November 17, 1923] 



NATURE 



729 



-IVculiar segregation of cementite at crystal 

 boundaries, x 50. 



very short and it is difficult to ascertain their orienta- 

 tion with exactitude. The disturbance occasioned in 

 the distribution of the structural elements which 

 separate the grain junctions by the simultaneous in- 

 fluence of the varying orientations of each grain is 

 thus m.anifest. Portevin remarks that some observers 

 will not fail to interpret them as arising from the 

 intervention of " amorphous material/' whereas they 



can easily be 

 interpreted as 

 the resultant 

 of two forces 

 acting in dif- 

 ferent direc- 

 tions. He has 

 observed a 

 similar in- 

 stance in the 

 case of alumi- 

 nium bronze 

 containing 90 

 per cent, of 

 copper which 

 has been hard- 

 ened and an- 

 nealed. Here 

 the intergranu- 

 lar elements of a separated along the grain joints have 

 a different orientation from that of the acicular intra- 

 granular element. 



Cementite, as is well known, is exceedingly sensitive 

 I to coalescence phenomena. The author has stimulated 

 the coalescence of the pro-eutectoid 

 cementite of the sample by heating ^^I'^'-^S^ 

 it for 1-5 hours at 950*^ C. followed ^ - - :^ 

 by oil-quenching, and then by one '^^^JJj^- 



hour's annealing at 600° C. This gives " "^^^C^^^ 



darkly-etching sorbite in which the ^' :J^ 



undissolved cementite appears white 

 and is very clearly distinguishable. ; 



Under these conditions the coales- 

 cence of the cementite prismoids is 

 shown by a rounding of the boundaries 

 and the splitting up of the elements 

 constituting the bundles, but in addi- 

 tion an agglomeration is observed 

 which gives the cementite a pitted 

 appearance and is misleadingly like v 



the eutectic of white pig-iron. (See • . -ft 

 Fig. 2.) This pseudo-eutectic ap- ..^^ ' "^ 

 pearance, due to coalescence, appears - ^.- 

 to be a new observation and shows 

 the intensity of the influence of sur- 

 face tension on cementite at the above 

 temperatures. The tension is, in this 

 instance, an important morphological 

 factor. 



Another unusual type of occurrence of cementite in 

 steel was described at the same meeting by Prof. 

 Edwards and Mr. Pfeil. In this case, however, the 

 phenomenon was observed in mild steel sheets, i.e. in 

 hypo-eutectoid steels. Defects are sometimes encoun- 

 tered in such sheets when subjected to moderately deep 

 stamping operations and consist of a series of corruga- 

 tions in the side walls of the dish. The degree of 



NO. 2820, VOL. I 12] 



corrugation increases on passing from the bottom to 

 the top, and is, in all probability, due to the greater 

 amount of cold work put upon the metal there. It 

 was found that the microstructure of the steel consisted 

 of two approximately equal parts : (a) a very coarsely 

 crystalline layer apparently free from carbon,.and {b) a 

 finely crystalline layer in which no pearlite was present 

 but the car- 

 bide was segre- 

 gated at the 

 crystal junc- 

 tions in irregu- 

 lar nodules. A 

 section cut 

 from the corru- 

 gated part of 

 the dish show- 

 ed very coarse 

 severely de- 

 formed crys 

 tals. Running 

 round the crys- 

 tal bound- 

 aries, however, 

 was an almost 

 continuous net- 

 work of what may be termed " beaded " cementite. 

 This constituent must have segregated from pearlite 

 and coalesced into this form under the influence of 

 surface tension during the annealing. Its appear- 

 ance is shown in Fig. 3 at a magnification of 250 

 diameters. So far as the writer is aware, this type of 



i 



Fig. 2.— Eutectic-like appearance of cementite. X500. 



1.^-n^^r-r^- -..^^^^Asr , 



Kk;. ). — Hrailed ccnientiic ai crystal i)imiuiaries o) Icrrite. x 250. 



occurrence of cementite in a mild steel has not been 

 previously described. The authors have not proposed 

 any explanation of how it is brought about, but are 

 endeavouring to produce it intentionally. Clearly 

 much work still remains to be done to explain 

 the various forms of cementite which may and do 

 occur in both hypo- and hypcr-eutectoid steels. 



H. C. H. C. 



