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Citation: Miguel, I.;
Berriozabalgoitia, I.; Artola, G.;
Macareno, L.M.; Angulo, C. Small
Punch Test on Jominy Bars for
High-Throughput Characterization
of Quenched and Tempered Steel.
Metals 2023,13, 1797. https://
doi.org/10.3390/met13111797
Academic Editor: Ramakanta Naik
Received: 22 September 2023
Revised: 18 October 2023
Accepted: 21 October 2023
Published: 25 October 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
metals
Article
Small Punch Test on Jominy Bars for High-Throughput
Characterization of Quenched and Tempered Steel
Ibon Miguel 1, 2, * , Itziar Berriozabalgoitia 1, Garikoitz Artola 1, Luis María Macareno 2and Carlos Angulo 2
1
Azterlan, Basque Research and Technology Alliance (BRTA), 48200 Durango, Spain; iberrio@azterlan.es (I.B.);
gartola@azterlan.es (G.A.)
2
Department of Mechanical Engineering, University of the Basque Country (UPV/EHU), 48013 Bilbao, Spain;
luismaria.macareno@ehu.eus (L.M.M.); carlos.angulo@ehu.es (C.A.)
*Correspondence: imiguel@azterlan.es
Abstract:
Studying the effect of quench and tempering heat treatments on steel, more specifically
screening the effect of the austenitizing, quenching, and tempering conditions on mechanical prop-
erties, can be extremely material- and time-consuming when standard tensile testing specimens
are employed. Jominy bar end quench testing has been used as a standard method to reduce the
resources that are required for this type of screening. Jominy bar testing by itself shows, though, the
limitation of yielding only hardness and microstructure as a result. In the last few years, the small
punch test (SPT) standard has been developed. This technique can obtain an estimation of tensile
mechanical properties with miniaturized specimens, which can be dissected from Jominy bars. The
paper proposes a new testing methodology for screening the outcome of heat treatment conditions by
combining the Jominy bar testing and SPT. Quench and tempering of API 5L X65Q pipe steel is used
as a case study to describe the proposed methodology. The ability of the Jominy with SPT to detect
variations in the mechanical properties produced by heat treatments is shown. This methodology can
be directly applied as a high-throughput testing approach in the optimization of heat treatments.
Keywords:
small punch test; quenched and tempered steel; Jominy test; heat treatment; mechanical
properties; high-throughput testing
1. Introduction
High-throughput testing is employed in materials research for screening a high num-
ber of experimental conditions, which involves minimal sample preparation, reduced
sample volumes, high automatization, parallelization of measurements, and miniature
specimens [
1
]. Regarding miniaturized techniques, several have been reported for me-
chanical property characterization [
2
,
3
], though not all of them are standardized. This lack
of standardization causes difficulties when comparing and interpreting the results in the
literature. The small punch test (SPT) is a good example of the variations that can be found
in testing conditions prior to standardization [
4
]. The SPT shows the advantages of being
standardized both by the American Society for Testing Materials (ASTM E3205 test method)
and the European Committee for Standardization (EN 10371 test method), in 2020 and 2021,
respectively. Both standards fully describe the testing methodology, involved tooling, load
carrying and transfer mechanism, and specimen features.
The SPT is a very versatile technique as it allows for the characterization of a broad
spectrum of materials, testing conditions, and mechanical properties. The estimation of
tensile properties such as yield strength, tensile strength, elongation, and the ductile-to-
brittle transition temperature is standardized in both ASTM E3205 and EN 10371. Many
other properties have been explored in terms of the coverage of both standards, such as the
elastic modulus, fracture toughness, creep response, and fatigue behavior [
4
,
5
]. In terms
of testing conditions, the SPT has been used for characterizing the mechanical response
Metals 2023,13, 1797. https://doi.org/10.3390/met13111797 https://www.mdpi.com/journal/metals
Metals 2023,13, 1797 2 of 14
of materials submitted to magnetic fields [
6
], at cryogenic temperatures [
6
–
8
], at elevated
temperatures [9–12], and immersed in embrittling media [13].
Regarding material conditioning and processing, the SPT has been reportedly used
for studying the effects of neutron-ion irradiation [
14
], exposure to hydrogen embrittle-
ment [
15
], thermal ageing [
16
,
17
], annealing [
18
], hot isostatic pressing [
19
], and thermal
cutting [
20
]. These thermal ageing, annealing, hot isostatic pressing, and thermal cutting
studies are related to the field of heat treatment. In this field, a high-throughput test known
as the Jominy test is standardized in ASTM A255 and ISO 642 and allows the generation
of materials that have been quenched at different cooling rates from a minimum material
amount whose dimensions are Ø25
×
100 mm. The ability of screening quenching rates has
proved to be valid for the traditional application of high-throughput tests, such as for alloy
design [
21
]. The material characteristics that the Jominy test allows one to assess are hard-
ness and microstructure for a broad range of cooling rates at a given temperature [
22
–
24
],
which explain why this technique has been used not only for steels but also for non-ferrous
alloys [23,24].
In this scenario, a combination of the Jominy test with SPT offers the possibility to
increase the throughput of the testing even further, as the standard diameter of Jominy
bars allows one to obtain at least three SPT specimens from any desired quenching rate
present in the bar. Following the quenching rate, sampling positions of ASTM A255 and
ISO 642, SPT specimens could be used to explore the mechanical properties generated
by three tempering conditions for at least 22 quenching rates with a single Jominy bar.
Despite some works that studied the effect of the cooling rate on mechanical properties
by employing the SPT, including research on the heat affected zones (HAZs) on thermal
cutting and welds [
20
,
25
] and on pressure tubes combining dilatometry and the SPT [
26
],
we have not found any instances in the literature regarding the combination of the Jominy
test and SPT for the same purposes.
This study proposes a new methodology that combines the Jominy test with the SPT
to overcome the drawbacks of the approaches mentioned above: the lack of control on the
cooling rate and tempering in previous HAZ studies and the high resource demand for
performing a dilatometric test for each thermal condition of interest on SPT specimens.
With the new methodology proposed here, a Jominy bar easily yields over 60 heat treatment
conditions with a continuous distribution of controlled cooling rates from which to choose.
This means extremely reduced material needs and lower specimen manufacturing effort.
Furthermore, the new testing methodology presented here can be used to screen across
a continuous range of cooling rates with a high spatial resolution along the Jominy bar
(0.5 mm). In order to show the feasibility of employing the methodology, a case study is
presented where the influence on the mechanical properties of a pipe steel grade of the
austenitizing temperature, quenching rate, and three tempering conditions is characterized.
Beyond the case study selected for this manuscript, this new experimental approach can
also be applied to study other phenomena of interest for heat-treating technology, such as
solution annealing and ageing behaviors or temper embrittlement.
2. Materials and Methods
The work in this study has been carried out using API 5L X65Q pipe steel. Its
chemical composition was analyzed using an automatic combustion analyzer (Leco
CS744, Leco Corporation, St. Joseph, MI, USA) and spark emission spectrometry
(Spectrolab M10, Spectro, Kleve, Germany) to determine any elements therein in
addition to carbon and sulfur. The results of the analyses are shown in Table 1together
with the API 5L X65Q specifications.
Metals 2023,13, 1797 3 of 14
Table 1. Chemical composition of the steel sample employed for the experimental tests.
API 5L X65Q C Mn Si P S Cr Ni Mo Fe
Standard specification
<0.18 <1.70 <0.45 <0.025 <0.015 <0.50 <0.50 <0.50
Bal.
Sample 0.17 1.22 0.17
0.014 0.001
0.07 0.09 0.02 Bal.
Uncertainty (K = 2) ±
0.01
±
0.02
±
0.01
±
0.001
±
0.001
±
0.01
±
0.01
±
0.01
-
2.1. Jominy Test
The Jominy test consists of heating the specimen above the austenitizing temperature
and quenching with a controlled flow of water applied on one of the ends of the sample.
This cools the specimen from one end and allows one to obtain the hardenability of the
steel sample under a set of given conditions. For this work, two Jominy test specimens
were machined following the Jominy testing standard UNE EN ISO 642. The specimens
were 25 mm in diameter and 100 mm in length. In this work, the Jominy test temperatures
for each specimen were 890 and 920
◦
C. The two specimens were heated and maintained at
the corresponding temperatures for 30 min to ensure heating of the core. The tests were
performed in a Jominy testing machine (Remet S.A.S., Bologna, Italy). During the test, a
water jet at 20
◦
C was applied to one of the ends of the specimen and maintained for 10 min,
and after that time the specimen was submerged completely in cold water. After quenching
both Jominy samples and before extracting the SPT samples, hardness tests at different
positions were performed with a semi-automatic Rockwell durometer (Emco-test DJ10G5,
Emco-test, Kuchl, Austria) following the distances given in the Jominy testing standard
UNE EN ISO 642. The obtained results are summarized in Figure 1, with 26 trials in total.
Metals 2023, 13, x FOR PEER REVIEW 3 of 14
Table 1. Chemical composition of the steel sample employed for the experimental tests.
API 5L X65Q C Mn Si P S Cr Ni Mo Fe
Standard specification <0.18 <1.70 <0.45 <0.025 <0.015 <0.50 <0.50 <0.50 Bal.
Sample 0.17 1.22 0.17 0.014 0.001 0.07 0.09 0.02 Bal.
Uncertainty (K = 2) ±0.01 ±0.02 ±0.01 ±0.001 ±0.001 ±0.01 ±0.01 ±0.01 -
2.1. Jominy Test
The Jominy test consists of heating the specimen above the austenitizing temperature
and quenching with a controlled flow of water applied on one of the ends of the sample.
This cools the specimen from one end and allows one to obtain the hardenability of the
steel sample under a set of given conditions. For this work, two Jominy test specimens
were machined following the Jominy testing standard UNE EN ISO 642. The specimens
were 25 mm in diameter and 100 mm in length. In this work, the Jominy test temperatures
for each specimen were 890 and 920 °C. The two specimens were heated and maintained
at the corresponding temperatures for 30 min to ensure heating of the core. The tests were
performed in a Jominy testing machine (Remet S.A.S., Bologna, Italy). During the test, a
water jet at 20 °C was applied to one of the ends of the specimen and maintained for 10
min, and after that time the specimen was submerged completely in cold water. After
quenching both Jominy samples and before extracting the SPT samples, hardness tests at
different positions were performed with a semi-automatic Rockwell durometer (Emco-
test DJ10G5, Emco-test, Kuchl, Austria) following the distances given in the Jominy testing
standard UNE EN ISO 642. The obtained results are summarized in Figure 1, with 26 trials
in total.
Figure 1. Hardness test for the Jominy samples at different distances from the quenched face.
The quenching at 890 °C is slightly harder in the millimeters closest to the quenched
face. For 9 mm and greater, the hardness of both quenching temperatures is the same and
below the regular Rockwell C scale. The first 10 mm is where the drop in mechanical
properties is most clearly observed.
2.2. SPT Specimen Extraction
The SPT samples were extracted from the tested Jominy specimens with the
following methodology (Figure 2). The quenched end of the Jominy sample was marked
for traceability. From each Jominy sample, three cylinders with a small “shoulder” along
all the length were extracted with electrical discharge machining (EDM). The cylinders
were 8 mm in diameter, with a shoulder of 1 × 3 mm and length of 100 mm. The shoulder
was used to stop the SPT specimens from detaching from the cylinder during machining,
thus allowing for their traceability. From each set of three cylinders, one was processed in
quenched conditions, one was tempered at 620 °C, and the last one was tempered at 420
Figure 1. Hardness test for the Jominy samples at different distances from the quenched face.
The quenching at 890
◦
C is slightly harder in the millimeters closest to the quenched
face. For 9 mm and greater, the hardness of both quenching temperatures is the same
and below the regular Rockwell C scale. The first 10 mm is where the drop in mechanical
properties is most clearly observed.
2.2. SPT Specimen Extraction
The SPT samples were extracted from the tested Jominy specimens with the following
methodology (Figure 2). The quenched end of the Jominy sample was marked for traceabil-
ity. From each Jominy sample, three cylinders with a small “shoulder” along all the length
were extracted with electrical discharge machining (EDM). The cylinders were 8 mm in
diameter, with a shoulder of 1
×
3 mm and length of 100 mm. The shoulder was used to
stop the SPT specimens from detaching from the cylinder during machining, thus allowing
for their traceability. From each set of three cylinders, one was processed in quenched
conditions, one was tempered at 620
◦
C, and the last one was tempered at 420
◦
C. The heat
treatments for the total of six cylinders are summarized in Table 2. They have been coded
in the table for reference in the results section.
Metals 2023,13, 1797 4 of 14
Metals 2023, 13, x FOR PEER REVIEW 4 of 14
°C. The heat treatments for the total of six cylinders are summarized in Table 2. They have
been coded in the table for reference in the results section.
Figure 2. Sketch of the SPT samples extracted from the Jominy samples.
Table 2. Heat treatments applied to each cylinder.
Heat Treatment Code Description
Q890 Jominy quenching from 890 °C
Q890-T420 Jominy quenching from 890 °C + tempering at 420 °C for 5 h
Q890-T620 Jominy quenching from 890 °C + tempering at 620 °C for 5 h
Q920 Jominy quenching from 920 °C
Q920-T420 Jominy quenching from 920 °C + tempering at 420 °C for 5 h
Q920-T620 Jominy quenching from 920 °C + tempering at 620 °C for 5 h
After the corresponding heat treatments, the SPT specimens were pre-machined from
each cylinder using EDM to a thickness of 0.6 mm, leaving 1 mm of shoulder uncut. Thus,
10 SPT samples per cylinder were obtained, yielding 60 in total. As the first 10 mm is where
the strongest hardness drop is observed, ten SPT samples were extracted from this area
close to the quenched face. Specimens in each cylinder were extracted with approximately
1 mm distance of separation from each other.
2.3. SPT Specimen Conditioning
The samples were identified for traceability according to their corresponding posi-
tion on the Jominy bar. They were ground and polished to a final thickness of 0.5 mm.
First, one of the faces was ground with abrasive paper, starting with P240, followed by
P600, and finishing with P1200. Second, the sample was turned over, and the process was
repeated until a thickness of 0.500 ± 0.005 mm was obtained. Before the test, the diameter,
thickness, and roughness (Ra) of samples were measured. For the diameter, two measure-
ments were taken at 90° from each other, and the thickness was measured at four positions
around the perimeter at 90° intervals from each other and in the middle.
In total, 60 samples were tested with a universal testing machine (Zwick All Round
Z100, Zwick, Ulm, Germany), coupling a load cell of 5 kN and a SPT tool with the dimen-
sions in accordance with the standard ASTM E3205-20. The characteristic geometrical di-
mensions of the SPT tool are described as follows; the diameter of the receiving die is 32
mm, the diameter of the receiving die bore is 4 mm, the punch diameter is 2.5 mm, the
corner radius of the receiving die is 0.2 mm, and a ball of 2.5 mm with a hardness of 55
HRC is employed to force the central portion of the specimen through the hole in the re-
ceiving die.
Figure 2. Sketch of the SPT samples extracted from the Jominy samples.
Table 2. Heat treatments applied to each cylinder.
Heat Treatment Code Description
Q890 Jominy quenching from 890 ◦C
Q890-T420 Jominy quenching from 890 ◦C + tempering at 420 ◦C for 5 h
Q890-T620 Jominy quenching from 890 ◦C + tempering at 620 ◦C for 5 h
Q920 Jominy quenching from 920 ◦C
Q920-T420 Jominy quenching from 920 ◦C + tempering at 420 ◦C for 5 h
Q920-T620 Jominy quenching from 920 ◦C + tempering at 620 ◦C for 5 h
After the corresponding heat treatments, the SPT specimens were pre-machined from
each cylinder using EDM to a thickness of 0.6 mm, leaving 1 mm of shoulder uncut. Thus,
10 SPT samples per cylinder were obtained, yielding 60 in total. As the first 10 mm is where
the strongest hardness drop is observed, ten SPT samples were extracted from this area
close to the quenched face. Specimens in each cylinder were extracted with approximately
1 mm distance of separation from each other.
2.3. SPT Specimen Conditioning
The samples were identified for traceability according to their corresponding position
on the Jominy bar. They were ground and polished to a final thickness of 0.5 mm. First,
one of the faces was ground with abrasive paper, starting with P240, followed by P600, and
finishing with P1200. Second, the sample was turned over, and the process was repeated
until a thickness of 0.500
±
0.005 mm was obtained. Before the test, the diameter, thickness,
and roughness (Ra) of samples were measured. For the diameter, two measurements were
taken at 90
◦
from each other, and the thickness was measured at four positions around the
perimeter at 90◦intervals from each other and in the middle.
In total, 60 samples were tested with a universal testing machine (Zwick All Round
Z100, Zwick, Ulm, Germany), coupling a load cell of 5 kN and a SPT tool with the di-
mensions in accordance with the standard ASTM E3205-20. The characteristic geometrical
dimensions of the SPT tool are described as follows; the diameter of the receiving die is
32 mm, the diameter of the receiving die bore is 4 mm, the punch diameter is 2.5 mm,
the corner radius of the receiving die is 0.2 mm, and a ball of 2.5 mm with a hardness of
55 HRC is employed to force the central portion of the specimen through the hole in the
receiving die.
All specimens processed with this conditioning methodology comply with the require-
ments of the SPT testing standard ASTM E3205-20.
Metals 2023,13, 1797 5 of 14
3. Results
All the samples analyzed in this study were extracted from a known distance to the
quenched end of the Jominy bar, so with this distance and the data shown in Figure 3,
obtained from UNE EN ISO 642, the cooling rates corresponding to each sample were
obtained; see Table 3.
Metals 2023, 13, x FOR PEER REVIEW 5 of 14
All specimens processed with this conditioning methodology comply with the re-
quirements of the SPT testing standard ASTM E3205-20.
3. Results
All the samples analyzed in this study were extracted from a known distance to the
quenched end of the Jominy bar, so with this distance and the data shown in Figure 3,
obtained from UNE EN ISO 642, the cooling rates corresponding to each sample were
obtained; see Table 3.
Figure 3. Correlation between distance to the quenched end and the cooling rate adapted from UNE
EN ISO 642.
Table 3. Corresponding cooling rates for each distance to the quenched face.
Distance to Quenched Face (mm) 1.1 2.0 2.9 3.8 4.7 5.6 6.5 7.4 8.3 9.2
Cooling Rate (°C/s) 370 195 93 74 60 46 37 30 26 20
After performing the SPT, a force–displacement curve for each specimen was ob-
tained. The data processing was performed with a Microsoft Excel, macro-enabled spread-
sheet as employed by National Institute of Standards and Technology (NIST) in [27],
which provides the characteristic points of the curve (Fe, Fm, um, uf, ESP, Em, and EPL). The
Fe value used in this study is based on the calculation of the vertical projection of the point
of intersection of the two tangents on the curve test, Fm is the maximum force of the curve,
and um is the displacement corresponding to the maximum force Fm. These three values
obtained from the SPT curve are used for estimating the mechanical properties.
Even though there are approximations for the mechanical property estimation of the
yield strength (RP0.2) and ultimate tensile strength (Rm) prior to the ASTM E3205-20 [28],
the ASTM E3205-20 proposed expressions for RP0.2 Rm, are, respectively:
𝑅. =𝛽
. ∙
, (1)
𝑅=𝛽
∙
∙, (2)
where 𝛽. and 𝛽 are material-dependent empirical constants. The empirical con-
stant 𝛽. used to estimate the yield strength is obtained from the recommended value
given by EN 10371, which for steels with RP0.2 values in the range of 200 MPa to 1000 MPa
is 𝛽. = 0.479, as is the case in this study. For the empirical constant 𝛽, the values of
the Vickers hardness number (HV) were obtained in each specimen with a Vickers micro-
durometer (FM-700, Future Tech Corp., Tokyo, Japan), and with the ASTM E140 hardness
conversion table, the tensile strength was obtained (see Table 4). The values of the tensile
strength and Fm/(h0·um) are correlated, allowing us to obtain an equation to estimate the
tensile strength from the SPT.
Figure 3.
Correlation between distance to the quenched end and the cooling rate adapted from UNE
EN ISO 642.
Table 3. Corresponding cooling rates for each distance to the quenched face.
Distance to Quenched Face (mm)
1.1 2.0 2.9 3.8 4.7 5.6 6.5 7.4 8.3 9.2
Cooling Rate (◦C/s)
370 195
93 74 60 46 37 30 26 20
After performing the SPT, a force–displacement curve for each specimen was obtained.
The data processing was performed with a Microsoft Excel, macro-enabled spreadsheet
as employed by National Institute of Standards and Technology (NIST) in [
27
], which
provides the characteristic points of the curve (F
e
, F
m
, u
m
, u
f
, E
SP
, E
m
, and E
PL
). The F
e
value used in this study is based on the calculation of the vertical projection of the point of
intersection of the two tangents on the curve test, F
m
is the maximum force of the curve,
and u
m
is the displacement corresponding to the maximum force F
m
. These three values
obtained from the SPT curve are used for estimating the mechanical properties.
Even though there are approximations for the mechanical property estimation of the
yield strength (R
P0.2
) and ultimate tensile strength (R
m
) prior to the ASTM E3205-20 [
28
],
the ASTM E3205-20 proposed expressions for RP0.2 Rm, are, respectively:
RP0.2 =βRp0.2 ·Fe
h2
0
, (1)
Rm=βRm·Fm
h0·um, (2)
where
βRp0.2
and
βRm
are material-dependent empirical constants. The empirical constant
βRp0.2
used to estimate the yield strength is obtained from the recommended value given
by EN 10371, which for steels with R
P0.2
values in the range of 200 MPa to 1000 MPa is
βRp0.2 =0.479
, as is the case in this study. For the empirical constant
βRm
, the values of
the Vickers hardness number (HV) were obtained in each specimen with a Vickers micro-
durometer (FM-700, Future Tech Corp., Tokyo, Japan), and with the ASTM E140 hardness
conversion table, the tensile strength was obtained (see Table 4). The values of the tensile
strength and F
m
/(h
0·
u
m
) are correlated, allowing us to obtain an equation to estimate the
tensile strength from the SPT.
Metals 2023,13, 1797 6 of 14
Table 4. Obtained results from the ASTM E140 conversion table to convert HV to Rm.
Cooling Rate
(◦C/s)
Q920 Q920-T620 Q920-T420 Q890 Q890-T620 Q890-T420
HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa)
370 421 1359 257 824 336 1078 372 1196 223 716 321 1030
195 415 1339 252 808 310 994 372 1196 239 767 333 1069
93 365 1173 238 764 303 972 353 1133 241 773 297 952
74 333 1069 242 776 276 885 295 946 217 696 297 952
60 288 923 243 780 266 853 284 910 237 760 266 853
46 281 901 242 776 255 818 243 780 230 738 262 840
37 264 847 244 783 238 764 256 821 210 674 235 754
30 248 796 235 770 234 751 247 792 212 680 232 744
26 242 776 226 725 226 725 246 789 204 655 228 732
20 237 760 230 738 235 770 238 764 206 661 237 760
In Figure 4, the correlation for the tensile strength is presented. The data have been
fitted with a straight line that passes through the origin, following the recommendations of
ASTM E3205-20. This correlation, where βRmrepresents the slope, is:
y=0.39xR2=0.99. (3)
Metals 2023, 13, x FOR PEER REVIEW 6 of 14
Table 4. Obtained results from the ASTM E140 conversion table to convert HV to Rm.
Cooling Rate
(°C/s)
Q920 Q920-T620 Q920-T420 Q890 Q890-T620 Q890-T420
HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa) HV Rm
(MPa)
370 421 1359 257 824 336 1078 372 1196 223 716 321 1030
195 415 1339 252 808 310 994 372 1196 239 767 333 1069
93 365 1173 238 764 303 972 353 1133 241 773 297 952
74 333 1069 242 776 276 885 295 946 217 696 297 952
60 288 923 243 780 266 853 284 910 237 760 266 853
46 281 901 242 776 255 818 243 780 230 738 262 840
37 264 847 244 783 238 764 256 821 210 674 235 754
30 248 796 235 770 234 751 247 792 212 680 232 744
26 242 776 226 725 226 725 246 789 204 655 228 732
20 237 760 230 738 235 770 238 764 206 661 237 760
In Figure 4, the correlation for the tensile strength is presented. The data have been
fied with a straight line that passes through the origin, following the recommendations
of ASTM E3205-20. This correlation, where 𝛽 represents the slope, is:
𝑦 = 0.39𝑥 (𝑅=0.99). (3)
In addition, we also fit the data with a linear function where the y-intercept was al-
lowed to vary freely during the fit; see Figure 5. In this case, Equation (4) is obtained:
𝑦 = −122.36 + 0.44 𝑥 (𝑅=0.81). (4)
Comparing the two fiings for mechanical resistance, a beer correlation is obtained
by making the linear fit pass through the origin. Consequently, the tensile strength esti-
mation in this study was carried out using 𝛽 = 0.39. The tensile and yield strengths for
all the samples were estimated, and the obtained values are presented in Tables 5 and 6.
Figure 4. Relationship between Rm and Fm/h0 ·um, which passes through the origin.
Figure 4. Relationship between Rm and Fm/h0·um, which passes through the origin.
In addition, we also fit the data with a linear function where the y-intercept was
allowed to vary freely during the fit; see Figure 5. In this case, Equation (4) is obtained:
y=−122.36 +0.44 xR2=0.81. (4)
Comparing the two fittings for mechanical resistance, a better correlation is obtained by
making the linear fit pass through the origin. Consequently, the tensile strength estimation
in this study was carried out using
βRm
= 0.39. The tensile and yield strengths for all the
samples were estimated, and the obtained values are presented in Tables 5and 6.
With the mechanical properties obtained by means of the SPT, along the length of
the Jominy bar and for different heat treatments, the mechanical properties as a function
of the cooling speed were plotted, as shown in Figure 6. The results of the tensile and
yield strength estimation, compared between different tempering temperatures for each
quenching temperature, can be seen in Figure 6.
Metals 2023,13, 1797 7 of 14
Metals 2023, 13, x FOR PEER REVIEW 7 of 14
Figure 5. Relationship between Rm and Fm/h0 ·um where the y-intercept is allowed to vary during
the fit.
Table 5. Rm estimation.
Rm (MPa)
Cooling
Rate (°C/s) Q920 Q920-T420 Q920-T620 Q890 Q890-T420 Q890-T620
370 1182 1026 776 1339 989 825
195 1118 1007 802 1149 988 822
93 1007 928 770 1096 921 775
74 946 945 777 999 908 815
60 875 862 740 917 890 782
46 814 827 751 909 842 748
37 838 824 728 858 806 728
30 802 812 711 865 820 731
26 808 767 733 813 773 745
20 762 793 718 820 781 706
Table 6. RP0.2 estimation.
Rp0.2 (MPa)
Cooling Rate
(°C/s) Q920 Q920-T420 Q920-T620 Q890 Q890-T420 Q890-T620
370 1085 956 628 1383 959 771
195 1002 907 647 1229 976 774
93 867 816 640 1130 919 749
74 728 761 574 957 814 693
60 767 690 625 811 769 648
46 729 646 576 751 704 617
37 666 617 492 669 674 624
30 643 601 459 670 664 598
26 604 577 488 619 663 579
20 629 588 441 543 639 557
With the mechanical properties obtained by means of the SPT, along the length of the
Jominy bar and for different heat treatments, the mechanical properties as a function of
the cooling speed were ploed, as shown in Figure 6. The results of the tensile and yield
strength estimation, compared between different tempering temperatures for each
quenching temperature, can be seen in Figure 6.
Figure 5.
Relationship between Rm and Fm/h
0·
um where the y-intercept is allowed to vary during
the fit.
Table 5. Rm estimation.
Rm(MPa)
Cooling
Rate (◦C/s) Q920 Q920-T420 Q920-T620 Q890 Q890-T420 Q890-T620
370 1182 1026 776 1339 989 825
195 1118 1007 802 1149 988 822
93 1007 928 770 1096 921 775
74 946 945 777 999 908 815
60 875 862 740 917 890 782
46 814 827 751 909 842 748
37 838 824 728 858 806 728
30 802 812 711 865 820 731
26 808 767 733 813 773 745
20 762 793 718 820 781 706
Table 6. RP0.2 estimation.
Rp0.2 (MPa)
Cooling
Rate (◦C/s) Q920 Q920-T420 Q920-T620 Q890 Q890-T420 Q890-T620
370 1085 956 628 1383 959 771
195 1002 907 647 1229 976 774
93 867 816 640 1130 919 749
74 728 761 574 957 814 693
60 767 690 625 811 769 648
46 729 646 576 751 704 617
37 666 617 492 669 674 624
30 643 601 459 670 664 598
26 604 577 488 619 663 579
20 629 588 441 543 639 557
For the specimens quenched from 890
◦
C and their different tempering conditions
(Figure 6a,b), differences in the mechanical properties, both Rm and R
P0.2
, can be observed
for each cooling rate. This difference becomes greater as the cooling rate increases and
decreases to almost equal the mechanical properties at slow cooling rates. The specimens
quenched from 920
◦
C and their corresponding tempering conditions (Figure 6c,d) show
a similar behavior to the specimens quenched from 890
◦
C. The mechanical properties of
the samples subjected to the different cooling rates from 920
◦
C and those quenched and
tempered at 420
◦
C are similar, and only small differences can be observed at high cooling
Metals 2023,13, 1797 8 of 14
rates. Also, the results compared between the different quenching temperatures for the
same tempering conditions are shown in Figure 7.
Metals 2023, 13, x FOR PEER REVIEW 8 of 14
Figure 6. SPT estimated strength vs. the cooling rate grouped by the austenitizing temperature. (a)
Rm for quenching at 890 °C and all tempering conditions. (b) RP0.2 for quenching at 890 °C and all
tempering conditions. (c) Rm for quenching at 920 °C and all tempering conditions. (d) RP0.2 for
quenching at 920 °C and all tempering conditions.
For the specimens quenched from 890 °C and their different tempering conditions
(Figure 6a,b), differences in the mechanical properties, both Rm and RP0.2, can be observed
for each cooling rate. This difference becomes greater as the cooling rate increases and
decreases to almost equal the mechanical properties at slow cooling rates. The specimens
quenched from 920 °C and their corresponding tempering conditions (Figure 6c,d) show
a similar behavior to the specimens quenched from 890 °C. The mechanical properties of
the samples subjected to the different cooling rates from 920 °C and those quenched and
tempered at 420 °C are similar, and only small differences can be observed at high cooling
rates. Also, the results compared between the different quenching temperatures for the
same tempering conditions are shown in Figure 7.
Figure 7a,c,e show that there is only a small difference in Rm between the different
quenching temperatures studied, provided that tempering is subsequently applied at 420
°C or 620 °C. On the other hand, for RP0.2 (Figure 7b,d,f), greater differences can be ob-
served between the quenching temperatures applied, which remain constant for the dif-
ferent cooling rates. From Figures 1 and 7, it can be observed that in both cases, i.e., the
SPT and the hardness test, the quenching at 890 °C yields higher mechanical properties.
In summary, the SPT technique shows its capability for studying the mechanical proper-
ties of steels with different heat treatment conditions by employing only a very small
amount of material.
For each heat treatment condition, the microstructure of SPT specimens was studied
at two different cooling rates, which are representative of the boundaries of study, see
Figures 8 and 9 (magnification bar dimension corresponds to the length of the white rec-
tangle). The selected cooling temperatures correspond to the quenched end face of the
Figure 6.
SPT estimated strength vs. the cooling rate grouped by the austenitizing temperature.
(a) Rm
for quenching at 890
◦
C and all tempering conditions. (
b
) R
P0.2
for quenching at 890
◦
C and
all tempering conditions. (
c
) R
m
for quenching at 920
◦
C and all tempering conditions. (
d
) R
P0.2
for
quenching at 920 ◦C and all tempering conditions.
Figure 7a,c,e show that there is only a small difference in Rm between the different
quenching temperatures studied, provided that tempering is subsequently applied at
420
◦
C or 620
◦
C. On the other hand, for R
P0.2
(Figure 7b,d,f), greater differences can be
observed between the quenching temperatures applied, which remain constant for the
different cooling rates. From Figures 1and 7, it can be observed that in both cases, i.e., the
SPT and the hardness test, the quenching at 890
◦
C yields higher mechanical properties. In
summary, the SPT technique shows its capability for studying the mechanical properties
of steels with different heat treatment conditions by employing only a very small amount
of material.
For each heat treatment condition, the microstructure of SPT specimens was studied
at two different cooling rates, which are representative of the boundaries of study, see
Figures 8and 9(magnification bar dimension corresponds to the length of the white
rectangle). The selected cooling temperatures correspond to the quenched end face of the
Jominy (370
◦
C/s), where the greatest difference in mechanical properties for each heat
treatment temperature is found, and to specimens far from the quench zone (37
◦
C/s), an
area where mechanical properties between heat treatments temperatures are more similar.
Metals 2023,13, 1797 9 of 14
Metals 2023, 13, x FOR PEER REVIEW 9 of 14
Jominy (370 °C/s), where the greatest difference in mechanical properties for each heat
treatment temperature is found, and to specimens far from the quench zone (37 °C/s), an
area where mechanical properties between heat treatments temperatures are more similar.
Figure 7. Quenching temperature comparison. (a) Rm for Q920 vs. Q890. (b) RP0.2 for Q920 vs. Q890.
(c) Rm for Q920—T420 vs. Q890—T420. (d) RP0.2 for Q920—T420 vs. Q890—T420. (e) Rm for Q920—
T620 vs. Q890—T620. (f) RP0.2 for Q920—T620 vs. Q890—T620.
Figure 8 shows the microstructures of the quenching at 920 °C and their respective
temperings at 420 °C and 620 °C for two different cooling rates, 37 °C/s on the right side
and 370 °C/s on the left side.
The quenched and tempered microstructures for the specimens with high cooling
rate (370 °C/s) show sharper differences in the evolution of the microstructure from the
as-quenched condition to the tempered conditions, when compared to the specimens with
low cooling rate (37 °C/s). The quenched sample for high cooling rate (Figure 8a) is com-
posed entirely of martensite and traces of bainite. When the sample is tempered at 420 °C
(Figure 8b), tempered martensite and tempered bainite are observed in the
Figure 7.
Quenching temperature comparison. (
a
) R
m
for Q920 vs. Q890. (
b
) R
P0.2
for Q920 vs.
Q890. (
c
) R
m
for Q920—T420 vs. Q890—T420. (
d
) R
P0.2
for Q920—T420 vs. Q890—T420. (
e
) R
m
for
Q920—T620 vs. Q890—T620. (f) RP0.2 for Q920—T620 vs. Q890—T620.
Figure 8shows the microstructures of the quenching at 920
◦
C and their respective
temperings at 420
◦
C and 620
◦
C for two different cooling rates, 37
◦
C/s on the right side
and 370 ◦C/s on the left side.
The quenched and tempered microstructures for the specimens with high cooling rate
(370
◦
C/s) show sharper differences in the evolution of the microstructure from the as-
quenched condition to the tempered conditions, when compared to the specimens with low
cooling rate (37
◦
C/s). The quenched sample for high cooling rate (Figure 8a) is composed
entirely of martensite and traces of bainite. When the sample is tempered at 420
◦
C
(Figure 8b), tempered martensite and tempered bainite are observed in the microstructure.
On the other hand, when tempered at 620
◦
C (Figure 8c), the grains recrystallize into ferrite.
Metals 2023,13, 1797 10 of 14
Metals 2023, 13, x FOR PEER REVIEW 10 of 14
microstructure. On the other hand, when tempered at 620 °C (Figure 8c), the grains re-
crystallize into ferrite.
(a)
(d)
(b)
(e)
(c)
(f)
Figure 8. Microstructures of SPT samples quenched at 920 °C. (a) Q920—370 °C/s. (b) Q920—T420—
370 °C/s. (c) Q920—T620—370 °C/s. (d) Q920—37 °C/s. (e) Q920—T420—37 °C/s. (f) Q920—T620—
37 °C/s.
For the slower cooling rate (37 °C/s), the quenched sample (Figure 8d) shows mostly
bainite. The samples tempered at 420 °C and 620 °C (Figure 8e,f) show a similar structure,
composed of tempered martensite and bainite.
Figure 8.
Microstructures of SPT samples quenched at 920
◦
C. (
a
) Q920—370
◦
C/s. (
b
) Q920—
T420—370
◦
C/s. (
c
) Q920—T620—370
◦
C/s. (
d
) Q920—37
◦
C/s. (
e
) Q920—T420—37
◦
C/s.
(f) Q920—T620—37 ◦C/s.
For the slower cooling rate (37
◦
C/s), the quenched sample (Figure 8d) shows mostly
bainite. The samples tempered at 420
◦
C and 620
◦
C (Figure 8e,f) show a similar structure,
composed of tempered martensite and bainite.
Metals 2023,13, 1797 11 of 14
Metals 2023, 13, x FOR PEER REVIEW 11 of 14
(a)
(d)
(b)
(e)
(c)
(f)
Figure 9. Microstructures of SPT samples quenched at 890 °C. (a) Q890—370 °C/s. (b) Q890—T420—
370 °C/s. (c) Q890—T620—370 °C/s. (d) Q890—37 °C/s. (e) Q890—T420—37 °C/s. (f) Q890—T620—
37 °C/s.
Figure 9 shows the microstructures of the quenching from 890 °C and the respective
temperings at 420 °C and 620 °C for the two selected cooling rates, 37 °C/s on the right
side and 370 °C/s on the left side. The observed behavior shows the same paern as the
one in Figure 8, but in this case the recrystallization is less appreciable.
Figure 9.
Microstructures of SPT samples quenched at 890
◦
C. (
a
) Q890—370
◦
C/s. (
b
) Q890—
T420—370
◦
C/s. (
c
) Q890—T620—370
◦
C/s. (
d
) Q890—37
◦
C/s. (
e
) Q890—T420—37
◦
C/s.
(f) Q890—T620—37 ◦C/s.
Figure 9shows the microstructures of the quenching from 890
◦
C and the respective
temperings at 420
◦
C and 620
◦
C for the two selected cooling rates, 37
◦
C/s on the right
side and 370
◦
C/s on the left side. The observed behavior shows the same pattern as the
one in Figure 8, but in this case the recrystallization is less appreciable.
Metals 2023,13, 1797 12 of 14
4. Discussion
The SPT results show that this miniaturized technique can be used to estimate and
distinguish the mechanical properties of X65Q steel under different heat treatment condi-
tions. Also, the mechanical properties were compared with the microstructures, and good
correlation was obtained. The specimens with high cooling rates (370
◦
C/s) show a clear
evolution in the microstructure, where martensite was obtained for the specimens with
higher mechanical properties. With the tempering at 420
◦
C, the mechanical properties
decrease, and this can also be observed with the appearance of tempered martensite and
tempered bainite. When tempering at 620
◦
C, the mechanical properties are the lowest, and
the microstructure shows ferrite, which has low hardness.
For the specimens with lower cooling rates of 37
◦
C/s, the mechanical properties of
the different heat treatment temperatures show similar behavior, and there are no major
differences in the microstructures either.
The main advantage of the small sample size is the low volume of material needed
to carry out studies where a wide variety of conditions are analyzed. A study like the
one presented here, using traditional methods such as tensile tests, would require larger
volumes of material, tests, and time. To achieve these results, 60 tensile specimens would
be required, which, with a pre-machining dimension of Ø17
×
20 mm, would result in a
total of 12.7 kg of steel. While using the methodology presented in this paper, the materials
used were two Jominy specimens, with a total of 800 g of steel. This methodology, in
addition to reducing the volume of material used by 94%, simplifies the heat treatments
to be carried out. Each tensile specimen would need an independent heat treatment, and
obtaining the cooling rates obtained with the Jominy specimen is not easy. For this, different
media are needed, such as water, oil, or air at different temperatures. With the presented
methodology, the number of HTs is also reduced, from 20 quenchings and two temperings
to two quenchings and two temperings.
These results also provide relevant information about the mechanical properties of
thick-walled tubes, showing how the properties of the tube can change as a function
of thickness due to cooling rate gradients. With this methodology, the heat treatment
conditions of tubes, flanges, and components of high thickness can be optimized according
to the required mechanical properties for each application.
An example of a practical application where this methodology can be applied is in
the manufacturing process of very thick pipes. During the manufacturing process, the
tubes are heat-treated in furnaces, where they are heated to high temperatures and then
introduced into water to quench them to improve their mechanical properties. The problem
of these heat treatments is controlling the cooling of the tube inward from the outer surface
since the outer surface of the tube is permanently surrounded by water and quenches faster
than the inner surface. When water is introduced inside the tube, steam is generated and is
not readily evacuated from the tube cavity, leading to a steam cushion in some areas that
impedes contact of the inner surface with the quenching water. All these factors generate
inhomogeneous mechanical properties in the tube.
Using the method of the present study combined with heat treatment simulations,
the manufacturing process can be optimized to generate tubes with more homogeneous
mechanical properties. By means of finite elements, is possible to calculate the temperature
of any point in the tube’s thickness after leaving the furnace. Knowing these temperatures
and armed with graphs such as those shown in Figure 7, it is possible to predict the cooling
rates necessary for each zone of the tube, thus allowing one to perform the quenching in
optimal conditions to homogenize the tube.
5. Conclusions
With the obtained results, it can be concluded that:
•
A very good correlation was obtained for the tensile strength in the SPT versus tensile
data obtained from the conversion of the HV test (R2= 0.99).
Metals 2023,13, 1797 13 of 14
•
Estimation of mechanical properties with the SPT technique allows one to distinguish
between changes in different steel heat treatment conditions.
•
This methodology allows one to obtain a lot of information about the material with
very little quantity of material being used in the test.
•
Optimization of heat treatment processes can be carried out with the results obtained
from this methodology.
•
This methodology achieves a 94% reduction in the volume of material used compared
to traditional methods, such as tensile test.
•
The number of HTs employed is also reduced and simplified compared to the tensile
test at a ratio of over 10 to 1.
Author Contributions:
Conceptualization, G.A. and C.A.; methodology, I.M. and I.B.; formal analysis,
I.M.; investigation, I.M., I.B. and G.A.; resources, C.A. and L.M.M.; data curation, I.M. and I.B.;
writing—original draft preparation, I.M. and G.A.; writing—review and editing, I.M., I.B., G.A.,
L.M.M. and C.A.; supervision, G.A. and C.A.; project administration, I.B.; funding acquisition, I.M.
and C.A. All authors have read and agreed to the published version of the manuscript.
Funding:
This work was partially funded by the Basque Government under grant number 004-
B2/2021, corresponding to the BIKAINTEK Program for PhD students. This work was also partially
funded by the Department of Research and Universities of the Basque Government under grant
number IT1542-22.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Acknowledgments:
We would like to highlight the invaluable contribution of Marcelo López-Belver
from Tubos Reunidos Group, who has invested his time and expertise to enhance the quality of
the article.
Conflicts of Interest: The authors declare no conflict of interest.
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