MECHANICS AND USEFUL ARTS. 67 



STEEL UNDER THE MICROSCOPE. 



An experienced steel-maker can estimate very closely the pre- 

 cise quality, chemical composition, tensile and compressive 

 strength, ami even the mode of treatment which a steel has 

 undergone, by looking at its fracture. We have already drawn 

 attention in this journal to the interesting researches made by M. 

 Schott, the manager of Count Stdllberg's foundery at Ilsenburg, 

 upon the appearance of liquid and solidifying cast iron under the 

 microscope, and we can quote the experience of this metallurgist 

 as to the advantages to be obtained from microscopic observation 

 of various kinds of steel. M. Schott, at his visit to the Paris Ex- 

 hibition, made some most remarkable "guesses," as some steel- 

 makers would call his conclusions, with regard to the qualities 

 and method of manufacture of many hundreds of steel samples 

 exhibited there, and of which he, in many cases, had no other 

 knowledge than that which he could gather through the aid of a 

 small pocket microscope, made of two pieces of rock-crystal, 

 formed into a very powerful single lens. A pocket microscope, 

 of this kind, ought to be the companion of every man interested 

 in steel manufacture. Lenses of the usual kind, even if piled up 

 in sets of three or four, are entirely insufficient. The lens must 

 be of a very small focus, and properly achromatic. M. Schott 

 contends that each crystal of iron is an octahedron, or rather a 

 double pyramid raised upon a flat, square base. The heights of 

 the pyramids in proportion to their bases are not the same in 

 different kinds of steel, and the pyramids become flatter and 

 flatter as the proportion of carbon decreases. Consequently, in 

 cast iron and in the crudest kinds of hard steel, the crystals 

 approach more to the cubical form from which the octahedron 

 proper is derived, and the opposite extreme, or the shaft wrought 

 iron, has its pyramids flattened down to parallel surfaces or 

 leaves, which, in the arrangement, produce what we call the 

 fibre of the iron. Between these limits, all variations of heights 

 of pyramids can be observed in the different kinds of steel in 

 which these crystals are arranged, more or less regularly and 

 uniformly, according to the quality and mode of manufacture. 

 The highest quality of steel has all its crystals in parallel po- 

 sitions, each crystal filling the interspaces formed by the angular 

 sides of its neighbors. The crystals stand with their axes in the 

 direction of the pressure or percussive force exerted upon them 

 in working, and consequently the fracture shows the side or sharp 

 corners of all the parallel crystals. In reality good steel under 

 the microscope shows large groups of fine crystals like the points 

 of needles, all arranged in the same direction, and parallel to each 

 other. If held against the light in a particular direction, each 

 point reflects the light completely, and a series of parallel 

 brilliant streaks is shown all over the surface. JSIow, the exact 

 parallelism of the pointed ends, or of the streaks of light, is one of 

 the most decisive tests for a good quality of steel, and this is not 

 visible quite so frequently as might be generally imagined. On 

 the contrary, a great majority of steel fractures show crystals 



