25.7.2021
Investigation of the effect of boronizing on cast irons - ScienceDirect
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Materials Research Bulletin
Volume 37, Issue 5, April 2002, Pages 971-979
Investigation of the effect of boronizing on cast irons
Salim Sahin, Cevdet Meric
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https://doi.org/10.1016/S0025-5408(02)00697-9
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Abstract
Gray iron, ductile iron and compacted graphite iron were boronized with solid boron-yielding substances by box-boronizing method.
Commercial EKabor® 3 powder is used as the boronizing agent and the treatments are carried out at 850, 900 and 950°C for 2, 3, 4, 5 and 6 h.
Thickness and microhardness of the boride layer, and the microstructure of the boronized specimens are reported.
Introduction
Considerable amount of economical loss occurs because of corrosion and wear in mechanical parts of machinery and equipment. 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. In nearly eutectic compositions, the melting temperature is low (1150–1250°C) and the soliding
interval is narrow. Since the volume increases during the dissociation of carbon as graphite, the shrink of the materials is low. 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 most used material in machinery manufacturing. Carbon is generally found in the
form of graphite flakes in the microstructure. The main mass can be ferritic, ferritic–pearlitic, or pearlitic depending on the dissociation amount
of carbon. The amount, distribution and geometric shape of the dissociated graphite affect the characteristics of gray cast iron, so called, because
of the view of its broken surfaces. The existence of graphite in the microstructure reduces the strength since it decreases the size of the efficient
section and makes 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 three or four times its tensile strength. Heat treatment is not applied to the coarse flakes of the gray cast
iron, because cracks may occur in the notches at the flake tips due to the internal stresses in hardening. However, in case only compressive
stresses are effective, there is no such risk that hardening may be applied; 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 amounts of manganese and sulfur are less than 0.5–0.02%, respectively and
the material is purified from the elements like Pb, As, Sb, Ti and Al, as well. A 0.5% Ce or 0.5% Mg, which is cheaper, is mixed into the liquid
metal before casting in order to assure the spherical dissociation of graphite. The spherical form of graphite provides the lowest surface/volume
ratio and accordingly the largest efficient section. Since the notch effect is dissipated apart from the increase in strength, the ductility of the
material is satisfactory (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 compacted graphite particle shape may appear worm-like when viewed with 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,
https://www.sciencedirect.com/science/article/abs/pii/S0025540802006979 1/5
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Investigation of the effect of boronizing on cast irons - ScienceDirect
can be specified to suit the wear, machinability and high temperature performance consideration requirements of the component. Alloying
elements can also be specified to improve selected properties [9].
Boronizing is a thermochemical surface hardening treatment, which enriches in boron atoms the material surface by the diffusion of boron
elements into the surface of material under contact at high temperatures. This treatment is similar to other surface hardening treatments like
carburization and nitriding in 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 based materials are boronized at temperatures of 800–1000°C for periods varying between 1 to 8 h, (Fe2B+FeB) or
Fe2B iron boride phases are formed at the material surface and a boride layer of up to 2000 HV hardness 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 time, 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 boride 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 into the heat resistant box and specimens are embedded into 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 amount of alloying elements as well as the treatment temperature and time. 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. The 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 a 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].
The aim of the present study is to improve the surfaces of cast irons by boronizing. For this purpose gray iron, ductile iron and compacted
graphite iron were used as cast irons, and the surfaces of specimens were borided. The effect of boronizing time and temperature on the
development of boronized layer thickness were investigated. Microstructure and microhardness in boronized specimens were examined.
Figures
Microstructure of gray iron boronized with EKabor® 3 at 900°C for 3h.
Microstructure of ductile iron boronized with EKabor® 3 at 950°C for 4h.
Microstructure of compacted graphite iron boronized with EKabor® 3 at
Variation of boride layer thickness as a function of temperature of gray iron
850°C for 5h.
boronized at different temperatures.
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Investigation of the effect of boronizing on cast irons - ScienceDirect
Variation of boride layer thickness as a function of temperature of ductile iron
Variation of boride layer thickness as a function of temperature of compacted
boronized at different temperatures.
graphite iron boronized at different temperatures.
Section snippets
Materials and experimental study
Three different materials, gray iron, ductile iron and compacted graphite iron were selected to be used in the experimental study. Surfaces of the
cylindrical specimens were prepared for boronizing treatment. The chemical compositions of materials used in the experiments are given in
Table 1.
The specimens for boronizing treatment were prepared cylindrically, 10 mm in diameter and 7 mm in length, from each material. Box-boronizing
method is preferred because of the ease of treatment, availabity of …
Results and discussion
Microstructure of the base metal and borided zone of cast irons are shown in Fig. 1, Fig. 2, Fig. 3. As a result of metallographical investigation of
boronized materials, it has been determined that the boride layer has a tooth-shaped structure and this structure is homogeneously distributed
over the surface. FeB and Fe2B phases in the boride layers of the specimens investigated by the optical microscope are distinguished by the
contrast difference and microhardness values. In addition, X-ray…
Conclusions
In this study, gray iron; ductile iron and compacted graphite iron were boronized with solid boron-yielding substances by solid boronizing
method in box. Commercial EKabor® 3 powder was used as the boronizing agent and the treatments were carried out at 850, 900, 950 and 1000°C
for 2, 3, 4, 5 and 6 h. Thickness and microhardness of the boride layer, and the microstructure in boronized specimens were investigated.
1. In boronizing treatment carried out at different temperatures, the boride layers…
…
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Investigation of the effect of boronizing on cast irons - ScienceDirect
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