Mechanical properties of boronized AISI W4 steel

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Abstract

A series of experiments was performed to evaluate some mechanical properties of boronized AISI W4 steel. Boronizing was carried out in a solid medium consisting of EKabor powders at 850, 950 and 1050°C for 2, 4, 6 and 8 h. After boronizing, FeB and Fe2B phases were formed on the surface of the steel substrate. A boride layer was revealed by a classical metallographic techniques and X-ray diffraction (XRD) analysis. Depending on the process temperature and boronizing time, the thickness of the coating layers ranged from 8 to 386 μm. Metallographic studies revealed that the boride layer has a lenticular morphology. The hardness of the boride layer was measured using a Vickers indenter with loads of 0.5 and 1 N. It was found that the hardness of the boride layers ranged from 1407 to 2093 HV. The fracture toughness of borided surfaces was measured via a Vickers indenter with a load of 10 N. It was observed that the fracture toughness of the boride layer ranged from 1.39 to 6.40 MPa m1/2. A longer boronizing time results in a greater boride layer thickness. Lengthwise cracks were formed on the samples that were borided at 1050°C for 6 and 8 h. The distribution of alloying elements from the surface to the interior was determined using energy-dispersive X-ray spectroscopy (EDS). The main aim of present study was to increase the service life of AISI W4 plain carbon tool steel.

Introduction

Mechanical components and tools are facing higher performance requirements. The use of surface coatings opens up the possibility for material design in which the specific properties are located where they are most needed. The substrate material can be designed for strength and toughness, while the coating is responsible for the resistance to wear, thermal loads and corrosion. Surface treatments offer remarkable choices for a wide range of tribological applications where the control of friction and wear are of primary concern [1], [2]. Boriding is a thermomechanical surface-hardening process, in which boron atoms are diffused into the surface of a workpiece to form borides with the base materials. Industrial boriding can be applied to most ferrous materials, such as structural steels, cast steels, Armco iron, gray and ductile iron, and sintered iron and steel, as well as to non-ferrous materials, such as nickel-, cobalt-, titanium-, and molybdenum-based alloys and cemented carbides. Both conventional (i.e. salt bath, paste and pack boriding) and advanced (i.e. plasma-based) boronizing techniques are available [3], [4], [5], [6], [7]. Depending on the potential of the medium and the chemical composition of the base materials, single or duplex (FeB+Fe2B) boride layers may be formed. A single-type (Fe2B) layer is generally desirable for industrial applications, owing to the difference between the specific volume and coefficient of thermal expansion of the boride and the substrate [8], [9], [10]. The strong covalent bonding of most non-oxide ceramic borides is responsible for their high melting point, modulus and hardness values. Borides, in general, have high free energy of formation, which gives them excellent chemical and thermal stability under many conditions [11], [12]. The main objective of this study was to investigate some mechanical properties of non-oxide ceramic borides formed on the surface of water-hardening AISI W4 tool steel, which is one of the cheapest tool steels. These steels, which are shallow hardening and have low resistance to softening, are widely used for woodworking tools, wear-resistant machine-tool uses and cutlery [13]. The boride layer formed on the surface of steel is also very hard at elevated temperatures, and is resistant to wear and corrosion. Each sample was studied by microhardness measurements, optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis.

Section snippets

Substrate materials and boronizing

The substrate material, AISI W4 water-hardening tool steel, essentially contained: 1.25 wt.% C; 0.27 wt.% Cr; 0.26 wt.% Mn; 0.24 wt.% Si; 0.001 wt.% S; and 0.018 wt.% P. The square-shaped test pieces measured 10×10×2.5 mm3. Boronizing was performed in a solid medium consisting of EKabor-I powders [14] at a temperature of 850, 950 or 1050°C for 1, 2, 4, 6 or 8 h. Boronizing was carried out in an electrical resistance furnace under atmospheric pressure, followed by cooling in air. A subsequent

Microstructure

Optical and scanning electron microscopy cross-sectional examinations of borided AISI W4 steel surfaces revealed that boride formed on the surface of the substrate has a lenticular morphology (Fig. 4). The thickness of the boride layer ranged from 8 to 386 μm, depending on boronizing time and temperature (Fig. 2). At higher magnifications, three distinct regions were identified on cross-sections of the borided steel surface; these are: (i) a surface layer, primarily consisting of FeB and Fe2B

Discussion

Optical microscopy and SEM cross-section examinations of borided AISI W4 steel showed that the characteristic sawtooth morphology of the boride layer is dominant. However, the morphology of borides formed on the surface of AISI W4 steel are compact and smooth compared to borides formed on the surface of low- and medium-carbon steels. The microstructure and mechanical properties of borided steels depend strongly on the chemical composition and structure of the boride layer and the composition of

Conclusions

  • 1

    Boride layers formed on the substrate steel have a lenticular structure. It was also observed that boride layers have three different regions: (a) boride layer; (b) transition zone; and (c) matrix.

  • 2

    It was observed that a single-phase (Fe2B) boride layer formed on the surface of steel boronized for up to 4 h. When the boronizing time was over 4 h, a double-phase (FeB+Fe2B) boride layer was formed.

  • 3

    It was noted that a higher boronizing temperature results in higher hardness and a higher layer

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