Another promising area is the study of the strength of the forging tool in order to increase its durability and increase the overall productivity of the deformation process. These studies are aimed at increasing the wear resistance of the forging tool by controlling the content of carbon and vanadium in the tool material together with implementing the specified thickness of the nitride layer [17][18][19]. In addition, an important factor affecting the wear resistance of the deforming tool is its preheating technology [20].…”

Structural Features of Fatigue Crack Propagation of a Forging Die Made of Chromium–Molybdenum–Vanadium Tool Steel on Its Durability

“…Nitriding is a relatively known and effective thermo-mechanical treatment technology. We can find many studies referring to increasing the operation time with the use of a nitrided layer [ 46 , 47 , 48 ]. In turn, hybrid layers are a much younger method enabling a durability increase.…”