Steel Treatment in the News
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Prominent Among Metallurgical Developments are Those Concerned With the Amelioration of Nominally Lowergrade Steels to Enable Them to Undertake Abnormally Stringent Duties
ANTI-FRICTION bearings made of cast iron, which, in practice, has proved to be a good replacement for bronze, make use of a metal with total carbon 8.2 to 3.0 per Cent., silicon 2.2 to 2.4 per cent., manganese 0.0 to 0.9. per cent., phosphorus 0.15 to 0.25 per cent., sulphur 0.12 maximum, chromium 0.2 to 0.35 per cent., niekel 0.3 to 0.4 per cent., and copper 0.2 to 0.3 per cent.; 0.1 to 0.15 per cent, of aluminium is sometimes added,
" InoCulated " cast iron, designed to replace hematiteiron in the making of large castings, such as machine-tool worktables, is expanding its fields of
application. The advantage of the " inocnisted " iron is that it possesses improved machinability and minimum tendency to distortion.. Improvements in ordinary iron obtained by inoculation with ferro-silicoa are of the order of 63 per cent, better impact value, 30 per cent. better transverse strength, 28 per cent, better deflection, 23 pei cent, better tensile strength, and 2per cent, higher hardness.
War-time developments in the beat treatment of steel are seeing the light ofday. In isothermal heat treatment, transformation takes place at about the same time throughout the part, thus preventing high stresses. The designer is, therefore, able to take for granted that, in numerous instances,
• the grinding of surfaces to be subjected to high loads may be carried out in advance of heat treatment, or. elirninated altogether.
Ascertaining Case, Depth
A novel way of ascertaining the case depth in ease-hardened steel is being
employed; it . is most valuable for examining .a large number of specimens
of equal size. A gradual taper is ground on a specimen hardened in the standard Manner, and the Rockwell C hardness numbers are determined at brief intervals along the surface. The readings thus give, the hardness at • different depths from the original surface.
• Low-frequency induction heating is being applied to the expansion of engine parts, in order to shrink them on, to bushes` or spindles. The current employed is 60-cycle a.c., and the heating period necessary is less than five minutes.
Ferro-manganese (75 per cent,) has been used with some success as a powdered brazing alloy in the tipping of high-speed steels containing chromium, tungsten, molybdenum and vanadium in relatively low percentages. The most satisfactory results were obtained when the ferro-manganese contained 20 to 25 per cent, of copper.
The tipping of cutting tools with hard materials is still the subject of much research, and recently' a. number of experiments has been made with forged tips of low-alloy steel containing 0.9 to 1.0 per cent. carbon, 4.5 to 5.3 per cent. chromium, 3.5 to 4.0 per cent. molybdenum, 2.0 to 2.5 per cent. vanadium, and 0.4 per cent. max.
silicon. These tips are secured to the tool shank by mechanical means, and are said often to be equal to and, in certain respects, better than, tungsten high-speed steels.
A revolutionary and most importantdevelopment Is the application of atomic hydrogen welding to the building up of worntools, the repair of damaged tools, and to hard facing tools. Until recently, the atomic hydrogen process had not successfully . been applied to high-speed steel because the carbon content of the deposited cutting-tool steel could not be controlled, and, in consequence, the steels lost some of their carbon and were too soft for efficient cutting.
The new patented process, known as " Athyweld," has solved the problem of carbon control, with the result that the best "putting on" tool known is now .being commercially applied on an
ever-increasing scale. Results in use are extremely satisfactory, and worn tools have been rebuilt many times and at extremely small cost.
There are nertain limits to the applicability of the process, but it is undoubtedly one of the most remarkable and attractive of all the existing tool-tipping processes, and bids fair to revolutionize both the use of cutting tools and the cost of such tools as dies, press teols, etc.
Press-quenched Gears To eliminate the warping of gears in engines, press-quenching in a press is being employed, and a machining procedure is being developed whereby the teeth of the gears are shaved in advance of heat treatment to dimensions allowing for the extent of the distortion that will result. This shaving operation is much speedier than grinding.
Automobile makers in the United States are establishing a common code system 'for the identification of tungsten and molybdenum high-speed steels, Each code consists of letters and numbers indicating the composition of the steel and the name of its manufacturer.
Modern pickling plant incorporates ingenious Mechanism for causing the material being treated to move while immersed in the tank. Where this movement is provided and is sufficient, pickling time is halved as compared with " still " pickling. In one Such Mechanism a rocking beam is pivoted between two baths, the charges being hung from the beam
ends, thus giving them an up-anddown motion. Another arrangement of cranks gives, motion in a horizontal plane, Whilst in a third type the suspended baskets are swung like a pendulum, Investigations into the best method of producing satisfactory, wearresistant chromium platings have been carried out, and the most effective results -nere obtained when an electrolyte containing 150 g. of Cr03 and 1.5 g. of 11,SC% per litre of water was employed at 50 degrees C., with a current density of 40 amp. per sq. in. X-ray examination is. proving a good non-destructive Method of checking the ability of -threanium platings to withstand wear,
C.,arhon In Galvanizing Sheet
The influence of the carbon content of -steel sheets on their galvanizing pro
perties has been studied. 'It appears
that if the steel sheets be first sandblasted, no effect is exercised by the
carbon, but if they be hot-dipped alter
only pickling, uneven coatings with dark patches are produced, and.qualitY
falls off with -increasing carbon. This is no doubt the result of nen-uniform attack by the pickling solution. Zinc coatings have also been found to improve if not more than 0.25 per cent, of copper be present.
Attempts have been made to increase the fatigue strength of the teeth of already 'finished engine gearwheels.
The steps taken include enlargement of the bevelling at the shoulder of the tooth, altetation of the thickness of the rim, and elimination of the grooves in the tooth gullet caused by grinding and polishing.
A novel instrument, of use in testing hardness, is based on measurement of the coercive force or the_demagnetizing current. The instrument is calibrated, using specimens of known hardness, and with a known microstructure. The test occupies inue'h less time than the
13rinell hardness test, and, in contrast with it, is not affected by surface roughness, oxide -films, or surface deearburization.
Centrifugal casting continues to make great progress, and already
centrifugally cast steels have been produced which possess strengths of the orderof 85 tons per sq. in., and a ductility of 10 per cent.
Presses have been devised for forming parts out of sheet metal by placing the blank over a block of the required form which rests on a plate on the bolster. The -descending ram has layers of rubber cemented togetheron its face. • The sheet nietal is formed around the block by the rubber under pressure. The advantage of this process is that it produces shapes that cannot be had by pressing with metalto-metal dies.
Siliconizing of steel is finding favour for giving a special case to this metal and to iron. The parts are packed in either silicon carbide .or ferrO-silicon and heated to approximately 1,000 degrees C. Chlorine is introduced into the container, or into the chamber of the furnace, in order to speed up the reaction. The siliconized case is typically soft and ductile, -and has a Rockwell hardness of 80-85 B. The case retains oil in microscopic pores in the surface, and, in this way; parts that have been siliconized can be made selflubricating merely by boiling them in a suitable medium.
Lithium is being used as a means for maintaining furnace atmospheres in a neutral condition and quite free from water vapour in the heat treatment of steel parts. A partly burned gaseous fuel is first employed to vaporize the lithium so that the mixture may enter the muffle containing the parts to be treated. There can be no water vapour present in the atmosphere when lithium is introduced, and in this dry condition the atmosphere is inert. Thus, steels having varying carbon percentages may be heat treated without oxidation or loss of carbon. , An interesting process has been devised for the tinning of cast iron, so as to give a bond for babbitt metal in . making bearings. The graphitic carbon in the iron stops the tin from adhering, but in this process the part is immersed in a molten salt bath at 370 to 510 degrees C. This bath contains a catalyst to promote oxidation of the carbon to carbon dioxide, whieb combines with constituents of the bath to form carbonates. The part is then rinsed, placed in a Second alkaline bath at 120 degrees C„ again rinsed, plunged for about 30 seconds in 20 to 30 per cent, hydrochloric acid, passed into a bath of flux, and then the tinning bath.
Electro-leaded Steel Lead is being electro-plated on to steel using solutions of either fluosilicate or fluoborate. the former being the more economical for large-scale opera
tions. The latter gives a successful deposit straight on •to steel, and the plating is finer in grain and denser than that given by the iluosilicate solutions.
In Russia, a phosphatizing process for protectively coating steel is being used with manganese phosphate as the basis. The bath is a 3 per cent. solution of manganese phosphate and . iron phosphate in the ratio of manganese to iron between 6 and 9.
Two methods of testing casehardened bars have developed. One is a bend test, the bar being freely supported on 8-in. :.entres, and pressure being applied to a test-piece 101 ins. long by /76in. diameter by a 12-in. radius bend block. The first crack in the case is accompanied by a clear and distinct sound, and the deflection in inches when this sound is beard is called the " case break point." The second test comprises allowing a 45 lb. blow to strike the bar midway between the two supports.
The hardness and service life of highspeed cutting-tool steels is being improved by treatment in a ,cyanide bath before the ordinary hardening process. The treatment comprises maintaining the Steel for 90 minutes in a sodium cyanide bath at 790 degrees C.,' cooling in air, cleaning, heating the steel to 1,260 degrees C. (slightly lower for the molybdenum high-speed steels), oil quenching and tempering twice for one hour at. 565 degrees C. A hardness of Rockwell C 68-70 is obtainable at the core ot 1-in, round bars in this manner.
The patenting of wire in sodium nitrate baths, instead of in molten lead, is being successfully achieved in Germany. The warping of steel components by heat treatment is being prevented in gears, shafts and shafted parts, and in symmetrical flat forms by controlled quenching while the parts are .still within the die. Die-quenching machines of the rotating type are used, contact between the quenching medium and the part being centrifugally controlled. In another type of quenching machine the part is lowered mechanically into the quenching medium. Boron introduced into the coating of monel metal (high nickel-copper alloy) welding electrodes seems to function as a flux, and allows thorough binding between the monel metal and the 'cast iron, so producing a successful weld, A new process allows aluminium cooling fins to be permanently bonded to the steel cylinders of engines, but as this is one of the many "official secrets," details of the process cannot be given. (See " Light . Metals," 1943/6/327.) A method of cutting sheet steel with an oxy-acetylene flame, by which distortion is eliminated, is another new -° advance. The plates are actually cut under water, and it is suggested that the water carries away more' swiftly the heat generated by the flame, thus lessening the hardening of the cut edge that normally obtains.
Parts to be tested by the magneticpowder method method are sometimes given a flash coating of cadmium to provide a bright background, against which the magnetic particles may stand out sharply. Cadmium having become.less plentiful, a substitute has had to be found, and an organic coating has been developed that is simply prepared, costs less than cadmium, and is applicable with a spray gun, whilst it has
proved suitable for the purpose.
Nitriding Straight Steels It has been discovered that nonferrous parts on gas-welding plant that have to withstand corrosion by moist oxygen on heating may be satisfactorily replaced by plain carbon steel rendered corrosion-resistant by nitrid
ing under suitable conditions. The plain carbon-steel parts are usually nitrided at 550-620 degrees C, for 30 to 90 minutes in ammonia dissociated to the extent of 25 to 40 per cent. The parts are permitted to cool to 60 degrees C. in the gas. The thickness of the nitrided layer amounts to 30 to 40 A. Resistance to both corrosion and wear is satisfactory. Nitrided carbon steel becomes passive in water, its potential approaching that of stainless steel.