Speaker
Description
High Cr cast irons have excellent wear resistance because a large amount of chromium carbides is distributed in the matrix. Mechanical properties of the cast iron required for practical uses are generally obtained by the heat treatment. In order to get the guideline for the appropriate heat treatment, however, the transformation behavior must be understood in each cast iron. Since various kinds of alloying elements are added as a single or a combination to improve the heat treatment characteristics, it is important to reveal the effects of their elements on the transformation behavior. The effect of a single alloy addition on the transformation have been reported quite a lot but that of combined addition is not always clarified sufficiently. Both the Mo and W elements have similar properties and not only form carbides mainly but also dissolve into the matrix at the same time.
In this study, the different ratios of Mo and W contents were simultaneously added to the high Cr cast iron and the effect of combined addition on the behavior of continuous cooling transformation (CCT) was investigated. The hypo-eutectic 16% Cr cast irons with various combinations of 0.5-3% of each Mo and W content were prepared for this experiment. In order to discuss the behavior of continuous cooling transformation, CCT curve of each specimen was constructed from the thermal expansion curves with various cooling rates using an automatic transformation recorder. The austenitizing temperatures adopted were 1273K and 1323K.
Pearlite (P), bainite (B), Ms and Mf transformation were clearly distinguished on the dilatation curve of each specimen regardless of Mo and W contents.
As the Mo and W contents increase, the nose times of P and B transformations showed a tendency shifting to the long-time side but the ratio was much greater in the P transformation. As the austenitizing temperature also rises, the P and B transformations shifted toward the long-time side and the Ms temperature lowered.
Here, a parameter of tungsten equivalent (Weq=%W+2x%Mo) was introduced and the effect of the Weq value on the transformations were evaluated. It is found that the P transformation is delayed in proportion to an increase in Weq value regardless of the ratio of Mo and W additions. As for the B nose time, it can be also related to the Weq value in the same way as the P nose time. However, the delaying rate by increasing of Weq value was small compared with the case of P transformation. Additionally, the Ms and Mf temperatures fell corresponding to the Weq value.
It is considered that the transformation behavior depends on the concentrations of alloying elements in the matrix. Therefore, the tungsten equivalent value of the matrix (Weq-mat) was calculated using Thermo-Calc in each austenitizing temperature and the Weq-mat value was connected to the nose time of each transformation. The P nose time enlarged with an increase in the Weq-mat value regardless of the austenitizing temperature. Therefore, it may reasonable to evaluate the P transformation behavior by means of the Weq-mat.
From the CCT diagrams of the specimens with various Weq values, the transformation diagrams of matrix were constructed in the relation of the Weq values vs. the critical cooling rates for each transformation of specimens. It was found that the matrix structures at various cooling rate can be predicted well fromthe proposed diagrams.
Keywords: High Chromium cast iron, tungsten equivalent, CCT curve, critical cooling rate, hardness, structure diagram of matrix.
Summary
High Cr cast irons have excellent wear resistance because a large amount of chromium carbides is distributed in the matrix. Mechanical properties of the cast iron required for practical uses are generally obtained by the heat treatment. In order to get the guideline for the appropriate heat treatment, however, the transformation behavior must be understood in each cast iron. Since various kinds of alloying elements are added as a single or a combination to improve the heat treatment characteristics, it is important to reveal the effects of their elements on the transformation behavior. The effect of a single alloy addition on the transformation have been reported quite a lot but that of combined addition is not always clarified sufficiently. Both the Mo and W elements have similar properties and not only form carbides mainly but also dissolve into the matrix at the same time.
In this study, the different ratios of Mo and W contents were simultaneously added to the high Cr cast iron and the effect of combined addition on the behavior of continuous cooling transformation (CCT) was investigated. The hypo-eutectic 16% Cr cast irons with various combinations of 0.5-3% of each Mo and W content were prepared for this experiment. In order to discuss the behavior of continuous cooling transformation, CCT curve of each specimen was constructed from the thermal expansion curves with various cooling rates using an automatic transformation recorder. The austenitizing temperatures adopted were 1273K and 1323K.
Pearlite (P), bainite (B), Ms and Mf transformation were clearly distinguished on the dilatation curve of each specimen regardless of Mo and W contents.
As the Mo and W contents increase, the nose times of P and B transformations showed a tendency shifting to the long-time side but the ratio was much greater in the P transformation. As the austenitizing temperature also rises, the P and B transformations shifted toward the long-time side and the Ms temperature lowered.
Here, a parameter of tungsten equivalent (Weq=%W+2x%Mo) was introduced and the effect of the Weq value on the transformations were evaluated. It is found that the P transformation is delayed in proportion to an increase in Weq value regardless of the ratio of Mo and W additions. As for the B nose time, it can be also related to the Weq value in the same way as the P nose time. However, the delaying rate by increasing of Weq value was small compared with the case of P transformation. Additionally, the Ms and Mf temperatures fell corresponding to the Weq value.
It is considered that the transformation behavior depends on the concentrations of alloying elements in the matrix. Therefore, the tungsten equivalent value of the matrix (Weq-mat) was calculated using Thermo-Calc in each austenitizing temperature and the Weq-mat value was connected to the nose time of each transformation. The P nose time enlarged with an increase in the Weq-mat value regardless of the austenitizing temperature. Therefore, it may reasonable to evaluate the P transformation behavior by means of the Weq-mat.
From the CCT diagrams of the specimens with various Weq values, the transformation diagrams of matrix were constructed in the relation of the Weq values vs. the critical cooling rates for each transformation of specimens. It was found that the matrix structures at various cooling rate can be predicted well fromthe proposed diagrams.
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