excellent service and quality.
Alloy steel casting make a Comeback
As regards chromium, it acts to improve the resistance to oxidation at elevated temperature and also the wear resistance. However, if the chromium is employed in an amount in excess of 30 wt%, the toughness and castability of the resultant alloyed steel casting will adversely be affected. On the other hand, if it is employed in an amount not more than 26 wt%, improvement in wear resistance can not substantially be expected.
As regards nickel and cobalt, each of these elements is necessary in order that the matrix be transformed into an austenite structure thereby improving the toughness and consequent improvement of the physical strength at elevated temperature. Of these elements, cobalt is known as an expensive element. Though the employment of cobalt in an amount as small as possible is preferred in view of the brittle fracture, this element is necessary to improve the solid-solubility of tungsten and also to improve the hardness of the austenite structure and, therefore, the uppermost and lowermost limits of the amount to be employed are determined to be 22 wt% and 18 wt%, respectively. The other element, that is, nickel is effective to stabilize the austenite structure if employed in an amount not less than 18 wt%. However, if nickel is employed in an amount in excess of 22 wt%, no further improvement can be expected.
As regards tungsten, it is necessary to improve the wear resistance at elevated temperature. It has been found that, in order to improve the wear resistance at 1,100กใ C., the amount of tungsten to be employed is recommended within the range of 0.5 to 3 wt%. However, in order to improve the wear resistance at the temperature of 1,100กใ to 1,200กใ C., 0.5 to 6 wt% is recommended.
As regards niobium, this element is necessary to improve the physical strength at elevated temperature. However, with the resistance to oxidation being taken into consideration, 2 to 4 wt% is necessary in order to improve the physical strength at 1,100กใ C. and 0.5 to 2 wt% is necessary in order to improve the physical strength at 1,100กใ to 1,200กใ C. Therefore, the uppermost and lowermost limits of the amount of niobium to be employed are respectively fixed at 0.5 to 4 wt%.
The remainder employed is iron and other industrially inevitable impurities.
The present invention will now be described by way of example which is not intended to limit the scope of the present invention.
A sample of alloyed steel according to the present invention which contains carbon in an amount of 0.32 wt%, silicon in an amount of 1.88 wt%, manganese in an amount of 1.24 wt%, phosphorus in an amount of 0.007 wt%, sulfur in an amount of 0.006 wt%, chromium in an amount of 27.17 wt%, nickel in an amount of 20.17 wt%, cobalt in an amount of 20 wt%, tungsten in an amount of 1.5 wt%, niobium in an amount of 1.5 wt% and an aluminum deoxidizing agent in an amount of 12.01 wt%, was cast.
The sample of alloyed steel in the above composition, was tested to determine the physical strength, the creep strength at rupture and the hardness at elevated temperature, the results of these tests being shown in the respective graphs of FIGS. 1 to 3.
For the purpose of comparison, the UM Co-50 alloy was also tested and the results of the tests are shown in the graphs of FIGS. 1 to 3.
In the graphs of FIGS. 1 to 3, the sample of alloyed steel according to the present invention and that of the UM Co-50 alloy are respectively identified by A and B.
From the graphs of FIGS. 1 to 3, it is clear that the alloyed steel according to the present invention substantially eliminates the disadvantages inherent in the UM Co-50 alloy and which have hereinbefore been described.
Although the present invention has been fully described by way of example, it should be noted that various changes and modifications are apparent to those skilled in the art, such changes and modifications being to be understood as included within the true scope of the present invention unless they depart therefrom.
