Eva Mazancová a Zdeňka Rucká a Karel Mazanec a

Transcription

1 CONTRIBUTION OF ACICULAR FERRITE TO THE INCREASED HYDROGEN RESISTANCE OF LOW ALLOY STEEL PŘÍNOS ACIKULÁRNÍHO FERITU KE ZVÝŠENÍ ODOLNOSTI PROTI PŮSOBENÍ VODÍKU V NÍZKOLEGOVANÝCH OCELÍCH Eva Mazancová a Zdeňka Rucká a Karel Mazanec a a VŠB-TU Ostrava, Tř. 17. listopadu 15, Ostrava-Poruba, ČR, eva.mazancova@vsb.cz Abstract The refined acicular ferrite microstructure represents an excellent contribution to mechanical properties and the toughness level detected in low-alloy steel. The majority of neighbouring plates (laths) have the mutual high-angle disorientation in contradistinction to the bainite microstructure. In this microstructure, the high-angle interfaces are only formed between the packets consisting of the low-angle plates (laths) set. The cleavage unit crack path (UCP) has been found to be a distance between two grains of high-angle ferrite regions (corresponding to the two crystallographic bainite packets boundaries). In acicular ferrite the UCP value is defined as a distance between two neighbouring highly misorientated plates. It shows the UCP values are shorter what results in higher frequency of deviations and consequently in limited (retarded) cleavage crack propagation. The conditions for acicular ferrite nucleation in austenite matrix after application of the optimized thermomechanically controlled process consisting of the consecutive straining process realized in recrystallization and in non recrystallization regions have been determined. The applied nucleation mechanism (based on the nucleation process realized in structured matrix) represents the second nucleation variant resulting in acicular ferrite formation. The beneficial resistance of acicular ferrite particles to hydrogen embrittlement can be held for a very important property of this microstructure what demonstrates valuable contribution of this one to its engineering application. 1. INTRODUCTION The acicular ferrite is held for a very important microstructural component because of its relatively high strength and beneficial toughness response. Experimentally, this microstructure finds the large commercial application by above given reason of mechanical metallurgy properties detected in pipeline steels. In presented contribution it is accepted that acicular ferrite microstructure results from the realization of bilateral processes, namely from mixed carbon diffusion and the displacive transformation mechanism. This process is beginning at a temperature usually lying in the transformation range corresponding to the boundary level of the upper bainite nucleation in austenite matrix. The acicular ferrite growth involves a coherent or semicoherent austenite/ferrite (γ/α) interface fulfilling the Nishyama- Wasserman or Kurdjumov-Sachs crystallographic relationship [1]. The both considered microstructures (bainite and acicular ferrite) are characterized with a different behaviour, predominantly in the achieved toughness values even though the temperature range of their initiation is identical. The different nucleation sites in steel matrix have the decisive influence on the formation of the final austenite decomposition mode. The difference in the formation of the acicular ferrite and bainite consists in their different nucleation sites in steel matrix. As being known, acicular ferrite plates (laths) are usually nucleated at non-metallic particles (inclusions) fulfilling the special nucleability parameters and having the preferential nucleation capacity in comparison with austenite grain boundaries.

2 In contradistinction to acicular ferrite nucleation sites, in case of bainite formation, the ferritic grains are nucleated at austenite grain boundaries. Concerning the nucleability of non-metallic inclusions their nucleation activity is based on the competition of acicular ferrite with B nucleation parameters. Recently, it has been demonstrated, the dominated acicular ferrite microstructure has been developed using the special form of thermomechanical control process (TMCP) after the hot plate rolling of pipeline steels. The considered process consists of two independent stages of controlled rolling. The complex process can be divided in two basic parts, namely. The deformation realized in the austenite microstructure accompanied with the matrix recrystallization and the rolling realized at lower temperature ( C) what leads to the recrystallization suppressing. After the TMCP, a controlled (accelerated) cooling (temperature range C) has been applied and at 500 C the soaking annealing for 1 hour ha been followed. The deformation performed at the temperature range in which the austenite recrystallization takes part has been around 27%, while the deformation level realized at the temperature range corresponding to recrystallization suppressing has been mildly higher (31 47%). The aim of this work is to contribute to the physical metallurgy analysis of the beneficial acicular ferrite effect to the achievement of higher resistance to hydrogen degradation effect in the structural steels [2]. 2. THEORETICAL CONCEPT Acicular ferrite particles exhibit different orientations what leads to the fine grained interlocking morphology formation. Bainite packets usually consist of plates (laths) having the low-angle interface orientations. In contrast to this morphology arrangement the detected beneficial mechanical properties of acicular ferrite microstructure are connected with the high-angle interfaces being effective as obstacles to the cleavage crack propagation. It represents very important contribution of acicular ferrite microstructure to the achievement of higher mechanical metallurgy properties in comparison with the bainite microstructure having larger unit crack path for the cleavage crack propagation [1]. Acicular ferrite high angle interfaces force the cleavage crack to change the microscopic propagation plain in order to accommodate the new crack crystallography [3]. The above discussed beneficial AF properties are asking in large scope inclusive of the imposed steel resistance to hydrogen induced cracking (HIC). From point of view of fracture mechanics, it is useful to apply the evaluation concept of the crystallographic packets defined as the continuous set of plates having misorientation lover than the angle of 15. The plate interface occurrence characterized with misorientation higher than 45 is prevailing in acicular ferrite microstructure. 3. MICROSTRUCTURE AND FRACTOGRAPHY ANALYSIS Characteristic microstructures of compared AF and B are presented in Figs.1a,b and 2a,b. Figure 3 shows the typical feature of fracturing deviation of steel containing investigated acicular ferrite microstructure in majority and/or B one in minority. The presented different features of the microstructures confirm the unlikeness in response of the investigated microstructures in dependence on loading parameters. The presented analysis is devoted to the physical metallurgy elucidation of processes leading to the acicular ferrite and bainite microstructures formation during austenite decomposition. Simultaneously, the obtained results contribute to the getting deeper knowledge concerning the relation between microstructures and the resistance to hydrogen brittleness (hydrogen induced cracking HIC) namely in this case between the influence of acicular ferrite and bainite microstructures. Acicular ferrite microstructure, due to its interlocking arrangement, will be probably more resistible than is the influence of the bainite microstructure.

3 Fig.1a. Micrograph of acicular ferrite (x2800) Fig. 2a. Fracture characteristics of acicular ferrite microstructure (x500) Fig.1b. Micrograph of bainite (x1900) Fig. 2b. Fracture characteristics of bainite microstructure (x500)

4 deviation acicular ferrite plate (lath) Fig. 3. Schematic feature of fracturing deviation in acicular ferrite microstructure The interfaces having the high misorientation angle are held for very important characteristics from point of view of higher toughness values obtained in acicular ferrite microstructure. The interface misorientations higher than 45 are taken for very beneficial. On the contrary, the interface misorientations lower than 15 are considered as critical because they are not contributing to an improvement of detected mechanical properties. It has been found in majority measurements performed in acicular ferrite microstructures. Nearly all interfaces among the primary acicular ferrite plates are of the high-angle type. The low misorientation angles are observed among the secondary (sympathetic) acicular ferrite plates. In summary, it is possible to conclude, the realized EBSD analysis demonstrates the acicular ferrite microstructure is consisting of intricately misorientated plates having internal low angle plates filling the space among neighbouring plates of high-angle interfaces [3]. The crystalline packets are considered as the microstructural units controlling the cleavage crack propagation in the bainite microstructure [1, 3]. The most of the packet interfaces are high-angle ones, characterized with a misorientation angle higher than the accepted limit of the 45. Analogously, as in the acicular ferrite microstructure, the low angle interfaces are found inside the crystallographic packets [4]. In comparison to acicular ferrite frequency of high-angle misorientation detected in the bainite microstructure (misorentation angle determined among crystallographic packets) is substantially lower while in case of acicular ferrite the high misorientation angles can be usually found among the primary AF plates (laths) with the substantially higher frequency than it is observed in the bainitic microstructure. 4. DISCUSSION The determination of the crystallographic grain size represents the very important parameter. This value influences the strength and the cleavage crack propagation resistance of ferritic steel as it is well known. The application of the EBSD technique represents a method making possible to evaluate the grain size in a very fine microstructures [5]. The results obtained after application of the above mentioned evaluation technique contribute to the local

5 estimation of the fracture behaviour in dependence on the microstructural type. The above mentioned evaluation method makes likewise possible to give account for the physical engineering causes contributing to the achievement higher cleavage crack propagation resistance in comparison with the behaviour of the investigated bainite microstructure. The detected unlike behaviour of compared microstructures (acicular ferrite and bainite) reflects the different level of unit crack path (UCP) dimension according the Pickering s concept [6]. The UCP value is equal to the distance between the neighbouring high angle interfaces (boundaries). This parameter is defined as a region in which the cleavage crack propagates in a nearly straight line. On the contrary to the acicular ferrite (high-angle interface is located to single primary plates - laths) the UCP values are corresponding to the distance between packet interfaces in bainite. These microstructure constituents are composed of low-angle plates (laths) interfaces which are more or less one another arranged parallelly what does not contribute to the strength increase and to the cleavage crack resistance. This result is a very important characteristic and reflects the distinct acicular ferrite behaviour from bainite microstructure. By means of the UCP value (X UCP ) application, it is possible to estimate the transition temperature (T) in steel using the equation presented by Pickering formerly [6]: T = T 0 K (X UCP ) -1/2 (1) In above mentioned equation K corresponds to constant and T 0 depends on the steel tensile strength. The equation (1) shows the transition temperature proportionality to the square root of the distance between high-angle interfaces. The acicular ferrite microstructure makes crack propagation difficult due to presence of great number of high-angle plates (laths) interfaces and due to shorter distance of this microstructural parameter. The high-angle boundaries (interface) force the crack deviations and evidently leave ligament fractured finally at development of ductile mechanism. The increased crack propagation energy detected in the acicular ferrite microstructure is a result of fracture behaviour. In this connection, it is interesting to remark that the same fracture processing takes part in the increased resistance to cleavage crack propagation in case of lover bainite microstructure. In acicular ferrite microstructure, the density of plates (laths) having higher misorientation is enhanced as it is also found under increased dislocation density in work hardened steel matrix after the application of the consecutive thermomechanical processing. The cleavage crack is realized along 100 planes. At this process the noticeable deflection is only observed for a misorientatin higher than 15, approximately. The set of adjacent plates (laths) having the misorientation range limited with 15 is considered to be a criterion of the microstructural unit controlling the cleavage crash propagation in the AF microstructure. The realized analysis shows, the acicular ferrite microstructure is intrinsically higher than the bainite one although both are formed at the same temperature range. The compared microstructures differentiate in the UCP values. In acicular ferrite microstructure the absence of carbides contributes to the beneficial effect of this microstructure. The recent result show, the acicular ferrite microstructure can be obtained after the complex set of sequential deformation operations realized in the temperature region corresponding to the optimized thermomechanical control process. This processing is composed of a properly arranged sequence of two mul-pass deformation rolling stages in austenite region in which the recrystallization of deformed austenite matrix is realized and at lower temperature when the recrystallization mechanism is suppressed [2]. The discussed deformation set is composed of five stages. The schematic diagram of considered deformation operations is presented in Fig.4. The presented dependence temperature-time demonstrates the deformation is necessary to finish at temperature situated above the Ar 3 with the subsequent controlled cooling to the temperature of austenite

6 transformation in acicular ferrite (by displacive mechanism) along with the superposed isothermal holding (0.5-1 hour) at this temperature. Fig. 4. Schematic diagram of mechanical control process [2] The mechanism can be considered as a second variant to the usually applied nucleation conception based on the nucleability capacity of some non-metallic inclusions. Probably, the application above described deformation process results in the modification of nucleability of the austenite grain boundary. This limits the bainite formation and the intragranular acicular ferrite nucleation activity, achieves a certain probability level in comparison with bainite microstructure. By this occasion, it is useful to remark the frequency of above discussed second nucleation variant is lover and is conditioned with the application of complex deformation processes having partially eliminated the prior grain boundary network in the austenite matrix. The increased dislocation density acts as a strong hydrogen trap in the AF plates (laths), resulting in the imposed HIC resistance. This mechanism can be taken for additional process to the basic idea consisting in beneficial UCP in the acicular ferrite-microstructure in comparison with the investigated bainite microstructure due to high-angle misorientation among the acicular ferrite plates. This positive acicular ferrite properly is kept under simultaneous hydrogen effect what is held for very important contribution to the behaviour of acicular ferrite microstructure from point of view of steel resistance to the HIC and SSC [2]. 5. CONCLUSIONS The AF microstructure is characterized with an excellent combination of mechanical (strength) properties and achieved toughness values. The acicular ferrite plates (laths) contain high-angle of the mutual misorientation in majority cases while the bainite microstructure low-angle one in majority. The high-angle occasions are usually found between neighbouring packets consisting of the low-angle plate set. The cleavage UCP has been detected to be a distance between two grains of high-angle ferrite regions. It corresponds to the two

7 recrystallization packet boundaries. In austenite the conditions for acicular ferrite nucleation after application of the optimized thermomechanical control process consisting of the consecutive deformation process realized in recrystallization has been determined. This acicular ferrite formation is considered as a second nucleation variant of the acicular ferrite. Its existence is conditioned by the realization of deformation set leading to the modification of the prior austenite grain boundary network. The beneficial resistance of the acicular ferrite microstructure to hydrogen embrittlement represents a very attractive property reflecting the basic arrangement of this micristructural component. REFERENCES [1] GORGUES, A.F., FLOWER, H.M., LINDLEY, L.C.: EBSD study of acicular ferrite, bainite and martensite steel microstructures. Mater. Sci. Technol., 2000, vol. 16, p [2] 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., 2003, vol. 19, p [3] DIAZ-FUNTÉS, M., IZE-MENDIA, A., GUTIÉRREZ, I.: Analysis of different microstructures in low carbon steels by EBSD study of their toughness behaviour. Met. Mater. Trans. A, 2003, 34A, [4] BOUYUE, 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., 1998, vol.39, p [5] GUTIÉRREZ-LORENZON, A.F.: Application of EBSD to study of phase transformation. Inter. Mater. Reviews, 2007, vol. 52, [6] PICKERING, F.R.: The structure and properties of bainite in steels. In Proceed. Transformat. and Hardenability of Steels. Ann Arbor, Michigan: Climax Molyb. Company, Univ. of Michigan, 1967, p ACKNOWLEDGEMENT Authors acknowledge the Ministry of Industry and Commerce of Czech Republic for financial support of project FI-IM 3/159.