Microstructural and properties evaluation of M2 high speed steel after inoculating addition of powder W and WC

A S Chaus

doi:10.1179/1743284714y.0000000558

Abstract

Melt treatment methods that involve the addition of small amounts of surface-active or/and inoculating elements have permitted the development of improved high speed steels (HSSs) with greater carbide control. In this work, addition of powdered W and WC into the melt was used to improve the as cast microstructure of AISI M2 HSS and, as consequence, its properties after full heat treatment. Under the action of inoculants, there was a refinement of the primary grains of the matrix and a transition from a columnar dendritic mode of solidification (typically found in uninoculated AISI M2) to a predominantly equiaxed morphology in the inoculated steels. SEM and EDS confirmed that the inoculation favoured the formation of the M6C eutectic at the expense of the M2C and MC variants, which prevailed in the as cast microstructure of the uninoculated steel. The relationship between microstructure and the properties is discussed.

Highlights

Three types of eutectic could be observed in the as cast microstructure of the uninoculated steel, i.e. MC with specific branched petal-like morphology, M6C with a typical fishbone morphology and M2C; M2C eutectic appeared in two morphological varieties – rod-like, dominating over other types of eutectic, and lamellar.[25]
8 a products of M2C eutectic carbide decomposition, b V distribution, c M6C fishbone eutectic, d V distribution and e, f overall microstructure in studied high speed steels (HSSs) after heat treatment attributed to its specific chemical composition, as discussed above, and this is further demonstrated by it keeping the initial fishbone morphology without changes after heat treatment, as shown in Fig. 8c for the steel inoculated with 0?1 vol.-%WC
The effect of inoculating additions, which were added to the melt in the ranges 0?3 and 0?6 vol.-% for W and 0?1, 0?3 and 0?6 vol.-% for WC, on the microstructure and properties in AISI M2 HSS have been examined

Summary

Introduction

In the case of cast high speed steels (HSSs), the structure refinement, and resulting toughness improvement, can be achieved using a small addition of different elements into the melt, which can act as inoculants or as surface active agents, for example, Ti1–6, Nb,[2,3,4,6] Zr,[2,6] Ca,[6,7] rear earth metals,[1,6,8,9,10,11,12] Mg6,9 and others.[13,14,15,16] Taking into account such criteria as melting temperature, surface energy, specific heat of sublimation, entropy in the standard condition, statistic generalised moment, and the total electron potential barrier of iron and other elements, it has been shown that Ti, Zr, Hf, Nb and Ta can be theoretically considered as effective inoculants.[6,13] This theoretical prediction has been experimentally proven.[1,2,3,4,5,6] With regard to fundamentals, it is known that the higher melting point and the similarity of a crystal lattice of the inoculant phase compared with the solidifying primary solid solution phase (ferrite in the case of AISI M2 HSS), the higher its potential inoculating effectiveness. 8 a products of M2C eutectic carbide decomposition, b V distribution, c M6C fishbone eutectic, d V distribution and e, f overall microstructure in studied HSS after heat treatment attributed to its specific chemical composition, as discussed above, and this is further demonstrated by it keeping the initial fishbone morphology without changes after heat treatment, as shown in Fig. 8c for the steel inoculated with 0?1 vol.-%WC.

Results

Conclusion