| Popis: |
The growth and scaling resistance of six spheroidal graphite cast irons of varying silicon content has been investigated. The range of silicon content selected was 1 to 6% since it is very difficult to make spheroidal graphite irons with less than 1% silicon and it is unlikely that these irons could even be regularly commercially produced with more than about 5. 5% silicon. Three series of growth tests have been carried out:- (a) repeated heating and cooling through the temperature range 650°-900°-65°C for all the irons, (b) progressive heatings at several different temperatures 710°, 815°, 890° and 1000°C for all the irons and (c ) repeated heating and cooling through the temperature range 650°-1000°-650°C for the two highest silicon irons. Heat resistance results obtained under conditions of repeated heating and cooling through the critical range are shown to be substantially different from those obtained by holding at one or a number of fixed temperatures. The effect of a prior heat treatment or annealing on the growth resistance has been investigated in two out of the three series of growth tests on those irons which initially contained pearlite in their as cast condition. The two highest silicon irons which were completely ferritic in the as cast condition were not given any previous heat treatment. An extensive metallographic examination has been made of the growth bar specimens. The sealing resistance of all the irons over the temperature range 550-900°C has been investigated with both continuously and intermittently exposed specimens and again a very extensive metallographic examination has been made of the sealing test specimens. The mechanical properties of the original as cast material and of the growth bars after treatment in the three series of growth tests have also been considered in relation to their microstructures. The critical temperature ranges of the six nodular irons have been determined and the growth and scaling results have been discussed in relation to these determinations. |
