Effects of constrained groove pressing, severe shot peening and ultrasonic nanocrystal surface modification on microstructure and mechanical behavior of S500MC high strength low alloy automotive steel

Highlights

• CGP achieves volumetric grain size reduction.

• S500MC HSLA fine grained steel is achieved SPD by CGP.

• SSP and UNSM contribute nanocrystallization (below 100 nm) on the surface.

• UNSM contributes the improvement both ductility and strength.

• CGP affectively increase the strength regardless of the pass number.

• SSP and UNSM showed better wear and friction resistance particularly at lower loads.

Abstract

S500MC high strength low alloy automotive steel is exposed to bulk severe plastic deformation (SPD) via constrained groove pressing (CGP) and surface severe plastic deformation (S²PD) via severe shot peening (SSP) and ultrasonic nanocrystal surface modification (UNSM). SSP and UNSM could create a nanocrystallization layer till 50–100 µm away from outmost surface. EBSD investigations showed average nano-grain size obtained via SSP and UNSM was found to be below 100 nm regime. The strength was improved via 1st to 4th pass of CGP, but elongation percentage decreased abruptly. UNSM achieves both strength-ductility improvement with gradient structure. SSP improves the total elongation however a slight decrease on strength is observed. SSP and UNSM showed better wear and friction resistance particularly at lower loads compared to CGP and untreated specimens. Nevertheless, wear and friction behavior at higher loads showed better responses for Bulk-SPD applications regardless of pass numbers. The frictional load increase played a detrimental role in removing a nanocrystallization surface layer and diminishing the positive influence of SSP and UNSM.

Introduction

S500MC high strength low alloy (HSLA) steels are selected for automotive, transport and offshore applications for high strength, effective weldability and limited carbon content [1]. The advantageous of the steel is the combination of three pods: strength-toughness-formability. Therefore, it is attractive for light bodies where the strength is indispensable [2].

Grain size is a crucial microstructural factor for determining physical and mechanical properties of metallic materials [3]. Reducing grain size to ultra-fine and nano-regimes imparts the metallic materials superior properties and attracts a great deal of attention [4]. Severe plastic deformation (SPD) methods are attractive techniques for obtaining ultra-fine and nanosized grains by imposing large plastic strain for bulk materials [5]. SPD methods provide high angle grain boundaries formed by deformation and allows nanostructural evolution [6]. Nanocrystallization, high number of grain boundaries and large grain boundary fraction by SPD may enhance wear resistance, fatigue performance and strength-hardness level [7], [8]. Several SPD methods such as equal channel angular pressing (ECAP) [5], [6], [9], accumulative roll bonding (ARB) [7], [8], [10], [11], high pressure torsion (HPT) [12] and constrained groove pressing (CGP) are also able to produce an ultrafine grains [13]. CGP is involved in bulk SPD methods and used grooved-flat dies. Grooved dies for exposing the strain and flat dies for smoothing the sheets are employed. Deformation is occurred by purely shear forces within the conditions of plain strain. The strains are deposited without negligible changing the geometrical properties of the molds and samples. The process could be able to achieve grain refinement down to 100–200 nm regime [14]. The CGP achieves to modify not only yield strength-hardness and grain size but also corrosion, cell adhesion and hydrophobic-hydrophilic characteristics of bio-based materials [15].

The literature studies presented the tensile and yield strength of the treated materials up to two times has been accomplished within the nano-regime grain size. CGP is applied to the materials like aluminum, copper, magnesium, Al-Cu, Al-Mg and Cu-Zn alloys due to the lower strength and high ductility. Since the press forces could only be able to achieve the process effectively for these alloys. In recent years, some scientists tried to apply the bulk and S²PD processes to low carbon steel owes the lowest strength-hardness capacity among iron based structural alloys [16].

SPD processes are not only used for grain refinement in bulk materials, but also applied to the surfaces to form a nanocrystalline layer having a depth of several microns at the surface materials. The SPD for surface nanocrystallization is named as surface severe plastic deformation (S²PD) methodology. Severe shot peening (SSP) performed by enhanced shot peening parameters [17], surface nanocrystallization and hardening (SNH) [18], [19], ultrasonic nanocrystal surface modification (UNSM) [20], [21] are involved in S²PD and lead to improve the mechanical [22], [23], [24] and fatigue [25], [26] properties of the materials. SSP is employed to create nanocrystalline layer on the surface and expose superior hardness and residual stress for a decade [27]. The metallurgical process is gradually improved by Almen intensity and surface coverage change. Residual stress distribution is influenced from intensity and coverage, as well. However, the limit is observed for fatigue resistance although the increase of intensity and coverage. The surface drawbacks subsequent to SSP is overcome by post-grinding, re-peening and over-peening processes[28].

UNSM is a novel surface modification process for improving the wear, corrosion and fatigue resistance via deeper plastically deformed layer with precisely controlled surface roughness and integrity [29]. UNSM is a type of mechanical cold-forging process to produce a high stressed and ultrafine grained layer. The layer owes higher fatigue and tribological resistance than that of the untreated ones. The superior characteristics are attributed to impact force that can be able to adjust strength-hardness-ductility three pod junctions [30]. Due to the high velocity impact of shots via SSP, the process can achieve remarkable hardness and compressive residual stress on the surface and effective nanocrystallization with several microns through interior from surface. Besides, the sustainability of UNSM contributes deeper residual stress creation that prevents sub-surface crack initiations [29].

The goal of this study is to compare the S²PD processes SSP and UNSM with one another CGP of bulk SPD process in terms of the mechanical and tribological properties of HSLA S500MC steel. To this end S500MC sheet metal specimens were subjected to UNSM, SSP and CGP processes. Tensile, micro-hardness and wear tests were performed on the untreated and treated specimens. For microstructural investigations optical microscopy (OM), scanning electron microscopy with electron back scatter diffraction (SEM-EBSD) and X-ray diffraction analysis (XRD) were carried out. Grain refinement and distributions based on bulk and S²PD were comparatively investigated.