US2174740A - Sensitivity controlled steel and the manufacture thereof - Google Patents

Description

Oct. 3, 1939. H. w GRAHAM Er AL 2,174,740

SENSITIVITY CONTROLLED STEEL AND THE MANUFACTURE THEREOF %EL ONG/1710 (Com Wann) d J2 l .5. Z0 .ZZ iv l' v w I Z 3 4 V5 6 7 6 0 INVENTOR Ad/4% YM/f@ Oct. 3, 1939. H. w. GRAHAM ET AL 2,174,740

sENsITrvITY coNTRoEEED STEEL AND THE MANUFACTURE TEEREOF FiledMarch 17, 1934 s sheets-sheet 2 SGN/19001 al/dw/ l Q N" N INVENTOR .Oct 3, '1939. H, w` GRAHAM r AL 2,174,740

SENSITIVITY CONTROLLED STEEL AND THE MANUFACTURE THEREQF Filed March 17, 1934 s sheets-sheet s 8. Q 7 M K om W w 5@ W 6 M 4 n A G N a w F. n/m Z J. 0 0 0 0 0 3 6 M .6 5 4 3 2 m. wwwlbbok .olx

INVENTORS Patented Oct. 3, 1939 UNITED sTATEs PATENT OFFICE SEN SITIVITY CONTROLLED STEEL AND THE MANUFACTURE THEBEOF Pennsylvania Application March 17 1934, Serial No. 716,164

13 Claims.

'I'his invention relates to the manufacture of steel and particularly to the predetermining and accurate controlling of certain qualities thereof. The present application is a continuation in part of our application Serial No. 524,334, filed March 21, 1931.

Certain properties of steel, for example, the hardness or softness, and the toughness vor brittleness, are well recognized and, generally speakl ing, may be controlled with reasonable accuracy, athoughsuch qualitiesare often regarded as interdependent. Soft steel, for example, is oftentimes regarded as inherently tougher than a hard steel. There have been observed, however, increasing manifestations of a heretofore uncontrolled property in steel, and, most unfortunately, wide variations in the characteristics of two pieces of metal have been observed, although such pieces respond equally to all the usual physical tests, appear alike microscopically, and have the same chemical analysis. Despite these likenesses, we have found cases where one such steel will prove entirely satisfactory under certain conditions of fabrication or service, while another piece will be totally unsuited for the purpose.

In our study of the problem We have found it necessary to take into account the predetermination of the initial qualities and, in addition thereto, the problem of maintenance of such initial qualities, or, if the conditions of fabrication or service require it, the treatment of the steel in such fashion that a change of the initial qualities, substantially in a predetermined amount, may be had.

We may, for example, wish to produce steel which is hard and tough, as may be used, for example, in the manufacture of rails; or hard and brittle, as may be desired in shear blades; or soft and tough, as may be desired for rivets; or soft and brittle, as is desirable in screw stock. This control of initial qualities, however, is not suflicient. We have found, for example, that certain screw stocks which initially have the desired qualities of softness and brittleness, and which respond perfectly well to all the usual criteria, may be of such character that these qualities change very materially during machining; lor steel used in the making of rails may be initially hard but become increasingly brittle in use. In the case of the screw stock the action ofthe cutting tool has served to harden the material, and in the case of the rails the cold work occasioned by train traine has made the steel more brittle.

We term this quality in steel the "strain sensitivity and provide for the control of such quality, together with the control of the initial qualities of the steel, so as to initially obtain the de- A sired properties for any particular use and to maintain reasonably constant, or to determine the amount of variation from such initial properties in fabrication or service, as may be demanded by the circumstances of the particular case. We are thus enabled to materially enlarge the field of usefulness of various steelmaking processes, permitting of the use of their products in places where they have been heretofore considered undesirable, resulting in econo- .mies of manufacture, and obtaining a more satisfactory .product than it has been possible heretofore to secure.

Various test methods may be employed for the purpose of determining how the metal will perform under such fabricating operations as bending, pressing, cutting or shearing, or how it will perform in service. The'test which we prefer to use has been found to be quite accurate. Briefly stated, our test method comprises subjecting a piece of material to increasing amounts of cold work along its length and then testing the impact quality or hardness of the piece at different points. We prefer to form a tapered bar and cold draw it through a die so as to form a rod of uniform diameter, although it may be reduced by other methods as hereinafter described. The bar is notched at intervals and subjected at each notch point to a standardIzod test.

Our discoveries are peculiarly valuable in connection with the Bessemer or Duplex steel-making processes, although by no means limited thereto. It is generally accepted by steel makers and metallurgists that there are serious limitations on certain steel-making processes, notably, the acid processes such as the acid Bessemer, because they are not effective for removing phosphorus or sulphur. Phosphorus is usually regarded as prejudicial in that phosphoritic steel is likely to be brittle under shock. In general, it is referred to as being treacherous. We have observed that the apparent treachery of Bessemer steel finds a counterpart (although perhaps to a lesser degree) in steel which has been made by the Duplex process. In other words, pig iron which has been initially Bessemerized may still be treacherous despite the fact that the Bessemerizing step has been followed by an open-.. hearth melting for the specific purpose of reducing the phosphorus content. Our work, including' experience with Duplex processes, indicates that, the deleterious effects generally attributed to gested procedures which depend upon'a change'in l the ordinary analysis of the steel. Steel is ordinarily analyzed for carbon, manganese, phosphorus, sulphur, silicon, and any desired metallicadditions such as are made in alloy steels.v Our invention contemplates a change in the strain sensitivity without any change in the composition as regards said elements and said metallic additions. Y

It has heretofore been suggested that the gaseous content of steel has certain, effects upon its properties, For example, it has been proposed to super-deoxidize" steel for the purpose of making it non-aging. We have discovered that there may be wide variations in the strain sensitivity of a steel having a given analysis, even though the total gas content or the total content of any given gaseous element is substantially constant. Our researches indicate that the critical factor is not so much the gaseous content as the form in which the gas is present, and specifically that the form of the contained nitrogen is of the utmost importance. We can vary the strain sensitivity by varying the amount of nitrogen in combined form while the total amount of nitrogen remains constant or substantially so.

We believe that the combined forms of nitrogen which aiect the quality kof strain sensitivity most profoundly are the nitrides of aluminum, iron, silicon and manganese.

Our work indicates that aluminum nitride decreases the strain sensitivity; that is to say, the steel becomes less sensitive to the effect of cold work; that the nitrides of iron, silicon and manganese tend to increase the strain sensitivity; and that by adjusting the relative amounts of oppositely effective nitrides practically anystrain sensitivity may be secured. It is apparently important to put the nitrogen into nascent form in order to adjust the strain sensitivity.

An important factor of our invention resides in the fact that the addition of aluminum nitride to the steel has no apparent effect on the fatigue limits, but the effect of the oppositely effective nitride additions is to greatly improve the fatigue resistance of the steel. It is therefore possible by the use of the invention not only to adjust the strainsensitivity of the steel but also to adjust the fatigue value thereof.

ing which is as fast as that obtained by normalizing, or faster, will aid.

The invention may best be understood by reference to diagram which illustrate the variations of the properties here under consideration. In the accompanying drawings:-

Figure 1 is a diagram illustrating various quali"- ties which may be combined in different steels and indicating the manner in which such qualities may change during the fabrication of -the steel or in service:

Figure 2 is a two-dimensional diagram showing the effect of cold work upon the Izod reading for steels of different strain sensitivity;

Figure 3 is a perspective of a three-dimensional diagram showingthe strain sensitivity of diner- A ent steels;

Figure 4 isadiagram illustrating the manner of forming a test piece; Figure 5 is a side elevation of such test piece; and i Figure 6 isa two-dimensional diagram showling the 'eect of aging on steels of different strain sensitivity.

In Figure 1 of the drawings there is diagrammatically shown the manner in which the propperties of a piece of steel may vary during fabrication or in service. Moving from left to ,right on the diagram we cover varying stages of hardness as measured by a Brinnell test. Moving vertically we represent the change in toughness of the steel as measured by the Izod test. Different pieces of steel will occupy diner-ent positions on this chart, but we have marked at A a point to represent by way of example the initial condition of a certain specimen.

As above pointed out the steel may depart very materially from the initial condition represented by the point A because of its strain sensitivity. Generally speaking, it will tend to move toward the hard-brittle corner of the diagram. We have indicated this by a line connecting the point A with a point A' which represents the condition of the steel after fabrication or cold work. With different pieces .of steel the amount of movement, as well as the direction thereof, may diil'er widely, and this despite the fact that such different specimens of steel may analyze the same and initially respond alike to the usual physical tests. We are able to control the magnitude and direction of movement of the point on the diagram of Figure 1 representing the change in the quality of the steel.

In order to get an accurate measure of the strain sensitivity it is desirable to plot Izod readings against varying amounts of cold work, and in order to do this we prefer-to form a specimen as indicated in Figure 5.. Such specimen consists of a bar I of uniform diameter having notches Il cut therein at intervals in accordance with the usual practice in forming Izod test specimens. The bar I0, however, is made by taking a tapered bar l2, sueh as indicated in Figure 4, and cold drawing the same through a die I3. Since the bar I2 increases in diameter, the amount of cold work done in drawing it through the die increases from pointvto point. Therefore the test specimen of Figure represents a piece of steel whose resistance to the Izod impact test at different points along its length is representative of the strain sensitivity of the steel. In Figure 5 we have marked the diierent notches from 0 `to 8 inclusive, which iigures in this particular specimen represent the percentage of elongation at the corresponding points in the bar.

It proves very satisfactory in practice to form the specimens in the fashion indicated. However, they may be formed in other ways. For instance, we have taken a tapered piece of steel and cold rolled it to uniform thickness. We have also formed tapered test specimens and pulled them in a testing machine. It will be seen that in each case the specimen is subjected to progressively varying amounts of cold work along its length. Successful results have been attained in practice with a specimen such as illustrated in l the Izod value determined on specimens of the type illustrated in Figure 5 has been plotted against the amount of cold work done. A comparison of the curves I4 and I5 is particularly striking. The curve I4 is representative of regular Bessemer practice and shows a very brittle steel. The maximum Izod value ln foot pounds is 3, and this is for the case where no cold work has been done. Where cold work has been effected the Izod reading is only about 2 footpounds. The curve I5 represents the strain sensitivity of a Bessemer steel of like analysis which, however, has been subjected to our treatment. For zero cold Work the Izod value is 100 footpounds, dropping oif to approximately 89 footpounds for 10 percent elongation by cold work. The startling increase in impact quality is obtained despite the fact that 'the two steels would analyze the same and would also show the same gaseous content if subjected to the analyses ordinarily employed for steel. 'It will be apparent, however, that the steel represented by the curve I4 is valueless for such uses as structural members or rails, whereas the steel represented by the curve I5 is free of the deficiencies made known by our invention.

Even in the case of the best open-hearth scrap practice our tests show that different heats, although of the same chemical analysis, vary widely in strain sensitivity. Steel made in the electric are furnace is, generally speaking, more sensitive than open-hearth scrap heats, as is steel made by the Duplex process. One characteristic of these steels which isparticularly noticeable is that the steel may have an initially high impact value but that this may drop oi very rapidly with increasing cold work. We have represented such a condition by the curve I6. This steel has a reasonably high'initial impact value but is so sensitive that it may be expected to rapidly become brittle when subjected to cold work in fabrication or in service. At I'I We have shown for purposes of illustration a line representing a piece of steel of substantially the same chemical analysis as the steel represented by the line I6, but subjected to a. treatment so as to decrease the strain sensitivity. It will be noted that there is a relatively small decrease in the toughness of the steel as represented by the Izod reading despite the large amount of cold work which has been done. For simplicity of explanation we have illustrated the curve I1 as having the same initial value as the curve I 6.

Generally speaking, when our invention is utilized to decrease the strain sensitivity, it serves to raise the initial Izod value as well. If We project the curves IB and I1 on the vertical axis as represented by a-a and a-b, we have a distance representing the total change in toughness under cold work. The distance A-A' on the chart of Figure 1 may be conceived of as a projection of the cold work- Izod diagram of.

Figure 2. We have plotted on the chart of Figure 1 a point B corresponding to the point b of Figure 2 so as to show how much more closely the initial condition of the steel -may be maintained by the use of our invention.

Extended investigations wherein Izod values have been plotted against Brlnnell values, as in the chart of Figure l, show thatthe curve con- .necting points such as A and A' may have a variety of shapes. The 4shape of this curve may be controlled to some degree by changing the strain sensitivity of the steel. For instance, we may treat a piece o1 steel whose condition is represented at A so that its condition after cold work will be represented by the point VC or by the point D, these representing marked departltlres from the usual trend of strain sensitive s eel.

The line A--C may be taken as representativel of a treatment which causes the steel to retain its toughness despite increases in hardness, as is desirable, for instance, in rails or other members subjected to cold work. The line AD, on the other hand, may be taken as representative of a steel which increases relatively rapidly in brittleness without such a marked change in hardness. This is desirable in free-cutting steels.

As above indicated, the processing of the steel has a material bearing on its strain sensitivity. Figure 3 represents diierent curves of the character illustrated in Figure 2 spaced an arbitrary distance apart along the axis Z-Z. The curves 20 may be considered as illustrative of Bessemer heatsthe curve 2| as illustrative of a Duplex heat; the curve 22 as illustrative of an openhearth scrap heat; and the curve 23 as representing a heat of steel which has been subjected to our treatment for decreasing the strain sensitivity.

It is our belief that a large amount of the research which has beenV done in connection with the brittleness of steel, notably, in investigations relating to phosphorus, has failed to take into account the amount of cold work done on the tested specimens, which failure explains the apparent treachery of phosphoritic steel in its response to physical tests. In the rolling of bars, for example, the finishing temperature may vary materially from time to time during a single run. A slight drop in the finishing temperature may mean such an increased amount of cold work as to represent a shift from point A to the point A' on the diagram of Figure 1 resulting .in a more brittle piece of steel. By our process we are able' to overcome these defects and produce metal articles subject to cold work in fabrication or service and having a. very slight strain sensitivity, regardless of the process of manufacture which maybe employed. Conversely, we may increase the tendency of the steel to become brittle under cold work.

-As an example of our invention we may take steel formed, say, by the Bessemer process and, if it is desired to. reduce the strain' sensitivity of the steel, add a material which will bring about the formation of aluminum nitride in the steel. Metallic aluminum itself is ideal for the purpose, but easily decomposable aluminum compounds may also be used. The aluminum thus added reacts with the dissolved or combined nitrogen in the steel to form aluminum nitride, and the result is to change the strain sensitivity of the steel. It is important that the aluminum, or the equivalent weight thereof in case aluminum compounds are used, be added in the proper amount. Our experience shows that aluminum up to the amount of 11/2 to 3 pounds per ton serves to'decrease the strain sensitivity. The curves I6 and I1 of VFigure 2 fairly represent the change in strain sensitivity obtained by an aluminum addition in this amount. If aluminum in any materially greater quantity is added the eiect is reversed and the'strain sensitivity is increased.

If it is desired to increase the strain sensitivity, this can be done by increasing the amount of nitrides in the steel acting in opposition to the aluminum nitride. It is not sufficient to merely add nitrogen in the ordinary form. We have found that it should be in nascent form in order to have it combine with the iron to form iron nitride. We prefer, therefore, to add the nitrogen in the form of a readily decomposable nitrogen compound, and preferably in the form of a compound of nitric or hydrocyanic acid. Examples of nitrogen compounds which may be added are sodium nitrate, potassium nitrate, ammonium nitrate, sodium cyanide, potassium cyanide and ammonium cyanide. When these compounds are added to the molten steel they dissociate and form nitrides. In Figure 2 `we have illustrated a curve 30, the effect of a sodium cyanide addition to a steel whose normal strain sensitivity is represented by the curve I6. It will be noted that the effect of the sodium cyanide has been to depress the strain sensitivity curve; that is to say,

to render the steel much more sensitive. In the specific example given the result was obtained by an addition of sodium cyanide in the amount of 2 pounds per ton.

The relative amount of iron nitride and aluminum nitride in the steel may be varied as desired by additions to the molten steel and the strain tal.

sensitivity thereby varied over Wide limits. It is interesting to note that it is not only possible to locate the strain sensitivity curve at practically any desired point between the lines I1 and 30 of Figure 2, but that it is also possible to carry the curve above the line I1. We have illustrated at 3| the strain sensitivity curve obtained by treating steel, whose normal strain sensitivity is represented by the line i6, withv aluminum in the amount -of 11/2 pounds per ton and sodium cyanide in the amount of 2 pounds per ton.

As stated, aluminum in the amount of 11/2 to 3 pounds per ton gives about the minimum strain sensitivity for a sole aluminum addition. It will be understood, however, that if it is desired to adjust the position of the curve further amounts may be added. Aluminum in the amount of 1 to 3 pounds per ton, or preferably in the amount of 11/2 pounds per ton will be found a very-use ful addition. It has heretofore been considered inadvisable to use aluminum in large amounts because of the cost, because of piping of the ingot, and because its use results in the production of A1203,

.Steel made by our process is highly superior to that made in the ordinary fashion since -it possesses qualities which have heretofore been unappreciated and uncontrolled. The metal may have desired initial qualities and the departure vof the metal from such predetermined initial qualities under cold work may also be accurately predetermined, thus rendering the metal far.

more suitable for particular uses than has heretofore been possible.

Our improved product demonstrates its advantages from various other points of view. For example, in Figure 6 we have illustrated by a curve 24 a piece of steel made according to usual practice. The line 25 illustrates a piece of steel of like analysis which has been treated so as to render it less strain sensitive. It will be noted which is generally considered detrimen-l that the treatment has been eective for raising its initial Izod value. It the steels represented by the lines 24 and 25 are subjected to accelerated aging their conditions are represented by the lines 24a and 25a. It will be noted that the curve 24a lies a short distance below the curve 24, whereas the curve 25a shows but a relatively slight departure from the curve 25 with no appreciable reduction in impact toughness.

Our investigation has also covered the effect of quenching our improved steel with illuminating results. Pieces of metal of structural steel grade were quenched from 1550 F. in water and cold rolled into work-brittleness test pieces. In the quenched condition our improved steel was very tough even after being cold worked to 40 per cent elongation, while steel made according to normal practice was very brittle under the same conditions. As above stated, any cooling rate equal to or more rapid than that attained in normalizing has considerable effect.

A further marked advantage of our invention arises where the amount of nitrides of iron, sili con or manganese in the steel has been increased. This advantage arises from the fact that such addition markedly increases the fatigue strength of the steel. The fatigue strength is usually expressed as a percentage of the ultimate strength representing the maximum ligure to which the steel can be subjected indefinitely to repeated stresses without breakage. In the steel represented by the line I6 the fatigue strength will be approximately 45 percent of the ultimate, but the sodiumcyanide addition serves to raise this figure to 63 percent, and aluminum additions for the purpose of decreasing the strain sensitivity do not materilaly atleet this enhanced fatigue strength.

We have illustrated` and described various aspects of our invention, but it will be understood that this is by way of example only, and that the invention may be otherwise embodied or practiced within the scope of the following claims.

l. -In the process of making steel, the steps consisting in forming a molten heat of steel of the desired composition and providing therein aluminum nitride and iron nitride in an adjusted ratio whereby a desired strain sensitivity is obtained.

2. In the process of making steel, the steps consisting in making a heat of steel of the desired composition and adjusting the strain sensitivity by adding to the molten steel a compound of the group sodium cyanide, potassium cyanide. ammonium cyanide, sodium nitrate, potassium nitrate, ammonium nitrate, and aluminum.

3. In the process of making steel, the steps consisting in making a heat of steel of the de- -sired'composition and adding thereto a readily ing the amount of such compound in accordance with the sensitivity desired.

5. In the process of making steel, the steps consisting in making a heat -of steel of the desired composition, adjusting the strain sensitivity by controlling the aluminum nitride content, and adjusting the fatigue strength by controlling the nitride content of at least one of the elements iron, silicon and manganese.

6. In the process of making steel, the steps consisting in forming a heat of molten steel of a desired composition, treating the same in the molten condition to adjust the metallic nitride content thereof for a desired strain sensitivity characteristic in the solid condition, causing the molten steel to solidify, and quenching the steel in solid form from approximately its upper critical transformation point.

'7. In the process of making steel, the steps consisting in forminga heat of steel of substantially the desired composition, the metal being bessemerized during the making of the heat, and, While the steel is in the molten state, changing the-strain sensitivity of the steel by providing therein aluminum nitride and iron nitride in an adjusted ratio.

8. In the process of making steel, the steps consisting in forming a heat of steel of substantially the desired composition, the metal being bessemerized during the making of the heat, and, while the steel is in the molten state, changing thestrain sensitivity by adding aluminum thereto in ,amounts up to three pounds of aluminum per ton of steel, the aluminum addition being adjusted in ,accordance with the amount of strain sensitivity desired.

9. In the process of making steel, the steps consisting in forming a heat of steel of substantially the desired composition, the metal being bessemerized during the making of the heat, and, while the steel is in the molten state, changing the strain sensitivity of the steel by varying the content of a .nitride of the group aluminum, iron, silicon, manganese, the strain sensitivity being less than that obtained by adding aluminum in excess of three pounds of aluminum per ton of steel.

10.`In the process of making steel, the steps consisting in forming a heat of steel of substan# tion, the metal being bessemerized during the making of the heat, and, while the steel is in the molten state, adjusting the ratio of aluminum nitride and iron nitride in the steel.

12. In the process of making steel, the steps consisting in forming a heat of steel of substantially the desired composition, the metal being bessemerized during the making of the heat, and, while the steel is in the molten state, changing the strain sensitivity of the steel by adding thereto a compound containing the CN radical in an amount suicient to impart to the steel a desired adjusted strain sensitivity.

13. The herein described steel having an adjusted strain sensitivity obtained by forming a heat of steel of substantially the desired composition, and, while the steel is in the molten state,

adjusting the ratio of aluminum nitride and iron nitride in the steel.

HERBERT W. GRAHAM. SAMUEL L. CASE.

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