The microstructure, texture evolution and plasticity anisotropy of 30SiMn2MoVA high strength alloy steel tube processed by cold radial forging
Introduction
As a unique precision forging process, radial forging processing is extensively used in the production of round and tubular components, with or without an internal profile [1]. Fig. 1 illustrates that workpiece flowing axially results from a large number of short-stroke and high-speed pressing operations by four hammer dies, arranged radially around the workpiece [2]. When the hammer dies are raised, the workpiece will have the axial feed and rotational feed. The internal profile in a tube workpiece is created by the morphological characteristics of the mandrel.
Considering the large plastic deformation of the material in the cold radial forging process, the change of mechanical and metallurgical properties cannot be ignored. Domblesky et al. [3] obtained that temperature gradients generated during forging due to contact with the mandrel were thought to give rise to variation in the mechanical properties in the forged tube by the finite-element method. Taherizadeh et al. [4] compared the mechanical and metallurgical properties of hollow and solid long radial forged products. The results indicate that the mechanical and metallurgical properties are more uniform and homogeneous through the thickness direction in hollow or mandrel forged products. In the solid forged parts, there is a gradient in the micro- and macro-structural properties from the surface to the internal areas. Arreolaherrera et al. [5] focused on the effect of various degrees of plastic deformation generated by cold radial forging on the mechanical properties and the fracture morphology of 32 CDV 13 steel and found the tensile strength, yield strength and hardness increase with the increase of cold forging percentage due to the energy stored in the material during cold forging. Nikulin et al. [6] investigated the structure and tensile properties of a three-layer “steel/vanadium alloy /steel” radial forged tube and reported that there was a transition zone of diffusion interaction near the “steel/vanadium alloy” interface which provided a high strength of bonding of the materials after forging. Panov et al. [7] presented the influence of structure formation on the properties of 321 metastable austenitic stainless steel under the different deformation degrees in the cold radial forging. They found that the band austenitic-martensitic structure was formed in the longitudinal section. Hardness, ultimate tensile strength and yield strength uniformly increased. But the impact strength of V-notch specimens decreased with the development of the deformation degrees. Nevertheless, none of these studies focused on the texture of materials after radial forging. For a large number of polycrystalline materials, the preferred orientation of crystallites(texture) is an intrinsic feature. It influences on physical properties such as strength, electrical conductivity, piezoelectricity, and so on, particularly in the anisotropy of these properties [8]. The plastic deformation can change the orientation of crystallites, forming the texture. The anisotropy also attracts the attention of researchers who study the plastic forming process. There is a lot of research about the texture and anisotropy of metal products processed by cold rolling [[9], [10], [11]], pilgering [12,13], cold drawing [14] and extruding [15,16]. Cold radial forging is usually used to produce barrel which works under the huge internal pressure due to an explosion of powder. Hence the circumferential mechanical properties which directly relate the ability of barrel against internal pressure should be paid more attention. Most current barrel design theories suppose that the forged gun barrel is isotropic and neglect the effect of deformation on the mechanical properties of the forged barrel. According to the analysis above, it is not comprehensive. However, the investigations about the anisotropy in cold radial forging are rare. Xu et al. [17] have proposed that there is an anisotropy in the steel tube processed by cold radial forging in their research. Nonetheless, the reason for the anisotropy in the cold radial forged steel tube is not clear.
This study presents the evolution of the microstructure, texture and plastic anisotropy of 30SiMn2MoVA steel tube processed by cold radial forging under different forging reductions. In this work, the tensile test and bulging test were employed to obtain the axial and circumferential mechanical properties of the forged steel tube. The microstructure and texture were measured by a field emission scanning electron microscope and an X-ray diffractometer. Meanwhile, the relationship between anisotropy and texture was built base on Taylor polycrystalline deformation theory [18].
Section snippets
Material
The experimental material is 30SiMn2MoVA high strength steel. Its component content is shown in Table 1. The metallographic structure of this material is Sorbite which is a fine pearlite consist of ferrite and cementite. The small grain size makes this steel have better strength and machinability.
Cold radial forging
The gradient forging ratios were introduced to process steel tubes. The forging ratio, ŋ, is also called area reduction ratio, which has the form as followwhere Ro and Ri are
The plastic strain
The effective plastic strain distribution is illustrated in Fig. 3. The effective plastic strain of the forged tube is inhomogeneous. The maximum equivalent strain is on inner and outer surfaces due to the friction. In this study, the specimens for microstructure tests come from the middle layer of the forged tube. At a forging reduction of 15%, the effective plastic strain of the middle layer is 0.165. For the 22% forging ratio, the plastic strain at the corresponding position is 0.273. At 35%
Grain refinement
Since the radial forging process works at room temperature, there is no recrystallization in the deformation grains. The mechanism of grain refinement under cold deformation has been well investigated. Argon and Haasen [35] reported the microstructural evolution after cold rolling: a high dislocation density is introduced, which leads to the formation of a lamellar structure consisting of dislocation cells with thick cell walls and low angles of misorientation. As the strain increases, the
Conclusions
A study of the microstructure, texture evolution and plasticity anisotropy of 30SiMn2MoVA alloy steel tube processed by cold radial forging under the different forging reductions was carried out. The major findings of the research are summarized below.
- (1)
After forging, the axial and circumferential strength all increases with the rise of the forging ratios. The axial strength is stronger than the circumferential. This phenomenon is obvious with the growth of the forging reduction. At a 35% forging
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest
The author(s) declare no competing interests.
Acknowledgements
This work was supported by the National Defense Basic Scientific Research Program of China (grant numbers: JCKY2016209A002).
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