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Carburizing[1] or carburising (chiefly British English) is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of making the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures lead to greater carbon diffusion into the part as well as increased depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.[2]

This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm).

Method

Carburization of steel involves a heat treatment of the metallic surface using a source of carbon.[3] Carburization can be used to increase the surface hardness of low carbon steel.[4]

Early carburization used a direct application of charcoal packed onto the metal (initially referred to as case hardening), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide or methane). The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.

Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion resistance, hardness and load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels. The process is used as it is environmentally friendly (in comparison to gaseous or solid carburizing). It also provides an even treatment of components with complex geometry (the plasma can penetrate into holes and tight gaps), making it very flexible in terms of component treatment.

The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution (dissolved within the metal crystalline matrix — this normally occurs at lower temperatures) or react with the host metal to form ceramic carbides (normally at higher temperatures, due to the higher mobility of the host metal's atoms). Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion. However, each different hardening mechanism leads to different solutions to the initial problem: the former mechanism — known as solid solution strengthening — improves the host metal's resistance to corrosion whilst imparting its increase in hardness; the latter — known as precipitation strengthening — greatly improves the hardness but normally to the detriment of the host metals corrosion resistance. Engineers using plasma carburization must decide which of the two mechanisms matches their needs.

Gas carburizing is normally carried out at a temperature within the range of 900 to 950 °C.

In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lower-temperature flame. It is often used to anneal metal, making it more malleable and flexible during the welding process.

A main goal when producing carbonized workpieces is to insure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the workpieces are often supported in mesh baskets or suspended by wire. In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickle-chromium alloy and sealed at all openings with fire clay.

Hardening agents

There are different types of elements or materials that can be used to perform this process, but these mainly consist of high carbon content material. A few typical hardening agents include carbon monoxide gas (CO), sodium cyanide and barium chloride, or hardwood charcoal. In gas carburizing, the CO is given off by propane or natural gas. In liquid carburizing, the CO is derived from a molten salt composed mainly of sodium cyanide (NaCN) and barium chloride (BaCl2). In pack carburizing, carbon monoxide is given off by coke or hardwood charcoal.

Geometrical possibilities

There are all sorts of workpieces that can be carbonized, which means almost limitless possibilities for the shape of materials that can be carbonized. However careful consideration should be given to materials that contain nonuniform or non-symmetric sections. Different cross sections may have different cooling rates which can cause excessive stresses in the material and result in breakage.[5]

Dimensional changes

It is virtually impossible to have a workpiece undergo carburization without having some dimensional changes. The amount of these changes varies based on the type of material that is used, the carburizing process that the material undergoes and the original size and shape of the work piece. However changes are small compared to heat-treating operations.[5]

Change in material properties[5]

Work material properties Effects of carburizing

Mechanical

Increased surface hardness Increased wear resistance Increased fatigue/tensile strengths

Physical Grain Growth may occur Change in volume may occur

Chemical Increased surface carbon content

Workpiece material

Typically the materials that are carbonized are low-carbon and alloy steels with initial carbon content ranging from 0.2 to 0.3%. The workpiece surface must be free from contaminates, such as oil oxides, alkaline solutions, which prevent or impede the diffusion of carbon into the workpiece surface.[5]

Comparing different methods

In general, pack carburizing equipment can accommodate larger workpieces than liquid or gas carburizing equipment, but liquid or gas carburizing methods are faster and lend themselves to mechanized material handling. Also the advantages of carburizing over carbonitriding are greater case depth (case depths of greater than 0.3 inch are possible), less distortion, and better impact strength. This makes it perfect for high strength and wear applications (e.g. scissors or swords). The disadvantages include added expense, higher working temperatures, and increased time.[5]

Choice of equipment

In general, gas carburizing is used for parts that are large. Liquid carburizing is used for small and medium parts and pack carburizing can be used for large parts and individual processing of small parts in bulk.[5]

Carburization

Abstract: Carburization is the formation of metal carbides in a material as a result of exposure to a carbon containing atmosphere. In some cases, materials are intentionally carburized to impart a high surface hardness and wear resistance. However, in service, it can result in loss of mechanical properties over time in addition to wastage of the material. The corrosion process referred to as "Metal Dusting" is a carburization process which results in surface attack and the formation of a powdery residue. Carburizing environments are commonly found in process piping and furnace tubes that contain an excess of carbon-containing species which includes hydrocarbon gases or carbon monoxide. Pyrolysis piping in ethylene and olefins plants are prime examples of equipment that must be designed to resist severe carburization. Carburization is the formation of metal carbides in a material as a result of exposure to a carbon containing atmosphere. In some cases, materials are intentionally carburized to impart a high surface hardness and wear resistance. However, in service, it can result in loss of mechanical properties over time in addition to wastage of the material. The corrosion process referred to as "Metal Dusting" is a carburization process which results in surface attack and the formation of a powdery residue. Carburizing environments are commonly found in process piping and furnace tubes that contain an excess of carbon-containing species which includes hydrocarbon gases or carbon monoxide. Pyrolysis piping in ethylene and olefins plants are prime examples of equipment that must be designed to resist severe carburization.

Resistance to carburization generally centers around two mechanisms:

the establishment of an effective barrier on the surface of the materials that limits the ingress of carbon into the material.

tying up carbon in the material using alloying elements which are strong carbide formers.

In the first case, the barrier is usually a Cr2O3 layer on the material. This is accomplished through the use of Cr as an alloying element. Secondly alloying additions of Ni also help by reducing carbide diffusion in the material.

Under severe carburizing conditions up to 800 C, the use of Type HK Alloys (Fe-25Cr-20Ni), Type HP (Fe-25Cr-35Ni) and Alloy 800H is common. Si additions have been found to be beneficial in enhancing these alloys through formation of a protective SiO2 layer on the surface of the material over the range of 1 to 2 wt percent Si (See Figure). Additions of niobium, a carbide forming element, is also used to extend the performance of these alloys by tying up carbon in the form of stable carbides to reduce its reaction with Cr.

Carburization can also occur in Cr-Mo steels at low temperatures (450 to 650 C). Several methods of reducing carburization can be utilized. These include the use of barrier coatings, reducing the carbon activity of the environment, lowering the service temperature by cooling and introduction of sulfur or other compounds which poison surface adsorption sites and thereby limit the entry of carbon into the material.

Figure

Pack Carburization of Mild Steel, using Pulverized Bone as Carburizer:

Optimizing Process Parameters

 

Fatai Olufemi ARAMIDE1,*, Simeon Ademola IBITOYE2, Isiaka Oluwole OLADELE1 and

Joseph Olatunde BORODE1

1Metallurgical and Materials Engineering Dept., Federal University of Technology PMB 704, Akure, Ondo State, Nigeria.

2 Materials Science & Engineering Department, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.

E-mails: fat2003net@yahoo.com

*Corresponding author: +2348038509288 or +2348051304583

 

 

Abstract

Investigation was conducted into the mechanical properties of mild steel

subjected to packed carburization treatment using pulverized bone as the

carburizer, carburized at 850°C, 900°C and 950°C, soaked at the carburizing

temperature for 15 minutes and 30 minutes, quenched in oil and tempered at

550°C. Prior carburization process, standard test samples were prepared from

the as received specimen for tensile and impact tests. After carburization

process, the test samples were subjected to the standard test and from the data

obtained, ultimate tensile strength, engineering strain, impact strength,

Youngs’ moduli were calculated. The case and core hardness of the

carburized tempered samples were measured. It was observed that the

mechanical properties of mild steels were found to be strongly influenced by

the process of carburization, carburizing temperature and soaking time at

carburizing temperature. It was concluded that the sample carburized at

900°C soaked for 15 minutes and the one carburized at 850°C soaked for 30

minutes followed by oil quenching and tempering at 550°C were better

because they showed a trend of hard case with softer core.

 

Keywords

Packed carburization; Carburizing temperature; Activated carbon; Hardness.

 

 

Introduction

 

Carburizing is the addition of carbon to the surface of low-carbon steels at

temperatures within the austenitic region of the steel concern, which generally is between

850°C and 950°C for mild steels. Within this temperature range austenite, which has high

solubility for carbon, is the stable crystal structure. Hardening is accomplished when the

subsequent high-carbon surface layer is quenched to form martensite so that a high-carbon

martensitic case with good wear and fatigue resistance is superimposed on a tough, low-

carbon steel core [1].

Carbon diffusivity in austenite varies both with carbon concentration and carburizing

temperature [2-5]. Considering that carbon concentration depends on its activity in austenite

and that the finite repulsive interactions exist between neighboring carbon atoms in

octahedral sites, Babu and Bhadeshia [6] modeled carbon diffusivity in accordance with

kinetic and thermodynamic behavior of carbon in austenite. Siller and McLellan [7]

suggested that the repulsive forces between the neighboring carbon atoms influence carbon

diffusivity by reducing probability of interstitial sites occupancy in the vicinity of the site

already occupied by carbon atom. Therefore, in a concentration gradient, carbon atom

attempting random motion faces exaggerated difference in the number of available sites,

which enhances carbon diffusion down the concentration gradient.

Interstitial carbon diffusivity is strongly affected by the atomic interactions with

substitutional solute, i.e., alloying elements present in the steel [8]. If these interactions are

positive, substitutional solute atoms tend to attract interstitial carbon atoms. Such deviation

from randomness in the interstitial atoms distribution impedes long range diffusion of carbon

in the austenite lattice, and therefore decreases the effective coefficient of carbon diffusion.

Similar effect but of the opposite nature will be expected with solute components of negative

interactions: as their binding energy decreases there will be localized volumes with increased

carbon diffusivity. The study of process parameters in metals during heat treatment has been

of considerable interest for some years [9-12] but there has been relatively little work on

process variables during the surface hardening process [13] since controlling parameters in

carburization is a complex problem. The major influencing parameters in carburization are

the holding time, carburizing temperature, carbon potential and the quench time in oil [14].

            The aim of research work is to improve the mechanical properties mild steels

applicable for constructing farm implements and in automobiles, machines, gears, springs and

high strength wires etc., by pack carburization using pulverized bone as a carburizer and by

optimizing carburizing temperature and soaking time.

 

 

Material and Method

 

The materials used for this research work are Mild steel (with the chemical

composition shown in Table 1. was sourced from Universal steel company, Ogba Industrial

Estate, Ikeja, Lagos.), bone (purchased from the abattoir Araromi, Akure), Industrial engine

oil as quenching medium.

 

Table 1. Chemical composition of the mild steel sample

Element C Si S P Mn Ni Cr Mo V Cu

Avg content0.199

9 0.15480.059

4 0.04590.582

6 0.131 0.1105 0.016 0.0013 0.384

Element W As Sn Co Al Pb Ca Zn Fe  

Avg content0.007

6 0.00510.037

7 0.00980.002

8 0 0.00010.004

8 98.2476  

 

Test Specimen Preparation

The mild steel was machined to standard test sample sizes of tensile and impact tests.

This was done according to ASTM’s specifications on standard  tensile and impact sample

dimensions.

 

Carburization of Mild Steel Samples

The prepared test samples were embedded in the activated carbon inside a steel pot

which was then tightly sealed with clay cover to prevent the C° from escaping and prevent

unwanted furnace gas from entering the steel pot during heating. The furnace temperature

was adjusted to the required temperature (850°C, 900°C and 950°C for each stage

respectively) and the loaded steel pot was charged into the furnace. When the furnace

temperature reaches the required carburizing temperature, it was then held/soaked at the

temperature for the required time (15 and 30 minutes). After the material was held at the

specified time, the steel pot was removed from the furnace and the material was quenched in

industrial engine oil.

 

Tempering of the Carburized Samples

The carburized test samples were then tempered at a temperature of 550°C, held for

one hour, and then cooled in air. After the cycles of heat-treatment, the test samples were

subjected to tensile test, impact test and hardness test.

 

Mechanical Test

 

Tensile Testing

The tensile tests were performed on various samples using Monsanto tensometer. The

fracture load for each sample was noted as well as the diameter at the point of fracture and

the final gauge length. The initial diameter and initial gauge length for each sample was noted

before uniaxial load. From the generated data the ultimate tensile strength and percentage

elongation of each sample was calculated.

 

Impact Test

The impact tests were performed on various sample determine the impact strengths by

the “V-notch method using the Honsfield Balance Impact Testing Machine. Prior to

mounting on the machine, the test sample is notched to a depth of 2mm with v-shaped hand

file. The notched test sample was then mounted on the impact-testing machine, which is the

operated to apply a (constant) impact force on the test sample. The impact strength (the

amount of impact energy the specimen absorbed before yielding) was then read off the

calibrated scale on the impact testing machine.

 

Hardness Test

In present experimental work Rockwell hardness was measured on carburized,

tempered mild steel samples which are carburized under different temperature range of 850,

900 and 950°C. For each of the sample, test was conducted for 3 times and the average of all

the samples was taken as the observed values in each case.

 

Results and Discussion

 

Table 2 shows the mechanical properties of the carburized, tempered samples: The

relationships between these properties are depicted as follows in Figures 1, 2, 3, 4, 5 and 6,

depict the effects of carburizing temperature on the impact energy, ultimate tensile strength,

engineering strain, Young’s modulus of elasticity, case hardness and core hardness of the test

samples respectively.

 

Table 2. Mechanical Properties of the Carburized, Tempered Samples

Time (Min)

Temperature (°C)

Impact Strength (J)

Core harness (HRA)

Case hardness (HRA)

UTS (N/mm2)

Engineering Strain

Modulus of Elasticity (N/mm2)

15 850 31.23 67 68.3 689.60 0.01942 35,509.7815 900 41.37 63.8 67.3 863.57 0.0652 13,244.9415 950 60.43 63.1 62.1 900.25 0.08479 10,617.41

30 850 20.69 61.6 67.6 751.81 0.02421 31,053.730 900 34.61 71 62.5 655.97 0.15987 4,103.1530 950 61.65 60.2 59.7 789.76 0.10977 7,194.68

 

Figure 1. The effect of carburizing temperature on the impact energy of the samples

 

From Figure 1, it is observed that the absorbed (impact) energy increases with

increase in the carburizing temperature, the impact energy of the samples soaked for 15

minutes (0.25 hours) were higher than those of the samples soaked for 30 minutes (0.50

hours).

 

Figure 2. The effect of carburizing temperature on the ultimate tensile strength of the

samples

 

In Figure 2, it is observed that for the samples soaked for 15 minutes, the ultimate

tensile strength (UTS) increases with increase in the carburizing temperature and reached the

maximum value of 900.25 Nmm-2 at 950°C. For the samples soaked for 30 minutes, the UTS

initially decreased from 751.81 Nmm-2 (at the carburizing temperature of 850°C) to 655.97

Nmm-2 when the carburizing temperature increased to 900°C. It then increased to its

maximum value of 789.76 Nmm-2 as the carburizing temperature increased to 950°C. This

shows that the UTS of the samples soaked for 30 minutes was reduced by the carburization

process, whereas the UTS of the samples soaked for 15 minutes improved as the carburizing

temperature increased. Similarly, for both series the carburization process reduced the UTS of

the treated samples at carburizing temperature of 850°C when compared with the UTS of the

untreated as received sample which has a UTS of 800.07 Nmm -2 and impact energy of 61.25

J. For the samples soaked for 15 minutes, a similar result was arrived at by some researchers

[15, 16] who studied the mechanical properties of cast 25Cr-20Ni steel subjected to pack

carburization. This relationship of the UTS with the carburizing temperature is similar to its

relationship with the percentage carbon content of steel in both annealed and tempered

conditions [17,18], it can be reliably assumed that the amount of carbon that diffused into the

samples increases with the carburizing temperature. The behaviour of the samples soaked for

30 minutes in Figure 2, is similar to the discoveries of Ward, (1981) [18] where he found that

the UTS of carburized samples of some steel grades were some reduced.

 

Figure 3. The effect of carburizing temperature on the engineering strain of the samples

 

In Figure 3, the strain for both series of samples initially increased. From its minimum

level of 0.02421 (i.e. 2.421 % elongation) at 850°C, increased to its maximum of 0.15987

(i.e. 15.987 % elongation) as the carburizing temperature is increased to 900°C which then

reduced to 0.10977 (i.e. 10.977 % elongation) with further increase in the carburizing

temperature to 950°C for the samples soaked for 30 minutes. Similarly, the strain increase

from its minimum level of 0.01942 (i.e. 1.942 % elongation) at 850°C, it further increased to

a value 0.0652 (i.e. 6.52 % elongation) as the carburizing temperature is increased to 900°C

which then increased slightly to 0.08479 (i.e. 8.479 % elongation) with further increase in the

carburizing temperature to 950°C for the samples soaked for 15 minutes [19]. The reason for

the initial increase in ductility as the carburizing temperature increases is due to an increased

interface area produced by the carbide formation at grain boundaries which lead to the

impurities (cavities and cracks) being redistributed, because their concentration is low the

problem of embrittlement is negligible [20].

 

Figure 4. The effect of carburizing temperature on the Young’s modulus of elasticity of the

samples

 

From Figure 4 it is clearly seen that for the samples soaked for 15 minutes, the

Youngs’ modulus at carburizing temperature of 850°C reduced from its maximum value of

35,509.78 Nmm-2 to 13,244.94 Nmm-2 as the carburizing temperature is increased to 900°C

and then reduced to minimum value of 10,617.41 Nmm-2 as the carburizing temperature is

increased to 950°C. Similarly, for the samples soaked for 30 minutes, the Youngs’ modulus

at carburizing temperature of 850°C reduced from its maximum value of 31,053.7 Nmm-2 to

the minimum value of 4,103.15 Nmm-2 as the carburizing temperature is increased to 900°C

and then increased to 7,194.68 Nmm-2 as the carburizing temperature is increased to 950°C.

Comparing these with the Youngs’ modulus of the as received sample which has a value of

12,973.86 Nmm-2 show that the stiffness of the carburized samples were initially higher than

the as received samples. This shows that the samples lost their stiffness with increase in the

carburizing temperature.

 

Figure 5. The effect of carburizing temperature on the case hardness of the samples

 

From Figures 5 and 6, it is observed that the for the samples soaked for 15 minutes

case hardness was 68.3 HRA (while the core hardness was 67 HRA), at the carburizing

temperature of 850°C; when the sample was carburized at 900°C, the case hardness reduced

to 67.3 HRA (while core hardness reduced to 63.8 HRA); when the sample was carburized at

950°C, the case hardness then reduced to 62.1 HRA (while the core hardness reduced slightly

to 63.1 HRA). Comparing these with the hardness of as received samples of 51.2 HRA it is

glaring that there was actually carbon enrichment in the carburized samples. Moreover, for

those samples soaked for 30 minutes at different carburizing temperatures the case hardness

was 67.6 HRA (while the core hardness was 61.6 HRA), at the carburizing temperature of

850°C; when the sample was carburized at 900°C, the case hardness reduced to 62.5 HRA

(while core hardness increased to 71 HRA); when the sample was carburized at 950°C, the

case hardness then reduced to 59.7 HRA (while the core hardness reduced to 60.2 HRA).

 

Figure 6. The effect of carburizing temperature on the core hardness of the samples

 

Summarily, looking at the hardness profile of the carburized samples, for those soaked

for 15 minutes, the sample carburized at 900°C is considered the best, while for the samples

soaked for 30 minutes, the sample carburized at 850°C is considered the best because they

showed a trend of hard case with softer core.

 

 

Conclusions

 

From the discussions so far it can be concluded that:

1.      Carburization process reduced the impact toughness of mild steels, but contrary to the

trend observed where activated carbon is the carburizer the impact toughness increases

with the carburizing temperature where pulverized bone is used as the carburizer.

2.      The stiffness of the mild steel is increased by the carburization process, but it decreases

with increasing carburizing temperature.

3.      The carburization treatment followed by the oil quenching and tempering at 550°C

strongly influence the hardness and tensile strength of mild steels.

4.      The carburization process decreases the impact energy (toughness) of the mild steels.

And the toughness is decreases with increase in the carburization temperature.

5.      The mechanical properties of mild steels were found to be strongly influenced by the

process of carburization, carburizing temperature soaking time at carburizing

temperature.

6.      The sample carburized at 900°C soaked for 15 minutes and the one carburized at 850°C

soaked for 30 minutes followed by oil quenching and tempering at 550°C are considered

better because they showed a trend of hard case with softer core.

Carbon Monoxide reaction:CO2 + C ---> 2 CO

Reaction of Cementite to Carbon Monoxide:2 CO + 3 Fe --->Fe3C + CO2

Carburisation

Carburisation is the process by which carbon is diffused into the surface of steel in order to increase its hardness. The carbon forms carbide precipitates (particularly if the steel contains carbide forming elements such as manganese or molybdenum) which pin dislocations and prevent slip, thus making the material harder. However, the increased carbon content reduces the toughness of the material. In most applications it is important that the surface of the steel is hard, but the bulk material can remain softer without detriment to the properties of the component. Thus, carbon is often diffused in from the outer surfaces to obtain a material that is hard on the surface but tough in the bulk.

A carburised steel, showing increased carbon content on the outer surface See Micrograph No 289

This is done by heating the steel in a carbon atmosphere, so that there is a concentration gradient of carbon across the interface. Carbon diffuses into the steel, and the elevated

temperature speeds up the process. The concentration profile of carbon is governed by Fick’s second law, as there is effectively an infinite source of carbon.

PLASMA CARBURIZING

PLASMA CARBURIZINGThe workpiece is heated up to the processing temperature by the external heaters in the vacuum furnace. Then, rare gas that contains hydrocarbon gases, such as methane or propane, is introduced into the furnace to the pressure of 2 to 3 Torr. After that, several hundreds volts DC voltage is applied between the workpiece and the furnace as the former to be cathode.Thus, glow discharge generates int the furnace and the gas ions in the plasma react with the workpiece and it is carburized.

PLASMA CARBURIZING

FEATURES

(1) No oxidation takes place at grain boundaries, and the finished surface is clean and glossy.

Model JIC-6SC-SVP lasma Carburizing Equipment

(2) Even carburized layer is obtained because of plasma reaction, and no sooting is encountered.

(3) Due to fast carburizing rate and low temperature/short time treatment, no grain growth is observed and residual strain is very little.

(4) Carburizing of high concentration and the one into the hard-to-carburized materials, such as stainless steel, sintered metals, etc., can easily be made. �B

(5) This is a pollution-free and energy-saving processing. �B

PLASMA CARBURIZING EQUIPMENTDemands for high quality industrial parts are very high these days along with the innovation of technology.Plasma carburizing is one of the way of surface modification by forming carbides on the metal surface in the vacuum utilizing thermal energy and electrochemical reaction of DC plasma.A new plasma carburizing method developed by NDK provides high quality products of good precision in clean environment and with high efficiency.Further, it makes possible high density carburizing and easy carburizing of hard-to-carburize materials.