The Nitrocarburizing Process: Part Two

Many surface-hardening processes, including galvanic deposition and chemical/physical vapor deposition, have been developed and are widely implemented to this day. Among these processes, the steel nitrocarburizing treatment was regarded as an effective, low cost method with many advantages, such as low treatment temperature, short treatment time, high degree of shape and dimensional stability, and reproducibility.

The aim of the work of L.-H. Chiu et al. was to study the effects of the substrate composition, nitrocarburizing holding time, surface hardness, and surface contact pressure on the nitrocarburized case morphology and the wear behavior of the JIS SKD61 tool steel. The experiment was conducted in a liquid salt bath to nitrocarburize the tempered martensitic substrate (TMS) SKD61 steel and, for comparison, the ferritic substrate (FS) specimens.

The experiment was conducted in a liquid salt bath to nitrocarburize the tempered martensitic substrate (TMS) SKD61 steel and, for comparison, the ferritic substrate (FS) specimens. The aim was to study the effects of the substrate composition, nitrocarburizing holding time, surface hardness, and surface contact pressure on the nitrocarburized case morphology and the wear behavior of the JIS SKD61 tool steel.

The hardness profiles of the nitrocarburized specimens exhibited maximum hardness in the range of 900–1000HV_0.05 at the surface layer. The white layer of the nitrocarburized specimen consist of ε-phase (Fe₃N) and γ´-phase (Fe₄N), and the phases trapped of nitrogen and carbon atoms were shown in the diffusion layer. These hard constituents at the surface layer accumulated on the grain boundaries and resulted in the hardness increase.

As the nitrocarburizing holding time increased, friction coefficients of the nitrocarburized specimens of both substrates gradually increased. For higher contact pressures, friction coefficients of both substrates with or without nitrocarburizing treatment decreased. Therefore, the nitrocarburizing treatment increased the surface roughness and hardness, as well as the friction coefficients of the specimens. The wear track analysis revealed that abrasive wear occurred on the surfaces of the nitrocarburized specimens.

The wear loss condition was greatly improved after nitrocarburizing the specimens. Wear behaviors of the specimens nitrocarburized for 1, 3, and 5 h did not varied significantly. Statistical analysis of the wear loss data indicated that the effects of the substrates and the nitrocarburizing holding time were significant to the wear resistance. The nitrocarburized TMS specimens showed better wear resistance than the FS specimens. Nevertheless, the nitrocarburizing process for both substrates improved the wear resistance.

In the study of D. Cherng Wen is examined the influence of nitrocarburizing holding time on the surface microstructures and erosion behavior of JIS SKD11 modified cold-work tool steel (DC11 tool steel). The steel was nitrocarburized at 570°C for varying durations of 1, 3, and 5 h. The microstructures and hardness of the nitrocarburized coatings were then analyzed. Particle erosion was examined at different impinging angles (15–90°) and impact speeds (20.2–45.6 m/s).

The results show that a single diffusion zone is formed on the specimens at 1 h nitrocarburizing while a compound layer together with diffusion zone are formed on the specimens for the nitrocarburizing time beyond 3 h. In addition, the compound layer formed on the specimens exhibits a higher erosion resistance.

The nitrocarburizing treatment not only increases the surface hardness but also improves the erosion resistance of the experimental steel. This improvement in erosion rates is more obvious at higher impact speeds and lower impinging angles. The maximum erosion rate appears at an impinging angle of 30° for all specimens.

In this condition, plough grooves and cutting lips appear in the eroded surface; however, the erosion tracks are more superficial for nitrocarburized specimens than untreated specimen. The exponent in the power law Ė=kVⁿ varied between 1.9–2.3 for impinging angles between 15° and 90°.

The main conclusions from these investigations can be summarized as follows:

(1) During nitrocarburizing treatment, a single diffusion zone is formed on the specimens at 1 h nitrocarburizing while a compound layer together with diffusion zone are formed on the specimens for the nitrocarburizing time beyond 3 h. The microhardness profiles of nitrocarburized specimens show surface hardnesses in the range of 970–1 110HV_0.05, which is increased by a factor of approximately 1.3 to 1.5 in comparison to that of substrate material. As the nitrocarburizing holding time increases, both the surface hardness and the hardness-profile depth increase.

(2) The nitrocarburized specimens show a much better erosion resistance than that of untreated specimen. This improvement in erosion rates is more obvious at higher impact speeds and lower impinging angles. In addition, the erosion resistance is higher for the specimens having compound layer than the specimens having a single diffusion zone. Nevertheless, as the compound layer is formed on the top of the nitrided layer, the erosion properties including erosion rate and erosion mechanism do not change significantly with nitrocarburizing holding time.

(3) The maximum erosion rate appears at an impinging angle of 30° for substrates both with and without nitrocarburizing treatment. In this condition, obvious plough grooves and cutting lips appear in the eroded surface, but the erosion traces on the untreated specimens are wider and deeper. As the nitrocarburizing holding time increases, the erosion traces of the specimen become more superficial. This feature leads to the decrease of erosion rate.

(4) The impact speed has a pronounced effect on the erosion wear of the experimental steel. The steady state erosion rate (Ė) has been related to the impact speed (V) as Ė=kVⁿ. The speed exponents at different impinging angles (15–90°) and impact speeds (20.2–45.6 m/s) are in the range of 1.9–2.3.