RWTH Aachen Institute of Ferrous Metallurgy MASTER THESIS Master Student Alireza Saeed-Akbari Matr. –Nr. 268696 Subject: Determination of Steels Microstructural Components Based on Novel Characterization Techniques Supervisors: Univ. Prof. Dr.-Ing. W. Bleck M.Sc. Malek Naderi
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RWTH Aachen Institute of Ferrous Metallurgy
MASTER THESIS
Master Student Alireza Saeed-Akbari
Matr. –Nr. 268696
Subject:
Determination of Steels Microstructural
Components Based on Novel Characterization
Techniques
Supervisors: Univ. Prof. Dr.-Ing. W. Bleck M.Sc. Malek Naderi
To mum and dad,
for the whole love they gave me in my life…
Acknowledgment The achievements of the current work are the results of helps
and supports of all technicians, assistants, and colleagues at the
Institute of Ferrous Metallurgy (IEHK), RWTH Aachen
University.
Among all, I should appreciate Professor Wolfgang Bleck –
head of the Institute of Ferrous Metallurgy – for his kind
considerations during the project; Mr. Malek Naderi for his
invaluable indications, instructions, and cooperation, and IEHK
technicians for their quick and accurate experiments.
I would like to give special thanks to Mrs. Aida Nonn for her
kind devotion to this investigation; without her helps, the
examination of weld structures was impossible. The
microstructural and relevant primary hardness data of welds in
the current work are parts of her assistance to broaden the
discussions of the project.
Alireza S. Akbari
Abstract In the present work, hardness mapping and dilatometry experiments were
employed as the novel methods for the quantitative and qualitative
assessments of the heterogeneous microstructures in different steels. The
results were initially compared with the optical microscopy images to
show the lack of accuracy in case of handling the heterogeneous
compressive specimens using optical microscopy. It was then
demonstrated that dilatometry and hardness mapping results are
consistent.
In another attempt, a mathematical formula was developed to predict the
fraction of martensite based on the bulk hardness of the investigated
specimens. The formula gave reliable results due to both steels studied.
Surface hardness mapping and optical microscopy methods were also
utilized to study the microstructural phases within the fusion zones and
the heat affected zones of steel welds. Both macro- and microstructures
were examined in this regard. It was concluded that the parallel
application of hardness measurements and optical microscopy was
4.3. Prediction of Martensite Fraction Using Hardness Criterion, 38 4.3.1. Developing the Formula, 38 4.3.2. Checking the Consistency of Results, 42
4.4. Microstructural Study of Weld Zones, 45
5. Conclusions 58 References i
Chapter 1 – INTRODUCTION 1
CHAPTER
ONE
INTRODUCTION
The fast yet accurate estimation of microstructural phases in steels might
be interesting due to both scientific and industrial points of view. In
addition, developing the reliable alternative experiments for the
calculation of phase distribution and phase fraction which can replace the
conventional microscopy techniques is the matter of interest based on its
time and cost saving impacts.
It has been well established that each single microstructural phase in steel
has its own hardness range. The distribution of such hardness values in
the pre-assumed hardness intervals is interconnected with the
arrangement and amount of different phases within the microstructure.
There are many research works and reports regarding the relationship
among the hardness of each phase, its chemical composition, and its
related mechanical properties [1-3]. In some reports [4,5], hardness
profiles were used to study the homogeneity of microstructures. Some
others [6-8] used surface hardness data to correlate hardness maps and
macrostructure of the entire weld zones, including heat affected zone and
the steel base plate.
Phase transformations occur during cooling or heating. Any
transformation results in a specific phase and causes a definite contraction
or expansion. For instance, transformation of austenite to martensite
during cooling yields a certain dilatation. It should be pointed out that the
dilatation term is mainly the magnitude of plastic deformation resulting
from martensitic transformation. This dilatation is an invariant plane
Chapter 1 – INTRODUCTION 2
strain, which is the combined effect of a uniaxial dilatation and a simple
shear. The higher volume fraction of martensite results in higher
dilatation magnitudes.
It is known that the relative length changes are proportional to the relative
volume changes if the system undergoes a macroscopically isotropic
phase transformation. In the case of anisotropy that may occur in the
specimens exhibiting preferred orientation for the constituent
crystallites/grains, distinction should be made between the length and the
volumetric changes [9]
In the present work, hardness mapping and dilatometry experiments were
employed as the novel methods for the assessment of microstructural
phases in different steels. The results were initially compared with the
optical microscopy images to show the lack of accuracy in case of
handling the heterogeneous compressive specimens using optical
microscopy. It was then demonstrated that dilatometry and hardness
mapping results are consistent.
In another attempt, a mathematical formula was developed to predict the
fraction of martensite based on the bulk hardness of the investigated
specimens. The formula gave reliable results due to both steels studied.
Surface hardness mapping and optical microscopy methods were also
utilized to study the microstructural phases within the fusion zones and
the heat affected zones of steel welds. Both macro- and microstructures
were examined in this regard. It was concluded that the parallel
application of hardness measurements and optical microscopy was
essential when handling the weld zones.
Chapter 2 – FUNDAMENTALS 3
CHAPTER
TWO
FUNDAMENTALS
2.1. Introduction
Phase transformations can be well studied using quantitative and
qualitative microstructural investigations as well as consideration of
dilatation data. The first method helps to recognize present phases and
fraction of each phase within the microstructure. The latter not only aids
to specify the start and finish temperatures of the phase transformations,
but also can be used to quantify the volume fraction of the produced
phases.
Based on the above mentioned reasons, the microstructural investigations
using light optical and scanning electron microscopy techniques are
carried out. The problem which has to be taken into account is that, the
compressive deformation is heterogeneous, i.e. the effective strain and the
cooling rate along the horizontal centerline of the samples are varied.
Consequently, the distribution of the produced phases is not
homogeneous when handling the compression specimens. It means that
light optical or scanning electron microscopy investigations on few
selected points of the heterogeneously deformed surfaces are not proper
techniques for the recognition of the phase fractions inside the
experimental specimen.
In this chapter brief statements are given regarding the concept and the
application of hardness measurement in the materials investigations.
Chapter 2 – FUNDAMENTALS 4
In addition, the dilatometry technique is introduced to measure the
temperature and the amount of the possible transformations during
cooling from the austenitization temperature.
Moreover, the fundamentals of martensitic and bainitic transformations
are reviewed. It is shown that similarities between certain bainitic and
martensitic microstructures may also reduce the accuracy of the
conventional optical microscopy techniques to assess the fraction of
microstructural phases in steels.
It is then concluded that due to both mentioned problems, i.e.
microstructural heterogeneity and distinguishing between fine bainitic
and martensitic structures, hardness mapping and dilatometry can be
employed as the alternative characterization methods.
The chapter is closed by notes regarding the development of
microstructures in the weld heat-affected-zone (HAZ) and the weld fusion
zone (FZ). It is also shown that in this case, hardness mapping can be
utilized as a reasonable method to study the microstructural state.
2.2. Hardness Testing
2.2.1. Background
With regard to metallic materials, hardness has always been (and still is) a
subject of much discussion among metallurgists, and material scientists.
Attributes like wear resistance, deformation behavior, tensile strength, as
well as modulus of elasticity are connected with the term hardness.
An exact description of the method must be made if one wishes to
compare the obtained readings with each other in order to achieve a
usable hardness value.
Chapter 2 – FUNDAMENTALS 5
Hardness testing is almost nondestructive and in many cases used for
determining parameters to differentiate and describe materials. For
example, hardness values can easily provide data on the strength
properties of a material.
The term hardness is generally understood as being the resistance of a
material against the penetration of a body made of a stronger material.
Hardness is therefore not a fundamental quantity of a material but always
a response of the material to certain load or test method. A hardness value
is calculated on the basis of the response of the material to this load [10].
2.2.2. Microstructural Aspects
The concept of hardness in steels is divided into two major categories
namely hardening capacity, i.e. the maximum achievable hardness level
in the selected steel, and hardness penetration, i.e. the change in hardness
as dependent on the distance from the surface. While the first item is
dependent on the carbon content, the latter is strongly affected by the
content of alloying elements [11].
As the hardness penetration is out of the discussions of the current work,
the hardening capacity is outlined here.
Maximum hardness in steels is obtained by producing a fully martensitic
structure. This can be done by austenitizing the steel and then quenching
it. During the austenitizing treatment all of the carbides dissolve and the
ferrite transforms into austenite. Quenching this structure causes the
austenite to transform via a shear mechanism into martensite. This
transformation is so fast (martensite needles grow at close to the speed of
sound) that there is no time to the carbon to diffuse out of the martensite
Chapter 2 – FUNDAMENTALS 6
grains or to form carbide phases. The martensite, supersaturated with
carbon, is very hard and also very brittle.
Carbon, being a very effective solid solution strengthening agent,
essentially determines the hardness of martensite. Cases where a lesser
degree of hardening can be attributed to the presence of other alloying
elements, but these elements tend to also make it more difficult to obtain
a fully martensitic microstructure. So while maximum hardness in a given
steel is dependent on our ability to produce a fully martensitic
microstructure, the hardness of the martensite is largely determined by its
carbon content [12].
In some instances, especially for hot rolled steels requiring enhanced
capability to resist stretching on a blanked edge (as typically measured by
hole expansion capacity), the microstructure can also contain significant
quantities of bainite [13].
The hardness of bainite also linearly increases with carbon concentration
by approximately 190 HV per wt%. This contrasts with a change of about
950 HV per wt% in the case of carbon supersaturated martensite.
For mixed microstructures, the hardness depends on the transformation
temperature and composition. This is because the stability of the residual
austenite to martensitic transformation changes with its carbon
concentration, the limiting value of which depends on the transformation
temperature [3].
Chapter 2 – FUNDAMENTALS 7
2.3. Dilatation Experiment
Changes in physical properties can be determined through differential
thermal analysis, measurement of magnetic or electrical properties, or
through measurements using a dilatometer, which is the most prevalently
used method.
The dilatometer measures the change in length ( L∆ ), which is dependent
on temperature and time (Figure 2-1). According to SEP 1681, test
evaluation is either isothermal (T=const.) or continuous [11].
Figure 2- 1. Change in length during isothermal and continuous
transformation for the determination of TTA and TTT
diagrams, after SEP 1681 [11].
Figure 2-2 shows how to evaluate an experiment involving continuous
cooling. The beginning of a transformation range is signified by the
deviation from a straight line in the graph of change in length over time.
If the curve continues into another straight line, then the transformation in
that field is finished. If different microstructural components form
directly after one another, then a turning point in the curve signifies the
boundary between transformation ranges.
Chapter 2 – FUNDAMENTALS 8
Figure 2- 2. Example of how to evaluate experiments using continuous
cooling, after SEP 1680 [11].
Finally, the temperature is graphed over the logarithmic time axis in a
nonequilibrium diagram. This makes it possible to investigate the
transformation behavior of the steel from short to long periods of time.
The accuracy of nonequilibrium diagrams varies for temperature by +10K
and for time by +10%. Variations due to different chemical compositions
and initial states can further increase the range of distribution. All
diagrams are valid, in the strictest sense, only for the chemical
compositions observed and the conditions given for each specific case
(austenitizing temperature and austenitizing time).
Chapter 2 – FUNDAMENTALS 9
In every nonequilibrium diagram, a multitude of transformation points are
given along with the specifications for the material and experimental
procedures, in accordance with the Stahl-Eisen-Prüfblatt 1680
(SEP 1680).
2.4. Martensitic Transformation and Microstructure
Martensitic transformation in steels occurs when austenite is cooled
rapidly (quenched) below Ms temperature so that diffusional
transformation is suppressed. Even pure iron may transform
martensitically if the cooling rate is sufficiently high.
Crystallographic characteristics of martensite transformation involve
shape change, surface relief, internal defects and existence of habit
planes. Other characteristics such as athermal transformation,
stabilization and autocatalytic effect also make the mechanism of
transformation difficult to understand.
The diffusionless transformation accompanies lattice distortion and
supersaturation of carbon. Inner microstructure of martensite plates
consists of either dislocations or twins and each has differently shaped
surface, consequently named as lath martensites and plate martensites.
Lath martensite has waved surface whereas plate martensite a flat one as
schematically depicted in Figure 2-3 [14].
Chapter 2 – FUNDAMENTALS 10
Figure 2- 3. Schematics of two typical kinds of martensite
microstructures observed by transmission electron
microscopy. Plate martensite (left) has twins inside the plates
of flat boundaries, whereas lath martensite (right) contains
dislocations inside the lath of wavy boundaries [14].
The changes in volume and in configuration associated with martensitic
transformation cause strong lattice distortions, which naturally counteract
the transformation. At high carbon content values (>1.4%) and low
transformation temperatures, these distortions can be reduced only to a
small degree through gliding and recovery mechanisms, so that the
transformation from austenite to martensite does not always fully
proceed. Metallographically, martensite forms in plates as seen in
Figure 2-4 [11].
Chapter 2 – FUNDAMENTALS 11
Figure 2- 4. Different morphologies of martensite in Fe-C alloys; Left:
Plate martensite, and Right: Lath martensite [11].
At lower carbon content values (<<0.5%) and higher transformation
temperatures, the elastic lattice stresses can be removed more easily
through dislocation movements. Martensite then no longer consists of
single crystal plates but rather of lath-shaped crystals that are bunched
into blocks and oriented in various directions [11].
2.5. Bainitic Transformation and Microstructure
In contrast to the diffusional characteristics, the surface relief during
bainitic transformation is very similar to that of martensitic
transformation. Bainite also forms as isolated plate unlike lamellae of
ferrite and cementite in pearlite. The speed of the formation of plates,
however, is much lower than that of martensitic transformation.
Chapter 2 – FUNDAMENTALS 12
The morphological characteristics of bainite are divided into two kinds,
upper bainite and lower bainite according to the microstructural
characteristics of carbides. Upper bainite, on {111} habit planes of
austenite, is obtained at a temperature range of 350-550°C and the major
microstructural constituent is either needle or laths of ferrite. Carbides,
mainly Fe3C, grow at the interface of the ferrite plates. The
transformation mechanism of the upper bainite is similar to that of
Widmanstatten side plates. Lower bainite, which forms at the lower
temperature than upper bainite and on irrational habit planes of austenite,
has a similar microstructure to that of tempered martensite. Due to the
lower diffusion rate of carbon, carbides, either Fe3C or epsilon carbides
grow mostly inside the ferrite plates with a characteristic angle with
respect to the ferrite plate and with characteristic habit planes. Schematic
features of both types of bainite plates are shown in Figure 2-5 [14].
Figure 2- 5. Two carbide shapes in bainite: left: upper bainite, right:
lower bainite. Note that the carbides in the upper bainite form
at the plate boundaries and are coarser than those in the lower
bainite. Also, the carbides in the lower bainite are aligned
along a certain crystallographic direction within the plate
[14].
Chapter 2 – FUNDAMENTALS 13
Lower bainite structure that is usually obtained in the temperature range
of 250-350°C, shows higher tensile strength than that of upper bainite.
This is a consequence of the higher dislocation density in the lower
bainite plates. Also, the lower bainite, due to the fine carbide structure,
has a higher toughness than upper bainite. Therefore, the ductile
transition temperature for the lower bainite is lower than that of upper
bainite [14].
It is also possible to distinguish the bainitic structures according to the
various metallographic appearances as:
• fine acicular,
• coarse acicular,
• granular.
Figure 2-6 gives examples of bainitic microstructures based on this
classification.
Chapter 2 – FUNDAMENTALS 14
Figure 2- 6. Optical microscope photos of bainitic microstructures: a) fine
acicular bainite, b) coarse acicular bainite, and c) granular
bainite [11].
Chapter 2 – FUNDAMENTALS 15
2.6. Characterization Difficulties
The macroscopic mechanical properties of steel are highly dependent
upon the microstructure, morphology, and distribution of each phase
present [13].
As stated before, there are certain situations where the conventional
optical microscopy techniques and/or electron microscopy are not proper
or sufficiently accurate methods for the evaluation of steels'
microstructures [15].
The examples are the situations where an inhomogeneous distribution of
strain and cooling rate along the horizontal centerline of the experimental
specimen are varied, thereby influencing the microstructural distribution
of phases; also cases where a fine bainitic-martensitic phase mixture
makes it impossible to distinguish between the present phases in a precise
manner (see Figures 2-4 and 2-6).
However, the variations of hardness values when passing through
bainitic-martensitic phase boundaries (see section 2.2.2), and the position
of sharp dilatation peaks during the dilatometry experiments
(see section 2.3) make it possible to indirectly detect and quantify the
present phases, getting precise information from the whole experimental
body.
Chapter 2 – FUNDAMENTALS 16
2.7. Microstructural Aspects of Weld Zones
Austenite formation and decomposition in both the weld heat-affected
zone (HAZ) and the weld fusion zone (FZ) is a well-studied phenomenon
in the welding metallurgy of steels [16].
In the HAZ, the room temperature microstructure transforms to 100%
austenite on heating above the Ac3 temperature. This transformation may
or may not reach completion depending upon the peak temperature
(between Ac1 and Ac3) attained in the HAZ and the time over which the
material experiences this temperature. Interestingly, the temperature near
the fusion line may be high enough to form the high temperature
δ –ferrite phase.
The austenite and δ –ferrite phases that form in the FZ and HAZ during
heating will transform to several different low-temperature phases during
cooling. For example, the δ –ferrite in the HAZ may transform back to
austenite, which may then transform to different morphologies of
α –ferrite, namely, allotriomorphic ferrite, Widmanstatten ferrite, and
bainite. At rapid cooling rates, the austenite will transform to martensite.
In the fusion zone during cooling, the liquid solidifies as δ –ferrite, which
then transforms to austenite. With continued cooling, the austenite
transforms to the different α –ferrite morphologies observed in the HAZ
microstructure.
Due to steep temperature gradients and dynamic cooling conditions of
welds, the resulting material contains significant microstructural
gradients. These microstructural gradients affect the strengths, ductility,
toughness, fatigue and creep rupture properties of steel welds. The
knowledge of these gradients is well developed and is routinely
considered in the design of weldments. However, there is a need to
Chapter 2 – FUNDAMENTALS 17
develop predictive models to describe these gradients as a function of
steel composition and weld thermal cycles [17].
The microstructure obtained as the weld cools from the liquid phase to
ambient temperature is called the as-deposited or primary microstructure.
As stated before, it may consists of allotriomorphic ferrite α ,
Widmanstatten ferrite Wα , acicular ferrite aα , and the so-called
microphases, which might include small amounts of martensite, retained
austenite or degenerate pearlite (Figure 2-7).
Figure 2- 7. An illustration of the essential constituents of the primary
microstructure of a steel weld deposit. The diagram is
inaccurate in one respect, that inclusions cannot be expected
to be visible in all of the acicular ferrite plates on a planar
section of the microstructure. This is because the inclusion
size is much smaller than that of an acicular ferrite plates, so
that the chances of sectioning an inclusion and plate together
are very small indeed [18].
Chapter 2 – FUNDAMENTALS 18
Bainite is also found in some weld deposits, particularly of the type used
in the power generation industry. Allotriomorphic ferrite is sometimes
called polygonal ferrite or proeutectoid ferrite, but polygonal simply
means many sided (like all ferrite morphologies) and Widmanstatten
ferrite can also be proeutectoid. Widmanstatten ferrite (Figure 2-8) is
sometimes included under the general description 'ferrite with aligned
MAC', the abbreviation referring to martensite, austenite and carbide.
However, bainite plates can also form in a similar shape, although their
thermodynamic and kinetic characteristics are quite different.
Among others, acicular ferrite is a phase most commonly observed as
austenite transforms during the cooling of low-alloy steel weld deposits.
It is of considerable commercial importance because it provides a
relatively tough and strong microstructure. It forms in a temperature
range where reconstructive transformations become relatively sluggish
and give way to displacive reactions such as Widmanstatten ferrite,
bainite and martensite.
The term 'acicular' means shaped and pointed like a needle, but it is
generally recognized that acicular ferrite has in three-dimensions the
morphology of thin, lenticular plates (Figure 2-9).
Chapter 2 – FUNDAMENTALS 19
Figure 2- 8. (a) Schematic diagrams illustrating the development of
microstructures in weld deposits. The hexagons represent
cross-sections of columnar austenite grains whose boundaries
first become decorated with uniform, polycrystalline layers
of allotriomorphic ferrite, followed by the formation of
Widmanstatten ferrite. Depending on the relative
transformation rates Widmanstatten ferrite and acicular
ferrite, the former can grow entirely across the austenite
grains or become stifled by the intragranularly nucleated
plates of acicular ferrite. This diagram takes no account of
the influence of alloying additions on the austenite grain
structure. (b) Actual optical micrographs illustrating the
unhindered growth of Widmanstatten ferrite in a weld
deposit. (c) Optical micrograph showing how the growth of
Widmanstatten ferrite is stifled by the formation acicular
ferrite [18].
Chapter 2 – FUNDAMENTALS 20
Figure 2- 9. Replica transmission electron micrograph of acicular ferrite
plates in a steel weld deposit [18].
The experimental data to date indicate that acicular ferrite is essentially
identical to bainite. Its detailed morphology differs from that of
conventional bainite because the former nucleates intragranularly at
inclusions within large austenite grains whereas in wrought steels which
are relatively free of nonmetallic inclusions, bainite nucleates initially at
austenite-austenite grain surfaces and continues growth by the repeated
formation of subunits, to generate the classical sheaf morphology.
Acicular ferrite does not normally grow in sheaves because the
development of sheaves is stifled by hard impingement between plates
nucleated independently at adjacent sites. Indeed, conventional bainite or
acicular ferrite can be obtained under identical isothermal transformation
conditions in the same (inclusion rich) steel. In the former case, the
austenite grain size has to be small in order that nucleation from grain
surfaces dominates and subsequent growth then swaps the interiors of the
austenite grains. For a larger austenite grain size, intragranular nucleation
on inclusions dominates, so that acicular ferrite is obtained (Figure 2-10).
Chapter 2 – FUNDAMENTALS 21
Figure 2- 10. An illustration of the effect of austenite grain size in
determining whether the microstructure is predominantly
acicular ferrite or bainite. A small grain sized sample has a
relatively large number density of grain boundary nucleation
sites and hence bainite dominates, whereas a large number
density of intragranular nucleation sites leads to a
microstructure consisting mainly of acicular ferrite [18].
Hence, the reason why acicular ferrite is not usually obtained in wrought
steels is because they are relatively free of inclusions and because most
commercial heat treatments aim at a small austenite grain size. It is ironic
that bainite when it was first discovered was referred to as acicular ferrite,
and that the terms acicular ferrite and bainite were often used
interchangeably for many year after 1930 [18].
Chapter 3 – EXPERIMENTAL PROCEDURE 22
CHAPTER
THREE
EXPERIMENTAL PROCEDURE
3.1. Materials Characterization
3.1.1. General Considerations
Five different steels in shapes of compression specimens and welded
sheets were employed. The compression specimens were made of
22MnB5 and MSW1200 steels which were received in form of plates,
while the welded sheets were composed of S355, EH36 and RQT701
steels. The S355 sheet had a thickness of 12mm, while EH36 and
RQT701 sheets had the thickness of 20mm.
3.1.2. Chemical Composition
Chemical compositions of the investigated steels are given in tables 3-1
and 3-2 in case of compression and welded specimens.
Table 3-1. Chemical composition of the compression specimens (wt-%).
C Si Mn Cr Ni Al Ti B
22MnB5 0.23 0.22 1.18 0.16 0.12 0.03 0.040 0.002
MSW1200 0.14 0.12 1.71 0.55 0.06 0.02 0.002 -
Chapter 3 – EXPERIMENTAL PROCEDURE 23
Table 3-2. Chemical composition of the welded sheets (wt-%).
S355-12 EH36-20 RQT701-20 C 0.054 0.121 0.144 Si 0.018 0.389 0.436
Mn 1.085 1.290 1.360 P 0.011 0.012 0.012 S <0.001 0.002 <0.001
Cr 0.015 0.015 0.016 Mo <0.005 <0.005 <0.005 Ni 0.019 0.024 0.021 Al 0.021 0.037 0.036 Nb 0.023 0.032 0.031 Cu 0.010 0.016 0.014 Ti 0.009 0.001 0.026 V 0.003 0.004 0.068 B <0.0005 <0.0005 0.002
3.1.3. Microstructure
The 22MnB5 steel contains ferrite and pearlite phases (together with
carbide) in as-received condition. Figure 3-1 shows the microstructure of
the 22MnB5 steel in the rolling direction.
The image analysis data shows that the microstructure contains around
78% ferrite besides 22%, combination of pearlite and carbide. Ferrite
grain size was measured to be comparable with 11 ASTM grain size
standard.
Chapter 3 – EXPERIMENTAL PROCEDURE 24
Figure 3-1. Microstructure of the as-received 22MnB5 sheets in the
rolling direction a) 500X, and b) 1000X.
The microstructures of the base metal of the investigated welded sheets
are given in Figure 3-2. As seen in this illustration, in case of S355 and
EH36 specimens, the microstructures are predominantly ferritic-pearlitic,
while RQT701 has a ferritic-bainitic microstructure.
Figure 3-2. Microstructure of the base metals: a) S355, b) EH36, and
c) RQT701.
Chapter 3 – EXPERIMENTAL PROCEDURE 25
3.2. Isothermal and Non-isothermal Compression Tests
The isothermal and non-isothermal compressive deformations were
exerted on the 22MnB5 and MSW1200 compression specimens.
The whole deformations and dilatometry experiments were carried out
using a Baehr 805 dilatometer (Figure 3-3) with a deformation unit
designed to perform a simple uniaxial deformation. In all cases, the
geometry of the sample used was cylindrical with a length of 10mm and a
5mm diameter (Figure 3-4). In all of the tests, the temperature was
measured using a surface mounted Pt/Pt-Rh10% thermocouple located at
the mid-length relative to the specimen. Non-isothermal deformations
were established through several simultaneous forming and quenching
tests at the temperatures between 600°C–850°C and by the strain rates of
0.05s-1–1.0s-1. Regarding the duration of the experiments, the samples
were deformed up to the strains of 0.1-0.5 in a single step. All samples
were austenitized at 900°C for five minutes and quenched to the
compression temperatures at a cooling rate of 50K/s.
In case of isothermal experiments, deformations were carried out at the
temperature range of 600°C – 850°C by the strain rates of
0.1 s-1 – 10.0 s-1. Due to the technical limitations in the Baehr deformation
dilatometer, higher strain rates could not be applied in the simultaneous
forming and quenching tests.
Chapter 3 – EXPERIMENTAL PROCEDURE 26
Figure 3-3. Baehr DIL 805 deformation dilatometer, and the sample
set up.
Figure 3-4. Schematic illustration of the cylindrical specimen used
during the dilatation experiments.
3.3. Execution of Welding
The aforementioned steel sheets (see 3.1), were joint together using arc
welding and laser welding methods. Different parts of a welded structure
Chapter 3 – EXPERIMENTAL PROCEDURE 27
are depicted in Figure 3-5. The caps and roots of fusion zones (FZ) and
heat-affected-zones (HAZ) are plotted as well.
In the current work, the joining sheets were put together before welding
using two methods: either with a simple touching of the two 'V' notched
neighboring sheets, or making a gap between them for further
improvement of the final attachment.
Figure 3-5. Schematic illustration of the welded structure.
3.4. Hardness Mapping and Metallography
In the present study, an innovative surface hardness mapping technique
was developed by the Department of Ferrous Metallurgy (IEHK), RWTH
Aachen University. In this technique, the surface of the sample is scanned
using an indenter which exerts a 0.8g force on the surface of the sample
Chapter 3 – EXPERIMENTAL PROCEDURE 28
and records hardness of the points in Vickers or Rockwell hardness
scales. Figure 3-6 shows the setup of the mentioned surface hardness
mapping device and relevant specimen insertion.
Figure 3-6. Surface hardness mapping relevant instruments at IEHK,
RWTH Aachen University.
Surface hardness measurements were performed on the previously
deformed and welded samples. To do this, in case of the deformed
samples, they were cut lengthwise. The samples were mounted after
cutting. The Vickers hardness of the whole deformed surfaces was
measured in 0.3mm steps using an exerted force of 0.8g. In case of
welded sheets 0.5mm steps were used. Afterwards, the surface hardness
maps of the samples were plotted.
Chapter 3 – EXPERIMENTAL PROCEDURE 29
Surface hardness maps of the deformed, cut and mounted samples were
started one millimeter out of the sample, i.e., from the polymeric
mounting material. The hardness of mounting material was recorded as
999 and was ignored. Due to the boundary conditions between the
mounting material and the sample, hardness values of the sample's edge
must not be considered. Accordingly, the quantitative measurements
using surface hardness mapping data were performed by means of the
reliable hardness data which were taken from inside the sample. To
evaluate the consistency of the hardness mapping results regarding the
determination of the present phases, optical microscopy investigation
were also executed on the whole deformed specimens.
The welded sheets were also scanned using hardness mapping method
throughout the fusion zones and heat-affected-zones. Four sets of optical
microscopy images were taken from caps and roots of FZ and HAZ in
every single specimen. In addition, the macrostructures of the welds were
revealed using lower magnification microscopy.
Chapter 4 – RESULTS AND DISCUSSION 30
CHAPTER
FOUR
RESULTS AND DISCUSSION
4.1. Revealing Heterogeneous Microstructures
As stated in the previous sections, the compressive deformation results in
an inhomogeneous distribution of microstructural components. To clarify
this issue, and by consideration of various hardness levels due to different
phases in steels, a complete surface hardness map a compressed 22MnB5
specimen deformed up to the strain of 0.1 is plotted in Figure 4-1.
Figure 4-1. The deformed 22MnB5 sample with maxε =0.1, ε& =0.05s-1,
TiD =750°C; the complete surface hardness map with the
scale of 25HV0.8 representing heterogeneity.
Chapter 4 – RESULTS AND DISCUSSION 31
In the present study, the hardness values of more than 400HV0.8g were
assumed as martensite. The hardness between 200 and 400HV0.8g were
assumed as bainite and below 200HV0.8g as ferrite. These assumptions
were calculated using the CCT diagram, microscopic investigations and
some reports [1-2,19]. Pearlite was not considered because no pearlite
was seen in the final microstructure using LOM and SEM images.
For further observation of the microstructural state, the whole surface of
the investigated specimen was scanned using optical microscopy. The
scanning sequence followed a pattern as depicted in Figure 4-2. Red spots
demonstrate the approximate positions where an optical image was taken.
Figure 4-2. The pattern corresponding to scanning the deformed surface
using optical microscopy.
Figure 4-3 shows the relevant microstructures of points 'X' and 'Y'
illustrated in Figure 4-1 using this scanning method.