"Spot Hardening."
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A Remarkable but Simple Method Adopted by Vickers, Ltd., which Likely to Supersede Much Present-day Case-hardening Practice.
The scientific discussion of any new heat treatment of steel undoubtedly introduces a subject which is of too highly technical a nature to justify its presentation to the reader who is no more expert than the average user or owner of commercial motors. Nevertheless, an occasional diversion to the fields of -mechanical science in this way is not inappropriate for our columns, because in effect not only is it a. fact that a considerable proportion of users, from sheer necessity, is rapidly acquiring a more intimate mechanical knowledge, but there is also always a large body of properly-qualified men whose interests are intrinsically bound up in the industry with which we are specifically concerned.
No further preamble, therefore, is necessary in the present case : we intend briefly to set before our readers particulars of a most interesting method of surface hardening of steel, as distinct from bulk casehardening, which has been perfected, and is now being employed commercially by the famous steel firm of Vickers, Ltd.
Armour-plate manufacture, as well as the production of special forms of steel which have been demanded by the particularly-difficult requirements of the automobile industry, have between them focussed a great deal of attention upon the most modern methods of securing the hardest possible skin on the surfaces of steel components.
Case-hardening Where Not Wanted.
It is almost unnecessary for us to remind our readers that the hardening processes to which certain gearwheel parts and similar pieces have to be subjected are not unlikely to result in distortion of the parts to a serious extent. It has always been a matter of some difficulty so to design certain components in such a way that it shall only be necessary to harden those areas of which the ultimate duty will demand surface treatment of this kind. Very often, owing to the necessities of economical machining and pro duction, it has been impossible to avoid the automatic hardening of areas for which in reality no such treatment is necessary or indeed desirable. With Any High-tensile Steel.
The Vickers patent surface-hardening process (Patent No. 5588/1910) provides an effective and indeed a very simple method of procuring local hardening, or, as we have heard it described with considerable aptitude, " spot hardening" of steel surfaces. The method enables the operator to effect the necessary hardening modification to the surface of the steel at any exact spot or other defined area, of a piece which he may desire. This is practicable with any high-tensile steel forging or casting of any size, and without distortion and without destroying the effects of the usual previous heat treatment to which the steel of which they are composed may have been subjected. Any article may be completely finish-machined throughout before hardening, no straightening or similar correction, and no grinding being necessary after hardening by the Vickers process.
It will be readily appreciated that the larger the article the greater is the value of the process. Examples which have been cited to us, in which the new method is of inestimable value, arc: turbine helical gearing ; traction and haulage gears ; gun-mounting forgings ; railway crossings ; chain sprockets, etc. ; and, as a matter of fact, in the majority of the above-mentioned cases, the Vickers system is the only really practical method of effecting the desired result.
Rapid Heating and Equally Rapid Cooling.
The essential characteristic of the process is the speed of the heating. The heating flame is used not in the ordinary way with which we are familiar, but more, if we may adopt the analogy, as if it were a paint brush. In the case of a gearwheel, for instance, the tooth is not heated up as a whole and then quenched, but the surface is traversed by the flame. The intense heat, and the rapidity with which this is transferred to the surface of the steel instantly raises the surface to a hardening temperature. As the flame passes along this surface, the latter is instantly chilled by the cold remainder of the tooth, so that the part which has been treated is of the maximum hardness of which the steel of which it consists is capable after heating and quenching in cold water.
The Flame Drives the Water Away Locally.
The actual process is a simple one. The equipment consists of the usual outfit supplied for oxy-acetylene welding purposes, with the customary burners. The work which is to be hardened locally is placed in a tank of water which has an adjustable overflow. The latter is necessary, in order that the water level can be regulated, as, while the portion of the surface which is being hardened is instantly heated to the required temperature, the whole of the remainder of the article is kept as cool as possible by being immersed in water. Should this be quite impossible, the part which is being hardened must be kept cool by water flowing over the surface.
Relative Positions of Flame and Water.
The burner is held so that the outer portions of the flame flow in the direction along which i he burner is travelling, and the portion of steel heated should be cooled, when intense hardness is required, by arranging that cooling water shall follow as closely as possible the moving burner, but this, of course, must not be so close as actually to disturb the flame or to run the risk of putting it out. Also, undue proximity of the oncoming covering film of water may just pre vent the necessarily-rapid heating and so cause irregular hardening. The cost for hardening with this method is estimated approximately at one farthing i per sq. n Highest Possible Temperature of Flame.
The flame required must. be of the highest possible temperature, and must burn close to and even under the water. This is obtained by adjusting the flame as for welding, and then increasing the pressure of the oxygen, so that the flame changes colour and burns a very white cone to a bluish streaky tongue when observed through smoked glasses.
Special Vickers Steels if Desired.
We understand that Vickers, Ltd., is supplying special steels which are particularly amenable to this treatment and show the very finest of results, although, as we have said, the process is not limited to such special material. Cast-iron and malleableiron castings of certain kinds, we understand, can be given a glass-hard surface by the Vickers spothardening method. It is claimed that the process will be found to be exceptionally useful for such articles as worm wheels, chain sprockets. and accurately-cast helical gears made from malleable-iron castings with this special chilled surface.
Can Harden three-sixteenths of an inch.
The normal depth of hardening is about one-sixteenth of an inch, but a greater depth up to an eighth or three-sixteenth inch can be obtained by slightly prolonging the heating, a small wavy or rotary movement being given to the flame to avoid burning the surface of the steel. The degree of surface hardness can be very easily varied on the same article to suit any requirements, and this can be done either by drawing the flame very rapidly again across the surface, so drawing the temper, or by arranging that the cooling is less rapid in the hardening. For instance, for sliding gears, such as those which are used in change-speed assemblies, it is usual to leave the surfaces of the teeth glass hard, but only to toughen the rounded ends of the teeth, which otherwise are liable to chip when left hard by the more ordinary methods of case hardening.
Case Hardening versus Spot Hardening.
It may be useful to compare briefly the relative advantages of case hardening and this new method of surface hardening. Case hardening would appear to be more suitable for the treatment of small articles that are required in quantities and where distortion is of no moment, or where grinding after hardening can readily correct it. Again, case hardening is probably the better method in the majority of cases where large articles require to have the major proportion of their exposed surface hardened. It would appear, however, that the new spot-hardening process offers special advantages where larger articles are being treated, upon which only a small proportion of the surface requires to be hardened. Spot hardening, again, is certain to be useful in the case of the treatment. of all articles where distortion in hardening is a, serious objection, and where it is difficult to correct by subsequent grinding. For such examples as the links of silent chains, where the metal round the holes has to be hardened, for the boles in templets, and work of that class, that is to say; small articles requiring a very small portion of hardening, this Vickers system has undoubted merits. In regard to the material which is most suitable for hardening purposes, it has to be admitted that difficulties not infrequently arise when it is desired to case-harden steel which is not originally comparatively mild. High-tensile steel has not been treated with uniform success. But with this new method, any high tensile steel can be used with impunity. It can receive its correct heat treatment, and after being finish-machined, it can be surface hardened in exactly the right place. In certain cases, a combination of casing as for case hardening, arid finally of surface hardening, may be advantageous. Contrary to what might have been expected even with the hard steels, no tendency has been detected for the hardened surface to crack or flake away from the unhardened portion. In a num ber of fractures made for examination not a single instance has shown any parting between the hardened and unhardened portions. Case hardening is cheaper for the classes of work we have suggested above, but for larger articles, requiring only small portions of their surfaces hardening, the new process is very much cheaper and very much quicker. The method we have described is not entirely new in practice, experimental operations having been conducted for some considerable while past. It is, however, known to very few and has only just reached the practical stage of exploitation.