DOI 10.1007/s11003-016-9971-8 Materials Science, Vol. 52, No. 3, November, 2016 (Ukrainian Original Vol. 52, No. 3, May–June, 2016)

LOW-CYCLE FATIGUE OF STEELS AFTER ION NITRIDING IN HYDROGEN-FREE ATMOSPHERES P. V. Kaplun1 and V. A. Gonchar1,2

UDC 621.785.6

We present the results of investigation of low-cycle fatigue in bending for different types of steel after ion nitriding in hydrogen-free saturating atmospheres. The recommendations concerning the field of applicability of this technology of surface hardening in the process of operation are given. We make a conclusion that the procedure of ionic hardening in hydrogen-free atmospheres makes it possible to change the physicomechanical properties of the surface layers in broad ranges and substantially increase the low-cycle fatigue strength of structural steels. The maximum durability is attained by optimizing the parameters of the process of diffusion saturation. Keywords: fatigue, bending, ion nitriding, deformation, surface.

The results of investigations of high-cycle fatigue in the process of bending of structural steels after ion nitriding revealed a substantial increase in their strength [1, 2]. The fatigue limit of specimens subjected to preliminarily ion nitriding in a hydrogen-free atmosphere (60 vol.% N 2 + 40 vol.% Аr) becomes 1.9 times higher (increases from 190 to 370 МPа) in the case of testing in air and 3.6 times higher (increases from 30 to 110 МPа) in a 3% NaCl solution as compared with the nonnitrided specimens. At the same time, the fatigue strength of the specimens preliminarily nitrided in a hydrogen-containing atmosphere (60 vol.% N 2 + 40 vol.% H 2 ) and tested in 3% NaCl was lower by 25% than for the specimens preliminarily nitrided in a hydrogen-free atmosphere. This effect is caused by the detrimental influence of hydrogen leading to the decohesion of the crystal lattice of the metal, the interaction of its atoms with dislocations in the metal, the pressure of molecular hydrogen in the microcavities of steel, its chemical interaction with the components of the alloy, and the precipitation of hydrogen-containing phases [3]. The hydrogen-free saturating atmospheres (mixtures of nitrogen and argon) improve the plastic properties of nitrided layers as a result of a more intense backward cathode spraying and the formation of less brittle phases and even of the α -phase on the surface [4]. At the same time, for practical purposes, it is important to know for what levels of elastoplastic strains and relative elongations, it is reasonable to apply the proposed technology. This is the main aim of our investigation. Testing Methods We used an IP-2 testing machine [5], which enables one to test plane specimens by pure bending of constant sign under elastoplastic deformation at a frequency of 23 min –1 . 1 2

Khmel’nyts’kyi National University, Khmel’nyts’kyi, Ukraine. Corresponding author; e-mail: [email protected].

Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 52, No. 3, pp. 95–98, May–June, 2016. Original article submitted September 8, 2015. 402

1068-820X/16/5203–0402

© 2016

Springer Science+Business Media New York

LOW-CYCLE FATIGUE OF STEELS AFTER ION NITRIDING IN HYDROGEN-FREE ATMOSPHERES

403

Fig. 1. Low-cycle durability of unhardened (curves 1–4) and nitrided ( T = 560°С, P = 265 Pа, τ = 4 h) (5–8) 45Kh steel (1, 8), 38KhMYuA steel (2, 7), 45 steel (3, 6) and 20 steel (4, 5) in the acid medium.

Table 1. Physicomechanical Characteristics of Specimens Prior to and After Hardening

Steel

Yield strength, σ y

Ultimate strength, σ u

Elongation, δ

МPа

Relative narrowing, ψ %

20

234/332

385/493

21/15

55/39

45

342/448

580/668

15/11

43/32

45Kh

846/834

1020/1003

10/6

51/26

38KhMYuA

875/948

1051/1182

9/5

42/22

Comment: In the numerator and denominator, we present the data obtained prior to and after nitriding, respectively,.

Specimens of 20, 45, 45Kh, and 38KhMYuA steels subjected to preliminary normalizing and martempering were studied in different media: in an acid medium (5 g/liter of a buffering solution of citric acid and 10 g/liter of disodium hydrogen phosphate) with рН 6.5; in an alkaline medium (aqueous solution of calcium oxide) with рН 13, and in a neutral medium (condensate of evaporators) with рН 7.0. Their physicomechanical characteristics prior to and after ion nitriding at T = 560°C under P = 265 Pа for τ = 240 min in a 75 vol.% N 2 + 25 vol.% Ar atmosphere are presented in Table 1.

P. V. KAPLUN

404

AND

V. A. GONCHAR

Fig. 2. Low-cycle durability of martempered (1–4) and nitrided (T = 560°С, P = 265 Pа, τ = 4 h) (5–8) 45Kh steel tested in different media: (1, 5) alkaline medium (рН 13); (2, 6) air; (3, 7) neutral medium (рН 7); (4, 8) acid medium (рН 6.5).

Table 2. Influence of the Technological Parameters of Ion Nitriding on the Number of Cycles to Fracture for Specimens Subjected to Elastoplastic Deformation ( ε = 0.75%) in the Acid Medium Modes of ion nitriding Steel

N 2 , P = 265 Pа, τ = 4 h at T , °С

T = 560°С, P = 265 Pа, τ = 4 h for various N 2 contents (%) of the N 2 + Ar atmosphere

T = 560°С, τ = 4 h, 75 vol.% N 2 + 25 vol.% Ar atmosphere, under P , Pа

520

560

600

45

60

75

90

100

80

265

450

20

2694

2536

2416

2518

2524

2581

2553

2536

2519

2581

2540

45

2843

2621

2482

2619

2642

2694

2661

2621

2596

2694

2598

45Kh

3156

2395

1693

2408

2425

2492

2394

2315

2388

2492

2428

38KhMYuA

3191

2484

2028

2415

2427

2485

2459

2484

2395

2485

2446

Results of Investigations It was established (Fig. 1) that, in the investigated range of variation of the amplitudes of deformation ( ε = 0.5–4.0%), the durability of nitrided and nonnitrided specimens varies within the range 10 2 –10 5 cycles to fracture and its relationship with the level of strains is described by a straight line on double logarithmic coordi-

LOW-CYCLE FATIGUE OF STEELS AFTER ION NITRIDING IN HYDROGEN-FREE ATMOSPHERES

405

nates of the number of cycles and amplitudes of deformation in the case of testing materials in different media. If ε > 0.3%, the durability of nitrided specimens is smaller than that of nonnitrided specimens, and this decrease rises with deterioration of the plastic properties of the steels (Table 1) and with increase in the strain. This is a general regularity under low-cycle fatigue of the tested steels [6]. The corrosive medium decreases more noticeably the low-cycle durability of high-strength steels [7]. At equal strains, high stresses, which accelerate corrosion at crack tips, arise in these steels. Thus, at substantial strain, steel 45 and steel 20 have a larger durability than 45Kh and 38KhMYuA steels (Fig. 1). The number of cycles to fracture of nitrided and unhardened specimens of 45Kh steel in the alkali medium is larger than that in air, the neutral medium, and acid medium (Fig. 2). This can be explained by the fact that, under these conditions, a hydroxide layer, which promotes an increase in the durability, forms on their surface [6]. In the process of substantial electrochemical dissolution of steels in the acid medium, we observe the intense formation of stress concentrators, which decrease their fatigue strength. In the less corrosive medium, the low-cycle durability of nitrided and nonnitrided 45Kh steel is higher. However, with increase in the amplitude of deformation, the influence of the medium on the durability becomes weaker, and the role of plastic deformation increases. The durability of nitrided specimens for ε ≈ 2.5% and nonnitrided specimens for ε ≈ 4% in air and corrosive media coincide. It was established (Table 2) that the temperature of the process of nitriding exerts the most pronounced effect on the low-cycle fatigue of steels. Thus, as the indicated temperature increases from 520°С to 600°С, the durability of the specimens tested in the acid medium for an elastoplastic strain ε ≈ 0,75% decreases by a factor of 1.1–1.9, and the higher value of the durability corresponds to more high-strength 45Kh and 38KhMYuA steels. The decrease in durability under the conditions of low-cycle fatigue observed as the nitriding temperature increases is connected not only with the changes in the physicomechanical characteristics and phase composition of the nitrided layer but also with the changes in the structure of the base metal and, especially, at temperatures higher than the eutectoid temperature [1, 2, 6]. Note that the higher the level of elastoplastic strains under cyclic loading, the stronger the influence of the indicated parameters. The influence of the nitrogen potential and the pressure of the saturating medium on the low-cycle durability is less significant and has an extreme character with maxima in the 75 vol.% N 2 + 25 vol.% Ar atmosphere under a pressure of 265 Pа. CONCLUSIONS The results of the investigation of the low-cycle fatigue of steels demonstrate that it is not reasonable to perform nitriding for elastoplastic strains with ε ≥ 0.3% because, in this case, their durability is lower than the durability of the nonnitrided steels under the same conditions. The procedure of ion nitriding in hydrogenfree atmospheres makes it possible to change the physicomechanical properties of the surface layers in broad ranges and substantially increase the low-cycle fatigue strength of structural steels subjected to bending in different media. The maximum durability is attained by optimizing the parameters of the process of diffusion saturation. REFERENCES 1. V. G. Kaplun, A. V. Kapinos, and Yu. I. Babei, “Influence of ion nitriding on the fatigue resistance of steel 20 in bending,” Fiz.Khim. Mekh. Mater., 24, No. 5, 108–109 (1988); English translation: Mater. Sci., 24, No. 5, 108–109 (1988). 2. V. G. Kaplun, A. E. Rudyk, Ya. N. Gladkii, S. M. Stechishin, “Influence of the parameters of ion nitriding on the endurance of 45Kh steel in an acid medium,” Fiz.-Khim. Mekh. Mater., 22, No. 5, 101–103 (1986); English translation: Mater. Sci., 22, No. 5, 535–538 (1986).

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3. V. V. Panasyuk (editor), Fracture Mechanics and Strength of Materials: A Handbook, Vol. 1: V. V. Panasyuk, A. E. Andreikiv, and V. Z. Parton, Fundamentals of Fracture Mechanics [in Russian], Naukova Dumka, Kiev (1988). 4. V. G. Kaplun and I. M. Pastukh, “Plasma-diffusion processes of modification of the metal surfaces: technology and equipment,” in: Proc. of the Sci.-Pract. Symp. “Equipment and Technologies of Heat Treatment of Metals and Alloys in Mechanical Engineering” [in Russian], OTTOM, Kharkov (2000), pp. 145–154. 5. V. I. Tkachev and Yu. I. Babei, “Type IP-2 machine for short-endurance fatigue tests on metals in liquid media,” Fiz.-Khim. Mekh. Mater., 2, No. 2, 228–229 (1966); English translation: Mater. Sci., 2, No. 2, 167–168 (1966). 6. G. V. Karpenko, Physicochemical Mechanics of Structural Metals [in Russian], Vol. 1, Naukova Dumka, Kiev. 7. H. H. Uhlig, Corrosion and Corrosion Control: an Introduction to Corrosion Science and Engineering, Wiley, New York (1963).