Increased resistance of acicular ferrite microstructure to hydrogen embrittlement - physical metallurgy cause of this effect

• Autorzy: Mazancová E. , Slavíková I. , Mazanec K.
• Konferencja: Metody oceny struktury oraz własności materiałów i wyrobów/sympozjum ( XXII ; 6-8.XI.2007 ; Svratka, Czechy)
• Języki publikacji: EN

Abstrakty

EN

The grain refinement leading to the strength and ductility increase represents important parameter resulting in improved achieved physical metallurgy properties. The acicular ferrite (AF) can be held for a promising microstructure contributing to the accomplishment of above mentioned requirement. The AF is initiated in the same temperature range as bainite (B) by the same transformation mechanism. The difference between AF and B consists in their different nucleation sites in steel matrix. In case of B the ferritic grains are nucleated at the austenite (A) grain boundaries. On the contrary, AF plates are nucleated on non-metallic particles which fulfil special conditions of their nucleability. The AF particles exhibit different orientations in the formation of the fine grained interlocking morphology. The bainitic packets consist of parallel plates (laths) having low angle interface orientations. The beneficial mechanical properties of the AF microstructure are related to the high frequency of high angle interfaces acting as effective obstacles to cleavage cracks propagation in contradistinction to the weak influence of the B having low angle plates interfaces. In B, only packets show high angle boundaries what results in larger free path for cleavage crack propagation [1]. The above discussed beneficial AF properties are acting in large scope inclusive of improved resistance of steels to hydrogen induced cracking. The fracture process analysis has shown the strength and toughness behaviour of AF microstructure are different from that conventional microstructure by reason of its particular microstructure characteristics. In the AF microstructures, the density of micro-orientated plates is enhanced by a profused direct intragranular nucleation on non-metallic inclusions. In the bainitic microstructure a decisive role is ascribed to morphological packets representing the microstructure unit that controls the cleavage cracks propagation without their deflecting. In the B microstructure, the morphological packets size (de) is related to the dimension of unit crack path (UCP) corresponding to the distance between neighbouring high angle boundaries [2]. The AF analysis shows, the microstructure unit is directly related to the set of the AF plates having high angle interface, what is corresponding to the unit crack path in the AF microstructure. In the AF, this value is shorter in comparison to the dimension of packets in investigated B. In the AF, it is 4-5 jim while in the B this distance is substantially greater and attains up to 20jam, approximately. Using the UCP value - dB, it is possible to determine the transition behaviour in steel T = To - K.dB"1/2, where K represents constant value, To depends on the tensile strength. Above presented equation shows, the transition temperature is universally proportional to the square root of distance between high angle interfaces. The AF makes the crack propagation difficult due to presence of a great number of high angle interfaces. The presented AF microstructure is stable and is kept after hydrogen charging process. The increased deflection numbers of the AF plates contribute to the achievement of high resistance to hydrogen induced cracking in this microstructure. The positive AF properties on the resistance to hydrogen induced cracking and the SSC formation in oil and gas pipe-line steels (X60-X80) confirm above presented idea [3]. The investigated steel with dominating AF microstructure, having optimum mechanical properties consists of the fine non-equiaxed and interwoven ferritic plates containing the ultra-fine carbonitrides, eventually. In some case the formed small M/A islands are not excluded absolutely. The AF plates formed by displacive transformation mechanism contain increased dislocation densitiy. This substructure contributes to the trapping of hydrogen atoms and in this way limits the detrimental hydrogen effect on steel properties. The combination of increased resistance to cleavage crack propagation and partial limitation of hydrogen activity (due to dislocation trapping of hydrogen atoms) result in the improvement of steel properties. These results demonstrate, the AF microstructure represents a perspective way how to increase steel durability under simultaneous hydrogen action.

Słowa kluczowe

EN

acicular ferrite microstructure; hydrogen embrittlement; physical metallurgy

• Wydawca: Oficyna Wydawnicza Politechniki Opolskiej
• Czasopismo: Zeszyty Naukowe. Mechanika / Politechnika Opolska
• Rocznik: 2007
• Tom: Vol. 321, z. 89
• Strony: 117--118
• Opis fizyczny: Bibliogr. 7 poz., il., wykr.

Twórcy

autor

Mazancová E.

autor

Slavíková I.

autor

Mazanec K.

• VŠB-TU Ostrava, Inst. of Mater. Science

Bibliografia

• [1] ZHAO M.C., SHAN Y.Y., XIAO F.R., YANG K.: Acicular Ferrite Formation During Hot Plate Rolling for Pipe-line Steels. Mater. Sci. Technol., 19, 2003, 355-359.
• [2] GOURGUES A.F., FLOWER H.M., LINDEY T.C.: Electron Backscattering Diffraction Study of Acicular Ferrite, Bainite and Martensite Steel Microstructures. Mater. Sci. Technol., 16, 2000, 26-40.
• [3] DIAZ-FUNTÉS M., IZE-MENDIA A., GUTIÉRREZ I.: Analysis of Different Ferrite Microstructures in Low Carbon Steels by Electron Backscattered Diffraction Study of Their Toughness Behaviour. Met. Mater. Trans. A, 34A, 2003, 2505-2516.
• [4] SMITH Y.E., COLDREN A.P., CRYDERMAIN R.L.: Manganese-Molybdenum- Niobium Acicular Ferrite Steels with High Strength and Toughness. Towards Improved Ductility and Toughness. The Iron and Steel Inst. of Japan, Climax Molybdenum Development Company, LTD, Tokyo, 1972, 119-142.
• [5] BOUYNE E., FLOWERS H.M., LINDLEY T.C., PINEAU A.: The Use of EBSD Technique to Examine Microstructure and Cracking in a Bainitic Steel. Scr. Mater., 39, 1998, 295-300.
• [6] GOURGUES-LORENZON A.F.: Application of Electron Backscatter . Diffraction to the Study of Phace Transformations. Internat. Materials Reviews, 52, 2007, 65-128.
• [7] PICKERING F.R.: The Structure and Properties of Bainite in Steels. Transformation and Hardenability of Steels, Climax Molybdenum Company, Univ. of Michigen, Sympos. Held, in September 27-28, Ann Arbor, Michigen, 1967, 109-129.