HARDENING AND DISTORTION.
Page 19
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The Relative Advantage of Hardening by Treating a Small Area at a Time.
IN CONNECTION WITH most engineering products there is an increasing tendency to adopt case-hardened material and, as the particular parts which appear to require hardening cannot really be said to be very highly stressed, the only object
to be gained in many cases is a glass-hard surface which will resist wear and avoid the necessity for frequent renewals. The making of these parts in this manner involve-s additional expense and, although the extra outlay is justified, the best results are frequently missed when
the question of distortion has not received sufficient attention and by the time the moving parts have worn themselves down to approximate concentricity the comparatively thin hard envelope has been worn away.
When the articles have been in a furnace for probably five hours at. 900 degrees C., afterwards being
quenched at about 860 degrees (1, and finally reheated and quenched at about 770 degrees C., the stresses which existed in the original forgings or bars become relieved, and this results in alteration in shape.
Having in view economy and uniformity of results, probably the most suitable steel to use in the cases suggested above is as follows :—
Iron, 99.2 per cent. ; phosphorus, 0.05 per cent. ; sulphur, 0.05 per cent. ; silicon, 0.02 per cent. ; manganese, 0.50 per cent. ; carbon, 0.18 per cent.
The above chemical analysis must not be taken as being absolutely necessary or in any way final, but the main point to watch is the percentage of phos phorous and sulphur. Sulphur should not be present in a greater degree than stated or the material will be subject to "red-shortness," or in other words will be brittle at red heat. Phosphorus in a greater degree than stated would produce "cold-shortness,'! that is, the material would be brittle when cold.
Material with either fault is-not to be relied upon and, if the above chemical analysis be followed ap
proximately, the results of the hardening process will be most satisfactory. There is not much danger of an excess of phosphorus in the case of Bes semer-acid or Siemens-acid steels, but, as the ma terials in these cases must be pure and somewhat costly, the basic steels are frequently more available. . The latter can be made from materials which are cheaper and more easily procured, but care must be
taken to ensure that the phosphorus contained is•reduced to alAnt 0.05 per cent. during the conversion In steel. Steel of similar analysis can, of course, be made by both basic and acid processes.
Having determined on a make of steel to correspond with the above data, it is advisable to carry out a few experiments in hardening—paying particu lar attention to the results obtained by only quenching once. For parts which are not particularly highly stressed, this method will give the required glass hard surface, but the core is not very refined. As most moving parts in general engineering work are really designed for wear rather than strength, the additional quenching operation may in Many cases be eliminated, as this only adds to the trouble of distortion, although the greater portion of this distortion takes place in the carburizing operation. This fault of distortion is a very serious matter, and in many cases necessitates the parts being machined slightly larger than is required, and then finally corrected, after hardening, by grinding. In order to reduce this distortion to a. minimum, in many cases it will be found advisable to machine the article only on the places where a hard surface is required, leaving as much bulk of metal as possible to keep the part rigid. After the carburizing process, the places w_hiCh were left thus must have the carbon machined out befere quenching and these surfaces will then be soft enough to machine to their final dimensions when the hardening is complete.
In camshafts we frequently have three or four bearings which all must be hard and perfectly concentric. Sufficient grinding allowance must be left to ensure concentricity and the shaft must be most carefully straightened after carburizing and must be tested again after quenching.
Mast heavy petrol engine camshafts would be thoroughly satisfactory if quenched once as they are generally of ample proportions and a hardener accustomed to one quenching can be more reliable with regard to uniformity of hardness. Distortion in the case of a sliding castellated gear is also a source of serious trouble and when the hardness is obtained by the cementation process it is most difficult to devise a satisfactory method of minimizing it. The castellated hole 'cannotbe left soft and finished after hardening as the chances of getting the bearing surfaces in the hole concentric with the pitch line of the gear are most remote, and in addition it is advisable to have the castellations hard. If the whole gear be finished, the teeth being cut on a splined mandrel and hardened all over, then it is only convenient to grind the inside of the hole. This appears to be most suitable in case-hardened gears, but, to make a good job, it is necessary .to have the bottom of the splines in the shaft ground to fit the hole in the gear.
As the trouble of -distortion is caused almost en
tirely in the cementation process, the advantages of a system of hardening without this process are obvious, and such a process has been brought to an advanced stage by Messrs. Vickers. Steel, cast iron and malleable iron can readily be hardened by their method, which only heats a small area at one krue, and that is immediately quenched. Thus -the process does not give rise to excessive internal stresses and the conse. quent distortion.
The article to be hardened is placed under water,
and the particular area required to be hardened is brought very close to the surface The area is then heated by oxy-acetylene blowpipe, the force of the jet blowing the water away from the point attacked by the flame. The flame is moved to another area, and the water at once quenches the portion first attacked. There is a danger of having hard and soft streaks alternately,as the whole area cannot be covered entirely with the same effect.
The system is exceptionally handy in many cases, however, and fully justifies its use.
Crown wheels can be hardened on the teeth only, and as the remainder of the gear is under water, one tooth being done at a time, distortion is avoided. Castellated holes can be surface-hardened, and as the holes retain their original shape anddimensions the trouble of fitting afterwards is reduced to a minimum. Sliding gears can have the teeth hardened on the working portion and left soft where profiled for sliding into mesh. This adds to the life of the gear, as the slamming into mesh of a case-hardened gear causes the surface to crack and chip.
Large forging.s or castings can be hardened on small areas only, and this is sometimes better than having to harden the whole article. Jigs can be made of malleable iron, and have the holes hardened locally instead of fitting bushes, and occasionally hardening can be carried out whilst the article is still in its place, providing the water can be supplied to it for
quenching. HOWARD ENSAW.