US3729344A - Process for making thermally hardened wrought stainless steel shaped member having a duplex ferritic/martensitic microstructure - Google Patents

Abstract

THERMALLY HARDENED AND STRENGTHENED MEMBERS MADE FROM STAINLESS STEEL CONTAINING 0.04-0.10% CARBON, 0.040.08% NITROGEN, UP TO 1% EACH OF MANGANESE AND SILICON, 16-17.5% CHROOMIUM, UP TO 1.0% NICKEL AND 0.75-1.5% MOLYBDENUM, WHICH RESPOND TO TEMPERING, CAN BE COLD WORKED UP TO ABOUT 50 TOO 75% REDUCTION IN AREA AND HAVE A DEPLEX MICRO-STRUCTURE OF BOOTH FERRITE AND MARTENSITE.

Description

United States Patent PROCESS FOR-MAKIN G THERMALLY HARDENED WROUGHT STAINLESS STEEL SHAPED MEM- BER HAVING A DUPLEX FERRITIC/MARTEN- SlTlC MICROSTRUCTURE Stephen M. Lukes, West Reading, and Robert L. Caton, Sinking Spring, Pa., assignors to Carpenter Technology Corporation, Reading, Pa. No Drawing. Filed Mar. 12, 1971, Ser. No. 123,813

Int. Cl. C21d 1/18, 7/14 U.S. Cl. 14812.4 8 Claims ABSTRACT OF THE DISCLOSURE Thermally hardened and strengthened members made from stainless steel containing 0.040.10% carbon, 0.04- 0.08% nitrogen, up to 1% each of manganese and silicon, 16l7.5% chromium, up to 1.0% nickel and 0.751.5% molybdenum, which respond to tempering, can be cold worked up to about 50 to 75% reduction in area and have a duplex micro-structure of both ferrite and martensite.

This invention relates to wrought-shaped members made from chromium-molybdenum bearing stainless steel and, more particularly, to such shaped members which have been thermally hardened and strengthened and which also respond to tempering.

Because of the high cost of nickel and the fact that at time it has been in short supply, a chromium-iron stainless steel alloy designated A.I.S.I. Type 430 was developed containing 16.0018.00% chromium, a maximum of each of 0.12% carbon, 1.00% manganese, 1.00% silicon, 0.040% phosphorus, and 0.030% sulfur, and the balance iron except for incidential impurities. Type 430 (often designated as 17% Cr steel) was very successful and in much demand, particularly for forming trim in the automotive field. Nevetheless, to improve its resistance to atmospheric corrosion in the presence of materials frequently used on roads, Type 430 was modified by the addition of 0.75-1.25% molybdenum, and this modification is known under the designation A.I.S.I. Type 434. Type 430 and Type 434 are known in the trade as ferritic stainless steels which could not be hardened (and thus could not be strengthened) by heat treatment. Such alloys can be hardened to some extent by mechanical deformation, and such hardening is usually carried out by cold working the material. Because of the limited extent to which such alloys could be mechanically hardened without objectionally affecting their ductility, other more expensive alloys, usually more expensive both in composition and to form and shape, are used when tensile strengths in excess of about 90,000 p.s.i. and good ductility are desired.

In spite of the fact that Type 434 stainless steel had been known as a ferritic steel which was not hardened and strengthened by heat treatment, but rather was subjected to objectionable grain coarsening when heat treated, we have unexpectedly discovered that by carefully adjusting the amounts of chromium and the elements silicon and molybdenum present in the composition which, like chromium, form ferrite and the amounts of nickel and the elements carbon, nitrogen and manganese present in the composition which, like nickel, form austenite, then the alloy can be hardened by heat treatment.

It is therefore a principal object of this invention to provide shaped members made of Type 434 stainless steel which are hardened by heat treatment.

A more specific object of this invention is to provide thermally hardened and strengthened members formed of Type 434 stainless steel which also respond to tempering 3,729,344 Patented Apr. 24, 1973 ICC and which combine a unique degree of hardness, strength and ductility.

The foregoing, as well as further objects and advantages of this invention are obtained by forming and heat treating shaped members from a composition which by weight contains about 0.04-0.10% carbon, up to about 1.0% manganese, up to about 1.0% silicon, about 0.04-0.08% nitrogen, about 16.0-17.5% chromium, up to about 1.0% nickel, about 0.75-1.5% molybdenum, and the balance iron except for incidental impurities which, among those elements usually present in relatively small quantities in such stainless steels, should include no more than about 0.04%, preferably no more than about 0.025%, phosphorus, no more than about 0.03%, preferably no more than about 0.015%, sulfur and no more than about 0.50%, preferably no more than about 0.25%, copper.

Not only must the elements carbon, manganese, silicon, nitrogen, chromium, nickel and molybdenum be maintained within the ranges stated, but in each heat these ele ments must be carefully balanced so that the total amount of chromium available (Cr av.) ranges from about 12.5 to 14.5 when calculated as the difference, in weight percent, between the amount of chromium equivalent (Cr eq.) and the amount of nickel equivalent (Ni eq.) present with chromium equivalent defined as Cr eq.=Percent Cr+l.5 (percent Si)+percent Mo and with nickel equivalent defined as Ni eq.+30 (Percent C+percent N) +percent Ni+0.5 (Percent Mn) Within the range stated and when the balance of the elements is such that the amount of chromium available, as was just defined, equals about 12.5 to 14.5 and when heat treated, as will be pointed out hereinbelow, at least a substantial part of the alloy undergoes transformation to martensite. Depending upon how the composition is adjusted, somewhat less or more than a major amount of the alloy microstructure will be martensitic, and the remainder will be ferritic so that a duplex microstructure in which both ferrite and martensite are present is an essential characteristic of thermally hardened and strengthened members formed in accordance with the present invention. Such thermally hardened and strengthened members are cold worked to provide finished articles which retain their duplex microstructure and combine a unique degree of strength and ductility as compared to similar articles heretofore produced from A.I.S.I. Type 434.

Shaped members which have been prepared and heat treated in accordance with the present invention can be provided with a wide range of desirable properties. Such members, when heat treated and quenched, have a duplex microstructure made up of ferrite and untempered martensite. In that condition, such members can have a hardness from about Rockwell B95 to Rockwell C38 and a corresponding room temperature ultimate tensile strength of about 100,000 p.s.i. to about 175,000 p.s.i. When tempered, such members can have an ultimate tensile strength of about 95,000 p.s.i. to 150,000 p.s.i., and then, when cold worked to effect about 25% reduction in area, finished articles can be provided having an ultimate tensile strength of about 120,000 p.s.i. to 175,000 p.s.i. After being hardened and tempered, such heat treated members can be subjected to more than 25 percent reduction in cross-sectional area, and up to about 50 to percent cold reduction can be provided with a corresponding increase in strength and with no more than a tolerable reduction in ductility.

Carbon and nitrogen are both very powerful austenite formers, each being about 30 times more effective than nickel on a weight-for-weight basis. Thus a minimum of about.0.04%. of each of these elements is present in the composition of this invention. Above about 0.10% carbon tends to make the alloy too hard; it tends to adversely affect the corrosion resistance of the material and to make it difficult to form. Preferably carbon is maintained at about ODS-0.07% for best results and to facilitate maintaining the preferred chromium available level of about l3.5-14.5%. Nitrogen in an amount above about 0.08% cannot readily be maintained in solution in this alloy and in the absence of relatively expensive procedures would result in blowy metal. Larger amounts of nitrogen would also be undesirable because, like carbon, it would tend to make the alloy too hard and too diflicult to form. Preferably nitrogen is limited to no more than about 0.06% for best results.

, From a consideration of the manner in which the amount of nickel equivalent is calculated, it is evident that the carbon and nitrogen content of this alloy must be adjusted within the limits stated and in accordance with whether the larger or smaller amounts of the chromium equivalent elements are present in order to permit attainment ofthe required amount of chromium available of from 12.5 to 14.5%. Maintaining carbon or nitrogen or both of them within the preferred ranges of 0.05-0.07% and 0.04-0.06% respectively facilitates attainment of the required balance of the alloy both within the broad and preferred ranges thereof.

Manganese and silicon are used as deoxidizers, and manganese also functions to remove sulfur during the melting of the steel. In keeping with good metallurgical practice, up to 1% of each can be present in the composition, and unless special precautions are taken, small amounts of these elements will be present in the steel. Because of their effect on the microstructure of the composition, best results are attained when the amounts of manganese and silicon retained in the composition on completion of melting and casting of the alloy are controlled and are kept within the preferred range of 04-06% manganese and 0.3-0.5 silicon. By stabilizing the manganese and silicon contents in this way, control of the chromium available in the alloy is facilitated especially when it is desired to maintain the chromium available in the preferred range of about 13.5-14.5.

At least about 16% chromium is required to provide the oxidation and corrosion resistance desired and also to contribute to the strength of the alloy. Chromium is the main alloying addition and, because it is a ferrite former, must be carefully adjusted so that the desired martensite reaction by which the alloy is hardened and strengthened will take place. When too much of such a ferrite-forming element is present, the alloy cannot be hardened and strengthened by heat treament to the desired degree, if at all. Thus, to facilitate maintaining the required balance of the alloy of 12.5 to 14.5 chromium available, the chromium content is limited to about 17.5%. Preferably 16.3- 16.9% chromium is used because of the way it works with the other ferrite-forming elements and the austenite-forming elements in establishing the desired balance of the alloy to ensure a duplex microstructure.

Molybdenum in an amount of 0.75-1.25% was added to A.I.S.I. Type 430, as was seen, to provide the hitherto known A.I.S.I. Type 434 alloy with improved corrosion resistance over A.I.S.I. Type 430. Molybdenum, like chromium, is a ferrite-forming element and, when present, in too large amounts, may upset the balance of the alloy and adversely affect its strength. However, the determination of chromium available in the alloy by the relationship as defined hereinabove permits the addition of as much as 1.5% molybdenum. On the other hand, when the amount of molybdenum included becomes less than about 0.75%, there is not enough present to provide the desired resistance to corrosion as in the case of the corrosive media to which automobiles are exposed in use. Preferably 0.85- 1.0% molybdenum is included.

Because of its cost and its effect on the microstructure of this composition, nickel is not a desirable addition to this alloy. However, small amounts up to about 1.0% can be present and can usefully be added to assist in stabilizing the desired duplex microstructure whenever it appears that the amount of the ferrite-forming elements as measured by the amount of chromium available is too high and needs to be offset. Preferably, no more than 0.5% nickel is present, and best results are achieved when about 0.3- 05% nickel is used in conjunction with the preferred amounts of the remaining elements.

Thus, for convenience the preferred composition of this invention is stated in weight percent as Carbon 0.05-0.07. Manganese 0.4-0.6.

Silicon 0.00.5. Phosphorus 0.025 max.

Sulfur 0.015 max. Nitrogen 0.04-0.06. Chromium 16.3-16.9.

Nickel 0.3-0.5. Molybdenum 0.85-1.0.

Copper 0.25 max.

Iron Balance except for incidental impurities.

Cr Av 13.5-14.5.

It is to be noted that some of the advantages of the preferred composition can be obtained by combining the preferred ranges of some of the elements with the broad ranges of the remaining elements. It is also to be understood that if desired, the limits of the preferred and broad ranges of each element can be interchanged as for example in the case of manganese and silicon so as to provide intermediate ranges for manganese of 0.41% and for silicon of 0.31%.

The composition of the present invention is melted using known basic electric arc furnace melting techniques such as are used in melting and casting ingots of such alloys as A.I.S.I. Types 430 and 434. The composition is readily hot worked to desired shapes from a furnace temperature of about 1900 to 2050 F., preferably from a furnace temperature of about 1950 F. Having in mind the finished cross-sectional area of the articles to be made, and that only a total reduction in area of about 75% can be accomplished by cold working, the amount of reduction by hot working beyond that required for metallurgical considerations will be readily apparent. Upon completion of hot working, the hot worked members are then heat treated at a temperature ranging from about 1700 to 1900 F., preferably 1800 to 1900 F., for about onehalf hour to one hour followed by quenching in water or oil. Generally, the higher the temperature at which the heat treatment is carried out, the higher the attained asquenched hardness. The preferred composition is preferably heat treated at about 1850 F. for 1 hour and then quenched.

After being hardened and quenched, tempering is carried out at by heating at a temperature between about 900-1l00 F. for up to about 16 hours, preferably about 1 to 8 hours followed by cooling in air. While some increase in strength and hardness can be obtained by tempering between about 900 to 950 F., ductility is improved by tempering between 950 and 1100 F. Though hardness and strength are reduced as the tempering temperature is increased above about 950 F, the best combination of strength and ductility, as measured by room temperature tensile tests, is obtained in the case of the preferred composition by heat treating for 1 hour at 1850 F. followed by quenching and tempering for 8 hours at about 1000 F.

Within the broad range stated and with Cr av. equal to about- 12.514.5, the hardness (and the corresponding strength) varies inversely as the chromium available. In other words, if it is desired to shift the hardness or strength of an analysis falling within the broad range of this composition, the chemistry can be adjusted to shift the chromium available downward to increase, or upward to decrease the attainable as-hardened hardness. Assuming no change in hardening temperature, the as-hardened and quenched hardness will be correspondingly affected.

The temperature at which heat treatment is carried out also provides a convenient way in which the hardness and strength, and the ductility can be adjusted. For any given composition within the ranges stated, hardness and strength can be increased or decreased depending upon whether the hardening temperature used is raised or lowered. On the other hand, increasing the tempering temperature above 950 to a limit of about 1100" F. serves to increase ductility while reducing hardness. Reducing the tempering temperature in that range has the reverse effect.

Following cleaning in a suitable solution, such as an aqueous solution of 15% nitric and 3% hydrofluoric acids, the thus prepared members are cold worked as by drawing, stamping, or flattening to the desired finished cross-sectional area. In this condition, the material has good ductility and can be bent, punched or otherwise formed to the finished product.

The example having the analyses in Table I was prepared using conventional electric arc furnace melting practices. Before the metal was cast into ingots, at any suitable stage and preferably near completion of refinement of the melt, a check analysis was made and such adjustments in the composition were made as were required to adjust the value of the chromium available in accordance with Cr av.=percent Cr+1.5 (percent Si) +percent) Mo 30 (percent C-l-percent N)percent Ni0.5 (percent Mn) to within the range of 12.5-14.5.

TABLE I aver- Mn Si N Cr Ni Mo age The balance of each of Examples 1-4 was iron except for incidental impurities which included less than 0.025% phosphorous, less than 0.015% sulfur, and less than 0.25% copper. Ingots of each example were hot worked into billets which were in turn hot rolled to 0.312 in. diameter wire which was then hardened by heating at 1850 F. for one-half hour and quenched in oil. The thus thermally hardened members were then tempered at 1000 F. for 8 hours followed by cooling in the air. The hardened and tempered members were cleaned, and then room temperature ultimate tensile strength specimens having a gage diameter of .252 in. and gage length of 1 in. were prepared and tested with the results indicated in Table 11 under UTS for ultimate tensile strength, .2% Y for .2% yield strength, percent e1. for percent elongation, and percent RA for percent reduction in area. The average hardness of the hot worked, hardened and tempered members is also given in Table II.

TABLE III Percent of .2% Y8, Ex. No. p.s.i. UTS, p'.s.i. E1 RA Hardness The thus heat treated and cold drawn .275 in. diameter members of Example 4 were further cold reduced in 3 passes to .120 in. x .360 in. rod by flattening and in that condition had an average hardness of R 29.5.

The thermally hardened wrought members of the present invention are especially well suited to be cold worked into articles such as automobile windshield wiper arms which must have good corrosion resistance to airborne corrodents and to the corrosive media usually encountered along roads. An important advantage of the present invention resides in the flexibility afforded by the way the hardness and ductility required of the finished wiper arms can be anticipated by adjusting the chromium available and the hardening and tempering temperatures to cooperate with the amount of cold reduction to be carried out in finishing the wiper arms.

When greater free machinability is desired than afforded by the composition as thus far described, then any of the usual free machining additives used in the 400 series type stainless steels and in the customary amounts can be used in the present composition so long as whatever impairment of corrosion resistance that may result from such additions can be tolerated in the end use of the finished products. Thus, for example, up to 0.5% of either sulfur or selenium or of both combined could be included for that purpose.

It is also to be noted that, for many purposes, tungsten is the equivalent of molybdenum differing primarily in its effect upon the density of the composition. Thus, tungsten can be substituted for all or part of the molybdenum content in the ratio by weight of 1.2% to 1.6% tungsten to 1% molybdenum.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

We claim:

1. In the process for making a thermally hardened Wrought stainless steel shaped member having a duplex microstructure containing ferrite and martensite consisting essentially by weight of about 0.040.10% carbon, up to about 1.0% manganese, up to about 1.0% silicon, about 0.04-0.08% nitrogen, about 16.0-17.5% chromium, up to about 1.0% nickel, about 0.751.5% molybdenum, the balance essentially iron and incidental impurities which includes the steps of controlling the elements in said steel so that the chromium available is about 12.514.5 as calculated from the relation Cr av.=percent Cr+1.5 (percent Si) +percent Mo30- (percent C-l-percent N) percent Ni0.5 (percent Mn) hot working said steel to form a shaped member, hardening said shaped member by heating at about 1700 to 1900 F., and then quenching and tempering said shaped member by heating at about 900 to 1100" F.

2. The process of claim 1 wherein said steel contains up to 0.5% of sulfur-{selenium as free machining additives.

, ing essentially by Weight of about 0.05.07% carbon,

about 0.40.6% manganese, about 0.3-0.5 silicon, about 0.04-0.06% nitrogen, about 16.316.9% chromium, about 03-05% nickel, about 0.851.0% molybdenum, the balance essentially iron and incidental impurities which includes the steps of controlling the elements in said steel so that the chromium available is about 13.5-14.5 as calculated from the relation Cr av.=Percent Cr+1.5(percent Si)+percent Mo-30- (percent C+percent N)-percent Ni-0.5 (percent Mn) hot Working said steel to form a shaped member, hardening said shaped member by heating at about 1700 to 8 1900 F., then quenching and tempering said shaped member by heating at about 900 to 1100 F.

6. The process of claim 5 wherein said steel contains up to 0.5% of sulfur-l-selenium as free-machining additives.

7. The process of claim 5 wherein said hardening is carried out by heating for up to about 1 hour at about 1800 to 1900 F.

8. The process of claim 7 wherein said tempering is carried out by heating at about 950 to 1100 F.

References Cited UNITED STATES PATENTS 2,772,992 12/1956 Kiefer et a1 14812 3,139,358 6/1964 Graziano 14812 3,183,080 5/1965 Harpster 75126 C 3,201,231 8/1965 Harpster 75-126 C FOREIGN PATENTS 1,145,280 3/1969 Great Britain 14812 WAYLAND W. STALLARD, Primary Examiner Patent No.

Ap ril 24 1973 Stephen M. Lukes and Robert Lo Caton It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column Column Column Column Column line line

line line line line line line line for example read examples and for "was" read were after percent", delce ")"p after "in", delete "the".

6, for as set read are set in Table III, for "7.1." read'--- 71.1

Signed and sealed this 18th flay of February 1975.

(SEAL) Atteet:

Ca KARSHALL DANN Commleeioner of Patents and Trademerks RUTH C. MASON Attesting Officer