Too much attention cannot be paid to the selection of a suitable steel for casehardening work. The requirements are, probably, more stringent than for any other class of steel.

For most casehardening work a low carbon steel, of low carbon content, is the most suitable and economical material. It is essential that the steel should be free from heavy slag inclusions and segregates of any description. The presence of dissolved oxides is highly detrimental in a steel for casehardening, and gives rise to soft spots and instability of the hardened case, that is, the case very easily loses its hardness under grinding stresses or moderate heat. Dissolved oxides are the result of improper methods of steel manufacture and are difficult of detection. Thus the steel must be made with great care and the ingots well cropped to eliminate pipe.

The acid open-hearth process probably produces the most suitable steel for this work, although basic open-hearth steel, of low sulphur and phosphorus contents may equal it in quality if skilfully made. The old criterion of a steel for case-hardening purposes was that it should exhibit a fibrous fracture in the core after heat treatment. The selection of steel on such a basis is likely to lead to serious trouble unless consideration is given to the fundamentals of the process.

The Cause And Effect Of Fibre In Steel

Pronounced " fibre " in a steel is due to the presence of either elongated slag streaks or segregates. The best example of a fibrous fracture is exhibited by wrought iron, which, due to the process of manufacture, contains large quantities of slag, usually between 1 and 3 per cent. There is no doubt that a fibrous steel or wrought iron is often tougher than a purer steel when the stresses are at right angles to the direction of rolling, but it is considerably weaker when the stresses are parallel to the direction of rolling. Further" a steel which shows a fibrous fracture, indicating the presence of slag, gives a much less efficient and uniform case than one which is purer. The slag lines do not, of course, carburise and the final case consists of hard, high carbon material, with soft, weak slag lines running through it. These slag lines are potential cleavage planes and often cause the case to chip or crack, due to the fact that under heavy loads the case collapses about these soft streaks.

The Selection Of Carburising Material

The correct selection of carburising materials is of fundamental importance, since different compounds vary enormously in their action.

Carburising materials consist of carbonaceous matter in various forms, such as charcoal, bonemeal, leather, petroleum, coke and gas coke. One or more of these are mixed with an " energiser." An energiser is to a carburising compound very much what a catalyst is to a chemical reaction ; that is, in a manner as yet imperfectly understood, it materially hastens the process of carburisation. Many substances possess this property in a greater or less degree, notably the carbonates of the alkaline metals. The proportion of energiser present must be strictly regulated, since too large an amount gives an undesirable type of case, whilst too little gives a slow acting -and thus uneconomical-compound.

Naturally, the cheaper forms of carbon, such as coke, enter very largely into the composition of low-grade compounds, and frequently these are loaded with cheap heavy substances which are of no possible assistance to the carburising process. Very frequently, also, compounds are met with containing extremely deleterious energisers, such as cyanides and ferro-cyanides, which not only render the compounds very poisonous, but impart a case which does not meet even the least stringent requirements.

Essentials Of A Carburiser

The ideal carburiser should give :-

(1) ;High rate of penetration.

(2) ;Case containing not over 0-90 per cent, carbon.

(3) ;Maximum thermal conductivity.

(4) ;Uniform penetration.

(5) ;Good mechanical support to the work during carburisation ; And should be free from :-

(6) ;Clagging or sticking.

(7) ;Blistering.

(8) ;Any tendency towards a sharp line of demarcation between case and core.

(9) Shrinkage or deterioration in use.

Naturally, all these characteristics are not attainable at the same time. For instance, a compound which gives the maximum thermal conductivity, that is, is open and porous in texture, will be most likelv to shrink in carburisation and will not afford the necessary mechanical support to the work to prevent sagging and other distortion.

Common Faults In Carburisers

Perhaps the most common fault with commercial carburisers is that a rapid rate of penetration is obtained at the expense of a case which is too high in carbon at the exterior. Such compounds, also generally give a sharp line of demarcation ; a defect, the seriousness of which is often insufficiently realised. The cause of this defect lies in the composition of the gases evolved by the compound at the temperature of carburisation. Appreciable percentage of certain hydro carbon gases, for instance, ethylene, give rise to this type of case. Shelling off of the case under working stresses is nearly always due to this defect.