1938: Metallurgy

Stainless Steel.

On Dec. 13, 1938, more than a hundred industrial leaders gathered in Pittsburgh to celebrate the twenty-fifth anniversary of the discovery of stainless steel. This was a significant fact in many ways, particularly in that it dramatically marked a new trend in the science of metallurgy. For years man had practised the art of extracting metal from ores, thereby creating the science of metallurgy. With the discovery of stainless steel, however, the effort of the metallurgist has been turning more and more to the art of combining various basic metals to create new products or a product capable of performing a specified and predetermined use.

Twenty-five years ago two men, one in England, Harry Brearley, and one in the United States, Elwood Haynes, started the era of chrome-bearing stainless steel. A third, Dr. Benno Strauss of Germany, almost simultaneously with Brearley and Haynes, conceived an alloy containing not only iron and chromium but also nickel. All had this one thing in common — stainless properties of alloys. Out of those discoveries has come a great new era of metallurgy.

From this discovery, too, have come innumerable brands of compositions, variously estimated at one thousand or more different combinations. One of the most significant recommendations in this twenty-fifth anniversary gathering was that to standardize some thirty types, thereby making possible greater economies and more stable products. Whether or not this is done, the fact stands out that the achievements to date have been primarily to concentrate attention on the great variety of products and the versatility of metallurgy in meeting today's commercial demands. An overlapping of types and a confusion of specifications may have threatened, but the movement now set on foot should go a long way towards correcting those unfortunate tendencies. No stabilization is expected which will rob alloyed metals of their prime virtue, namely of commanding a specification best suited to the needs of the resulting product.

Refining and Processing Iron Ores.

Next to oxygen, silicon and aluminum, iron is the most widely-distributed metal and the largest part of the solid material in the earth. It constitutes probably a little over 4 per cent of the earth's crust. That most notable advances have been made in the refining and processing of iron ores should, therefore, occasion no surprise.

Among the more recent discoveries of vital importance are those relating to the microscopic study of metals, high-temperature and creep properties, as well as many others affecting not only the development of new alloys but the improvement of existing alloys and the adaptation of these materials to the special requirements of modern engineering. Especially noticeable has been the work of the research metallurgists in contributing to engineering knowledge of such important factors as mechanical and corrosion fatigue, creep under stress at both normal and high temperatures, factors which influence brittleness in steels and machining properties and, above all, the intensive study of reactions of corrosive agents, whether from atmospheric or from other causes.

Great improvements have been made in the furnaces in which metals are refined. The use of forced draft and then the application of electricity have each marked advances. Today practically all stainless steels, so-called, are made in electric-arc furnaces, most of which have a capacity of thirty tons. Thus tonnage production for some of the stainless types is being ushered in.

Uses of Alloys.

Nickel steel was introduced about 1890. It was a decided innovation and had important results upon the industry. Today alloy steels are working an even greater metamorphosis. They are making possible better automobiles and airplanes, new light-weight trains, and causing a tremendous revolution in all lines of transportation for these steels are making possible stronger and lighter carriages. These are but the beginning, for metallurgists foresee even wider and still newer uses to which the metal may be put.

Pure aluminum, while possessing many valuable engineering properties, is too weak and soft for a number of engineering purposes for which it is otherwise naturally suitable. Many efforts have been made to improve its application by adopting alloys. In 1911 duralumin was developed and has found extensive use in the construction of Zeppelins. There has followed what are known as the 'Y' and 'RR' aluminum alloys. Magnesium alloys have been developed to provide an ultra-light material possessing reasonable strength.

Iron and Steel Alloys.

Out of some forty possible elements which might be intentionally added to iron or steel, there is a surprisingly small number which, up to the present time at least, have been found to have sufficiently beneficial efforts, at a reasonable cost, to warrant investigation and commercial development. Without regard to relative importance, these are manganese, silicon, chromium, nickel, molybdenum, tungsten, vanadium, titanium, aluminum, zirconium, copper, cobalt, and the non-metallic elements, phosphorus, nitrogen, sulphur, selenium and oxygen. Carbon is invariably present in all steels except in the instance of certain grades of the highly-specialized stainless alloys, where the carbon content is only a few hundredths of one per cent.

Chromium is the only alloying element which has been found to produce in iron alloys a condition approaching complete resistance to atmospheric corrosion. Chemical and metallurgical tests indicate that more than 11 per cent chromium is necessary to provide this characteristic even in pure iron. The presence of carbon in the commercial alloy necessitates an additional amount of chromium because, under certain conditions, each unit of carbon may combine with as many as eighteen units of chromium and render that portion of the alloying element ineffectual in its protective function. The remainder of the chromium forms a homogeneous solid solution with the iron, and imparts to the alloy its notable resistance to corrosion.

Of all the alloys, however, the greatest progress in recent years has been made with the use of chromium and nickel. Chromium mixed in small amounts with steel improves its hardening qualities. In a quantity of more than 10 per cent of the whole mix it prevents rust. This type of steel is used in the making of tools, machinery parts, and stainless, heat- and acid-resisting steels.

Nickel increases the toughness, stiffness, strength and ductility of steel. When used in large amounts the steel will resist heat and acids. This steel is used in the making of tools, machinery parts, and stainless heat- and acid-resisting steels.

When using both chromium and nickel in the steel it acquires unusual characteristics. It is this type of steel which is today commonly known as stainless. The most common prescription is 18 per cent chromium and 8 per cent nickel. A variation of these percentages is used in order to obtain special results. This steel may be grouped under three classifications.

Classification of Stainless Steel.

The austenitic steels are those in which nickel exceeds 7 per cent, and nickel plus chromium exceeds 24 per cent, with or without moderate additions of other elements. These steels are normally non-magnetic. While they cannot be hardened by quenching, they may be hardened materially by cold working and in such cases become slightly magnetic. In the annealed condition they possess higher ductility and toughness than ordinary steels.

The ferritic steels contain chromium in excess of 16 per cent and 0.12 per cent maximum carbon, with or without small additions of other elements. These steels show no significant transformation on heating or cooling and hence remain essentially ferritic at all times, and are magnetic. They are only slightly hardened by quenching, and are hardened only moderately by cold working. They possess relatively high strength and when properly annealed are quite ductile.

The martensitic steels have chromium of 15 per cent maximum, carbon 0.12 per cent maximum (although in stainless cutlery steels the carbon may run as high as 0.30 to 0.40 per cent), and other elements, if present, not exceeding 2 or 3 per cent. These steels are ferritic in the annealed condition, but when cooled rapidly in air or liquid media from above the critical point, they are definitely hardened and hence called martensitie. They may be hardened and tempered as ordinary carbon steel, and possess excellent physical properties of strength and toughness. They are magnetic.

It is significant that these more recent and outstanding developments in the metallurgy of steel have resulted from a more profound study of the uses to which the product is to be put. This approach to metallurgy is destined to have its important effect upon all phases of its practice.