Vehicle Reliability Starts with Metallurgy
Page 27
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How Bad Treatment of Metals in the Foundry and in Hardening May Affect Life of Finished Components
"ATTGUE," as a term applied
to a metal component that
failed after long service, was misleading, and a better expression would be "endurance," said Mr. T. G. Strong, A.R.I.C., A.I.M., of Albion Motors, Ltd., in a paper which he read, last week, to the Scottish Centre of the Institute of Road Transport Engineers..
Fatigue suggested that metals became tired, which was not so. The endural-ice limit of a metal or alloy was the stress below which the material would not fail if subjected to an infinite number of stress reversals, and was determined by subjecting test specimens to alternate tensile and compressive. stresses, decreasing in stress until there was no fracture at 10,000,000 reversals.
In onening his paper, the author dealt with the natural and mechanical properties of steel. A crystalline substance, it might be regarded, he said, as an alloy of iron and carbon, the latter constituent being less than 1.7 per cent. Steel always contained sulphur, phosphorus, manganese and silicon in proportions varying with the method of manufacture, and the type of steel being produced. When there were additions of nickel, chromium, molybdenum, etc., which imparted properties not forthcoming from ordinary carbon steel, it was known as alloy steel.
The Physical Side of Hardening In explaining the physical change which took place when steel was hardened by heating and quenching, Mr. Strong said that the carbon in steel existed as a compound of iron and carbon, known as iron carbide, or cementite. Up to a temperature of 700 degrees C., the iron and iron carbide preserved their separate identities, but from a temperature range of 700 degrees C. to 900 degrees C., depending upon the carbon content, the iron matrix absorbed or took into solution, the iron-carbide constituent.
On being cooled rapidly, the iron matrix had not sufficient time to eject all the iron carbide and it was compelled to hold a substantial quantity in solution. This unnatural state of affairs imposed a. severe stress on the crystal structure of the steel, which was outwardlY manifested by a charac teristic rise in hardness. .
The mechanical properties of a particular steel which . the designer wished to. know were: (a), ultimate tensile stress;-i.e., the maximum stress in tons per sq. in. that it can withstand before rupture; (b), its elastic limit, which is the -maximum stress it will withstand without taking a permanent set; (c), its: degree of resistance to shock loading. The last-named was found• by an hod test, in which a standard specimen, BO Mtn, long by 10 mm. square, having a notch with a 45-degree included angle and a 2,5 mm.
root radius, was broken by impact. The amount of energy in ft-lb. absorbed in breaking the specimen, served as a guide to the toughness of the material.
It had been estimated, said the speaker, that 90 per cent, of all metal failures were caused by dynamic loads. Farts failed even though the stresses were relatively low, but under repeated stress, the metal yielded at one or more local areas of weakness, resulting in the formation of Minute cracks, at the end of which the stress concentration was frequently high.
The cracks usually spread inwards from the surface, and, after a time, there was so little sound metal left that the normal stress in the remaining piece was sufficiently high to cause fracture. This led Mr. Strong to the use of the word " fatigue," which he preferred to refer to as the endurance limit.
Endurance Limit of Steels
The endurance limit for most carbon and alloy steels was about half their tensile strength, he added, whilst that of cast iron was between 0.3 and 035 of the tensile strength. In the case of high-tensile steels, the endurance limit might be reduced by as much as 50 per cent, by the presence of a notch, which pointed to the necessity of relieving all sharp corners, the removal of tool marks, and the provision of generous radii wherever possible.
One of the most effective means for lowering the endurance limit almost to nil was by subjecting a component to stress under corrosive conditions. Ways of avoiding failure were to use a stainless steel, or to protect the surface withzinc or cadmium plating.
Grey cast iron, although inherently brittle, was relatively insensitive to notches, arid unless these were very sharp, their effect on steels having a tensile strength up to 28-32 tons per sq. in., was very little. If, therefore, exigencies of design made it necessary to use a square or V notch under conditions where endurance limit was important, there was no advantage in employing a high-alloy steel_ of the 100ton type, as in such circumstances its endurance limit did not usually exceed 15 tons per sq. in.
-Dealing with the effects of understressing, Mr. Strong said that if a specimen were stressed for a few million cycles; to an extent slightly below its endurance limit, the material became stronger, and might withstand loads in service which could cause early failure had it not been_ -improved in that way.
_Having dealt at length with metal failures as the result of reaching tile limit of endurance, the speaker referred to the defects which could arise in
steels ' during their manufacture, mechanical working, and their heat treatment.
The volume of slag associated with melting was considerable, he said, and whilst it floated on the surface of the molten metal, the removal of all the particles was difficult. Compounds of silicon, manganese, and iron with oxygen were trapped in the steel, and, together with the slag, could become centres of corrosion or adversely affect impact values.
Another serious defect in steel ingots was segregation, which was the concentration 'of insoluble impurities in certain zones, and was aggravated by casting the steel at too high a temperature. 'Steel, said the author, should be cast at the lowest possible temperature: If present in any appreCiable quantity. sulphur became a real nuisance, but if there were sufficient manganese present, the compound formed, if well distri. buted, made the steel easier to machine. Blow holes were caused by gases which became trapped as the steel solidified.
The mechanical working of steel was carried out mainly by rolling and forging, of. which the former was the least costly. Forging operations were usually carried out at temperatures between 1,000 degrees C. and 1,250 degrees C., and the temperatures at which the work was begun and finished were of paramount importance.. If the steel were heated too much, it was said to be burned, and if the final temperature of the rolling or forging were too low, the product might be full of stresses, and lacking in ductility and , toughness.
How Steel Becomes Decarburized
The steel might be worked at a temperature not sufficiently high to cause burning, but high enough to permit the carbon in the outer layers of the steel to combine with the oxygen in the air, leaving a very soft skin. Steel in this condition was said to have been decarburized, which was a serious defect.
On the subject of heat treatment of steel, Mr. Strong said that this might take the form of softening the material by annealing, which eliminated the stresses induced by machining, and the restoration of the steel to a standard condition by normalizing, grain relining, hardening, tempering, case-hardening and nitriding.
On breaking badly treated material, said the author, a brittle crystalline and coarse fracture was obtained, instead of the silky tough fracture indicative of good treatment. It was easy to ruin good material by faulty heat treatment, particularly if the furnace did riot produce uniform temperature conditions, or the temperature checking devices were snowing an incorrect reading.
In the field of high-tensile alloy steels, nickel was the most important alloying element, and this was closely followed by chromium.