US3370994A

US3370994A - Method of softening steels

Info

Publication number: US3370994A
Application number: US510356A
Authority: US
Inventors: Paul J Konkol
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1965-11-29
Filing date: 1965-11-29
Publication date: 1968-02-27
Anticipated expiration: 1985-02-27
Also published as: DE1508454A1; NL6616799A; BE690161A; ES333614A1; FR1501652A

Description

United States Patent 3,370,994 METHOD OF SOFTENING STEELS The initial. hardness of the carbon-containing, high-alloy steels described above is so high that softening such steel compositions to between 20 and 25 R would be highly desirable for Wire drawing and even reducing the hard- Paul J. Konkol, Plum Borough, Pa., assignor to United 5 11688 to less a 30 Be would be satisfactory- States Steel Corporation, a corporation of New Jersey The present mvention provides a method of softening No Drawing. Filed Nov. 29, 1965, Ser. No. 510,356 high yield strength, carbon-containing, alloy steels of the 5 Claimstype described, which comprises a series of temperature excursions to within the intercritical region followed by a subcritical anneal. This is accomplished by subjecting such steels to at least two heating and cooling cycles This nven o r t to a method of Sofienlllg wherein the steel is heated to the intercritical temperature containing, high-alloy steels, More particularly, the invenrange f f b t 1150 F, to 1400" F. and then {i011 relates a method of Softening high-alloy Steels cooled to the martensite transformation range, after ing carbon, nickel and other y g elemmtswhich the steel is annealed in the subcn'tical range of An alloy system which has recently received a great from about 1050 Rt 0 about 1175 F. for a time suitideal of attention because of its high yield strength, i.e. cient to further reduce the hardness. After annealing, the

180-210 K.s.i., is a group of steels that btain t eir steel is cooled from the annealing temperature to permit strength through a combination of carbide precipitation handling. The method is particularly beneficial in softenand age hardening. Such steels contain a large amount of ing carbon-containing, high-alloy steels having at least nickel, i.e. at least 4%, and other alloying elements such 0.06% carbon, at least 4% nickel, and other alloying as chromium, molybdenum, cobalt and vanadium, as elements (usually at least two) from the group consisting Well as a significant amount of carbon, i.e. about 0.06% of chromium, molybdenum, vanadium and cobalt.

or more. Because of the relatively high carbon and alloy The invention will be more fully explained by the folcontent, these steels are quite hard in the as-rolled condilowing description and examples which demonstrate the tion, making cold reduction very difiicult. Moreover, the critical processing conditions and beneficial results obsteels do not respond to conventional softening treattained thereby.

ments, i.e. annealing, due to their hardenability which As discussed above, the high yield strength alloy steels prevents formation of high temperature transformation with which the invention is concerned are not amenable products and also due to their depressed lower critical to softening by conventional annealing practices. To

temperature which prevents an effective subcritical andemonstrate this, a series of compositions described in neal. Table I were given five different conventional softening The mechanical properties of steels of this type make treatments, the results of which are reported in Table it very tempting to employ them as weld filler metals in II. It should be noted that among the compositions are high strength (180-210 K.s.i.) weldments. However, if three which do not contain both chromium and molybthese steels are to be used as filler metal for weldments, denum, and one which does not contain either. Steels they must be softened sulficiently to be drawn into wire without one or both of chromium and molybdenum can for deposition by conventional welding processes. Wire be softened by conventional treatments. The other steels drawing operations can best be performed when the hardof this type, however, which do contain chromium and mess of the rod stock is in the range of R 10 to 20; howmolybdenum are not softened materially by the convenever, wire can be cold drawn from rods as hard as 29 R tional annealing practices.

TABLE I beslgnation Heat No. 0 Mn P S St Ni Cr 00 Mo A1 N. 0. 2s 0.019 0. 002 0. 00s 0. 093 10.1 1. 02 0.00 0. 9s 0.004

R T5990-2 0.20 0.005 0. 001 0. 000 0. 084 7.05 3.13 0113 1 e2 0. 000

1 Ladle Analyses.

TABLE II. ROCKWELL C HARDNESS C onventional Treatments Inter-Critical Sub-Critical Anneal Anstenitic Cyclic Anneal Cyclic Anneal Designation On i ls-Received On Quenchcd Rod On Quenchcd Rod 1,400 B, cycled 4 Times Same, plus 1,300 F., W.Q.

Rod After Htg, From l,700 F., From 1,700 F., Between 1,150 F. and Annealing at (4 Times),

1,150 F./24 hr. 1,150 F./48 hr. 1,100 F./24 hr. 1,250 1 Furnace-Cool +1,150 F./48 hr. +1,100 F.l16 hr.

1 As-Reoeived (hot-rollcd) hardness for composition P, 48.0 R Hardness after normalizing (1,900 F., Furnace Cool) composition P, 42.0 Re.

The softening practices reported in Table II include three subcritical anneals and two austenitic cyclic anneals and a normalizing treatment. The first subcritical anneal involved heating as-received hot-rolled rods to 1150" F. for 24 hours. The second subcritical anneal involved heating rod quenched from a typical hot rolling temperature (l70=0 F.) to 1150 F. for 48 hours. The third subcritic-al anneal involved heating rod quenched from 1700 F. to 1100 F. for 24 hours. The first austenitic cyclic anneal involved heating as-received rod to 1400 F. and then, after cooling to 1150 F., subjecting the rod to four heating and cooling cycles wherein the steel is heated to between 1150 F. and 1250 F. and furnace cooling after cycling. The second austenitic cyclic anneal involved the same conditions as the first austenitic cyclic anneal; however, in addition, the rod was given a final anneal at 1150 F. for 48 hours. The typical hardness of as-received steel and after normalizing is given in the footnote (Table II).

For comparison of the effect of conventional softening treatments with the method of softening high yield strength alloy steels according to the invention, steels of the same composition were given treatments in accordance with the invention and the hardness results are reported in the last column of Table II. In the latter treatment, the steels were cycled four times by heating to 1300 F. and water quenching, after which they were given a final anneal at 1100 F. for 16 hours. As can be seen, the steel compositions are generally considerably softer after processing according to the invention than by conventional annealing practices. The data in Table II also indicates that the hot-rolled rods are quite hard in the as-received condition, as shown by a hardness of 48 R of a typical steel P (see footnote). A subcritical anneal (heating to a temperature below the lower transformation temperature of 1150 F.) on the as-received rods failed to soften the steels below 29 R with the exception, noted above, of steel A which contained neither chromium nor molybdenum. It is also seen that doubling the annealing time to 48 hours does not significantly reduce the hardness. Some reduction in hardness was obtained by doubling the annealing time on pieces of rods that had been water quenched from 1900 F., but as shown in the table, most steels were still too hard for effective wire drawing. As will be hereinafter discussed, annealing temperatures lower than 1100 F. have been found to harden the steels because of the age hardening effect of chromium and molybdenum. The effort to produce a favorable carbide morphology by austenitic cycling as shown in Table II was also unsuccessful since the hardnesses obtained were higher than the as-received rods.

The new softening treatment is believed to make use of the softening effect of large amounts of stable reverted austenite. This structure occurs in high nickel steels that are thermally cycled to within the intercritical temperature region producing patches of alloy-enriched austenite that are stable upon cooling to room temperature. Thus, the series of temperature excursions to within the intercritical region according to my process produces a significant amount of alloy-enriched reverted austenite. The final, subcritical anneal over-ages the transformed martensite and. any fresh martensite formed from quenching the unstable reverted austenite. As will be hereinafter described, optimum results are obtained by cycling between the temperature range of 1150 to 1400 F. and the martensite transformation range and annealing at a temperature in the range of 1050 F. to 1175 F. Although some softening, satisfactory for many purposes, can be achieved by annealing for as low as two hours, it is preferred to perform the final anneal for at least about 10 hours, preferably at least 16 hours. Generally, the longer the annealing time, the lower the hardness.

The effect of variation in intercritical cycle temperature is shown by the results reported in Table IV of compositions described in Table III. As can be seen, greater reduction in hardness occurs within the range of from about 1200 F. to about 1350 F. with the hardness increasing somewhat over 1350" F.

1 Check Analyses.

TABLE IV Inter- Hardness, Rockwell C Desigcritical nation Cycle After After 1 After 4 After 4 Tempera- Furnace Cool Cycle 1 Cycles Cycles and ture, F. from 1,900 F. Anneal l, 150 37. 32.0 29.0 1, 200 37.0 35. 30.5 31.0 1, 250 37.0 38.0 33. 5 23.0 1, 300 37.0 40.0 38. 5 28.0 1, 350 37. 0 43. 0 43. 0 29. 5 l, 400 37. 0 44. 0 42.5 33. 5 1, 150 27. 5 28. 5 27. 5 l, 200 27. 5 30. 0 27. 0 26.0 1, 250 27.5 31.0 29. 5 26. 5 1, 300 27. 5 38.0 36. 5 25.5 1, 350 27. 5 42.0 37. 5 28. 5 l, 400 27. 5 43. 0 42. 0 32. 0

1 Cycle=1 hour at temperature and water-quenched to room temperature. Anneal=16 hours at 1,100 1". and water-quenched to room temperature.

Additional results are shown in Table V to demonstrate that the initial condition of the steel does not materially effect final hardness. In these examples, various initial treatments were performed on samples of composition S; it should also be noted that no satisfactory reduction in hardness resulted from the initial treatments and only after subjecting the steel to cycling and annealing in accordance with the invention (last column, Table V) was the hardness reduced materially.

1 1=Water-quenched from 1,500 I. 2=Furnace-cooled from 1,500 F. 3=Weter-quenched from 1,900 F. 4=Furnace-cooled from 1,900 F. 5= Furnace-cooled from 1,900 F. to room temperature and liquid nitrogen quenched.

2 Cycle=1 hour at 1,300 F. and water-quenched to room temperature. Annea1=16 hours at 1,100 F. and water-quenched to room temperature.

The effect of variation in the final annealing temperature is shown in Table VI. In these examples, samples of compositions S and T were cycled four times at 1250 F. (heated to 1250" F. and then cooled to below the martensite transformation range) and then annealed at various temperatures between 1050" F. and 1200 F. As can be seen, optimum results are obtained in the range 1050" F. to about 1175 F. At 1200 F., the hardness begins to increase.

TABLE VI Annealing Rockwell C Rockwell C Designation Temperature, Hardness Alter Hardness Alter F. 16-hour Anneal 48-hour Anneal InitIi al treatment urnace-cooled from 1,900 F. cycled 4 times at If the steel is not cycled but merely furnace cooled and annealed, unsatisfactory results are obtained. This is shown by Table VII which reports the results of treatments on samples of compositions S and T. It is seen that the hardness is not reduced materially after annealing for 16 hours or even 48 hours following furnace cooling from 1900 F. However, when the samples are cycled prior to annealing, the resulting hardness is very much lower.

The importance of the final annealing treatment in softening these steels is shown by the data reported in Table VIII. In these examples, samples of the steel compositions were heated to the intercritical region, held one hour and water quenched. Additional cycling (2 more times, middle column) generally lowered the hardness. In almost every case, however, the hardness of the steel was reduced the most after the final anneal at 1100 F. for 16 hours.

TABLE VIII Heat to 1,300 Same, 1,300 Same, 1,100

Water-Quench Water-Quench 2 Water-Quench Designation cresc e es ps z wrps f s i s ws aows ps s i mww cnrowhmoowoorotoueodimlv 1 2 Times. 2 2 Additional Times.

Quenching after each temperature excursion to the intercritical range causes fresh martensite to form from unstable reverted austenite and the martensite is then converted to over-aged martensite and reverted austenite in the subsequent intercritical cycle. The 1050 to 1175 F. anneal further softens the steels due to over-aging the fresh martensite formed from the final intercritical cycle. The time at intercritical temperature during annealing is not critical and satisfactory results can be obtained with only a few seconds exposure. Times greater than one hour per cycle may not be necessary. Virtually all steels of the alloy systems concerned will be softened by employing the annealing conditions described above.

It appears from the foregoing description that the initial condition of the steel does not appreciably affect its final hardness. Although some softening in these steels may occur through coallescence of carbides, the major contribution to softening is thought to be the formation of stable reverted austenite. It is believed that the presence of significant amounts, that is at least about 10% of stable reverted austenite, results in a softer steel because it is softer than ferrite and because it absorbs the secondary hardening elements thereby keeping them in solution. I have found that a maximum amount of stable reverted austenite will produce the softest structure and that cycling refines the grain size and increases the grain surface area. It is possible that the smaller grains provide more imperfections and greatly increase the nucleation sites for the austenite. This accelerates the austenite nucleation rate and time for the thermodynamic equilibrium amount of austenite to form since more austenite is nucleated and formed with each cycle until an equilibrium amount exists. It appears that the sluggishness of the reaction at the temperatures involved causes less formation of austenite after one relatively long cycle than will form after many shorter cycles. During annealing, the martensite is tempered to ferrite and carbide and forms an austenite of a higher alloy content. Relatively longer anneals allow the sluggish transformation to occur and enough diffusion for some of the high alloy austenite to form. The high alloy austenite TABLE VIII Rockwell G Rockwell C Rockwell C Rockwell C Rockwell C Hardness After Hardness After Hardness Alter Hardness After Hardness After Designation Furnace Cool 16-hour Anneal 48-hour Anneal 4 Cycles at 1,250 F., 4 Cycles at 1,250 F.,

From 1,900 F. at 1,100 F. at 1,100 F. followed by llowed by Iii-hours at 1,100 F. 48-hours at 1,100 F.

7 forms very little, if any, martensite upon quenching. The final microstructure consists of ferrite, stable reverted austenite and carbides.

In view of the foregoing, it is apparent that various changes and modifications may be made within the purview of the invention and, accordingly, the scope of the invention should be limited only by the appended claims.

I claim:

1. A method of softening high alloy steels having at least 0.06% carbon, at least 4% nickel, and other alloying elements from the group consisting of chromium, molybdenum, vanadium and cobalt comprising subjecting said steel to at least two heating and cooling cycles wherein the steel is heated to the intercritical temperature range of from about 1150" F. to about 1400 F. and then cooled to the martensite transformation range, annealing said steel in the subcritical temperature range of from about 1050 F. to about 1175 F. and cooling to permit handling.

2. A method according to claim 1 wherein said intercritical temperature is in the range of from about 1200 F. to about 1350 F.

3. A method according to claim 1 wherein said steel is subjected to four heating and cooling cycles prior to annealing.

4. A method according to claim 1 wherein said annealing is conducted for at least 10 hours.

5. A method according to claim 1 wherein said annealing is conducted for at least 16 hours.

References Cited UNITED STATES PATENTS 2,664,369 12/1953 Kiefer 148l34 3,178,324 4/1965 Grange et a1 148-443 3,288,657 11/1966 Morita et a1. l48--l43 CHARLES N. LOVELL, Primary Examiner.

Claims

1. A METHOD OF SOFTENING HIGH ALLOY STEELS HAVING AT LEAST 0.06% CARBON, AT LEAST 4% NICKEL, AND OTHER ALLOYING ELEMENTS FROM THE GROUP CONSISTING OF CHROMIUM, MOLYBDENUM, VANADIUM AND COBALT COMPRISING SUBJECTING SAID STEEL TO AT LEAST TWO HEATING AND COOLING CYCLES WHEREIN THE STEEL IS HEATED TO THE INTERCRITICAL TEMPERATURE RANGE OF FROM ABOUT 1150*F. TO ABOUT 1400*F. AND THEN COLLED TO THE MATENSITE TRANSFORMATION RANGE, ANNEALING SAID STEEL IN THE SUBCRITICAL TEMPERATURE RANGE OF FORM ABOUT 2050*F. TO ABOUT 1175*F. AND COOLING TO PERMIT HANDLING.