Metal Processing Techniques Make Headway
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Surface Treatment for Hardness or Corrosion Resistance, and Mechanical Treatment to Give Improved PhysicalProperties at Metallic Surfaces, Feature Largely in Current Advances
KNOWN as toe Fraser-Heller system, a novel method has been evolved for degreasing metals. in this process, the first' stage is to take the parts to be degreased and treat them with trichIorethylene. vapour. then subject theth to a concentrated rain of the condensed solvent. This condensate is specially treated to prevent it froth carrying any grease acquired during the previous opera-. -Eons.
An American • method is said to lengthen the service life of high-speed steel tools by means of a superior chromium-plating process. A frequent difficulty with chromium plating in the past has been that the metal absorbed hydrogen during deposition and became brittle. In the new process, a water solution is used containing 50 or. of 99.75 per cent. chromium trioxide, plus 0.5 oz. of concentrated sulphuric acid.
Allowance has to be made for the sulphate content of the chromium trioxide when adding sulphuric acid to the bath. When plated, the tool is washed well in cold water and heated in an oil bath at about 175 degrees C. for mm hour, then cooled in the air. The oil treatment, it is claimed, pre-. vents the hydrogen from causing brittleness. • Adhesion Improved by "Spark" Etching
A simple way of preparing hard steel surfaces for metallization, i.e., spraying with metal, has been developed. This preparation stage comprises roughening to give better adhesion. A range of six high nickel:alloy electrodes is clamped between cooling •discs in a holder, and these and the steel to be treated are connected by flexible leads to the secondary coil of a small transformer. The surface to be. coated is stroked lightly and rapidly with the electrode ends. Heat is generated at the points of contact and causes a "metallic foam " to be fused on to the base metal. This roughened surface then makes an extremely. satisfactory foundation. It is known as the Fusebond process.
Etching-part numbers, trade marks, etc , on metals such as tool steel; rustless steel, • stellite, etc., has. been facilitated by a recent invention using a standard waxed stencil on which are typewritten or printed the desired words or Marks. This is inserted between the metallic part. constituting the anod'e, and an absorbent pad, peoviiled with a supply of etching liquid linked up with the cathode ..)f a 15-volt d.c. circuit Three further methods of producing a black finish on steel are now available. In one, the steel, cleaned and' pickled, is dipped into 'a concentrated alkali bath at approximately 165 degrees C., which, in 10 minutes, pro
climes 'a film of iron oxide -(Fe3Cil4), unaffected by indoor moisture. The next method is phosphatizing. The third comprises coating the steel in the normal way with zinc, and afterwards. blackening the zinc by immersing the work in a proprietary Solution (e.g., niolybdic acid), which does not give higher resistance to corrosion, but gives a superior surface for painting.
An instrument has been devised for continuously testing the composition of heat-treatment furnace atmospheres. It -detects, electrically, any variations in the heat conductivity of the gaseous iniXtirre.
Sprayed or brushed on wax coatings. are being more and more employed in the United States as a means for giving finished and semi-finished metallic parts increased resistance to corrosion.
The liquid nitriding of tool steel is said to raise the hardness from 850 to 1,100 -13rinell. A mixture of 60 per cent.. sodium cyanide with 40 per cent. by weight of potassium cyanide is employed, and must be aged before use. Maximum surface hardness is attained after ithmersion for a period of two to three hours at 565 degrees C.. A longer period gives a deeper case, but rather lower surface hardness.
Abrasive Compound with Novel Binder
In America, a type of proprietary abrasive compound is being employed as a means for giving engine parts subjected to severe stresses.a polished surface. This eliminates fatigue failures. The material used is a Cold glue with a sodium silicate base, and contains aproportion of abrasive grains. A. polishing wheel is brushed with this compound, which takes less than eight minutes to set, and whenever desired a fresh coat will restore the wheel.
In a new method of paint spraying, the part to he sprayed is surrounded by an electric field arid earthed, so as to attract the sprayed particles. The system is also adaptable to methlspraying plant.
An induction hardening plant has been installed at one large .works in 'which a range of .oil-engine parts from
• 7/16-in. dia. up to • 6-ins. dia. are quickly heat treated by the employment of various fixtures and coils in a single machine.
Three methods of mending cools by welding and brazing are being used in Canada. One is low-temperature silversoldering of fractured high-speed steel cutting tools of comparatively small cross-section. Another is welding the shanks of tools. The third is building up worn, or fractured tool edges by means of electrodes of high speed steel.
A porous coating on the side face of piston rings causes the retention of oil, which is advantageous. Such a coating is being obtained by depositing chromium to form a coating 0.004-0.006-in, thick, using. a high current density. The current is afterwards reversed to set up a stripping action, which produces a porous layer 0.001-0.0015-in. thick. '
To prevent the Corrosion of zinc coatings on steel, the part iS first Lleansed, and then dipped for 5-20 secs. in a eMution of 200 g, .-5odium diehrunate and 6-9 ce, of concentrated su/phuric acid per litre of water. The work is then lensed and dried.
Manganese in the Stainless Steels High-manganese stainless steels are now attracting fresh attention,, although they were developed in Germany as long ago as 1936. They may be of either cltromium-manganese or nickel-chromium manganese type. They are readily worked, welded and built up, and are claimed to be more 'resistant to intercrystalline corrosion than the ordinary nickel-chromium stairdeSs steels, particularly whet welded.
Among the developments' in conUrinous case-carburizing may be mentioned the production of a carburizing furnace heated by radiant tubes and fitted with fourlines of independent muffles, each with its own pusher and timing mechanism, so. that .parts of different dimensions, necessitating different cartnirizing times, may he operatedthrough.. the furnace at
different speeds. • A novel quenching machine presses batches of steel parts emerging from a hardening furnace against a Axed upper platen by means of a movable lower platen. At the same time a water-spray at high pressure is directed against each side of the part for a period of approximately one minute. .
Dry cyaniding case-hardening is a new process tieing adopted in some works. It is done by raising the parts to be treated to the correct temperature in a furnace or muffle with a neutral producer, town, or coke-oven gas atmosphere, to which anhydrous ammonia is added. The heating period is about two hours, at about 845 degrees C. Small parts are shovelled on to low-sided perforated trays and pushed througn the furnace. The case depth, obtained is 0.007-in. min. The period may be abbreviated by using more ammonia and slow cooling. The process depends on the formation of cyanogen compounds by reaction between the ammonia and carbon.
high-speed steel is being -welded to• carbon steel by a method in which the parts are preheated and afterwards welded with electrodes of stainless steel. The type of work done includes lathe tools, lathe centres, screw threading taps and. gauges
One of the most fascinating developments of recent lines is the introduction of cold treatment for metals. Sub-zero temperatures are being employed as a means for obtaining shrinkage fits in assembling engine parts, as well as for
seasoning gauges and for giving tool steels additional hardness. There are many advantages in shrinking the inner part to obtain a fit rather than expanding the outer. For example, the metallic structure is less likely to be harmfully affected by cooling than by heating. There is less. likelihood of warping, whilst ths refrigerated parts are easier to handle than heated parts. Special industrial . refrigerators are being built :for this work. Care should be taken to avoid the use of excessively low temperatures as irreversible changes (i.e., austenitento Martensite)may be provoked.
Crack Detection by Fluorescent Medium " Hyglo " is a new crack-detection process for metals, plastics and ceramic materials. The piece to be tested is immersed in a fluorescent liquid and afterwards studied by ultra violet light. No washing after immersion is needed, as the parts may simply be dried in • a wire basket for about 30 secs.
Machine flame-cutting is bein g improved bY the development of machines for cosy-acetylene cutting of stenl plate Special methods of pre • parb'mg the plate edges are adopted. Some machines .possess as many as three nozzles in close sequence, thus enabling " multiface " cutting to be performed.
Band saws for cutting steel give the maximum service life if theteeth be hardened down to their base, i.e.,. to a line just touching the bottom of the gullets. The requisite depth of hardness is secured by subjecting the steel strip from •w hich the saws are manufactured to two forms, of heattreatment.
• Forging dies have been developed for forging differential-gear bevel pinions requiring no later machining. The dies are made to fit a coining press of 2,000-tons capacity, making 80 strokes a minute. By leaving a depression in the rear face of each tooth,' large enough to absorb the displaced metal, fracture of the clie caused by the swift cooling of the thin flash is prevented.
Decarbinization of the surface of the molybdenum alternative high-speed steels liaVbeen greatly decreased by the expedient of using carbon blocks or by
coating -the steel with compounds having a base of borax or copper-oxide.
In Germany, chromium-manganese steels are being used as substitutes for nickel-bearing -steels, and these are heat-treated in electrically heated salt baths.
Bolts have been investigated from , the point of view of preventing their faijure from fatigue. It has been found that' a bolt or stud should be tightened to a load exceeding the maximum .work-, jug load. This lessens the fatigue stress range and lengthens the service life of the bolt. Pre-stressing the bolt by peening,' and pre-stresSing the roots of threads by roffing•likewise increase -die fatigue strength. An interesting observation is that pitting of gear teeth may occur even when no load is transmitted, if the gears be running submerged in oil, for the reason that the oil is trapped between the teeth, sometimes at very high pressure.
"' Steels treated with certain new hardeners, Bortam and ,Carbortarn, containing boron and titanium, have their depth of hardening increased. The boron-containing steels are virtually as strong, as similar untreated steels containing 0,25 per cent. molybdenum, and have . greater toughness combined with equal ductility, though at relatively high temperatures are not so strong.
Local casehardening, i.e., where chosen areas are required soft, is best done by means of copper, which protects against pack carburizing, gas carburizing and salt-bath carburizing. The thickness of the copper plating necessary depends on the period of immersion in the cyanide. bath, and ranges from 0.0005 in. for one hour to 0.002 in. for four hours. Even better than copper plating is to leave extra material on the part, but this is not always feasible.
New Design for Vertical Furnace
A new 'vertical cylindrical heat treatmentfurnace enables temperatures up to 955 degrees C. to be obtained. It is manufactured in various standard sizes up to 72 ins, diameter by 84 ins. deep. It may be heated by gas, oil or electricity, and heat is swiftly transferred tct the charge by means of a circulating fan in a separate chamber. Long parts are hung from a special fixture and can he heat-treated without warping. Cylindrical furnaces of this type are being used for nitriding cylinder liners, refining and hardening gears, hardening steel, etc.
A stainless steel containing approximately 16 per cent. chromium, 2 per cent. nickel, -and 0.25 per cent. (maximum) carbon has been adopted Officially in this country, aud has great Corrosion resistance. .