UDK 669.14.018.252:539.42	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 45(1)67(2011)
FRACTURE TOUGHNESS OF THE VACUUM-HEAT-TREATED SPRING STEEL 51CrV4
LOMNA ŽILAVOST VAKUUMSKO TOPLOTNO OBDELANEGA VZMETNEGA JEKLA 51CrV4
Bojan Sencic1, Vojteh Leskovšek2
1Štore Steel, d. o. o., Železarska cesta 3, SI-3220 Štore, Slovenia 2 Institute of metals and technology, Lepi pot 11, 1000 Ljubljana, Slovenia bojan.sencic@store-steel.si
Prejem rokopisa - received: 2011-01-12; sprejem za objavo - accepted for publication: 2011-01-17
In the work the possibilities of the vacuum heat treatment of spring steel grade 51CrV4 are presented. Charpy-V notch (CVN) impact-test values are widely used in toughness specifications for spring steels, even though the fracturing energy is not directly related to the spring design. The plain-strain stress-intensity factor (KIc) at the onset of unstable crack growth can be related to the spring design; however, Kic values are not used in the toughness specifications. This is surprising since to the designer Kic values are more useful than CVN values, because the design calculations for springs from high-strength steels should also take into account the strength and the toughness of materials to prevent rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture-toughness testing (ASTM E399-90), which is difficult to perform reliably for hard and low ductility materials, could be replaced with a non-standard testing method using circumferentially notched and fatigue-precracked tensile specimens. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell-C hardness -Fracture toughness KIc - Tempering temperature) for the vacuum-heat-treated spring steel grade 51CrV4. Fractographic and metallographic analyses of the KIc-test specimens used shows in steel the presence of positive and negative segregations. It was found that the width of the segregations bands and the distance between the positive and negative segregations influence significantly the fracture toughness due to the presence of bainite in the negative segregations.
Keywords: spring steels, vacuum heat treatment, hardness, fracture toughness, microstructures
V članku so predstavljene možnosti vakuumske toplotne obdelave vzmetnega jekla 51CrV4. Vrednosti Charpy-V udarne žilavost (CVN) so pogosto navedene v specifikacijah za vzmetna jekla, čeprav energija loma ni neposredno povezana z dimenzioniranjem vzmeti. Ravninsko deformacijski faktor intenzitete napetosti (KIc) na začetku nestabilne razpoke lahko uporabimo pri dimenzioniranju vzmeti, vendar pa vrednosti Kic ne najdemo v specifikacijah. To je presenetljivo, saj so za konstruktorje vrednosti Kic bolj uporabne od vrednosti CVN za visokotrdnostne vzmeti, pri katerih je treba upoštevati poleg trdnosti tudi žilavost materiala, da preprečimo lom. Opravljena je bila preiskava, s katero smo želeli ugotoviti, ali lahko standardizirano preizkušanje lomne žilavosti (ASTM E399-90), ki je težko izvedljivo za trde in krhke materiale z majhno duktilnostjo, nadomestimo z nestandardnim postopkom preizkušanja lomne žilavosti s cilindričnim nateznim preizkušancem z zarezo po obodu in utrujenostjo razpoko v dnu zareze. Rezultati raziskave so pokazali, da s predlagano metodo lahko konstruiramo diagram popuščanja (trdota Rockwell-C - lomna žilavost KIc - temperatura popuščanja) za vakuumsko toplotno obdelano vzmetno jeklo 51CrV4. S fraktografsko in metalografsko analizo Kic preizkusnih vzorcev smo ugotovili prisotnost pozitivnih in negativnih izcej v jeklu. Ugotovljeno je bilo, da širina izcej in razdalja med njimi pomembno vplivata na lomno žilavost, in sicer zaradi prisotnosti bainita v negativnih izcejah.
Ključne besede: vzmetno jeklo, vakuumska toplotna obdelava, trdota, lomna žilavost, mikrostruktura
of springs, the information relating to the heat treatment 1 INTRODUCTION	of a specific spring steel is important. Assuming that the
chemical composition and initial microstructure of the
A manufacturer of spring steels must provide a tech-	-.jf ,, jn^^r^t
steel correspond to prescribed for steel grade 51CrV4
nical steel description in their production program.
_ ^ ^u A ur» f^u ■ ■ V aL %	(DIN 17221 and DIN 17222), then the mechanical Namely, the durability of the springs is limited by plastic
deformation, fatigue and fracturing. From this point of	properties for a specific application depend mainly on
view, the use of spring steel with the following properties	the appropriately selected parameters of the heat treat-
is recommended: high ductility and toughness at ope-	ment.
rating temperatures from-40 °C to+50 °C, good harde- Hypo-eutectoid steels, such as the spring steel
nability that provides the required mechanical properties,	5lCrV4, are usually heat treated conventionally in a
even at the maximum dimensions.	furnace with or without a protective atmosphere at a
Steels with a similar chemical composition may	temperature of 30 °C to 50 °C above the Ac3 point soaked
behave differently due to various mechanical properties	at the austenizing temperature to obtain "homogeneous
as a consequence of the manufacturing route. The	austenite", and oil cooled to the temperature of the
description of the spring steel includes the chemical	quench oil. The quenching is followed by a tempering at
composition and the basic mechanical and physical as	a selected temperature. The conventional tempering
well as technological properties. For the manufacturers	diagram for spring steel gives us information about the
mechanical properties, i.e., the tensile strength Rm, yield strength Rp0.2, elongation A5 and necking Z as a function of the tempering temperature in the range from 350 °C to 700 °C for a specific austenitizing temperature. In the case of spring steels, a minimum impact toughness Charpy-V measured by standard CVN-specimens, must be given. Lately, more and more users also request measured values of the fracture toughness Kic for the springs after heat treatment, which with the known ultimate tensile strength allows us to calculate the fracture stress Of and the critical defect size Ua at the applied stress.
The trend of the heat treatment of machine parts, including springs, is in the direction of vacuum heat treatment. Namely, cooling rates that can be achieved in modern vacuum furnaces with cooling in a stream of N2 or in a mixture of He and N2 under a pressure up to 25 bar, already reach cooling speeds close to the cooling rates in oil. One can expect that the obtained micro-structure and mechanical properties of the springs after vacuum quenching and tempering will be comparable to those achieved with a conventional heat treatment. In vacuum heat treatment, there is no risk of decarburi-zation, no formation of the oxide layer and no residues of quench oil on the surface. It offers then a significant advantage when compared to conventional heat treatment. The objective of our work was to create a specific tempering diagram for the investigated steel 51CrV4 from which the relations between the Rockwell-C hardness, the fracture toughness KIc, the austenizing and the tempering temperature could be determined.
2 THEORETICAL PART
2.1 Hardness, ductility and toughness
Factors contributing to the isotropic mechanical properties of steel are a high cleanliness, a low level of residual elements, of non-metallic inclusions (NMI) and primary carbides without segregations. This can be achieved with an appropriate process route of the continuous cast spring steel through the Clean Steel Concept1 using the Continuous Soft Reduction (CSR)2 process.
The properties used to evaluate the heat treatment of spring steel are the Rockwell-C or Brinell hardness. In the hardened state the hardness may be an indication of the austenitizing temperature, from which the spring steel has been quenched. In the tempered state, the hardness is important for users, although based on the hardness, it is not possible to distinguish between the springs that were quenched and tempered in various ways. For example, the same hardness can be achieved by varying the austenitizing and tempering temperature, but the toughness can be different. From this point of view, it has sense to introduce into the tempering diagram an additional criterion such as toughness. Unfortunately, for the determination of toughness there are no
generally accepted test methods. Consequently, the literature often refers to the data determined by different test methods. The comparison of these data may sometimes lead to confusing discussions.
The Charpy-V notch (CVN) impact test is widely used in toughness specifications for spring steels, even though the fracturing energy is not directly related to the spring design. The user requirements for springs are in the direction of the operation of springs at a higher tensile strength (1800 MPa and more), which allows the use of only a single parabolic leaf spring, resulting in a lower weight of the vehicle. However, with such a high strength, the toughness is low, and therefore it is necessary to measure the toughness of such springs with a more sensitive method than the Charpy-V impact toughness. The plain-strain stress-intensity factor (Kic) at the onset of unstable crack growth can be related to the spring design; however, the test values are not used in the toughness specifications.
Introducing a linear elastic fracture mechanic (LEFM) for the evaluation of the heat treatment of springs makes it necessary to draw a distinction between the ductility and the toughness. In this context, the ability of a material to resist the initiation and spread of fracture at gross plastic deformation is ductility and the correct meaning of the toughness is the ability to resist the growth of an existing crack at a tensile load 3,4. The ductility and toughness are, consequently, two different material properties, even though both, unfortunately, are denominated as toughness. The opposite of both properties is, however, the same, i.e., brittleness.
The most reliable measure of toughness is the plain-strain fracture toughness (KIc)5. The minimum size of the specimens depends on the yield stress and the fracture toughness of the material, both of which are required for the plain-strain deformation. A fatigue crack of defined length is propagated from a mechanical notch in the specimens, ensuring that the notch effect is at a maximum and equal for all tests. The same value of fracture toughness should be found for tests on specimens of the same material with different geometries and with a critical combination of crack size and shape and fracture stress. Within certain limits, this is indeed the case, and information about the fracture toughness obtained under standard conditions can be used to predict the failure for different combinations of stress and crack size, and for different geometries6.
To determine the fracture toughness KIc, of spring steels, standard CT (Compact Tension) and SENB (Single Edge Notch Bend) specimens and non-standard circumferentially notched and fatigue-precracked tensile-test specimens (KIc-test specimen) can be used7.
2.2 Relationship between ultimate fracture stress and
defect size
In metal materials there are always defects present, with their importance increasing with increasing strength
and reducing ductility and toughness. In springs the two-dimensional defects are critical. The fracture strength of brittle materials depends on the size and the orientation of the defect. It usually decreases rapidly with the increasing defect size. If LEFM can be applied to calculate the material behaviour, the fracture strength can be calculated with the following equation8-10(1): KIC
o f =,- for: CTf< CTys	(1)
Y wn-a
KIc fracture toughness a defect size
of fracture stress (tensile strength) oys yield stress
Y correction factor (depending on the geometry: Y = 1.11 for surface cracks)
Equation (1) can be applied if fracture occurs prior to reaching the yield stress. As spring steels are relatively hard (brittle) materials, they can be compared to tool steels with low ductility and toughness. For this reason LEFM can be applied over a wide range of defect sizes.
According to the literature, equation (1) is suitable for describing the fracture behaviour of tool steels for a typical crack-nucleating defect size (e.g., carbide diameter, NMI) above 10 pm to 20 pm. In the case of conventional manufactured tool steels with a banded microstructure, the width of the carbide agglomerates has to be taken as the critical defect size and it has been further noted that the fracture occurs along a path with a high carbide density. The fracture toughness of conventionally produced tool steels depends, therefore, on the specimen orientation8. It is highest for a crack propagation perpendicular to the rolling direction and it is lowest for a crack propagation in the rolling direction.
If the fracture stress according to equation (1) approaches the yield stress (e.g., if no larger crack-nucleating particles are present) elastic-plastic methods have to be applied. In this region the material behaviour
can be described with equation10 (2), if the ultimate fracture strength is known.
Figure 1: Circumferentially notched and fatigue-precracked KIc-test specimen. All dimensions are in mm.
Slika 1: Cilindrični natezni preizkušanec za merjenje lomne žilavosti z zarezo po obodu in utrujenostno razpoko v dnu zareze. Vse dimenzije so v milimetrih.
o f =n o U • cos
exp
-n
K
8• oU • a
(2)
Ou final tensile fracture stress (tensile strength Rm)
For a large defect size equation (2) approximates the results from equation (1). Deviations from equation (1) are found for small defect sizes. Especially for very small defect sizes, the influence of the defect size is almost negligible. A decrease of the fracture strength is observed if a critical defect size is exceeded.
3 EXPERIMENTAL
3.1 Hardness and fracture-toughness tests
The Rockwell-C hardness (HRc) was measured on individual groups of Kjc-test specimens using a Wilson 4JR hardness machine. Circumferentially notched and fatigue-precracked tensile-test specimens11 with the dimensions indicated in Figure 1 were used for the investigation.
The advantage of the KIc-test specimens used here over standardized CT specimens (ASTM E399-90) is in the radial symmetry, which makes them particularly suitable for studying the influence of the microstructure of metallic materials on the fracture toughness. The advantage of these specimens is related to heat transfer, ensuring a completely uniform microstructure.
Due to the high notch sensitivity of hard and brittle metallic materials, such as continuous casted spring steel grade 51CrV4, it is very difficult - sometimes even almost impossible - to create a fatigue crack in the test specimen. However, with KIc-test specimens the fatigue crack can be created with rotating-bending loading before the final heat treatment11; the second advantage of such test specimens is that plain-strain conditions can be achieved using specimens with smaller dimensions than those of conventional CT test specimens12.
For the linear elastic behaviour up to fracture of such specimens13 the following equation is applied:
KIC D 3/2
^	D
-1.27+1.72-d
(3)
where P is the load at failure, D is the outside diameter, and d is the notched-section diameter of the test specimen, i.e., the diameter of the ligament next to the crack. Equation (3) is valid as long as the condition 0.5 < d/D < 0.8 is fulfilled.
Measurements of the fracture toughness were performed at room temperature using an Instron 1255 tensile-test machine. The cross-head speed was 1.0 mm/min for standard tensile tests on specimens with a nominal
Y
Table 1: Chemical composition of continuous-cast spring steel grade 51CrV4 in mass fractions, w/% Tabela 1: Kemična sestava kontinuirno litega vzmetnega jekla 51CrV4 v masnih deležih, w/%
Steel grades 51CrMoV4	C	Si	Mn	P		Cr	Mo	V	Ni	AL	Cu	Ti	Sn	Ca	N
Heat 1 Heat 2	0.49 0.50	0.30 0.33	0.93 0.94	0.008	0.004 0.008	0.97 1.00	0.08	0.1	0.11 0.14	0.006 0.009	0.17 0.20	0.018 0.019	0.011 0.010	0,0010 0.0015	0.013
test length of 100 mm. In the tests two specially prepared cardan fixed jaws, ensuring the axiality of the tensile load, were used. During the tests the tensile-load/ displacement relationship until failure was recorded. In all cases this relationship was linear, and the validity of equation (3) for the tests was confirmed.
3.2 Material, sampling and vacuum heat treatment
Commercial, continuous-cast spring steel grade 51CrV4, delivered as rolled bars of dimensions (20 x100 X 4000) mm from two heats with similar chemical compositions (Table 1) were used. As can be seen from the table there is no difference in the chemical composition of the two heats. The rolled bars were delivered after hot plastic deformation in the as-rolled condition with a mixed martensitic-bainitic microstructure and a hardness of HRc 42-45. Before the sampling, the bars were soft annealed. The bars of each heat were selected at the beginning, middle and end of the rolling.
^ic-test specimens in the form of circumferentially notched and fatigue-precracked tensile-test specimens were cut from soft annealed bars with HB2.5/187.5274-277 in the rolling direction with the fatigue crack at the notch root in the transverse direction.
The specimens were heat treated in a horizontal vacuum furnace with uniform high-pressure gas-quenching using nitrogen (N2) at a pressure of 5 bar. After the first preheat (650 °C) the specimens were heated (10 °C/min) to the austenitizing temperature of 870 °C, soaked for 10 min, gas quenched to a temperature of 80
Figure 2: Tempering diagram for two heats of continuous-cast spring steel grade 51CrV4
Slika 2: Diagram popuščanja za dve šarži kontinuirno litega vzmetnega jekla 51CrV4
°C, and then single tempered for one hour at different temperatures between 200 °C and 575 °C, as shown in Figure 2. Sixteen Kic-test specimens were tested for each tempering temperature.
4 RESULTS AND DISCUSSION
The average measured hardness and fracture-toughness data are shown for the range of hardness (HRc 30-56 ; i.e. Rm = 1275-2161 MPa) in a so-called combined tempering diagram (Rockwell-C hardness - Fracture toughness Kic - Tempering temperature for austenitizing temperature 870 °C) in Figure 2.
From the diagram it is clear that the highest hardness of HRc 58 and the associated fracture toughness KIc i.e., 18.8 MPa m1/2, are achieved in the as-quenched condition after vacuum quenching from the austenitizing temperature of 870 °C. In examining the course of tempering, it is observed that the minimum scatter of results within ±2ct = 3.2 MPa m1/2 is in the as-quenched state and after single tempering at 200 °C. At higher tempering temperatures between 300 °C and 575 °C, the scattering of the Kic results slightly increased up to ±2ct = 9.4 MPa m1/2, while the scatter of the Rockwell-C hardness is up to ±2ct =1.2 HRc in the whole range of used tempering temperatures. Within each group of KIc-specimens, which were quenched and tempered at the same temperature, this can be attributed to the kinetics of carbide precipitation during tempering at selected temperatures as well as to the heterogeneity of the investigated steel. Since the differences in chemical composition (Table 1) are minimal and the austenitizing temperature is the same, it is clear that the fracture toughness KIc is a very selective mechanical property with regard to the tempering temperature. It should be noted that the KIc-test specimens were taken from the middle of the bar, and therefore the microstructures of the KIc-test specimens with the lowest and highest fracture toughness are comparable.
In the as-quenched condition the microstructure consists of untempered martensite and bainite, Figure 3. Grain boundaries of the prior austenite grains are not pronounced, and therefore the determination of the austenite grain size according to ASTM E112 was not possible.
Strong positive (bright) and negative (dark) segregations are visible due to the lower etching intensity of the untempered martensite.
Figure 3: The microstructure of the ^Ic-test specimen in the as-quenched condition: a, b) specimen with the lowest fracture toughness (KIc = 16.2 MPa m1/2 and HRc 58.9) c, d) specimen with the highest fracture toughness (KIc = 22.3 MPa m1/2 and HRc 58.1), transverse direction, natal
Slika 3: Mikrostruktura KIc-preizku{anca v kaljenem stanju: a, b) z najmanj{o lomno žilavostjo (KIc= 16,2 MPa m1/2 in HRc 58,9) c, d) z največjo lomno žilavostjo(KIc = 22,3 MPa m1/2 in HRc 58,1), prečna smer
The fractured surface of the Kic-test specimens from the same heat with the highest and lowest fracture toughness tempered at 457 °C examined in binocular at 16-times magnification, Figure 4.
From Figure 4, it is evident that on the fractured surface of the KIc-test specimen the highest fracture toughness is higher with the density of positive segregations (black features), the size of segregation is smaller and the distribution is more even in comparison to the
Figure 4: Fractured surfaces of KIc-test specimen, tempered at 475 °C: a, b) specimen with the lowest fracture toughness (KIc = 75.7 MPa m1/2, HRc 43.8) c, d) specimen with the highest fracture toughness (KIc = 82.2 MPa m1/2, HRc 43.2)
Slika 4: Prelomna povr{ina KIcpreizku{ancev, popu{čenih na 475 °C: a, b) z najmanj{o lomno žilavostjo (KIc = 75,7 MPa m1/2 in HRc 43,8) c, d) z največjo lomno žilavostjo(Kic = 82,2 MPa m1/2 in HRc 43,2)
Figure 5: Typical microstructure of KIc-test specimens a, b) specimen with the lowest fracture toughness (KIc = 75.7 MPa m1/2, HRc 43.8) in transverse direction and c, d) in longitudinal direction and e, f) specimen with the highest fracture toughness (KIc = 82.2 MPa m1/2, HRc 43.2) in transverse direction and g, h) in longitudinal direction Slika 5: Značilna mikrostruktura KIc -preizku{anca: a, b) z najmanj{o lomno žilavostjo (KIc = 75,7 MPa m1/2 in HRc 43,8), prečna smer c, d) vzdolžna smer; e, f) z največjo lomno žilavostjo (KIc = 82,2 MPa m1/2 in HRc 43,2), prečna smer; g, h) vzdolžna smer
fractured surface of the KIc-test specimen with the lowest fracture toughness with nearly the same hardness.
The microstructure of the same KIc-specimens just below the fractured surface was examined in the optical microscope, Figure 5.
The microstructure in the quenched and tempered condition consists of tempered martensite and bainite (~ 20 %). In the microstructure, non-metallic inclusions of the sulphide type can be observed that are located in positive segregations and oriented in the rolling direction.
The microhardness HV0.025 on the positive (bright) and negative (dark) segregations of the KIc-test specimens with the highest fracture toughness is shown in Figure 6. The microhardness in the positive segregations
Figure 6: Microhardness measurements on positive (bright) a) and negative (darker), b) segregations of the Kic -test specimens with the highest fracture toughness
Slika 6: Mikrotrdota a) na pozitivnih (svetli pasovi) in b) na negativnih (temnej{i pasovi) izcejah KIc-preizku{anca z najve~jo lomno žilavostjo
was 554-584 HV0.025, in negative segregations it was ^458 HV0.025. From the difference of the microhardness one can conclude that the microstructure in positive segregations consists of tempered martensite and in negative segregations with a lower microhardness of martensite and bainite.
The microstructure of the KIc -test specimens with the highest fracture toughness was examined in the SEM on positive and negative segregations in the longitudinal direction, Figure 7.
At higher magnification it can be seen that the microstructure in the positive segregations consist of tempered martensite and in negative of tempered martensite with islands of bainite, Figure 8.
From the comparison of the microstructures of the Kic-test specimens with the lowest and highest fracture toughness from the same heat and nearly the same hardness and fracture toughnesses, it can be concluded that the density, the size and the distribution of segre-
Figure 8: Bainite grain from Figure 7 Slika 8: Kristalno zrno bainita s slike 7
gations have a significant influence on the fracture toughness. The increase in the fracture toughness can be ascribed to the presence of bainite in the negative segregations.
As mentioned above, if the fracture stress according to equation (1) approaches the yield stress (e.g., if no larger crack-nucleating particles are present), elastic-plastic methods have to be applied. In this region the material behaviour can be described with equation (2), if the ultimate fracture strength is known, Figure 9.
In the diagram the decrease of the fracture strength is observed when the critical defect size Acr is exceeded, i.e., Acr = 50 pm and 100 pm, respectively.
5 CONCLUSIONS
The investigated spring steel grade 51CrV4 was successfully vacuum quenched in a horizontal vacuum furnace with uniform high-pressure gas-quenching using nitrogen (N2) at a pressure of 5 bar. The obtained as-quenched Rockwell-C hardness was HRc 58.4 ± 0.8,
Figure 7: SEM Microstructure of the KIc-test specimens with the highest fracture toughness: a, b) positive segregations and c, d) negative segregations, longitudinal direction
Slika 7: SEM-mikrostruktura Kic-preizkusanca z največjo lomno žilavostjo: a, b) pozitivna izceja, c, d) negativna izceja
Figure 9: Fracture stress o^ calculation for the investigated spring steel tempered at various temperatures, ultimate fracture strength (Ou = Ruc) was calculated from the Rockwell-C hardness according to DIN
50150.
Slika 9: Lomna napetost Of za preiskovano vzmetno jeklo, popusčeno pri različnih temperaturah; končna natezna napetost (ou = Ruc) izračunana iz trdote Rockwell-C, DIN 50150
i.e., high enough to obtain, after a single tempering, the required hardnesses from HRc 30 to HRc 50.
The obtained microstructure consists of tempered martensite and bainite (~ 20 %). Such a microstructure is allowed according to the criteria TB1402 (Scania Standard STD512090 and STD4153) permitting that the microstructure consists of volume fraction (p ~ 80 % of martensite in the middle of the spring. This condition was met for the investigated spring steel. From the results we can conclude that the investigated spring steel 51CrV4 with a thickness up to 20 mm is suitable for heat treatment in a vacuum furnace with uniform high-pressure gas-quenching using nitrogen (N2) at a pressure of 5 bar or higher.
The fracture toughness of the steel was determined with a non-standard circumferentially notched and fati-gue-precracked tensile-test specimen (^Ic-test specimen). In all cases the load-displacement relationship was linear and the validity of equation (3) for the tests was confirmed. Because of the radial symmetry of the heat transfer, the microstructure of the metallic material along the circumferential area is completely uniform; therefore, the scatter of the fracture toughness could be ascribed to the heterogeneity of the investigated steel only.
For three tempering temperatures and the associated ultimate fracture strength calculated from the Rock-well-C hardness according to DIN 50150, the critical defect size da was estimated, i.e., da = 50-100 pm, respectively.
The fractured surfaces of the Kjc-test specimens show positive segregations (black features) and confirmed with metallographic analyses as well as with the microhard-ness HV0.025 measurements. The microstructure of the positive segregations consists of tempered martensite with a microhardness HV0.025554-584. In the microstruc-ture, non-metallic inclusions of the sulphide type can be observed, which are located in the positive segregations and oriented in the rolling direction. The microstructure in the negative segregations with a lower microhardness consists of low-alloyed tempered martensite and bainite.
In analyzing the fracture surfaces of the Kjc-test specimens of the same heat with the highest and lowest fracture toughness, tempered at the same tempering temperature, as well as from the examined microstructure, which confirmed the presence of positive and negative
segregations, we found that specimens with a higher density of positive segregations (black features), which are smaller in size and evenly distributed over the fractured surface, exhibit up to 27 % higher fracture toughness. This finding, which is also very interesting for the automotive industry, will be the subject of a further investigation.
Acknowledgement
Štore Steel, d. o. o., Železarska cesta 3, SI-3220 Štore, Slovenia, is thanked for financial support as well as for the supply of test materials. Thanks also to Prof. dr. Franc Vodopivec and dr. Matjaž Godec for helpful discussions.
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