Special Steels or Motor Vehicles?
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The test we have already given of nickel-chrome steel is somewhat remarkable on account of its high percentage it elongation in proportion to the tensile strength of the material; but, as we have previously pointed out, in judging the merits of a steel the three principal factors of the tensile test, viz., the maximum stress, elastic limit, and elongation, must all be considered in relation to each other; and it, therefore, does not necessarily follow that the steel giving the highest tensile strength is the best to use, even putting on one side the question of machining. After seeing the breaking load, look at the elongation. Neither, by the way, if the stretch is poor in proportion to the tensile, should the quality of the steel be condemned; this may be merely a question of heat treatment. The tests we give here will show our meaning clearly, although it is perhaps going to the extreme; but so much the better for our purpose.
In explanation, the tests were both made from the same bar of nickel-chrome steel, rolled tin. diameter.
Obviously, the steel as rolled, in this instance, was unsafe for use, but, heat treated, Lhe same material is, equally obviously, a very good all-round steel. As we have said earlier, all steels are, in a greater or lesser degree, affected by very slight variations of heat, but none are more susceptible than those containing nickel. And it is well, for reasons not difficult to comprehend, that this should be understood. The steel manufacturer, after finishing the hammering on a nickel-steel forging, is most careful to remove any " forging strains " that may have been set up unequal heating during the process, by some method of treatment. The methods may differ somewhat in detail, but the essential part is to re-heat the forging till it obtains, as nearly as practicable, an even temperature over every part ; it is then cooled down slowly. In the works fitted with the necessary plant, this heating is regulated by means of a pyrometer; although this is not an absolute necessity, still it makes for that regularity, which is a desideratum. In order further to illustrate the susceptibility to heat treatment of steels containing nickel, and to introduce another point, we give a further series of tests, all made from precisely the same steel (nickel-chrome), in fact, from the same ingot, although the test-pieces were submitted to a slightly different heat treatment.
We think this series will show the importance of each test being considered in its entirety, and, at the same time, help to convince the motor engineer of the necessity of allowing a little latitude in the results of tests from steel supplied to his specification. The treatment of No. 2 and No. 3 was, for all practical purposes, the same, but, as will be seen, there is considerable difference in the result, particularly in the elongation ; the cold bend of No. 3 test, however, proves that even the reduced percentage of elongation is perfectly safe; and, indeed, it would be difficult to say which of the three tests, for purposes of motor-vehicle construction, is the best. The tests will also serve to show how extremely essential it is that a forging made from these special steels shall be heat-treated after the hammering, as the same variation may exist in different parts of a forging due to one portion taking, probably, more heats to " get down " than the remainder of the piece.
* The preceding articles aireared in our isLues of the 28th February and the '7th inst.
Coming under the heading of heat-treatment is the proems of annealing. This is a most important operation and one demanding great care, if the best results are to be obtained; if the annealing is only necessary for the purpose id facilitating the machining, it may, however, be done satisfactorily by several quick, if rather crude, methods. It is, also, well to bear in mind that, as the process differs slightly, there. is, also, some difference in the results, and not a little trouble has been caused from time to time—this remark is not confined exclusively to the motor industry— by a misunderstanding as to how many tons the annealing will reduce the tensile strength of the material. This varies with the temper, i.e., carbon content, and composition of the steel, and also very largely with the process of annealing employed. For instance, crucible tool steel of a high carbon (say, a drill temper about noo per cent. carbon) which, in the rolled bar, will give from 6o to 65 tons tensile, will, after a thorough annealing, give only 35 to 40 tons. Again, in some works, the method of annealing a medium-carbon, Siemens-steel forging would only reduce the tonnage by from 2 to 4 tons, whilst the process, in other works, would reduce the same temper as much as to tons. Consequently:, if the finished article is required to give a certain test, after annealing, it is very necessary for the steel maker to be cognisant, to some extent, of the method to be employed, so that he may supply a material of a suitable temper. As we have explained, it is not possible to work within such fine limits in dealing with some alloy steels, but the following tests may be of sonic help, and act as a guide, at least.
Annealing by ordinary methods appears to have very little effect on the high percentage nickel steels. The annealing of the following test was done by precisely the same method as employed in the above tests.
We refrain here from going into the details of annealing, on precisely the same ground as mentioned above; different manufacturers have their own methods of conducting this operation, and are very willing to give their customers advice as to the proper manner of treating the steel they supply. Under these circumstances, we think it better to leave the question of annealing, now that we have touched upon it in a general manner, and call attention to a few points which should not be overlooked. For the same reason, we do not propose to deal in detail with the question of case-hardening.
Many of the motor engineers have their method of " casing," and prefer that steel should be supplied to suit own particular method rather than modify their process to suit the steel. There is some rcee.on for this as, in many instances, a process is finally adopted after numerous tests, and, naturally, there is a tendency " to leave well alone." There is no doubt, however, that, in some quarters, this process has not had the attention it certainly merits, and the results have been disastrous. Some of the greatest troubles have arisen from no precautions being taken to ensure regularity in the " casing " or " potting," with the result that the depth varies; if too deep, it may easily be detected, but, if not sufficiently deep, it may only be found out by the actual wear. The object is, of course, to get the greatest possible surface hardness combined with great toughness of the core, and one of the quickest and most conclusive tests is a tensile.
The appearance of a broken test-piece is rather peculiar, as will be seen from the accompanying photograph (Fig. 1). This is a very fair sample of case-hardening. Owing to the troubles that have been experienced, there is a general desire -LI obtain a steel that may be used without casing, but, as yet, no very great success has been met with. Most experiments have been made on nickel-chrome steel, for gears, but, obviously, steel having sufficient hardness to be at all suitable presents very great practicable difficulties in machining and cutting the gears. This steel may be used in a moderately—comparatively—soft state because, as in all steels, it acquires a certain surface hardness by the friction occasioned by the gears running in mesh. There is no doubt, however, that at present, a carbon steel for case-hardening is most satisfactory; the chief need is to Fe sure of the steel and to take every precaution to obtain regular results.
In these articles, it has only been our idea to touch upon several points connected with metallurgical sides of the steel question ; to go into all details would need much more space than we have at our present disposal, but we shall be very pleased to answer in our columns, when possible, questions dealing with steel for motor vehicle construction in which any of our readers are interested.