Effect of Combined Addition of Molybdenum and Tungsten on Continuous Cooling Transformation Behavior of High Chromium Cast Iron

The effect of the combined addition of molybdenum (Mo) and tungsten (W) on the behavior of continuous cooling transformation (CCT) for 16%Cr cast iron was investigated. In the CCT diagrams, pearlite (P), bainite (B), and martensite (M) transformations appeared in each specimen regardless of Mo and W contents except for a specimen with small amount of Mo and W in which the B transformations did not occur. The nose time of both transformations was postponed as the Mo and W contents increased. When the effects of the combined addition of Mo and W on each transformation were evaluated using a parameter of tungsten equivalent (W_eq=%W+2x%Mo) of the specimen, the nose time of P transformation was delayed with an increase in the W_eq value, regardless of the combined or single addition of both elements. The nose time was also sifted to longtime side when the austenitizing temperature rose to 1050 ºC. The nose time of B transformation showed the same tendency as that of P transformation. However, the delaying degree of B nose time toward a rise of W_eq value was small compared with that of P nose time. The Ms temperature rose with increasing the W_eq value, but it dropped about 50 ºC by elevating austenitizing temperature to1050 ºC. It is in actual fact that the transformation behavior is related to the concentration of alloying elements in the matrix. Therefore, the W_eq value in the matrix (W_eq-mat) at each austenitizing temperature was calculated and the relations of the P and B nose times versus their W_eq-mat value were obtained. The relations are expressed by the following equations, regardless of the austenitizing temperature.

\({t}_{\text{P}-\text{n}}\left(s\right)=856.7\text{exp}(0.8\times {W}_{\text{eq}-\text{mat}})\) $t_{\text{P}-\text{n}}\left(s\right)=856.7\text{exp}(0.8\times W_{\text{eq}-\text{mat}})$

\({t}_{\text{B}-\text{n}}\left(s\right)=3714.9\text{exp}(0.07\times {W}_{\text{eq}-\text{mat}})\) $t_{\text{B}-\text{n}}\left(s\right)=3714.9\text{exp}(0.07\times W_{\text{eq}-\text{mat}})$

The hardness of transformed cast iron is closely connected to the matrix microstructures identified by the CCT diagram. The highest hardness (HVmax) can be estimated from the values of W_eq-mat and matrix C at each austenitizing temperature. The HVmax decreases with an increase in W_eq-mat value, irrespectively of the austenitizing temperature. As the matrix C content increases, the hardness rises first to the maximum and then lowers.