J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
647–655
EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE
SECONDARY HARDENING OF Mo-W TOOL STEEL
VPLIV MIKROLEGIRANJA Nb, Ta IN Ti NA SEKUNDARNO
UTRJEVANJE Mo-W ORODNEGA JEKLA
Jaka Burja
1,2*
, Ale{ Nagode
2
, Klemen Grabnar
2,3
, Jo`ef Medved
2
, Tilen Bala{ko
2
1
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
2
Faculty of Natural Sciences and Engineering, University of Ljubljana, A{ker~eva cesta 12, 1000 Ljubljana, Slovenia
3
Metal Tech Solutions d.o.o., Trubarjeva ulica 91, 3000 Celje, Slovenia
Prejem rokopisa – received: 2024-06-04; sprejem za objavo – accepted for publication: 2024-09-04
doi:10.17222/mit.2024.1210
This work investigates the influence of microalloying elements on the tempering process of a hot-work tool steel. The study ex-
amines the austenitisation temperatures and grain growth of a high thermal conductivity hot-work tool steel, with the addition of
various microalloying elements: Nb+0.06 w/% Nb, Ta+0.03 w/% Ta, and Ti+0.006 w/% Ti, compared to a reference sample.
Thermodynamic calculations are used to analyse the influence of microalloying on the transformation temperatures and carbide
formation. Austenitisation temperatures of (1030, 1060, 1080, and 1100) °C are selected, and subsequent tempering is per-
formed at (540, 580, 600, 620, and 640) °C. The investigation focuses on analysing the microstructure, hardness, and grain size.
The results show that the microalloying elements have a positive influence on the retention of the grain size during
austenitisation and on the enhancement of hardness during tempering. Electron microscopy is utilized to analyse the
microstructure, which indicates the prevalence of Mo-W carbides. The carbides exhibit coarsening and morphological changes
during high-temperature tempering. The secondary hardening peaks occur at temperatures between (600 and 620) °C, and are
more pronounced by microalloying.
Keywords: hot work tool steel, microalloying, tempering, heat treatment, carbides
Raziskovali smo vpliv mikrolegirnih elementov na postopek utrjevanja orodnega jekla za delo v vro~em. Analizirali smo vpliv
temperature avstenitizacije na rast zrn visoko toplotno prevodnega orodnega jekla ob dodatku razli~nih mikrolegirnih
elementov: Nb+0,06 w/% Nb, Ta+0,03 w/% Ta in Ti+0,006 w/% Ti v primerjavi z referen~nim vzorcem. S termodinami~nimi
izra~uni smo analizirali vpliv mikrolegiranja na temperature premen in tvorbo karbidov. Izbrane temperature avstenitizacije so
bile (1030, 1060, 1080 in 1100) °C, nato smo vzorce popu{~ali pri (540, 580, 600, 620 in 640) °C. Analizirali smo
mikrostrukturo, trdote in velikosti zrn. Rezultati ka`ejo, da mikrolegirni elementi pozitivno vplivajo na ohranjanje velikosti zrn
med avstenitizacijo in na pove~anje trdote med popu{~anjem. Z elektronsko mikroskopijo smo analizirali mikrostrukture, v
kateri prevladujejo Mo-W karbidi. Ti karbidi pri visokih temperaturah popu{~anja rastejo in spremenijo obliko. Vrhovi
sekundarnega utrjevanja se pojavijo pri temperaturah med 600 °C in 620 °C ter so bolj izraziti pri mikrolegiranju.
Klju~ne besede: orodno jeklo za delo v vro~em, mikrolegiranje, popu{~anje, toplotna obdelava, karbidi
1 INTRODUCTION
Hot-work tool steels are high-quality steels valued
for their exceptional mechanical properties at elevated
temperatures. These properties are essential for their ap-
plications in demanding environments of high tempera-
tures and high heat fluxes. The combination of strength,
hardness, toughness, wear resistance, thermo-cyclic sta-
bility, and thermal conductivity make hot-work tool
steels indispensable.
1–6
Widely used in the manufacturing
of dies for extrusion, forging, and especially light-metal
die casting, these steels are subjected to severe cyclic
thermal and mechanical stresses, adhesion, abrasion, and
thermal dissolution. Degradation of these steels primar-
ily results from a combination of low-cycle fatigue and
thermal fatigue, arising from frequent temperature fluc-
tuations during service.
7,8
Achieving the desired mechanical properties in
hot-work tool steels requires precise control of the steel’s
chemical composition and the formation of a suitable,
stable microstructure through heat treatment. Tempered
martensite and alloy-carbide precipitates typically form
within the microstructure of these steels.
9–12
The heat-treatment process for hot-work tool steels
involves three stages: austenitisation, quenching, and
tempering. Fine, undissolved carbides, nitrides, or
carbonitrides play a crucial role in the austenitisation
stage by preventing the growth of austenite crystal grains
through their pinning effect at grain boundaries.
5,13,14
Small crystal grains are essential for achieving the de-
sired mechanical properties. Conversely, coarse prior-
austenite crystal grains negatively impact the steel’s
strength and impact toughness, especially at low temper-
atures.
15–17
To suppress grain growth, microalloying elements
(e.g., Nb, Ta, V, Ti) are added to the steel.
5,12,18
Addi-
tionally, alloyed carbide precipitates are instrumental in
Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655 647
UDK 621.785.54:691.714 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(5)647(2024)
*Corresponding author's e-mail:
jaka.burja@imt.si (Jaka Burja)

secondary hardening during the tempering process. The
type of carbide precipitates formed in hot-work tool
steels depends on the steel’s chemical composition and
the tempering temperatures. Common carbide types
found in hot-work tool steels include V-rich (MC), Mo-
and/or W-rich (M
6
C, M
2
C), and Cr-rich (M
3
C, M
7
C
3
, and
M
23
C
6
) carbides.
4,5
However, at temperatures exceeding
the maximum for secondary hardening, alloy carbides
gradually coarsen, and the dislocation density decreases.
Ultimately, the martensitic matrix transforms into a fer-
ritic structure, leading to a significant softening of the
steel.
7
The relationship between steel softening and carbide
coarsening was first described by Hollomon and Jaffe.
19
By delaying carbide coarsening, the deterioration of the
martensitic microstructure can be slowed down.
9,12,20,21
This results in the retention of hardness and high-temper-
ature fatigue strength at higher temperatures and for ex-
tended durations. Consequently, understanding the
microstructure evolution during tempering and service is
crucial for optimizing the tool life.
22
While the effect of microalloying elements (V, Nb, or
Ti) on the quenching and tempering processes of low-al-
loy steels has been extensively studied, their impact on
the tempering process of higher alloy steels exhibiting
secondary hardening peaks remains less under-
stood.
14,23–25
This is particularly true for Mo-W tool steels
with increased thermal conductivity values up to
60 W·mK
–1
, where we have briefly described the effect
in previous papers but have paid more attention to the
thermodynamics and characterization in the current
work.
26–28
The objective of this study is to further investigate the
influence of microalloying elements (Nb, Ta, and Ti) on
the tempering process of Mo-W hot-work steel. The role
of these microalloying elements in the microstructural
evolution and the development of secondary hardening
peaks during tempering will be examined.
2 EXPERIMENTAL PART
A total of four steel compositions were melted and
cast into a 60 × 60 × 400 mm
3
ingot. The first charge was
made as a reference without additional alloying, to better
describe the base properties (steel composition 0). The
second composition had 0.06 w/% Nb added (Nb – Ta-
ble 1), the third composition had 0.03 w/% Ta added (Ta
– Table 1) and the composition had 0.006 w/% Ti added
(Ti – Table 1). The chemical composition (Table 1)was
measured by wet chemical analysis and infrared absorp-
tion after combustion with an ELTRA CS-800.
The ingots were cooled to room temperature in the
air after casting. Then they were annealed at 720 °C for
2 h to relieve stress, and soften for further processing.
Any possible surface defects were ground off, and the in-
got head was cut off to remove the shrinkage porosity.
The ingots were then homogenised at 1200 °C for 1 h
and hot rolled into billets measuring (40 × 40) mm. After
hot rolling, the billets were slowly cooled to room tem-
perature in ceramic wool. The billets were then heated to
1100 °C and hot forged into square bars measuring (18 ×
18) mm. The bars were then slowly cooled to room tem-
perature in ceramic wool. After cooling to room temper-
ature, the samples were ground to remove the oxide
scale. Then the samples were soft annealed for 1.5 h at
770 °C in a vacuum hardening furnace type Ipsen Turbo
XL treater under vacuum.
Then the samples were cut to the dimensions (18 ×
18 × 60) mm. The hardening process was carried out in a
Bosio EUPK 6/1200 electric laboratory furnace without
a protective atmosphere followed by oil quenching. The
samples were placed in the furnace at room temperature,
heated to 650 °C in 1 h and held at 650 °C for 20 min,
then heated to 850 °C in 30 min and held at 850 °C for
20 min, the final heating step to the quenching tempera-
ture was carried out in 45 min and the samples were held
at the quenching temperature for 20 min, before quench-
ing in oil. The quenching temperatures were (1030,
1060, 1080 and 1100) °C. The oil temperature was
60 °C. The samples were then tempered in a vacuum fur-
nace under a vacuum at temperatures of (540, 580, 600,
620 and 640) °C with a holding time of 2.5 h. The sam-
ples were milled to remove 2 mm of the surface layer to
avoid the influence of decarburization.
The metallographic samples were ground, polished
and etched with Nital 5%. The microstructure was ana-
lysed with an Olympus DP70 light microscope and a
Thermo Scientific Quattro S field-emission scanning
electron microscope with energy-dispersive X-ray spec-
troscopy (EDS). Hardness values were measured on all
the investigated samples using Vickers hardness method,
HV10.
The commercial software Thermo-Calc version
2022b was used for the CALPHAD calculations. The
Thermo-Calc Software TCFE10 Steels/Fe-alloys data-
base was selected to obtain the thermodynamic data for
the calculations. We used the Equilibrium Calculator and
selected the Property Diagram calculation type, from
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
648 Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655
Table 1: Chemical compositions of the experimental charges (w/%)
Sample C Si Ni Mo W Nb Ta Ti Fe
0 0.32 0.04 0.03 3.2 1.7 / / / balance
Nb 0.32 0.05 0.03 3.2 1.7 0.06 / / balance
Ta 0.32 0.05 0.03 3.2 1.7 / 0.03 / balance
Ti 0.32 0.04 0.03 3.2 1.7 / / 0.006 balance

which we obtained diagrams showing the amount of
thermodynamically stable phases in the samples studied.
3 RESULTS AND DISCUSSION
3.1. CALPHAD calculations
The CALPHAD calculations (Figures 1–4) show the
amount of thermodynamically stable phases in the ana-
lysed samples in the temperature range between 500 and
1300 °C. Table 2 also lists all transformation and precip-
itation temperatures for all the investigated samples. In
the reference sample 0 (Figure 1), the M6C ((Mo, Fe,
W)
6
C) carbides start to precipitate first at 1087 °C, fol-
lowed by MC (WC) carbides at 810 °C. At 858 °C we
have the Ae
3
at which the transformation from austenite
to ferrite begins. The transformation ends at 797 °C, i.e.,
at the A
e1
temperature.
In the sample Nb (Figure 2), the MC (NbC) carbides
start to precipitate first at 1253 °C, followed by M6C
((Mo, Fe, W, Nb)
6
C) carbides at 1096 °C and the last
carbides to precipitate are also MC (WC) carbides at 809
°C. At 860 °C we have the A
e3
temperature at which the
transformation from austenite to ferrite begins. The
transformation ends at 797 °C, i.e., at the A
e1
tempera-
ture.
In the Ta sample (Figure 3), the MC carbides (TaC)
begin to precipitate first at 1172 °C, followed by the
M6C carbides ((Mo, Fe, W, Ta)
6
C) at 1087 °C and the
MC carbides (WC), which precipitates last at 810 °C. At
859 °C, we have the A
e3
temperature at which the trans-
formation from austenite to ferrite begins. The transfor-
mation ends at 797 °C, i.e., at the A
e1
temperature.
In the Ti sample (Figure 4), the M6C ((Mo, Fe, W,
Ti)
6
C) carbides start to precipitate first at 1087 °C, fol-
lowed by MC (TiC) carbides at 1057 °C and the last car-
bides to precipitate are also MC (WC) carbides at 810
°C. At 859 °C, we have the A
e3
temperature at which the
transformation from austenite to ferrite begins. The
transformation ends at 797 °C, which corresponds to the
A
e1
temperature.
Table 2 lists all the equilibrium phases that are stable
in the analysed samples. The Thermo-Calc designations
are given with the corresponding phase and precipitation
or transformation temperatures. It is obvious that the ad-
dition of microalloying elements (Nb, Ta and Ti) has no
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655 649
Figure 4: CALPHAD calculations of the equilibrium amount of
phases in sample Ti between 500 °C and 1300 °C
Figure 1: CALPHAD calculations of the equilibrium amount of
phases in sample 0 between 500 °C and 1300 °C
Figure 3: CALPHAD calculations of the equilibrium amount of
phases in sample Ta between 500 and 1300 °C
Figure 2: CALPHAD calculations of the equilibrium amount of
phases in sample Nb between 500 °C and 1300 °C

influence on the transformation temperature between fer-
rite and austenite (A
e3
and A
e1
). It also has no influence
on the precipitation temperature MC (WC), which is ba-
sically the same for all the samples analysed. There is a
slight influence on the precipitation temperature of the
M6C carbides, with the addition of Nb increasing the
temperature by 9 °C. In the contrast, the addition of Ti
and Ta has no effect on the precipitation temperature of
the M6C carbides.
29,30
The main difference is the precipi-
tation of additional carbides based on the microalloying
elements. In the Nb sample, there is an additional precip-
itation of MC (NbC) carbides, which begins to precipi-
tate at 1253 °C. The same applies to the Ta and Ti sam-
ples, where TaC carbides begin to precipitate at 1172 °C
and TiC at 1057 °C.
Table 1: Precipitation temperatures for carbides and austenite – ferrite
transformation temperatures (A
e3
and A
e1
) for all investigated samples
in the temperature range between 500 and 1300 °C.
Sample
Thermo-Calc
designation
Phase
Precipitation or
transformation
temperature (°C)
0
FCC_A1#2 / /
M6C_E93 (Mo, Fe, W)
6
C 1087
MC_SHP WC 810
BCC_A2 ferrite ( ) 858–A e3
FCC_A1 austenite ( ) 797–A
e1
Nb
FCC_A1#2 NbC 1253
M6C_E93
(Mo, Fe, W,
Nb)
6
C
1096
MC_SHP WC 809
BCC_A2 ferrite ( ) 860–A e3
FCC_A1 austenite ( ) 797– A
e1
Ta
FCC_A1#2 TaC 1172
M6C_E93
(Mo, Fe, W,
Ta)
6
C
1087
MC_SHP WC 810
BCC_A2 ferrite ( ) 859–A e3
FCC_A1 austenite ( ) 797–A
e1
Ti
M6C_E93
(Mo, Fe, W,
Ti)
6
C
1087
FCC_A1#2 TiC 1057
MC_SHP WC 810
BCC_A2 ferrite ( ) 859–A e3
FCC_A1 austenite ( ) 797–A
e1
3.2. Quenching and tempering
Samples were quenched from austenitising tempera-
tures of (1030, 1060, 1080 and 1100) °C. The austeni-
tisation holding time was 25 min. The samples were then
tempered for two hours at different temperatures (540,
580, 620 and 640) °C. Under all conditions, a bainitic-
martensitic microstructure developed after quenching.
Furthermore, all samples were free of primary carbide.
The hardness increases in most cases with increasing
austenitisation temperature (Figure 5). At an austeniti-
sation temperature of 1030 °C, the sample without added
microalloying elements (0) reaches an average hardness
value of 425 HV10. The addition of niobium (Nb) and
tantalum (Ta) has no effect on the increase in hardness at
the selected austenitisation temperature. The addition of
titanium (Ti) also proved effective even at a lower tem-
perature and caused an increase in the hardness of the
steel after quenching to an average value of 446 HV10.
The relatively low hardness after quenching is attributed
to a relatively high amount of bainite in the micro-
structure.
Increasing the austenitisation temperature to 1060 °C
did not increase the hardness of the sample without
added microalloying elements (0) and it remained at
425 HV10. A greater effect on the hardness increase is
observed in the case of niobium additions (Nb), where
the hardness increases to an average value of 459 HV10.
An even greater increase in hardness after quenching was
obtained in the case of the sample alloyed with tantalum
(Ta), where the hardness after quenching is 468 HV10.
The addition of titanium (Ti) at an austenitisation tem-
perature of 1060 °C had no significant effect on the in-
crease in hardness.
At an austenitisation temperature of 1080 °C, the
hardness of the sample without added microalloying ele-
ments (0), increases to 484 HV10. This increase is re-
lated to an increase in the concentration of carbon and
dissolved alloying elements in the steel matrix. The trend
of increasing hardness after quenching is also observed
when the sample is alloyed with titanium (Ti). The hard-
ness increases to 461 HV10. The addition of niobium
(Nb) and tantalum (Ta) at the austenitisation temperature
had no significant effect on the increase in hardness.
This remains the same at the values obtained as at the
lower austenitisation temperature of 1060 °C.
When the austenitisation temperature rises to
1100 °C, a smaller drop in hardness to 473 HV10 is ob-
served in the unmodified sample (0). At the selected tem-
perature, the addition of niobium (Nb) has the greatest
effect on the increase in hardness, which after quenching
is 502 HV10, which is also the highest hardness
achieved. The addition of titanium (Ti) also increases the
hardness to a value of 485 HV10. The addition of tanta-
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
650 Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655
Figure 5: Hardness as a function of austenitisation temperature

lum (Ta) has the least effect on the increase of hardness
among all the added alloying elements at the selected
austenitisation temperature. The average hardness value
is 475 HV10 and this is higher than the hardness of the
reference sample (0).
The increase in the austenitisation temperature af-
fected the increase in the size of the primary austenite
crystal grains (Figure 6), especially of the reference
sample (0) and the titanium-alloyed sample (Ti). Sam-
ples alloyed with niobium (Nb) and with tantalum (Ta)
had a lesser effect on growth. At the highest austeniti-
sation temperature of 1100 °C, all samples developed a
coarse-grained microstructure. The hardness is most in-
fluenced by the change in the chemistry of the austenite
matrix before quenching, the resulting bainite/martensite
ration and the grain size.
The addition of niobium (Nb) and tantalum (Ta) had
a large influence on the size of the crystal grains (Fig-
ure 6), which is also consistent with findings from litera-
ture sources.
26,29,30
Due to the small amount added, the
addition of titanium (Ti) has been shown to be partially
effective at lower austenitisation temperatures of up to
1060 °C, but has no major effect at higher temperatures.
The austenitisation temperature range considered was
between 1030 °C and 1100 °C. At the lowest austeniti-
sation temperature selected, all the samples reach the
same crystal grain size, i.e., about 30 μm, while the
coarse grain size at 1100 °C was 90 μm, as shown in Fig-
ure 7. Increasing in the austenitisation temperature leads
to the growth of the crystal grains, although there are
large differences between the samples due to the influ-
ence of the additional alloying elements. In the case of
sample (0) used as a reference, the more intensive
growth of the crystal grains already begins at an
austenitisation temperature of 1060 °C, at which the size
increase more than doubles, i.e., from 30 μm to 65 μm.
The effect of the titanium addition (Ti) has a lesser effect
on inhibiting the growth of the crystal grains up to a tem-
perature of 1060 °C, where the grains are 45 μm in size
compared to the reference sample (0). As the austeniti-
sation temperature increases, the effect of titanium on
inhibiting the crystal growth decreases. The addition of
tantalum (Ta) and niobium (Nb) successfully inhibits the
growth of crystal grains up to a temperature of 1080°C,
which remains unchanged in the case of niobium (Nb),
i.e., 30 μm in size. Tantalum (Ta) also has a very similar
effect to niobium (Nb), although the crystal grains in the
Ta sample only partially grow to a size of 38 μm. The
reason for the smaller crystal grain sizes is related to the
formation of niobium- and tantalum-based precipitates or
carbides, which precipitates at growth-inhibiting crystal
grain boundaries. The addition of niobium proved to be
most effective in inhibiting the crystal grain growth, even
up to a temperature of 1100 °C. A more detailed discus-
sion on the grain-growth mechanism in the samples is
available in our previous work.
5
The samples were hardened at austenitising tempera-
tures of (1030, 1060, 1080 and 1100) °C and tempered at
temperatures of (540, 580, 600, 620 and 640) °C, which
are most commonly used to temper tools to the required
hardness. As mentioned before, the tempering times
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655 651
Figure 7: Representative microstructures of a) fine grain sample Nb – 1030 °C and b) coarse grain sample Ti – 1100 °C
Figure 6: Grain size as a function of austenitisation temperature
5

were 2.5 h after reaching the temperature. With increas-
ing tempering temperature, a secondary hardness peak
was reached in all the samples. With increasing
austenitisation temperature, the hardness after tempering
and the value of the secondary peak hardness also in-
creased. The highest secondary peak hardness values
were achieved in the austenitisation temperature interval
between 1080 °C and 1100 °C.
The alloying elements niobium, tantalum and tita-
nium had an influence on the increase of the strength
properties in the temperature range between (540 and
640) °C.
At an austenitisation temperature of 1030 °C (Fig-
ure 8), the Ti sample exhibits the maximum secondary
peak hardness of 506 HV10. On the other hand, the Ta
sample has the highest yield strength at 640 °C with a
hardness value of 444 HV10. A predominantly bainitic
microstructure with martensite was achieved by quench-
ing from 1030 °C, as shown in Figure 12a and 12b. The
samples that were quenched from 1030 °C contained the
most bainite of all the samples.
At the austenitisation temperature of 1060 °C (Fig-
ure 9), the maximum secondary peak hardness of the Nb
sample is 533 HV10. Furthermore, this sample has the
highest yield strength at 640 °C with a hardness value of
464 HV10. The microstructure contained both bainite
and martensite; however, the bainite is still dominant
(Figure 12c and 12d).
At an austenitisation temperature of 1080 °C (Figure
10), the maximum secondary peak hardness of the Ti
sample is 540 HV10. Furthermore, this sample has the
highest yield strength at 640 °C with a hardness value of
493 HV10. The martensite content is higher than at 1030
°C or 1060 °C, but the sample still contains a fair amount
of bainite (Figure 12e and 12f).
At an austenitisation temperature of 1100 °C (Figure
11), the maximum secondary peak hardness of Nb and Ti
samples is 542 HV. On the other hand, the Ta sample has
the highest yield strength at 640 °C with a hardness
value of 504 HV10. The bainite content is diminished, as
the martensite becomes the dominant phase, as seen in
Figure 12g and 12h.
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
652 Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655
Figure 10: Tempering diagram for all the samples at austenitisation
temperature of 1080 °C
Figure 8: Tempering diagram for all the samples at austenitisation
temperature of 1030 °C
Figure 11: Tempering diagram for all the samples at austenitisation
temperature of 1100 °C
Figure 9: Tempering diagram for all the samples at austenitisation
temperature of 1060 °C

J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655 653
Figure 12: Microstructures of corresponding samples: a) and b) sample 0, c) and d) sample Nb, e) and f) sample Ti and g) and h) sample Ta

As the tempering temperatures rise the carbides
coarsen and become rounder, as seen in Figure 13. Fur-
thermore, tempering eliminates most of the needle-like
carbides that form during quenching, mainly in the
bainite and self-tempered martensite, as the carbon is
combined with Mo-W carbides.
5
The changes in mor-
phology are especially evident at 640 °C, where the typi-
cal round carbides, that have a very limited hardening
ability, occur. These carbides are mostly found in tem-
pered martensite.
4 CONCLUSIONS
The following conclusions can be drawn:
• Microalloying elements (Nb, Ta, and Ti) have a posi-
tive effect on grain-size retention during austeniti-
sation and on the enhancement of hardness during
tempering of Mo-W hot-work tool steel.
• CALPHAD calculations showed that the addition of
microalloying elements influences the precipitation
of additional carbides, depending on the element
added (NbC, TaC, TiC).
• All samples exhibited a bainitic-martensitic micro-
structure after quenching and were free of primary
carbides.
• The hardness increased in most cases with increasing
austenitisation temperature.
• The addition of Nb and Ta showed the greatest effect
on inhibiting the growth of austenite grains during
austenitisation.
• The Ti addition had a limited effect on grain size re-
finement compared to Nb and Ta.
• Secondary hardening peaks were observed at temper-
ing temperatures between 600 °C and 620 °C, and
were more pronounced with microalloying.
5 REFERENCES
1
A. Eser, C. Broeckmann, C. Simsir, Multiscale modeling of temper-
ing of AISI H13 hot-work tool steel - Part 1: Prediction of
microstructure evolution and coupling with mechanical properties,
Comput. Mater. Sci., 113, (2016), 280–291, doi:10.1016/
j.commatsci.2015.11.020
2
J.Y. Li, Y. L. Chen, J. H. Huo, Mechanism of improvement on
strength and toughness of H13 die steel by nitrogen, Mater. Sci. Eng.
A, 640, (2015), 16–23, doi:10.1016/j.msea.2015.05.006
3
R. Wu, W. Li, M. Chen, S. Huang, T. Hu, Improved mechanical
properties by nanosize tungsten-molybdenum carbides in tungsten
containing hot work die steels. Mater. Sci. Eng. A 812 (2021)
141140, doi:10.1016/j.msea.2021.141140
4
R. Marke`i~, N. Mole, I. Nagli~, R. [turm, Time and temperature de-
pendent softening of H11 hot-work tool steel and definition of an
anisothermal tempering kinetic model. Mater. Today Commun. 22
(2020), doi:10.1016/j.mtcomm.2019.100744
5
K. Grabnar, J. Burja, T. Bala{ko, A. Nagode, J. Medved, The influ-
ence of Nb, Ta and Ti modification on hot-work tool-steel grain
growth during austenitization. Mater. Technol. 56 (2022) 331–338,
doi:10.17222/mit.2022.486
6
M. Von~ina, T. Bala{ko, J. Medved, A. Nagode, Interface Reaction
between Molten Al99.7 Aluminum Alloy and Various Tool Steels,
Materials, 14 (2021) 24, 7708, doi:10.3390/ma14247708
7
B. C. De Cooman, J. G. Speer, Fundamentals of Steel Product Physi-
cal Metallurgy; AIST, Association for Iron & Steel Techno-logy:
Warrendale, PA, USA, 2011
8
B. Podgornik, M. Sedla~ek, B. @u`ek, A. Gu{tin, Properties of tool
steels and their importance when used in a coated system, Coatings,
10 (2020) 3, 265, doi:10.3390/coatings10030265
9
Q. Zhou, X. Wu, N. Shi, J. Li, N. Min, Microstructure evolution and
kinetic analysis of DM hot-work die steels during tempering. Mater.
Sci. Eng. A, 528 (2011), 5696–5700, doi:10.1016/j.msea.2011.
04.024
10
A. Medvedeva, J. Bergström, S. Gunnarsson, J. Andersson, High-
temperature properties and microstructural stability of hot-work tool
steels. Mater. Sci. Eng. A, 523 (2009), 39–46, doi:10.1016/
j.msea.2009.06.010
11
N. B. Dhokey, S. S. Maske, P. Ghosh, Effect of tempering and cryo-
genic treatment on wear and mechanical properties of hot work tool
steel (H13). Mater. Today Proc., 43 (2021), 3006–3013,
DOI:10.1016/J.MATPR.2021.01.361
12
E. Cabrol, C. Bellot, P. Lamesle, D. Delagnes, E. Povoden-
Karadeniz, Experimental investigation and thermodynamic modeling
of molybdenum and vanadium-containing carbide hardened
iron-based alloys. J. Alloys Compd., 556 (2013), 203–209,
doi:10.1016/j.jallcom.2012.12.119
13
K. Chen, Z. Jiang, F. Liu, J. Yu, Y. Li, W. Gong, C. Chen, Effect of
quenching and tempering temperature on microstructure and tensile
properties of microalloyed ultra-high strength suspension spring
steel. Mater. Sci. Eng. A, 766 (2019), doi:10.1016/j.msea.2019.
138272
14
G. Yang, X. Sun, Z. Li, X. Li, Q. Yong, Effects of vanadium on the
microstructure and mechanical properties of a high strength low al-
loy martensite steel. Mater. Des., 50 (2013), 102–107, doi:10.1016/
j.matdes.2013.03.019
15
C. Zhang, Q. Wang, J. Ren, R. Li, M. Wang, F. Zhang, K. Sun, Effect
of martensitic morphology on mechanical properties of an
as-quenched and tempered 25CrMo48V steel. Mater. Sci. Eng. A,
534 (2012), 339–346, doi:10.1016/j.msea.2011.11.078
16
C. Wang, M. Wang, J. Shi, W. Hui, H. Dong, Effect of micro-
structural refinement on the toughness of low carbon martensitic
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
654 Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655
Figure 13: Coarsening of carbide microstructure of steel Ta hardened from 1100 °C, tempered at 540 °C

steel. Scr. Mater., 58 (2008), 492–495, doi:10.1016/j.scriptamat.
2007.10.053
17
O. Haiko, V. Javaheri, K. Valtonen, A.; Kaijalainen, J. Hannula, J.
Kömi, Effect of prior austenite grain size on the abrasive wear resis-
tance of ultra-high strength martensitic steels. Wear, 454–455 (2020),
13–16, doi:10.1016/j.wear.2020.203336
18
A. Karmakar, S. Kundu, S. Roy, S. Neogy, D. Srivastava, D.
Chakrabarti, Effect of microalloying elements on austenite grain
growth in Nb-Ti and Nb-V steels. Mater. Sci. Technol., 30 (2014),
653–664, doi:10.1179/1743284713Y.0000000386
19
J. H. Hollomon, L. D. Jaffe, Time–temperature relations in tempering
steels, Transactions of the American Institute of Mining and Metal-
lurgical Engineers, 162 (1945), 223–249
20
J. Mazurkiewicz, L. A. Dobrzañski, E. Hajduczek, Comparison of
the secondary hardness effect after tempering of the hot-work tool
steels, Journal of Achievements in Materials and Manufacturing En-
gineering, 24 (2007), 119–122
21
V. Leskov{ek, B. [u{tar{i~, G. Jutri{a, The influence of austenitizing
and tempering temperature on the hardness and fracture toughness of
hot-worked H11 tool steel. J. Mater. Process. Technol., 178 (2006),
328–334, doi:10.1016/j.jmatprotec.2006.04.01
22
H. Yang, J. H. Zhang, Y. Xu, M. A. Meyers, Microstructural charac-
terization of the shear bands in Fe-Cr-Ni single crystal by EBSD. J.
Mater. Sci. Technol., 24 (2008), 819–828
23
J. Janovec, M. Svoboda, A. Výrostková, A. Kroupa, Time-tempera-
ture-precipitation diagrams of carbide evolution in low alloy steels.
Mater. Sci. Eng. A, 402 (2005), 288–293, doi:10.1016/j.msea.2011.
03.086
24
J. G. Jung, J. S. Park, J. Kim, Y. K. Lee, Carbide precipitation kinet-
ics in austenite of a Nb-Ti-V microalloyed steel. Mater. Sci. Eng. A,
528 (2011), 5529–5535
25
J. Dong, X. Zhou, Y. Liu, C. Li, C. Liu, Q. Guo, Carbide precipita-
tion in Nb-V-Ti microalloyed ultra-high strength steel during temper-
ing. Mater. Sci. Eng. A, 683 (2017), 215–226, doi:10.1016/
j.msea.2016.12.019
26
J. Burja, A. Nagode, K. Grabnar, J. Medved, T. Bala{ko, Effect of
Microalloying on Tempering of Mo-W High Thermal Conductivity
Steel. Metallurgia Italiana, 114 (2023) 22–27
27
E. Kaschnitz, P. Hofer, W. Funk, Thermophysical properties of a
hot-work tool-steel with high thermal conductivity, Int. J.
Thermophys., 34 (2013) 5, 843–850, doi:10.1007/s10765-012-
1162-8
28
I. Valls, A. Hamasaiid, A. Padré, High thermal conductivity and high
wear resistance tool steels for cost-effective hot stamping tools, J.
Phys. Conf. Ser., 896 (2017) 012046, doi:10.1088/1742-6596/896/
1/012046
29
J. Chen, C. Liu, Y. Liu, B. Yan, H. Li, Effects of tantalum content on
the microstructure and mechanical properties of low-carbon RAFM
steel. J. Nucl. Mater, 479 (2016), 295–301, doi.:10.1016/j.jnucmat.
2016.07.029
30
J. Foder, J. Burja, G. Klan~nik, Grain size evolution and mechanical
properties of Nb, V–Nb, and Ti–Nb boron type S1100QL steels.
Metals, 11 (2021) 1–16, doi:10.3390/met11030492
J. BURJA et al.: EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 647–655 655