Influence of different plasma nitriding treatments on the wear and crack behavior of forging tools evaluated by Rockwell indentation and scratch tests

Abstract

Forging tools are often showing short lifetimes compared to cold forming tools e.g. for sheet metal forming. This is based on the process conditions where high local surface temperatures are alternating with chilling conditions due to the spray cooling with water based cooling lubricants. The resulting thermal shock is provoking fatigue of the tool material in the near surface regions. Crack initiation and crack growth due to thermal shock exposure then causes chipping of the tool steel material in the surface regions. These are starting points for extensive wear.

Hardness and wear resistance of tool surfaces at elevated temperatures can be dramatically enhanced with nitriding pretreatments. This has become state-of-the-art for hot forming tool steels in many forging applications. With inappropriate adjustments of the nitriding parameters a decrease of the ductility can occur and will reduce the crack resistance of the tool surface especially under thermal shock conditions.

The hot working steel DIN-1.2367 (X38CrMoV5) is currently one of the most often used chromium–molybdenum tool steels in the field of forging. Exemplary for this material is the influence of the nitriding parameters like temperature, nitrogen supply and plasma parameters on the nitriding depth, the maximum hardness and the crack sensitivity will be discussed.

Nitrided samples will be investigated with methods developed for the adhesion measurement of hard coatings. It could be shown that this is also appropriate for a qualification of the crack sensitivity of tools. Comparative application tests in the production of automotive components show the influence on the wear behavior and lifetime of forging tools in an industrial environment.

Introduction

Today the economical production of steel parts is still using hot forging processes as high tech mass metal forming processes with high requirements of contour accuracy of the produced parts.

Forging processes are suitable for cost-effective manufacturing with high reliability where a certain strength of the steel products is required. Forged parts are commonly found at shock and stress exposed applications. Typical automotive applications are wheel spindles, axle shafts, torsion bars, ball studs or idler arms. Other applications are found in modern diesel injection systems where increasing nozzle pressures have led to high strength requirements for the parts [14].

Forging dies are exposed to high mechanical loads, which are superposed by extreme thermal and tribological loads within the near surface areas. As a result of the preheating of the steel parts above 1000 °C, hard oxide particles like scale are arriving in the shaping contact zone and causing severe abrasive wear. With elevated temperatures the tribological conditions are promoting adhesive processes and material is transferred to and from the die surfaces. During processing cycles the heat transfer of the preheated parts is followed by spray cooling with lubricants at room temperature. This causes thermal shock conditions within the dies where high internal stress initiates cracks. Crack propagation parallel to the surface can cause the chipping of surface near parts when converging with cracks normal to surface. In case of insufficient die cooling, soft-annealing of the tool steel is possible [11].

If the damage reaches a critical value it causes the failure of the tool and it has to be replaced or refurbished. The resulting costs for tools, necessary production interruptions and down-time for shutting down the production machinery are lowering the efficiency of the production processes.

To improve hot forming tools different techniques of surface engineering such as weld surfacing, thermal spraying, electrodeposition, diffusion treatments and combined techniques have been applied. In the meantime gas or plasma nitriding of the tools is state-of-the-art. These special surface hardening treatments will increase hardness, wear and corrosion resistance of tools at elevated temperatures [1], [2], [3]. Plasma nitriding can generate compound layers at the tool surface consisting of different iron nitrides (gamma′-phase, Fe₄N/epsilon-phase, Fe_2–3N and mixed gamma′–epsilon-phase) at elevated nitrogen contents in the process atmosphere (Table 1). The most important hardening mechanism is the dispersion of nitrides of alloyed elements in surface near regions which forms the diffusion zone. This causes the generation of inherent compressive stress and thus can reduce the tendency of crack formation and elevate the fatigue strength of the material.

There are also existing combined processes as so called duplex treatments consisting of plasma nitriding and subsequent plasma coating processes (PVD and PACVD). The nitriding pretreatment will improve the coating adhesion especially under hot forming conditions and is described elsewhere [4], [5], [6], [7]. But for these combination processes adequate nitriding parameters are important as well because faults will lead to a decrease in the ductility of the tool surface and cracks in the underground of the coatings will occur [8].

Different works at the IST in Braunschweig in the field of hot forming operations showed that crack formation is a decisive factor for tool life [12], [13]. Economic production processes assume short forging cycles where the following spray cooling cycles are generating massive thermal shock conditions. In a short time a crack network appears and destructive pitting, crevice corrosion, abrasive and adhesive wear are the consequences.

The presented work wants to analyze the mechanisms of the described wear processes leading to reduced tool lifetime. It will show approaches for optimized nitriding processes to improve the forging tool performance.