Vital Steel Research Work
Page 49
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Efforts to Control the Grain Size of Carbon Steels, to Improve Annealing, Resistance and Ductility, as well as in Other Important Directions
SERS of steels do not always appre ciate the enormous amount of quiet research work which is continually taking place in the modern steel inanufac rers' works. These researches do not invariably produce immediately applicable results, but they often indicate ways and means by which specific problems may be attacked and overcome.
A. typical example concerns the grain size of carbon steels. Grain size is the term applied 'to the size of the individual crystals of which -steel is composed. Grain size exerts an important influence on the properties and performance of any specific steel, and it is all the more vital because it is not revealed by mere chemical analysis. I rsers of steels Will often have observed that steels with .identical analyses frequently give Widely differing results in machining or working.
Coarse and Fine Grains.
When the grains are coarse, it can usually be taken for granted that a carbon steel will be machinable without difficulty, and can be case-hardened to a depth which would not be feasible with a carbon steel possessing a fine grain. On the other hand, when the grains are fine, the steel is not so easy to machine, does not case-harden to an equal depth, but is more easily forged and is much tougher.
Now, it will be obvious that, if the steel maker, knowing the purpose for which his material was ultimately to be used, could arrange. matters so as to produce steel with either coarse or fine grain, whichever might be the better in the individual instance ; if he could, in short, control the grain size with a considerable measure of accuracy, he would have done a great deal towards giving his customers greater satisfaction.
A great deal of experimental work has been carried out, therefore, in an endeavour to find some means for accurately controlling the grain size of "plain 'carbon Steels.A method that is being employed with a measure of subcess is to tap the steel in the molten condition, while it still retains a portion of iron oxide in a dissolved condition, and afterwards to deoxidize it by means of aluminium added to the ladle.
It cannot be said that this process definitely gives the desired solution, because eccentric results are occasionally
encountered. Nevertheless, this discovery constitutes a marked advance towards a legitimate answer to the problem, and suggests that the right path is being taken by metallurgists.
Another process which has been the subject of investigation is the anneal
lug of tool steels. Annealing is a softening process, applied to bars of. steel prior to their leaving the factory. Steel thus softened is then in a condition in which it can be made into the tools required. If, however, the annealing be irregular, difficulties in tool making occur.
One of the chief causes of these irregularities has been the type of gasfired furnace in which the material was heated up for annealing, and left to cool. So soon as the doors of these furnaces were opened for any purpose, air entered and reduced the temperature at one end, thereby affecting the condition of the steel.
To prevent this, a new type. of gasfired annealing furnace has been designed. It is constructed on a special patented recuperative system, and has gas screens at both ends. Air is thus excluded from the furnace chamber even when the doors are open, and uniformity of annealing is thereby assured.
In respect of tool steels, a great dealof time and thought has been devoted to the improvement of tool-holder bits of high-speed steel. Originally, these were made from a standard tungsten high-speed steel, containing 18 per cent. or 14 per cent, of tungsten. Then, with the arrival of the modern cobalt super-high-speed steels, an attempt was made to employ" them for tool-holder bits.
Brittle Cobalt Steels.
Unfortunately, these steels are much more brittle than is ordinary high-speed steel and considerable trouble was caused by cracking under the pressure of the tool-holder. Men frequently left the bit screwed up in the tool-holder overnight, and in the morning it would • be found broken, largely, because the physical characteristics of the -material prevented it from' withstanding the prolonged stress.
Investigation proved that much of the difficulty, was due to the fact that the bits, as produced, were not a perfect fit in the tool-holder, thus increasing the stress caused when they were screwed up. By grinding them all over by special methods, it was found pos.. sible to make the bits bed properly in the holder, with the result that breakages were minimized. It is now possible to employ the super-high-speed steels for tool-holder bits, but the steel manufacturers still recommend that the tools should not he left in the holder overnight, if this can be avoided, as there
is always a possibility of fracture.
The nitriding process, which was dealt with in an earlier article, has led to a great deal of experimental work in connection with the effect of nitrogen gas on various steels and other metals. As the result of a number of important investigations recently made, primarily into the effect of nitrogen on steels with a high chromium content, it has been established that nitrogen improves the resistance and ductility of castings, into the composition of which 20 per cent, of chromium enters.
Ordinary nickel-chrome alloy-steel castings are improved in grain and in resistance without any loss of ductility, whilst there is,•at the same time, a marked increase of machinability. The mild steels containing percentages of chrome or nickel-chrome become more suitable for hot work when nitrogen is added to theni; and can be thus worked under much better conditions.
Stainless-steel Experiments.
The stainless steels of this type are particularly affected in this way, and reveal a distinct improvement in resistance and ductility. As a result of these experiments, it is also apparent that the introduction of nitrogen into the steel is equally to be recommended whenever a rustless, non-magnetic steel, with a high elastic limit, but capable of being machined, is desired.
While discussing stainless steels, it is interesting to note that an American . foundry has recently inaugurated a new process for the centrifugal casting of stainless and other steels, such as 18/8, 26/10 and 10/23, The principle of the process is the same as in the standard centrifugal-casting operation, but, the mould itself is fixed during casting and is only afterwards subjected to rotation.
The chromium plating of tools continues td exercise research. workers, So far as cutting tools are concerned, it seems fairly well established that, thLi process is beneficial, if at all, only when light cuts are being taken and when a plentiful supply of coolant is employed. Measuring tools, however, have been effectively, plated, but there has been
trouble due to stripping.
The latest device in this connection is directly to .chromium-Plate, -Without a previous. nickelling of the surface. This process has been found to raise the resistance of the fool to stripping (i.e., splitting), and thereby to prolong the life Of the toot The hardness of the chromium layer has been proved to rise with increasing current intensity (amp/dm2). As against this, however, a hardening of the basic material cannot be avoided.