US2900250A - Free-machining austenitic alloys - Google Patents

Description

' Patented'Aug. 18, 1959 FREE-MACHINING AUSTENITIC ALLOYS Kenneth Metcalfe, Bridgeville, Pa., assignor to Universal Cyclops Steel Corporation, Bridgeville, 'Pa., a corporation of Pennsylvania No Drawing. Application June 18, 1958 Serial No. 742,728

Claims. 01. 75-124 This invention relates to austenitic steels that are of the free-machining type.

While substantially all austenitic stainless steels that are capable of being worked also are capable of being machined, at least to some degree, there are known in 'the art grades of steel denominated as free-machining steels. This designation indicates relative ease of machinability, as compared with austenitic steels ingeneral, using standard techniques, 1 The common method of producing a free-machining steel is to melt the steel to a high sulfur content. For example, free-machining 18-8 chrome-nickel stainless steel has been melted as high as .35 weight percent sulfur content and the resulting steel is a free-machining steel. However, that advantage is obtained at the expense of other properties of the steel. In these high sulfur steels a low yield in hot-workability is experienced as the freemachining-characteristics are improved by sulfur additions.

In recent years, fabricators have insisted on even greater ease of machinability and have set sulfur specifications as high as 0.40 to 0.45 percent. As is apparent, the sacrifice of other properties at those sulfur levels is increasingly greater and, in some cases, cannot be tolerated.

It is a major object of the present invention to provide austenitic steels that are of a free-machining grade without sacrifice of other properties and that even may show improvement in other properties.

I have discovered, and it is on this discovery that the invention is in large part predicated, that free-machining austenitic steels can be produced with a lesser sulfur content than heretofore necessary by including in such sulfur-bearing steels a small but effective amount of aluminum. Surprisingly, the resulting product evidences superior machining characteristics, as compared to the results when using sulfur or aluminum alone, thereby showing that the combination of these elements exerts a synergistic effect. Moreover, the resulting products are free-machining without sacrifice in product yield while hot-working the material.

Steels to which my discoveries apply are austenitic chromium-nickel steels. By austenitic I intend to indicate steels that contain at least 90 percent of austenite with the balance ferrite as well as those that are substantially 100 percent austenite. 'steels that ordinarily form the base-stock for free-maching steels and, of course, these constitute the most important materials to'which my invention is to be applied in view of the experience fabricators have with these lwell-known materials. The 18-8 type stainless steels are representative of the usual steels that are modified by sulfur additions to produce free-machining character- 'istics.

The steel compositions embodying thepresent invention and by which its stated objects are attained contain about There are now several .15 to 30 percent of chromium, 6 to 40 percent of nickel, up to .25' percent of carbon, 0.05 to 0.20 percent of sulphur and 0.25 to 1.35 percent of aluminum. The remainder of the steels consists of iron together with impurities and elements in amounts that do not adversely affect the properties that characterize the steels of my invention. For example, the following elements in the amounts stated can be present to the extentthat they do not affect the austenitic characteristics of the steels:

up to 2 percent manganese, up to 3 percent silicon, up

to 4 percent molybdenum, up to 2 percent tungsten, up to 1 percent vanadium, up to 1 percent columbium, up to 1 percent tantalum, up to 1 percent titanium, up to 5 percent cobalt, up to 4 percent copper and up to 1 per.- cent zirconium.

Within the foregoing broad range, intermediate ranges are definitive of steels that in instances are of particular interest for such reasons as economics or as a consequence of an intended application. A suitable intermediate range of the steel compositions contains 17 to 20 percent of chromium, 8 to 35 percentof nickel, 0.05 to 0.15 percent of sulphur, 0.25 to 1.0 percent of aluminum, 0.01 to 0.20 percent of carbon'and the remainder iron and incidental impurities andelementsas stated above.

As also noted above there are now manufactured larg quantities of conventional base-stock that find use in forming free-machining steels, and the preferred compositions according to the present invention include those steels. My compositions that encompass those materials contain about 17 to 26 percent of chromium, 8 to 22 percent of nickel, 0.08 to 0.14 percent of sulphur, 0.50 to 1 percent of aluminum, 0.05 to 0.15 percent of carbon, and the remainder iron and incidental impurities and elements as aforestated. It will be noted that in the preferred range of compositions, the nickel content and chromium content encompass the 18-8' and 25-20 compositions that now are made in tonnage quantities. 7

Aluminum is used in the invention generally in amounts of at least about 0.25 weight percent based on the total weight of the compositions. Many diflerent aluminum contents have been tested and I have found that it should not exceed about 1.35 percent. Beyond about 1.35 percent of aluminum, ferrite begins to form in the alloy in sufficient quantity for it to become noticeably magnetic. The ferritic constituent is undesirable in that it has a detrimental effect upon the physical' properties of the alloy and in particular'decreases the oxidation and corrosion resistance Moreover, the aluminum alloy containing ferrite would be a two-phase alloyand, therefore, would present -difficulties in hot working. Aluminum can be incorporated in the steel in this invention in the samemanner as other alloying constituents being careful, of course, to avoid loss of aluminum, as by vaporization, under the conditions of production. I I i The invention will be described further in conjunction with the following examples.- It should be understood that the details are not to be constructed as limiting the invention.

A thirty pound heat of 18-8 stainless steel was prepared by melting a low carbon iron, along with chromium and nickel in an induction furnace. Sulfur in a predetermined amount was then added to the melt. An ingot was poured from part of the melt and when it solidified, it was hot worked to dimensions of 1 in. ,sq and then annealed at 1950 F. for hour. and water quenched.

. 3 A chemical analysis and Brinell hardness test of this steel were made, and its composition was as follows:

Table 1 Bar 1: Weight percent C 0.12

Fe Balance Brinell hardness 163 Table 11 Bar No. C 8 Cr N1 Al Brinell hardness 0.11 0.12 18. 1s 9. s 0. 27 163 0.12 0. 12 18.14 9. 0s 0. 53 149 0.12 0.13 18.06 9. 04 0. 63 149 The machinability of the bars was tested by a standardized procedure heretofore found to give a reliable measure of this characteristic. The machinability test consists of drilling a standard bar both before and after drilling the material being tested and the results obtained are corrected by the use of a formula, for dulling of the drill. The time required to drill a A diameter hole 0.3 deep is measured with a stopwatch. The use of a standard bar makes it possible to change or resharpen drills and reproduce the results of the test. The results are reported as percent machinability of the standard bar.

The standard bar is made of annealed .90/ 1.00 carbon tool steel. The bar is 1%" x 4" x 6" approximately. It is annealed at 1700 F. one hour and furnace cooled.

A weight is mounted on the top of the spindle of a drill press so as to produce a constant load of 275 pounds on the drill. One hole is drilled in the standard bar, then a hole is drilled in the test specimen (or specimens) and another hole is then drilled in the standard bar. The time in seconds required to drill each hole is recorded and the results, corrected for dulling of the drill, are calculated in accordance with the following formula:

1 D.+% D.D. Maehinability index= IOOX D -lnitial drilling time in standard block D Final drilling time in standard block N-Sequence number of sample TTotal number of tests including initial and final holes in standard block 'SObserved drilling time in each sample The machinability index of each of the bars was determined by the test just described and the data obtained are:

These data show clearly that the addition of aluminum to a free-machining steel markedly improves the machinability of the steel even at low concentrations of aluminum. Indeed, as will be apparent from data hereinafter presented, the elfect of the aluminum and sulfur in combination exceeds that attainable with a comparable amount of either of those elements alone in most steels to which the invention applies, and results in high machinability ratings in the remaining steels while improving the general, physical and working characteristics thereof.

A series of tests were conducted that demonstrated the synergistic effect produced upon the use of sulfur and aluminum in combination in accordance with my invention. Test bars were made according to standard melting practice for 18-8 stainless steels. A reference bar substantially free of sulfur and aluminum was prepared. In addition, a bar containing sulfur in an amount within the range normally used in producing a freemachining steel was made. Then a bar substantially free of sulfur but containing a significant amount of aluminum was produced. Other bars containing combinations of sulfur and aluminum within the range according to the present invention were also prepared. The machinability rating of each of the above bars was then made in accordance with the test procedure described hereinbefore. The data obtained in this series of tests are as follows:

Table IV Machin Bar 0 8 Or N1 Al ability rating 0.12 0. 017 18.58 9. 27 0. 02s 60 12 .25 1s. 40 9. 06 .013 87 12 .006 19.13 3. 89 .72 67 12 .12 1s. 22 9. 0a .023 84 11 12 18.18 9. 0s .27 12 12 1s. 14 9. 0s 53 109 12 .13 18.06 9. 04 .63 115 12 12 1s. 26 9. 04 .73 110 11 .10 17. 75 8.56 .86 113 Referring to the data in Table IV above, it may be noted that the standard bar (bar 5), which was substantially free of sulfur and aluminum, had a machinability rating of 66. The addition thereto of a large amount of aluminum (bar 7) did not materially change the machinability rating. The use of sulfur alone as shown by bars 6 and 8, produced a significant rise in the rating and such bars are representative of commercial free-machining steels. In bars 9 through 13, sulfur and ammonium were used conjointly. The machinability rating of each of the resulting steels is shown to be markedly beyond the rating for the sulfur-containing free-machining steels, namely bars 6 and 8. From the data, as shown upon comparing bars 5 and 7, it would not be expected that aluminum additions would bring about significant changes in the machinability rating. The fact that a pronounced improvement resulted therefrom is an entirely unexpected result. It is further to be noted that since the rat ing found on steels containing both aluminum and sulfur far exceeded that of steels containing high sulfur alone or high aluminum alone (compare 6 and 8 with any of 9 through 13) it is apparent that an unexplainable phenomenon producing this synergistic effect must have occurred as a consequence of the presence of both the sulfur and aluminum at the same time.

These significant results can be attained in steels of compositions other than those of the 18-8 stainless steels. To demonstrate this, a series of tests were made on standard 25-20 stainless steel compositions. In this group of tests a bar substantially free from sulfur and aluminum was made along with a second bar high in sulfur but low in aluminum. The third bar was controlled to provide a high aluminum content with a low sulfur content. Two

additional bars containing aluminum and sulfur within the range according to the present invention were also made- The analyses of these steels along with their machinability rating are shown in the following table:

In evaluating these data it should first be noted that this type of steel is exceedingly diflicult to machine. The regular composition (bar 14 above) shows the typically low machinability rating for this class of steel. The addition thereto of a large amount of sulfur raised the machinability rating markedly as shown by the data at bar 15 above. The machinability rating where aluminum alone was present was not a significant improvement though it did raise the rating somewhat. Bars 17 and 18 conformed to the ranges of the present invention and in each it will be noted that the machinability rating is on the order of that of a high sulfur steel even though their actual sulfur content is on the low side of the sulfur range for this invention. Hence, in accordance with this invention, it is possible to obtain practically the same machinability with a low-sulfur plus aluminum combination as with a high sulfur contert alone. Indeed, when the sulfur content is more in accord with the preferred level, such as was used in the 18-8 compositions as shown in Table IV, bars 9 to 13, machinability would be expected to be at least equal to that of bar 15 of table V, yet not have its disadvantages. The low-sulfur content alloys are decidedly improved in workability as is evidenced by hot upset tests performed on the various compositions. The high sulfur material exhibits definite tendencies for hot brittleness while the low-sulfur material was capable of being upset to a much more satisfactory degree. The hot upset test is a common laboratory test to determine hot workability characteristics.

In addition to the advantages manifest in the above data, steels made in accordance with this invention are characterized by improved cleanliness as compared with steels having a high sulfur or aluminum content. By cleanliness I refer to the inclusion contents of the steels. Steels produced in accordance with my invention would have inclusions that would be both smaller in size and in number than those in steels having only a high sulfur content or a. high alumium c ntent.

From the foregoing discussion and data it is apparent that free-machining steels which can be worked at high yields have been provided in accordance with my discovcries. Steels with maximum machinability can be provided without encountering the low yields in hot-working operations that would tend to limit their present uses. Moreover, these results are achieved merely by including a combination of alloying constituents in the basic compositions and do not require special melting practices, heat treatment or other change in the normal production of the steel.

In accordance with the provisions of the patent statutes, I have explained the principle of my invention and have described what I now believe to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. Free-machining, austenitic chromium-nickel steels consisting essentially, by weight, of 15 to 30 percent of chromium, 6 to 40 percent of nickel, up to 0.25 percent of carbon 0.05 to 0.20 percent of sulfur, 0.25 to 1.35 percent of aluminum, and the remainder iron and incidental impurities and alloying elements that do not deleteriously affect the character of the resulting steels.

2. Free-machining, austenitic chromium-nickel steels consisting essentially, by weight, of 17 to 26 percent of chromium, 8 to 35 percent of nickel, 0.01 to 0.20 percent of carbon, 0.05 to 0.15 percent of sulfur, 0.25 to 1 percent of aluminum, and the remainder iron and incidental impurities and alloying elements that do not deleteriously affect the character of the resulting steels.

3. Free-machining, austenitic, chromium-nickel, steels consisting essentially, by weight, of 17 to 26 percent of chromium, 8 to 22 percent of nickel, 0.05 to 0.15 percent of carbon, 0.08 to 0.14 percent of sulfur, 0.50 to 1.0 percent of aluminum, and the remainder iron and incidental impurities and alloying elements that do not deleteriously afiect the character of the resulting steel.

4. Free-machining, austenitic 18-8 chromium-nickel stainless steels including 0.08 to 0.14 weight percent of sulfur and 0.50 to 1.0 weight percent of aluminum.

5. Free-machining, austenitic 25-20 chromium-nickel stainless steel including 0.08 to 0.14 Weight percent of sulfur and 0.50 to 1.0 weight percent of aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 2,496,245 Jennings Ian. 31, 1950 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patenfi N00 l8; Kenneth Metcalfe It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 61., for "constructed" read construed column 4, 1 ins 51, for "emmonium" read aluminum Signed and sealed this 22nd day of March 1960.,

(snail) Attest:

KARL no AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Officer

Claims (1)

1. FREE-MACHINING, AUSTENITIC CHROMIUM-NICKEL STEELS CONSISTING ESSENTIALLY, BY WEIGHT, OF 15 TO 30 PERCENT OF CHROMIUM, 6 TO 40 PERCENT OF NICKLE, UP TO 0.25 PERCENT OF CARBON 0.25 TO 0.20 PERCENT OF SULFUR, 0.25 TO 1.35 PERCENT OF ALUMINUM, AND THE REMAINDER IRON AND INCIDENTAL IMPURITIES AND ALLOYING ELEMENTS THAT DO NOT DELETERIOUSLY AFFECT THE CHARACTER OF THE RESULTING STEELS.