Material Mastery By Temperature Control
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The Urge to Substitution and the Use of Nominally Lower-alloyed Steels has -Prompted Full Advantage to be Taken of Control of Physical Properties by Specialized Heat-treatment IN the U.S.A. much has been dune on the use of boron as a means for shortening the time necessary. for rnalleabilizing iron castings. An addition of 0.001 per cent, of boron in the form of ferro-boron to the iron as it is discharged from the cupola is quite sufficient, and will lessen the time 'required for the malleabilizing operation by as much as 10 per cent.
Research has been carried out in Sweden on the rate of formation of surface-reaction products, when specimens of copper are exposed to iodine vapour and sulphur fumes, and specimens of iron to oxygen. The apparatus employed enabled the specimens to he subjected simultaneously to supersonic waves produced by a quartz oscillator, and it was found that these waves markedly increased the reaction rates. This might have been expected as a result of the breaking down of impermeable surface films.
Determining Hot Workability
Apparatus • has been developed for determining the hot-working properties of steel bars. The test bar, A-in. diameter by 22 ins: long, is placed in a • small electric furnace, with each end projecting, one being clamped in a chuck and the other to a stop that prevents rotation but allows of expansion. Furnace temperature can be strictly controlled and the bar is held at the desired heat for half an hour. The motor driving the chuck is then started, together with a revolution counter, which stops when the specimen breaks. The number of twists of the bar before breakage is taken to indicate its hot workability. * Bars may he tested at a range of temperatures, of which that permitting the maximum number. of twists before fracture is the best heat for piercing or other hot-working processes, Another new machine has been designed for establishing the mechanical properties of materials under alternating torsional stress. This is of the oscillatory type, and evacuates both damping capacity and the dynamic modulus of rigidity of both plastics and metals. The resonance-amplification factor for the plastics tested is abut one-tenth that of the metals. Sustained cyclic stress below the endurance limit increases the resonance-amplification factor pf the metals tested by as much as 25 per cent. .A pronounced reduction 'in both the resonance-amplification factor and the dynamic modulus of elasticity occurs at impending fatigue failure in all the materials tested.
Important work has been done on the influence of melting conditions on the physical properties of steel castings. The combined phosphorus and sulphur percentage affects the mechanical properties mitrb more than do other lac tors, the property most impaired being impact value if the contents of these elements be too high.
Unsoundness, caused by blowholes or porosity, also exercises a marked influence, particularly on elongation and
R.O.A. values Casting temperature has a slight effect on mechanical properties, but can, for practical purposes, he ignored by the purchaser.
Salt-bath heat treatment of highspeed steels has been greatly developed, and although the capital cost of the equipment is high, it is believed that no other method exists whereby full hardness, combined with almost perfect surface conditions, can be obtained.
Stress-relieving tests at various temperatures have indicated clearly that cast-iron, stressed at about 5 tons per sq, in, before annealing, can have 80 per cent of this stress relieved by 'heating to 600 degrees C. Soaking for six hours at this temperature relieves an additional 17i per cent, of the stress. Heating to 550 degrees C. releases about 95 per cent, of the stress, and growth is only about 0.001 in. per in: Caststeel, soaked at 650 degrees C. for one hour, has its initial residual stress relieved by 97 per cent.
Tests have been carried out with welding rods i in, to irk in, diameter to decide the welding conditions that will produce a weld of the greatest possible ductility. Satisfactory welds may he carried out with a traverse or "weaving motion" equal to six or seven times the electrode diameter. , Weav'nig at about 100 times a minute across this distance gives rise to a weld with a rather higher elongation and impact resistance than when traversing at 15 times per minute, with the other conditions kept uniform.
Light on Mn-Si Steels
Silicon-manganese steels are largely employed for the springs of commercial and 'other vehicles, and the effect of the silicon content on these steels has been studied. it appears that silicon is almost as effective as manganese in increasing the tensile strength in the normalized state. Silicon is superior to carbon or manganese as a strengthener Of normalized steels for welding, because there is less weld-hardening and higher weld ductility irt the silicon steels. Carbon content must not be too low. •
The " browning ' , of steel is being carried out to produce a hard blackoxide coating. 'The parts to be treated are first cleansed to eliminate all grease, dirt, and oil. They are then sandblasted and coated with the browning solution by dipping, sponging, or spraying. The next stage is heating in a dry atmosphere to 55 degrees C., followed by heating in a humid atmosphere to 65 degree3 C., in order to peoduce a red-oxide sin ting. The parts
are then boiled in water to transform. the red oxide to black oxide, and buffing follows to eliminate superfluous oxide. The last five operations are repeated until four coats have been formed.
It has been found that a thin refractory wash or facing mixture applied to tool steel results in a considerably improved quenching operation, because it increases the rate at which the steel cools, and lessens the initial yapper stage, when vapour surrounds the tool steel and tends to reduce the hardening effect of the quench. It has also been found that oil-quenching .baths are better if slightly warmed within the temperature range of 30 to 60 degrees C. There must he sufficient volume of oil and adequate circulating means; one • gallon of oil per lb. of steel quenched per hour is a good rule for fixing the capacity of the quenching tank.
Practice of Weld Hammering The peening or hammering of welds has been investigated, and it has been determined that peening in the " bluebrittle" temperature range increases the hardness by 25 per cent., he hardening being greater than that occurring in the heat-affected zone of the parent metal. Adding a further layer of weld metal to the peened layer has a beneficial tempering effect and has led to the practice of peening all except the final layer of a welded joint. Peening must not however, be carried out at temperatures below 40 degrees C.
Oxy-acetylene cutting equipment of novel design enablee four identical sprockets to be cut out of heavy steel plate frbin a single template. The machine has four burners.
An air-hardening chromium-nickelmolybdenum steel is being increasingly employed for castings,having thick and thin sections which are to be hardened throughout, with little or no distortion. A preliminary stress-relieving treatment is usually given before the gates and risers of the castings are removed. To allow of machining, a . carefully controlled.annealing treatment, first at 870 degrees C., and then at 675 degrees C., is applied After the castings have been machined, they are hardened by cooling them in air from 900, and then tempered at 480-595 degrees C.
In some quarters, airor oil-hardening alloy s eels are replacing case
hardened steels. It is admitted that many thousands of case-hardened parts have carried heavy loads and have resisted wear, but this is not regarded as necessarily a criterion of excellence for such parts. It may, • the' argument goes, only be evidence of the provision
of a very large load safety factor by nervous, ingenious designers, iand, In many. instances, it is believed that a step forward would be made in the automobile industry by the discarding of case-hardened steels. This matter was investigated some 12 years ago in the laboratory of Clement Talbot, Ltd., when it was found that, by replacing case-hardening nickel-steel by case.: -hardened oil-hardening nickel-chromium. steel, an excellent air-hardening core was obtained. The resulting gear was less " sonorous " in operation than the usual .oil-quenched case-hardened type.
The magnetic-powder process is being extensively used for testing welds for cracks. In the latest method, two " prods " are employed, comprising bars ol manganese bronze or copper 1 in. in diameter, bent at right angles
2 ins, from one end. These are connected to the generator and 'the ends are placed 6 ins, apart on the surface to be inspected. The current is -then switched on and the magnetic powder is distributed over the area between the prods.
Fabricating the Stainless Steels
. More attention is being paid to the mechanical and fabricating properties of the stainless steels. An alloy into the composition tit which enter 25 per cent. ot chromium and 12 per cent of nickel, although excellent where resistance to high temperatures is desired, is not always the best possible material to use, because it may have a harmful effect on some of the materials with which it-is brought into contact. In these instances there is a tendency to lower the nickel content (as it is the
nickel thatexercises this deleterious influence) and replace it by nitrogen. Steels containing apprOximately 16 per cent, of manganese. which have excellent mechanical properties when subjected to cold working and are relatively easily fabricated by the normal methods, are finding increased use for specialized purposes, on account of their low cost.
Flash-welding machines are being employed for the upset forging of round bars and tubing. Strongly constructed machines able to exert mechanical pressures of 8,000 lb. per sq. in. are proving extremely suitable for the economic production of large numbers of upset forgings in medium-carbon steel.
The air-operated drop stamp is being increasingly used for the formation of sheet-metal components. The principal advantages of this stamp are that all operations can be readily controlled and that the sensitiveness of the mechanism allows the operator, without fatigue, to move the ram quickly and with the requisite amount of pressure, . while keeping the work closely under his eye.
A temperature-measuring and control unit has been designed for use with electric salt-bath hardening furnaces for high-speed-steel tools, in which the rate of heating is extremely high. A wrought-iron tube has a socket at one end and into this a Sillimanite target tube 12 ins, long is inserted, A radiation-measuring cell is fixed in the upper end. This is connected to a potentiometer recorder, which operates an " on and off '• controller in the furnaceelement.circuit When in use, the Sillimanite tube is dipped 4 or 5 ins, below the surface of the bath. . This apparatus
is designed for barium-chloride baths and is not suitable for cyanide salts.
Full-hardening gear steels., such as are replacing in many quarters case-hardened steels, are being hardened in cyanide baths, particularly when cf the medium carbon 1 per cent, chromium gear-steel type. The case depth obtained is of the order of 0.002 in, to 0.005 in. according to the mass of the
part. • The Micro-hardness Tester
A new micro-hardness testing instrument carries a _diamond penetrator, so mounted as to be attachable to a microscope, in place of the object lens. The microscope has 'cross-hairs in the eyepiece, and, by correct alignment of the diamond, it can be made to touch the sample under examination on the precise spot indicated previously by she crosa-hairs. The diamond is lowered rsn to the specimen by turning the tineadjustment screw of the microscope. The exertion of force quickly suffices to overcome that of a carefully calibrated spring. When this occurs, two contacts held by the spring are opened, a small lamp is illuminated, and the penetrator is raised. This ensures that the same force is always applied. The instrument is being found specially valuable in powder metallurgy.
Centrifugal casting is being employed for the mass production of cast-steel cylinder liners. The steel is poured at a minimum temperature of 1,615 to 1,625 degrees C. into the mould. which is spun at approxmately 800 r.p.m. The rate of production is 400 liners an hour, with 15 men working on the moulding line.. L.S.