Fatigue and Fatigue Crack Growth Behavior of Tool ...

Proceedings of the 1st WSEAS International Conference on MATERIALS SCIENCE (MATERIALS'08)

Fatigue and Fatigue Crack Growth Behavior of Tool Steel Z. SAJURI*, J. SYARIF, M. Z. OMAR, M. M. ALLAFI and M.F. SUDAR Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, MALAYSIA

Abstract: - Fatigue and fatigue crack growth (FCG) behaviors of tool steel were investigated at room temperature. Fatigue tests were performed under a load control with sinusoidal waveform of stress ratio R=0.1 at a frequency of 10Hz. The fatigue crack was identified nucleated from inclusion presents at specimen surface. The inclusion became the stress raiser and acts as the fatigue fracture origin. The fatigue limit value was observed at 396Mpa. The threshold stress intensity factor range obtained from the fatigue crack growth test was 8.06MPam1/2. The fatigue crack growth curve was then used to predict the fatigue life based on modified Paris equation. The predicted lives were in good agreement with the experimental results. Key-Words: - Fatigue, FCG, Inclusion, Stress intensity factor, Life prediction, Tool steel In this study, fatigue and fatigue crack growth behavior of KRUPP 2510 steel was investigated in laboratory environment. Based on the fatigue crack growth curve obtained, fatigue life of round bar specimens under constant amplitude loading was predicted.

1 Introduction Materials used for dies in the forging process always operate under conditions such as very high pressure and repeating large impact loads. These severe conditions may result in brittle fracture of the dies. To prevent from sudden brittle fracture, die materials must have high strength and toughness. The commonly used commercialized tool steel selected for hot and cold dies including SKD6 [1], SK4, SKS3, SKD11 and SKH9 [2]. These tool steels give good hardening under normal heat treatment, produce small dimensional changes and exhibit sufficient toughness. Thus, they are recommended for cold forging, blanking and bending dies applications. Dies for hot forging operations are subjected to couple mechanical and thermal cycles. Crack initiation and propagation on die surface are induced both by thermal gradients acting in the layer near the contact surface with the billet and by the superimposed stresses due to the mechanical cycles, which deeply influence their thermo-mechanical fatigue life [3]. Cyclic stresses in long-term operation, often leads to mechanical failures of the dies such as fatigue fracture, tensile fatigue, fracture contact fatigue fracture and bending fatigue fracture. Therefore, the lifetime of such dies is mainly controlled by fatigue damage, comprising of the fatigue crack initiation and fatigue crack growth stages. The fatigue cracks are simultaneously generated by cracking of carbides, debonding of carbide / metallic matrix interfaces or by crack initiation in the matrix [4].

ISSN: 1790-2769

2 Experimental Procedures The material used in this study was KRUPP 2510 steel that typically used as cutting tools, punchers and dies etc. The chemical composition of the material used is shown in Table 1. The microstructure of the steel is shown in Fig. 1. Axialcylindrical fatigue specimens with threaded ends were machined from cylindrical rod following ASTM E466. Table 1 Chemical composition of tool steel (wt %) C 0.95

Si 0.3

Mn 1.2

Cr 0.5

W 0.5

V 0.1

Fig.1 Microstructure of the steel used.

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ISBN: 978-960-474-024-6

Proceedings of the 1st WSEAS International Conference on MATERIALS SCIENCE (MATERIALS'08)

Crack growth curves (crack growth rate da/d vs. stress intensity factor range ∆K) were obtained by ∆K-decreasing and ∆K-increasing test procedures. The decreasing and increasing load steps was below 10% of the previous loading. The crack lengths were measured on the front and back surfaces of the specimen by a replica technique. The threshold value of stress intensity factor range ∆Kth was determined when a crack growth was not observed for 106 cycles. The stress intensity factor values for SECT specimen were calculated according to the following equation:

The specimen gauge diameter and gauge length were 4 mm and 15 mm, respectively. After machining, the specimens were polished in the longitudinal direction of the specimen with 500 to 1500 grit emery papers to get a smooth surface. All fatigue tests were performed under a load control with sinusoidal wave form of stress ratio R=0.1 at a frequency of 10Hz. The cyclic loading was continued until specimen failure. The test was stopped when the specimen did not fail up to 107 cycles. The stress level at this life was then defined as the fatigue limit. The fracture surfaces and specimen surfaces were observed under scanning electron microscope (SEM) to determine the fatigue fracture origin and fatigue crack growth behavior. Prior to the fatigue and fatigue crack growth tests, Vickers hardness and tensile properties of the material was investigated. The tensile tests were performed at constant strain rate of 2×10−3 sec−1. A single edge cracked tension (SECT) specimen was used for fatigue crack growth test. A screw type fixture was used so that tension-compression loading can be applied. Figure 2 shows the specimen geometry. To avoid the excessive lateral deflection or buckling of SECT specimen during the test, the gage length and thickness of gage position was limited to 40 and 4 mm, respectively. The gage part was polished with 500 to 1500 grit emery papers to obtain a smooth surface. An edge notch was introduced at one edge of the specimen to facilitate fatigue pre-cracking. The stress ratio R during precracking was the same as the stress ratio used in the fatigue crack growth test. The pre-crack was attained at crack growth rates less than 10-8 m/cycle. The pre-cracking was stopped after a pre-crack length reached 0.1B (B is the thickness of gauge part). The test was conducted on a servo-pneumatic universal testing machine with a maximum capacity of 14kN. The test was performed under the same loading condition as in the fatigue test.

∆K = ∆σ πa ⋅ Y

(1) Here, Y is a boundary correction factor which is a function of the ratio of crack length a to the width of the specimen W. The boundary correction factor is given as: Y = 1.12-0.231α+10.55α2-21.72 α3+30.39 α4 (2) where

α = aW

(3)

3 Results and Discussion 3.1 Hardness and tensile properties The measured hardness of the KRUPP 2510 was 205Hv. Table 2 shows the tensile properties of the above material. The tensile properties, i.e. yield stress, ultimate tensile strength and percentage of elongation were 410MPa, 688MPa and 22%, respectively. Table 2 Tensile properties of tool steel Elastic Yield Ultimate tensile Elongation strength (%) modulus stress (MPa) (GPa) (MPa) 198 410 688 22

Stress range (MPa)

600 550 500 450 400 350 300 4 10 105 106 107 108 Number of cycles to failure, Nf(cycles) Fig. 3 S- curve for KRUPP 2510.

Fig.2 SECT test specimen used.

ISSN: 1790-2769

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ISBN: 978-960-474-024-6

Proceedings of the 1st WSEAS International Conference on MATERIALS SCIENCE (MATERIALS'08)

3.2 Fatigue and fatigue crack growth behavior The relationships between stress range ∆σ and number of cycles to failure f is shown in the form of S- curve in Fig. 3. The fatigue life increased with decreasing stress range. The fatigue limit value was observed at 396Mpa. The ratio of fatigue limit to the corresponding tensile strength was about 0.58. This value was in the same range to the values generally observed for other steel materials. Figure 4 shows the scanning electron microscope (SEM) images of fracture surface for the specimen tested at stress range of 450MPa under the frequency of 10 Hz. The figure shows a typical fatigue fracture surface. A crack initiates from a site on circumference as shown in Fig. 4(a). The fracture surface is divided into fatigue crack propagation and the rapid ruptured region, which is marked with A and B, respectively. In Fig. 4(a), the appearance of circular arc on the fracture surface corresponds to the transition boundary from the fatigue crack propagation to rapid fracture. The fatigue fracture surface is relatively flat in comparison with that in the final ruptured region. The fatigue fracture origin was identify to be nucleated from an inclusion at specimen surface as shown in Fig. 4(b). From the figure, it is also can be identified that many inclusions presented at near specimen surface or at sub-surface. This inclusion became the stress raiser and acts as the fatigue fracture origin. The size of the inclusion was about 10 to 20 µm. Figure 5 is the fatigue crack growth rate vs. stress intensity factor range. Vertical axis is the fatigue crack growth rate and horizontal axis is the stress intensity factor range. Arrow in the figure indicate the threshold value of stress intensity factor range ∆Kth. From the result it was found that the threshold stress intensity factor range obtained was 8.06MPam1/2. At the top right side of the crack growth curve, the ∆K became very high and approaching the critical stress intensity factor range value KIC. The final ∆K value measured during the test was 150 MPam1/2. This value was then considered as the KIC value for determination of fatigue life. The Paris’s power law parameters to draw a fitting curve C and m, were 3.29x10-12 and 2.71, respectively. The solid line in Fig. 5 represented the fitting curve.

A

B

(a) Overview of fracture surface

(b) Fatigue fracture origin Fig.4 SEM micrographs showing (a) the overview of specimen fracture surface and (b) the fatigue crack initiation site of specimen tested at stress range of 450 Mpa.

10-4

Crack growth rate, da/d (m/cycle)

10-5

Fitting curve Experimental data

10-6 10-7 10-8 10-9 10-10 10-11 10-12 1

10

100

Stress intensity factor range, ∆σ (MPam1/2) Fig.5 Relationship between da/d and ∆K for the KRUPP 2510 tool steel.

ISSN: 1790-2769

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ISBN: 978-960-474-024-6

Proceedings of the 1st WSEAS International Conference on MATERIALS SCIENCE (MATERIALS'08)

Fatigue life for the tool steel was predicted based on the fracture mechanics, where the fatigue life was assumed to be equal to the crack growth life and the size of the defect or inclusion observed on the fracture surface origin was assumed as the initial crack length, ao. It was also assumed that final failure occurred when the crack length reached the critical crack length, ac where the ΔK will be exceeded the fracture toughness or the critical stress intensity factor range value KIC. Fatigue life can be predicted based on the Paris equation modified by the threshold stress intensity factor range, ∆Kth, as follows:

(

da = C ∆K m − ∆K thm d

)

4 Conclusion Fatigue and fatigue crack growth behavior of KRUPP 2510 tool steel were investigated. It is found that inclusion served as the fatigue crack initiation site. The fatigue limit was measured at stress range level of 396MPa. From the fatigue crack growth test, the threshold stress intensity factor range was 8.06MPam1/2. The fatigue life prediction based on the modified Paris equation for the tool steel was in good agreement with the experimental results.

References: [1] Y. Sun, S. Hanaki, H. Uchida, H. Sunada and N. Tsujii, Fatigue Strength of Laser-processed Hot Work Tool Steel, ISIJ International, Vol. 43, No. 1, 2003, pp. 127–131. [2] Y. C. Lee and F.K Chen, Fatigue Life of ColdForging Dies With Various Value of Hardness, Journal of Materials Processing Technology, Vol. 113, 2001, pp. 539–543. [3] A. Berti and M. Monti, Thermo-Mechanical Fatigue Life Assessment of Hot Forging Die Steel, Fatigue & Fracture of Engineering Materials & Structures, Vol. 28, No.11, 2005, pp. 1025-1034. [4] G. Jesner, S. Marsoner, I. Schemmel, K. Haeussler, R. Ebner, R. Pippan, Damage Mechanisms in Materials for Cold Forging Dies Under Loading Conditions Typical for Dies, Proceedings of the 7th International Tooling Conference "Tooling materials and their applications from research to market", 2006, pp. 29-36. [5] Raju IS, Newman JC. Stress-intensity Factors for Circumferential Surface Cracks in Pipes and Rods Under Tension and Bending Loads. Fracture Mechanics: ASTM Special Technical Publication 905, No.17 1986, pp. 789-805.

(4)

The defect was assumed as a semi-elliptical crack and the K-value was estimated by the equation proposed by Newman and Raju [5]:

∆K = 1.12 ∆σ π a

(5)

Q

where, Δσ is the stress range, Q (=2.464) is the shape factor for an ellipse. By substituting Eq. (5) into Eq. (4), the number of cycles to failure can be calculated by the following equation:

1

ac

 =∫

ao

(

C 1.12∆σ

π Q

)

m

a

m

da 2

− C∆K

m th

(6)

where, ao and ac are the initial defect size and the critical fatigue crack length to failure, respectively. The result of fatigue life prediction for the tool steel is shown in Fig. 6. The predicted lives were in good agreement with the experimental results.

Stress range (MPa)

600 Experimental data Prediction curve

550 500 450 400 350

300 4 10 105 106 107 108 Number of cycles to failure, Nf (cycles) Fig.6 Fatigue life prediction of tool steel

ISSN: 1790-2769

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ISBN: 978-960-474-024-6