This material, in addition to resisting corrosion by the atmosphere, can "withstand attacks by various acids and alkalis. There is an impression in some quarters that the stainless features are only superficial ; this is not correct ; it is stainless right through.

The stainless properties are obtained bv addintr to the steel the alloy chromium in quantities of 12 to 14 per cent, and this composition persists through the body of the metal and renders the material stainless throughout. The addition of this alloy is also attended by other characteristics besides the resistance to corrosion, and consequently special precautions in manipulation are necessary. Chromium imparts to the steel a very dense and close - grained structure, and the mechanical tests are affected to a very marked degree, as will be seen later.

In order to meet the very wide range of purposes for which stainless material is being used, Edgar Allen & Co., Ltd., have found it necessary to introduce a variation in the carbon content. This has been accomplished without interfering with the stainless properties, and for purposes of classification one is called Imperial Stainless Steel, and the other Imperial Stainless Iron.

These two productions have many characteristics in common, such as their stainless properties, density, resistance t o tempering by the application of heat up to 500° C, but the different carbon content fits each one for its own particular sphere of service. For instance, Imperial Stainless Steel is suitable for cutlery, because the high carbon content enables a greater degree of hardness to be obtained, and in consequence a knife made from it will keep a keen cutting edge, whereas Imperial Stainless Iron, having great malleability, is used for sheets from which deep pressed articles are obtained with greater facility and freedom from cracking during the pressing operation.

Both these materials will resist oxidation to a marked degree when subjected for lengthy periods to temperatures up to 700° C, and, even at temperatures well above this,, scaling is scarcely observable.

To roll stainless steel satisfactorily, a very high temperature is found to be necessary.

Forging

It should be heated very slowly, and allowed to soak well at the forging temperature. Forging from the ingot is commenced at 1275° C. and not continued below 1I00°C, but it has been found that if the material has had much work already put into it, as in the production of bars from the ingot, forging may be carried out at temperatures down to 950° C. On no account should it be continued below this, as the steel hardens and may split. Operations in the nature of a drop forging should be carried out at 1250° to 1300° C.

Softening For Machining

To soften forgings or stampings made from Imperial Stainless Steel they should be heated to a temperature of 750-780° C, and may be allowed to cool in the air.

Full Annealing Temperature

Heat to a temperature of 850° C. and allow to cool in the furnace.

Hardening Temperature

Heal slowly and evenly to 950-1000° C. and quench in oil for a large section, or allow to cool in air for a small section. If extreme hardness is required and section of piece is plain, water may be used for quenching. However, there is risk of cracking by water quenching and consequently it is not recommended.

Tempering

When ordinary steel is heated in the atmosphere a thin film of oxide is formed on the surface of the metal. This varies in colour in direct relation to the tempering temperatures and forms a rough but ready means of measuring such temperatures. However, chromium steel, having stainless properties, is not nearly so susceptible to oxidation as ordinary carbon steel, and requires a much higher temperature to produce a given colour as the following particulars will show.

Stainless Properties

Although for domestic purposes stainless steel may be considered actually stainless, and although the same holds pood for many applications outside the domestic sphere, it must not be thought that stainless steel resists all acids. For instance, there are several well-known acids which have an effect on stainless steel e.g.,

Sulphuric Acid .... (H2SO4) Hvdrochloric Acid (HCL) Aqua Regia .... (3HCL HNO3) Citric Acid .... (though lemon juice which contains citric acid does not) Acetic Acid .... (CH3COOH) in certain concentrations ; though vinegar, which contains acetic acid, does not.

Polishing

Imperial Stainless Steel is capable of taking a very high polish, which is known generally as " mirror polish." This is obtained by submitting the steel to successive operations, each one being done by finer emery, and the final polish is obtained by a special preparation which is largely comprised of chromic oxide and mutton suet.

Cutlery

Imperial Stainless Steel is now used extensively throughout the cutlery trade, and many of the leading cutlery firms and goffers have adopted it exclusively, owing to its regularity of composition, ease of working, and the keen cutting edge which is retained by knives made from this popular brand. Knife blades are hardened by heating to a temperature of 950-1000° C. and quenching in wa ter. Tempering should be carried out at 150-200° C.

It is necessary that stainless steel be hardened in order that it may maintain its stainless properties, this does not apply to stainless iron.