Elsevier

Materials & Design

Volume 31, Issue 8, September 2010, Pages 3915-3921
Materials & Design

Short Communication
Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

https://doi.org/10.1016/j.matdes.2010.03.008Get rights and content

Abstract

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N2–15% H2 and that for plasma nitrocarburizing was 85% N2–12% H2–3% CO2. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.

Introduction

The need of high wear-resisting materials with acceptable corrosion resistance for engineering components, has increasingly led to use of different types of surface treatment methods. From the industrial and economical perspective, nitriding and nitrocarburizing treatments of steels play an important role in improving the wear and corrosion properties of engineering components [1]. Nitriding and nitrocarburizing are thermochemical surface engineering processes, which involve enrichment of surface of steel parts with nitrogen in nitriding and nitrogen plus carbon in nitrocarburizing. The microstructure of nitrided/nitrocarburized sample is generally characterized by a thin compound layer which is supported by a diffusion zone underneath. The compound layer provides corrosion, wear and anti-scuffing properties to the surface whilst the diffusion zone improves the load bearing capacity and fatigue strength. Plasma nitriding and nitrocarburizing treatments can be classified into ferritic and austenitic treatment categories. Ferritic nitrocarburizing is mainly applied to steels to improve surface properties by a compound layer which consists mainly of γ′ and ε phases and is supported by a diffusion zone underneath [2]. The diffusion zone in ferritic nitrocarburizing is relatively poor due to its low load bearing capacity. Austenitic treatment is used to solve this problem. Treatment temperature in austenitic treatment is above 590 °C (the Fe–N eutectoid point) and this higher treatment temperature increases the diffusion rate. This process usually produces a triple layer consists of a top compound layer, an intermediate iron–carbon–nitrogen austenite in the subsurface and a diffusion zone underneath [3].

Basso et al. [4] studied the effects of the treatment parameters on the nitrocarburized layer properties such as thickness, porosity, composition and microstructure and consequently, on the corrosion resistance improvement of the nitrocarburized layer. They concluded that the parameters which affect the above properties are very important to the enhancement of the corrosion resistance. Furthermore, the change of the thickness and the porosity of the nitrocarburized layer are the most important factors which control the corrosion resistance of the treated steel. Teimouri et al. [5] compared the corrosion resistance of the nitrocarburized layers produced during nitrocarburizing treatment for different durations. They demonstrated that a nitrocarburized layer with the proper thickness and compactness can improve the corrosion resistance.

Epsilon phase has a better corrosion resistance with respect to the γ′ phase [6] and the denser the nitride and carbonitride phases in the compound layer and also the thicker compound layer, the more the corrosion resistance improvement [7]. Fils et al. [8] studied the corrosion behavior across the nitrided AISI 431 steel with Cr content more than 12 wt.%. They reported that although the anodic current in the compound layer was more than that in the untreated steel, the corrosion resistance was fairly good. Nevertheless, the diffusion zone showed weak corrosion resistance due to the precipitation of CrN phase and depletion of Cr in the steel matrix.

Some data relating properties of plasma nitriding and nitrocarburizing of AISI 4140 low alloy steel have been published in the past [9], [10], [11]. The objective of this research was to understand the effect of both ferritic and austenitic plasma nitriding and nitrocarburizing processes on the improvement of the corrosion resistance of the AISI 4140 low alloy steel.

Section snippets

Methods

Specimens in the form of discs 20 mm in diameter and 8 mm thick were provided from a commercial grade of AISI 4140 low alloy steel which is widely used in industry. The nominal composition of the steel is given in Table 1. One of the surfaces of all specimens was mechanically polished sequentially by 120, 400, 600, 800, and 1200 grit wet SiC emery paper followed by fine polishing with alumina slurry to yield a mirror finish before being placed in the chamber. The experiments were performed in a 5 

Microstructure and X-ray diffraction

Fig. 1 shows the optical micrographs from the cross-section of the nitrided and nitrocarburized specimens. From Fig. 1, it can be seen that the microstructure of the nitrocarburized and nitrided samples are affected by the treatment temperature. The micrographs show that increasing the treatment temperature from 570 to 620 °C, results in the formation of an intermediate transformed austenite zone next to the compound layer. However, in austenitic nitriding and nitrocarburizing a transformed

Conclusions

From the investigation of ferritic and austenitic plasma nitriding and nitrocarburizing treatments at 570 and 620 °C, performed on AISI 4140 steel, the following conclusions can be made:

  • The corrosion resistance improves after both ferritic plasma treatments with respect to the untreated sample due to the noble nitride and carbonitride phase formation. The amount of ε phase, the characteristics of the compound layer such as thickness, microstructure, continuity and the amount of porosity in the

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