I 22 



NATURE 



[November 23, 19 



1 1 



IHE CASE-HARDENING OF STEEL. 

 'PHE autumn meeting of the Iron and Steel Institute 

 ^ was to have taken place in Turin, but unfortunate 

 circumstances rendered this impossible. None the less, a 

 series of important papers by Italian authors were pre- 

 sented — and taken as read— at the meeting held recently 

 in London. Most of these papers were of the nature often 

 met with at foreign meetings of this institute — i.e. records 

 of metallurgical resources and achievements of the country. 

 On this occasion, however, two Italian papers of a different 

 character were laid before the institute. These deal with 

 the case-hardening of ste il, and are both from the pen of 

 the well-known Turin metallographer Prof. F. Giollitti, 

 whose name proclaims his close relationship to the present 

 Prime Minister of Italy. 



The two papers in question are entitled " On Case- 

 hardening by Means of Compressed Gases " and on a 

 " New Industrial Process for the Case-hardening of Steel." 

 The striking fact brought out by these papers is that the 

 Italian metallographcrs who have worked on this subject 

 have evolved order out of chaos by treating the whole 

 question as one of physico-chemical equilibrium between 

 the various solids and gases present during the process. 

 It is to be regretted that Prof. Giollitti did not carry out 

 his avowed original intention of giving a detailed summary 

 of the more important researches on this subject conducted 

 by him and his collaborators at Turin, but the mere 

 bibliography of some fifteen memoirs is sufficiently 

 impressive. The results of these researches are, however, 

 summed up by the author in his present paper in a series 

 of conclusions which are very definitely laid down. If we 

 may accept Prof. Giollitti 's statement that his views are 

 fully established, the fundamental facts of case-hardening 

 are as follows : — 



(i) Solid carburising agents, without the intervention of 

 gases, have only a slight action. 



(2) The specific effect of nitrogen is very weak, and only 

 in the presence of cyanides, ferrocyanides, &c., does the 

 effect of volatile nitrogen compounds become marked. 



(3) The specific carburising effect of carbon monoxide is 

 enormously greater than that of any solid cementing agent. 

 Pure carbon monoxide carburises iron at all temperatures 

 between 700° C. and 1300° C, and it produces a greater 

 depth of carburisation in a given time than any other 

 carburising agent. Both the depth and intensity of the 

 cementation can be acpurately regulated, as they are 

 governed by equilibrium conditions, which can be definitely 

 ascertained and adhered to. 



(4) The use of carbon monoxide as the principal cement- 

 ing agent makes it possible to obtain softer and better 

 graded cementation than is obtainable by other means. 



The new industrial process which Prof. Giollitti bases 

 on his experimental results is, in effect, a specially 

 mechanically arranged furnace, which makes it possible to 

 charge and discharge a vertical muffle with the articles to 

 be case-hardened in a very short time. So soon as the 

 muffle is charged with the steel, which is introduced at a 

 red heat, the remaining free space of the muffle is filled 

 with hot granulated carbon, which, the author tells us, 

 flows into the interstices like a liquid. Then a current 

 of carbon monoxide or of dioxide is passed in at a 

 measured rate for a definite time and at a measured 

 temperature, and any desired degree of hardening can be 

 obtained. The effect of the direct contact of the steel with 

 the solid granular carbon appears to be the production of 

 a thin outer skin of very highly carburised steel, while the 

 effect of the gases gives a less highly carburised region 

 extending for some depth into the metal. In the Giollitti 

 muffle it is possible to withdraw the solid granulatfed 

 carbon at any desired stage, and to continue the cementa- 

 tion with the gas alone, merely leaving enough carbon in 

 the muffle to secure equilibrium of the gases (CO and CO,) 

 with solid carbon. Tables of analyses are given which 

 show that the result of such treatment is somewhat to 

 lower the carbon concentration of the extreme outside layer 

 and to reduce the carbon concentration gradient inwards 

 to almost any desired extent. This treatment, therefore, 

 removes the risk of cracking and shelling which arises 

 from the rapid changes in carbon content which occur in 

 articles case-hardened in the ordinary way, and more 



NO. 2195, VOL'. 88] 



especially by the use of cyanides or ferrocyanides 

 new method it is claimed that cementation of very 

 depth can be safely obtained, even with special alloy 

 which tend to become " rotten " on tlie surface whea 

 by other means. Whether the Giollitti muffle ar--* — 

 will realise all these expectations practical 

 alone can show, but there is no doubt that tlv 

 investigators have thrown a flood of new light i; 

 formerly obscure region of steel metallurgy. 



The character of this new light is perhaps more t 

 shown, so far as the scientific point of view is com; 

 in the paper by Giollitti and Carenvali on cas*^^ ' 

 in compressed gases. The work described in tli 

 based on the researches of Schenk on the equil... ...... u 



the systems consisting of Fe, C, CO, and CO, ; Fe, Fe,C, 

 CO, CO,; Fe, FeO, CO, CO,, and other systems con- 

 sisting of the mutual compounds of the elements iron, 

 carbon, and oxygen at various temperatures and pressures. 

 The experiments of 

 the present authors 

 were conducted by 

 means of a small 

 electric resistance 

 furnace placed in- 

 side a steel cylinder, 

 and partially filled 

 with both granular 

 carbon and steel 



specimens, into 

 which compressed 

 carbon dioxide was 

 fed at known rates. 

 The authors found, 

 in general, that the 

 rate of cementation 

 increased with th<^ 

 pressure employed . 

 but they also found 

 that when certain 

 pressures were ex- 

 ceeded, in spite of 

 their intimate con- 

 tact with incan- 

 descent solid carbon, 

 the surfaces of the 

 steel specimens be- 

 came thickly coated 

 with oxide, although 

 vigorous cementa- 

 tion had taken place 

 in the metal imme- 

 diately beneath the 

 oxide layer. Thus 

 they show the 

 photomicrog r a p h s 

 which are repro- 

 duced in Figs. I 

 and 2 ; in No. i we 

 see the section of 

 the region of a 

 carbon steel close to 

 the surface, which 

 was covered with a thick layer of oxide, and the higlily 

 carburised nature of the steel close to this surface is at 

 once evident. In No. 2 we see the section of an alloy steel 

 (in this case a nickel-chrome steel) which exhibits an 

 altered, highly carburised layer close to a deeply oxidised 

 surface ; the magnification in both cases is 65 diameters. 



In the case of chrome-steels such a paradoxical result 

 had already been observed by Charpy, who expressed his 

 results by stating that apparently in the action of carbon- 

 monoxide on iron-chromium alloys the two metals behaved 

 independently, the iron becoming carburised, while the 

 chromium is oxidised. The present authors show that such 

 an opinion is not justified ; the explanation of the apparent 

 paradox lies in the equilibrium conditions of the systems 

 referred to above. For every temperature and concentra- 

 tion there is a critical pressure above which oxide, as well 

 as carbide, of iron is present in equilibrium with CO, 

 CO2, and C. The addition to iron of a baser metal, such 

 as chromium or manganese, lowers this critical pressure 



Fig. 



