o90 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Metallography, etc. 



The Defects in Ingot-Iron Castings.* — K. H. "Wedding classifies 

 the defects in ingot-iron castings, usually termed steel castings, as (1) 

 blowholes, (2) shrinkage cavities, (3) gas cavities from other sources, 

 (4) surface markings, (5) cracks. The ingot-iron is generally made in 

 the open hearth furnace, or in the crucible, seldom in the Bessemer 

 converter. Blowholes are caused by the liberation of gas during solidi- 

 fication of the molten metal, all carbonised iron when fluid having the 

 property of absorbing gases. The addition of silicon or aluminium 

 prevents the formation of blowholes by causing the gases to remain 

 alloyed with iron, manganese having a contrary effect. Shrinkage 

 cavities — " pipes " — are a consequence of the contraction of iron during 

 solidification and cooling, and are usually unavoidable. Small pores 

 between the crystals, only visible by means of the Microscope, may be 

 attributed to the separation of gas, and are essentially harmless. Surface 

 markings have been attributed to segregation. Cracks are caused by 

 contraction, and their formation is influenced by chemical composition. 

 Cavities may be filled by electric welding, by pouring molten iron over 

 the defective part, by thermit treatment, or by hammering in iron at a 

 welding temperature. The results of filling by these methods are fre- 

 quently not satisfactory. 



Notes on the Etching of Steel Sections.! — W. C. Smeaton 

 distinguishes the processes by which the micro-constituents may be 

 differentiated on the polished surface of a metal, as (1) heat-tinting, 

 (2) electro-deposition, (3) polishing in bas-relief, (4) use of solvent 

 etches. The last method is the only one fully dealt with by the author. 

 The nature of the polished surface affects the etch : crystals of the same 

 constituent may be differently coloured by a reagent owing to the plane 

 surface of the section cutting them in different relations to their crystallo- 

 graphic axes. Solid solutions are attacked most rapidly, a-, f3-, and y- 

 iron are attacked at different rates by the same reagent. Beilby has 

 shown that surface flow on metals, caused by the mechanical work involved 

 in polishing, results in the formation of a surface film, differing from 

 the mass of the metal. This film must be removed by the etching 

 agent in order to develop the true micro-structure. Carborundum and 

 wet rouge used as polishing agents on surfaces at high speeds, lead to the 

 formation of pronounced films. Alumina is not so liable to cause films. 

 Surface flow may be diminished by care in polishing. 2 p.c. sulphuric 

 acid, acting at 60° C. for 2 minutes, removes films, producing only a 

 very light etch. The etching action is approximately proportional to 

 the degree of electrolytic dissociation of the active etching agent in the 

 case of water solutions of nitric acid, ammonium nitrate, etc. The 

 author adds an indifferent substance with a common ion, e.g. potassium 

 or sodium nitrate with nitric acid, to alter the electrolytic dissociation. 

 Solutions of potassium and sodium salts are without noticeable etching 

 action. Ammonium salts have an etching action ; concentrated solutions, 



* Iron and Steel Mag., ix. (1905) pp. 209-21. 

 t Tom. cit., pp. 222-30 (1 fig.). 



