Abstract
Deep nitriding is used to obtain a nitriding hardness depth beyond 0.6 mm. The long nitriding processes, which are necessary to reach the high nitriding hardness depths, mostly have a negative influence on the hardness and strength of the nitrided layer as well as on the bulk material. The compound layer often is considered less, because in most practical cases, it is removed mechanically after nitriding, to avoid spalling in service. However, in former investigations, it was shown, that thick and compact compound layers have the potential for high flank load capacity of gears. The investigations focus on the simultaneous formation of a high nitriding depth and a thick and compact compound layer. Beside the preservation of the strength, a challenge is to control the porosity of the compound layer, which should be as low as possible. The investigations were carried out using the common nitriding and heat treatable mild steel 31CrMoV9, which is often used for gear applications. The article gives an insight on the development of multistage nitriding processes studied by short- and long-term experiments aiming for a specific compound layer build-up with low porosity and high strength of the nitride layer and core material.
Highlights
High performance gears are usually subjected to heat treatment in order to achieve optimum performance for the respective application
The short-term treatments initially carried out for nitriding and nitrocarburizing were intended to give a clue for the design of the process stages of the two-stage treatments in order to influence the compound layer structure
In order to influence the thickness and phase composition of the compound layer, a two-stage process control is suitable, in which the diffusion layer and the nitriding hardness depth is first adjusted in a long process stage with a comparatively low nitriding potential
Summary
High performance gears are usually subjected to heat treatment in order to achieve optimum performance for the respective application. Thermochemical surface layer heat treatment, such as case hardening, carbonitriding, nitriding or nitrocarburizing are common treatments, since the highest stress is in the surface layer of the teeth and a tough core is positive for the service behavior [1]. Studies have shown that nitriding treatment can improve the load-bearing properties of gears with regard to different stresses. The necessary hardening depth of the surface layer depends on the normal modulus of the gearing [3]. Case hardening is frequently used for larger gears, since higher case depths can be achieved in a shorter time. Nitriding offers some process-related advantages compared to case hardening, such as better protection against scuffing, wear and corrosion, higher surface hardness, a temperature-stable surface layer for use at higher temperatures, as well as less dimensional and shape changes and generally no need for reworking [4]
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