Studies on the Formation of Intermetallic Compound Layers …
663
an optimum of 6–6.13 wt% C. Lower levels of carbon result in a higher amount of
dissolved W in the binder phase, which increases the density of Co and furthermore
leads to a deceleration of the solution-reprecipitation process during the sintering
step. Therefore, the grain size of these cemented carbides remains smaller. Higher
amounts of W in cobalt result in a stabilization of the cubic structure, which can
not be converted to the hexagonal form by extensive annealing. Hence, cemented
carbides with smaller grain sizes exhibit a higher content of fcc-Co in the binder
[5, 12, 13].
At the end of their product life cycle, cemented carbides like inserts, drills, compacts from mining, oil and gas industry, wear parts, and hot rolls are recycled for
example by the so-called zinc process. Due to the low energy consumption, high
tungsten recovery, eco-friendliness, and high amount of treated scrap (12,000 t/a in
2010), these methods offer a number of advantages. In this study, we concerned ourselves with the fundamental mechanisms within the zinc process. Thus, a detailed
description will be provided [14–16].
In the first step of the zinc process, the scrap is treated with molten or vaporous zinc
at temperatures of 800–1050 °C in graphite crucibles under Ar or N 2 atmosphere,
whereas the ratio of masses of cemented carbide to zinc typically corresponds to
1:1.3. The zinc infiltrates the material and forms various intermetallic phases with
the Co binder according to the Co–Zn binary phase diagram. Compared to Co, these
intermetallic compounds exhibit a higher volume. As a result, the initially compact
and solid material swells and disintegrates to a porous cake. The reaction between
Co and Zn corresponds to a diffusion controlled process, which proceeds from the
cemented carbide surface inwards. The second step includes the removal of zinc
from the disintegrated material by distillation based on elevating the temperature to
1000–1050 °C and applying of a vacuum in the range of 10 Pa. The product of the
zinc process constitutes a porous and spongy cake with a zinc content < 10 ppm,
which is milled and homogenized to obtain a high quality WC-Co powder. This
zinc reclaim serves as feed material in the production of ready-to-press powder for
cemented carbides [14, 15, 17, 18].
The exact mechanism of the progress of disintegration of the cemented carbides
in the first stage of the zinc process is not known yet, therefore, the purpose of
this investigation was to generate fundamental knowledge regarding the formation
and growth of the intermetallic Co–Zn phases. As aforementioned, a wide range of
cemented carbide grades with varying Co content and WC grain size exists. Since the
structure of the feed material affects the disintegration of the material significantly,
this study takes a step back and puts the focus on the interactions between the two main
components Co and Zn, in order to gain basic understanding of the phase formation by
avoiding the challenges arising from the application of WC-Co composite material.
The tungsten content of the binder material strongly depends on the structure of
the cemented carbide, the carbon balance, sintering process, etc.; hence, this study
intended to describe the influence of the W content in Co on the Co–Zn phase
formation and to allow conclusions regarding the impact on the recyclability of
certain cemented carbide grades. Therefore, this article provides basic information
concerning the growth of intermetallic Co–Zn phases and the effect of dissolved
663
an optimum of 6–6.13 wt% C. Lower levels of carbon result in a higher amount of
dissolved W in the binder phase, which increases the density of Co and furthermore
leads to a deceleration of the solution-reprecipitation process during the sintering
step. Therefore, the grain size of these cemented carbides remains smaller. Higher
amounts of W in cobalt result in a stabilization of the cubic structure, which can
not be converted to the hexagonal form by extensive annealing. Hence, cemented
carbides with smaller grain sizes exhibit a higher content of fcc-Co in the binder
[5, 12, 13].
At the end of their product life cycle, cemented carbides like inserts, drills, compacts from mining, oil and gas industry, wear parts, and hot rolls are recycled for
example by the so-called zinc process. Due to the low energy consumption, high
tungsten recovery, eco-friendliness, and high amount of treated scrap (12,000 t/a in
2010), these methods offer a number of advantages. In this study, we concerned ourselves with the fundamental mechanisms within the zinc process. Thus, a detailed
description will be provided [14–16].
In the first step of the zinc process, the scrap is treated with molten or vaporous zinc
at temperatures of 800–1050 °C in graphite crucibles under Ar or N 2 atmosphere,
whereas the ratio of masses of cemented carbide to zinc typically corresponds to
1:1.3. The zinc infiltrates the material and forms various intermetallic phases with
the Co binder according to the Co–Zn binary phase diagram. Compared to Co, these
intermetallic compounds exhibit a higher volume. As a result, the initially compact
and solid material swells and disintegrates to a porous cake. The reaction between
Co and Zn corresponds to a diffusion controlled process, which proceeds from the
cemented carbide surface inwards. The second step includes the removal of zinc
from the disintegrated material by distillation based on elevating the temperature to
1000–1050 °C and applying of a vacuum in the range of 10 Pa. The product of the
zinc process constitutes a porous and spongy cake with a zinc content < 10 ppm,
which is milled and homogenized to obtain a high quality WC-Co powder. This
zinc reclaim serves as feed material in the production of ready-to-press powder for
cemented carbides [14, 15, 17, 18].
The exact mechanism of the progress of disintegration of the cemented carbides
in the first stage of the zinc process is not known yet, therefore, the purpose of
this investigation was to generate fundamental knowledge regarding the formation
and growth of the intermetallic Co–Zn phases. As aforementioned, a wide range of
cemented carbide grades with varying Co content and WC grain size exists. Since the
structure of the feed material affects the disintegration of the material significantly,
this study takes a step back and puts the focus on the interactions between the two main
components Co and Zn, in order to gain basic understanding of the phase formation by
avoiding the challenges arising from the application of WC-Co composite material.
The tungsten content of the binder material strongly depends on the structure of
the cemented carbide, the carbon balance, sintering process, etc.; hence, this study
intended to describe the influence of the W content in Co on the Co–Zn phase
formation and to allow conclusions regarding the impact on the recyclability of
certain cemented carbide grades. Therefore, this article provides basic information
concerning the growth of intermetallic Co–Zn phases and the effect of dissolved