Difficulties he was greatly embarrassed by the brittleness or " redshortness " of his steel, which he did not know how to cure.
Brittleness and ready inflammability.
But the invaluable and rather delicate art of tempering the hardened steel by a very careful and gentle reheating, which removes its extreme brittleness though leaving most of ifs precious hardness, needs such skilful handling that it can hardly have become known until very long after the art of hot-forging.
The casting of the molten cast iron into shapes which were useful in spite of its brittleness, naturally followed.
The presence of a small quantity of the hard cementite ought naturally to strengthen the mass, by opposing the tendency of the soft ferrite to flow under any stress applied to it; but more cementite by its brittleness naturally weakens the mass, causing it to crack open under the distortion which stress inevitably causes.
The fact that this decrease of strength begins shortly after the carboncontent rises above the eutectoid or pearlite ratio of o 90% is natural, because the brittleness of the cementite which, in hypereutectoid steels, forms a more or less continuous skeleton (Alloys, Pl., fig.
The carboncontent of steel is rarely greater than this, lest the brittleness be excessive.
This brittleness has therefore in general to be mitigated or " tempered," unfortunately at the cost of losing part of the hardness proper, by reheating the hardened steel slightly,
But in most such cases, in spite of the annealing, this hardness is accompanied by a degree of brittleness too great for most purposes.
This coarsening and the brittleness which accompanies it increase with the temperature to which the metal has been exposed.
This coarseness and the consequent brittleness are removed, by reheating them much above Ac 3, which also relieves the internal stresses due to the different rates at which different layers cool, and hence contract, during and after solidification.
Although the presence of 1.50% of manganese makes steel relatively brittle, and although a further addition at first increases this brittleness, so that steel containing between 4 and 5.5% can be pulverized under the hammer, yet a still further increase gives very great ductility, accompanied by great hardness-a combination of properties which was not possessed by any other known substance when this remarkable alloy, known as Hadfield's manganese steel, was discovered.
They serve a given purpose more cheaply, in the long run, than either rolled or cast steel, in spite of their need of being so massive that the brittleness of the material itself shall be endurable.
Indeed this high carbon-content, 3 to 4%, in practice actually leads to less brittleness than can readily be had with somewhat less carbon, because with it much of the carbon can easily be thrown into the relatively harmless state of graphite, whereas if the carbon amounts to less than 3% it can be brought to this state only with difficulty.
That it cannot be hammered from one shape into another, yet its degree of brittleness differs as that of soapstone does from that of glass, so that there are the intensely hard and brittle cast irons, and the less brittle ones, softer and unhurt by a shock which would shiver the former.
Now this matrix itself is equivalent to a very low-carbon steel, strictly speaking to a carbonless steel, because it consists of pure ferrite, which is just what such a steel consists of; and the cast iron as a whole is therefore equivalent to a matrix of very low-carbon content greater than r so%lest its 111E brittleness should be excessive, yet -(1 cast iron with be U H c ° ?
Second, though the brittleness should be lessened somewhat by the decrease in the extent to which the continuity of the strong matrix is broken up by the graphite skeleton, yet this effect is outweighed greatly by that of the rapid substitution in the matrix of the brittle cementite for the' very ductile copper-like ferrite, so that the brittleness increases continuously (RS), from that of the very grey graphitic cast irons, which, like that of soapstone, is so slight that the metal can endure severe shock and even indentation without breaking, to that of the pure white cast iron which is about as brittle as porcelain.
To sum this up, as graphite is replaced by carbon combined as cementite, the hardness, brittleness and density increase, and the expansion in solidification decreases, in both cases continuously, while the tensile strength increases till the combined carbon-content rises a little above I %, and then in turn decreases.
That strength is good and brittleness bad goes without saying; but here a word is needed about hardness.
- Phosphorus has, along with its great merit of giving fluidity, the grave defect of causing brittleness, especially under shock.
Fortunately its embrittling effect on cast iron is very much less than on steel, so that the upper limit or greatest tolerable proportion of phosphorus, instead of being o.10 or better 0.08% as in the case of rail steel, may be put at 0.50% in case of machinery castings even if they are exposed to moderate shocks; at 1.60% for gas and water mains in spite of the gravity of the disasters which extreme brittleness here might cause; and even higher for castings which are not exposed to shock, and are so thin that the iron of which they are made must needs be very fluid.
The permissible phosphorus-content is lessened by the presence of either much sulphur or much manganese, and by rapid cooling, as for instance in case of thin castings, because each of these three things, by leading to the formation of the brittle cementite, in itself creates brittleness which aggravates that caused by phosphorus.
While the work is often cleverly done as to matching and manipulation of the pelt which is very soft, there are great objections in the odour and the brittleness or weakness of the fur.