Strain hardening and nanocrystallization behaviors in Hadfield steel subjected to surface severe plastic deformation

Abstract

A gradient nanocrystalline layer with a thickness in a range of millimeter magnitude was successfully produced on the surface of Hadfield steel by a novel severe plastic deformation technology, high speed pounding. The surface hardness was measured, and the microstructure evolution during nanocrystallization process was characterized by X-ray diffraction and transmission electron microscopy. Results showed that the hardness increment and nanocrystallization in Hadfield steel were obtained at different stages under high speed pounding. The first stage was strain hardening, where surface hardness of Hadfield steel increased gradually during high speed pounding until a steady-state value was obtained. The hardening degree and rate of Hadfield steel were determined by deformation stress and strain rate, respectively. The second stage was microstructure nanocrystallization, at which twin boundaries interacted with dislocations to form general high angle grain boundaries. In this stage, the surface hardness of Hadfield steel remained basically the same. Moreover, a physical model was established to explain the strain hardening and surface nanocrystallization behaviors in accordance with the microstructure evolution at different stages in Hadfield steel.

Introduction

Surface severe plastic deformation (S²PD) procedure can be used as a typical surface nanocrystallization technology to refine the surface of conventional coarse-grained materials to obtain gradient microstructure throughout the thickness [1], [2]. This procedure has been used to produce nanocrystalline layer without any voids and defects at the surface of various metals and alloys, such as iron alloy [3], copper [4], [5], carbon steel [6], stainless steel [7], [8] and nickel/nickel-based alloys [9], [10]. Lu et al. [11], [12] prepared cylindrical gradient nanocrystallized samples in nickel and copper by surface mechanical attrition, displaying excellent mechanical properties. However, producing nanocrystalline material in large scale remains challenging.

The excellent mechanical properties of gradient nanocrystalline materials prepared by S²PD can be attributed to strain hardening and microstructure refinement. Many studies have analyzed the microstructure refinement mechanisms through S²PD processing in different metals and alloys [2], [7], [12], [13], [14], [15], [16]. Dislocation slip is known as the main deformation mechanism for face-centered cubic (fcc) structured alloys with high stacking fault energy (i.e. > 35 mJ/m² [17]), displaying complicated dislocation interaction mechanisms, namely dislocation generation, rearrangement and annihilation, which contribute to grain refinement. On the other hand, strain induced martensitic transformation is another refinement mechanism, which is mainly observed in materials with a low stacking fault energy (i.e. < 20 mJ/m² [18]). However, no systematic research has reported about the relationship between strain hardening and microstructure refinement behaviors for Hadfield steel with medium-low stacking fault energy during plastic deformation with different degrees, during which deformation twins coordinate with dislocations.

Stacking fault energy is the main factor that affects plastic deformation mechanisms such as dislocation slip, martensitic transformation and mechanical twining. Hadfield steel has medium-low stacking fault energy, whose plastic deformation mechanism is different from iron and copper metals [19]. Hadfield steel possesses good work hardening capacity, high strength and toughness after hardening, and it has been widely used in industrial applications. Nanocrystalline layers have been obtained on the surface of railway crossings and wear-resistant liners made from Hadfield steel subjected to rolling contact fatigue and friction-wear service [20], [21]. The size of the nanocrystalline on the surface of the Hadfield steel railway crossings is about 30 nm, and the hardness of the nanocrystalline layers is about 550 HV [20]. Meanwhile, the size and hardness of the nanocrystalline formed on the surface of the wear-resistant liners are 20 nm and 600 HV, respectively [21]. Nanocrystalline with a size of 3–10 nm could be obtained on the surface of an austenitic manganese steel after shot peening treatment [22]. However, the formation mechanism of nanocrystalline and strain hardening behavior in austenitic manganese steels have not been thoroughly studied.

High speed pounding (HSP) is a novel type of surface optimization technique in metals. It has been utilized in surface hardening of Hadfield steel crossings to produce surface nanocrystalline layer [23], [24]. HSP can introduce high-frequency impact stress to the sample surface, and cause plastic deformation with a large strain rate in deep surface layer. In the present study, HSP treatment with different parameters was conducted on the Hadfield steel, and nanocrystalline layer with different levels was obtained on the sample surface. Strain hardening and microstructure refinement behaviors were systematically studied by analyzing the microstructures, hardness, dislocation density, deformation twin density, and grain size on the surfaces of Hadfield steel samples subjected to HSP for different times. The relationship between strain hardening, nanocrystallization and pounding treatment parameters (strain rate, deformation stress, and pounding time) were also analyzed.