334 



NATURE 



[June 27, 1918 



Sir Henry Newbolt, who followed, likewise em- 

 phasised the vital importance of true education, both 

 of the mind and of character, to the future of this 

 country. It might be true that science had not yet 

 received the centuries of devoted effort bestowed on 

 the humanistic studies, and was in process of develop- 

 ment. But ultimately the impression that there was 

 any antagonism between humanistic and scientific 

 study would disappear. In both cases there was a 

 search after truth and a similarity in general aims, 

 and both demanded gifts in the teacher not only of 

 intellect, but also of character. 



A vote of thanks to the Lord Mayor and speakers 

 was moved by Sir William Beale, and this terminated 

 the proceedings. 



THE IMPORTANCE OF THE NON- 

 METALLIC INCLUSIONS IN STEEL. 



TT is impossible to manufacture steel which does 

 ■^ not contain non-metallic inclusions to a greater 

 or less extent. These have an important effect on 

 its properties, particularly in producing defects and 

 causing failures to a degree which is not sufficiently 

 realised. Mr. A. McCance, who presented a most 

 able study of this subject at the May meeting of the 

 Iron and Steel Institute, states that much defective 

 steel is bad solely because of the number of non- 

 metallic particles which it contains, and that fully 

 90 per cent, of the failures due to faulty material 

 which have come under his notice are traceable to 

 this cause alone. He states further that when 

 material has cracked under a stress which experience 

 shows it should safely have carried^ it is advisable to 

 examine the crack along its whole' length, and when 

 this is done, in many cases it will be found that the 

 crack passes through groups of inclusions, while in 

 cases in which it can be traced to its origin it is not 

 unusual to find that it has started from a segregation 

 of non-metallic particles. He treated a piece of steel 

 in such a way as to produce slight intercrvstalline 

 brittleness, and then stressed it above the elastic limit. 

 A number of small cracks appeared, and in nearly 

 every case they started from one or more non-metallic 

 inclusions. 



He next heat-treated a heavy slab known to con- 

 tain inclusions, and carried out tensile tests on pieces 

 machined along the length in the direction of rolling, 

 and also at right angles to this direction through the 

 thickness of the slab. The length-test was in the 

 same plane as the centre portion of the thickness-test, 

 so that they were in every way comparable. The 

 results obtained were as follows : — 



Elastic Ultimate „ 



** . rnnfr.qrfinn 



on 2 in. 



cnntraction 



Tons per Tons per 

 sq. in. sq. in. 



Length (A) ... 24 432 270 658 



Thickness (B) ... 18 345 4-0 16-8 



These remarkable differences in properties, par- 

 ticularly as regards the ductibility of the steel, are 

 due solely to the presence of the non-metallic in- 

 clusions, which in the fractured tensile surfaces of (B) 

 appear as thin circular discs. McCance goes on to 

 point out that these have acted as small areas of 

 zero strength which have lowered the effective area 

 of the test piece, though this is not the only effect 

 they have, and he considers the distribution of stress 

 in such a composite material. It has been proved 

 experimentally by two different methods that the stress 

 at the edge of a circular hole is thre*; times that of the 

 average. In the case of inclusions the elastic pro- 

 NO. 2539, VOL. lOl] * 



perties of which differ from those of the isurroiinding 

 steel, the differences in stress at the edges will not 

 be so great as for holes, but the edge-stress will still 

 be greater than the average. " In steel, therefore, 

 which possesses even slight brittleness the presence of 

 inclusions may give rise to cracks when such material 

 is stressed, though in steel which has received proper 

 thermal treatment during rolling, forging, etc., in- 

 clusions, so long as they are evenly distributed and 

 small, will have an effect which is quite negligible. 

 It is only when they begin to form groups that they 

 have a detrimental effect, and this power which they 

 have to segregate is, unfortunately, without control 

 in the existing state of our knowledge, so that the 

 only way to minimise the chance of segregation is to 

 lessen the number of inclusions present." 



In his paper McCance considers the method of 

 occurrence and composition of the various non-metallic 

 inclusions and how they are formed. According to 

 him, there is no evidence that anv of them are soluble 

 in molten steel. In other wordlf they exist as sus- 

 pensions, and therefore do not obey the laws 

 governing the segregation of elements soluble in 

 liquid steel. Being lighter, they tend to rise to the 

 surface. Assuming, as he does, that the particles 

 exist as spherical globules (density = 4), and that the 

 viscosity of liquid steel is about the same as that of 

 mercury, he calculates their velocity of rising (undis- 

 turbed) as follows : — 



lo-o X 10 

 10 

 01 



of r articles 



cm. 



Vdociiy of rising 



80 cm. per minute. 

 08 

 0008 ,, 



Taking for purposes of illustration an ingot of 

 140-cm. length, which set in twenty minutes from the 

 time the mould was filled, and ignoring convection 

 currents, he calculates the percentage of the number 

 of particles of each size which would be entrapped in 

 the solid metal thus : — 



Diameter of particles 



All over 30 x io~'' 

 „ „ 20 „ 

 „ „ i-o „ 

 ,, under 05 ,, 



apped 



Convection currents play an important, though un- 

 controllable, part in determining both the position and 

 size of the inclusions in every steel ingot. Inasmuch 

 as the viscosity of the steel diminishes as the tem- 

 perature rises, the metal should be cast as high above 

 that of the liquidus as is practicable. 



The greater part of the paper contains a detailed 

 study of the identification and mode of occurrence of 

 the inclusions commonly met with, e.g. manganese 

 sulphide and its oxidation products, manganese sili- 

 cates, iron oxide scales and silicates, acid open-hearth 

 slags and their reduction products, fluxed refractory 

 materials, and oxide inclusions. Iron sulphide, which 

 is scarcely ever encountered, and the action of 

 aluminium on the sulphides of iron and manganese 

 are also dealt with. By means of various etching re- 

 agents any inclusion can be classified as a sulphide, 

 a silicate, or an oxide, though research is required for 

 the working out of more suitable reagents than at 

 present exist. 



In his final section the author discusses the equili- 

 j Ijrium conditions in liquid steel. He considers that 

 i ferrous oxide plays a most important rdle ' in deter- 

 j mining the origin and occurrence of inclusiors, and 

 that all the evidence favours the view that this sub- 

 ! stance is present in the liquid. The addition of man- 

 j ganese in the form of ferro-manganese causes the re- 



