TH STEEL IN YOUR CAR
AND HOW IT IS TES ED.
By B. G. MANTON, B.Sc., A.M.I.CE.
ACHAIN is only as strong as its weakest link and a car is only as strong as its steel. Sound design, careful workmanship and perfect finish are merely wasted if the material is faulty, and when one considers what is happening to the various parts of a car’s mechanism; even at such a moderate jog-trot as forty m.p.h., it is amazing that metal can be endowed with sufficient endurance to survive for lengthy periods the drastic conditions under which it has to function.
The sports car, with its higher maximum speed, lightning acceleration and superior braking power naturally requires a higher standard of material than the more sedate tourer or family saloon and, in a car of good quality, there will usually be a surprisingly large number of different steels, many of the expensive alloy type, each particular variety being chosen for its special suitability under certain kinds of stress.
The British Engineering Standards Association has drawn up a number of specifications for a wide range of plain and alloy steels for use in automobile construction, and these materials are submitted to standard tests which give a clear indication of quality and characteristics, so that the conscientious car manufacturer, who employs the right types of steel, made by a reputable British firm to suit the appropriate standard specification, may be reasonably certain that his products will not fail through faulty material.
The Constituents of Various Steels.
A ” plain ” steel, as distinct from an alloy, will contain iron, carbon, silicon, manganese, sulphur and phosphorus. The percentage of carbon has a profound influence on the properties of the metal and it may be anything from 0.15 to 0.45 for ordinary automobile purposes, the lower percentages giving a softer material than the high. Where a special hardness is required, as in the case of clutch plates, the carbon content may be as much as 0.8 per cent., a figure which is also suitable for a plain spring steel ; the percentage of both sulphur and phosphorus is usually limited to the range 0.05 to 0.07, as an excessive amount of either of these substances would make the stet]. brittle ; a common figure for the silicon percentage is 0.3—this ingredient appears to give a certain degree of hardness to the metal, but an excess would result in brittleness ; the manganese is a beneficial constituent—it improves the structure of the steel and assists in counteracting the bad effect of the sulphur and its percentage in a plain carbon steel may be anything from 0.3 to 1.0. When superior steels are required for highly stressed parts, an alloy is usually adopted, containing an ad
ditional metal or a combination of metals, those most frequently used being nickel, chromium, vanadium and tungsten. In this way a considerably higher strength is obtained, combined with good ductility (or, in other words, a complete lack of brittleness) enabling a great saving in weight to be effected while the parts will still be adequately strong and durable. The higher quality of alloy steels is well illustrated by a comparison of’ tensile strengths, i.e., the pull,
expressed in tons per square inch, required to snap a small bar of the steel by tugging at its ends :
A plain steel, containing 0.3 per cent. carbon will give a tensile strength of 28/30 tons per square inch. An alloy steel with the same amount of carbon, but
with 3.5 per cent. of nickel will have a figure of 35/38 tons per square inch.
The further addition of 0.75 per cent. of chro. mium to the nickel steel referred to above will result in an improved tensile strength of 50/55 tons per square inch. These figures apply to steels which have not been ” heat treated,” or submitted to a series of processes which include heating to a high temperature and cooling with more or less suddenness, followed by re-heating to a lower temperature and again cooling. On the temperature of the second heating depends the final characteristics of the steel and the resulting product may have either a comparatively low tensile strength combined with great toughness (or ability to stand shocks),
or a higher tensile strength with an inferior toughness, according as the re-heating temperature is relatively high or low.
Alloy steels lend themselves admirably to treatment of this nature.
Some Tests Described.
The principal test applied to steels is the ” tension ” test, in which a small sample test piece, machined in the form of a thin bar, is fractured by the pulling apart of its ends. The standard test pieces are usually turned down to a diameter of .564 inches, although smaller sizes are also used, and collars are left at the ends of the bar for clamping in the grips of the testing machine, the edges of the collars being rounded off so that no sudden change in section occurs. In a specimen of the above diameter, the length of the narrow turned portion is 21 inches and within this length two lines are carefully scribed, exactly 2 inches apart, this distance being termed the
gauge length.” The specimen having been clamped in the testing machine, one of the grips is pulled outwards, usually by means of a crosshead threaded on to two massive screwed rods. These rods are really extremely large and lengthy bolts, on which the crosshead forms a nut, and a rotation of the rods results, of course, in a gradual movement of the crosshead along the thread. The rotation is produced by gearing, driven from an electric motor
and the pull set up by the moving crosshead is transmitted through the test piece to the second grip which is attached to a long pivoted beam. This is kept in a balanced position by a travelling weight and, as the tension increases, the weight is run out further and further from the fulcrum point, its distance therefrom providing a means of measuring the pull applied to the specimen.
At a certain period of the test it is always noticed that the beam drops suddenly, although the specimen has not been fractured and this dropping of the beam indicates that the metal has reached its ‘yield point.” Previous to this stage, the test piece, although stretched under the tension, would return to its original length if the load were released, but after this particular point, the stretch becomes a permanent deformation and is followed ultimately by a complete fracture.
A specially calibrated scale, reading in pounds, is attached to the beam and the travelling weight carries a pointer indicating the load on the test piece corresponding to any position along the length of the beam.
After breaking, the specimen is micronaetered at the fractured section and then held with the broken edges in close contact while the stretched length, between the original gauge marks is scaled off. The test piece will be drawn out somewhat,into a narrow neck, at the vicinity of the break and the dimensions taken after testing enable both the reduction in cross-sectional area and the elongation in the gauge length to be ascertained.
These figures, to which limiting values have been given in the British Standard Specifications, form valuable guides as to the characteristics of the metal, a tough, ductile steel drawing out to a greater extent than a hard and comparatively brittle one.
It may be of interest to note that a high-class chromevanadium steel will give a maximum tensile strength of 65 tons per square inch (about twice the strength of mild steel), with a yield point of 45 tons, and a steel of this type, in a car of first-class quality, would be used for such parts as crankshafts, connecting-rods, axle and propeller shafts, steering arms and front axles, while air-hardening nickel-chrome steels, with a tensile strength as high as 100 tons per square inch, are sometimes used for gear-wheels, acquiring under correct heat treatment, the requisite properties of extreme hardness on the surface to resist the abrasive action on the faces of the teeth, combined with great toughness in the body of the material to withstand the heavy shock stresses, which are occurring constantly within the gearbox.
Another standard test is the “Brinell hardness test,” in which a hardened steel ball, ten millimetres in diameter, is pressed into the specimen for at least fifteen seconds under a load of 3,000 kilogrammes (roughly 3 tons). The surface of the test piece is carefully prepared by filing, grinding, or machining and the area of the impression formed by the ball is accurately determined.
The ” Brinell hardness number” for the steel under test is found by dividing the load on the ball by the area of the impression and this number bears a definite relationship to the tensile strength.
The “notched bar impact test” is another interesting method of investigating the quality of a steel. The specimen, as the name denotes, is formed into a small bar, either square or circular in section and one or more ” V ” shaped notches are cut in it, the angle of the ” V ” being forty-five degrees.
The testing machine consists of a massive framework, triangular in shape and some five feet high, the specimen being clamped rigidly at about the middle point of the horizontal base. A heavy pendulum, swings from the apex and can be locked in a raised position and then released by a trigger, when it thereupon swings down and strikes the test piece at a point a short distance above the notch. After fracturing the specimen, it continues its swing in an upward direction and reaches a certain height on the, opposite side to its starting point.
In fracturing the test bar, a portion of the energy of the falling pendulum is naturally absorbed and the extent of the swing, after the collision point, is correspondingly curtailed. By a simple automatic device, the amount of the absorbed energy is indicated on a specially calibrated scale, giving a measure of the strength of the specimen.
It will be realised that each of these tests is forla different purpose and each determines different qualities in the steel ; therefore while the results of one test are of value, they must be considered in conjunction with the results of the other tests to give a clear idea of the full characteristics of the material in question, and it is to the development of these methods of testing that we owe in great part the extraordinary reliability of modern engines.
As the tempering (or re-heating) temperature increases, the material becomes tougher and more ductile, as shown by the increase in elongation and percentage area reduction, but the tensile strength and yield point are lowered.
The modern sports car, with its high efficiency and wonderful capacity for revs, has called for the best efforts of the steel maker and the demand has been met, unfailingly, from the laboratories and the furnaces of Sheffield, where, unobtrusively, but none the less effectively, the metallurgists have played their part and rendered possible the vast developments in every sphere of high-speed motoring, ranging from the remarkable performances of the sporting light car to the glorious achievement of the “Golden Arrow.”