Microstructural evolution during laser cladding of M2 high-speed steel
- PDF / 1,938,493 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 106 Downloads / 235 Views
I. INTRODUCTION
LASER cladding involves the deposition of a solid layer onto a substrate surface, with an excellent metallurgical bond at the interface. This process has been used to improve corrosion, wear, and oxidation resistance at the surface of a component.[1] Recently, laser cladding combined with a three-axis computer numerical controlled (CNC) table has been used to fabricate fully dense and near-net-shaped metal components directly from powders using a specific computer-aided design (CAD).[2] This offers an excellent opportunity to reduce the time and cost of manufacturing functional metal parts. In addition, the inherent rapid heating and cooling associated with laser cladding can be expected to provide grain refinement and in-creased homogeneity in the resultant microstructure. High-speed steels (HSSs) are important engineering materials with a desired combination of wear resistance, hot hardness, and toughness. They are commonly used in the cutting tools, automotive, and space vehicle industries.[3] According to the phase diagram of M2 HSS, solidification transformation of M2 involves the following four stages:[4,5,6] L→L1d L1d→L1d1g
1435 8C 1330 8C
L 1 d 1 g → L 1 d 1 g 1 Mx C L 1 d 1 g 1 MxC → g 1 MxC
1260 8C
1235 8C
g 1 MxC → a 1 MxC where L is the melt, d is the ferrite, g is the austenite, and MxC is the carbide, with “M” denoted as the metallic constituent elements. H.J. NIU, formerly Postgraduate Student, School of Metallurgy and Materials, The University of Birmingham, is Research Associate in Department of Physics, University of Durham, Durham DH1 3LE, United Kingdom. I.T.H. CHANG is Lecturer in the School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, United Kingdom. Manuscript submitted October 27, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
The solidification of M2 begins with primary crystallization of d ferrite and is followed by a peritectic reaction between the carbon-rich liquid and the d-ferrite dendrites to form austenite. This reaction transforms some of the original d ferrite into g austenite, but generally leaves behind d ferrite at the core of the dendrites. Subsequently, the last interdendritic liquid decomposes into austenite and carbide through a eutectic reaction. As the temperature continues to drop, the cores of the ferritic dendrites decompose to a darketching austenite-carbide aggregate known as d eutectoid. Finally, some of the remaining austenite is transformed into martensite during further cooling. Since M2 HSS contains an inherently high alloy content and exhibits a complex carbide structure, conventional processing of HSSs, such as casting and hot working, gives rise to carbide segregation at grain boundaries and a coarsegrain microstructure.[7] This can reduce toughness, with the consequent risk of premature failure by chipping or breakage of tools during service at highly stressed conditions.[7] Therefore, HSSs need complicated, expensive, and carefully controlled processing, such as hot working, heat treating, and machi
Data Loading...
