• Heat-treatment Shop and Foundry
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Aid Production Drive Among Other Points Reference is Made to Eleetrolytic Descaling of High-chromium Steel in Fused Caustic and to the "Colouring" of
Stainless Steel
MANY lorry repair shops and other smaller ■.Tehicle manufacturing works do not have sufficient regular work of a type warranting the installation of a turret lathe, yet do occasionally need this kind of machine. It iS, therefore, interesting to note that a machine-tool builder in the United States has developed an attachment than can quickly be applied to an engine lathe to convert it into a turret lathe.
Various types are made, designed to be fitted either to the bed or the carriage. The turret body itself is in an alloy cast iron, cast with a cored vertical centre hole, the average section being about 6 ins. Horizontal tool holes are drilled in. The iron contains 3.1 per cent, total carbon, 1.4 per cent. silicon, 0.8 per cent, manganese, and 1.25 per cent. nickel.
Forging Cylinder Liners Forging of internal combustion engine cylinders is now being done in some works on heavy forging presses.
a The method is said to be cheaper than machining from seamless steel tubes or extruding in an hydraulic process. The press used is of the crankshaft-operated type, driven through multiple V-belts by a 150 h.p. motor. The forgings themselves are usually of nitriding steel and range from 41-64 ins, internal diam., weighing 25-125 lb.
Shafts and axles for automobiles are being kept perfectly straight during the process of heat-treatment by having all furnaces and. tanks designed so as to take the Shafts in the vertical position, and attaching each batch of shafts to a perforated carrier. From this they remain suspended during the entire treatment processes.
Considerable development has taken place' in the employment of ultrasonics for the examination of metals.
The Air Ministry has introduced a specification governing sandor chillcast cast-iron piston ring pots. The chemical composition of the iron is specified as total carbon, 3.4 per cent. max.; combined carbon, 0.6-0.9 per cent.; silicon, 1.6-2.4 per cent.; sulphur, 0.12 max. per cent.; phosphorus, 0.6 per cent. max.; manganese, 0.5-1.5 per cent.; nickel, 0.254.0 per cent.; chromium, 0.26 per cent. max.; copper, 0.2-1.0 per cent.; molybdenum, 0.5-1.2 per cent. Elasticity and transverse breaking tests arc also laid down.
Chromium molybdenum steel is being increasingly adopted for such tools as spanners. High hardness combined with a high yield point are developed by this steel suitably heattreated. The spanners are normalized and quenched, normalizing being carried out at 885-900 degrees C. to remove forging strains. Quenching is done in oil, from 845 degrees C. The tempering temperature is below 540 degrees C., and is designed to produce a hardness of about 46-48 Rockwell C (445-470 Brinell). This treatment gives higher tensile strength with rather lower impact strength. The steel is free from temper brittleness, is relatively inexpensive in the light of the properties it develops, .has free scaling during forging, thus reducing pickling costs, is easy to machine, and has few surface imperfections.
In some instances, carburetters are now being made as die castings, as are other commercial motor parts. • A special rod has now been dev4loped for welding white metal of die-casting. type, previously regarded as unweldable. The rod is said to give a weld strength equal to that of the parent metal. Another interesting fact is that oil pump gears for lorries, etc., are being made from pressed and sintered iron powder.
A new method has been introduced for loosening hard and adhesive scale on hot-rolled or forged material. The. chromium oxides formect at high temperatures are difficult for the pickling acid to attack, and, for this reason, the scaled metal is suspended for a few minutes in a large tank of molten caustic at about 400 degrees C, and made cathodic by aril electric current of about 25 ampeisq. ft. Sodium platesout on the metal and has the effect of reducing the oxides without attacking the underlying metal. Withdrawn and given a water quench, the steel appears fairly clean, and when dipped in nitrichydrofluoric acid comes out with a perfectly clean surface.
Free-cutting Stainless Steels As free-.machining stainless steels._ become popular, other new analyses are introduced. There are now as many as six different compositions marketed commercially. These cover low and high carbon, 12-14 per cent. chromium stainless steels, low and high carbon 16-18 per cent, straight chromium steels; and two types of 18/8 stainless steels. The straight chromium steels are usually rendered free machining by adding sulphur (in combination with molybdenum); . the austenitic stainless steels require either sulphur or molybdenum, or selenium and phosphorus additions.
A new lithium compound is being marketed for the special purpose of neutralizing furnace gases. It is
claimed that alloy' and carbon steels can be heated without decarburization, carburization, or scaling, whilst, as there is a surplus of the neutralizing compound, any oxygen due to water vapgur, air, carbon dioxide, carbon monoxide, nitrogenous oxides, absorbed or occluded gases, etc. is absorbed. The compound is contained in a cartridge, ;which' is placed in the furnace, and a carrier gas, generated within the furnace, carries the vapour evolving continuously' from the cartridge, and envelops the part being heated in the neutralizing atmosphere.
An American, Clements Batcheller, has invented a. simple treatment with sulphuric acid whereby stainless steel can be given a wide range of colours, ranging from gold to brown black. The colour depends on the temperature and the actual duration of the acid application. The steel is soaked in a solution of sulphuric acid at a concentration that would, ordinarily, strongly attack the metal, but by means of an inhibitor, e.g., potassium permanganate, dissolution is prevented. A film of the required colour forms on the surface and is due to oxidation of elements in the steel.
Light-alloy Blocks A silicon-aluminium alloy is being widely used for cylinder blocks and heads, and contains 5 per cent, silicon, with 1.2 per cent, copper and 0.5 per cent. magnesium. The mechanical properties of this metal are obtained and varied by suitable forms of heat treatment. It has a high degree of corrosion resistance, together with good
• mechanical strength at temperatures up to about 200 degrees C.
For pistons, another silicon alloy is of value, and contains 13 per cent.. silicon, plus small quantities of magnesium, nickel and copper. This metal has high strength at elevated temperatures, together with an extremely low coefficient of thermal expansion. It is readily anodized; which is important in connection with piston-wear reduction. Low thermal expansivity enables the piston to he simply designed, without compensating struts.
By adding magnesium to copper and nickel an alloy was produced of excellent quality for pistons. The material contains 4 per cent, copper, 2 per cent, nickel, and 1.5 per cent. magnesium. It has a tensile strength as cast and heat-treated of 18 tons/sq. in., and retains a considerable degree of strength even at temperatures as high as 500 degrees C., when it still possesses a tensile strength of 10 tons/sq. in. The . Brinell hardness' over the same temperature range of 0-500 degreesaC. falls from 100 to 65. The cast pistips are heated in an air muffle at 525 degrees C., quenched in boiling water, and maintained in the water for three or four hours.
Cast crankshafts for internal-corn bustion engines are being made from a cast metal containing 1.35-1.6 per cent. carbon, 0.7-0.9 per cent. manganese, 0.85-1.1 per cent. silicon, 1.5-2.0 per cent. copper, 0.4-0.5 per cent. chromium. Another development is the production of large quantities of barrelshaped valve tappets for internal-combustion engines by die-casting in a zinc-base alloy.
Greater use is being made of flamecutting machines as a substitute for sawing metal bars. It takes only half as much time as sawing and less metal is lost. It should be borne in mind, however, that certain metals are not suitable for flame-cutting owing to the possibility of ernbrittlement or cracking. Carbon and nickel steels appear to be the most satisfactory.
Electrolytic Polishing The electrolytic polishing of stainless steels is making great stiides. Much research work has gone into this process, and it is now stated that the best polish is obtained with an electrolyte composed of about 42 per cent. phosphoric acid, 47 per cent, glycerine, and 11 per cent, water, used at 100 degrees C., or higher, with an anodic c.d. of at least 0.1 amp./sq. in. Some organic additions to the phosphoric acid also prove useful. Mild steel and chromium steel can also be polished by the same electrolyte.
A new form of heat-treatment for grey cast iron has been introduced, and resembles the austempering treatment applied on occasion to steel. The iron ,is heated to 845 degrees C., held at this heat for a quarter of an hour, quenched and held at 265 degrees C. for another quarter of an hour, then allowed to
cool in air to room temperature. The damping capacity of the iron is not affected, but mechanical properties -are improved.
The nickel nitriding steels for gears, deserve special mention. They produce an exceptionally hard sutiace at a relatively low temperature (480-540 degrees C.), so that distortion is reduced to a minimum. Nickel nitralloy —a regular nitriding steel containing
3.5 per cent, nickel—is also a precipitation-hardening steel, developing in
the core, as a result of exposure to the nitriding temperature, a strength and hardness much above the original properties.
The steel can be advantageously machined to finished dimensions in a comparatively soft state after normalizing, quenching and a tempering at 650705 degrees C. The nitriding treatment then gives a wear-resistant case of 9501,100 Vickers, and the core is automatically strengthened to values exceeding 85 tons. per sq. in. tensile. The depth of the nitrided case varies from 0.01-0.02 in., according to the time of treatment.
To obtain economy of floor space in motor coaches, German manufacturers have been employing the horizontal type of oil-engined unit. These engines are principally two-row c.i. units, the two sets of cylinders driving a common crankshaft.
A magnetic thickness gauge has been developed for measuring the thickness of steel sheets within 1 mil, for any thickness in the 10-25-mil, range, and within 2 ma. in the 20-45-miL range. The instrument is not of large dimensions and is readily portable, whilst it has the advantage that it can be employed when there is only a single side of the sheet accessible.
A new corrosion-resisting steel contains 9 per cent, chromium and 1.5 per cent. molybdenum. It is claimed to be intermediate between 18/8 austenitic stainless steel and steel containing 5 per cent. chromium and 0.5 per cent. molybdenum. Another interesting stainless-steel development is the possi
bility. of using carbon monoxide as an inhibitor for preventing the corrosion of stainless steel by sulphur-bearing gases.
.Research and practice have now shown beyond question that the etching Of oil-engine cylinder liners by suitable agents will create on the surface of the bore of the liner a form of non-metallic non-abrasive coating which will prevent metal-to-metal seizure under borderline conditions of lubrication. Copper paint has often been used to act as a local protection of steel against carburization. It is also being used to-day for the reverse process, i.e., to prevent decarburization or soft skin. It is said to be just as effective as borax, but cleaner.
Hard-chromed Liners --An American development is the production of a thick and adherent, but porous and corrosion-resistant, tough chromium plating for preventing wear in engine cylinders. The porosity is designed to provide anchorage for the oil film, and the corrosion resistance is to prevent attack by chemical reaction. One concern has used a thick dull plate of similar type for cylinder lining in this country. Attempts are now being made to apply the same plating to different kinds of piston rings.
Tappets of chilled cast iron are now being produced in large quantities. Valve inserts are also being made from cylinder iron with the addition of about 1.5 per cent, of chromium. Molybdenum is likewise employed in quantities of 0.5-5.0 per cent.
Nitrided liners are being made from an iron alloyed with aluminium, chromium and molybdenum (about 1 per cent, of each element). Vanadium is also sometimes present. Thesliners are heat-treated and machined before the nitriding treatment, and are ground to dimensions as a final operation.