Investigation of the boronizing effect on the abrasive wear behavior in cast irons

Abstract

One of the methods used to improve the surface properties of iron and steel is boronizing. Gray iron, ductile iron and compacted graphite iron were boronized with solid boron-yielding substances by pack-boronizing method. Commercial EKabor^®3 powder was used as the boronizing agent and the treatment was carried out at 900 °C for 2, 3, 4, 5 and 6 h. Thickness, microhardness and microstructure of the boride layer are investigated. Abrasive wear behavior of the boronized and unboronized cast irons were investigated. For this purpose, the specimens were tested on a pin-on disk test apparatus. SAE 1040 steel was used as the moving surface member. Abrasive wear tests were carried out at a fixed load and a fixed sliding speed. The weight loss was measured and worn surfaces were examined.

Introduction

Considerable economic loss occurs because of corrosion and wear in mechanical parts of machine and equipment during service. In order to reduce this loss, properties of the surface region of materials should be improved. One of the methods used to improve the surface quality is boronizing [1], [2].

Cast irons are iron based cast alloys containing 2–5% carbon, which have extremely high mould filling and pourity in casting, vibration damping properties and resistance against corrosion. For nearly eutectic compositions, the melting temperature is low (1150–1250 °C) and the soliding interval is narrow. Since the volume increases during dissociation of carbon as graphite, shrinkage in the material is less. Cast irons are widely used in the casting industry.

Gray cast iron, which contains 2.5–5% C and 0.8–3% Si, is the commonly used material in machinery manufacturing. Carbon is generally found in the form of graphite flakes in its microstructure. The main structure could be ferritic, ferritic–pearlitic, or pearlitic depending upon the dissociation amount of carbon. The amount, distribution and geometric shape of the graphite affects the characteristics of gray cast iron, so called, because of the gray appearance of its broken surface. The existence of graphite in the microstructure reduces the strength since it decreases the size of the efficient section and causes notch effect as well. The deformation ability is negligibly small and the elongation at rupture is less than 1%. Still, its compressive strength is approximately 3 or 4 times of its tensile strength. Heat treatment is not applied to the coarse flakes of the gray cast iron, because cracks may occur in at the flakes tip due to internal stresses in hardening. However, for applications where compressive stresses are effective, there is no need of hardening, e.g. slipways of the tool machinery [3].

In ductile cast iron, which contains 3.2–3.8% C and 2.4–2.8% Si, the concentration of manganese and sulfur are less than 0.5% and 0.02%, respectively, and the material is purified from the elements like Pb, As, Sb. Ti and Al, as well. 0.5% Ce or 0.5% Mg, which is cheaper, is mixed into the liquid metal before casting in order to assure nodular dissociation of graphite. The nodular form of graphite provides the lowest surface/volume ratio and accordingly the largest efficient section. Since notch effect is dissipated apart from increase in strength, ductility of the material is improved (≅10%) [4], [5].

The graphite in compacted graphite iron (sometimes referred to as vermicular iron) appears as individual “worm shaped” or vermicular particles [6]. Although the particles are elongated and randomly oriented as in gray iron, the compacted graphite particles are shorter and thicker and have rounded edges. While the particles shape may appear worm-like when viewed under a conventional light microscope, deep etched SEM micrographs show that the “worms” are connected to their nearest neighbors within the eutectic cell. This complex graphite morphology, together with the rounded edges and irregular bumpy surfaces, results in strong adhesion between the graphite and the iron matrix [7].

Ultimately, the compacted graphite morphology inhibits both crack initiation and propagation and is the source of the improved mechanical properties relative to gray cast iron [8]. Flake graphite inadmissible. The pearlit content, which is linearly related to hardness and tensile strength, can be specified to suit the wear, machinability and high temperature requirements of the component. Alloying elements can also be specified to improve selected properties [9].

Boronizing is a thermochemical surface hardening treatment, which enriches boron in the material surface by diffusion of boron atoms into the surface of the material at high temperatures. This treatment is similar to other surface hardening treatments like carburization and nitriding in respect of physical and chemical characteristics. It is successfully applied to all ferrous materials, nickel alloys, titanium alloys, and sintered carbides [1], [10]. Boronized steels and cast irons are characterized by their increased surface hardness and increased wear resistance [11]. When ferrous materials are boronized at temperatures in the range of 800–1000 °C for periods varying between 1 and 8 h, (Fe₂B + FeB) or Fe₂B iron-boride phases are formed at the material and a boride layer having hardness up to 2000 HV hardness and thickness in the range of 40–270 μm is produced. The characteristics of this boride layer depends on the physical state of the boride source used, boronizing temperature, treatment period, and properties of the boronized material [1], [12]. Industrial applications of wear and corrosion resistant materials include drive shafts, camshafts, pulleys, machine slide-ways, tanks, weapons and part for agricultural machinery [13].

The boron source may be in solid, liquid, or gaseous state. However, boronizing in solid state has technical advantages. This method, in which the boronizing agent is in powder form, has a wide range of applications because of its advantages such as ease of treatment, achieving a smooth surface, and simplicity of the required equipment. Solid state boronizing, that is similar to pack cementation, can be carried out under inert atmosphere as well as in tightly closed boxes. Boronizing agent is placed in a heat resistant box and specimens are packed in this powder [1]. A large contact surface is desired between the material and boronizing agent to allow a better diffusion of boron atoms into the material surface. Thus, the grain size of the powder is an important factor in the formation of boride layer [14].

Tooth-shaped structure is a characteristic property of the boride layer. The degree of toothing between the layer and the base material depends on the concentration of alloying elements as well as the treatment temperature and period. Strong toothing occurs in steels and cast irons [15]. It depends on the ratio of alloying elements in steels and cast irons; the higher the ratio of alloying elements, the less is the degree of toothing. Boride layers join the base metal better because of their tooth shape. Fragility of the boronized layer increases with increasing thickness [16].

The advantage of boronizing over other types of surface hardening methods is that, the surface layer is very hard, friction coefficient is very low, no extra heat treatment is required after boronizing, it has considerable resistance against some acid, base, metal solutions and high temperature oxidation. Boronized steels and cast irons can resist wear and oxidation without losing their tribological properties starting from surface temperatures up to 1000 °C [2], [13].

In wear mechanisms, some important factors are hardness, shape and size of abrasive grit or roughness, attack angle, normal load applied, sliding speed and fracture toughness of the material [4], [17], [18]. It is possible to reduce abrasive wear by various methods such as development of different materials, undertaking appropriate heat treatment and surface process and use of composite materials [1].

The most important reason for damage and consequent failure of machine parts is wear [1], [19]. According to DIN 50320 and ASTM G 40–93 standard wear is unwanted surface damage as a result of separation of small pieces from the material surface due to the interaction of other materials such as liquid, solid or gas. The mechanism of wear means physical and chemical processes that occur during wear. The term of abrasive wear is removal of pieces from one of the two rubbing bodies. The abrasive wear occurs in the test apparatus moving in touch with various abrasives [20], [21].

The aim of this study was to investigate the effect of boronizing on wear behavior of cast irons. Gray iron, ductile iron and compacted graphite iron were used as cast irons and surface of specimens were borided. The effect of boronizing period on boride layer thickness were investigated. Microstructure and microhardness in boronized specimens were examined. The wear behavior of boronized and unboronized cast iron specimens were investigated. For this purpose, the specimens were worn in pin-on disk lest apparatus. SAE 1040 steels was used as moving surface member. Abrasive wear tests were a fixed load and a fixed sliding speed. It were measured amounts of weight loss and examined worn surfaces.