EVALUATION OF THERMAL FATIGUE PROPERTIES OF HSS ROLL MATERIALS. Jong Il Park a Chang Kyu Kim b Sunghak Lee b

Transcription

1 EVALUATION OF THERMAL FATIGUE PROPERTIES OF HSS ROLL MATERIALS Jong Il Park a Chang Kyu Kim b Sunghak Lee b a Hot Rolling Department, Pohang Iron and Steel Co., Ltd., Pohang b Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang Abstract Rolling conditions are gradually complicated in hot rolling mills because of increase in production of thin plate and high strength steel plates. The high speed steel(hss) rolls are introduced in hot strip mills to meet these requirements. In this study, thermal fatigue life of five different HSS rolls with different chemical composition was investigated by conducting the constraint thermal fatigue test. Five work roll materials which were manufactured by a centrifugal casting method were investigated quantitatively by microstructures, mechanical properties and thermal fatigue test. The basic microstructures of their shell regions were observed to be composed mainly of coarse primary carbides and tempered martensite matrix. The cracks formed by thermal fatigue initiated on primary carbides located on surface of the specimen and propagated along the primary carbides. The thermal fatigue life of each roll decreased with increasing the temperature range of the thermal fatigue cycles. These results were interpreted in terms of the morphology of primary carbides and by cyclic softening phenomena associated with the exposed time at elevated temperatures during the thermal fatigue test. For the improvement of thermal fatigue property of HSS rolls, roll materials should have a fine and homogeneous distribution of primary carbides, which reduces fatigue crack initiation sites and lowers matrix hardness. 1. INTRODUCTION The steel industries are currently exerting efforts to devise a new and advanced technique for the quality improvement of the products in hot rolling process. The rolls with improved mechanical properties such as wear resistance, high strength, fracture toughness, and thermal fatigue have been required because the rolling conditions become severe as the demands are increasing for the rolled plates with more uniform and lower thickness, smoother surface, and higher strength[1-2]. Developing such rolls is mainly for the quality improvement of the rolled products and for the roll durability. Thus, understanding of the thermal fatigue behavior is essential since surface roughening caused by the surface cracks are closely related with the thermal fatigue. In this study, the microstructure, mechanical properties, and thermal fatigue properties of the five High Speed Steel (HSS) rolls were investigated to obtain the fundamental data for the establishment of hot rolling conditions and the extension of the roll durability. In addition, the mechanism of thermal fatigue and the microstructural factors influencing on thermal fatigue properties and fracture behavior were analyzed and compared using the thermal fatigue results and microstructure. 2. EXPERIMENTALS Five HSS rolls used in this study were centrifugally cast, and their chemical compositions including carbon equivalent (CE=C+1/3Si) and tungsten equivalent 1

2 Table 1. Chemical compositions of the HSS rolls (wt.%) Roll CE * W eq ** V Cr Mn P S Ni A B C D E *CE = C + 1/3Si ; carbon equivalent ** W eq = W + 2Mo ; tungsten equivalent (W eq =W+2Mo) are listed in table I. Based on A roll, which has the basic chemical composition of 1.9CE-5.5V-4.5Cr (wt.%), other rolls were designed with varying carbon equivalent, tungsten equivalent, and vanadium contents in order to examine their respective effects. B roll has a same W eq with A roll by substituting Mo to W, while C roll has higher Mo content than the basic A roll. D and E roll have higher carbon equivalent and lower vanadium content than A roll, respectively. A small horizontal centrifugal caster was used for fabricating experimental HSS rolls. Specimens were obtained from the outer shell. They were austenitized at 9~1 C for 5 hours, followed by air cooled to room temperature, and the double tempered at the temperature range from 47~6 C. For analysis of the carbides of the HSS roll, Murakami etchant (3 g K 3 Fe(CN) g NaOH + ml H2O)[3], through which the carbides can be identified by their color and contrast, was used. The specimens were observed by using optical microscope and the chemical compositions of carbides were analyzed by EDS(Energy Dispersive Spectroscopy). The carbide volume fractions were quantitatively measured by using image analyzer. The hardness of the matrix and carbides were measured by using a Vickers hardness tester under the load of g and 5 g, respectively. The overall hardness was measured under the load of 1 kg. The fracture toughness were tested using compact tension (CT) type specimen with sharp notch of 3~5 µm by ASTM-E399. The loading speed was 1 MPa m/sec. The thermal fatigue test was performed by varying the maximum temperature (T max ) with the fixed minimum temperatures (T min ) of 2 C. T max was in the range of 556 C under the heating and cooling rate of 1 C /sec. The thermal fatigue test was performed at a complete constraint state [5]. The fractured thermal fatigue specimens were examined by using SEM to identify the fracture mode of each roll. In addition, the thermal fatigue process was investigated by observing the morphology of the cracks formed in the region beneath the fracture surface after sectioning of the fatigue tested specimens longitudinally. The quantitative analysis of the thermal fatigue damage was performed by sequentially observing the cracks of the region from the fracture surface to about 4 mm down. The number and length of the cracks were counted and measured using an image analyzer. 3. RESULTS AND DISCUSSIONS 3.1 Microstructure Figures 1(a) through (e) are optical micrographs of the HSS rolls, which were etched by Murakami etchant to reveal the carbides morphology. Figures 1 reveals that there are 2

3 a MC b M 6C M 2C c M 2C M 6C M 7C M 7C 3 M 7C M 6C d e M 7C 3 M 7C Fig. 1. Optical micrographs of the shell region of the (a) A, (b) B, (c) C, (d) D and (e) E rolls. Etched by Murakami etchant. four types of carbides, i.e., MC (white), M 7 C 3 (gray), M 6 C (black), and M 2 C (black) in needle-like shape[3]. A roll shows a homogeneous formation of MC carbide inside the cells and M 7 C 3 and M 6 C carbides along cell boundaries (figure 1(a)). B and C roll show MC carbide inside the cells and M 7 C 3, M 6 C and M 2 C carbides along cell boundaries, more M 2 C carbide compared with A roll (figure 1(b)-(c)). D-roll consists of MC carbide inside cells and coarse M 7 C 3 carbide along cell boundaries, and M 6 C and M 2 C carbide are not almost observed (figure 1(d)). E roll is mainly composed of M 7 C 3 and coarse MC carbide along cell boundaries (figure 1(e)). The matrix is composed of tempered martensite, together with fine and spheroidal M 23 C 6 carbides precipitated in the matrix Hardness and fracture toughness Table 2 lists the overall hardness, the matrix hardness, and the fracture toughness of HSS rolls. The matrix hardness is the greatest in the E roll, followed by the D and A roll. The overall hardness is mostly dependent on the matrix hardness due to the additional effect from the total carbide fraction. The value deducting the Stoichiometric Carbon Equivalent(SCE) from the overall carbon content roughly equals the amount of carbon contained in the matrix. The tendency of matrix hardness is well consistent with SCE (C%-SCE) shown in table 2. It indicates that the matrix hardness is greatly affected by the carbon content remained in the matrix. The fracture toughness decreases in the order of A to E rolls. This value decreases with increase in the overall hardness and the matrix hardness increases, and with increase in the amount of M 7 C 3, M 2 C, and M 6 C carbides formed in network along cell boundaries. 3.3 Thermal fatigue properties The rolls in hot rolling mills experience thermal fatigue damage due to the repeated temperature fluctuation as they come into contact with the rolled plates at high 3

4 Table 2. Volume fraction of carbides, SCE, hardness, and apparent fracture toughness data. Roll Volume Fraction of Eutectic Carbides (%) MC M 7 C 3 * Total SCE** C%- SCE*** Overall Matrix Fracture Hardness Hardness Toughness (Hv) (Hv) (MPa m) A B C D E * Small amount of M 2 C, M 6 C carbides are included in this value. ** SCE=.6Cr(%)+.33W(%)+.6Mo(%)+.2V(%) ; Stoichiometric Carbon Equivalent *** This value estimates the maximum carbon content contained in the matrix. temperature followed by water cooling. These thermal fatigue properties are closely connected with the roll life. In this study, the specimen temperature is sequentially controlled so that it can simulate the temperature variation of the rolls in hot rolling. The specimen in the thermal fatigue test was put under repeated compressive and tensile stresses until they eventually failed. That is, the specimen experiences a compressive stress during heating and tensile stress during cooling due to the volume expansion and contraction of the specimen at the complete constraint state. To restrain the volume change of the specimen, compressive and tensile stress is applied on the specimen repeatedly. The stress in elastic limit for a specimen at a complete constraint state can be expressed as:[6] σ = E α T (1) where E and α are elastic modulus and liner expansion coefficient, respectively. Figures 2 are the results of the thermal fatigue life at a complete constraint state by varying T max with T min set at 2 C. At a given T, A roll shows the longest thermal fatigue life followed by the order of C, B, D, and E roll. The thermal fatigue life of HSS rolls decreases as the T increases. The thermal fatigue life is coincident with the apparent fracture toughness and increases as the fracture toughness improves. Figures 3(a) to (e) show the variations of the tensile stresses for five HSS rolls as the temperature range changes. The tensile stress gradually increases as the number of thermal fatigue cycle increases. This is associated with the cyclic softening of the materials. The tensile stress at each cycle shows generally high values as T increase, and at the same roll have a high value as T increase because the σ increase with an increase in T as Eq.(1) indicates. 4

5 625 6 T max A-Roll B-Roll C-Roll D-Roll E-Roll Fig. 2. Effect of the (T max ) on thermal fatigue life(t min = 2 C) A Number of Cycles to Failure (N f ) 2-6 B maximum temperature Number of Cycles Number of Cycles C 2-6 D Number of Cycles Number of Cycles E Fig. 3. Internal tensile stress Number vs of fatigue Cycles cycles in the HSS rolls. 3.4 Hardness and microstructural change upon thermal fatigue Figures 4 show the matrix and the overall hardness after thermal fatigue test. In A roll, the decreasing rate of hardness in the matrix is similar with that of the overall because the high volume fraction of the matrix (%) dominates the hardness of the overall bulk. Since the hardness is related with the microstructural evolution of the matrix, the microstructural change of the matrix at each temperature range is shown in Figures 5(a) and (b). In the temperature range of 255 C (Figure 5(a)), the small amount of martensite is 5

6 decomposed, and the hardness of the matrix was reduced by 12% of initial hardness. However, the substantial amount of martensite in the temperature range of 26 C is decomposed as shown in Figure 5(b), thereby reducing the hardness considerably 3%. In B roll, the amount of overall hardness change is smaller than that of matrix hardness due to the smaller volume fraction of the matrix than A roll (table 2). Figures 5(c) and (d) show the microstructural change of the matrix at each temperature range. In the temperature range of 255 C (Figure 5(c)), the small amount of martensite is decomposed and carbides are precipitated locally. However, the small amount of martensite in the temperature range of 26 C were decomposed compared to the temperature range of 255 C (figure 5(d)). The variations of hardness with temperature change of A and B roll is different. This is related with the exposed time in high temperature. Thus, the results up to this point indicate that the hardness decrease is directly related to the decomposition of martensitic phase by the change of T and expose time in high temperature. Vickers hardness 7 6 Matrix Bulk 4 A-RT A-55 A-6 B-RT B-55 B-575 B-6 Tem perature Fig. 4. Graphs showing Vickers hardness of the matrix and the overall bulk vs temperature range in the A and B rolls Observation of fracture surface and thermal fatigue cracks Figures 6 shows the Surface crack initiation sites were investigated by observing the region beneath the fracture surface by sectioning the specimen longitudinally. The coarse carbides formed along cell boundaries were found to be the preferred crack initiation sites since they have little ductility and high Young s modulus in addition to the large difference in the expansion coefficient with that of the matrix [7]. The number of cracks and their average length on the specimen surface for each roll was measured by using image analysis. The number of cracks and their length increase as T max or T increases at a given T min. The length of the surface cracks is mostly several micrometers, which agrees with reports that the crack length leading to fracture is about 1 µm [-9]. Therefore, the microcracks formed at surface are an important factor to determine the thermal fatigue life. Cracks were propagated along the hard and brittle coarse carbides a b 5 5 c d 6

7 Fig. 5. SEM micrographs of the matrix at T max of (a) 55 C and (b) 6 C for the A roll and (c) 55 C and (d) 6 C for the B roll. A B C D E Fig. 6. SEM micrographs of the surface crack initiation site of the thermal fatigue specimens subjected to thermal fatigue cycles in the temperature range of 2~575 for the HSS rolls (M 7 C 3, M 2 C, and M 6 C carbide), and no crack propagation through the matrix was observed except when cracks in adjacent carbides were connected through the matrix. The thermal fatigue process is very complicated in the HSS rolls that consist of hard and coarse primary carbides and the matrix of tempered martensite. The factors that determine the thermal fatigue property in those rolls are 1) the distribution and volume fraction of primary carbides, 2) the increase of tensile stress by cyclic softening, 3) the matrix hardness, and 4) the frequency of crack formation in the surface region under thermal fatigue. The fracture toughness and thermal fatigue properties were improved as the volume fraction of coarse M 7 C 3, M 2 C, and M 6 C carbides decrease, which is because the frequency of crack formation in the surface region is reduced. Therefore, the thermal fatigue life is dependent on the amount of coarse carbides located in cell boundaries. According to those results, the thermal fatigue life of the rolls are determined by the collective effect of the size and volume fraction of the coarse primary carbides, the frequency of surface crack formation, and the increase of tensile stress by cyclic softening. The centrifugally cast HSS roll has cellular solidification structure, and coarse primary carbides segregated along the cell boundary may provide an easy crack propagation path. Therefore, to improve the thermal fatigue property, it is desirable to reduce the segregated cell-boundary carbides and to reduce the cell size for refining the carbides and to inducing more uniform distribution of them. 7

8 4. CONCLUSIONS The thermal fatigue properties of the centrifugally cast five HSS rolls are quantitatively evaluated and the results were analyzed in relation to the microstructure and mechanical properties. 1) Cracks were formed on the specimen surface by the repetitively applied tensile and compressive stresses, and the thermal fatigue life decreased with an increase in the number density of the surface cracks. 2) The thermal fatigue life of the HSS rolls was reduced as the temperature range increased. Fatigue failure occurs when the tensile stress applied to the specimen approaches to the tensile strength of the roll materials since the tensile strength of the material decrease with an increase of exposure time at elevated temperature by cyclic softening. Also, the thermal fatigue life increases as the facture toughness increases and the volume fraction of the coarse carbide along cell boundaries decreases. 3) The factors that influence the thermal fatigue property in those rolls are the distribution and volume fraction of primary carbides, cyclic softening that results in the increase of tensile stress, the matrix hardness, and the frequency of crack formation in the surface region. To improve the thermal fatigue properties, it is desired to reduce the intergranular carbide precipitation, and to refine the carbides REFERENECES 1. J. C. Werquin and J. C. Cailand : Roll for the Metal Working Industries, R. B. Corbett (ed.), Iron and Steel Society, Inc., (199) W. M. Betts and H. L. Baxter : Roll for the Metal Working Industries, R. B. Corbett (ed.), Iron and Steel Society, Inc., (199) L. A. Dobrzanski : Steel Res., 57 (196) ASTM Standard test Method for Plane-Strain Fracture toughness of Metallic materials, ASTM E399-3, ASTM, Philadelphia, PA 5. G. Stevens, K. P. Ivens and P. Harper : JISI, 29 (1971) 1, 1 6. G. E. Dieter : Mechanical Metallurgy, Mc-Graw Hill, (197) J. J. debarbadillo : Iron and Steel Engineer (191) 63. B. Tomkins and J. Wareing: Met. Sci., 11 (1977) J.C. Grosskreutz: J. Appl. Phys., 33, 5 (1962) 177.