materials

Article
Effect of Sandblasting on Static and Fatigue Strength of Flash
Butt Welded 75Cr4 Bandsaw Blades
Andrzej Kubit 1 , Łukasz Lenart 2 , Tomasz Trzepieciński 1, * , Andrzej Krzysiak 3 and Wojciech Łabuński 4

1 Department of Manufacturing and Production Engineering, Faculty of Mechanical Engineering

and Aeronautics, Rzeszow University of Technology, Al. Powst. Warszawy 8,
35-959 Rzeszów, Poland; akubit@prz.edu.pl
2 Walter, Pustyny, ul. Ksi˛eża 83, 38-422 Krościenko Wyżne, Poland; lukas.lenart@gmail.com
3 Department of Aerospace Engineering, Faculty of Mechanical Engineering and Aeronautics,
Rzeszow University of Technology, Al. Powst. Warszawy 8, 35-959 Rzeszów, Poland; a.krzysiak@prz.edu.pl
4 Department of Applied Mechanics and Robotics, Faculty of Mechanical Engineering and Aeronautics,
Rzeszow University of Technology, Al. Powst. Warszawy 8, 35-959 Rzeszów, Poland; w.labunski@prz.edu.pl
* Correspondence: tomtrz@prz.edu.pl

Abstract: The aim of the research presented in this article is analysis of the effect of the surface
treatment method on the static and fatigue strength of flash butt welded bandsaw blades. A 1-mm-
thick 75Cr1 cold-work tool steel sheet used for bandsaw blades was used as the test material.
Fractographic studies of the fatigue fractures and fractures formed in static tests were also carried
out. The static strength tests showed sandblasting the weld surface had no significant effect on
the load capacity of the joint. However, the sandblasted specimens showed a higher repeatability
of the load capacity (lower standard deviation). In the case of both analyzed sample variants



of specimens, sandblasted and non-sandblasted, the number of cycles at which the sample was


damaged decreases with the percentage increase of the stress amplitude. When loading the samples
Citation: Kubit, A.; Lenart, Ł.; with a stress amplitude value in the range between 400 and 690 MPa, sandblasting of the weld
Trzepieciński, T.; Krzysiak, A.; surface increased the average value of destructive cycles by about 10–86% (depending on the stress
Łabuński, W. Effect of Sandblasting
amplitude) compared to non-sandblasted joints. The sandblasting process introduces compressive
on Static and Fatigue Strength of
stresses in the surface layer of the welds, therefore the variable tensile load acting on the sample
Flash Butt Welded 75Cr4 Bandsaw
requires a greater number of cycles before the fatigue cracks initiate and propagate. In the case of
Blades. Materials 2021, 14, 6831.
all specimens, a ductile fracture was observed. It was also found that, regardless of the variable
https://doi.org/10.3390/ma14226831
stress amplitude, sandblasting has a positive effect on reducing the standard deviation of fatigue
Academic Editor: Andrea Spagnoli test results.

Received: 10 October 2021 Keywords: flash welding; sandblasting; static strength; surface engineering; tool steel
Accepted: 9 November 2021
Published: 12 November 2021

Publisher’s Note: MDPI stays neutral 1. Introduction

with regard to jurisdictional claims in Welding technology is widely used in various industries because it is much cheaper
published maps and institutional affil- than other joining technologies. Welded joints have much wider applications; it is still one
iations.
of the basic methods of joining materials in the automotive [1] and machine industries [2].
In the engineering industry, spot welded joints are of great use because machine covers are
often large and made of thin sheets [3]. In the production of shields from thin sheets, the
use of resistance welding is much cheaper than, for example, the use of fusion welding [4].
Copyright: © 2021 by the authors. It is a similar situation in the automotive industry, where the use of welded joints is more
Licensee MDPI, Basel, Switzerland. economic, and often used for complex shape components.
This article is an open access article Bandsaw machines are the basic equipment of machining plants, allowing for the
distributed under the terms and precise cutting of a wide range of materials, including wood [5], stone [6], non-ferrous
conditions of the Creative Commons
metals, stainless steels, cast iron, heat-resistant alloys and structural steels [7]. Taking into
Attribution (CC BY) license (https://
account the functionality of cutting machines, it can be stated that the following models are
creativecommons.org/licenses/by/
available on the market: manual, gravity, semi-automatic and automatic controlled devices.
4.0/).

Materials 2021, 14, 6831. https://doi.org/10.3390/ma14226831 https://www.mdpi.com/journal/materials

Materials 2021, 14, 6831 2 of 18

The bandsaw blade is usually guided on the basis of ball bearings or hydraulically pressed
sintered carbide guides. An inverter provides the optimal bandsaw blade speed in relation
to the processed material for a wide range of values, most often 10–120 m/min.
So far in world industry, the known and used method of permanently joining bandsaw
blades is the process of flash butt welding (FBW). FBW represents an attractive welding
process due to its high productivity and wide applicability [8]. The main advantage of the
using FBW for bandsaw blades is the possibility of making a permanent joint of very high
quality whose mechanical properties are not less than those of the base material [9]. Apart
from the proper welding of the bandsaw blade, its service life is largely determined by
the proper conduction of the running-in and cooling processes [10]. Bandsaw blade steel
must have a good balance between strength, toughness and material elasticity in the weld
joint [9]. Workpieces sensitive to water can be lubricated with an oil spray which is applied
directly to the bandsaw blades. During the bandsaw blade’s running-in period, reducing
the optimal working speed of the belt to 70% and the feed rate to 50% should be taken into
account. Bandsaw blades are produced in a thickness range of about 0.65–1.3 mm and a
width of 3–80 mm, with various tooth profiles, as well as having a constant or variable
pitch, various blade types (uniform, bimetal and with sintered carbide blades) and various
protective coatings [11].
Analysis of the FBW of bandsaw blades is the subject of a number of research works.
Kalincová [12] analyzed the influence of different annealing temperatures on the struc-
tural and mechanical properties of C75 steel welded bandsaw blades. The results of
microstructure evaluation confirmed the need for annealing after welding bandsaw blades.
Gochev [13] investigated the processes of tempering bandsaw blades after welding ac-
cording to both weld methods—Metal Inert Gas (MIG) and Metal Active Gas (MAG).
It was found that the homogeneity of a joint and the diffusion between the basic metal
(the bandsaw blade) and the secondary material (the welding wire) is improved using
the MIG and MAG methods. Bodea et al. [9] flash butt welded 51CrV4 steel strips used
for manufacturing bandsaw blades and concluded that minor changes in the welding
parameters or in the post-welding treatment can cause significant changes in the bandsaw
blade’s durability and performance. Ichiyama and Kodama [14] studied the effects of
welding conditions and base metal chemical compositions on the flash butt weld defects of
high strength steels. They concluded that although flash welding is an efficient welding
method, it has limited applications because of the difficulties involved in ensuring the
required weld quality. Krishnaraj et al. [15] studied the quality of flash butt welded joints
in mild steel. The results indicated that increases in the preflashing energy and preheating
energy improve the weld quality significantly. There are many methods of hardening to
increase the durability of the bandsaw blades: electro-contact hardening [16], rigging the
tool teeth with carbide plates and tempering the teeth in a high-frequency current field [17]
and electro spark processing [18].
In addition to flash butt welding, endless bandsaw blades can also be joined by
brazing, gas welding, as well as MIG and TIG welding [13]. In the MIG and TIG methods,
a very high temperature is produced, which weakens the microstructure of the material,
thus reducing the fatigue life of the joint. The TIG welding of the endless saw blades is
economically efficient in the case of small series production, as well as for repairing broken
blades during their exploitation [19]. The ends of the bandsaw blades are joined by means
of an overlay brazing process, which requires high level brazing skills, but still produces
a weak joint on account of the foreign material introduced into it. The electric resistance
butt welding is the modern method where the joint strength is 25% higher than that of
the base metal. This process is also characterized by high speeds and is automatic, thus
eliminating human error and producing a perfectly strong joint [20]. The resistance butt
welding machine does not require additional flux or solder. After setting the bandsaw
blades and achieving a correct clamping by means of the special, quick-acting clamps with
which the machine is equipped, the welding process takes place automatically with the
assumed welding parameters. Compared to the brazing and MIG processes, the flash
Materials 2021, 14, 6831 3 of 18

butt welding technology has the advantages of being very low-priced and of lacking any
foreign material introduced into the weld joint. Moreover, the preparation of the weld
surface is not required. Metals with different melting temperatures can be welded using
this flash welding process. Flash welding is mostly used for welding steel but can also be
used for aluminum alloys, magnesium alloys, stainless steels, low-alloy steels, tool steels,
heat resisting alloys, Ni-based alloys, Cu-based alloys and Ti-based alloys. Compared to
other joining methods, flash butt welding is suitable for mass production. A solid phase,
forge weld is made, and any molten metal and contaminants formed at the interface during
heating are squeezed out into the upset. Thus, solidification cracking and porosity are not
normally an issue [21].
Bandsaw blades work under specific load and stress conditions [22]. The blade of
the bandsaw machine is subjected to many dynamic, cyclically repetitive forces resulting
from the resistance of the cut material. Additionally, it usually works in very variable
temperature ranges because of the strong heating of the blade material due to friction [23].
These extremely severe conditions for the bandsaw blade material can cause the blades to
break in the places where they are joined. Therefore, the durability of the flash butt weld
directly affects the efficiency of production processes which use bandsaws [24].
The fatigue strength of the bandsaw blades is the basic parameter that determines the
efficiency and failure-free unfolding of the cutting process. Currently, scientific investiga-
tions are focused on ensuring the adequate strength of the joint by optimizing high-energy
methods such as MIG and TIG. Meanwhile, the flash butt welding process is still the most
economic method of joining bandsaw blades in mass production. The introduction of
a fast, non-energy-consuming and low-priced method that would increase the strength
of welds and would prevent the formation of by-products is desirable in the machine
industry. The methods of mechanically increasing the strength of the material in cold
forming conditions fit perfectly into the above-mentioned quality indicators. Due to the
obvious need to increase the fatigue life of welded joints using low-cost surface treatment
methods in the joint area, the heat-affected zone and the base material, it is important to
understand the mechanisms influencing the increase in fatigue strength. As a consequence
of the surface treatment methods leading to changes in the value of the residual stress,
which in turn affects the initiation and development of fatigue cracks, it is important to
conduct a thorough analysis resulting in a specific determination of the optimal values
and characteristics of the applied stresses in the subsurface area. This, in turn, should be
precisely correlated with the parameters of the surface treatment used. Considering the
above statements, it is justified to undertake research works aimed at methods that improve
the durability and fatigue strength of joints, and at the same time accurately characterize
the parameters leading to such improvement.
The purpose of this work is to analyze the effect of sandblasting the flash butt weld
surface on the static and fatigue strength properties of the joints made of 75Cr1 steel,
which is commonly used for bandsaw blades. The thickness of the strips does not exceed
1 mm [16], therefore FBW is currently the dominant technology for producing bandsaw
blades. Moreover, which is equally important in the production of bandsaws, FBW is cheap,
and in addition, the joints are characterized by adequate strength and are very easy to make.
This article considers the possibility of increasing the strength of 1-mm-thick metal sheet
joints by sandblasting, which, according to the best of the authors’ knowledge, has not been
studied so far. The sandblasting process, by elastic-plastic deformation, creates compressive
stress in the surface layer of the flash butt weld. It also leads to strain hardening in the
outer layer of the weld material. The samples were subjected to fatigue tests with different
levels of stress amplitude.
Materials 2021, 14, 6831 4 of 18

Materials 2021, 14, 6831 4 of 18

2. Materials and Methods

2.1. Material
2. Materials
The testand Methods
material used was 1-mm-thick 75Cr1 cold-work tool steel. This steel is com-
2.1. Material
monly used in the wood industry for the production of bandsaws, circular saws and
The testwhich
equipment material used high
requires was 1-mm-thick 75Cr1 cold-work
abrasion resistance. tool steel.
The chemical This steel
composition is
of the
commonly used in the wood industry for the production of bandsaws,
75Cr1 steel, according to the ISO 4957:2018 standard [25], is shown in Table 1.circular saws and
equipment which requires high abrasion resistance. The chemical composition of the 75Cr1
steel,
Table according
1. Chemicaltocomposition
the ISO 4957:2018 standard
of the 75Cr1 [25], is shown in Table 1.
steel (%wt.).

C Si Mn P
Table 1. Chemical composition of the 75Cr1 steel (%wt.).
S Cr Fe
0.70–0.80 0.25–0.50 0.60–0.80 max. 0.03 max. 0.03 0.30–0.40 remainder
C Si Mn P S Cr Fe
2.2. Flash Butt Welded
0.70–0.80 Specimens
0.25–0.50 0.60–0.80 max. 0.03 max. 0.03 0.30–0.40 remainder
The samples for the static and fatigue testing of the welded joints, in the form of dog-
boneFlash
2.2. specimens withSpecimens
Butt Welded dimensions of 174 mm × 30 mm (Figure 1), were cut using laser
processing on a STX 2500 machine
The samples for the static and (Yamazaki Mazak
fatigue testing Corporation,
of the Takeda,
welded joints, Japan).
in the formThe
of
laser processing parameters were:
dog-bone specimens with dimensions of 174 mm × 30 mm (Figure 1), were cut using laser
• laser power
processing P = 1700
on a STX 2500 kW,
machine (Yamazaki Mazak Corporation, Takeda, Japan). The
•
laser frequency f = 500 Hz, were:
processing parameters
•• laser
cutting speed
power P= v 1700
= 2700 mm/s,
kW,
•• frequency
gas pressuref = 0.4
500bar.
Hz,
• cutting
The laserspeed v = 2700
cutting mm/s, were consistent with the conditions for cutting band-
parameters
•sawsgas
andpressure
circular0.4
sawsbar.made of 75Cr1 sheets.

Figure 1.
Figure 1. Dimensions (in mm) of the
the flash
flash butt
butt welded
welded specimen
specimen for
for strength
strength testing.
testing.

The
The laser
FBWcutting
machine parameters
for bandsawwereblades
consistent
usedwith the experiments
in the conditions forwascutting bandsaws
a Viscat VC 4
and circular
(Fulgor s.r.l.,saws made
Torino, of 75Cr1
Italy). sheets.of the bands to be joined must be clean in the area
The edges
of theThe
butt.FBWAftermachine
carefullyforsetting
bandsaw bladesofused
the edges in thetoexperiments
the bands waswelding
be joined in the a Viscatdevice
VC 4
(Fulgor s.r.l.,with
(Figure 2a), Torino, Italy).ensuring
the force The edges of theposition
a stable bands toofbe thejoined
bandsmust be clean
during in the area
the welding pro-
of the butt. After carefully setting the edges of the bands to be joined
cess, the Cu-Cr alloy electrode is pressed with appropriate force against the upper surface in the welding
device (Figure 2a),
of the bandsaw with
blade. thethe
After force ensuring
welding a stable
pressure position
is exerted, of the bands
a current during the
of appropriate in-
welding
tensity is passed through the electrodes and the joint (Figure 2b). Under the influencethe
process, the Cu-Cr alloy electrode is pressed with appropriate force against of
upper
currentsurface
flow, theof the bandsaw
resistive blade.
heating After the
transforms welding
the pressure
joint area is exerted,
into a highly a current
plasticized of
state,
appropriate
and the pressure intensity
forceisupsets
passedthethrough
welding thearea
electrodes
(Figure and the joint (Figure
2c), ensuring 2b). Under
a high-quality joint
the influence of current flow, the resistive heating
with mechanical properties not lower than the base material. transforms the joint area into a highly
plasticized state, and the pressure force upsets the welding area (Figure 2c), ensuring a
high-quality joint with mechanical properties not lower than the base material.
The parameters of the FBW process (power absorbed 4.5 kW, welding time 5 s) corre-
sponded to the parameters used in the production of bandsaw blades by the manufacturer
Walter (Krościenko Wyżne, Poland). After welding, the faces of the welds were grinded.
Materials 2021, 14, 6831 5 of 18
Materials 2021, 14, 6831 5 of 18

Figure
Figure 2. Flash butt
2. Flash butt welding
welding of
of aa bandsaw
bandsaw blade:
blade: (a)
(a) fixing
fixing the
the sheets
sheets in
in the
the device,
device, (b)
(b) switching
switching on
on
the current flow with pressure, (c) ending the welding process.
the current flow with pressure, (c) ending the welding process.
2.3. Fatigue Strength Testing
The parameters of the FBW process (power absorbed 4.5 kW, welding time 5 s) cor-
Fatiguetostrength
responded tests of flash
the parameters usedbutt welded
in the specimens
production were carried
of bandsaw bladesoutbyon anmanufac-
the HT-9711
Dynamic Testing Machine (Hung Ta Instrument Co., Taichung
turer Walter (Krościenko Wyżne, Poland). After welding, the faces of the welds City, Taiwan). The fatigue
were
tests were
grinded. carried out at room temperature with a limited number of cycles equal to 2 × 106
and a frequency of 50 Hz. The coefficient of the stress cycle of R = 0.1 was used which
corresponds to a tension-tension
2.3. Fatigue Strength Testing cycle in which σmin = 0.1σmax [26]. In order to compare
the fatigue strength, sandblasted and non-sandblasted samples were tested. All variants of
Fatigue strength
the specimens tests for
were tested of flash butt welded
five levels specimens
of dynamic were
loading. At carried out on
every level, theantests
HT-9711
were
Dynamic Testing Machine (Hung Ta Instrument Co., Taichung City, Taiwan).
repeated four times. The lowest level of dynamic load was the value at which the specimen The fatigue
testsnot
did were
failcarried out loaded
after being at roomby temperature withFive
2 × 106 cycles. a limited number
specimens wereoftested
cyclesfor
equal
eachto 2×
level
10 6 and a frequency of 50 Hz. The coefficient of the stress cycle of R = 0.1 was used which
of amplitude.
corresponds to a tension-tension cycle in which σmin = 0.1σmax [26]. In order to compare the
fatigue
2.4. strength,Procedure
Sandblasting sandblasted and non-sandblasted samples were tested. All variants of
the specimens were
Sandblasting was tested for five
carried outlevels of dynamic
on a KCW loading. At
1000 machine every level,
(New-Tech, the tests were
Dobrzykowice,
repeated four times. The lowest level of dynamic load was the
Poland). Processing parameters: sandblasting pressure p = 2 atm, abrasive—GH50 value at which the speci-
cast
men did not fail after being loaded by 2 × 10 6 cycles. Five specimens were tested for each
steel shot, nominal fraction d = 0.3 mm, abrasive hardness—approx. 60–68 HRC. The
level of amplitude.
sandblasting treatment was aimed not only at cleaning the surface, but also (in the places
where the weld was made) introducing compressive stresses in the weld subsurface to
2.4. Sandblasting
strengthen Procedure
the material.
Sandblasting was carried out on a KCW 1000 machine (New-Tech, Dobrzykowice,
2.5. Fractographic
Poland). Analysis
Processing parameters: sandblasting pressure p = 2 atm, abrasive—GH50 cast
steel Fracture morphologies
shot, nominal fraction dof=selected
0.3 mm,specimens were analyzed using
abrasive hardness—approx. an S-3400
60–68 HRC. Thescanning
sand-
electron
blastingmicroscope
treatment (SEM) from Phenom
was aimed not onlyProX (Nanoscience
at cleaning Instruments,
the surface, Phoenix,
but also AZ,places
(in the USA).
where the weld was made) introducing compressive stresses in the weld subsurface to
3. Results and
strengthen the Discussion
material.
3.1. Static Strength
2.5. Fractographic Analysis
In the static tests five specimens were tested for each of the two variants of specimens:
sandblasted and non-sandblasted.
Fracture morphologies of selected Basedspecimens
on the results
wereforanalyzed
all repetitions
usingthe
an average load
S-3400 scan-
capacity has been determined. The average load capacity (LC) of non-sandblasted
ning electron microscope (SEM) from Phenom ProX (Nanoscience Instruments, Phoenix, flash
butt welded joints was approximately 23.8 kN (Figure 3). The static strength tests of the
AZ, USA).
sandblasted sheets did not show any significant influence of this type of treatment on the
joint load capacity.
3. Results However, sandblasted specimens exhibit greater repeatability for load
and Discussion
capacity. The standard deviation of the load capacity of these joints was two times smaller
3.1. Static Strength
than for non-sandblasted samples. All samples were damaged in the weld zone (Figure 4).
In the static
The results of thetests five specimens
statistical were
analysis of thetested for each
static tests of the
results fortwo
thevariants of specimens:
non-sandblasted and
sandblasted specimens
sandblasted and non-sandblasted.
are shown inBased
Tableon2.the results for all repetitions the average load
capacity has been determined. The average load capacity (LC) of non-sandblasted flash
butt welded joints was approximately 23.8 kN (Figure 3). The static strength tests of the
sandblasted sheets did not show any significant influence of this type of treatment on the
 joint
jointload
loadcapacity.
capacity.However,
However,sandblasted
sandblastedspecimens
specimensexhibit
exhibitgreater
greaterrepeatability
repeatabilityfor
forload
load
capacity.
capacity.TheThestandard
standarddeviation
deviationofofthe
theload
loadcapacity
capacityofofthese
thesejoints
jointswas
wastwo
twotimes
timessmaller
smaller
Materials 2021, 14, 6831
than
thanforfornon-sandblasted
non-sandblastedsamples.
samples.AllAllsamples
sampleswereweredamaged
damagedininthe theweld
weldzone
zone(Figure
(Figure
6 of 18
4).
4).The
Theresults
resultsofofthe
thestatistical
statisticalanalysis
analysisofofthe
thestatic
statictests
testsresults
resultsfor
forthe
thenon-sandblasted
non-sandblasted
and
andsandblasted
sandblastedspecimens
specimensare areshown
shownininTable
Table2.2.

Figure
Figure3.3.Load
Figure Loadcapacity
Load capacityofof
capacity offlash
flashbutt
flash buttwelded
weldedjoints.
joints.

(a)
(a)

(b)
(b)
Figure
Figure4.4.View
Figure Viewofofthe
View thefracture
fracturemode
modeofof(a)
(a)sandblasted
sandblastedand
and(b)
(b)non-sandblasted
non-sandblastedbutt
buttwelds.
welds.

Table2.2.
Table
Table Statistical
2.Statistical analysis
analysisofof
Statisticalanalysis the
ofthe static
thestatic tests
statictests results.
testsresults.
results.
Parameter Non-Sandblasted Specimens Sandblasted
Non-Sandblasted
Non-Sandblasted Speimens
Sandblasted
Sandblasted
Parameter
Parameter
25.5Specimens
Speimens
Specimens
Speimens
20.247
25.047 25.5
27.056
20.247
25.5
20.247
LC of joint, kN 20.967 24.324
25.047
27.056
25.047
27.056
24.148 19.513
LC
LCofofjoint,
joint,kN kN 23.547 20.967
24.324
20.967
24.324
28.174
Average value of LC, kN 23.842 24.148
24.148 19.513
19.513
23.869
Standard deviation s, kN 1.59 23.547
23.547 28.174
28.174
3.498
Coefficient
Average
Average valueW
of variation
value s ,LC,
ofof %
LC,kNkN 6.669 23.842
23.842 14.655
23.869
23.869
Value ta for confidence level p = 95% 3.182 3.182
Standard deviation
Standard deviation s, kN s, kN 1.59
1.59 3.498
3.498
ta × s 5.059 11.131
Coefficient
Coefficientofofvariation
variationWW s, s%
,% 6.669
6.669 14.655
14.655
Value
Valuetatafor
forconfidence
confidencelevel levelp p= =95%
95% 3.182
3.182 3.182
3.182
3.2. Fatigue Strength
tata× ×s s 5.059
5.059 11.131
11.131
The common method of characterizing the fatigue performance of welded joints
under cyclic loading it to use the Wöhler’s curve. Comparison of the fatigue strength of
sandblasted and non-sandblasted specimens is shown in Figure 5. Specimens were tested at
five levels of stress amplitude σ: 690 MPa, 575 MPa, 460 MPa, 400 MPa and 345 MPa which
correspond to the 100%, 83%, 67%, 58% and 50% of the assumed maximum stress amplitude.
At every level, the tests were repeated four times. No sample loaded with an amplitude
of 373 MPa was damaged after 2 × 106 cycles. The results of the quantitative analysis of
 The common method of characterizing the fatigue performance of welded joints un-
der cyclic loading it to use the Wöhler’s curve. Comparison of the fatigue strength of sand-
blasted and non-sandblasted specimens is shown in Figure 5. Specimens were tested at
five levels of stress amplitude σ: 690 MPa, 575 MPa, 460 MPa, 400 MPa and 345 MPa which
Materials 2021, 14, 6831 7 of 18
correspond to the 100%, 83%, 67%, 58% and 50% of the assumed maximum stress ampli-
tude. At every level, the tests were repeated four times. No sample loaded with an ampli-
tude of 373 MPa was damaged after 2 × 106 cycles. The results of the quantitative analysis
of
thethe parameters
parameters ofof the
the fatiguetests
fatigue testsfor
fornon-sandblasted
non-sandblastedand andsandblasted
sandblasted specimens
specimens are
shown in Tables 3 and 4, respectively.
respectively. TheThe coefficient
coefficient of
of variation
variation WWss has been determined
according to the formula:
s
W 𝑠= × 100%
𝑊𝑠 = s
× 100% (1)
𝑙𝑜𝑔𝑁̅ logN
where s is the standard deviation
deviation and N
𝑁̅ isisthe
theaverage
averagevalue
valueofofthe
thedestructive
destructivecycles.
cycles.

Figure 5. Comparison of the fatigue strength of sandblasted and non-sandblasted specimens.

Table 3. Results
Table 3. Results of
of the
the statistical
statistical analysis
analysis of
of the
the fatigue
fatigue tests
tests for
for non-sandblasted
non-sandblasted specimens.
specimens.
Parameter
Parameter Values for
Values forIndividual
Individual Specimens
Specimens
Stress
StressAmplitude
Amplitude MPa
σ, σ, MPa 690690 575
575 460
460 400 345
345
7.586 26.970
7.586 26.970 121.903
121.903 110.263
110.263 2000
2000
3 14.570
14.570 67.508
67.508 67.031
67.031 473.251
473.251 2000
2000
Number
Numberofofdestructive cycles
destructive N ×N10× 103
cycles 3.483 80.451 100.492 363.04 2000
3.483 80.451 100.492 363.04 2000
18.246 19.635 198.564 732.53 2000
18.246 19.635 198.564 732.53 2000
3.880 4.431 5.086 5.042 6.301
Logarithmic number of destructive cycles 3.880 4.829
4.163 4.431 4.826 5.086 5.042
5.675 6.301
6.301
Logarithmic number
logNof destructive cycles 3.5424.163 4.905
4.829 5.002 4.826 5.675
4.559 6.301
6.301
logN 4.261
3.542 4.293
4.905 5.298
5.002 4.865
4.559 6.301
6.301
Average value of destructive cycles N 4.261 48,641
10,971 4.293 121,997
5.298 173,268
4.865 6.301
-
Standard deviation s 0.27984 0.2588 0.1696 0.40755 -
̅
Average value of destructive cycles 𝑁 10,971 48,641 121,997 173,268 -
Coefficient of variation Ws , % 7.063 5.608 3.356 8.093 -
Standard
Value ta for deviation
confidence level p =s 95% 0.27984 3.182
3.182 0.2588 0.1696
3.182 0.40755
3.182 -
Coefficienttaof×variation
s Ws, % 7.063
0.89 5.608
0.823 3.356
0.539 8.093
1.297 --
Value ta for confidence
logNup level p = 95% 3.182
4.852 3.182
5.438 3.182
5.592 3.182
6.332 --
ta × s 0.89 0.823 0.539 1.297 -
Nup × 103 cycles 71.139 274.294 391.516 256.204 -
logNup 4.852 5.438 5.592 6.332 -
logN 3 3.071 3.791 4.513 3.738 -
Nup × 10low cycles 71.139 274.294 391.516 256.204 -
Nlow × 103 cycles 1.178 6.182 32.614 5.479 -
Fatigue strength Zg at 2 × 106 cycles 345
Materials 2021, 14, 6831 8 of 18

Table 4. Results of the statistical analysis of the fatigue tests for sandblasted specimens.

Parameter Values for Individual Specimens

Stress Amplitude σ, MPa 690 575 460 400 345
10.067 42.285 190.749 249.901 2000
Number of destructive cycles N × 103 18.723 50.760 98.032 494.720 2000
12.104 85.458 307.645 351.386 2000
8.547 36.574 294.482 198.640 2000
4.003 4.626 5.280 5.397 6.301
Logarithmic number of destructive cycles
4.272 4.705 4.991 5.694 6.301
logN
4.083 4.931 5.488 5.545 6.301
3.932 4.563 5.469 5.298 6.301
Average value of destructive cycles N 12,360 53,769 222,727 323,661 -
Standard deviation s 0.12717 0.13940 0.19960 0.15006 -
Coefficient of variation Ws , % 3.123 2.962 3.761 2.736 -
Value ta for confidence level p = 95% 3.182 3.182 3.182 3.182 -
ta × s 0.405 0.443 0.635 0.477 -
logNup 4.477 5.150 5.942 5.961 -
Nup × 103 cycles 30.004 141.331 875.749 915.155 -
logNlow 3.667 4.263 4.672 5.006 -
Nlow × 103 cycles 4.654 18.326 46.999 101.506 -
Fatigue strength Zg at 2 × 106 cycles 345

In the case of both analyzed sample variants, the number of cycles at which the sample
is damaged decreases with the percentage increase of the set stress. Thus, as the amplitude
of the stress increases, the strength of the welded joint decreases, and thus the service life of
the bandsaw blades is reduced. Under the load of the samples with the highest amplitude
value (σ = 690 MPa), sandblasting of the sample surface increased the average value of the
destructive cycles by 12.7%. A similar increase in the mean value of the destructive cycles
was observed for sandblasted samples loaded with a stress amplitude of σ = 575 MPa. The
load of the sandblasted samples with an amplitude of 460 MPa increased the average value
of destructive cycles by about 82% compared to the samples not subjected to sandblasting.
The largest difference in the average value of destructive cycles was observed for samples
loaded with a stress amplitude of σ = 375 MPa. Sandblasting increased this number by
over 86% compared to non-sandblasted samples loaded with the same stress amplitude.
This phenomenon can be explained by the differences in the mechanisms of low- and
high-cycle fatigue. In the low-cycle fatigue range, plastic deformations resulting from the
load hysteresis occur with each cycle, thus plastic deformations accumulate. On the other
hand, the sandblasting process introduces compressive stresses in the subsurface layer of
the weld, therefore, due to these additional stresses, the tensile variable load acting on the
sample requires a greater number of cycles before the fatigue cracks initiate and propagate.
The range of high-cycle fatigue, on the other hand, is characterized by cyclically repeat-
ing elastic deformations, in this case, the compressive stresses introduced by sandblasting
in the subsurface zone are not of significant importance. At the same time, high-cycle fa-
tigue phenomena are more sensitive to any surface defects. The increase in the repeatability
of the fatigue life for the high-cycle fatigue load range can be explained primarily by the
uniformity of the surface properties of the samples in terms of geometry and surface rough-
ness. Samples fabricated under industrial conditions and not subjected to sandblasting are
characterized by the presence of various types of surface defects with little repeatability,
such as micro-grooves and scratches of various shapes and directions, which constitute
Materials 2021, 14, 6831 9 of 18

surface stress concentrators significantly affecting the high-cycle fatigue mechanism. The
variety of surface defects may influence the phenomenon of fatigue crack initiation; hence,
the non-sandblasted samples exhibit large dispersion of average value of destructive cycles
The aforementioned defects are minimized or removed in the sandblasting process, which
contributes to increasing the repeatability of the fatigue test results for this sample variant.
Based on the research, it was shown that, regardless of the variable load level, sand-
blasting has a positive effect on reducing the scatter of the test results, and the samples are
subject to fatigue failure in a more reproducible manner. This is due to the standardization
of the surface topography and the state of the stresses in the weld subsurface.

3.3. Morphology of Fractured Samples upon Quasi-Static Fatigue Loading

Observation of the fatigue fractures of samples tested with a stress amplitude of
σ = 690 MPa showed the destruction is as a result of ductile fracture mode (Figures 6 and 7).
During ductile fracture, the formation and joining of cracks takes place due to the plastic
flow of the material. Ductile cracking occurs by nucleation and void growth and usually
begins with particles of a different phase [27]. Voids are created during solidification stage.
Cooling of the nugget takes place immediately after the end of the heating cycle. The
solidification front begins from the periphery and moves toward center [28]. Dendrites
growing in electrode direction will experience a higher cooling rate and would grow faster
than the dendrites growing in the direction of interface/bulk. As a result, the dendrites
growing in electrode direction obstruct the interdendritic feeding during the final stages of
solidification owing to dendrite coherency [29]. The coherency causes an acute shortage in
liquid feeding to the nugget center. This shortage along with metal contraction is believed
to cause large pores in the nugget known as shrinkage voids [30,31]. The void growth
occurs by the emission of shear dislocation loops from the void surface, which evolve
by cross slip to prismatic loops leading to an increase in the void dimensions [32,33].
Turnage et al. [27] found that tensile results indicate that the microstructural damage
accumulation due to the change in number density of voids is much faster in the fusion
zone than in the parent material, where the change in void growth is the more dominant.
The effect of void growth from inclusions is much more prominent in the parent material
than in the fusion zone, which shows less ductile behavior (nucleation dominant).
The diameter of the voids is a characteristic structural dimension for the cracking
mechanism through the growth and merging of voids [34,35]. This mechanism is deter-
mined by the law of the evolution of voids in the stress field in the presence of plastic
strains [36,37]. Voids are formed around heterogeneity in the microstructure of the material,
that is, carbides and non-metallic inclusions. At room temperature, the voids increase as a
result of the development of plastic deformation.
Differences in the strains of the hard particles and the matrix cause the generation
of dislocations in the matrix during deformation. If brittle particles of a different phase
are present in the tough matrix, such particles are unable to accommodate large plastic
deformations of the matrix. Therefore, even when the plastic deformations of the matrix
are not very large, the stress caused by external forces reaches a value sufficient for particle
fracture [38]. In the near-edge layer of sandblasted joints (Figure 7b), a clear flattening of
the traces caused by grinding is visible (Figure 7a). Virtually the entire fracture surface
is composed of dimples characteristic of ductile fracture. Dimples in the near-edge layer
are spread over large flat surfaces, while in the middle area of the weld, the fracture
surfaces show a random character (Figures 6d and 7d) with a few void-initiated fractures
(Figures 6c and 7c).
Materials 2021, 14, 6831 10 of 18
Materials 2021, 14, 6831 10 of 18

Figure 6. SEM micrographs of the fracture surface of non-sandblasted flash butt welded joints tested
Figure 6. SEM micrographs of the fracture surface of non-sandblasted flash butt welded joints tested
atataastress
stressamplitude
amplitudeof of690
690MPa:
MPa:(a)
(a)cross-section
cross-sectionof
ofthe
thefractured
fracturedsurface,
surface,(b)
(b)view
viewof
ofthe
thenear-edge
near-edge
layer of the weld, (c,d) magnification of the middle area of the
layer of the weld, (c,d) magnification of the middle area of the weld.weld.
Materials 2021, 14, 6831 11 of 18
Materials 2021, 14, 6831 11 of 18

Figure
Figure7. 7. SEM
SEM micrographs
micrographs of the fracture surface
surface of
of sandblasted
sandblastedflash
flashbutt
buttwelded
weldedjoints
jointstested
testedatata
astress
stressamplitude
amplitude ofof 690
690 MPa:
MPa: (a)(a) cross-section
cross-section of the
of the fractured
fractured surface,
surface, (b) view
(b) view of theofnear-edge
the near-edge
layer
layer
of theofweld,
the weld, (c,d) magnification
(c,d) magnification of theofmiddle
the middle area
area of theofweld.
the weld.

3.4. Morphology
The diameter of Fractured Samples
of the voids is a upon Low-Cyclestructural
characteristic Fatigue Loading
dimension for the cracking
mechanism through the growth
The morphologies and merging
of the fatigue of formed
fractures voids [34,35]. This mechanism
in low-cycle is deter-
fatigue conditions
mined
(Figuresby8theandlaw of the
9) are evolution of
characterized byvoids in the stress
a non-uniform field in
random the presence
structure along of
theplastic
entire
strains [36,37]. Voids are formed around heterogeneity in the microstructure of the mate-
width of the fatigue fracture (Figure 8a). At high magnification, voids appear, which were
formed
rial, during
that is, the and
carbides process of joining
non-metallic thermallyAtplasticized
inclusions. materials.theThis
room temperature, superficial
voids increase
cracking
as a resultisofpossibly caused byofhydrogen
the development which is deposited in the weld [4]. Hydrogen,
plastic deformation.
diffused from theinwelding
Differences zoneoftothe
the strains thehard
heatparticles
affected zone,
and theaccumulates in the
matrix cause thediscontinuities
generation of
under the grains.
dislocations in the At the same
matrix time,
during hydrogen is
deformation. If pressurized as aof
brittle particles gas that generates
a different phasehigh
are
internal stresses. Such a type of fracture is common in flash butt welded
present in the tough matrix, such particles are unable to accommodate large plastic defor- high strength
structural
mations of steels [39]. Therefore, even when the plastic deformations of the matrix are not
the matrix.
very large, the stress caused by external forces reaches a value sufficient for particle frac-
ture [38]. In the near-edge layer of sandblasted joints (Figure 7b), a clear flattening of the
traces caused by grinding is visible (Figure 7a). Virtually the entire fracture surface is com-
posed of dimples characteristic of ductile fracture. Dimples in the near-edge layer are
spread over large flat surfaces, while in the middle area of the weld, the fracture surfaces
 width of the fatigue fracture (Figure 8a). At high magnification, voids appear, which were
formed during the process of joining thermally plasticized materials. This superficial
cracking is possibly caused by hydrogen which is deposited in the weld [4]. Hydrogen,
diffused from the welding zone to the heat affected zone, accumulates in the discontinui-
Materials 2021, 14, 6831
ties under the grains. At the same time, hydrogen is pressurized as a gas that generates
12 of 18
high internal stresses. Such a type of fracture is common in flash butt welded high strength
structural steels [39].

Figure
Figure8.8.SEM
SEMmicrographs
micrographsof ofthe
thefatigue
fatiguefracture
fractureof
ofnon-sandblasted
non-sandblastedflash
flashbutt
buttwelded
weldedjoints
jointstested
tested
at
at a stress amplitude of 575 MPa: (a) cross-section of the fatigue fracture, (b) magnification of the
a stress amplitude of 575 MPa: (a) cross-section of the fatigue fracture, (b) magnification of the
subsurface
subsurfacearea,
area,and
and (c,d)
(c,d) view
view of
of the
the near-edge
near-edge layer
layer of
of the
the weld.
weld.

The sandblasted samples in the vicinity of the fracture edge contain a network of
dimples smaller in size than in the center of the flash butt weld (Figure 9a,b). Compressive
stresses occur in the near-edge zone subjected to sandblasting. These stresses add to
those resulting from plastic deformation and the resulting stress sign is reoriented in the
subsurface layer. During further deformation of the weld material, local necks are formed
between the micro-voids, and when breaking cause, the joining of the voids formed on
the particles [38,40]. The processes associated with ductile fracture are usually related
to particles of a different phase and the strength of the particle-matrix interface. This
type of fracture, characterized by the presence of micro-voids, is due to the coalescence of
microcracks that form the nuclei of microcracks in discontinuous areas and are associated
with dislocations, second-phase particles, grain boundaries and inclusions [41]. As the
deformation increases, the microcracks increase and eventually form a continuous fracture.
Materials 2021, 14, 6831 13 of 18
Materials 2021, 14, 6831 13 of 18

Figure
Figure 9.9. SEM
SEM micrographs
micrographs of the fatigue fracture
fracture of
of sandblasted
sandblastedflash
flashbutt
buttwelded
weldedjoints
jointstested
testedatata
astress
stressamplitude
amplitude ofof
575575 MPa:
MPa: (a)(a) cross-section
cross-section of the
of the fatigue
fatigue fracture,
fracture, (b) view
(b) view of theofnear-edge
the near-edge
layer
layer of the
of the weld. weld.

3.5. Morphology
The sandblastedof Fractured
samples Samples
in theupon High-Cycle
vicinity of theFatigue
fracture Loading
edge contain a network of
dimplesThe fatigue fractures of non-sandblasted samples tested(Figure
smaller in size than in the center of the flash butt weld under9a,b). Compressive
high-cycle fatigue
stresses
conditions can be divided into two clear zones (Figure 10a): the area adjacentadd
occur in the near-edge zone subjected to sandblasting. These stresses to those
to the edge
resulting
of the flashfrom
buttplastic
weld anddeformation
the middle and zonethewith
resulting
a mixedstress sign mode
fracture is reoriented in the sub-
which corresponds
surface layer. During
to two ductile fracturefurther deformation
mechanisms. In theofcentral
the weldpartmaterial, local fracture,
of the fatigue necks arethe formed
crack
between
propagates theaccording
micro-voids, to theandvoid when breaking
growth cause, the
and merging joining of
mechanism the voids
(Figure 10d).formed
In general,on
the
the particles
center of[38,40].
the weld Theisprocesses
devoid ofassociated
inclusionswith thatductile
could be fracture
a sourceare of
usually
crack related to
initiation.
particles of a of
The studies different
Siddiqui phase
et al.and
[42]the strength
showed of the
that the inclusions
particle-matrix interface.
are pushed outThis
of thetype of
area
fracture, characterized
to be welded towards the by outer
the presence
surface of of micro-voids,
the weld during is due
the to the coalescence
upsetting process. of Atmi-
the
crocracks
edges of the that formonthe
weld, nucleiplanes,
inclined of microcracks
the crack in discontinuous
develops according areas
to theand are mechanism
shear associated
(Figure
with 10a). These
dislocations, inclined planes
second-phase are called
particles, grainshear lips. and inclusions [41]. As the de-
boundaries
In theincreases,
formation sample that theismicrocracks
stretched, beforeincrease formation of the neck
and eventually form begins, micro-voids
a continuous may
fracture.
form in the entire volume of the sample [43]. During ductile fracture, the tensile strength of
the Morphology
3.5. material is less than the stress
of Fractured Samples required to propagate
upon High-Cycle the fracture,
Fatigue Loading therefore the specimen
first The
deforms
fatigue fractures of non-sandblasted samples testedhas
uniformly, then a neck is formed. Once the neck underbegun to form, fatigue
high-cycle further
deformation and the merging of the voids is confined to this zone.
conditions can be divided into two clear zones (Figure 10a): the area adjacent to the edge A crack is then formed
inthe
of theflash
central
buttpart
weldof and
the sample
the middle due zone
to thewithvoids merging,
a mixed and the
fracture mode final separation
which of the
corresponds
material is achieved by the fracture in the outer areas of the weld.
to two ductile fracture mechanisms. In the central part of the fatigue fracture, the crack
The ultimate
propagates according tensile strength
to the of flashand
void growth buttmerging
welded mechanism
joints is usually 10 to
(Figure 20%Inlower
10d). gen-
eral, the center of the weld is devoid of inclusions that could be a source of crack significant
than that of the base material, due to the presence of impurities in the weld and initiation.
grain growth in the relatively wide welding zone [44]. Furthermore, deeper zones in the
The studies of Siddiqui et al. [42] showed that the inclusions are pushed out of the area to
weld cool down more slowly and microstructural transitions take place with some delay.
be welded towards the outer surface of the weld during the upsetting process. At the
In this way, the material located below the subsurface of weld is stretched, which causes
edges of the weld, on inclined planes, the crack develops according to the shear mecha-
additional compression of the material located directly below this zone. The tensile and
nism (Figure 10a). These inclined planes are called shear lips.
compressive stresses add up, causing a specific state of resulting stresses, which depends
on the type of material and the welding parameters.
The fatigue fracture of a sandblasted specimen subjected to high-cycle fatigue con-
sists of evenly distributed voids (Figure 11) with decreasing size towards the weld edge
(Figure 11b) and clear slip planes (Figure 11d). If the majority of the particles on which
the voids are formed are located at the grain boundaries, cracking occurs along the grain
boundaries (Figures 10c and 11d) and is called intercrystalline.
Materials 2021, 14, 6831 14 of 18
Materials 2021, 14, 6831 14 of 18

Figure 10. SEM micrographs of the fatigue fracture of non-sandblasted flash butt welded joints tested
Figure 10. SEM micrographs of the fatigue fracture of non-sandblasted flash butt welded joints
at a stress amplitude of 460 MPa: (a) cross-section of the fatigue fracture, (b) magnification of the
tested at a stress amplitude of 460 MPa: (a) cross-section of the fatigue fracture, (b) magnification of
subsurface area, (c,d) magnification of the middle area of the weld.
the subsurface area, (c,d) magnification of the middle area of the weld.
The fracture mechanism along the slip planes during the development of the ductile
In the sample that is stretched, before formation of the neck begins, micro-voids may
fracture is characteristic for high plasticization of the material in front of the crack [45].
form in the entire volume of the sample [43]. During ductile fracture, the tensile strength
Several changes in the fracture mechanism from ductile to brittle can occur during crack
of the material is less than the stress required to propagate the fracture, therefore the spec-
growth. With this type of crack development, a brittle fracture is realized only in limited
imen first deforms uniformly, then a neck is formed. Once the neck has begun to form,
areas of the material, surrounded by the dominant ductile fracture mechanism (Figure 10c).
further deformation and the merging of the voids is confined to this zone. A crack is then
formed in the central part of the sample due to the voids merging, and the final separation
of the material is achieved by the fracture in the outer areas of the weld.
The ultimate tensile strength of flash butt welded joints is usually 10 to 20% lower
than that of the base material, due to the presence of impurities in the weld and significant
grain growth in the relatively wide welding zone [44]. Furthermore, deeper zones in the
weld cool down more slowly and microstructural transitions take place with some delay.
In this way, the material located below the subsurface of weld is stretched, which causes
additional compression of the material located directly below this zone. The tensile and
compressive stresses add up, causing a specific state of resulting stresses, which depends
on the type of material and the welding parameters.
 The fatigue fracture of a sandblasted specimen subjected to high-cycle fatigue con-
sists of evenly distributed voids (Figure 11) with decreasing size towards the weld edge
Materials 2021, 14, 6831 (Figure 11b) and clear slip planes (Figure 11d). If the majority of the particles on which the
15 of 18
voids are formed are located at the grain boundaries, cracking occurs along the grain
boundaries (Figures 10c and 11d) and is called intercrystalline.

Figure
Figure11.11.SEM
SEMmicrographs
micrographs ofof the
the fatigue
fatigue fracture of sandblasted
sandblasted flash
flashbutt
buttwelded
weldedjoints
jointstested
testedatata
astress
stressamplitude
amplitudeofof 460 MPa: (a) cross-section of the fatigue fracture, (b) view of the near-edge
460 MPa: (a) cross-section of the fatigue fracture, (b) view of the near-edge layer
layer
of theofweld,
the weld, (c) magnification
(c) magnification of the of the middle
middle area ofarea of the (d)
the weld, weld, (d) magnification
magnification of the subsur-
of the subsurface area
face area of the weld.
of the weld.

The fracture mechanism along the slip planes during the development of the ductile
4. Conclusions
fracture
Theisconducted
characteristic for high plasticization
experimental studies on the of the material
fatigue strengthinof front of the
flash butt crack joints
welded [45].
Several changes in the fracture mechanism from ductile to brittle can
in 75Cr1 cold-work tool steel allow the following conclusions to be drawn: occur during crack
growth. With this type of crack development, a brittle fracture is realized only in limited
• Static strength tests showed no significant effect of sandblasting of the flash butt weld
areas of the material, surrounded by the dominant ductile fracture mechanism (Figure
surface on the load capacity of the joint.
10c).
• The sandblasted samples were characterized by a greater repeatability of the static
load capacity determined by the value of the standard deviation.
• In the case of both analyzed sample variants (sandblasted and non-sandblasted), the
number of cycles at which the sample is damaged decreases with the percentage
increase of the stress amplitude.
• Depending on the stress amplitude value, sandblasting of the weld surface increased
the average value of destructive cycles by about 10–86% (depending on the stress
amplitude) compared to samples not subjected to sandblasting.
Materials 2021, 14, 6831 16 of 18

• Regardless of the variable amplitude level, sandblasting has a positive effect on

reducing the distribution of the test results, with the samples subjected to fatigue
failure in a more reproducible manner.
• The surfaces of the fatigue fractures formed in low-cycle fatigue conditions are charac-
terized by the ductile fracture mode with an uneven grain structure along the entire
width of the fatigue fracture.
• The fatigue fractures of non-sandblasted samples tested under high-cycle fatigue
conditions can be divided into those in an area adjacent to the flash butt weld edge
and those in a middle zone with a mixed fracture mode. In the central part of the weld,
the crack propagates according to the void growth and merging mechanism, and at
the near-edge layer of the weld the crack develops according to the shear mechanism.
The tests conducted to determine the suitability of the sandblasting process in increas-
ing the fatigue strength of bandsaw blades have in fact demonstrated, for both analyzed
sample variants, that the number of cycles after which the sample was damaged is higher
in the case of sandblasted specimens. Future studies should investigate the effect of a
wide range of changes in sandblasting parameters (nominal fraction of cast steel shots
and sandblasting pressure) on the work hardening of the weld subsurface. The next task
will be to determine the effect of sandblasting on the surface roughness of the joint, which
under the conditions of high-cycle fatigue may be the source of microcracks initiation.
The mechanical properties of the weld material and its microstructure may affect the
susceptibility of the joint to work hardening. An interesting research direction may be
to determine the fatigue strength of sandblasted joints made by other methods, such as
brazing, gas welding, MIG and TIG welding. More extensive research is also planned to
analyze the mechanism of strengthening the joints by surface treatment. For this purpose,
measurements of residual stresses will be carried out, making possible the determination
of the surface treatment parameters for the residual stress distribution, which will then
be correlated with the influence of these properties on the fatigue life of the joints. The
research plans also assume the determination of the influence of other surface treatment
methods on fatigue properties, such as pneumatic ball peening and brushing. Finally,
the analysis of the influence of residual stress distribution in subsurface layers on the
propagation rate of fatigue cracks will be carried out. The directions of these studies are
extremely important, as the relatively low-cost surface treatment can lead to a significant
increase in the fatigue life of structural joints. The research results presented in this paper
are an introduction to a broad analysis of the phenomena occurring during the fatigue
process of flash butt welded joints.

Author Contributions: Conceptualization, A.K. (Andrzej Kubit) and Ł.L.; methodology, A.K.
(Andrzej Kubit) and Ł.L.; validation, A.K. (Andrzej Kubit), Ł.L., T.T. and A.K. (Andrzej Krzysiak);
formal analysis, Ł.L., A.K. (Andrzej Krzysiak) and W.Ł.; investigation, A.K. (Andrzej Kubit), Ł.L.;
data curation, A.K. (Andrzej Kubit), Ł.L., T.T., A.K. (Andrzej Krzysiak), and W.Ł.; funding acquisition,
T.T.; writing—original draft preparation, A.K. (Andrzej Kubit), Ł.L., T.T., A.K. (Andrzej Krzysiak)
and W.Ł.; writing—review and editing, A.K. (Andrzej Kubit) and T.T. All authors have read and
agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
Materials 2021, 14, 6831 17 of 18

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