Revealing the maximum microhardness and thickness of hardened layers for copper with various grain sizes

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Abstract

Surface strengthened Cu samples with ultra-fine grain (UFG), fine grain (FG), and coarse grain (CG) matrixes were prepared by cold-rolling, controlled annealing, and then treated by surface spinning strengthening (3S) to reveal the effect of grain size on the feature of the hardened layer. The results indicate that the maximum microhardness of the hardened layer was slightly affected by grain size. The thickness of the hardened layer is significantly influenced by grain size. When decreasing the grain size of matrix from CG to UFG, the thickness of the hardened layer displays an increasing then deceasing trend, and approaches in the highest thickness at FG.

Introduction

In recent years, surface mechanical strengthening receives more and more attentions for the reason that it can modify the microstructure in the surface layer of metals and thus improve the service performance of metallic components, such as fatigue property and wear resistance [[1], [2], [3], [4], [5]]. A layer with gradient microstructure can be created on the surface strengthened metals, in which grain size decreases gradually along the depth direction. For the gradient microstructure in the hardened layer, nano-scale grains (NGs), ultra-fine grains (UFGs), fine grains (FGs), and the coarse grains (CGs) appear successively from the surface to the matrix [6]. Just because of this, the microhardness also decreases gradually along the depth direction from the topmost surface layer to the matrix and the maximum value usually appears at the topmost surface layer [[7], [8], [9]].

In the industrial field, surface strengthening is always employed to improve the service performance of metallic components, and therefore to reveal the feature of the hardened layer is of great significance [10,11]. As the hardened layer of surface strengthened metals is the foundation of improving service performance, how to describe and understand the properties of hardened layer is an important issue. It is well known that the stress-strain curve is often used to describe the tensile behavior of metal, which can provide us with yield strength, ultimate tensile strength and uniform elongation [12,13]. In a similar vein, the microhardness-depth curve is widely used to describe surface strengthening effect, which can also give us some important information about the hardened layer even the surface strengthening characteristics of metals [14,15]. In our previous study, an equation was proposed to describe the microhardness distribution in the surface hardened layer. Some important parameters were defined, such as the maximum microhardness (HM), the thickness of hardened layer (λ) and the matrix microhardness (Hm) [16]. It was reported that matrix strength affects the surface strengthening behaviors of four common steels treated by surface spinning strengthening, and HM is increasing and λ is decreasing with the increase of yield strength [17].

However, the effect of microstructure on the properties of the surface hardened layer is still rarely studied, especially the grain size of metals. Whether or not the grain size affects the characteristics of the surface hardened layer is a very important scientific issue. In this work, the pure Cu sample is used as a model material for studying the effect of grain size on the hardened layer properties to reveal the corresponding mechanisms.

Section snippets

Materials and methods

The as-cast pure Cu with the purity of 99.95% was first forged to a billet with a cross section area of 40 × 40 mm2, and then rolled to a plate with a thickness of 5 mm at room temperature. Next, the cold-rolled Cu plates were recrystallized with an annealing treatment, including 200 °C for 20 min with an oil bath furnace; 300 °C, 400 °C, and 500 °C for 20 min with a sault bath furnace; 600 °C, 700 °C, and 800 °C for 1 h with a muffle furnace to prepare Cu samples with different grain sizes,

Experimental results

The pure Cu samples with various grain sizes were fabricated by first cold-rolling and then controlled annealing treatments, and the microstructures of all samples are shown in Fig. 1. The CR sample has the elongated cold-rolling microstructures with a grain size about 1 μm, and CR + A200 sample has a partially recrystallized microstructure with a grain size about 3 μm. With the increase of annealing temperature from 300 °C, 400 °C–500 °C, the CR + A300, CR + A400 and CR + A500 samples have

Discussion

Grain refinement theory indicates that the yield strength of metal increases with the decrease of grain size. Therefore, from the traditional point of view, with the increase of the initial grain size, the thickness of the hardened layer should increase because plastic deformation gets easier. In fact, it is surprising that the thickness of the hardened layer increase first and then decreases, and it is a very different result from the traditional understanding. A very important scientific

Conclusion

In summary, the Cu samples with different grain sizes from UFG to CG were prepared by cold-rolling, controlled annealing treatments and then 3S treatment. In the topmost surface layer, the grains of all the 3Sed samples are refined to NSGs, and the corresponding HM of the hardened layer is approximate with a similar value of 162 HV. This result indicates that the maximum microhardness is slightly influenced by the initial grain size of Cu samples and it depends on the degree of grain

CRediT authorship contribution statement

C.X. Ren: Investigation, Methodology, Writing - original draft. Q. Wang: Methodology, Writing - review & editing. J.P. Hou: Project administration, Methodology. Z.J. Zhang: Conceptualization, Methodology, Project administration. H.J. Yang: Data curation, Resources, Investigation. Z.F. Zhang: Conceptualization, Funding acquisition, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) under grant Nos. U1664253, 51331007 and LiaoNing Revitalization Talents Program under grant No. XLYC1808027.

References (33)

Cited by (9)

  • The nature of the maximum microhardness and thickness of the gradient layer in surface-strengthened Cu-Al alloys

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    With an increase in the yield strength of the as-received metal, the maximum microhardness increases, as in pure Ti processed by a surface rolling treatment (SRT) and IN 718 alloy processed by SP [48,51]. Furthermore, some metals with the same chemical compositions but different strengths have the same maximum microhardness after the surface strengthening, as in pure Cu [52] and in the Cu-Al alloys in this study. Likewise, the effect of the work-hardening exponent on the maximum microhardness in the gradient layer of the different metals processed by different surface strengthening methods was also examined as shown in Fig. 10b. With a large range of work-hardening exponents, the maximum microhardness changes irregularly and the distributions of the maximum microhardness values are essentially random.

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