In this study two new tests were developed, i.e., with continuous internal water cooling as well as discontinuous internal water and air cooling. It was proved that the first type of testing is appropriate for simulating the time course of the temperature at a selected depth of a thermally loaded, hot-working die surface layer, i.e., the temperature field on the die surface layer. The second type of testing is appropriate for a study of the thermal fatigue resistance of a tool material.U ovom istraživanju razvijeni su dva nova testa, tj. test sa kontinuiranom unutarnjim vodenim hlađenjem, kao i test sa diskontinuiranom unutarnjim vodenim i zračnim hlađenjem. Dokazano je da je prvi tip ispitivanja primjeran za simulaciju vremenske raspodjele temperature na odabranoj dubini termalno opterećene površine alata za vruću preradu. Druga vrsta ispitivanja je prikladno za proučavanje otpora termalnog zamora alatnog materijala

B. Žužek

M. Fazarinc

M. Terčelj

P. Fajfar

English

Hrčak - Portal of scientific journals of Croatia

453METALURGIJA 51 (2012) 4, 453-456P. FAJFAR, M. FAZARINC, B. ŽUŽEK, M. TERČELJIMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL STEEL SAMPLES FOR DIFFERENT APPLICATIONSReceived – Prispjelo: 2012-01-10Accepted – Prihvaćeno: 2012-04-10Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51 (4) 453-456 (2012)UDC – UDK 669.1:620.17=111P. Fajfar, M. Fazarinc, B. Žužek, M. Terčelj, Department of Materials and Metallurgy, University of Ljubljana, Ljubljana, SloveniaIn this study two new tests were developed, i.e., with continuous internal water cooling as well as discontinuous internal water and air cooling. It was proved that the fi rst type of testing is appropriate for simulating the time course of the temperature at a selected depth of a thermally loaded, hot-working die surface layer, i.e., the tem-perature fi eld on the die surface layer. The second type of testing is appropriate for a study of the thermal fatigue resistance of a tool material.Key words: diss/continuous internal cooling, tool steels, thermal fatigue, microhardness profi le.Primjena novo razvijenih testova sa grijanjem i unutarnjim hlađenjem uzoraka od alatnog čelika za različite primjene. U ovom istraživanju razvijeni su dva nova testa, tj. test sa kontinuiranom unutarnjim vodenim hlađenjem, kao i test sa diskontinuiranom unutarnjim vodenim i zračnim hlađenjem. Dokazano je da je prvi tip ispitivanja primjeran za simulaciju vremenske raspodjele temperature na odabranoj dubini termalno opterećene površine alata za vruću preradu. Druga vrsta ispitivanja je prikladno za proučavanje otpora termalnog zamora alatnog ma-terijala.Ključne riječi: ne/kontinuirano unutarnje hlađenje, alatni čelici, termalni zamor, mikrotvrdoća profi la.INTRODUCTIONHot-working dies are subjected to high thermal, me-chanical, tribological and chemical loadings. The cyclic temperature loading as the main load on the die surface layer leads to softening and consequently to a process of wear and thermal cracking [1-9].An accurate determination of the temperature ﬁ eld on the die surface layer during hot working is still the subject of many investigations [10-11]. An assessment of the thermal loading of an industrial die surface can be assessed using different ﬁ nite-element codes or ﬁ nite-difference codes, where the selection of a real value for HTC is usually a difﬁ cult task. A direct measurement of the temperature on massive industrial hot working dies with an embedded thermocouple is usually very de-manding (in some cases this is also not possible) and expensive, and the accuracy of the measured values is always questionable. Knowledge about the temperature ﬁ eld on the die surface is desired since this is utilized for the selection of appropriate parameters during die-steel processing, the parameters for die surface coat-ings, the nitriding parameters, etc., aimed at reducing the wear as well as the growth of cracks on the die sur-face [10-14].The Gleeble 1500D thermo-mechanical simulator has excellent computer control for the temperature on tested samples as well as the movement of the working jaws. For thermal fatigue testing on the Gleeble 1500D a specimen with a notch but without additional cooling is recommended [14]. It is assumed that the temperature change in the notch is rapid, which leads to the nuclea-tion of cracks and their growth. Since the temperature ﬁ elds in such testing usually differ from those encoun-tered in industrial applications the direct transfer of the obtained laboratory results to practice is limited. Thus, the above mentioned abilities of Gleeble 1500D enable the development of new tests aimed at a speciﬁ c appli-cation, which would better match the various thermo-mechanical conditions to which metallic materials in their applications are subjected [7-14].In this contribution, the possibility of applying new-ly developed tests with computer-controlled heating and internal cooling of the specimen for improving the assessment of temperature on the hot-working die sur-face layer and for an assessment of the thermal fatigue resistance has been studied.MATERIALS AND METHODSApplied specimens and materialsThrough the specimen (Figure 1) a hole with 4 mm in diameter was drilled in the axial direction that ena-bles cooling of the specimen with a stream of water or air. A thermocouple (TC) of type K to control the ex-454  METALURGIJA 51 (2012) 4, 453-456P. FAJFAR et al.: IMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL …periment was mounted in the middle of the reduced part of the specimen. Thus, the Gleeble 1500D, in addition to a computer-controlled temperature during heating of the sample, and, due to its internal water cooling, also allows the computer-controlled rapid decrease of its temperature. Two modes of testing, i.e., test with heat-ing and simultaneously continuous internal cooling (test mode 1) as well as discontinuous cooling of samples (test mode 2), respectively, were carried out on the Gleeble 1500D.jected to assumed thermal loads for each of the selected depths on the die surface layer will serve for the accu-racy of the assumed temperature ﬁ eld on die surface layer during its service time. The test cycle can be re-peated until sufﬁ cient agreement between the mentioned values for the microhardness is achieved, i.e., in the case of an accurate determination of the temperature ﬁ eld. Figure 2 shows the microhardness proﬁ le on a laboratory hot-forging die surface layer that was ob-tained after 1 250 forging cycles and the measured val-ues on heated and internally cooled specimens after Figure 1 Shape of the specimenSince the surface layer of each hot-working die (casting, forging, etc.) exhibits a decrease of the micro-hardness after their service time, it is assumed that on the base of the obtained microhardness proﬁ le an im-proved assessment of the temperature ﬁ eld on the die surface layer during its service time using test mode 1 can be carried out.The samples for test mode 1 were made from H11 tool steel with the following chemical composition: C 0,35 %, Cr 5,05 %, Si 0,85 %, Mn 0,32 %, Mo 1,37 %, V 0,38% and Fe balance. They were austenitized at 1010 °C for 30 min and quenched in vacuum. The ﬁ rst tempering was at a temperature of 550 °C for 120 min, while the second tempering was at a temperature of 590 °C for 120 min. The microstructure consists of temper-ed martensite and precipitated carbides. The achieved hardness was 520 HV0.1. The samples for test mode 2 were machined from an indeﬁ nite chill iron roll CIN-N-80 with the following chemical composition: C 3,15 %, Ni 4,50 %, Cr 1,77 %, Si 0,92 %, Mn 0,89 %, Mo 0,37 % and Fe balance. The initial microstructure of the outer shell of the indeﬁ nite chill iron consists of a ma-trix of dendrite grains from bainite, martensite, ledebur-ite and some free graphite.Cyclic heating and continuous internal cooling of the specimens (test mode 1)Cyclic heating and cooling on the hot-working die surface layer leads to a decrease of the microhardness, on the tested specimen as well as to the occurrence of cracks. For a test with a continuous internal cooling time the course of the temperature for each selected depth on the hot-working die surface layer can be simu-lated. This will lead to a decrease of the microhardness on the tested samples that can also correspond to a de-crease of the microhardness for a selected depth on the hot-working die surface layer. The agreement between the obtained microhardness proﬁ le on the die surface and the measured values on the tested specimens sub-Figure 2 Microhardness profi lesFigure 3  Measured and calculated time course of temperature for the hot forging1 250 cycles during a simulation of the temperature pro-ﬁ le for selected depths.Furthermore, on the basis of the agreement between the values of the microhardness of this proﬁ le with the obtained values of the microhardness on the tested sam-ples, the possibility of the application of this test for an improved assessment of the temperature ﬁ eld on the hot-working surface of the industrial dies during their service time was studied. Typical measured values of temperature at approximately 0,15 mm below the die surface during laboratory hot forging and calculated temperature ﬁ eld on die surface layer which procedure is described in [10] are given in Figure 3. Characteristic points on the measured curve (Figure 3) regarding the tool-workpiece position represent: 1–deformation, 2–decreasing of force, 3–slight contact, 4–no contact. The 455METALURGIJA 51 (2012) 4, 453-456P. FAJFAR et al.: IMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL …contact time was 0,5 s and the specimen temperature was 950 °C. Here we should emphasize that during em-bedding of the thermocouple close to the surface of the industrial die it is not entirely clear to which depth be-long the measured values of the temperature since the large size of the TC hot point as well as the response time of the TC inﬂ uence the accuracy of the measured values of the temperature. By computer-controlled cy-clic heating and continuous internal cooling of the spec-imens the temperature time courses for each selected depth on the die surface layer (Figure 3) can be simu-lated. On the basis of the obtained values of the micro-hardness on the tested samples and from their compari-son with the microhardness proﬁ le on the surface layer of a real die, a more accurate assessment of the tem-perature ﬁ eld can be carried out. The procedure with test mode 1 for all the selected depths on the die surface layer can be repeated until the applied temperature courses lead to sufﬁ cient agreement between the men-tioned microhardness proﬁ les.Additionally tests at temperatures of 650 °C, 750 °C and 850 °C for various numbers of cycles, i.e., 500, 1 000 and 3 000, were carried out. These tests served for the determination of area on specimen cross-section which is subjected to prescribed computer guided time courses and where measurements of microhardness were carried out.Cyclic heating and discontinuous cooling of the specimen (test mode 2)A higher temperature gradient on the tested surface is achieved during this type of testing. It seems that this test will be appropriate for a study of the thermal fatigue resistance of steels aimed at the production of hot-roll-ing rolls. Discontinuous cooling of the internal speci-men surface with water and air after heating simulates the cooling of the roll surface.In this batch of tests the specimens were tested un-der similar conditions found on the surface of rolls dur-ing the hot-rolling process. The specimens were heated to four different temperatures, 400 °C, 500 °C, 600 °C and 700 °C. After the specimens attained the test tem-perature, they were computer-controlled internally cooled with water which was then ejected by air. The experiments were interrupted at 500 and 1 000 cycles; the duration of each temperature cycle was 4,8 s.RESULTS AND DISCUSSIONContinuous internal cooling of the specimenThe measured values for the microhardness on the specimen radial cross-sections after 500, 1 000 and 3 000 cycles and for the test temperature of 750 °C re-veal that these values decrease from the cooled inner side to the outer side of the specimen, where consider-ably higher temperatures prevail. But at a distance of approximately 0,5 mm from outer surface of the speci-men about the same level of decrease of the microhard-ness for each number of the cycle and the testing tem-perature was obtained. Consequently, within the men-tioned depth on the specimen surface layer a measure-ment of the microhardness should be carried out. During testing at the maximum temperature of 850 °C this val-ue amounts to approximately 0,25 mm, while during testing at a temperature of 650 °C this value amounts to 0,75 mm. Furthermore, the micro-cracks also appeared only on the inner surface of the specimen at the highest temperature (850 °C) and the largest number of thermal cycles (3 000). At the outer surface of the specimen only a few cracks appeared. More cracks were found on the inner surface in intervals of 20 μm to 50 μm and their average length of 6 μm. At lower temperatures, the cracks did not appear.The measured values of the microhardness on a specimen for a simulation of the temperature time course for selected depths on the die surface (Figure 3) are given in Figure 2. Good agreement between the measured microhardness proﬁ les on the die surface lay-er with those obtained on tested samples was achieved. As mentioned in the case of the measured lower values of the microhardness on the tested samples higher tem-peratures for the next test mode 1 should be applied un-til sufﬁ cient agreement is achieved. Thus, applying test mode 1 is appropriate for a determination of the tem-perature ﬁ eld on the die surface layer and less appropri-ate for thermal fatigue testing.Discontinuous internal cooling of the specimenThermal cycling causes the appearance of cracks on the surface of the specimen. Carbides represent the main source of crack initiation due to the different thermal coefﬁ cients of expansion. At elevated temperatures, crack propagation is enhanced by the formation of car-bides and oxides. Because the volume of the oxide is Figure 4  SEM image of cracks after 500 cycles at the test temperature of 700 °C456  METALURGIJA 51 (2012) 4, 453-456P. FAJFAR et al.: IMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL …greater than that of the base metal, the oxides cause the formation of wedge-shaped cracks (Figure 4). The chemical composition of the oxide (x) is O 29,36 %, Cr 2,1 %, Ni 1,48 %, Mn 0,94 %, Si 0,43 % and Fe bal-ance. The crack propagation is trans-granular and inter-granular.Thermal fatigue cracks appear in all the specimens after testing. In the 2,5-mm-thick wall of the specimens the cracks reached a depth of 200 µm to 1 500 µm. The maximum lengths of the cracks are shown in Figure 5. They are larger at the temperatures of 500 °C and 600 °C in the case of 1 000 cycles for about 200 μm in com-parison with 500 cycles. At the temperatures of 400 °C and 700 °C the number of cycles has no inﬂ uence on the lengths of the cracks. With a higher thermal fatigue temperature and a greater number of cycles the cracks becomes wider and deeper, but they appear in a smaller number. At a temperature of 400 °C, there were on aver-age 5,1 cracks per millimeter of surface length and at the temperature 700 °C, the number of cracks was re-duced to 2,8 cracks per millimeter (Figure 5).CONCLUSIONSIn this research work the implementation possibili-ties for the results obtained on specimens subjected to computer-controlled heating and internal cooling simul-taneously, was studied. Two types of tests were carried out on a Gleeble 1500D. The following conclusions can be drawn:•  Tests with continuous internal cooling of the spec-imen are appropriate for the simulation of the tem-perature time course for selected depths on the hot-working die surface, i.e., for an improved as-sessment of the temperature ﬁ eld on the die sur-face layer during its hot working. Very good agree-ment between the measured microhardness proﬁ le on the die surface layer of the laboratory hot-forg-ing tool and the measured values of the specimens subjected to same temperature loading was ob-tained.•  Testing with discontinuous cooling is appropriate for thermal fatigue testing since the obtained lengths of the cracks on the internal surface for indeﬁ nite chill iron roll (CIN-N-80) were up to 1500 µm after 1000 cycles. The cracks for the tested indeﬁ nite chill iron roll (CIN-N-80) appear on the cooled surface only, usually on the carbides’ phase boundaries. With an increase in the number of cycles deeper and wider cracks occur, while with an increase of the testing temperature the number of cracks decreases, but their length in-creases. The crack propagation is enhanced by oxidation.REFERENCES[1] A. Persson, S. Hogmark, J. Bergström, Int. Journal of Fati-gue, 26 (2004) 1095-1107.[2] D. Klobčar, J. Tušek, B. Taljat, Mat. Science and Enginee-ring: A, 472 (2008) 198–207.[3] T. Pepelnjak, B. Barišič, J. of Mater. Process. Technol., 186 (2007) 111-119.[4] T. Pepelnjak, G. Gantar, K. Kuzman, J. Mater. Process. Te-chnol., 115 (2001) 122-126.[5] D. Klobčar, J. Tušek, Computational Material Science, 43 (2008) 11, 47–54.[6] M. Petrič, J. Medved, P. Mrvar, Metalurgija, 50 (2011) 2, 127-131.[7] A. Persson, S. Hogmark, J. Bergström, J. of Mater. Pro-cess. Technol., 152 (2004) 228-236.[8] A. Persson, J. Bergström, C. Burman, S. Hogmark, Surface and Coatings Technology, 146-147 (2001) 42-47.[9] L. Lavtar, T. Muhič, G. Kugler, M. Terčelj, Eng. Fail. Analysis, 18 (2011) 4, 1143-1152.[10] M. Terčelj, R. Turk, M. Knap, Applied Thermal Enginee-ring, 23 (2003) 2, 113-125.[11] J.O. Aweda, M.B. Adeyemi (HTC), J of Mat. Process. Te-chnol., 209 (2009) 3, 1477-1483.[12] S. Amiable, S. Chapuliot, A. Constantinescu, A. Fissolo, Int. J. of Fatigue, 28 (2006) 692-706.[13] M. Fazarinc, R. Turk, G. Kugler, P. Mrvar, M. Terčelj, RMZ–Materials and Geoenvironment, 54 (2007) 1, 33-48.[14] L. A. DobrzańskI, K. Golombek, , J. Kopač, M. Soković, J. Mater. Process. Technol., 157-158 (2004) 304-311.[15] Gleeble 1500 Operations Manual, Duffers Scientiﬁ c, Inc. 1989.Note:  The responsible translator for English language is Paul McGuiness.Figure 5  Maximum lengths of the cracks and the crack number for 1000 cycles

Primjena novo razvijenih testova sa grijanjem i unutarnjim hlađenjem uzoraka od alatnog čelika za različite primjene

HRČAK - Portal of Croatian Scientific and Professional Journals

453METALURGIJA 51 (2012) 4, 453-456P. FAJFAR, M. FAZARINC, B. ŽUŽEK, M. TERČELJIMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL STEEL SAMPLES FOR DIFFERENT APPLICATIONSReceived – Prispjelo: 2012-01-10Accepted – Prihvaćeno: 2012-04-10Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51 (4) 453-456 (2012)UDC – UDK 669.1:620.17=111P. Fajfar, M. Fazarinc, B. Žužek, M. Terčelj, Department of Materials and Metallurgy, University of Ljubljana, Ljubljana, SloveniaIn this study two new tests were developed, i.e., with continuous internal water cooling as well as discontinuous internal water and air cooling. It was proved that the fi rst type of testing is appropriate for simulating the time course of the temperature at a selected depth of a thermally loaded, hot-working die surface layer, i.e., the tem-perature fi eld on the die surface layer. The second type of testing is appropriate for a study of the thermal fatigue resistance of a tool material.Key words: diss/continuous internal cooling, tool steels, thermal fatigue, microhardness profi le.Primjena novo razvijenih testova sa grijanjem i unutarnjim hlađenjem uzoraka od alatnog čelika za različite primjene. U ovom istraživanju razvijeni su dva nova testa, tj. test sa kontinuiranom unutarnjim vodenim hlađenjem, kao i test sa diskontinuiranom unutarnjim vodenim i zračnim hlađenjem. Dokazano je da je prvi tip ispitivanja primjeran za simulaciju vremenske raspodjele temperature na odabranoj dubini termalno opterećene površine alata za vruću preradu. Druga vrsta ispitivanja je prikladno za proučavanje otpora termalnog zamora alatnog ma-terijala.Ključne riječi: ne/kontinuirano unutarnje hlađenje, alatni čelici, termalni zamor, mikrotvrdoća profi la.INTRODUCTIONHot-working dies are subjected to high thermal, me-chanical, tribological and chemical loadings. The cyclic temperature loading as the main load on the die surface layer leads to softening and consequently to a process of wear and thermal cracking [1-9].An accurate determination of the temperature fi eld on the die surface layer during hot working is still the subject of many investigations [10-11]. An assessment of the thermal loading of an industrial die surface can be assessed using different fi nite-element codes or fi nite-difference codes, where the selection of a real value for HTC is usually a diffi cult task. A direct measurement of the temperature on massive industrial hot working dies with an embedded thermocouple is usually very de-manding (in some cases this is also not possible) and expensive, and the accuracy of the measured values is always questionable. Knowledge about the temperature fi eld on the die surface is desired since this is utilized for the selection of appropriate parameters during die-steel processing, the parameters for die surface coat-ings, the nitriding parameters, etc., aimed at reducing the wear as well as the growth of cracks on the die sur-face [10-14].The Gleeble 1500D thermo-mechanical simulator has excellent computer control for the temperature on tested samples as well as the movement of the working jaws. For thermal fatigue testing on the Gleeble 1500D a specimen with a notch but without additional cooling is recommended [14]. It is assumed that the temperature change in the notch is rapid, which leads to the nuclea-tion of cracks and their growth. Since the temperature fi elds in such testing usually differ from those encoun-tered in industrial applications the direct transfer of the obtained laboratory results to practice is limited. Thus, the above mentioned abilities of Gleeble 1500D enable the development of new tests aimed at a specifi c appli-cation, which would better match the various thermo-mechanical conditions to which metallic materials in their applications are subjected [7-14].In this contribution, the possibility of applying new-ly developed tests with computer-controlled heating and internal cooling of the specimen for improving the assessment of temperature on the hot-working die sur-face layer and for an assessment of the thermal fatigue resistance has been studied.MATERIALS AND METHODSApplied specimens and materialsThrough the specimen (Figure 1) a hole with 4 mm in diameter was drilled in the axial direction that ena-bles cooling of the specimen with a stream of water or air. A thermocouple (TC) of type K to control the ex-454  METALURGIJA 51 (2012) 4, 453-456P. FAJFAR et al.: IMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL …periment was mounted in the middle of the reduced part of the specimen. Thus, the Gleeble 1500D, in addition to a computer-controlled temperature during heating of the sample, and, due to its internal water cooling, also allows the computer-controlled rapid decrease of its temperature. Two modes of testing, i.e., test with heat-ing and simultaneously continuous internal cooling (test mode 1) as well as discontinuous cooling of samples (test mode 2), respectively, were carried out on the Gleeble 1500D.jected to assumed thermal loads for each of the selected depths on the die surface layer will serve for the accu-racy of the assumed temperature fi eld on die surface layer during its service time. The test cycle can be re-peated until suffi cient agreement between the mentioned values for the microhardness is achieved, i.e., in the case of an accurate determination of the temperature fi eld. Figure 2 shows the microhardness profi le on a laboratory hot-forging die surface layer that was ob-tained after 1 250 forging cycles and the measured val-ues on heated and internally cooled specimens after Figure 1 Shape of the specimenSince the surface layer of each hot-working die (casting, forging, etc.) exhibits a decrease of the micro-hardness after their service time, it is assumed that on the base of the obtained microhardness profi le an im-proved assessment of the temperature fi eld on the die surface layer during its service time using test mode 1 can be carried out.The samples for test mode 1 were made from H11 tool steel with the following chemical composition: C 0,35 %, Cr 5,05 %, Si 0,85 %, Mn 0,32 %, Mo 1,37 %, V 0,38% and Fe balance. They were austenitized at 1010 °C for 30 min and quenched in vacuum. The fi rst tempering was at a temperature of 550 °C for 120 min, while the second tempering was at a temperature of 590 °C for 120 min. The microstructure consists of temper-ed martensite and precipitated carbides. The achieved hardness was 520 HV0.1. The samples for test mode 2 were machined from an indefi nite chill iron roll CIN-N-80 with the following chemical composition: C 3,15 %, Ni 4,50 %, Cr 1,77 %, Si 0,92 %, Mn 0,89 %, Mo 0,37 % and Fe balance. The initial microstructure of the outer shell of the indefi nite chill iron consists of a ma-trix of dendrite grains from bainite, martensite, ledebur-ite and some free graphite.Cyclic heating and continuous internal cooling of the specimens (test mode 1)Cyclic heating and cooling on the hot-working die surface layer leads to a decrease of the microhardness, on the tested specimen as well as to the occurrence of cracks. For a test with a continuous internal cooling time the course of the temperature for each selected depth on the hot-working die surface layer can be simu-lated. This will lead to a decrease of the microhardness on the tested samples that can also correspond to a de-crease of the microhardness for a selected depth on the hot-working die surface layer. The agreement between the obtained microhardness profi le on the die surface and the measured values on the tested specimens sub-Figure 2 Microhardness profi lesFigure 3  Measured and calculated time course of temperature for the hot forging1 250 cycles during a simulation of the temperature pro-fi le for selected depths.Furthermore, on the basis of the agreement between the values of the microhardness of this profi le with the obtained values of the microhardness on the tested sam-ples, the possibility of the application of this test for an improved assessment of the temperature fi eld on the hot-working surface of the industrial dies during their service time was studied. Typical measured values of temperature at approximately 0,15 mm below the die surface during laboratory hot forging and calculated temperature fi eld on die surface layer which procedure is described in [10] are given in Figure 3. Characteristic points on the measured curve (Figure 3) regarding the tool-workpiece position represent: 1–deformation, 2–decreasing of force, 3–slight contact, 4–no contact. The 455METALURGIJA 51 (2012) 4, 453-456P. FAJFAR et al.: IMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL …contact time was 0,5 s and the specimen temperature was 950 °C. Here we should emphasize that during em-bedding of the thermocouple close to the surface of the industrial die it is not entirely clear to which depth be-long the measured values of the temperature since the large size of the TC hot point as well as the response time of the TC infl uence the accuracy of the measured values of the temperature. By computer-controlled cy-clic heating and continuous internal cooling of the spec-imens the temperature time courses for each selected depth on the die surface layer (Figure 3) can be simu-lated. On the basis of the obtained values of the micro-hardness on the tested samples and from their compari-son with the microhardness profi le on the surface layer of a real die, a more accurate assessment of the tem-perature fi eld can be carried out. The procedure with test mode 1 for all the selected depths on the die surface layer can be repeated until the applied temperature courses lead to suffi cient agreement between the men-tioned microhardness profi les.Additionally tests at temperatures of 650 °C, 750 °C and 850 °C for various numbers of cycles, i.e., 500, 1 000 and 3 000, were carried out. These tests served for the determination of area on specimen cross-section which is subjected to prescribed computer guided time courses and where measurements of microhardness were carried out.Cyclic heating and discontinuous cooling of the specimen (test mode 2)A higher temperature gradient on the tested surface is achieved during this type of testing. It seems that this test will be appropriate for a study of the thermal fatigue resistance of steels aimed at the production of hot-roll-ing rolls. Discontinuous cooling of the internal speci-men surface with water and air after heating simulates the cooling of the roll surface.In this batch of tests the specimens were tested un-der similar conditions found on the surface of rolls dur-ing the hot-rolling process. The specimens were heated to four different temperatures, 400 °C, 500 °C, 600 °C and 700 °C. After the specimens attained the test tem-perature, they were computer-controlled internally cooled with water which was then ejected by air. The experiments were interrupted at 500 and 1 000 cycles; the duration of each temperature cycle was 4,8 s.RESULTS AND DISCUSSIONContinuous internal cooling of the specimenThe measured values for the microhardness on the specimen radial cross-sections after 500, 1 000 and 3 000 cycles and for the test temperature of 750 °C re-veal that these values decrease from the cooled inner side to the outer side of the specimen, where consider-ably higher temperatures prevail. But at a distance of approximately 0,5 mm from outer surface of the speci-men about the same level of decrease of the microhard-ness for each number of the cycle and the testing tem-perature was obtained. Consequently, within the men-tioned depth on the specimen surface layer a measure-ment of the microhardness should be carried out. During testing at the maximum temperature of 850 °C this val-ue amounts to approximately 0,25 mm, while during testing at a temperature of 650 °C this value amounts to 0,75 mm. Furthermore, the micro-cracks also appeared only on the inner surface of the specimen at the highest temperature (850 °C) and the largest number of thermal cycles (3 000). At the outer surface of the specimen only a few cracks appeared. More cracks were found on the inner surface in intervals of 20 μm to 50 μm and their average length of 6 μm. At lower temperatures, the cracks did not appear.The measured values of the microhardness on a specimen for a simulation of the temperature time course for selected depths on the die surface (Figure 3) are given in Figure 2. Good agreement between the measured microhardness profi les on the die surface lay-er with those obtained on tested samples was achieved. As mentioned in the case of the measured lower values of the microhardness on the tested samples higher tem-peratures for the next test mode 1 should be applied un-til suffi cient agreement is achieved. Thus, applying test mode 1 is appropriate for a determination of the tem-perature fi eld on the die surface layer and less appropri-ate for thermal fatigue testing.Discontinuous internal cooling of the specimenThermal cycling causes the appearance of cracks on the surface of the specimen. Carbides represent the main source of crack initiation due to the different thermal coeffi cients of expansion. At elevated temperatures, crack propagation is enhanced by the formation of car-bides and oxides. Because the volume of the oxide is Figure 4  SEM image of cracks after 500 cycles at the test temperature of 700 °C456  METALURGIJA 51 (2012) 4, 453-456P. FAJFAR et al.: IMPLEMENTATION OF NEWLY DEVELOPED TESTS WITH HEATED AND INTERNALLY COOLED TOOL …greater than that of the base metal, the oxides cause the formation of wedge-shaped cracks (Figure 4). The chemical composition of the oxide (x) is O 29,36 %, Cr 2,1 %, Ni 1,48 %, Mn 0,94 %, Si 0,43 % and Fe bal-ance. The crack propagation is trans-granular and inter-granular.Thermal fatigue cracks appear in all the specimens after testing. In the 2,5-mm-thick wall of the specimens the cracks reached a depth of 200 µm to 1 500 µm. The maximum lengths of the cracks are shown in Figure 5. They are larger at the temperatures of 500 °C and 600 °C in the case of 1 000 cycles for about 200 μm in com-parison with 500 cycles. At the temperatures of 400 °C and 700 °C the number of cycles has no infl uence on the lengths of the cracks. With a higher thermal fatigue temperature and a greater number of cycles the cracks becomes wider and deeper, but they appear in a smaller number. At a temperature of 400 °C, there were on aver-age 5,1 cracks per millimeter of surface length and at the temperature 700 °C, the number of cracks was re-duced to 2,8 cracks per millimeter (Figure 5).CONCLUSIONSIn this research work the implementation possibili-ties for the results obtained on specimens subjected to computer-controlled heating and internal cooling simul-taneously, was studied. Two types of tests were carried out on a Gleeble 1500D. The following conclusions can be drawn:•  Tests with continuous internal cooling of the spec-imen are appropriate for the simulation of the tem-perature time course for selected depths on the hot-working die surface, i.e., for an improved as-sessment of the temperature fi eld on the die sur-face layer during its hot working. Very good agree-ment between the measured microhardness profi le on the die surface layer of the laboratory hot-forg-ing tool and the measured values of the specimens subjected to same temperature loading was ob-tained.•  Testing with discontinuous cooling is appropriate for thermal fatigue testing since the obtained lengths of the cracks on the internal surface for indefi nite chill iron roll (CIN-N-80) were up to 1500 µm after 1000 cycles. The cracks for the tested indefi nite chill iron roll (CIN-N-80) appear on the cooled surface only, usually on the carbides’ phase boundaries. With an increase in the number of cycles deeper and wider cracks occur, while with an increase of the testing temperature the number of cracks decreases, but their length in-creases. The crack propagation is enhanced by oxidation.REFERENCES[1] A. Persson, S. Hogmark, J. Bergström, Int. Journal of Fati-gue, 26 (2004) 1095-1107.[2] D. Klobčar, J. Tušek, B. Taljat, Mat. Science and Enginee-ring: A, 472 (2008) 198–207.[3] T. Pepelnjak, B. Barišič, J. of Mater. Process. Technol., 186 (2007) 111-119.[4] T. Pepelnjak, G. Gantar, K. Kuzman, J. Mater. Process. Te-chnol., 115 (2001) 122-126.[5] D. Klobčar, J. Tušek, Computational Material Science, 43 (2008) 11, 47–54.[6] M. Petrič, J. Medved, P. Mrvar, Metalurgija, 50 (2011) 2, 127-131.[7] A. Persson, S. Hogmark, J. Bergström, J. of Mater. Pro-cess. Technol., 152 (2004) 228-236.[8] A. Persson, J. Bergström, C. Burman, S. Hogmark, Surface and Coatings Technology, 146-147 (2001) 42-47.[9] L. Lavtar, T. Muhič, G. Kugler, M. Terčelj, Eng. Fail. Analysis, 18 (2011) 4, 1143-1152.[10] M. Terčelj, R. Turk, M. Knap, Applied Thermal Enginee-ring, 23 (2003) 2, 113-125.[11] J.O. Aweda, M.B. Adeyemi (HTC), J of Mat. Process. Te-chnol., 209 (2009) 3, 1477-1483.[12] S. Amiable, S. Chapuliot, A. Constantinescu, A. Fissolo, Int. J. of Fatigue, 28 (2006) 692-706.[13] M. Fazarinc, R. Turk, G. Kugler, P. Mrvar, M. Terčelj, RMZ–Materials and Geoenvironment, 54 (2007) 1, 33-48.[14] L. A. DobrzańskI, K. Golombek, , J. Kopač, M. Soković, J. Mater. Process. Technol., 157-158 (2004) 304-311.[15] Gleeble 1500 Operations Manual, Duffers Scientifi c, Inc. 1989.Note:  The responsible translator for English language is Paul McGuiness.Figure 5  Maximum lengths of the cracks and the crack number for 1000 cycles

Primjena novo razvijenih testova sa grijanjem i unutarnjim hlađenjem uzoraka od alatnog čelika za različite primjene

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