steels for bearings - university of thessaly...gonidakis st. physical metallurgy/2015-2016...

UNIVERSITY OF THESSALY

MECHANICAL ENGINEERING

CONTEMPORARY METHODS OF

DESING AND ANALYSIS IN

INDUSTRY

STEELS FOR BEARINGS

Material Science Engineer: Civil Engineer: St.Gonidakis K.Christopoulos

Supervisor: Gregory N. Haidemenopoulos

VOLOS, JANUARY 2016

Gonidakis St. Physical metallurgy/2015-2016 Christopoulos K.

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Contents

1 Introduction

1.1 General Information

1.2 Types of bearings

1.3 Historical data

1.4 Rolling element of bearings

1.5 Life span of roller bearings

2 Microstructure

2.1 Overview

2.2 Grain

2.3 Crystal structure

2.4 Interstitial solid solutions

2.5 Substitutional solid solutions

3 Transformations

3.1 Phase transformation

3.2 Diffusion

3.3 Diffusion coefficient

3.5 Diffusion transformation

3.6 Martensitic transformation

4 Roller bearing technology

4.1 Low concentration of alloying elements steels

4.2 Aerospace

Conclusion


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1 Introduction

1.1 General information

The term bearing describes an element that constrains motion and supports moving

mechanical parts, such as axles and shafts. Their main objective is to receive the loads

that are imposed during machine operations and to transfer them to their fixed parts,

such as their foundation or their reduction gearboxes. Bearings are lubricated in order

to keep friction losses and temperatures to a minimum. Friction in bearings can either

be kinetic (that does not allow for relative movement to the opposite direction

between two surfaces) or static when two objects are not moving relative to each

other.

1.2 Types of bearings

Generally there are 3 basic ways of categorizing bearings

1. Depending on the position of the shaft that they are supporting:

o Radial

o Thrust

2. Depending on the type of friction that develops :

o Rolling Element

o Plain

3. Depending on the way it is operating :

o Fixed

o Self Aligning

1.3 Historical data

Bearings have been used since ancient times in the construction of the greatest

monuments of human history. This is a quick historical overview and some milestone

dates for the development of bearings.

2600 BC - The ancient Egyptians used a type of roller bearing to move huge

stone blocks during the building of the pyramids .

40 BC -An early type of ball bearing made out of wood was used by the

Romans to support tables.

1500 AD - Leonardo DaVinci describes a type of ball bearing.

1740 AD - John Harrison invents the first caged roller bearing

1794 AD - The welsh Phillip Vaughan files a patent application for the first

modern ball bearing design.

1980 AD - NASA uses needle roller bearings on its space shuttles.

Nonetheless ,the present paper focuses more on the materials used for

creating the rolling bearings , more specifically to 1C - 1.5 Cr steel alloys ,which are

widely used .According to H Bhadeshia ,professor of metallurgy at SFK Steel

Technology Center , the performance of the particular alloys has been studied

extensively by Stribeck (1901) displaying suitability for the required applications and

with progressive improvements ,became the main alloy to be used for bearings since

1905 .It is still used today due to its excellent fatigue performance and cleanliness


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1.4 Rolling element of bearings

Rolling element bearings are comprised of two rings or discs one inner and

the other outer, shaped so that the rolling elements can move between them. The

rolling elements can be formed as a sphere, cylinder ,toroid ,tapered cylinder or

needle . There are the following five types of rolling element bearings based on the

shape of the rolling elements (Fig 1) and below is the diagram that depicts the basic

parts of a rolling element bearing (Fig 2) :

Fig 1: Radial ,Cylindrical ,Spherical ,Tapered ,Needle -Roller

Fig 2: 1) An outer ring , 2) An inner ring, 3) Rolling Elements , 4) A cage

The rolling element bearings are preferably used for bearing under normal

loading and safe operating conditions, for example machinery, reduction gearboxes,

engines, vehicles, fans, pumps, machine moving parts etc. They must be

manufactured so that they offer high reliability, resistance and accuracy and also to be

able to rotate at high velocities while minimizing sound and vibrations.

Their life span is not infinite and depends on a number of factors, with the

most important of them being the manufacturing method, the method of transportation

and storage, the method of installation and finally the operating method of the

bearing. Below there is a quick overview of those factors:


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The manufacturing factor consists of the heterogeneity of materials in

use as well as the low tolerance of the product.

The transportation and storage factor includes bad packaging and

vibrations during transportation and storage.

The installation factor includes deformations of the bearing,

unappropriated loading and misalignment errors .

The operation factor includes overloading , inadequate lubrication ,dust

and chemicals occurrence , humidity and overheating

All the above factors in correlation with time lead to bearing failure. The main

causes of bearing failures are:

fatigue

corrosion

deformation

long term operation

1.5 Life span of roller bearings

Roller bearings that are being used in industrial applications are one of the

components dealing with huge dynamic and long term static strains. As a result they

suffer from mechanical wear. Apart from the mechanical wear, a second kind of

strain that they have to face is corrosion. Due to their use in a wide range of industrial

operations roller bearings can stay in touch with corrosive fluids, such as water, acids,

bases and various gases. Standardized industrial roller bearings are designed and

manufactured with low internal tolerances and therefore they are extremely sensitive

to corrosive pollutants. The most common corrosive agent of bearings is plain water.

It can even penetrate water proof ball bearings through the gap that is created when

the air trapped inside the bearing gets cooler and contracts every time the machinery

that uses the bearing halts its operation.

Usually roller bearings require special protection against corrosion when they

used as part of the production chain in chemical industries, in food industries, in steel

work industries and in semiconductor industries.

But as it was mentioned above, water is not the only corrosive agent that

threatens roller bearings, so in order to solve corrosion problems for every industrial

application of bearings special manufacturing techniques have been developed to

solve the problem for each case. More specifically:

1. When the bearings are operating in humid environments they are made out

of stainless steel or they are chrome coated. Stainless steel bearings are more suitable

for high humidity environments like the food industry, the chemical industry as well

as the semiconductor industry. Martensitic steels that are used for the manufacturing

of these bearings are also used for the manufacturing of the inner and outer rings of

the hybrid types of ball bearings that are installed in the above mentioned

applications. With only minor variations to the design such as using a different

material for their rolling elements and another type of lubricant, the life of martensitic


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roller bearings can be increased fivefold in comparison with the regular stainless

bearings.

2. In some special food industry applications where corrosive strain is greater

than normal the use of fluoride low temperature chrome plated roller element in

bearings is preferred (the inner and outer rings are made out of stainless steel). For

applications where grease lubrication is not possible then a fluoride resin cage is used.

In general this type of bearings has a greater life span not only from stainless steel

bearings but also from hard chrome plating.

3. In alkaline or slightly acidic environment the installation of either ceramic

or hardened stainless steel bearings or nickel plated bearings is recommended.

Ceramic bearings are mostly made of sulfur oxide or sulfur nitride therefore having a

greater life span in acidic or alkaline environment than stainless steel or hybrid

bearings. Sulfur oxide based bearings are cheaper to manufacture than the others,

while sulfur nitride based bearings are used in the most extreme and heat requiring

applications. In highly acidic, highly alkaline and highly corrosive gases

environments carbide based (instead of nitride) ceramic roller bearings are used

.Those types of bearings are most corrosive resistant than all other ceramic bearings

and all the rest types of bearings in general.

2 Microstructure

2.1 Overview

Steels that have desirable attributes and that are mostly used in industrial

applications have carbon concentrations of 0.8 - 1 wt %. They form martensitic

structures when the material from its austenitized structure is quenched in salt or oil

bath below its Ms( it is the temperature when martensitic structure is formed ). It is

followed by the process of shaping the retained austenite according to the desirable

properties of the final product, with hardness being the most important of them. In

small scale applications hardening occurs through tempering and quenching, while in

large industrial bearings the need for even greater hardenability is met by

implementing larger concentrations of alloy elements[1]

. Steel alloys with carbon

concetration of 1% and chromium of 1.5% are extremely popular in rolling element

bearing manufacture (table 1)


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TABLE 1: The composition of common roller bearings, 1%wtC and 1.5%wt Cr.[1]

GRADE C Mn Si Cr Ni Mo Cu S P others

En31 0.90-

1.20

0.30-

0.75

0.10-

0.35

1.00-

1.60 0.05 0.05

SAE_52100 0.98 0.38 0.16 1.39 0.07 0.02 0.12 0.06 0.12

100Cr6 0.90-

1.05

0.25-

0.45

0.15-

0.35

1.40-

1.65 ≤0.30 ≤0.30 ≤0.025 0.03

SUJ-2 1.03 0.37 0.23 1.35 0.51 - 0.023 0.018

AISI_52100 0.95-

1.10

0.20-

0.50 ≤0.35

1.30-

1.60 - - ≤0.025 ≤0.025 -

2.2 Grain

The steels that are used for making rolling element bearings are a ferritic

matrix (the metal) with percentages of added alloying elements. Another basic

characteristic of them is that their comprising elements contribute to the creation of a

common crystal structure, which is usually caused by the melting of the various

elements that are mixed together and the following solidification. The optical

representation of a metal or an alloy revealed by a microscope (e.g. metallographic or

scanning electron microscope), after suitable preparation is called microstructure.

Compound elements of microstructure are its phases and its grains. Grain is the

structural unit of metallic materials and is the structural unit of metal materials and

consists of each area of the material that has resulted from the repetition of the

elementary cell in the space with the same orientation. Grain structure is the most

commonly found structure, in case of metals and alloys, and comes from the

solidification of the material after casting, through nucleation and growth

mechanisms. Grain size determines the materials mechanical properties. Thus, a fine

grained material has greater hardness and greater resistance to tension compared to a

coarse grained material. Fig. 3 depicts various grains sizes of low carbon concentrated

soft steel.

Fig 3: Steel microstructures where grain size is visible from larger (α) to smaller (γ)


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Grain size control for the specific chemical compositions (table 1) of a metal can offer

special properties from the early stages of processing till the desired properties of the

final product according to the application. For example smaller grain sizes are created

at lower austenitization temperatures (Fig 4). In order to obtain those fine grains rapid

heat treatment is needed, such as molten metal baths and induction hardening [1]

. In

those steels austenitization temperature is obtained at 900 οC while full dissolution of

cementite is calculated at 20 minutes at 1040 οC (Fig 5). Taking into consideration the

Fe-C phase diagram, carbon concentration is at 1% which means that we must attain

greater temperatures above Αcm in order for the iron carbide to be dissolved (Fig 6)

Fig 4: Austenite grain for 52100 steel ,following austenitization for 1 hour at various temperatures[1]

Image 5: According to MTDATA results [Bhadeshia], for steel with concentrations of 1% carbon and

1,5% chrome. (M) responds to metallic atoms including Fe, C, Si, Mn, Ni, Mo, Cr[1]

.


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Fig 6: Phase diagramm Fe-C[4]

.

Apart from carbon these types of steel contain a variety of alloying elements

(table 1) with Cr dominating above others. The main reason for that is to affect the

properties of the metal in such a way that leads to its desired processing, for example

the thermodynamic phase stability, the speed of transformations and the formulation

of the microstructure. So the presence of Cr offers great oxidization and corrosion

resistance when the bearings are in contact with harsh environments, while the

carbide is used to give to the alloy toughness and antifriction attributes. Mn in those

concentrations is capable of desulfurising the material by creating MnS sulfides

favoring its workability. Also, it leads to the increase of hardenability, the ability to

harden after quenching (the capability of steel to grow its martensitic structure in

deeper layers under its surface ). Sulfur and phosphorus are present in small amounts

because they degrade the metals ductility and toughness[5]

.

2.3 Crystal structure

Metals are called crystalline solids because of their crystalline structure .A crystalline

structure is defined as the orderly geometric arrangement of atoms that is repeated in

all three dimensions. The smallest repeating array of atoms in metal is called unit

cell[6]

. The atoms of the unit cell are considered to be spheres with a certain diameter.

The area where a similar continuous crystal structure appears is called crystal or

crystallite or grain.

This repetitive presence of the unit cells creates a network of points, arranged

in such a way that their boundary points have the same layout along any direction,

thus creating a lattice.

In 1848, French physicist Auguste Bravais while researching the types of

lattices according to the aforementioned definition, he showed that there are just 14

ways of arranging points in space lattice which constitute 7 crystal systems. The two


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types of crystal systems that appear in steel structure are the body centered cubic

structure BCC and the face centered cubic structure (Fig 7, 8)

Fig 7: BCC crystal structure[6]

Fig 8: FCC crystal structure[6]

The atoms of the body centered cubic structure are located at each of the eight

corners of the cube plus one atom in the center of the cube. The structure of the cell is

presented in fig 7 where a fraction of the volume of each atom that is part of the

crystal structure, moreover the relation between the atomic radius and the side of the

cube. The same features are depicted on Fig 8 where a face centered cubic structure is

shown. The difference here is that the atoms are located at eight corners of the cell

and six of them are at the center of the cubes surfaces.

Some metals (Fe, Ti, Co) can crystallize in more than one crystal structures

under various circumstances (different temperatures or pressure) and this property is

called allotropy. In the present paper we are focusing mostly on steel alloys

containing 1% carbon and 1.5% chromium that is used in common bearing

applications. Iron (Fe) takes up most of the steel percentage, therefore under

atmospheric pressure and within -273 o

C till 92 o

C (a-Fe ) the cell structure is BCC ,as

the temperature rises from 912 o

C to 1394 o

C (γ-Fe) it is transformed to FCC and just

below its liquidus temperature ( 1394 o

C to 1539 o

C ) (δ-Fe), the metal transforms

back to its original BCC crystal structure (Fig 6). These transformations of the irons

crystal structure result in significant changes in its properties .It must also be noted

that transforming from BCC structure to a more dense FCC structure greatly reduces

the materials volume (Fig 9).


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Fig 9: Iron elongation with temperature

2.4 Interstitial Solid Solutions

An extra property that stems from steel allotropy is about the atoms that are

hosted in the crystal lattice. The spaces that are occupied by these atoms are called

interstitial sites. In other words is the space that can accommodate small atoms like

carbon between the solutes atoms of the crystal structure. The resulting solid solution

is called interstitial solid solution. The size of the atoms that can take up those sites

have smaller atomic radius than irons (H: 0.46Å, B: 0.97Å, C: 0.77Å, N: 0.71Å, O:

0.60Å). The magnitude of gap inside the interstitial sites, (the empty space between

the crystal structure atoms), is important for the formation of solid solutions .

In FCC structure there are two types of interstitial sites, octahedral and

tetrahedral. The octahedral sites (bands) are larger and are surrounded by six atoms,

located at the edges of an octahedron. The tetrahedral sites are surrounded by four

atoms that form a tetrahedron. It is possible to calculate the size of interstitial sites by

using geometry rules (Fig 8). Austenite has FCC structure and its octahedral type can

fit an atom with a radius of 0.52 Å while the tetrahedral allows a smaller radius of up

to 0.28 Å (Fig 10), given that RFCC-Fe: 1,27Å, RBCC-Fe: 1,24Å the atomic radius[6]

.

Fig 10: Interstitial sites of FCC structure (a) octahedral, (b) tetrahedral

[6]

It is easily deducted that in octahedral band exists the capability of carrying a

carbon atom with Rc: 0.71 Å with just a minor lattice deformation. Therefore in face

centered cubic structure carbon occupies sites that have the largest lattice distance that

leads to the reduction of the total lattice deformations (γ -Fe with carbon : austenite).

In BCC structure there are similar interstitial sites with the only difference

being that tetrahedral type of bands is spacer than octahedral (a-Fe with carbon :

ferrite). So the spacer interstitial site (tetrahedral) can accommodate an atom with

0.36 Å radius while the smaller octahedral an atom with 0.19 Å radius (Fig 11 ).


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Fig 11: Interstitial sites of BCC structure (a) octahedral, (b) tetrahedral

[6]

Therefore creating an interstitial solid solution α-Fe with carbon is much more

difficult than in γ-Fe (FCC) and that the formation of such a solution will be

accompanied by significant lattice deformations. The interstitial site size depends on

the crystal structure atomic size.

In body centered cubic structure these interstitial sites are smaller than those in

face centered structured. As a result the FCC structure can accommodate larger atoms

in interstitial sites thus having a greater solid solubility of carbon in iron. For

example, austenite can dissolve considerably more carbon (up to 2%) than ferrite

(0.02%). The increased hardenability of steels after heat treatment is based on this

difference in solubility.

2.5 Substitutional Solid Solutions

The alloying elements that have radius size similar to iron are not able to be

dissolved into the interstitial sites, nonetheless they can be dissolved into the crystal

lattice of iron by creating substitutional solid solutions. The Hume -Rothery rules

describe solid solubility during formation of substitutional solid solutions[5]

:

1 Maximun atomic radius difference at 15%

2 Similar electronegativity

3 Same valency

4 Similar crystal structures

Substitutional solid solutions can be divided into two categories, those that are

ferrite dissolved (Ni, Cu, P and Si) and those that form carbides Mn, Cr, Mo, V, Ti, W

and Nb. In low concentrations they can also be dissolved in the ferrite matrix and in

cementite. In roller bearings the basic alloying element that forms substitutional solid

solution is chromium which only comprises of 1.5 wt% of the alloy. Although

chromium is present at very low percentages it can form carbides with cementite (Fig

12, 13). Cementite thermodynamic stability is enchanced through chromium

enrichment, making its particles more tough resistant during heat treatment .This

results in an increase of the Αcm temperature (Fig 5, 6).


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Image12: Chromium percentage in cementite.

13: Chromium percentage in cementite at austenitization temperature in relation to time for 52100

steel.

3 Transformations

3.1 Phase Transformations

Phase transformations of metal materials play an instrumental role in

amplifying the properties of the alloys and in changing their microstructure. In most

metals the movement of atoms is activated through thermal treatment, these types of

transformations are called diffusive. There are also other types of transformations

where the diffusion of atoms is obstructed, called diffusionless and martensitic

transformation is one.

3.2 Diffusion

Metallic materials are more than often heat treated to enhance their properties.

These processes which take place during heat treatment are usually processes of mass

movement of atoms or atomic diffusion. Controlling the rate of this mass movement

can result in great improvement of the materials properties. Therefore diffusion is

defined the atomic movement through mass. In the present paper the diffusion

mechanism plays an important role in bearing construction because is a significant

method for hardening steel like case hardening. It is a basic technique for acquiring

the appropriate carbon percentage (carburizing) which improves not only the bearings

hardness but also its life span against fatigue failure.


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3.3 Diffusion coefficient

In a system where atoms are diffused in a particular manner of diffusion in a

specific crystal structure with lattice defects ,the ability of atoms to be diffused is

referred to as the diffusion rate .Temperature plays an important role to the

transformation and to the atoms diffusion speed. The diffusion coefficient of the

interstitial atoms and more specifically in carbon is and Arhenius equation[5]

:

D=D0*e–ΔG*

D/RT

D: diffusion coefficient

Do: constant characteristic to any diffusion system

ΔG*D: interstitial diffusion activation energy

R: universal gas constant

Τ: temperature

According to this equation, diffusion rate increases exponentially with an increase of

temperature .Therefore in order for carbon to make the leaps and be diffused in the

ferrite matrix, first must obtain enough energy to move to a neighboring interstitial

sites by uplifting iron atoms (Fig 14).

Fig 14: Diffusion Energy Barrier

Diffusion in lattice sites can occur when atoms of the solute move through

defects in the crystal lattice of the solvent. A necessary condition for this to happen is

that the atoms of both the solute and the solvent have similar sizes. (chromium:

166pm, iron: 156pm silicon: 111pm, nickel: 149pm, manganese :161pm,

molybdenum: 190pm)[8]

. Atoms that are diffused through steel crystal lattice (in

52100 steel ) with substitutional diffusion form a solid solution. In order for that to

happen a gap must be created from a driving force where the solvents atoms release

their bonds and be free from the near atoms. Then the free atom may leave the lattice

position leaving a hole behind (lattice walk)[5]

.Afterwards with a secondary driving

force the substitutional atom cover that hole. The sum of these energies is the

diffusion activation energy for lattice sites. It is concluded that substitutional atoms

diffusion requires smaller amounts of activation energy (Fig 15).


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Fig 15: Diffusion coefficients

[5].

The gradients of Fig 15 depict the activation energy that is needed for the

diffusion mechanism to take place. It is observed that although carbon is more soluble

in austenite it can be diffused faster in ferrite. The reason why this is happening is

atoms in BCC structures cover more volume than atoms in FCC structures

(APFBCC=0.68, APFFCC=0.74) resulting for the former structure being spacer. It is also

to be noted that that the diffusion activation energy in lattice sites is greater (looking

at Fe on α-Fe ). Diffusion is an important process, especially of carbon in steel,

because it affects significantly the materials properties through transformations

3.4 Diffusion transformations

Recrystallization of metals is an important diffusion process in steels due to

the fact that it changes their properties. The progress of phase transformations is

described by isothermal transformation diagrams (ΤΤΤ: Time, Temperature,

Transformation). First the metal is heated and then cooled in such a way so that it

acquires the desirable properties. This process is called heat treatment which happens

at a specific temperature and time, in case of tempering and cooling speed

Isothermal transformation diagrams capture the progress of a transformation.

Specifically the curves show the start and the end of a diffusion transformation rate,

while diffusionless transformations are depicted through a straight line (Fig 16).


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Fig 16: Isothermal transformation diagram Fe, 0.95C, 0.44Mn, 0.21Si, 1.42Cr wt%. Austenitization

occurs at 845οC for 20 minutes. Numbers show the start and the percentage of the transformation

[1]

In Fig 16 can be observed the transformation to perlite microstructure happens

at the 450-750 οC range, to bainite at 200-450

οC . the perlite microstructure is

consecutive ferritic and sementitic planes while the bainitic is semetitic needles inside

a ferritic matrix in a precipitated form. In roller bearing industry bainite structure is

important due to the fact that it can combine both hardenability and ductility

extending the bearings life span. Alloys such as 52100 steel in order to be transformed

to bainite, they are initially austenitized at a temperature of 1040 ο

C for 30 minutes

and then they remain isothermally at a temperature range of 200 - 450 οC .In

temperatures lower than 350 ο

C bainite is called lower bainite and its precipitates are

thinner. Full transformation of bainite can take up to four hours. Nonetheless it has

been tested on industrial scale cooling bainite at 20 ο

C below the Ms for less than a

minute and then continuing the rest of the transformation at the bainite temperature

range[1]

.This type of heat treatments (step quenching) aim to increase the

transformation kinetics without losing any material hardness. In addition austenit is

almost completely transformed minimizing the possibility of microcracking during

processing.

3.5 Martensitic Transformations

Martensitic structure is the toughest microstructure in steels .It is made by

rapid cooling from the austenitization temperature to the Ms temperature (Ms is the

temperature where formation of the microstructure starts) . The cooling speed must be

rapid in order to avoid the peaks which form the microstructures of perlite and bainite

(Fig 16). When the crystals are formed (needle shaped for small concentrations of

carbon and lenticular or lens shaped for larger the material is strained by an ordered

movement of a large number of atoms at the same time, resulting in the end of the

transformation without all of the austenite to be transformed into martensite. The

remaining austenite is called retained austenite. Martensite characteristics in steels

are:

There is no diffusion .Movement of atoms is minimal.

It adopts the same composition of austenite.


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It happens in allotropic metals

There is deformation strain.

4 Roller bearing technology

There are many research papers aiming at the improvement of bearing

operations, especially enhancing the bearing life span. Factors that cause wear failure

are stress loads, temperature loads and friction where operating under harsh

environments (humidity, dust) are capable of degrading the bearing races. Therefore it

is of great importance that the metal structure is able to remain operating flawlessly

for a long time.

The characteristics of the solid structure as described above should match

characteristics like tolerance, toughness and stability. Martensitic steel transformation

has the toughest structure steel can provide, but it must somehow keep the

microstructure stable because of its brittleness. This is why after quenching steel is

heat treated again in order for the cementite to lose its carbon atom surplus. It has

been studied that tempering at temperatures just above Ms for almost an hour

followed by cooling at room temperature forms a partial bainite structure and the

remained austenite transforms into martensite, leading to a strong structure that is

satisfactory for roller element bearings [1]

. Another example is the fine austenite grain

structure which is formed firstly by heating at temperatures of 1150 ο

C, quenching in

heated oil at 150 οC and cooling at room temperature, following austenitization at

900 οC and finally tempering at 250

οC

[1]. According to Hong -Seok Yang's paper

about correlating temperature when martensitic transformation starts and austenite

grains size, the Ms temperature is greater when the grains are more refined.

4.1 Low concentration of alloying elements steels[2]

Stainless steel with small percentages of alloying elements that range from

2,07 % and chromium of at least 10% are another type of ferrous material that have

superior mechanical properties to regular steels. The aim is to increase hardness after

heat treatment.

It has been proved that for the particular type of metal the heat treatment

process can set the temperature that martensitic transformation starts by shaping the

size of its grains .

Samples that have been homogenized at a vacuum furnace at 1200 ο

C for two

days ,while remaining at the austenitization temperature for a small period of time (

30 minutes ) ,are double quenched acquiring finer grain structure and by using a

thermal expansion apparatus the temperature at which the martensitic transformation

starts can be calculated (Fig 17) .Whereas in order for the grains to be better visible

acid it used and through chemical etching the structure of austenite is revealed and its

size is calculated using a simple linear method.


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Fig 17: Lγ refined austenite grain, M

os-Ms the fraction of martensite, quantitative

estimates of the dependence of the observed MS on the austenite grain size

4.2 Aerospace[3]

Modern technological advances increase the demand for reliable bearings

operating at harsh environments .In the aerocpace industry engines require bearings to

operate at high temperatures. The material that has proved to have hardness

,resistance and toughness at high temperatures is called M50 (Fig 18). Another

equally important property to this material is its anti friction ability, that could prevent

degradation of its operation in case of lubrication failure while it is being in use.

Fig 18: Μ50

In a paper by Prantik Mukhopadhyay for improving M50 steel microstructure

caused by wear due to friction. Significant differences in the metals microstructures

have been reported before and after hardening process. The hardening method

selected was a heat treatment where it was tempered at austenitization temperature ,

short term overheating at a greater temperature, quenching in oil baths, overcooling in

liquid nitrogen and finally tempered at 540 οC (Fig 19, 20). Following that the

samples underwent a series of surface abrasion tests by silica sand.

Fig 19: M50 Stress - Strain diagram.


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Fig 20: Microstructure before abrasion (ι) coarse carbides in ferrite matrix before hardening, (ιι) coarse

carbides after hardening (ιιι) closer view of ferrite, (ιν) martensite plates after hardening.

Fig 21: Microstructure after abrasion , (ι) before hardening, (ιι) after hardening, (ιιι) closer view after

hardening.

It can be inferred from Fig 21 that particles do not follow the direction of

movement and that the get stuck thus creating linear cavities visible on the image. In

addition they leave a coarser mark on the martensite than on soft ferrite, due to the

fact that martensite has a tougher surface. Therefore the martensitic crystal structure

suffers more damage compared to the ferritic structure, but ferrite keeps the structure


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stable during martensitic recrystallization. The deformation during abrasion has many

features. First occurs plastic deformation while the surface of the races is being

etched. The martensite creates dislocation in the crystal structure while ferrite being

ductile recovers from the deformation, occurring in lasting longer (the material)

Conclusion

The purpose of this paper is to comprise steel through bearings. As it is being

presented the right combination of iron and alloy elements can provide many versatile

products that can aid sciences evolution. The reader has the opportunity to understand

the transformations occurring inside the material in a magnified perspective using

some examples from other papers. The paper can not cover all the metallurgy science

from bearings but a fraction of it.


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References

[1] Bhadeshia HKDH. Steels for bearings. Met Sci 2012 p. 268-435

[2] Hong-Seok Yang, Bhadeshia HKDH. Austenite grain size and the martensite-start

temperature. Met Sci 2009

[3] Prantik Mukhopadhyay, PS Kannaki, M Srinivas, Manish Roy. Microstructural

developments during abrasion of M50 bearing steel. Met Sci 2014 p.31-37

[4] Abdlmanam, SA Elmaryami, Badrul Omar. Developing 1-Dimensional Transient

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