Nanostructured tool steel fabricated by combination of laser melting and friction stir processing

doi:10.1016/j.msea.2008.11.006

Materials Science and Engineering: A

Volume 505, Issues 1–2, 15 April 2009, Pages 157-162

https://doi.org/10.1016/j.msea.2008.11.006 Get rights and content

Abstract

The microstructural control of tool steel (SKD11) by laser melting and friction stir processing (FSP) was studied. A nanometer-sized microstructure consisting of fine M₇C₃ carbide (particle size: ∼100 nm) and a matrix (grain size: ∼200 nm) was successfully fabricated by the FSP on the laser treated SKD11. The nanostructured SKD11 had an extremely high hardness of about 900 HV even with its relatively high amount of retained austenite.

Introduction

Tool steels are indispensable materials in various industrial fields, and are used for cutting tools, dies, molds, and so on. Tool steels have been developed through the addition of large amounts of alloying elements in order to disperse many carbide particles in iron based matrix. However, the improvement of mechanical properties, such as hardness, by the compositional modification has become limited. Furthermore, the amount of alloying elements should be reduced because of resource savings, cost reduction, and recyclability requirements.

Recently, much attention has been paid to structural refinement, which improves the mechanical properties of metallic materials. It is well known that the grain refinement of the matrix increases strength and hardness in accordance with the Hall–Petch relation. Several severe plastic deformation (SPD) processes have been developed to reduce the grain size of various metallic materials [1], [2], [3], [4]. However, the grain refinement of tool steels by a representative SPD process, such as Equal-Channel Angular Pressing (ECAP) and Accumulative Roll Bonding (ARB), is quite difficult because of the high deformation resistance of tool steels. Compared with other SPD processes, friction stir processing (FSP), which is the same technique as friction stir welding (FSW), can be easily used for various metallic materials [5], [6], [7], [8], [9]. A selected area of a metal plate could be modified by a rotating tool which is inserted into the metal plate, hence producing a highly plastically deformed zone. The stir zone consisting of fine and equiaxed grains has excellent strength and hardness [10], [11].

Additionally, the uniform dispersion of the fine carbide particles is also important for increasing its mechanical properties as well as helping to extend the lifetime of various steel parts. Though the exfoliation of the large carbide particles leads to serious defects, that of the nanometer-sized carbide particles is negligible for general applications. Usually, the coarse carbide particles with particle sizes of several tens of micrometers are dispersed in the matrix of tool steels. It is difficult to refine the carbide particles by only the FSP because of their high hardness and thermal stabilities. In this study, the microstructural control of tool steel by the combination of laser melting and FSP is investigated in order to form a nanometer-sized microstructure.

Section snippets

Experimental procedure

A commercially available plate of tool steel (SKD11) was used in this study. The chemical composition of the SKD11 is shown in Table 1. The surface of the plate was melted by multi-pass laser heating (1 kW, LASERLINE LDF-1000–750) to produce a rapidly solidified zone for the FSP. The scanning rate of the laser beam and the beam diameter at the surface of the plate were 1000 mm/min and 1 mm, respectively. The overlap between the pass of the beam was 0.3 mm. The as-received SKD11 and the laser

Microstructure of the laser treated SKD11

The rapidly solidified zone fabricated by the laser melting could be clearly confirmed in Fig. 2(a). The unique shape of the zone was formed by the laser lapping. There were many coarse carbide particles in the matrix of the as-received SKD11 as shown in Fig. 2(b). On the other hand, no coarse carbide particles could be confirmed in the rapidly solidified zone. The carbide formed a fine dendritic structure as shown in Fig. 2(c). The microhardness was increased from 260 to 473 HV by the

Conclusions

The matrix grains and carbide particles of the SKD11 were significantly refined by the laser melting and the FSP. The microstructure and microhardness were evaluated by observations of the grain size and phase of the matrix, and the size and dispersion of the carbide particles. The obtained results can be summarized as follows.

The nanometer-sized microstructure consists of a fine carbide (particle size: ∼100 nm) and matrix (grain size: ∼200 nm) as fabricated by the combination of laser melting

Acknowledgements

The authors wish to acknowledge the financial support of a Grant-in-Aid for Young Scientists (B) and a Grant-in-Aid for Science Research (B) from the Ministry of Education, Culture, Sports, Science and Technology.

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Nanostructured tool steel fabricated by combination of laser melting and friction stir processing

Abstract

Introduction

Section snippets

Experimental procedure

Microstructure of the laser treated SKD11

Conclusions

Acknowledgements

Mater. Sci. Eng. A

Acta Mater.

Scripta Mater.

Mater. Sci. Eng. A

Scripta Mater.

Mater. Sci. Eng. A

Scripta Mater.