Spotlight on special Steels
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War-time Shortages of Alloying Elements have Prompted Fullest Investigation into Steels of Lower Alloy Content, Specially Treated to Attain Maximum Properties
MANY recent developments in materials, machines, and processes are designed to meet the needs of the world in the post-war years, From the spate of new technical advances, the following items of particular interest to the ,commercial-motor industry have been drawn. They come from many lands and from many different, indirstries, but all have' two common threads —progress in metallurgical Science and potential application to 'civilian peacetime needs.
Experiments have been carried out to reduce the time required for the heattreatment of alloy steels for forging ie to tools. Various'steels were tested, and. eventually a series of satisfactory beat– treatments was evolved. This comprises holding the tool at a temperature of 860-900 degrees C. for a period of one to one-and-a-half hours, which is followed by soaking at a temperature below the critical point for a period not exceeding four hours. The total heattreatment time has been reduced by approximately 60 per cent., as compared with former methods.
• Magnetic..devices are being fitted to roller hearth beat-treatment furnaces so as to give completely automatic operation of the vestibule floors of the charging end, the roller hearth drive and the discharging door.
-NitrIcling High NI/Cr Steels The nitrid:ng of a steel containing -12 per cent. nickel, 6 per cent. man-. ganese, 4,5. per cent, chromium, and 0.:3-'percent, vanadium was tried, at
first, without success,' but eventually the desired result was achieved by hold ing the steel parts at 600 degrees C for six hours; after cutting off the flow of ammonia to the nitriding chamber, tuns
giving the nitrogen time to diffuse inwards and lessen the concentration at the surface.
Electrical strain gauges are now boil' in units holding three offour gauges set at different angles. They are termed
rosette strain gauges. The calculation of the principal stresks from the readine:s obtained is a laborious and dull tar'sk, and, in consequence, a mechanical calculator has been developed for quick evaluation.
A new moulding process for grey-iron castings has been developed. This is the Use of a cement sand as a pattern sand. The rest of the mould is filled up with silver sand. No hand or pneumatic ramming is required.
Large quantities of steel 'scrap containing small amounts of copper, nickel and molybdenum will soon become available from unwanted guns, armament, etc., at the conclusion of she war, and already plans are being worked out to use these as a basis for the' manufacture of high-grade alloy steels. — '
High-speed .steel-cutting tool; found to be low in hardness' or incorrectly gr mnd . when received, are : being improved by 'refrigeration, followed by salt-bath tetrinejag. or nitriding. re
grinding and honing. Salt-bath nitriding is being advanced as ameans for lengthening the service life 'of taps, chasers and milling cutters. • Heavy low-frequency current is being used for the local induction heating of large plates and pipes before welding, and for normalizing after welding. The stress relieving of long welds is being carried out, by simultaneously heating two bands of the steel—one on each side of the weld—with special oxyacetylene torches. Tests have demonstrated that the temperature required to attain adequate stress relief does not exceed WO degrees C. '
A simple method is being employed for removing oil, dirt, oxide, etc., from tow carbon steel sheets in order. to ensure structurally sound welds and a surface to which paint or plating readily adheres. The method adopted resembles that employed for the pre-cleaning of aluminium in advance of spot welding.
The application of end-quench test data and cooling rate curves are being used to determine the choice of steels 'suitable for manufacturing heat-treated motor-vehicle parts. The curves' drawn from end-quench test results represent the high and low hardness values obtained from a number of heats representative of the normal manufacturing practice fox a particular grade of alloy steel. The area between the two curves is termed the hardenability band.
Electro-polishing 8/18 Stainless The electro-polishing of stainless steel
is finding increasing favour. Bettar reflectivity values are being obtained from electrolytes based on phosphoric acid or mixtures of phosphoric and sulphuric acid3 with an organic or inorganic. addition agent. A typical e!ectrolyte contains 42 per cent. phosphoric acid by weight. 47 per cent, glycerine by weight, and 11 per cent, water by weight.
A cast plastic tool for forming aluminium sheets is based on a hot melted ethyl-cellulose composition compounded with plasticizers, resins, pigment fillers, and other ingredients capable of variation to give many different formulations.
Secondary-aluminium compositions have developed with great -rapidityas • a result of the war. New alloys of this type have been introduced, complying .chemically and mechanically with many official specifications that.. formerly applied only to virgin or primary aluminium. These will, beyond question; prove a great factor. in post-war development of the aluminium industry, and, consequently, on all those industries employing aluminium in one form
or another._
War-time developments in induction hardening, flame hardening, iso-thermal heat-treatment, quenching, shot blasting_ and controlled atmosphere carburizing, when reviewed, show the possibility of improving the design of many mechanisms on the basis of engine experience, and emphasize the need for c:oser co-operation betw_een the designing engineer, the metallurgical engineer; and the 'productionengineer, during the prelimtnary development of any. important design.
A new unit for 'neat-treating bolts comprises an electrically heated Conveyor 22 ft. long, 6 ft, wide, and 10 ft. high, -through which a link „onveyor belt slowly moves1 At the discharge end the bolts fall down a covered chute into an oil tank. from which an inclined conveyor automatically lifts and discharges them into boxes. Furnace and chute to oil tank are filled with a oroteetive atmosphere. 'A high Output of bolts with extremely uniform properties results.'
Electric Hard-facing of Dies
Electric welding is being applied to the manufacture of die sets. For simple. dies, coatings are applied with a selfhardening tool7steel electrode to wear-. ing surfaces and edges. These 'need no heat treatment and require merely to be ground to dimensions. Two roughhole-punches for rough work Were made by inlaying hard cutting edges on mildsteel shanks, and proved highly successful.. A considerable economy in the amount of tool sEeel required and in costly machining operations reSulted.
When metal Parts are immersed in synthetic resin, or similar protective. solutions, and afterwards allowed to. drain, blobs of the solution format the points where it drains off. The removal of the final blobs inn, manner that leaves a smooth and uniform surface has been economically arid competently effected by subjecting the, drain-off point's to ,an-electrostatic field by means of an electrode charged to a very high voltage. An investigation of the strength of heat-treated 2.5 per .cent. nickel-steel .holts has been carried out, and certain important conclusions emerge. Failures under the head may be prevented by making a radius from the shank to the head. Superimposed bending by insert-, ing a 10-degree wedge Or .shim causes brittle fractures. Stripping of the threads occurs with .a shim under the nut. 'Bolts should be heat-treated to a -tensile strength of 112. torts per sq. in.. if the straining in service be in.pure tension,but '78. tons per sq.. in.' is the maximum safe limit if bending be: superimposed.. • • An investigation of the • causes of gear-tooth failure reveals that the finish• machining or grinding following on
rough machining rarely . goes deep enough to remove all the metal-containing residual stresses. Such residual stresses should be removed by heat. treatment, or by mechanical means.. Residual stresses of tensile type at the surface are advantageously converted into compressive stresses by a peening operation, such as hammering, swaging, shot,blasting, or tumbling. This increases the endurance of the surface layer.
Recommendations on the choice cf steel for gears have been made by some_ authorities, and it is said that for those 'running at low speeds under light to medium loads, an oil-hardened 0.4 to 0.45 per cent. carbon steel issatisfactory. For high-speed gears transmitting heavy loads, a 0.35 to 0.4 per cent, carbon, 1.75 per cent nickel, 1.25 pci cent, chromium steel, oil-hardened and tempered at 200 degrees C., has a tensile strength of 110 to 120 tons per sq. in., and gives excellent l'esults: For small pinions, however, the tempering temperatures should he increased to 450 degrees C., which lowers the tensile strength to 85 to 90 tons per sq. in., whilst increasing the toughness.
A method has been invented whereby all the information existing on a radiograph negative is brought out on a _single print. Use is made of a compensating filter to mask the printing light, and this enables quite high contrast to be retained, whilst avoiding the need for reproducing a wide range of densities throughout the picture as a whole.
Remarkable Six-station Rubber-pad Press
A six-station 2,500-ton vertical hydraulic process has been designed, which uses the rubber-pad technique. In this, instead of a pair of solid contoured dies, which haye to match exactly, only one die is needed, the sheet metal to be shaped being pressed into or over the die contour by means of a rubber pad, • which transmits tie pressure applied to it in all directions, instead of in one ;direction. The press has six columns and six openings, through which any of the dres and material can be inserted and withdrawn. There is a central hydraulic ram, and the dies, which may be of a very different character, are assembled on rectangular die slides. These slides, after being loaded, are run in under the press platen. Pressing can take place On one slide simultaneously with loading on the others.
Pressing and drawing processes have been developed for making compressed. gas lightweight cylinders from chromium-molybdenum steel. These were formerly machined out of forgings, which meant a high degree of waste. The amount of scrap has been reduced by 83 per cent, by the new method. Th.; punches and dies are made of graphitic steels.
Induction heating is being applied to carry out three operations simultaneously on oil-engine shafts, namely, to harden a bearing surface and a thrust face, and to braze a collar on the shaft.
Cleaning with solvent emulsions eliminates solid and organic dirt which contributes to dullness on " bright " coatings, and to pitting, peeling, and pomsity of electrolytic coatings on steel. The total time needed for cleaning is greatly lessened by precleaning with solvent emulsions.
Corrosion is often found to occur in the refining plant condensers of benzole plants. Research has shown that this is caused by sulphur dioxide, and methods of dealing with the trouble have been developed. These include modification of the refining process to produce minimum sulphur dioxide in the still; neutralization of corrosive vapours; the employment of corrosion-resisting Materials; a better designed condenser giving more rapid cleaning and the replacement of corroded parts; use of a readily corrodable material in the _vapour stream immediately before the condenser; and the employment of vacuum distillation.
Methods of reducing the ,ammonia consumption in nitriding steels have been introduced. The reduction of consumption by 60 per cent, can be achieved byadjusting the composition of the inflowing gases, so as to give 60 per cent, of dissociated ammonia in the exhaust gases. The nitriding of high-speed steel for tools in molten cyanide baths 'is being achieved. The bath consists 'of 70 per cent,. sodium cyanide and SO per cent, potassium cyanide, and is held at about 565 degrees C. Tool life is considerably increased by this liquid nitriding where relatively light cuts are taken. There has been a marked increase in the employment of low-alloy steels, 'and the war has both widened'their 'application and extended it to the field of heat-treated stocks. The majority of the new war-time steels is, however, too new for any but the most tentative predictions to be made, but there are indications that certain of them will occupy a permanent place, in the postwar period. Experiments have been carried out to determine the best steel for nitriding, and it was found that a steel ..containing 0.25 to 0.35 per cent. carbon, 0.75 to 0.9 per 'cent, silicon, 1.7 to 1.8 per cent. manganese, 0.75 to 0.9 per cent. chromium, and apprcetimately 0.25 per cent. vanadiuna was the most effective in giving a Vickers hardness of 650 with a good depth of case.
Softening Nitrogen hardened Surfaces
Parts that have been given a hard surface surface by the nitriding process can, if desired, be resoftened by heating them in a bath comprised of equal amounts oi sodium and potassium s,hlorides.
The employment of salt baths for quenching has been further investigated, and it has been established that an agitated salt bath maintained at 205 degrees C gives a cooling rate about equal to that obtained by quenching in a mineral-oil bath without agitation at room temperature.
A high-speed steel containing 5 per
cent. tungsten, 5 per cent. molybdenum, 4 per cent, chromium and 1.5 per cent, vanadium, with carbon 0.8 per cent., has been Jeveloped. The heat treatment producing the best results for this steel comprises heating to 1,220 degrees C., holding for four minutes, quenching in oil warmed to 26 degrees C. until cold, heating to 550 degrees C. for one and a half hoUrs, holding for one hour, cooling in air, and then repeating this tempering treatment.
A helium-shielded arc-welding process has been successfully applied to welding stainless steel parts for exhaust manifolds, etc. A carbon electrode is made the negative pole, and is held.in the torch, to which helium is fed at the rate of about five litres a minute. The filling rod is made of a stainless steel, and the procedure is largely akin to that of oxy-acetylene welding.
• A new development in the application of stellite, high-speed steel and special alloying materials to lathe tools, milling cutters, and similar cutting tools, is the employment of lowcarbon steel core wire electrodes with a heavy metallic coating containing the alloying elements. In this way, a range of properties greatly in excess of that of ordinary high-speed steel deposits is obtainable, and care can be taken to fulfil the fundamental requirements of hot hardness and abralion resistande.
• Surface Cold-working Improves Endurance
Shot-peening, i.e., the direction on to the surface of a metal part of a stream of chilled iron shot under heavy pressure, has been found to increase • the tensile strength of a metal just below the surface, and sets up longi
tudinalcompressive stress in the thin layer of peened metal. A restricted amount of peening improves the fatigue
strength, but it is possible to workharden too much, which will, of course, lower the fatigue resistance.
Corrosion fatigue of steel is also improved by peening, which can he applied to irregularly formed parts that might be warped by heat treatment, while local peening is beneficial in the. fillets of shafting, where stress concen, trations arise. Coil springs and gear teeth are other parts that have been advantageously treated in . this Man. ner. Alternative forms of shot include malleablized-chilled-iron shot and hardened and tempered iron shot.
For post-war motof-vehicle manefacture, metallurgists are inclined to believe that the majority of require ments can be fulfilled by securing the right combination of manganese and the residual chromium and nickel present in the steel as a result of melting down scrap resulting from munitions and armaments.
New methods of rust-proofing and packing, developed originally for war purposes and likely to be of great • value in peace-time, include a spray of a plastic compound based on "vinyl
its " resins. This forms a flexible coating that can be rapidly stripped from the part. A second method entails wrapping in a sealed, transparent film-like material, whilst a third comprises sealing the -part with a dehydrating agent in a moisture-proof wrapper.