Effect of Sulphur in Carbon Steel

Effect of Sulphur in Carbon Steel

I am sure we are indebted to Prof. Hayward for his addition to our knowledge of the influence of sulphur on steel. As he has said in this paper, sulphur has had many defenders in recent years. It seems to me, however, that, while it may be old school to believe in the detrimental action of sulphur, it is not yet old school to believe in segregation; that while it may be that 0.1 per cent, of sulphur has no very great effect on the physical properties of steel, a ladle analysis of 0.1 per cent, of sulphur may readily result in 0.2 or 0.3 per cent, sulphur being present in the segregated portion of the ingot, and that, it seems to me, might be a rather serious matter.

Prof. Hayward has presented the results of an investigation along a line to which I have devoted considerable time and attention, and I am particularly pleased to note that his results are in accord with my own and further tend to prove the fact that the old established prejudice against sulphur is based more on belief than actual facts.

I have felt for years that too much stress was placed on the harmful influence of certain elements in steel such as sulphur, phosphorus, copper, etc., but it has only been within recent years that efforts have been made to, establish the truth. Practically all the investigations made to date have proven that these elements within reasonable limits are harmless and, in fact, that for certain purposes the addition of these elements is beneficial.

In referring to copper, J. E. Stead, Vice-President of the British Iron and Steel Institute, said at their last meeting: “Even today one comes across steel specifications in which copper is barred, which can only be regarded as an indication of ignorance, if not stupidity, of those who prescribe the composition of steel, for it has been long ago proved that copper in steel, instead of being an evil, is quite harmless, and is sometimes distinctly beneficial.”

Sulphur owes its evil name to the early days of the manufacture of steel. Chemical analyses were crude, and failure due to either poor raw materials or metallurgical treatment were many. Sulphur being a comparatively easy element to determine with reasonable accuracy, particular attention was called to the amount of sulphur, and the failure was attributed to it, without giving any consideration to other causes. A prejudice against sulphur was thus formed, which has existed to this day. As a result, the permissible sulphur-content in steel is so low in some cases that it is questionable if the excessive purification necessary to produce such results is not detrimental to the steel.

It has been my experience in the working of steel, that occasionally a high-sulphur steel would roll or work much better than a lower-sulphur steel, yet the steels would be apparently the same in composition, excepting the sulphur. At other times the reverse would be true, and the lower-sulphur steel would roll better than the high-sulphur steel. If a certain amount of sulphur produces a particular effect, to be consistent it should always produce the same effect.

My investigation of the influence of sulphur in steel covered tests of considerable quantities of several kinds of steel in various sizes ranging from small roofing nails to 8-in. channels and railway axles, with sulphur, contents of from 0.025 to 0.254 per cent.

The products tested were rivets, machine- and hand-made chains, sheets, wire, nails tubes, pipe, drop forgings, channels, plates, axles, rails and wire cable. Owing to the variety of products investigated, it was sometimes necessary to use steel a little, softer or harder, than that ordinarily used for the same purpose. This, however, had no influence on the comparative results.

In the fabrication of these products, the usual works practice was followed in the production of the finished article, no preference being given in the treatment of steel of one sulphur content over that of another.

The material was subjected to the tests regularly prescribed for steel products of the varieties mentioned and, in addition, special tests were made on most of the products.

It was not my object to study the state in which the sulphur existed in steel, or its microstructure. What I wanted to, learn was what effect different amounts of sulphur had on the working of the steel and its effects on the properties of the finished article. This is what concerns the user of the steel.

A paper read before the Society of Automobile Engineers and published in their Proceedings for 1916 and in the Iron Age, January 13, 1916, gives the results of a great many of these tests, particularly those of interest to the automobile industry. Another paper presented before the, American Boiler Manufacturers’ Association and published in their Proceedings for 1916 and in the Blast Furnace and Steel Plant, July, 1916, describes an extensive series of tests on the influence of sulphur in rivet steel.

I will not review these results here, but will briefly refer to some of the tests which were not covered in these papers; such as tubes, plates, rails and wire products.

Referring first to the tests on tubes, a series of fifty 2-in. boiler tubes were made of each sulphur content, and all met the regulation hydraulic pressure test without failure. The welding practice on the tubes of 0.050 and 0.090 per cent, sulphur was better than the practice on the tubes containing 0.030 per cent, sulphur. A series of 4-in. seamless tubes were also made and subjected to different tests. The tubes of 0.068-sulphur as a whole gave better results than the 0.032 per cent, sulphur tubes. In the higher-sulphur steels there was a slight falling off in tensile strength, but an increase in ductility as shown by the elongation.

The investigation of plates consisted of an extensive series of longitudinal and transverse tensile, torsion and bend tests, welding and hot and cold flanging tests. The results of these tests were practically the same for plates of all sulphur contents.

Steel of 0.51 per cent, carbon with varying sulphur contents was rolled into 100-lb. rails. Tensile and drop tests were made. There was a slight decrease in the tensile strength in the high-sulphur rails, but an increase in ductility as shown by the elongation in both the tensile and drop tests. Rails of each sulphur content were then put into actual service in a railroad track, and although lower in carbon than steel ordinarily used for rails, which reduced the life of the rails, all of them, regardless of the sulphur content, showed about the same amount of wear.

In the investigation of wire products, one of the studies made was the influence of sulphur in the manufacture of roofing nails with a very large, thin head. The upsetting of the head is a particularly severe operation, yet the steel of the highest-sulphur content made as perfect a head as the lower-sulphur steels.

Another study was the electrical welding of wire. When welding the wires for continuous drawing or in the manufacture of fencing, where the wires are welded at right angles to each other, it was found that steels of all sulphur contents could be perfectly welded.

In making the regulation twist and bend test of galvanized wire, the wires all met the requirements of this test, and the coating adhered perfectly, regardless of sulphur content.

The tests of wire cable showed that cables containing up to 0.10 per cent, sulphur gave practically the same results in the tensile, twist and life tests.

Attention has been called by other investigators to the fibrous condition of high-sulphur steels. This fact was particularly noticeable in our study of case-hardening. After the steels had been case-hardened and quenched, all showed the same depth of case and hardness. When fractured, the low-sulphur steels would snap off square and show a crystalline fracture, while the higher-sulphur steels, instead of snapping off square, had to be bent backward and forward before they would break, and the fracture would be irregular and have a fibrous appearance.

The results of this investigation confirmed my belief that steel could contain a great deal more sulphur than that usually permitted and still be of good quality. The work of Wahlberg,. Arnold, Stead, Cooper, Hayward and other investigators has done much to strengthen this belief. There is still a large field for investigation and a much larger work in distributing the knowledge thus gained to all those interested. This seems to be the only way by which any erroneous beliefs can be replaced by actual facts, which later find a practical application.

The results of tests given in Prof. Hayward’s paper are interesting, and are worthy of serious attention by those who advocate an appreciable increase in the permissible limits of sulphur content in steel. It does not seem, however, that Prof. Hayward in his conclusions has rendered an entirely impartial verdict. The beginning of the paper would indicate that he started out with the intention of proving if possible that sulphur up to 0.1 or 0.15 per cent, does no harm to steel, and the tensile tests were extremely encouraging to such a conclusion. It should be noted, however, that the high-sulphur steels were no better on the whole in the tensile tests than the low-sulphur steels.

The impact tests, on the other hand, do show a marked difference in favor of the low-sulphur steels, but Prof. Hayward is apparently unwilling to admit in his conclusions that any particular importance attaches to this fact. The excuse is given that the shock test is new and its value not thoroughly appreciated by all engineers. This, I suppose, is a matter of opinion, but it surely would seem that a glance through the published records of technical societies interested in metals or their testing, both in this country and Europe, should be sufficient to show that the shock test is not new but is well known and extensively used. Looking at the matter from the viewpoint of the designing engineer, it might be asked how metals usually fail in service, whether by slow, gradually increasing tension, or from small repeated stresses which cause fatigue, or from sudden shocks? It seems obvious that failure from pure slow tension practically never occurs in service, and that fracture from shock is far more common, although probably not so common as failure from fatigue. When looked at in this light it is hard to see why Prof. Hayward’s shock test results should not be given serious attention in judging of the practical value of high-sulphur steels.

The actual results of the work described in this paper may be summarized as follows: Tensile tests showed on the whole no difference between high-sulphur and low-sulphur steels; impact tests showed a distinct difference in favor of low-sulphur steels; microstructure not discussed. In view of this summary, where nothing is shown in favor of the high-sulphur steels, but one point of decided superiority is brought out for the low-sulphur steels, is there any logical escape from the conclusion that steel low in sulphur is better and safer to use than steel with high sulphur?

The failure of the tensile tests to show a difference between these classes of steel is very similar to its results on steels with varying amounts of phosphorus, or on steels heat-treated differently. It has been shown by very eminent authorities that steel high in phosphorus may give good tensile strength and elongation, and still show great brittleness in shock tests. This is a case directly in line with Prof. Hayward’s results with sulphur, and if high-phosphorus steels are considered unsafe to use in spite of fairly good tensile results, then Prof. Hayward’s paper can only be regarded as showing that high-sulphur steels are equally unsafe.

I notice that Prof. Hayward does not discuss the microstructures of these samples, although photomicrographs were made and are reproduced in the paper. I would like to ask what conclusions are to be drawn from them? It always seems unfortunate to me that photomicrographs should be considered as proving anything in themselves in cases like this, for of course no two photomicrographs even of the same sample would be exactly alike. The true use of a photomicrograph is, I believe, in illustrating points that would be less clear without the actual views, and I would like to ask whether Prof. Hayward considers that these photomicrographs show the structures of high-sulphur and low-sulphur steels to be alike, or, if not, what differences are shown?

If photomicrographs had been made before the samples were etched, there is no doubt that much greater differences would have been shown in the cleanness of the metal. The excessive amount of non-metallic manganese sulphide in the high-sulphur steel could be easily seen in an unetched sample, and would illustrate well the reason why such steel is easy to machine, and also why it is brittle and unfit for severe service. Sulphide inclusions are as harmful as any other non-metallic impurities in steel, and more so than some because they tend to segregate. Unless we disagree entirely with the eminent authorities who maintain that non-metallic inclusions are harmful in steel, we cannot logically admit that high-sulphur steel is all right.

It is unfortunate that Prof. Hayward did not continue the investigation to include alternating stress tests, even if new stock would have been required. Surely he does not consider his results of no general value and inapplicable to any but the identical steels on which he worked, and it is not clear why the results of fatigue tests on high-sulphur and low-sulphur steels should have, any “uncertainty in their interpretation” merely because the stock was different from that used in the impact and tensile tests. It is to be hoped that such an investigation will soon be made, in order to show, more clearly even than Prof. Hayward’s present results show it, why high-sulphur steel is unsafe to use for severe service.

I recollect when Dr. Ungers’ paper on’the “Effect of Sulphur in Steel” appeared in the Iron Age, I intended to offer a criticism more with the hope of having some more competent authority challenge the rather revolutionary statements contained therein. It was, however, never, published, as I hoped to secure substantiating figures in defense of the current specifications for sulphur limits.

While connected with a steel works producing merchant open-hearth bars, splitting ingots in the process of rolling were repeatedly found to be caused by the high sulphur in the steel, the other constituents being normal. The ingots at the same time appear to have a fibrous appearance. This may perhaps be ascribed to the rolling out of the sulphides , into filaments. Sulphur exists in steel as MnS, or perhaps both as FeS and MnS, particularly when the manganese content is low. The melting points of these sulphides are considerably below the melting points of hypoeutectoid steel. Now if the steel is high in sulphur, it contains a considerable amount of MnS or MnS plus FeS; it is reasonable to suppose that as these impurities are of lower melting point, they would be thrown out to the grain boundary of the metal on cooling. Granting that, high; sulphur may even favor an increase in the tensile strength and elongation, the question rises in one’s mind, will such steel stand transverse strains or shocks? Prof. Hayward answers that question himself in his Charpy tests figures. From the above considerations of the discontinuity in the structure of such steels, one may well expect it.

One cannot help but wish that Dr. Unger would give us the secret of how to weld high-sulphur steel; it is the general experience that such steels are stringy and unweldable.

I would like to ask Dr. Unger about those different sulphur steel tests. Do I understand they were all less than 1 per cent, sulphur?

I had steels of about 0.09 per cent, carbon that ranged in sulphur from 0.030 to 0.254 per cent,; of 0.32 per cent, carbon, from about 0.032 to 0.230 per cent, sulphur; and of 0.51 per cent, carbon, that ranged from 0.025 to 0.230 per cent, sulphur. Those figures can, however, be obtained in my former paper. I do not recollect the exact figures, but in making these tests I want to say, if you are selling to a customer steel that he proposes to weld into chain, the man that finally uses the chain is not interested to a very large extent, as to what tensile strength a bar may possess: He is interested, however, in the strength of that chain.

In making this investigation I found that there was a greater possibility of making a poor weld in a chain, whether it was of low sulphur or high sulphur, than any slight change that might exist in the physical properties of the steel.

If we make a beam, channel or an angle, the usual method of testing is to make a tensile test. A beam is very rarely subjected to a tensile strain; it is either used as a column or as a girder and it is subjected to compressive or transverse loading. In testing the channels we made, we took full-sized channels, 6 ft. long, and tested them transversely to determine at what point they would take a permanent set, which imitated service conditions.

In testing the rails, we did not pay a great deal of attention to what they might show on tensile or drop tests; but we put them in a track that had to carry 2,000,000 tons of freight a month, and ran them for a year until they were practically worn out. We then compared them with other rails of a normal composition installed at the same time and location to find out whether they gave the same results. I assure you that until I learned they were safe, I watched the high-sulphur rails very carefully to observe the least signs of failure; so that they might be removed before an accident happened.

In making tests, the thing you must consider is, what is the customer going to do with the steel? You may make tensile, hardness, impact and all sorts of tests, but what you want to know is whether the steel is suitable for the purpose for which you are going to use it, not whether it is going to pass a particular physical or mechanical test.

G. Aertsen, Philadelphia, Pa.—As I understood Dr. Unger, some of these tests on high and low steel were made on plate steel?

J. S. Unger.—Yes.

G. Aertsen.—Did you notice any greater difference between longitudinal and transverse tests on the high-sulphur steel than on low-sulphur , steel?

J. S. Unger.—We know, of course, that there is a pronounced difference between the longitudinal and the transverse tests on all plates, but there was not any greater difference on the high-sulphur steels than on the low-sulphur steels between the longitudinal and transverse tests.

G. Aertsen.—Was there very much difference noticeable in the surface appearance of the high-sulphur plates? In other words, were they cleaner or less clean plates with low sulphur than with high?

J. S. Unger.—I will answer that in this way: I kept on adding sulphur, to the steel until I got to a point at which the steel was difficult to roll. I believe I could have rolled the highest-sulphur steels that I had, had I been willing to lower the rolling temperature, but my object was to roll these plates at a regular working temperature, not to make a special provision, on account of their being high in sulphur. I am satisfied that I could have rolled them if I had reduced the rolling temperature about 100°. But to answer your, question, after the sulphur had exceeded about 0.150, I obtained a surface that was inclined to be slivery, it was what we call snakey in the mill.

M. H. Medwedeff.—I believe you said it did not make any difference on the welding properties of the steel?

J. S. Unger.—You are correct. I brought out in my paper why it seems that sulphur was the only possible explanation, the reason being, that if we cannot find any other reason, we pick out the sulphur and blame it on that.

Leonard Waldo, New York, N. Y.—There is another connection in which the paper Dr. Unger published a year or so ago and this paper of Prof. Hayward are papers of great importance. The fuel near the Atlantic seacoast for open-hearth steel practice in the future will be Mexican crude oil. Mexican crude oil runs 3½ to 4 per cent, of sulphur; in the manufacture of many thousands of tons of steel, I found it practically impossible to prevent the addition of perhaps 0.01 per cent, of sulphur to the ingot steel, from the fuel alone? The education which has been persistently spread—whether because of gas fuel existing in certain localities (which has no sulphur) or not, I do not know—makes open-hearth superintendents hesitate over the addition of 0.01 per cent, of sulphur to a steel, which, in its ordinary product, would give 0.03. Dr. Unger has pointed out the error in this point of view, and he has only stated in words what every maker of steel rails has known for several years back, that the slight addition of sulphur has no practical effect on the product. I think, therefore, that we ought to be especially grateful to gentlemen like Dr. Unger and Prof. Hayward for putting this thing in a professional paper so that the facts and the reasons may be stated to inquirers who otherwise would be face to face with a difficult fuel situation as the years go on.