Processes that Improve the Performance of Steel
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A Metallurgical Authority Gives Some Details of the Valuable Results Obtained from the Hardfacing, Chapmanizing, Shorterizing and Nitriding Processes and Deals with Directions in which they are Employed SIDE by side with direct developments in steels go developments in processes designed either to improve the performance of a steel, or to give to it properties which alone it would not possess. These advances have resulted in some important advantages from the point of view of the designer and the manufacturer. As a characteristic example, the chromium plating of various parts and tools, referred to in a previous article, may be mentioned. There are, however, other processes of possibly even greater importance.
Among these, the hard-facing process is of definite value. This process is not absolutely new in origin, but it has, in the past, had its full development delayed by difficulties, the majority of which has now been completely overcome, so that hard-facing is rapidly becoming more and more popular as a means for increasing the resistance of steel parts to wear. .
Hard-facing is a method of welding a coating, edge, or point of a special type of wear-resistant metal directly on to the parts required. It is claimed that a hard-faced surface will outlast an ordinary steel surface by as much as 20 times or more, depending on the kind of facing metal employed. The hard-facing treatment is generally given to parts which_ have already been in use and subjected to wear, and it builds them up to a condition in which they are virtually new again.
Uses for the Hard-facing Treatment.
The same treatment, it is important to note, can also be given to new parts. Three different types of metal are generally used for facing purposes. For wear, combined with some abrasion, it is customary to use a high-alloy steel having a good degree of strength and toughneSs, especially where the parts are likely to suffer from shock and impact.
When parts have been restored by the hard-facing treatment, they possess at least twice their life at barely a fraction of the cost of new parts. If shock and impact are less likely to be encountered, tungsten carbides, or hard non-ferrous alloys containing cobalt, tungsten and chromium are used as the facing material. Guides, valve seatings, bearings for shafts and many other parts can be, and have been, successfully dealt with in this way.
Another method having an economic value is the Chapmanizing process, which is of American origin. The object of this process is to give to intricate steel parts manufactured from low-priced or medium-priced steels a surface hardness of glass-hard character with a practical freedom from warping or distortion.
There are certain hardening elements 8.36 which are capable of absorption by steel and ferrous alloys when subjected to a temperature above or below the critical. The process is based on this fact, and ordinary carbon or free-cutting steels can be employed as an alternative to expensive alloys, for such parts as demand a hard surface.
The treatment gives to the steel a glass-hard case, but this is somewhat ductile, with consequent minimizing of chipping or flaking of the case. Parts which have been subjected to the Chapmanizing treatment are said to withstand abrasion nine times better than ordinary case-hardened parts.
The Shorterizing Method.
Then there is the Shorterizing process, which was first introduced some years ago, and for some time was scouted, but which is now recognized as a perfectly satisfactory method of hardening the surface of metals. It is not in any way a case-hardening process proper, but a method of heating and quenching metals in order to produce a great increase in surface hardness.
The hardening is carried out by mechanically passing an oxy-acetylene flame, suitable in size and degree to the area under treatment, over the surface to be hardened, and following this with a cooling jet to quench the heat imparted to the surface. This cooling jet immediately succeeds the heating flame as it crosses the surface of the metal, so that quenching takes place the moment the steel is brought to the critical temperature.
Quenching Problems.,
As not every steel, and particularly not every alloy steel, can be properly quenched by means of a water jet, and an oil jet is not practicable, a nitrogen jet, or sometimes air, is occasionally employed. Thus, the quenching method is variable, according to the material. Worms, bevels, helicals, cams, gears, spindles and many other parts have been satisfactorily hardened by the Shorter process. A great saving in time is claimed over other processes, together with reduced labour costs, more accurate control and a lessaffected core.
A process of definitely growing interest and importance is the metallization process. This consists of the spray ing of pure metal on to virtually may metallic or non-metallic surface, without the employment of fluxes or acids, and with no prior heating of the part to be built up or coated.
The process it carried out by means of a small gun, made up of an air turbine linked up with reducing gears and feed 'rollers for automatically feeding wire through the gun to a nozzle supplied with cxygen and acetylene. When the wire comes into contact with the oxy-acetylene flame, it is melted, and compressed air then blows it at high velocity on to the surface to be metallized.
Surfaces which have been coated in this way can be built up to reasonable thickness and can be filed, ground or polished. One point that must, however, be borne in mind is that metal-. using cannot be applied, as hard-facing can, to parts which are already worn, but is applicable only to entirely finished components. •
It has the great advantage that, by its means, parts can be put into such a state, that they resist corrosion, erosion or abrasion. Shafts, in particular, lend themselves to metallizing, and bearings which have been coated by this process with babbitt metal will be _ found to last considerably longer.
The Nitriding Process.
The nitriding process has not developed quite so remarkably as seemed probable when it first appeared. The gas employed for nitriding purposes is rather expensive, and in the process of nitriding, a certain wastage of this gas seems unavoidable. Furthermore, there has been some difficulty in securing containers made from a material which would withstand, for a sufficiently long time, the conditions involved.
High-alloy containers have, to some extent, overcome this trouble, and the process of research and experiment is continuous. By the nitriding method it has now been found feasible to give a glass-hard surface to one or two of the cast and malleable irons, and to a few of the ordinary steels. Two interesting variations in the process have, furthermore, been the subject of patents.
In the one variation, ordinary steels, low in carbon, are subjected to active nitrogen led into a salt bath, after which a quenching treatment is given. In the other variation, molybdenum, chromium or vanadium alloy steels, or steels with one or more of these elements in combination, are subjected to activated nitrogen 'at a low temperature in a pressure-tight container. This produces an extremely hard case and no quenching treatment is afterwards necessary.
To review every recent development in the technique of a further range of sieel processes, the welding methods would require a separate article., or series of articles. Here, however, one may mention in detail one new develop
ment in this direction. This is a new gas-electric welding process, which has the advantage :that it produces no burr or ridge at the point of union.
When the welding of mild steel is in question, the joint is heated by means of an acetylene, oil, or petrol flame, which has an excess of carbon. The moment a specific temperature is reached, the surface, which has a thickness of only a few tenths of a millimetre, absorbs carbon, has its melting point lowered, and the surface metal becomes molten, while the rest of the material is still in the solid State. If the joint surfaces be close together, this molten metal is drawn up by capillary action into the joint, and thus completely fuses the parts without the creation of any burr or ridge. Welds made in this way have given favourable results under test.
In making steel, one of the most essential processes, in many instances, is the descaling operation. A new descaling process of electro-chemical character, known as the Bullard Dunn process, has been invented. By its means, all pitting, etching, smudge formation, etc., are eliminated.