Thermal shock resistance of electroslag cast steel for hot-working tools

Thermal shock resistance of electroslag cast steel for hot-working tools

Journal of Materials Processing Technology 155–156 (2004) 2122–2126 Thermal shock resistance of electroslag cast steel for hot-working tools Y.H. Moo...

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Journal of Materials Processing Technology 155–156 (2004) 2122–2126

Thermal shock resistance of electroslag cast steel for hot-working tools Y.H. Moon a,∗ , J.W. Kim b , C.J. Van Tyne c a

Department of Mechanical Engineering, Engineering Research Center for Net Shape and Die Manufacturing Technology, Pusan National University, Pusan 609 735, Republic of Korea b LG Electronics, ChangWon, Republic of Korea c Department of Materials and Metallurgical Engineering, Colorado School of Mines, Golden, CO 80401, USA

Abstract Thermal shock resistance of electroslag cast steel for hot-working tools has been studied. High resistance to thermal shock is an important requirement for hot-working tool steels, which are exposed, to both severe thermal and mechanical stresses. Electroslag casting is a method of producing shaped ingots in a water cooled metal mould by the heat generated in the current-conductive slag when electric current passes through a consumable steel electrode. The method provides the possibility of producing material for high quality hot-working tools cast into a variety shapes. Good ductility and toughness as well as high tensile strength at elevated temperatures are important properties for high thermal shock resistance. Direct measurements of quantitative values for the thermal shock resistance of hot-working tool steels is needed. Hence, a new testing procedure is proposed in the present study to measure the relative thermal shock resistance of electroslag cast steel. The reliability and reproducibility of the results from the new test are confirmed by measurements of the thermal shock resistance for several commercially available hot-working tool steels, which are consistent with high temperature mechanical properties. © 2004 Elsevier B.V. All rights reserved. Keywords: Electroslag casting; Thermal shock; Hot-working tool

1. Introduction Hot-working tools are often subjected to extreme thermal shocks, which sometimes can lead to cracking or premature fracture of the tool [1–4]. Therefore, a high resistance to thermal shock is an important requirement for hot-working tool steels, which are exposed to severe thermal-mechanical stresses. Good ductility, toughness and high yield strength at elevated temperature are important requirements for high thermal shock resistance. In this study, the electroslag casting (ESC) method [5–7] has been investigated as a possible method of producing steel having a high resistance to thermal shock. In general, cast metal is not only mechanically inferior to forged metal one but also it has less chemical and physical homogeneity and mechanical property uniformity. For the traditional methods of shaped casting, the molten metal is in a ladle or crucible prior to being poured into the mould. Properties of the poured metal are usually inferior to those of the original, since secondary oxidation of the metal stream occurs during pouring. Reactions with atmospheric gases ∗ Corresponding author. Tel.: +82-51-510-2472; fax: +82-51-512-1722. E-mail addresses: [email protected] (Y.H. Moon), [email protected] (J.W. Kim), [email protected] (C.J. Van Tyne).

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.274

and ambient air inside the mould and with other materials in the mould can cause contamination with inclusions. During solidification, flaws are initiated within the ingot due to segregation and shrinkage processes, which drastically deteriorate the chemical and the physical homogeneity of the ingot. In contrast, the problems listed above are eliminated or minimized in the electroslag casting (ESC) process, which is schematically shown in Fig. 1. Electroslag casting is a method of producing shaped ingots in a water-cooled metal mould. It is based on the electroslag process of melting a consumable electrode. Moreover, the ESC method provides the possibility of producing high-quality metal in a variety of shapes. Unlike all the other casting methods, it is also based on preparing and consuming the metal in a casting unit co-existent with the mould. The heat generated in the current-conductive slag, when electric current passes through it, melts the consumable steel electrode. In the ESC like in other methods of electroslag processes, the source of heat energy is the molten slag. ESC as a technology for billet production has many advantages over conventional casting methods including no melting furnaces, no casting ladles, no moulding mixtures and no sand moulds. The casting is produced without the need to crop the end since its shaping and solidification conditions minimize possible defects inherent to conventional casting.

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Fig. 3. Test procedure for the Uddeholm thermal shock test [8]. Fig. 1. Schematic drawing of electroslag casting process.

In general, since thermal shock resistance may differ considerably among different steels, it is important to be aware of methods to determine comparative quantitative values that can be used to assess the relative thermal shock of various hot-working tool steels. The relative thermal shock resistance of the material can be measured by the procedure developed by Uddeholm [8]. In brief, a securely clamped test specimen is forced to fracture by its own thermal shrinkage caused by a rapid lowering of the temperature as shown in Fig. 2. The total temperature change (T) that the specimen can withstand before fracture is the measure of the relative thermal shock resistance of the material. Fig. 3 shows the test procedure developed at Uddeholm to measure the thermal shock resistance. For the implementation of the Uddeholm thermal shock test method, prestraining to 0.5 mm (∼0.7% strain) is required. Although

the prestraining may be necessary to guarantee fracture during the cooling, application of equal amounts of prestrain to materials that have different mechanical properties can produce inconsistent and non-comparative results. In the case of materials having low uniform elongation, the application of 0.7% prestain may cause severe tensile load damage during the prestraining step. Furthermore, the wide variation in load carrying capacity during prestraining due to higher sensitivity of notched specimens may decrease the reproducibility of the test. In the present study, the thermal shock test proposed by Uddeholm is modified to increase the reliability and reproducibility of the test. The modified test is based on the Uddeholm test method, but the prestraining stage has been replaced by imposing tension on the specimen with simultaneous cooling.

2. Experimental details 2.1. Materials preparation

Fig. 2. Schematic drawing of Uddeholm thermal shock test [8].

The shape of the consumable electrode for electroslag casting process is shown in Fig. 4. The chemical composition of electroslag cast steel was adjusted to that of AISI-L6 tool steel for a comparison of an electroslag cast product with a forged product. Also, for the purpose of comparison, H13 tool steel was tested. All test specimens were held at 810 ◦ C for 1 h followed by oil quenching and tempering at 600 ◦ C for 1 h. The chemical compositions of test materials are shown in Table 1. In the Table 1, L6(ESC) means ‘Electro Slag Cast’ specimen having same chemical composition with AISI-L6 steel. Fig. 5 shows the shape of the electroslag cast ingot used in the study. The electroslag casting conditions are given in Table 2.

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Table 1 Chemical composition of test materials AISI-grade

C

Si

Mn

P

S

Ni

Cr

Mo

V

Al

L6 H13 L6(ESC)

0.51 0.36 0.51

0.22 1.0 0.24

0.97 0.44 0.92

0.015 0.028 0.012

0.002 0.009 0.005

1.56 – 1.95

0.94 5.3 0.96

0.47 1.3 0.47

0.11 0.98 0.12

0.002 – 0.008

testing was done with a high temperature micro-Vickers hardness tester at 300, 500 and 700 ◦ C. The load on the micro-Vickers indenter was 500 g. Averaged hardness values are reported based on five measurements for each material. To account for positional differences in the electroslag cast specimens, several spots on both the upper and lower surfaces of ingot were tested. 2.3. Thermal shock testing procedures

Fig. 4. Schematic drawing of consumable electrode.

Fig. 5. Schematic drawing of electroslag cast ingot.

2.2. High temperature tensile and hardness testing procedures

Fig. 6 schematically shows the modified Uddeholm test method, which is proposed. The specimen is inserted and preheated from room temperature to a predetermined high temperature called the top temperature. It is held for 5 min at the top temperature. Then simultaneously with cooling, a constant tensile displacement rate is applied until the specimen fractures. The temperature at fracture is recorded and the difference (T) between the top temperature and the fracture temperature is obtained. T is used as the quantitative measure of the thermal shock resistance of the material. Larger values of T indicate a higher thermal shock resistance. To evaluate the modified Uddeholm test method, experiments to determine the thermal shock resistance of the hot-working tool steels were performed. The test was done on a Gleeble3500 thermo-mechanical testing machine. The schematic loading mechanism for the Gleeble is shown in Fig. 7. The cylindrical V-notched specimen, 10 mm diameter, which was used in the test, is shown in Fig. 8. The notch has a depth of 2.5 mm, an angle of 60◦ and a root radius of 0.1 mm matching the Uddeholm test specimen. The specimen was inserted in the Gleeble and preheated from room temperature to top temperatures of 700, 800 and

Tensile testing was performed on a servo-hydraulic mechanical testing machine at the temperatures of 300, 500 and 650 ◦ C. The size of specimen conformed to ASTM A370. The strain rate for tensile testing was 2×10−3 s. Hardness Table 2 Electroslag casting conditions Test variable

Input value

Remarks

Flux

66% CaF2 –28% Al2 O3 –5% CaO 11 10–12 38–45



Slag height (cm) Current (kA) Voltage (V)

– – Voltage in slag pool

Fig. 6. Schematic drawing of modified thermal shock test method.

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Fig. 7. Schematic drawing of loading mechanism in the Gleeble.

Fig. 10. Comparison of area reduction at elevated temperatures.

Fig. 8. Geometry of test specimen for the thermal shock test.

900 ◦ C at a rate of 10 ◦ C/s. The specimen was held for 5 min at top the temperature and then the constant tensile stroke rate was applied along with simultaneous cooling from top temperature. The stroke rate was maintained at 10 mm/min and the cooling rate was about 25 ◦ C/s. It should be noted that the stroke rate may have a strong influence on the results and it can be adjusted for the material being tested.

materials. In general, the deformation resistance in temperature range of 600–750 ◦ C is known to be very important to avoid tool softening [4]. Reduction in area values, a measure of the ductility of the material, are shown in Fig. 10. As would be expected from temperature dependence of tensile strength, L6 and L6(ESC) show higher reductions in area as compared to H13. The high temperature hardness values are presented in Fig. 11. The hardness decreases with increasing temperature. The three materials have a similar hardness at 300 ◦ C but H13 has the highest hardness at 700 ◦ C. 3.2. Thermal shock resistance

The high temperature tensile properties of tensile strength and reduction in area have been determined. Fig. 9 shows the variation of tensile strength with temperature. As shown in Fig. 9, H13 has the highest strength among the three

The thermal shock resistance of hot-working tool steels is better in materials with the greater ability to accommodate both elastic and plastic deformation before fracture at elevated temperatures. Thus, a superior thermal shock resistance is accomplished by an optimal combination of strength and ductility in the material [1]. Fig. 12 shows temperature and load changes as a function of time for L6(ESC) material tested at top temperature of 800 ◦ C.

Fig. 9. Comparison of tensile strength at elevated temperatures.

Fig. 11. Comparison of hardness value at elevated temperatures.

3. Results and discussion 3.1. Mechanical properties

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Fig. 12. Results from thermal shock test.

As shown in Fig. 12, the specimen fractured during cooling due to both its thermal shrinkage and the simultaneously applied tensile force. The thermal shock resistance of L6(ESC) steel is estimated to be approximately 300 ◦ C. The thermal and mechanical properties in the temperature range of 500–800 ◦ C are responsible for the resultant thermal shock resistance. 3.3. Analysis of results Fig. 13 shows the measured thermal shock resistance for various top temperatures. As shown in Fig. 13, H13 has a higher thermal shock resistance than L6 and L6(ESC) for top temperatures of 700 and 800 ◦ C, while it has the lowest resistance for a top temperature of 900 ◦ C. Fig. 14 shows a plot of the fracture load for various top temperatures. The fracture load of H13 is higher than L6 and L6(ESC) for top temperature of 700 and 800 ◦ C, while it has a slightly lower fracture load for a top temperature of 900 ◦ C. For good thermal shock resistance the elastic and plastic deformation before fracture at elevated temperature needs to be accommodated by an optimal combination of mechanical properties. The thermal shock results of the steels are consistent with their high temperature mechanical properties, which are shown in Figs. 9–11. H13 has the highest

Fig. 13. Thermal shock resistance for various top temperatures.

Fig. 14. Fracture load for various top temperatures.

tensile strength with reasonable ductility (i.e. reduction in area) and it exhibits the best thermal shock resistance. The thermal shock resistance of L6(ESC) is higher than that of L6 because L6(ESC) has a higher ductility even though they have similar tensile strengths.

4. Conclusions With the experimental implementation of a modified test method, the critical temperatures at which fracture occurs have been measured in order to characterise the thermal shock resistance of the material. The numerical value of T, the temperature difference between top (holding) temperature and the fracture temperature, is a useful measure of the thermal shock resistance of the material. The measured thermal shock resistances confirm that the resistance of hot-working tool steels is favoured by the ability of accommodating both elastic and plastic deformation before fracture at elevated temperatures. The steel made by the electroslag casting process showed better resistance to thermal shock than the forged steel having a similar chemical composition due to the better ductility at similar tensile strengths.

References [1] M. Collin, D. Rowcliffe, Analysis and prediction of thermal shock in brittle materials, Acta Materialia 48 (2000) 1655–1665. [2] C.M.D. Starling, J.R.T. Branco, Thermal fatigue of hot work tool steel with hard coatings, Thin Solid Films 308-309 (1997) 436–442. [3] T.J. Lu, N.A. Fleck, The thermal shock resistance of solids, Acta Materialia 46 (1998) 4755–4768. [4] J.H. Song, J.K. Lim, H. Takahashi, Thermal shock/fatigue evaluation of FGM by AE technique, KSME J. 10 (1996) 435–442. [5] B.I. Medovar, G.A. Boyko, Electroslag Technology, Springer-Verlag, New York, NY, USA, 1990. [6] B.E. Paton, B.I. Medovar, G.A. Boiko, Electroslag Casting, Naukova Dumka Publisher, Kiev, Russia, 1981. [7] R.G. Baligidad, U. Prakash, A.R. Krishna, Effect of carbon addition on structure and mechanical properties of electroslag remelted Fe–20 wt.% Al alloy, Mater. Sci. Eng. A249 (1998) 97–102. [8] L.A. Norstrom, Performance of hot-work tool steels, Scand. J. Metall. 11 (1982) 33–38.