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  • 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 111 BEND CRACKING BEHAVIOR OF HYDROGENATED LOW STRENGTH STRUCTURAL STEEL UNDER DIFFERENT HEAT TREATMENT CONDITIONS Amjad Saleh El-Amoush1 and Salman A. Al-Duheisat2 1 Al-Balqa Applied University, College of Engineering, Materials and Metallurgical Eng, Al-Salt 19117, P. O .Box 7181, Jordan 2 Faculty of Engineering Technology, Al Balqa Applied University P.O. Box 15008, Amman – Jordan 1. ABSTRACT The bend cracking behavior of the low strength structural steel type A36 was studied under cathodic charging in three-point bend system. Different heat treatments regimes were applied to the material in order to obtain various grain sizes. The test results revealed that the low-angle grain boundaries in a structural steel samples of small grain size are less susceptible to hydrogen damage than those of the high-angle grain boundaries associated with large grains. Furthermore, it was found that the type and amount of hydrogen cracking depend on the grain size. Intergranular cracking (IG) was found to occur in the structural steel samples having both smaller and larger grain sizes. The amount and number of hydrogen cracks were found to increase with increasing the grain size. It was observed from the test results that the increase charging time resulted in an increase of a number of hydrogen cracks on the surface of the structural steel samples. Keywords: Three-Point Bending, Structural Steel Type A36, Hydrogen Charging. 2. INTRODUCTION Low strength structural steel type A36 is the principal steel for building constructions, bridges and other structural uses. However, this material must be used carefully in structures exposed to hydrogen. The combined action of stress and hydrogen environment may result in the so-called environmentally assisted cracking. Hydrogen effect is greater near room temperature and decreases INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2014): 7.8273 (Calculated by GISI) www.jifactor.com IJARET © I A E M E
  • 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 112 with increasing strain rate and hydrogen content or a charging rate [1]. It is well known that the hydrogen cracking changes from transgranular to intergranular with increasing yield strength. Environmental hydrogen cracking occurs when the material is being subjected to a hydrogen atmosphere. Improper use of cathodic protection for corrosion protection results in absorbing and/or adsorbing hydrogen into the structural steel. The effect strongly depends on the stress imposed on the steel and it is maximum at around room temperature. This may lead to the structural failure of the steel. The failure of steel by hydrogen is resulted from different propagation crack modes such as transgranular cleavage, brittle intergranular, quasi-cleavage. Some researchers found that the hydrogen degrade mechanical properties of steel without changing a fracture mode [2]. The effect of grain structure on the hydrogen cracking has been investigated by a numerous researchers. The tendency to induce hydrogen cracking in purified iron and in Fe-Ti alloys decreases with decrease in grain size [3-5]. The increase in ferrite grain size has enhanced toughening as well as embrittling (beyond a certain limit of hydrogen content) of the low alloy steel under the charging conditions [6]. The heavy-strain working after hydrogenation of Ti-6Al-4V Alloy composites produces a non-homogeneous microstructure containing such as micro bands, shear bands and lamellar boundaries. Moreover, it was found that a small increment of dislocation pile-up sources has an important role in inducing dense dislocation walls and cells during in the early stages of deformation of the material [7]. The effect of microstructure on the hydrogen embrittlement of steel was studied by a numerous investigators [8-11]. The susceptibility to hydrogen embrittlement was observed to be closely related to the microstructural state. Intercritical annealing at relatively low temperatures of hydrogenated dual- phase microalloy steel exhibited quasi-cleavage fracture with some ductile dimpling. While, the mode of fracture of the quenched hydrogenated steel from higher intercritical annealing temperatures was predominantly intergranular fracture along prior austenite grain boundaries and cracking of martensite laths [12]. The aim of this paper is to investigate the cracking behavior of the low strength under cathodic charging in three-point bend system. The steel specimens with different grain sizes were obtained by varying the heat treatment conditions. 3. EXPERIMENTAL PROCEDURE Bend experiments were conducted on a structural steel sheet of 1mm thickness. The chemical composition of the investigated material was analyzed using an energy dispersive X-ray (fig. 1). Table 1 lists the elemental composition of the structural steel used in this investigation. A number of specimens were cut from this material with dimensions of 10cm long and 1 cm wide. Different heat treatment temperatures and holding times were applied to the steel specimens in order to obtain different grain sizes. Table 2 lists the heat treatment temperatures and times for obtaining different grain sizes. The three-point bend system developed in the laboratory consists of a holder (metal block) in which the specimens were supported at the ends and bents into a glass chamber (contained an electrolyte and anode) by forcing a steel punch (equipped with a screw driven) against it at a point halfway between the end supports as shown in figure 2.
  • 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 113 Figure 1: EDX analysis of A36 structural steel investigated. Table 1: Chemical composition of A36 structural steel investigated, (wt%) C Mn Si Al Cu Ni Mo P S 0.19 1.21 0.23 0.025 0.005 0.022 0.01 0.028 0.01 Figure 2: Schematic of the experimental set-up used for three-point bending during cathodic hydrogen charging The specimen with the metal block holder was made the cathode (graphite anode). The electrolytic solution contained 75% (volume) methanol, 22.4% (volume) distilled water, 2.6% (volume) sulphuric acid and 10mg.l-1 arsenic trioxide to inhibit hydrogen recombination at the
  • 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 114 surface. A constant current density of 25mA.cm-2 was applied for different times. Since hydrogen- charged bend specimens were too ductile to bent to fracture in the bend test fixture, they were deformed during cathodic hydrogen charging. 4. RESULTS AND DISCUSSION It is well known that grain refinement improves resistance to hydrogen cracking (13-16), but no quantitative data relating measured hydrogen content to grain size and associated mechanical properties have been obtained. There have been attempts to explain why alloys of small grain size are less prone to hydrogen damage. The concentration of hydrogen required to saturate grain boundaries with a monolayer of hydrogen at various grain sizes was calculated and found that, by decreasing grain size from 100 to 10µm, the hydrogen coverage on grain boundaries with 10ppm of available hydrogen would decrease from saturation level to only about one site in ten covered (17). However, it was found that an increase of grain size from 10 to 160 µm raised the threshold stress intensity of an AISI 4340 steel from 20 to 30 MN m(-3/2) and the amount of hydrogen cracking increased (18). Metallographic examination of steel specimen with small grain size which had been cathodically hydrogen charged during bending showed that cracking occurred mainly along the grain boundaries i.e. intergranular cracking as can be seen from figure 3. It is believed that the more disordered and high-energy grain boundaries occluded a higher amount of hydrogen. Thus the presence of hydrogen increased the ease of cracking in these regions, either by building up localized pressure or by reducing the cohesion force. Figure 3: Intergranular and few transgranular cracks observed in a bent structural steel specimen having a small grain size (i.e. 22 µm) The surface of the steel specimen with larger grain size (i.e. 55µm) which had been cathodically hydrogen charged during bending is shown in the figure 4. It is clearly seen from this figure that the amount of intergranular cracking increased with grain size and the crack path is mostly intergranular. However, in contrast to the steel specimen having smaller grain size, the former specimen with larger grain size exhibits few transgranular cracks.
  • 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 115 Figure 4: Intergranular cracks observed in a bent structural steel specimen having a larger grain size (i.e. 55 µm) This shows that the low-angle grain boundaries, found in specimens of small grain size, were less susceptible to hydrogen damage than the high-angle boundaries associated with large grains. Bending of the steel specimen with larger grain size (i.e. 160µm) during cathodic hydrogen charging reveals large and thick intergranular cracks (Fig. 5). It is believed that specimens with small grains exhibited a certain amount of texture, with low mismatch between grains, whereas specimens heated to higher temperatures and/or for longer periods of times showed a larger extent of grain growth. Due to the high migration rate of the high-angle grain boundaries, the eventual microstructure consisted of grains with a large degree of mismatch separated by such high-angle grain boundaries. These grain boundaries have a high energy and the distortion along them is greater, so more hydrogen is trapped in them than in the low-angle ones. Figure 5: Intergranular cracks observed in a bent structural steel specimen having a larger grain size (i.e. 160 µm)
  • 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 116 It was observed from the test results that the increasing charging time resulted in an increase of a number of hydrogen cracks on the surface of the steel specimens. Figures 6(a) and (b) show the surface micrographs of the steel specimens with small grain size which had been cathodically hydrogen charged during bending for 12 and 24 hours respectively. The results showed that the hydrogen cracks formed in the specimens charged for shorter charging time, initiated in groups mainly along grains and they are relatively small, however, in the specimens charged for longer time, hydrogen cracks were connected to each other and propagated along and across the grains and therefore they are larger than those in the specimens charged for shorter time. (a) (b) Figure 6: Micrographs of surface of the structural steel specimens of with small grain size (i.e. 22 µm) hydrogen charged during bending for (a) 12 hrs, (b) 24 hrs charging Since with increasing time of charging, the grain boundaries are saturated more quickly and hydrogen cracks formed at grain boundaries of the steel specimens and then connected and propagated along the slip lines. This may explain why hydrogen induced cracks have been found to propagate transgranularly when the steel specimen charged for longer time during bending. The effect of the charging time on the number of hydrogen cracks formed was examined for the steel specimens with small grain size which had been cathodically hydrogen charged during bending for different charging times up to 24 hours. The crack density n, which is defined as the number of surface cracks per unit area, are counted on a fixed area of 0.3 mm2, which was randomly
  • 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 117 marked on each specimen. The results of this semiquantitative study of the hydrogen cracks are shown in figure 7. It should be noted that in the hydrogen-charged steel specimens with large grain size, the initial transgranular cracks observed in the charged specimens having small grain size were not presented. Instead, large hydrogen cracks along the grains were observed. Figure 7: Effect of the charging time on the number of hydrogen cracks formed for the bent structural steel specimens having a small grain size (i.e. 22 µm) From the test results it was observed that the length of the hydrogen cracks increased with increasing the charging time. Accordingly, the effect of the charging time on the crack length was also examined for three steel specimens with large grain size which had been cathodically hydrogen charged during bending for 3, 10 and 24 hours respectively. The results are shown in figure 8. The above results showed that the hydrogen cracks initiated in groups along grains when the specimens charged for shorter charging time, however, in the steel specimens charged for longer time, these cracks were connected to each other as discussed above. Figure 8: Effect of grain size on the length of hydrogen crack formed for the bent structural steel specimens hydrogen charged during bending for (a) 3 hrs, (b) 10 hrs, (c) 24 hrs
  • 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 111-118 © IAEME 118 5. CONCLUSIONS The low-angle grain boundaries in the bent steel specimens of small size are less susceptible to hydrogen cracking than are the high-angle grain boundaries. Therefore the high- angle grain boundaries associated with large grains provide an easy path for crack propagation. Intergranular cracking is more likely to occur with larger grain sizes and its amount and length increase with grain size. 6. REFERENCES 1. ASM Handbook. Vol. 11 Failure Analysis and Prevention. ASM International, 1986. 2. C. D. Beachen : Metall. Trans. 3 (1972), 437. 3. I M. Bernstein and B. B. Rath: Metall. Trans., 4 (1973), 1545. 4. I M. Bernstein : Metall. Trans., 1 (1970), 3143. 5. I M. Bernstein and A. W. Thompson: Int. Met. Rev., 21 (1976), 269. 6. S. K. Singh and B. Sasma : ISIJ international, Vol. 39 (1999), No. 4, pp. 371-379. 7. Naoya Machida, Masafumi Noda, Kunio Funami and Masaru Kobayashi, Materials Transactions, Vol. 45, No. 7 (2004) 2288-2294. 8. I. M. Bernstein, R. Garber and G. M. Pressouyre, in “Effects of Hydrogen on Behaviour of Materials”, edited by A. W. Thompson and I. M. Bernstein (TMS, New York, 1976) p. 27. 9. D. A. Ryder, T. Grundy andT. J. Davies, in Proceedings of 1st International Conference on Current Solutions to Hydrogen Problems in Steels, Washington, DC, November 1982, edited by C. G. Interrante and G. M. Pressouyre (ASM, Metals Park, 1982) p. 272. 10. P. Lacombe, M. Aucouturier andJ. Chene, in “Hydrogen Embrittlement and Stress Corrosion Cracking”, edited by R. Gibala and R. F. Hehemann (ASTM, Metals Park, 1984) p. 79. 11. T. Alp, B. Dogan andT. J. Davies,J. Mater. Sci. 22 (1987) 2105. 12. T. Alp, F.I. Iskanderani and A.H. Zahed, J. of Materials Science, Springer, Vol. 26 (1991) 5644-5654. 13. M. Martinez-Madrid, S.L. Chan and J.A. Charles, Mater. Sci. Techn., Vol. 1 (1985), p. 454. 14. A.W. Thompson and I.M. Bernstein, in Advances in Corrosion science and technology, Vol. 7, (ed. M.G. Fontana and R.W. Staehle), 53-175, 1980, New York, Plenum Press. 15. I.M. Bernstein and A.W. Thompson, Int. Met. Rev., Vol. 21 (1976), pp. 269-287. 16. W.M. Cain and A.R. Troiano, Pet. Eng., Vol. 37 (1965), pp. 37, 78. 17. J.K. Tien, in Effect of Hydrogen on Behavior of Materials, (ed. A.W. Thompson and I.M. Bernstein), 1976, pp. 305-321. 18. W.W. Gerberick and A.G. Wright, in Environmental Degradation of Engineering Materials in Hydrogen, (ed. Louthen et al.), 1981, pp. 183-206, Blacksburg, Va, Virginia Polytechnic Institute. 19. Asst. Prof. Samir A. Al-Mashhadi, Asst. Prof. Dr. Ghalib M. Habeeb and Abbas Kadhim Mushchil, “Control of Shrinkage Cracking in End Restrained Reinforced Concrete Walls”, International Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 1, 2014, pp. 89 - 110, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. 20. Salman A. Al-Duheisat and Amjad Saleh El-Amoush, “An Investigation of the Cracking Path of a Hydrogenated Tin Brass Heat Exchanger Tube”, International Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 3, 2014, pp. 202 - 208, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.