Photomicrography of Iron and Steel. By W. H. Merrett. 9 



liquorice or a solution of nitric acid. Cementite is very hard, and 

 stands in relief when the steel is polished on the finest rouge on wet 

 cloth. A good specimen of cementite is shown in fig. magnified 

 850 diameters. Free cementite, however, does not often occur in low 

 carbon steel, but usually assumes the form of " pearlite," which is an 

 intimate mixture of cementite and ferrite arranged in laminae which 

 are alternately hard and soft. These laminae are very minute, and it 

 is necessary to use a magnification of at least 300 diameters for their 

 identification. The laminae of pearlite often assume a more or less 

 granular form. Penrlite is so called on account of its resemblance to 

 mother-of-pearl. When pearlite is attacked with either an infusion 

 of liquorice or a solution of nitric acid, a voltaic action is set up which 

 causes the ferrite to become dark in colour. A pure steel containing 

 0*9 per cent, of carbon will consist entirely of pearlite ; if the carbon 

 be less than this amount, the mass will be composed of pearlite and 

 ferrite. If the carbon exceed • 9 per cent., it will consist of pearlite 

 and cementite. Fig. 5 shows a photomicrograph of a steel containing 

 0*6 per cent, of carbon. It consists of ferrite (the white portion) and 

 pearlite under a magnification of 850 diameters. Fig. 6 represents a 

 steel containing about 1 ■ 8 per cent, carbon, and is composed of cemen- 

 tite (the white raised portion) and pearlite. 



Cementite may be distinguished from ferrite by its greater hard- 

 ness. It will be seen that the cementite in figs. 6 and 7 appears to 

 stand in relief. Ferrite is easily scratched by an ordinary sewing 

 needle, while cementite is not. 



Cast iron contains more carbon than steel, the amount varying 

 from 2 to 5 per cent. It practically consists of three varieties, white, 

 mottled, and grey. The first variety is composed of pearlite and 

 cementite ; the second of pearlite, cementite, and a little graphite ; and 

 the third of pearlite together with either cementite or ferrite and 

 graphite. In both cast iron and steel it seldom happens that free 

 cementite and ferrite exist in the same specimen. 



Fig. 7 is a sample of white iron containing 2*2 per cent, of 

 carbon. Fig. 8 is a mottled iron containing 3 ■ 8 per cent, of carbon, 

 of which 2*6 per cent, is graphite. Fig. 9 (plate II.) is a grey iron 

 containing about 4'6 per cent, of carbon. As it has been before 

 stated, steel differs from cast iron by being capable of acquiring 



EXPLANATION OF PLATE II. 

 All figs, x SoO diameters. 



Fig. 9. — Grey cast iron containing 4*0 p.c. of carbon (pearlite, cementite, aiui 

 graphite). 

 „ 10. — Quenched steel (martensite). 



„ 11. — Steel which lias been quenched and tempered to straw colour. 

 „ 12. — Troostite (the black) in a matrix of ferrite and martensite. 

 „ 13. — Aubtenite (the white) and a variety of martensite. 



