Thermal shock behavior of cast irons with different graphite forms

Thermal shock behavior of cast irons with different graphite forms
Поведение чугунов с различными формами графита при изменении температуры
Поведението на чугуни с различни форми на графита при изменение на температурата

I. RIPOSAN
L. SOFRONI
M. CHISAMERA

The paper deals with the thermal shock behavior of flake, vermicular and spheroidal graphite cast irons as well as the different combinations and properties of graphite separations. As regards the flake graphite cast iron, the stringers of microcracks occur in the matrix at the peaks of the graphite flakes or at the contact with non-metallic inclusions and blow-holes in the cast iron.

The areas with B, D and E (ASTM) type nodular or star-like shape graphite prove to be prone to the formation and development of microcracks. The effect of these graphite forms is enhanced at the edge of the sample and in the proximity of the inclusions and the blow-holes being themselves stringers of microcracks. The lowest development of cracks is observed in the areas of the uniformly distributed flake (type A) graphite. As to the spheroidal graphite cast irons, the influence of the "black spots" is strong, these spots being stringers of cracks long before their occurring in the matrix, on coming into contact with the graphite separations. The porous chunk-graphite areas in the thermal centre of the castings increase the tendency to the occurrence of cracks under the conditions of thermal shock. A higher sensitiveness to the cracking is noticed in the event that the spheroidal graphite has an irregular shape and the graphite spheroids are agglomerating in certain areas. The occurrence of the degenerate
(N type - ASTM) graphite reduces the thermal shock resistance of the cast iron by more than 50 per cent. The occurrence of the free cementite facilitates the cracking of the iron to a higher extent than the degenerate (N) graphite or the "black spots". As far as the vermicular graphite cast irons are concerned, the microcracks occur in the matrix at the ends of the graphite separations in the event of lower compactness (P type - ASTM) and radially for a higher compactness (adjacent to spheroids). In case that the vermicular and spheroidal graphites are associated, the irons are cracking due to the join of the vermicular graphite separations (lengthwise), while the spheroids have only small-sized microcracks.

In the first stage of the thermal shock stress (formation of cracks), the relation of the crack maximum elongations, for the three types of irons are: spheroidal : vermicular : flake graphite - 1 : 3 : 5.

The thermal shock resistance is: spheroidal : vermicular : flake graphite: 1.8-1.6 : 1.4-1.25 : 1.

The relations are valid for unalloyed cast irons with a ferritic structure.

As for heavy castings (ingot - moulds), the thermal shock behavior of the spheroidal graphite irons is practically similar to that of vermicular graphite cast irons, while as for thin castings, the spheroidal graphite cast irons prove to be superior.

As regards some bimetal castings (spheroidal graphite cast iron - grey cast iron), the thermal shock behavior is intermediary, approaching that of the spheroidal graphite cast irons excepting that the two types of cast irons do not form D-type graphite and ferrite in the joined area, this fact facilitating the occurrence of cracks.