Investigations of fatigue behaviour and microstructure of titanium alloy and low-carbon steel

Revue de génie industriel 2010, 5, 80-88

Revue de Génie Industriel ISSN 1313-8871 http://www.revue-genie-industriel.info

Investigations of fatigue behaviour and microstructure of titanium alloy and low-carbon steel Lyubov Nikolova*, Zornitsa Angelova, Zoya Naydenova University of Chemical Technology and Metallurgy, Sofia, Bulgaria * Auteur correspondant : e-mail : [email protected] Révisé et accepté le 15 mars 2010/Disponible sur Internet le 15 juillet 2010 Résumé La fatigue de deux alliages métalliques différents est étudiée en fonction de leur microstructure. Des caractéristiques spécifiques de la fatigue, ainsi que l'influence de la microstructure locale sur la rupture et son développement sont obtenues et analysées. Dans la Partie A l’alliage étudié est un alliage de titane Ti-6242 et dans la Partie B - acier à faible teneur de carbone, 09Mn2. Alliage de titane Ti-6242: Il y a deux domaines d'application classiques pour les alliages de titane – dans les cellules et les moteurs d’avions. Les échantillons dans cette étude sont du deuxième type d’application et sont testés sous dwell fatigue. L’effet dwell est lié à l'initiation précoce d’une fissuration. Dans ce cas, les mécanismes qui gouvernent la fatigue et l'influence de la microstructure sont identifiés par la microscopie électronique à balayage. Acier à faible teneur en carbone 09Mn2 : Des essais de fatigue sont effectués sur un acier roulé -09Mn2 à faible teneur en carbone et bas-allié pour des applications en mer avec une microstructure indiquant une séquence des bandes de ferrite et de perlite, longue et uniforme. Les essais comprennent des essais de fatigue rotation flexion et des mesures de la longueur des fissures en état de propagation sur des éprouvettes lisses divisés en deux groupes de spécimens - original et avec une surface modifiée obtenues à des conditions particulières par de traitement électriques à décharge dans un électrolyte. Pour mesurer la longueur de la fissure une technique de réplication est utilisée et pour examiner la microstructure – la microscopie électronique à balayage 3D. Abstract Fatigue in two different metal alloys is investigated in terms of their microstructure. Specific fatigue characteristics and local microstructure influence on fatigue fracture and its development are found and analyzed. In Part A the investigated alloys is titanium alloy Ti-6242 and in Part B - low-carbon steel 09Mn2. Titanium alloy Ti-6242: There are two classical application areas for titanium alloys – airframes and aero engines. The specimens for this investigation are from the second kind of application and are tested under dwell-fatigue. Dwell-effect is related to early crack initiation. In this case governing fatigue mechanisms and microstructure influence are identified by means of scanning electron microscopy. Low-carbon steel 09Mn2: Fatigue tests are carried out on a rolled low-carbon, lowalloyed 09Mn2-steel for offshore applications with microstructure revealing a sequence of long and uniform perlite and ferrite bands. The investigations include short fatigue-crack rotation-bending experiments and length measurements of propagating crack on smooth specimens divided into two groups of hour-glass specimens – original and with modified surface obtained at special conditions of electrical-discharge treatment in electrolyte. A replication technique is used for the

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Revue de génie industriel 2010, 5, 80-88 crack-length measurements and scanning electron and 3D microscopes – for microstructural investigations. Mots-clés: Dwell-fatigue, fatigue en rotation-flexion, alliage d'acier à faible teneur de carbone, alliage de titane, dislocations, fissures; mécanisme microstructurale de fatigue Keywords: Dwell-fatigue, rotating-bending fatigue, low-carbon steel, titanium alloy, dislocations, cracks; fatigue microstructure mechanism

Part A: Titanium alloy Ti-6242 Introduction Titanium alloys are widely used to manufacture gas turbines and compressor discs. Many studies are performed to investigate the influence of parameters such as temperature, applied stress, microstructure, duration of dwell periods and R ratio. The material for the present study was forged Ti-6242 alloy. Hour-glass-shaped specimens were tested under dwell-fatigue using trapeze waveform. The dual-phase Ti-6242 microstructure was found purely lamellar.

Material, specimen and testing Titanium alloy, marked as Ti-6242, used in aerospace industry, was subjected to dwellfatigue. Ti-6242 is a quasi α-alloy. The elements Al, O2, N2 and C are α-stabilizers and Mo, Cr, Fe, H2, Mn, and Si are β-stabilizers. The chemical composition of Ti-6242 and its mechanical characteristics are given in Table 1. Chemical composition Ti

Al

Sn

Zr

Mo

Si

Fe

O

N

H

Impurity

Min

balance

5.5

1.8

3.6

1.8

-

-

-

-

0.01

-

Max

balance

6.5

2.2

4.4

2.2

0.8

0.25

0.15

0.05

0.0125

0.4

Characteristics Elastic limit

Young modulus

Temperature β-transus

σе, MPa

E, GPa

Tβ, ºC

900

120

970

Table 1. Chemical composition (wt %) and mechanical characteristics of Ti-6242

The mechanical-heat treatment is forging above the β-transus temperature, inducing a purely lamellar microstructure. Specimens are taken from a forged disk in a radial and axial direction. They are cut off in hour-glass shape. Tests were carried out on a Schneck hydro-plus PSB machine.

Experimental results The specimens were tested under dwell-fatigue. To analyze the microstructure, two groups of specimens were investigated: (i) small specimens extracted from notdeformed parts of the tested specimens; and (ii) deformed fatigue tested specimens, more precisely their fractured surfaces. The geometry of the specimens is shown on Fig.1. Both groups of specimens were mechanically polished with emery paper and finally polished with diamond paste. After polishing to reveal the microstructure it was used Kroll’s reagent (1–3 ml of HF, 2–6 ml HNO3 in 100 ml of H2O).

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Revue de génie industriel 2010, 5, 80-88 All microstructural investigations are own work of the authors and done by scanning electron microscopy [1-7] in the Laboratory LETAM, University “Paul Verlaine”, Metz, France.

Small specimens are extracted from not-deformed parts of the tested specimens

Deformed fatigue tested specimens

Figure 1. Geometry of specimens.

The differences in the microstructure of the not-deformed small specimens in radial and axial direction are shown in Figure 2.

a)

b) Figure 2. Microstructure.

a) not-deformed axial specimen – type “rugby ball”; b) not-deformed radial specimen – type “pancake”

The fractured surfaces of the deformed fatigue-tested specimens in axial and radial direction are shown respectively in Figs 3a, 4 and 5a and the polished surfaces, dislocations and cracks - in Figs 3b, 3c, 5b, and 5c.

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a)

b)

c) Figure 3. Deformed axial specimen

a) fractured surface; b) polished surface; c) A- part of the fractured surface, shown in a) – crack.

Figure 4. Deformed radial specimen (part 1) – fractured surface

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B - Part of the fractured surface, which later is polished parallel to long radial-specimen axis and shown in b)

B

a)

Pile-up (agglomeration) of dislocations find on the part B of the surface, shown in a)

Crack

b)

c) Figure 5. Deformed radial specimen (part 2) a) fractured surface with a part (B) polished later in parallel to long radial-specimen axis; b) polished surface of part B (shown in a)) – pile-up (agglomeration) of dislocations; c) parts of the fractured surface, shown in a) – cracks

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Evaluation of the results Microstructure investigations show differences in the microstructure of not-deformed (radial and axial) specimens and in the orientation of their grains, which is a result from the thermo-mechanical treatment of the forged disk. The existence of cracks on the fractured surfaces and in the depth of the material is due to forming of dislocation pileups (agglomerations). All the microscopy work is done by author Lyubov Nikolova.

Summary and conclusions Two hour-glass specimens machined from radial and axial direction of a forged disc of titanium alloy Ti-6242, are subjected to dwell-fatigue. The radial specimen shows higher deformation after the mechanical-heat treatment of forging, leading to more cracks. However the axial specimen shows shorter fatigue lifetime in comparison with the radial one.

Part B: Low-carbon steel 09Mn2 Introduction In the present work short fatigue crack propagation behavior in a low-carbon, lowalloyed construction 09Mn2-steel is investigated under rotating-bending and purebending symmetric loading conditions. Two different groups of specimens are used: one of original hour-glass specimens and another one with modified surface under rotatingbending fatigue [8]. Some corresponding microstructural observations are involved for clarifying fatigue crack development.

Material specimen and testing The rolled low-carbon, low-alloyed steel, marked as 09Mn2 (Bulgarian Construction Steel Standard) was subjected to rotating-bending fatigue. The chemical composition of 09Mn2 – steel and its characteristics are shown in Table 2. Chemical composition C,%

Si,%

Mn,%

Cr,%

Ni,%

P,%

S,%

Cu,%

Al,%

As,%

0.09

0.28

1.63

0.05

0.04

0.017

0.026

0.13

0.12

0.014

Characteristics Tensile Strength, σB, MPa

Proof Strength σ 0.2, MPa

Cross section Contraction, ψ,%

Hardness HB, MPa

Average grain size, d [µm]

482

382

62.3

148

25.6

Table 2 Chemical, mechanical and microstructural characteristics of 09Mn2 –steel.

Three specimens were under investigation: specimen 1 and 2 - with microstructure and geometry, shown respectively in Figs. 6, 7 a), and specimen 3 with modified-surface and microstructure, shown on Fig. 7 b); specimen 3 has the same geometry as specimens 1and 2. The modified-surface of the specimen 3 was obtained at special conditions of electrical-discharge treatment to improve his mechanical properties [9].

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Figure 6. Geometry of specimens

The average ferrite grain size was 25.6 µm. Preliminary investigations of microstructure of specimens of 09Mn2-steel showed a sequence of long and uniform perlite and ferrite bands, with a large number of non-metallic inclusions, which is a precondition for the generation of fatigue cracks. Due to the large number of non-metallic inclusions in combination with high applied maximum stress near the boundary of the yield for this steel, necessary conditions were created for forming and growing simultaneously several outbreaks of generation of short cracks in specimens 1, 2, 3.

а)

b) Figure 7. Microstructure of specimens.

a) microstructure of specimens without modified surface and b) microstructure of specimen with modified surface.

Experimental results Tests were carried out on a table model Fatigue Rotating Bending Machine, FATROBEM-2004, designed and assembled in Fracture and Fatigue Laboratory in UCTM Sofia, [10]. The hour–glass specimens 1, 2 and 3 were tested at stress ranges ∆σ= 550, 670 and 620 MPa under conditions of rotating-bending fatigue. The applied stress ratio was R=–1, and the frequency –11Hz. The specimens were tested to fracture. Cracks appearing on their surfaces were monitored by replication technique and crack’s length – measured under light microscope. The fractured surfaces of specimen 1 are shown in Figs. 8 a),b),c), of specimen 2 in Figs. 9 a),b),c) and specimen 3 in Figs. 10 a),b),c). All microstructural investigations are own and done by scanning electron microscopy [7] in the Laboratory LETAM, University “Paul Verlaine”, Metz, France.

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a)

b)

c)

Figure 8. Fractured surfaces of specimen 1 tested at stress range ∆σ = 550 MPa a) outbreaks of generation of cracks, b) striations resulting from the formation of extrusions and intrusions and c) Zones of final failure (not caused by fatigue)

a)

b)

c)

Figure 9. Specimen 2 tested at stress range ∆σ = 670 Mpa a), b), c) cracks appearing on the surfaces of specimen

a)

b)

c)

Figure 10. Fractured surfaces of specimen 3 tested at stress range ∆σ = 620 MPa a) outbreaks of generation of cracks, b) striations resulting from the formation of extrusions and intrusions and c) zones of final failure (not caused by fatigue)

In these pictures are shown typical fractured zones – zone of fatigue fracture of striations resulting from the formation of intrusions and extrusions during cyclic loading, and zone of final failure having prominent plastic characteristics.

Evaluation of the results Fatigue data – crack lengths and the corresponding number of cycles – are plotted in a presentation “Crack Length, a – Cycles, N” and shown in Figure 11.

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Crack lenght, a [10-6 m]

1,E+04

1,E+03

540 MPa 1,E+02

670 MPa 620 MPa 1,E+01 1,E+04

1,E+05

1,E+06

Number of cycles, N [cycles] Figure 11. Plot “Crack Length, a – Cycles, N”

Summary and conclusions Two groups of hour-glass specimens of low-carbon steel without and with a modified surface are subjected to rotation bending fatigue. The specimen with the modified surface shows shorter fatigue life in comparison with the basic specimens. More tests and experience are needed considering the usual large scatter in fatigue data.

Acknowlegments This study was supported by the Laboratory LETAM, University “Paul Verlaine” – Metz, France. References 1. Lutjering J. C. Williams, In Titanium, ed. Springer, 2007 2. Humphreys F. J., Journal of Materials Science 2001, 36, 3833 – 3854 3. Gerland M., P. Lefranc, V. Doquet , C. Sarrazin-Baudoux, Materials Science and Engineering A 2009, 507, 132–143 4. Bridier F., P. Villechaise,*, J. Mendez, Acta Materialia 2005, 53, 555–567 5. Lefranc P., V. Doquet, M. Gerland, C. Sarrazin-Baudoux, Acta Materialia 2008, 56, 4450–4457 6. Lefranc, P., C. Sarrazin-Baudoux, V. Doquet, J. Petit, Scripta Materialia 2009, 60, 281–284[7] Fractography, ASM Handbook, 1987, 12. 8. Krastev D., B. Stefanov, B. Yordanov, D. Angelova, International Virtual Journal for Science, Technique and Innovations, February 2009, MTM 09, 60-63. 9. Angelova D., R. Yordanova, In the Book “Fracture Mechanics of Materials and Structural Integrity”, 4th International Conference, 23-27 June 2009, Lviv, Ukraine, pp. 309-314, 2009. 10. Davidkov A., On factors influencing fatigue in 09Mn2 steel, PhD Thesis, University of Chemical Technology and Metallurgy, Sofia, 2007

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