The tensile properties of four steels have been determined as a quantitative function of the measured dimensions of the aggregate structures pearlite and spheroidite, and of the austenite decomposition temperature for the structures pearlite and bainite. Studies of the recalescence effect have been performed in connection with the measurement of the reaction temperature. The strength indices (stress at corresponding strains, tensile strength, hardness) vary linearly with the reaction temperature and the logarithm of the dimensions of the aggregate. Mixtures of pearlite and bainite are intermediate in strength. The ductility indices are low for mixed structures, coarse pearlite and low temperature bainite; higher for bainite and pearlite in the middle of the reaction temperature range for each. It has been observed that spheroidized eutectoid specimens have a typical mild steel yield point; pearlitic specimens of the same tensile strength do not. The spacing of pearlite is shown to be proportional to the carbon diffusion coefficient in austenite, the logarithm of the spacing plotting as a straight line against the reciprocal of the absolute reaction temperature, with the same slope as a similar plot for the diffusion coefficient. Because of this it is concluded that a measurement of the spacing at one temperature permits its calculation at another, using the measured energy of activation for the diffusion of carbon in the steel. A rule of strength for aggregates is proposed, based on these studies, as follows: The resistance to deformation of a metallic aggregate consisting of a hard phase dispersed in a softer one is proportional to the logarithm of the mean straight path through the continuous phase. The rule works for a comparison of the properties of pearlite and spheroidite, as well as for pearlite alone over a wide range of spacings, and extrapolates to reasonable particle sizes for the finest spheroidites (tempered martensite). A simple explanation of the semilogarithmic character of the relationship is advanced.Keywords Microstructure Á Tensile properties Á Mechanical properties Á Pearlite Á Bainite This paper is the second of a series dealing with the quantitative correlation of the microstructures observed in metals and alloys with the mechanical properties of these structures. It deals with the mechanical properties of structures consisting of a hard phase dispersed in a soft one, and particularly with those aggregate structures in steel known as pearlite, spheroidite, and bainite. The structure of pearlite is lamellar; the particles in spheroidite are approximately spheroidal, while the fine structure of bainite is still uncertain. Work on the specific effect of various alloying elements in solution in ferrite is in progress, and work on the effect of grain size on the properties of ferrite will be reported in the near future. It is hoped that these studies will help in the development of a theory of the strength of metals and alloys, and perhaps be of use in their practical utilization.Two y...

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An interpretive review is presented of the development of scientific knowledge of fracture processes and of the technological application of this information to the evolution of engineering principles for fracturesafe design. The review is in the format of a chronological exposition of the successive advancements in the state of knowledge relating to both the mechanical and metallurgical aspects of the subject. The consolidation of these aspects emphasizes that fracture-safe design practices are not separable into metallurgical and mechanical aspects, but rather involve detailed engineering consideration of both factors. The evolution of modern fracture-safe design technology has Its origins in the broad-scope research activity period of the 1940's. The results of the early research provided an enduring base on which more selective studies were evolved in the ensuing decades. The evolution of significant fracture toughness characterization test methods and of procedures for their analytical interpretation, with respect to both metallurgical quality and mechanical aspects, paced the rate of progress during this time period. In this report a detailed description is provided of these various tests, with separation as to types which were primarily of research interest as compared to those which emerged as suitable for general engineering usage. The theoretical bases for analytical translation of laboratory test data to structural performance factors are discussed, with particular reference to the subject of fracture mechanics. The role of metallurgical factors is described in relation to microfracture processes which determine the macroscopic engineering properties. PROBLEM STATUS This is a special summary and interpretative report covering the results of a wide spectrum of investigations within the Metallurg/Division of NRL. These investigations are aimed at the general problem of metallurgical optimization andfracture-safe design. The major portions of the studies are continuing under the established problems.

Trends in pressure vessel applications involving higher pressures, lower service temperatures, thicker walls, new materials, and cyclic loading require the development of new bases in the supporting scientific and technological areas. This report presents a “broad look” analysis of the opportunities to apply new scientific approaches to fracture-safe design in pressure vessels and of the new problems that have arisen in connection with the utilization of higher-strength steels. These opportunities follow from the development of the fracture analysis diagram which depicts the relationships of flaw size and stress level for fracture in the transition range of steels which have well-defined transition temperature features. The reference criteria for the use of the fracture analysis diagram is the NDT temperature of the steel, as determined directly by the drop-weight test or indirectly by correlation with the Charpy V test. Potential difficulties in the correlation use of the Charpy V test are deduced to require engineering interpretation of Charpy V test data rather than to involve basic barriers to the use of the test. The rapid extension of pressure vessel fabrication to Q&T steels is expected to provide new problems of fracture-safe design. These derive from the susceptibilities of steels within this family to tear fractures of low energy absorption. This fracture mode does not involve a transition temperature and is therefore relatively independent of temperature. It is emphasized that such susceptibilities are not inherent to the family of Q&T steels of low and intermediate strength levels, but are related to specific metallurgical conditions of the plate and particularly the HAZ (heat-affected-zone) regions of Q&T steel weldments.

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