Role of grain size in liquid-metal embrittlement of iron

Abstract

The phenomenon of liquid-metal embrittlement is determined to a significant degree by the structure of the material and the conditions of deformation. It has been experimentally established that with an increase in grain size of pure metals and low-alloy alloys the effect of a liquid-metal medium increases. For alloys a linear relationship has been revealed between the true tensile strength and D -~2 (D is the average grain size). However, the majority of investigations have been made at constant temperature. In studying the change in temperature of the ductile-brittle transformation on samples with different grain sizes [i] a linear relationship of it to log D was obtained. The tests were made on u-brass in mercury, and in th~s case there is a possibility of diffusion penetration of the medium. Therefore, it is of interest to investigate the influence of grain size on the temperature range of embrittlement for systems in which diffusion or corrosion solution is practically absent. Below are given the results of such investigations on Armco iron in molten Bi, Pb, and their eutectic. The greatest~umber of tests was made in Pb--Bi eutectic since as a result of its low melting point tests may be made at lower temperatures. Tension of the 9-mm-diam. cylindrical samples was on a reequipped MP-4G unit at a rate of movement of the clamp of 5%/min. Different grain sizes were obtained b~ appropriate heat treatment of the samples in vacuum: anneal at 1200=C for 5 h for 120 ~m; anneal at II00~ for 2 h for 35 ~m; normalize with subsequent tempering at 500~ for i h for 20 ~m. The samples were covered with a solder of the appropriate low-melting metal in the presence of flux, placed in a vessel with the liquid metal, and tested in a vacuum chamber.