US3846186A - Stainless steel having improved machinability - Google Patents
Stainless steel having improved machinability Download PDFInfo
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- US3846186A US3846186A US00289589A US28958972A US3846186A US 3846186 A US3846186 A US 3846186A US 00289589 A US00289589 A US 00289589A US 28958972 A US28958972 A US 28958972A US 3846186 A US3846186 A US 3846186A
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- the improved stainless steels are further enhanced in machinability, e.g. as to chip characteristics, by special heat treatment, including heating between the A and A critical points of temperature.
- Optional additions of rare earth elements can coact in establishing or enhancing the desired, e.g. sulfideselenide, inclusions, and can afford deoxidation function effective in avoiding unwanted aluminum addition, for corresponding cooperation in minimizing occurrence of aluminum oxide.
- This invention relates to stainless steel and is more particularly directed to the provision of stainless steels having superior machinability, and to methods of producing such stee s.
- requirements and conditions can be considered more critical than in the low or medium carbon grades of ordinary steel, in that: machined products of stainless steel, which is more costly, are often expected to have a much better finish and dimensional control than carbon steel products, while secondary operations such as reaming, threading, tapping and grinding on stainless grades can be very uneconomical at high volumes of production, as in shops using automatic screw machines; and stainless steels are inherently unsatisfactory to machine not only because of general cutting difficulties but especially because their thermal diffusivity is about twothirds to one-half that of plain carbon steel, causing higher tool chip temperature and consequently shorter tool life.
- Particular stainless steels that have heretofore been produced with special characteristics of machinability include 3,846,186 Patented Nov. 5, 1974 AISI type 303 of the austenitic 300 series, and A181 type 416 from the martensitic grades of the 400 series, as well as others, e.g. the ferritic type 430F.
- the principles of the present invention are primarily illustrated with the so-called free-machining varieties such as the 303 and 416 stainless steels, the same principles are deemed applicable to a broad range of stainless steels, embracing the 200, 300 and 400 series, and also the precipitation hardening grades of both the martensitic and semi-austenitic types, such broader range being inclusive of steels for which the end service properties might restrict the usages of free-machining additive to low levels.
- the invention has been developed with special applicability to the presently recognized machining grades, for example types 303 and 416, and corresponding particular objects are to provide improvement in such grades, notably in suiting the properties of these steels to the requirements of machining operations in industrial production.
- important aims of the present invention are to afford improved stainless steels, and methods of producing them, which have distinctly superior machining properties and which in presently preferred embodiments are well suited to the needs, in quality and production rate, of manufacturers of machined products.
- a further object is to provide such steels and such methods in an economical manner, and without significantly altering the other desired properties that characterize the grade of steel to which the invention is applied.
- a significant feature of the invention in its specific and preferred aspects, resides in the finding that instead of relying on sulfur as the sole addition to promote machining properties, improved characteristics of the inclusions are attained by incorporation of selenium, or in some cases alternatively or additionally by the incorporation of tel lurium.
- compositions which thus contain sulfur, and selenium in amounts less than 0.15%, preferably 0.04 to 0.1%, together with manganese in sufficient amounts to satisfy the theoretical stoichiometric requirements of MnS and MnSe as well as to account at least for other unavoidable or required utilization of manganese in the steel (such as constituting a mild deoxidizer and a matrix strengthener), are found to provide inclusions of superior and assured characteristics for machinability, not hetertofore or reliably achieved with inclusions predicated on sulfur addition alone.
- the steel is relatively economical to produce, for example as compared with the special grades last mentioned or as measured, in effect, against the attained improvement in machinability.
- the aluminum oxide appears as a distribution of minute, hard particles or inclusions, further evidence being that they tend to show up in manganese-sulfur inclusions and apparently even influence the very nature of precipitation of the latter in an adverse manner, as by promoting so-called eutectic or type II sulfides which become long, stringy configurations of the manganese-sulfur bodies in as-rolled steel bars.
- the invention in one related aspect, consists in steel of the stated character wherein alumina is kept to an unusually low maximum, such as 0.0025% or more advantageously 0.002%, and is preferably well below such values, a further feature being that the process of making the steel involves deoxidation otherwise than by the use of aluminum, as for instance by employing silicon (conveniently in the form of ferro-silicon), and indeed more specifically by using such agent in a form having no more than a very low impurity content of aluminum.
- Another aspect of the invention is that the incorporation of selenium along with sulfur, in the manner explained above, has been demonstrated to reduce materially the tool-destruc tive effect of aluminum oxide, it being further noted that at moderately small levels of A1 0 -(yet above 0.002%) the manganese-sulfur-selenium inclusions retain their desired substantially globular shape and appearance and have been observed as functioning, at least in part, to provide a sheath or enclosure for the aluminum oxide particles.
- rare earth metals such as lanthanum, cerium and others may be employed, with good efiect on the machinability of the steel in one or more of the respects of tool life, surface finish, ease of chip removal, and productivity.
- These elements can form sulfides and selenides, or possibly complex. compounds of such nature with manganese, and produce: the desired globular inclusions or appear in them, imparting characteristics that are similar to those afforded by compounds of manganese with sulfur or selenium.
- a further procedural feature is afforded by the step of adding a rare earth metal or metals, as at the end of a heat or in the ladle, in that such addition may serve the deoxidizing function in lieu of silicon or othersubstitute for aluminum, and then at least in part the rameearthaddition m y appear as sulfide or selenide compounds in-thle in.-
- martensitic grades of stainless steel produced to contain the desired, unusually effective inclusions, e.g. comprising manganese with sulfur and selenium, may be further benefited by a special heat treatment, particularly in that the chip formation, for various kinds of machining operations, can be greatly improved. That is to say, in some cases even with optimum form 'and volume fraction of the sulfide-type inclusions, machining chips may in fact be very long, tough ribbons or curls of difiiculty manageable type.
- the improved steel as for instance of the 416 grade, is subjected to heat treatment which includes heating to a temperature between the lower and upper critical points, i.e.
- the article can thereafter be tempered at a suitable, lower temperature, and also stressrelieved.
- the cutting or drilling chips may tend to be several feet long without this treatment, its effect is to cause the chips to break off short, e.g. at a few inches or less. It is believed that the treatment, notably if performed in a preferred manner as explained hereinbelow, results in a two-phase microstructure, partly martensitic and partly ferritic with carbides.
- FIGS. 1, 2 and 3 are microphotographic views respectively showing three different types of sulfide inclusions in steel.
- FIG. 4 is a microphotographic view showing inclusions, of the general type of FIG. 1, as appearing in a stainless steel embodying the invention.
- FIG. 5 is a view, to be compared with FIG. 4, showing undesirable sulfide inclusions.
- FIG. 6 is a graph illustrating the distribution of plane intercept dimensions corresponding to distribution of inclusions in an example of one type of stainless steel embodying the invention.
- FIG. 7 is a graph, like FIG. 6, showing such interceptdimension distribution for another type of stainless steel of the invention.
- FIG. 8 is a microphotographic view, comparable with FIG. 4, showing aluminum oxide particles in a stainless steel.
- FIG. 9 is another view, on further magnified scale, of inclusions in an example of the invention.
- FIG. 10 is a view, to be compared with FIGS. 8 and 9, showing aluminum oxide particles in a stainless steel embodying certain features of the invention.
- FIG. 11 is a graph of the results of screw machine tests of stainless steels, showing the effect of aluminum oxide in the steel.
- FIG. 12 is a graph of the results of screw machine tests, comparatively showing the effect of aluminum oxide in stainless steel without and with selenium in accordance with the invention, and in stainless steel produced in accordance with a further feature of the invention.
- FIG. 13 is a graph of the results of screw machine tests, showing the effect of tellurium addition in accordance with the invention.
- each test consisted of an 8-hour run on an automatic screw machine, supplied with l-inch diameter bars and automatically turning out small finished pieces, usually a total of 1,500 to 1,700 pieces for the 8-hour run.
- the general practice in preparing the test bars involved hot rolling from 4-inch billets, followed by a small amount of cold reduction (when the special heat treatment of the present invention was used, it was performed after the cold reduction), the bar being thereafter turned and ground, in other equipment, to have the finished l-inch dimension. Utilizing a 6-spindle automatic screw machine, it was charged with six bars, each 12 feet long, and in general each 8-hour run required three to four further charges depending on the cutting speed and feed chosen for the test.
- This surface finish measurement was made with a Brush surface analyzer, yielding a roughness determination, corresponding to the wave height or peaks of surface roughness.
- the measurement was thus determined, conventionally, as microinch R.M.S. (rootmean-square), one minimum criterion being to achieve a surface roughness less than microinches R.M.S. over a tool life of at least the stated 8 hours at a chosen combination of feed and cutting speed.
- the cutting speed as will also be understood, represented the relative speed of the surface of the work, i.e.
- the end slide feed of the machine i.e. the drill feed
- the cross slide feed was about one-seventh of the end slide feed.
- the finish forming operation was usually a plunge cut, employing a tool of appropriate width moved radially inward of the workpiece and thereby producing an annular machined area which has a width equal to that of the tool and is intended to be characterized by a finished surface.
- FIGS. 1, 2 and 3 of the drawing being microphotographs at about 500x magnification, showing the three generally recognized types of manganese sulfide inclusions in steel.
- the first are the so-called globular inclusions, being Type I as in FIG. 1.
- These are round or somewhat elongated bodies predominantly composed of a compound of manganese and sulfur and it has now been demonstrated not only that they are requisite for providing enhanced machinability by reason of sulfur addition, but also that they must persist in the final product form of the steel, e.g. after hot rolling or like reduction.
- Type II Another type of inclusion, shown in FIG. 2 and identified as Type II, usually consists of much smaller particles, often appearing along grain boundaries or in groups defined by such boundaries, these being sometimes called eutectic inclusions. In other cases, or particularly as a result of reduction by rolling, these Type II inclusions appear as long, thin or stringy bodies, but it has been determined that whichever their shape, these Type II inclusions are relatively undesirable and do not have the capability of improving machining properties in the manner of the globular type. It is believed, as indicated by studies, that where circumstances favor the formation of Type I inclusions, the latter are produced by segregation of manganese and sulfur in combination, usually throughout the steel and in approximately uniform though perhaps locally random distribution, at a stage prior to the completion of solidification of the cast ingot.
- Type II inclusions are understood to be formed or precipitated only on the freezing of that liquid phase or portion of the steel which, solidifies last, this stage being very rapid so as not to afford time, as is the case in the formation of the globular (Type I) bodies, for migration or diifusion of manganese sulfide or its constituents into collected, larger masses.
- Type III inclusions are illustrated in 'FIG. 3, being somewhat large, highly angular bodies of correspondingly irregular shape and often having a highly irregular distribution in the steel. They seem, at least in part, to be of the nature of individual crystals or shapes of crystal growth. Inclusions of this sort have also been found to be relatively ineffectual, especially because inclusions of this type or form occur, as has now been discovered, when the steel is excessively deoxidized with aluminum, with the resultant formation of undesirable A1 0 As will be understood, more than one type of inclusion can show up in a given steel; for example, some Type I inclusions may be formed even though the Type II particles are plentiful or greatly predominant, or there may be a number of Type III bodies.
- Type II groups may accompany inclusions of Type III, e.g. as in FIG. 3.
- a scale designation of 20 microns has been added to each of the above microphotographic representations, although it should be understood that the globular inclusions, for instance, may have a considerable range of dimensions, including sizes well above those seen in FIG. 1.
- a 416 grade steel having a manganese-sulfur ratio of only about 1.5 was relatively poor, whereas a heat with the ratio at 3.35 showed much more satisfactory results, eg a severalfold greater tool life, and surface roughness (in and 8-hour test) at about one-half the value for the steel with the lower ratio.
- microprobe examination revealed a significant chromium content in the inclusions of the low-ratio metal, which was essentially absent in the higher-ratio material.
- compositions of typical heats of the 416 and 303 types of stainless steel embodying the invention represent the values being weight percent, as is true for all percentages elsewhere herein except when otherwise specified, and the balance being, of course, iron except for incidental impurities:
- seleniumsulfur inclusions substantially retain their globular, i.e. ellipsoidal shape through conventional forming steps (e.g. hot rolling and cold reduction) intermediate between the casting of the ingot and ultimate production of a bar or the like, this being particularly true with respect to hot rolling.
- ordinary manganese sulfide inclusions even when globular in the as-cast metal, are sensitive to conversion to a long, thin, stringy configuration as a result of hot rolling, the sulfur-selenium inclusions of the present invention have been formed to retain their shape, i.e. by virtue of sufiicient hardness at the rolling temperatures of the order of l,800 F. and higher, so that they are still desirably globular in the ultimate bar or other stock.
- Mn(S, Se) globular inclusions are significantly harder than Mn(S) inclusions at the hot working temperatures, to the critical extent that they keep their characteristics and change only to an ellipsoidal or moderately elongated form, whereas the Mn(S) inclusions are at least in most cases changed by hot rolling to a thin, highly drawn-out, inferior type.
- Mn(S) bodies even when of suitable configuration, are relatively harder and correspondingly less appropriate than the present Mn(S, Se) inclusions, at the temperatures of 1,000 F. or thereabout, reached locally in the metal by the action of the tool in machining.
- the volume fraction of the globular inclusions in the steel is important, especially for the so-called free-machining grades, and, of course, is dependent on the proportions of manganese, sulfur and selenium in the compositions.
- the manganese content of the steel should in most cases be equal to or greater than the value of 2.5 times (often at least 3 times) the sulfur content, or advantageously, such value plus that of the total content of element or elements of the class consisting of selenium and tellurium.
- the inclusions or segregated bodies containing elements S, Se and/ or Te are predominantly and usually substantially all of globular type, provided by manganese in coaction with such elements, and indeed at least predominantly characterized by the presence of quantities of such elements in combination with manganese, the inclusions also being predominantly free of unwanted elements such as chromium.
- tellurium functions similarly to selenium and may in at least a number of instances be employed wholly or partially as an alternative, although selenium is presentely deemed to be of special l 1 advantage, economically and otherwise, and is therefore chiefly, considered in the exemplification of the invention.
- the volume fraction which is the ratio of the total volume of inclusions to the total volume of metal, measured in percent, may range from 0.1% to an upper convenient limit of about 4%.
- the optimum or ordinarily desired values differ for various grades or types of stainless steel, and depend on whether the steel is specially designed for machinability or whether the invention is employed as a supplemental improvement of machinability in steels where other characteristics are paramount.
- the obtainable volume fraction of inclusions for a given content of S, Se and/ or Te has been measured as lower for austenitic steel than for martensitic compositions, and the cause of this difference, e.g.
- a volume fraction of 0.3% to 1.5% has been found very suitable, the range of about 1% and above being especially preferred. In the above examples A and B of such steel, the volume fractions were about 0.9 and 1.1%. Likewise in the straight-chromium, martensitic, free-machining grade 416, best results have been achieved with a volume fraction of 0.9% to 2.5%, present special preference being for values in the range approaching 2% and upwards. In the stated examples A and B of 41 6, the volume fractions were about 1.8 and 1.5%.
- austenitic and martensitic grades are believed to be applicable in general to other so-called free-machining stainless steels, respectively as some may be of austenitic or martensitic character; more generally, in stainless steels designed to have high machinability, including ferritic grades of the 400 series and precipitation-hardening grades, useful results are achieved in the range of volume fraction, for inclusions, from 0.3 to 4%, preferably 1 to 2% as may be readily ascertained (by test if necessary) for any given composition of such steel.
- the improvements herein described are applicable for achieving some useful betterment in machinability, as at least to aid in some necessary drilling, cutting or shaping operations.
- the volume fraction of the inclusions may have to be relatively low, i.e. about 0.1 to 0.2%, but may be higher if possible.
- a special requisite of superior machinability in accordance with the present invention is the control of the inclusions to have the desired globular shape, this being primarily achieved by the stated incorporation of selenium (or tellurium) in the range upwards of 0.01% and for the special machining grades, preferably upwards of 0.04%.
- the selenium (or tellurium) content should, moreover, ordinarily be equal in amount to at least one-tenth of the sulfur; a range of one-half to one-eighth has been usefully employed in the special machining types with sulfur around 0.2% and above, but providing, of course, that a minimum absolute amount of this element, e.g. selenium, is present.
- Higher relative proportions of selenium are ordinarily requisite in stainless steel grades with very low sulfur, e.g. 0.03% S max.; with 0.01 to 0.03% Se, the latter may equal from one-half to twice or more of the sulfur content.
- FIGS. 4 and 5 of the drawings the first of these being a microphotographic showing of inclusions of the globular (specifically, ellipsoidal) character which are found in steels of the invention, such as the 303 and 416 examples noted above.
- the globular character specifically, ellipsoidal
- FIG. 5 shows long, thin inclusions which, as explained above, are undesirable and which are found, for example, in a 416 grade steel lacking selenium and processed, through hot rolling, from a heat having a manganesesulfur ratio of about 3.
- Examination of a number of heats made in accordance with the invention have indicated that the inclusions (in the final product, after hot working) are at least predominantly, preferably very predominantly, Type I as shown in FIG. 4, it being understood that advantageously or more of the total inclusion volume, and most usefully over is in this form.
- FIGS. 6 and 7 afford some information about the examples of 303 and 416 grades made in accordance with the invention, e.g. as above.
- These are computer-plotted graphs of information derived, by sensitive scanning instrument, from sections through the steels, and represent the size distribution of transverse and longitudinal dimensions of the measured intercepts of the inclusions. Since the intercepts or sections of the ellipsoidal inclusions will often or perhaps mostly occur at other than central localities, the measured dimensions have average values considerably smaller than those of the actual inclusions, which are believed to predominate, roughly, in a length range of 2 to 20 or 30 microns (with instances up to 50 or even 80), but the plotted data are deemed of some significance in a relative sense, e.g. for comparison with readings of other specimens made on intercepts in the same way. Moreover, the graphs exclude measurements less than 0.2 micron, as being below the reliable range of the measuring instrument, but it is likely that the curves would slope down to low values in such regions.
- the broken lines represent the transverse (narrower) dimensions and the solid lines, i.e. horizontal, represent the longitudinal dimensions or length of the inclusion intercepts.
- the solid lines i.e. horizontal, represent the longitudinal dimensions or length of the inclusion intercepts.
- FIG. 6 grade 416) about 45% of the measured inclusion-section widths (transverse) were between 0.4 and 0.8 micron, and about 33% of the measured lengths were in the same range; other particulars of size distribution for the intercepts are similarly readable in FIG. 6, and likewise in FIG. 7 for one example of grade 303.
- all this relates to the size of the inclusion sections that were intercepted, rather than to the actual inclusion sizes, which were much larger.
- a further feature of the invention embodied in the examples of stainless steel set forth above and in their method of production, embraces the control of such production, including special aspects of the treatment of the metal, so as to afford an output of produced steel wherein the content of aluminum oxide is reduced to and maintained at an extremely low value.
- free-machining steels for example of the 416 grade
- FIG. 11 shows the results of 8-hour screw machine tests on specimens from two heats of steel which are respectively designated as C and D, both being stainless grades of type 416 having manganese sulfide inclusions and manganese-sulfur ratios respectively of 2.9 and 3.2, i.e. within the range presumably requisite for useful machining.
- the compositions were, approximately, 0.13% C, 1.1% Mn, 13% Cr. other elements within A.I.S.I. maximum limits for this grade, S, respectively 0.38 and 0.33%, and no Se or Te.
- the surface finish is plotted against the produced number of pieces, for these two steel specimens. Heat C maintained a fairly level surface finish throughout about 1,500 pieces at 180 s.f.p.m.
- FIG. 12 where a series of heats of 416 types stainless steel having the same kind of basic composition as heats C and D are shown as subjected to the automatic screw machine test, being respectively as follows:
- a heat here designated E having a manganese-sulfur ratio of 2.92, but containing no selenium addition and thus in no way embodying the present invention.
- E A heat here designated E, having a manganese-sulfur ratio of 2.92, but containing no selenium addition and thus in no way embodying the present invention.
- ferrosilicon 75% Si, 1 to 2% Al
- the steel analyzed 0.004% A1 2.
- F selenium was added, in amounts of 0.05%, the manganese-sulfur ratio being still approximately 3, i.e. 3.12.
- the content of A1 0 was 0.0038%.
- FIG. 8 shows a steel such as heat E, with relatively imperfect sulfide inclusions, and large particles of aluminum oxide, being the very dark irregular masses.
- FIG. 10 which shows a steel containing selenium but with no effort to reduce aluminum oxide--thus corresponding to heat Fthe dark aluminum oxide particles are noted to have become incorporated with the sulfide-selenium inclusions, and indeed in part to be coated or covered by the material of such inclusions.
- FIG. 9 shows the highly desirable, alumina-free inclusions constituted by steel such as that of heat G or the specific examples of 416 and 303 (A and B for each) given above.
- the aluminum oxide in the finished billet of steel should be not more than 0.0025%, and indeed most advantageously and critically for best results, not more than 0.002%
- ferrosilicon containing 75% silicon by weight
- the aluminum content of this material e.g. as impurity in it
- the actual amount of ferrosilicon added for a given heat will, of course, depend on the amount of silicon already present, whether incidentally or otherwise, and available to coact with the ladle addition. As indicated below, other agents can be used instead of silicon.
- vacuum dexoidation should be appropriate, it has appeared to involve some difficulty because the preferred Mn(S, Se) inclusions apparently embrace some oxy-type combination of the elements for best effect and vacuum treatment depletes the available oxygen too much. It is nevertheless conceived that in some cases and with special control or other compensation, vacuum techniques may not necessarily be excluded.
- a particularly effective procedure for producing stainless steel in accordance with the invention thus includes the steps of deoxidizing the metal, as in the ladle, by addition of silicon or other agent having no more than a very low aluminum content.
- silicon or other agent having no more than a very low aluminum content.
- con taining 75% Si this should be not more than 0.5% aluminum.
- the deoxidizing agent of which other examples are rare earth elements such as lanthanum, cerium, and others, should not introduce more than about 0.003% aluminum, measured as weight percent of the steel, and preferably not more than 0.0025%.
- a further step in the production process is that each completed heat of steel is tested by analysis, for example in ingot or billet form, to determine the aluminum oxide content, and the actual production of finished metal to constitute a truly machinable prodnet is selected as those beats for which the analysis shows a content of alumina not greater than 0.002%.
- the product may be diverted to other uses, so that the controlling operation, as just explained, restricts production to the stated limit.
- Analysis of the aluminum oxide present may be achieved in any suitable fashion, i.e. in accordance with any of various available chemical and spectrographic procedures.
- One suitable mode of examination for instance, has involved obtaining a quantity of chips of the steel, including fine particles, by milling or drilling, e.g. l-l5 grams. These are dissolved in suitable acid (hydrochloric and hydrofluoric) and filtered. The residue containing the acid insoluble aluminum may then be analyzed for such aluminum by appropriate spectrographic technique. For instance, one convenient process involving fusing the residue in potassium pyrosulfate, and then dissolving the solidified fusion product in concentrated hydrochloric acid containing yttrium (dissolved therein as oxide) as internal spectrographic standard.
- a preferred treatment which is believed to result in a microstructure of martensite and ferrite that is peculiarly appropriate for machining, involved first heating the steel, in rod or other finished stock shape after all hot rolling and any cold drawing that may have been used, to the lower critical temperature (which might be, for example, 1500 F.), and holding at that temperature for one hour or more. The temperature was then raised about 100 F. and again held for a predetermined time, for example one hour or more, being thereafter air cooled to a suitable low value, such as room temperature, or more generally, below 200 F.
- the lower critical temperature which might be, for example, 1500 F.
- the steel was tempered in conventional fashion, with attainment of hardness in the range commonly desired for martensitic stainless steel of these grades.
- the tempering treatment involved heating at temperatures conventionally appropriate, for instance in the range l050 to 1l0O F. for 1 hours, then cooling to room temperature.
- desired hardness e.g. Brinell hardness values in the range of 195 to 250, preferably 195 to 220; nor was there difficulty in ultimately hardening machined products by standard procedure to satisfactory values such as Rockwell 40C to C.
- compositions of a number of steels which were subjected to the preferred type of heat treatment and which also serve to illustrate further compositional variations in this 416 grade, within the invention:
- the upper and lower critical points vary with the composition of the steel, in accordance with recognized principles, determinations for particular cases being thus readily achieved from known data, or by tests if necessary.
- the critical point values vary with composition, particularly manganese content in these martensitic grades of the 400 series. Both the lower and upper critical points fall with increase of manganese, the change in the lower critical temperature being considerably larger in proportion.
- the lower critical point A is in the range of 1560 to 1425 F. for 0.45 to 2.14% Mn
- the upper point (A or complete austenitizing temperature is in the range of 1775 to 1750 F. for the same Mn range.
- the critical points which define the range over which the structure of the steel undergoes change in the usual manner (being completely austenite above the upper point), are different for heating and cooling, being higher when attained in the course of heating.
- the critical points mentioned and exemplified above are those for heating, and for brevity these points are simply identified as A and A without a further qualifying designation.
- the basic or simpler heat treatment involves a temperature well within the A -A region, e.g. at least 50 F. above A and 50 F. below A and advantageously in a range departing by about 100 F. from each point, and where the two-stage operation is used, starting at A the second step is preferably 100 F. to 150 F. above it.
- the time at each selected temperature, for either mode, is usually one hour or more, preferably 1 /2 hours for bars and the like, and longer times for heavier sections.
- compositions embodying the improved Mn(S, Se) inclusion, of rare earth elements in general any one or more of this known class and in a specific, practical sense, combinations of lanthanum and cerium or a selection of one or more of the so-called cerium earths, notably lanthanum, cerium, and neodymium.
- cerium earths notably lanthanum, cerium, and neodymium.
- the rare earth metal additions in this selected case afforded an improvement in tool life, of notable advantage especialy in that tool life Without the additions happened to be somewhat less than optimum.
- This heat had excellent machined surface finish characteristics, which were not significantly affected by the rare earth additions.
- Some tests on steel of another specific composition tended to indicate that machinability improvement with rare earth metals may involve some correlation between the amount of such addition and the content of sulfur, or sulfur and selenium, in the steel. For instance, with a lower sulfur content than in the above heat, tool life improvement was selectively noted for an addition of 2 lbs. rather than 3 lbs. of the rare earth alloy per ton. There was also indication, in further tests, that with a larger content of manganese, machinability advantage with rare earth elements may be less.
- the procedure of makaing stainless steel may include the step of supplying rare earth metals, in suitable metallic form as above, to the melt at the time of pouring, for instance in the ladle in appropriate amount, as of the order of 2 to 4 lbs. per ton.
- the present indication is that the rare earth content is in the range of 0.02 to 0.3%, preferably 0.05 to 0.2%, the above additions to heat H, measured as 2 and 3 pounds per ton, being equivalent to about 0.1 and 0.15%, respectively.
- the compositions ranged approximately as follows: 0.07 to 0.12% C (mostly below 0.1%), 1.6 to 1.9% Mn, 0.025 to 0.04% P, 0.25 to 0.35% S (mostly above 0.3%), 0.3 to 0.7% Si, 0.15 to 0.35% Cu, 9.05 to 9.5% Ni, 17.0 to 18.2% Cr, 0.22 to 0.58% Mo, and 0.04 to 0.08% 'Se, with A1 0 below 0.002% in the several instances where it was controlled.
- A.I.S.I. 430F which is a ferritic steel of straight-chromium type with chromium 14-18% (usually about 17% the composition as to sulfur, selenium or tellurium and low content of aluminum oxide may be the same as for martensitic grade 416.
- Tecontaining specimens were found to machine to a sig- Machinability tests of these heats designated L and M showed that the properties were appreciably improved over those of ordinary heats of the standard compositions.
- the aluminum oxide content was, in each case, controlled to fall below 0.002%, but since these are austenitic steels, the special heat treatment was not employed.
- these grades are intended to have high corrosion resistance (very high, in the case of 316), thus limiting the amount of sulfur that might be included.
- Type 304 is also expected to be capable of highly polished or bright surface characteristics for decorative purposes.
- Still another type of stainless steel for which some tests of the invention have been made is the so-called 17-4 precipitation hardenable steel, as for example 0.045% C, 3.4% Cu, 4% Ni, 16% Cr.
- 17-4 precipitation hardenable steel as for example 0.045% C, 3.4% Cu, 4% Ni, 16% Cr.
- selenium where sulfur content was around 0.01 to 0.02%, afforded modest improvement, while in situations where the sulfur content was allowed to rise to 0.15% and above, correspondingly larger amounts of selenium, e.g. 0.03 to 0.1% were employed with significant advantage in the machining properties.
- the control of alumina to low values is readily applicable with corresponding advantage.
- Such feature cannot, of course, be employed in special types which require a significant aluminum content, such as grade 405 and the semi-austenitic grades of precipitation hardenable steels.
- the special heat treatment can be employed, i.e. in substitution for a conventional treatment, without substantial detriment to the ultimate precipitation hardening which thereafter involves heating at 900 F. or higher, and air quenching, and which is performed on the finished piece after all machining and forming.
- FIG. 13 shows the results of an 8-hour screw machine test for stainless steel of the 416 grade, utilizing tellurium instead of selenium.
- the composition was essentially similar to several 416 heats above, having about 0.11% C, 1.1% Mn, 12.6% Cr, and with sulfur about 0.35%, Te 0.04%, and no Se.
- the uppermost curve N in the figure is that for a complete test with steel from an ingot of such heat which did not have the Te addition, while the lower curve P represents an ingot in which the tellurium addition Was made, both tests being performed at 180 s.f.p.m. for a full run of 1500 pieces.
- the comparison pieces N showed a relatively high surface roughness, rising early in the run to a rather high value (in R.M.S. microinches),
- the results of the invention in respect to machinability are unusually good, especially in grades such as 303 and 416.
- the inclusions appear to function very effectively, and indeed appear to satisfy very well two specific aspects of their function, namely that in machining operations the inclusions produce microcracks in the shear zone, this promoting local fracture and reducing the energy consumed, and further, that the inclusion material deposits on the tool surface, in very small amounts, thus reducing tool wear.
- the attainment of these results as to machinability has been thoroughly established with the 8-hours screw machine tests and indeed with such tests of the several major features, notably in the case of grades 303 and 416, an embodied or carried out in large-scale heats, e.g. regular -ton electric furnace heats.
- stainless steels here contemplated include such as may contain 10-27% chromium and 0 to 22% nickel.
- the so-called straight-chromium grades e.g. up to 27% Cr usually have less than 3% Ni or in most cases substantially less than 1%, e.g. as in type 416, which has 12-14% Cr.
- the chromium-nickel grades may contain 14-26% Cr and 4-22% Ni with many types, among the 300 series, characterized in 1520% Cr and 6-15% Ni, the nickel content being 8-13% for 21 certain more common austenitic types.
- grade 303 is specified as 17-19% Cr and 8-10% Ni.
- manganese can range from 0.3 to 10% in stainless steels, a preferred minimum for the invention is 0.8%, with special advantage, in the free-machining grades, at 1% or more and in some instances, notably the austenitic series, 1.5% or above; not more than 3% is necessary in many cases, and indeed preferably not more than 2%, or advantageously less.
- Optional or incidental elements in stainless steels may include up to 4% molybdenum, though usually below 1% in the machining grades, up to 5% copper when desired, and up to 1.5 silicon, but preferably not over 1% Si.
- Total additions of minor, special-purpose elements up to 2% e.g. up to 1.5 of any one are conceivable, for instance such as Ti, Cb, Ta, C0 and Zr.
- the balance of the composition is iron (e.g. at least 50%) and incidental impurities, together with carbon 0.01 to 1.2%, more usually 0.05 to 0.2%. All percentages herein are by weight, except in reference to the volume content of inclusions.
- a minimum is 0.15%, but for best results with the invention, at least 0.2% and notably 0.3%, e.g. in the range to 0.45% or conveniently not more than 0.4%.
- the material of the selenium and tellurium class can range up to 0.3%, or with further cost, to 0.4%, there is special advantage in the economical lower ranges noted earlier above. Indeed some drill penetration-time tests have indicated little, if any, and advantage in that specific respect, in carrying selenium to as much as 0.15%, or indeed much over 0.1%.
- the content of this addition is preferably 0.02% or above, or advantageously at least 0.03%, to approach special machinability as evidenced by volume fraction of inclusions.
- Such inclusion volume content is advantageously 0.2% and preferably 0.3%, or more, a minimum of 0.5% by volume being greatly preferred to achieve a machining-type steel.
- Maintenance of an element such as selenium at the lowest weight-percent level consistent with optimum results is specially desirable, in that such element, in excess, may tend to have a harmful effect on surface properties of stainless steel.
- the invention affords notable improvement in machinability, attributed in significant part to the content of relatively large globular inclusions, which are substantially free of iron and chromium and which are understood to consist essentially, or at least predominantly, of the nature of sulfides, selenides and tellurides of manganese, such terms being employed to include so-called oxy compounds, e.g. oxysulfide.
- oxy compounds e.g. oxysulfide.
- Stainless steel which has been subjected to hot working to a thickness reduction of at least about 50% and which has improved machinability, said steel consisting essentially of: 0.01 to 1.2% carbon; 10 to27% Cr, 0 to 22% Ni, 0.3 to 10% Mn; 0.15 to 0.7% S and a total of 0.03 to 0.1% of element or elements selected from the class consisting of selenium and tellurium; balance iron and incidental impurities; said steel being characterized by absence of aluminum oxide above 0.002% of said oxide; and said steel being characterized by globular inclusions which are distributed throughout the steel in total quantity of 0.2 4% by volume and which are predominantly provided by and collectively contain Mn, S and said selected element or elements, the content of Mn in the steel being at least equal to 2.5 times the content of S, and the total of said selected element or elements being equal to at least 10% of the content of S.
- Stainless steel which has been subjected to hot Working to a thickness reduction of at least about 50% and which has improved machinability, said steel consisting essentially of: 0.01 to 1.2% carbon; 10 to 27% Cr, 0 to 22% Ni, 0.3 to 10% Mn; 0.15 to 0.7% S and a total of 0.03 to 0.1% of element or elements selected from the class consisting of selenium and tellurium; 0.02 to 0.3% of rare earth element or elements which consist predominantly of element or elements selected from the class consisting of lanthanum and cerium; balance iron and incidental impurities; said steel being characterized by absence of aluminum oxide above 0.002% of said oxide; and said steel being characterized by globular inclusions which are distributed throughout the steel in quantity of 0.2 to 4% by volume and which are predominantly provided by and collectively contain Mn, S and said element or elements selected from the class of Se and Te, the content of Mn in the steel being at least equal to 2.5 times the content of S, the total of said element or elements of the class of Se and Te being equal to at
- Stainless steel as defined in claim 1 which is essentially straight-chromium stainless steel having a content of 11 to 18% Cr and less than 3% Ni.
- Stainless steel as defined in claim 1 which is chromium-nickel stainless steel having a content of 15 to 21% Cr and 6 to 15% Ni.
- Stainless steel as defined in claim 1 which is a martensitic grade of stainless steel and in which the selection of element or elements from the aforesaid class of selenium and tellurium is Se, and the content of S, Se and Mn coacts to provide a content of said globular inclusions which is greater than 1% by volume of the steel.
Abstract
Description
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US00289589A US3846186A (en) | 1970-04-06 | 1972-09-18 | Stainless steel having improved machinability |
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US00289589A US3846186A (en) | 1970-04-06 | 1972-09-18 | Stainless steel having improved machinability |
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Cited By (13)
Publication number | Priority date | Publication date | Assignee | Title |
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US4091147A (en) * | 1975-11-07 | 1978-05-23 | Nippon Steel Corporation | Welded steel products having low sensitivity to weld cracking and a production method thereof |
US4219356A (en) * | 1977-09-20 | 1980-08-26 | Daido Tokushuko Kabushiki Kaisha | Machinable ferritic stainless steels |
FR2456785A1 (en) * | 1979-05-17 | 1980-12-12 | Daido Steel Co Ltd | DECOLLETING STEEL CONTAINING DETERMINED INCLUSIONS AND A PROCESS FOR THE PREPARATION THEREOF |
US4240827A (en) * | 1977-12-12 | 1980-12-23 | Sumitomo Metal Industries Ltd. | Nonmagnetic alloy steel having improved machinability |
US4279646A (en) * | 1978-12-25 | 1981-07-21 | Daido Tokushuko Kabushiki Kaisha | Free cutting steel containing sulfide inclusion particles with controlled aspect, size and distribution |
US4340424A (en) * | 1974-04-23 | 1982-07-20 | Daido Tokushuko Kabushiki Kaisha | Ferritic stainless steel having excellent machinability and local corrosion resistance |
EP0088814A1 (en) * | 1982-03-16 | 1983-09-21 | Carondelet Foundry Company | Alloy resistant to corrosion |
US5118341A (en) * | 1991-03-28 | 1992-06-02 | Alcan Aluminum Corporation | Machinable powder metallurgical parts and method |
EP0635581A1 (en) * | 1993-07-20 | 1995-01-25 | Toyota Jidosha Kabushiki Kaisha | Ferritic heat-resistant cast steel and process for producing the same |
WO1996001911A1 (en) * | 1994-07-07 | 1996-01-25 | Crs Holdings, Inc. | Free-machining austenitic stainless steel |
RU2635646C1 (en) * | 2017-03-20 | 2017-11-14 | Юлия Алексеевна Щепочкина | Steel |
US10260123B2 (en) * | 2014-07-03 | 2019-04-16 | Nippon Steel & Sumitomo Metal Corporation | Rolled steel bar for machine structural use and method of producing the same |
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1972
- 1972-09-18 US US00289589A patent/US3846186A/en not_active Expired - Lifetime
Cited By (16)
Publication number | Priority date | Publication date | Assignee | Title |
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US4340424A (en) * | 1974-04-23 | 1982-07-20 | Daido Tokushuko Kabushiki Kaisha | Ferritic stainless steel having excellent machinability and local corrosion resistance |
US4091147A (en) * | 1975-11-07 | 1978-05-23 | Nippon Steel Corporation | Welded steel products having low sensitivity to weld cracking and a production method thereof |
US4219356A (en) * | 1977-09-20 | 1980-08-26 | Daido Tokushuko Kabushiki Kaisha | Machinable ferritic stainless steels |
US4270950A (en) * | 1977-09-20 | 1981-06-02 | Daido Tokushuko Kabushiki Kaisha | Machinable ferrite stainless steels |
US4240827A (en) * | 1977-12-12 | 1980-12-23 | Sumitomo Metal Industries Ltd. | Nonmagnetic alloy steel having improved machinability |
US4279646A (en) * | 1978-12-25 | 1981-07-21 | Daido Tokushuko Kabushiki Kaisha | Free cutting steel containing sulfide inclusion particles with controlled aspect, size and distribution |
FR2456785A1 (en) * | 1979-05-17 | 1980-12-12 | Daido Steel Co Ltd | DECOLLETING STEEL CONTAINING DETERMINED INCLUSIONS AND A PROCESS FOR THE PREPARATION THEREOF |
EP0088814A1 (en) * | 1982-03-16 | 1983-09-21 | Carondelet Foundry Company | Alloy resistant to corrosion |
US5118341A (en) * | 1991-03-28 | 1992-06-02 | Alcan Aluminum Corporation | Machinable powder metallurgical parts and method |
EP0635581A1 (en) * | 1993-07-20 | 1995-01-25 | Toyota Jidosha Kabushiki Kaisha | Ferritic heat-resistant cast steel and process for producing the same |
US5470402A (en) * | 1993-07-20 | 1995-11-28 | Toyota Jidosha Kabushiki Kaisha | Ferritic heat-resistant cast steel and process for producing the same |
WO1996001911A1 (en) * | 1994-07-07 | 1996-01-25 | Crs Holdings, Inc. | Free-machining austenitic stainless steel |
KR100244374B1 (en) * | 1994-07-07 | 2000-03-02 | 바이샴펠 존 | Free machining austentic stainless steel |
US10260123B2 (en) * | 2014-07-03 | 2019-04-16 | Nippon Steel & Sumitomo Metal Corporation | Rolled steel bar for machine structural use and method of producing the same |
US10266908B2 (en) * | 2014-07-03 | 2019-04-23 | Nippon Steel & Sumitomo Metal Corporation | Rolled steel bar for machine structural use and method of producing the same |
RU2635646C1 (en) * | 2017-03-20 | 2017-11-14 | Юлия Алексеевна Щепочкина | Steel |
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