US3278298A

US3278298A - Chromium-nickel-aluminum steel and method

Info

Publication number: US3278298A
Application number: US334923A
Authority: US
Inventors: Perry D Cameron
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1963-12-31
Filing date: 1963-12-31
Publication date: 1966-10-11
Anticipated expiration: 1983-10-11
Also published as: BE649371A; DE1483177B2; GB1063374A; DE1483177A1

Description

United States Patent 3,278,298 CHROMIUM-NICKEL-ALUMINUM STEEL AND METHOD D Cameron Perry, Middletown, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Filed Dec. 31, 1963, Ser. No. 334,923 Claims. (Cl. 75128) My application for patent is a continuation-in-part of my copending application, Serial No. 227,731, filed October 2, 1962, and entitled Chromium-Nickel-Aluminum Steel and Method, now abandoned. The invention is concerned with the precipitation-hardenable stainless steels, more particularly the chromium-nickel-aluminum steels and the heat-treatment of the same.

One of the objects of the invention is the provision of a semi-austenitic chromium-nickel-aluminum stainless steel which may be shipped from the mill in severly cold weather without risk of premature hardening; which is readily formed or fabricated in one condition of heattreatment; and which is strong and yet tough and ductile in another condition of heat-treatment.

Another object of my invention is the provision of a precipitation-hardenable chromium-nickel-aluminum stainless steel which readily lends itself to hot-working in the mill as in the production of various flat-rolled products such as plate, sheet and strip, as well as in the production of bars, rods, wire and the like; and, which, in addition, is characterized by good cold-working properties as in the production of cold-rolled plate, sheet, strip and the like and in the production of cold-drawn rods and wire, and other products which are characterized by strength, toughness and ductility not only in the longitudinal direction but in the transverse and short transverse directions, as well.

A further object is the provision of a simple, direct, and effective method for heat-treating a corrosion-resistant chromium-nickel aluminum steel of the character desribed to develop optimum mechanical properties in the precipitation-hardened condition, more especially hightensile strength and high-yield strength in combination with good ductility and toughness, particularly sharpnotched toughness as evidenced by great resistance to crack propagation.

A still further object is the provision of particular articles of manufacture fashioned of the steel and the flatrolled products of my invention in Which there is had a high strength-to-weight ratio; notably, air frames, skin and other parts of supersonic aircraft, advanced missile motor casings, rocket casings and other pressure vessels where stresses are encountered along all three axes.

An additional object is the production of precipitationhardenable, corrosion-resistant chromium-nickel-aluminum steel castings, and various precipitation-hardened cast articles and products fashioned of the same, in which strength, durability and toughness are had.

Other objects of my invention in part will be obvious and in part will be pointed out during the course of the description which follows.

The invention, accordingly, consists in the chemistry of the steel in terms of the combination of ingredients employed and in the correlation of the same, in the temperature and cycles of heat-treatment employed, and in the articles and products fashioned of the steel, all as more particularly described herein, the scope of the application of which is more particularly set forth in the claims at the end of this specification.

As an aid to a better understanding of certain aspects of my invention, it may be noted at this point that a variety of precipitation-hardenable grades of stainless steel are 3,278,208 Patented Oct. 11, 1966 widely accepted in the art. Among the grades favorably accepted are the chromium-nickel-titanium steels. Others are the chromium-nickel-copper stainless steels. Still others are the precipitation-hardenable chromium-nickel-aluminum steels.

While the several precipitation-hardenable stainless steels noted above possess many desirable characteristics, depending upon specific analysis and heat-treatment, those characteristics of necessity are accompanied by others of less desirability. A case in point is the precipitation-hard ena'ble chromium-nickel-titanium steel. These steels incline to be dirty as a result of the titanium content and the ease with which titanium nitrides are formed in melting and in teeming; the mechanical properties are inclined to suffer. Similarly, the chromium-nickeLcopper stainless steels, while readily workable in one condition of heat-treatment, do not reach the hardness and strength necessary for many applications when subjected to precipitation-hardening treatment. And the known chromium-nickel-aluminum stainless steels in many instances, paricularly in fiat-rolled products, are somewhat lacking in toughness, especially where great stresses are encountered in the transverse direction.

It is the chromium-nickel-aluminum grades which are of particular interest here in that they possess a surprising combination of good forming and fabricating properties in the annealed or solution-treated condition of the metal; are readily hardened by heat-treatment at moderate temperatures; and in age-hardened or precipitation-hardened condition possess great strength and hardness. In this connection, attention is invited to US. Patents 2,505,- 762; 2,505,763; 2,505,764 and 2,506,558; all issued May 2, 1950 to George N. Goller.

Perhaps the greatest mechanical strength had in the chromium-nickel-aluminum grades of precipitation-hardenable steel are achieved in the grade which is modified by the presence of substantial quantities of molybdenum. Unfortunately, this steel lacks the toughness desired for many applications. And while toughness may be developed by overageing the steel, this results in a loss of tensile properties.

Accordingly, among the objects of my invention is the provision of a semi-austenitic chromium-mickel-aluminum stainless steel which is readily worked at the mill into a variety of flat-rolled and other products, notably plate, sheet, strip, billets, bars, rods and wire; which is of such composition balance that it may be shipped cross-country even in extremely cold weather, with-out prematurely hardening; which readily lends itself to fabrication as by bending, pressing, shrinking, stretching, cutting, drilling and the like and by riveting, brazing, welding and the like; and which steel and the products fashioned thereof may be hardened by a precipitation heat-treatment at moderate temperatures to a desired combination of mechanical properties, particularly high strength together with good ductility .and toughness, and even sharp-notched toughness.

Turning now to the practice of my invention, 1 provide a semi-austenitic chromium-nickel-aluminum stainless steel of especially low carbon content, especially low sulphur content and especially low nitrogen content. The steel of my invention essentially consists of carbon not exceeding 0.05% (preferably in the amount of 0.002% to 0.050% for desired strength and toughness as noted hereinafter), chromium in the amount of about 7.0% to 18.0%, nickel in the amount of about 6.0% to 12.0%, aluminum in the amount of about 0.5% to 2.5%, with a nitrogen content which must not exceed 0.05% (preferably not exceeding .03% for assured cleanliness, and especially not over 01% for best impact resistance and toughness) and a sulphur content which must not exceed 0.015% (preferably not exceeding 0.010% and most preferably not exceeding 0.005% for best results), and remainder essentially iron.

By remainder essentially iron it, of course, is meant that the remainder of the composition is iron together with such incidental ingredients and additives as not to detract from the new and surprising properties had in my steel. For example, manganese is present in the steel, this amounting to as much as 1.0%, although preferably being maintained at a value not exceeding 0.50% and most preferably not exceeding 0.1%. Similarly, silicon is present, this in amounts up to 1.0%, although preferably not exceednig 0.50% and most preferably not exceeding 0.1%. The steel additionally may contain molybdenum in an amount up to about 8.0%, titanium in an amount up to 0.10%, and boron in an amount up to 0.003%. Where desired, small amounts of tungsten, vanadium, zirconium and columbium may be employed in my steel. Cobalt may be substituted for nickel, this up to about one-half of the total nickel requirement, in the ratio of three parts cobalt for one part of nickel replaced.

Now the particular amounts of chromium, nickel and aluminum employed in the steel of my invention and the correlation between these ingredients generally may be viewed as critical. For where lesser amounts of chromium are employed the desired resistance to corrosion is not achieved. And where greater amounts are employed, the structural balance of the steel is upset, this with a loss of ultimate hardness in the precipitation-hardened condition. Similarly, where either lesser amounts of nickel are used than the prescribed minimum a tendency toward instability results and the metal inclines to prematurely harden in cold weather, or where greater amounts are used than the prescribed maximum, the structure of the steel is radically changed with resulting loss of mechanical properties. While perhaps there is some latitude in the aluminum content of the steel, here again any substantial decrease from the prescribed lower limit or any substantial increase above the prescribed upper limit disturbs the structural balance with the resulting undesired change in the overall mechanical properties.

While the chromium, nickel and aluminum contents of my steel may be viewed as generally critical as to the amounts employed and as to the correlation between the same, it is the carbon, sulphur and nitrogen contents which are particularly critical. The carbon content, as noted above, must not exceed 0.05%, for -I find that with greater carbon contents, the toughness of the steel de creases to an undesirably low level. And actually, from the standpoint of toughness, the lower carbon contents are more desirable in that with them maximum toughness is had. For example, I find that even with as low a carbon content as 0.002%, -I obtain a steel which is stable against cold transformation in the annealed condition, but can be heat-treated to high strengths and toughness. This steel, however, requires some 50 hours at the conditioning temperature, prior to refrigeration and precipitation-hardening, to develop these desirable properties. With a somewhat higher carbon content a shorter time is required for conditioning the metal. For example, at a carbon level of about 02%, the conditioning treatment must be of the order of several hours, and at carbon contents of about 025% to 0.045%, the conditioning treatment is reduced to about one hour, this prior to refrigeration and precipitation-hardening, to develop the desired strength and toughness.

While I do not care to be bound by the explanation, my views on the effect of carbon are as follows. The steels of my invention are stable against cold transformation in the solution-treated or annealed condition with all of the carbon being in solid solution in the austenite. Upon heating to the austenite conditioning temperature, a precipitate of carbides probably of the M C type (metal carbides), occurs in the phase boundaries. -In addition, there also is probably precipitated, especially in the very low carbon steels, an intermetallic compound, probably Ni Al At the higher carbon levels, these precipitates form in the phase boundaries much more rapidly than they do at the lower carbon levels. These precipitates, in removing from solid solution powerful austenite stabilizers such as carbon and nickel, unbalance the austenite so that upon cooling to refrigeration temperatures, transformation occurs. The precipitation-hardening heattreatment then follows to develop the desired strength and toughness.

As pointed out above, the precipitates noted normally occur in the phase boundaries as continuous networks. These precipitates are hard and brittle. By reducing the amount of these precipitates, and by eliminating the network, both accomplished by keeping the carbon below .05 the high toughness of these steels is obtained.

The sulphur content of my steel should not exceed 0.015%, as noted above, and preferably should not exceed 0.010%, is also noted, for the reason that this ingredient seems to appear as an interstitial in the crystal lattice of the metal. It causes dislocations. And while I do not care to be bound by the explanation, it is my view that by minimizing this interstitial there is had a critical reduction in the number of dislocations in the lattice structure and a resultant increase in the toughness of the metal. Whatever the explanation, however, I find that the sulphur content of the steel must not exceed 0.015% and desirably should not exceed 0.010%. Preferably, the sulphur content is maintained at a value not exceeding about 0.005%.

The nitrogen content of my steel should not exceed 0.05% and preferably should not exceed 0.01% in order to enjoy maximum toughness and resistance to impact. I am inclinde to feel that because of the large amounts of aluminum present, any significant amount of nitrogen gives rise to a strong inclination to form aluminum nitrides, these nitrides becoming dispersed throughout the steel with a sacrifice of the Welding properties of the steel as a result of the nitrides decomposing under the heat of the welding operation, causing porisity and possibility of failure under load. While soundness and freedom from porosity may be assured through the addition of titanium, I am generally disinclined to employ more than 0.10% of this ingredient, even though a small excess would not be harmful, since such an addition is but a corrective measure which is inclinded to give dirty metal with some loss of toughness. Best results are had by maintaining the low nitrogen content, nitrogen preferably not exceeding 0.1% and certainly not exceeding 0.05

The ingredient boron may bev employed in my steel as noted above. Actually, I prefer to employ this ingredient in an amount of 0.001% to 0.002%, in order to secure good hot-rolling and other hot-working properties. The amount of boron, however, should not exceed .003% because boron like sulphur discussed above, is inclinde to appear as an interstitial in the crystal lattice structure, with resultant dislocation and loss of toughness.

Phosphor-us commonly is present in my steel in small amounts. While not critical, the phosphorus content should not exceed 0.040%.

While, as indicated above, the ingredient molybdenum may be employed as an additional element in the steel of my invention, I actually prefer to employ this as an essential ingredient in the amount of some 2.0% to 6.50%. Molybdenum contributes to the tensile strength of my steel. Surprisingly enough, however, steels with molybdenum contents exceeding the 6.5% figure do not develop adequate toughness.

One preferred steel according to my invention essentially consists of carbon not exceeding 0.050%, chromium about 13.0% to 15.0%, nickel about 7.5% to 9.5%, molybdenum about 2.0% to 3.0%, aluminum about 0.75% to 1.50%, and remainder essentially iron. Manganese and silicon each are present in amounts up to 1.0%. The nitrogen content must not exceed 0.05%. Preferably the nitrogen content is maintained at a value not exceeding 0.03% and especially not over 0.01% for best impact resistance and toughness. The phosphorus content of this preferred steel does not exceed 0.040%, and the sulphur content does not exceed about 0.010%. Sulphur up to an amount of 0.015% is tolerated where the carbon content is not in excess of 0.03% (with nitrogen not exceeding 0.050%), or where the carbon content is up to 0.03% or even up to 0.05% (with the nitrogen content not exceeding 0.03%, especially not exceeding 0.01%). Titanium may be present in amounts up to 0.10% and boron in amounts up to 0.003%. This steel enjoys the combination of strength and toughness.

Another and more specific preferred steel essentially consists of carbon about 0.025% to 0.04%, chromium about 14.2% to 14.7%, nickel about 8.1% to 8.6%, molybdenum about 2.00% to 2.50%, nitrogen up to about .05%, aluminum about 1.05% to 1.30%, and remainder essentially iron. Manganese and silicon each is present in amounts up to .70%, preferably each in amounts of .20% to .70%. In this preferred steel the phosphorus does not exceed about 0.040% and the sulphus does not exceed about 0.010% and preferably not over about 0.005%. The nitrogen content does not exceed about 0.05 and preferably does not exceed 0.01% for the reasons given. Titanium and boron may be present, titanium in amounts up to 0.10% and boron in amounts up to 0.003%.

A further preferred steel according to my invention, this at somewhat different carbon, chornium and nickel balance, essentially consists of about 0.02% to 0.03% carbon, about 13.7% to 14.2% chromium, about 8.0% to 8.5% nickel, about 2.0% to 2.5% molybdenum, about 1.05% to 1.30% aluminum, and remainder essentially iron. The manganese content preferably does not exceed 0.50%, manganese in the amount of 0.20% to 0.50% being preferred. And the silicon content does not exceed 1.0% and preferably is in the amount of 0.60% to 1.0%. The phosphorus content does not exceed about 0.040%, and the sulphur content does not exceed about 0.010% and preferably does not exceed about 0.005%. Nitrogen does not exceed 0.05%; preferably it does not exceed 0.01%. Here, too, both titanium and boron, one in an amount up to 0.10% and the other in an amount up to 0.003%, may be present.

A still further preferred steel, this again at a somewhat different carbon, chromium and nickel balance, with a difference in molybdenum content too, essentially consists of about 0.02% to 0.03% carbon, about 12.0% to 12.5% chromium, about 8.5% to 9.0% nickel, about 4.0% to 4.5% molybdenum, about 1.05% to 1.30% aluminum, with remainder essentially iron. The manganese content does not exceed 0.50% and preferably is on the order of some .20% to .50%, while the silicon does not exceed 1.0% and preferably amounts to 0.60% to 1.0%. The phosphorus content does not exceed about 0.040%, and the sulphur content does not exceed about 0.010%; preferably it does not exceed about 0.005%. Here again, nitrogen does not exceed 0.05% and preferably it does not exceed 0.01%. Both titanium and boron may be included in this preferred steel, the one not exceeding 0.10% and the other not exceeding 0.003%.

An additional preferred steel, again at another different carbon-chromium-nickel-molybdenum balance, essentially consists of about 0.02% to 0.03% carbon, about 10.25% to 10.75% chromium, about 9.0% to 9.5% nickel, about 6.0% to 6.5% molybdenum, about 1.05% to 1.30% aluminum, and remainder essentially iron. Again, the manganese preferably does not exceed 0.50% and usually is in the amount of .20% to .50%. The silicon content should not exceed 1.0% and preferably amounts to .60% to 1.0%. The phosphorus content does not exceed about 0.040%, and the sulphur content of the steel does not exceed about 0.010%; preferably it does not exceed about 0.005%. Nitrogen, where present, is in amount not exceeding 0.05% and preferably not exceeding 0.01%. 'I-itanium may be present in the amounts of 0.10%, and boron is preferably present in amount up to 0.003%.

The steel of my invention conveniently is made in the electric arc furnace although, where desired, it may be vacuum-melted. In either event the steel in the form of ingots is converted into slabs, blooms and billets or it may be continuously cast into slabs, blooms and billets. And upon reheating, hot-rolled into plate, sheet, strip, bars, rods, wire and the like. As previously noted, the metal works well in the hot mill. Of course, it forges easily.

Now the steel in the form of plate, sheet, strip, bars, rods, wire and the like, may be supplied various customersfabricators in the annealed condition, or it may be supplied in the annealed plus box-stabilized condition. Where desired, it may be supplied in cold-rolled condition. The composition balance of the steel is such that partial transformatin as a result of cross-country shipping, even in the most severe cold, is effectively precluded.

In the annealing or solution-treatment, whether performed at the steel mill or in the plant of the customerfabricator, heating at some 1800 F. to 2000 F. is employed. Usually for most fiat-rolled products, and even for the other hot-Worked products, solution-treatment at about 1850 F. for a time of 3 minutes for each 0.1" thickness of the metal gives satisfactory results. The annealing or solution-treatment apparently places the metal in an austenitic condition in which the aluminum content of the steel is dissolved. And upon quenching the steel either in air, oil or Water, following the anneal ing treatment, the aluminum constituent remains in solution. The metal is essentially austenitic and is soft, ductile and readily workable.

In the solution-treated condition my steel has a hardness on the order of about Rockwell B to 90. It readily lends itself to forming and fabrication by cutting, bending, pressing, drilling, and the like, as well as by machining, tapping and threading. Additionally, the steel may be brazed and welded, the welding properties being superior to the known precipitation-hardenable chromium-nickel-aluminum steels. It is suited to the production of a variety of supersonic aircraft parts such as frames, skin and the like, and to the production of defense missile motor cases, rocket cases, pressure vessels and high pressure tankage. Additionally, it is suited to the production of fastening devices such as bolts, screws, studs and the like, and to the production of fluid valves, valve seats and valve parts generally.

Following fabrication the steel is given an austeniteconditioning heat-treatment at a temperature of 1300 F. to 1750 F. for a time of one hour or more. The metallurgy of the austenite conditioning heat-treatment is discussed above.

Following the austenite-conditioning treatment, the steel of my invention is cooled or refrigerated at a temperature between about 60 F. and a low of 200 F. Usually I find refrigeration at a temperature of some F. to F. to be eminently satisfactory; ordinarily a temperature of 100 F. for about 8 hours effects the desired transformation from an austenitic condition to a martensitic condition. There is a significant change in hardness with this treatment. Nevertheless, where desired, limited forming and fabricating operations actually may be conducted with the steel in the transformed con dition rather than in the annealed condition. Actually, certain of the machining operations, for example, sawing, drilling, threading and the like, may be best effected with the metal in the transformed condition, taking advantage of the hardening had as a result of the transformation treatment.

Final hardening of the steel is had by reheating at a temperature of some 700 to 1200 F. and cooling in air,

oil or water. Ordinarily, I treat the steel at a temperature of some 900 F. to 1050 F. for several hours, and cool, although for most purposes the reheating at a temperature of 950 F. for 1 hr. and quenching in air, oil or water, gives excellent results. The hardness had is on the order of some Rockwell C35 to 50.

Of particular importance, the steel of my invention in the age-hardened or precipitation-hardened condition possesses an excellent combination of mechanical strength and toughness. The strength realized is on the order of some 200,000 p.s.i. to 255,000 p.s.i., with an Allison Bend 1 figure indicative of toughness being at least .40 to .50 at an ultimate tensile strength level of 230,000 p.s.i. to 240,- 000 p.s.i. This degree f toughness at these levels of ultimate tensile strength is most surprising.

Additionally, the steel in the age-hardened condition is tough and peculiarly resistant to crack propagation. Center notched, fatigue cracked, specimens, .050" thick by 2.000" wide, with slow crack growth followed by ink stain, developed K0 values calculated to be near 230,- 000 p.s.i. /i n. The value developed, however, is not the true Kc value; the true Kc value may be greater than this since the net section stress immediately prior to fracture was greater than the tensile yield strength of the steel.

A typical fracture surface on the failed specimen is 100% shear where the fracture surface is at approximately a 45-degree angle to the sheet surface. The fracture surface of a brittle material is typified by a cleavage type fracture which is at an angle of about 90 degrees to the sheet surface.

By way of specific illustration of the steels of my invention, the chemical analyses of four of my steels analyzing about 14% to chromium, about 8% to 9% nickel, about 1% aluminum, about 2% to 3% molybdenum, manganese and silicon each not exceeding about .7%, carbon not exceeding about 04%, phosphorus and sulphur each not exceeding about 008%, nitrogen not exceeding about .04%, and remainder essentially iron, these to be compared with four others of differing chemical composition, are given in Table I(a) below:

and cold-rolling operations. The standard test samples were prepared and given a heat treatment consisting of austen-ite-conditioning, refrigeration and hardening. The samples of the various steels were tested for ultimate tensile strength and toughness (Allison Parameter). The results are given below in Table I b) Table 1(b) ULTIMATE TENSILE STRENGTH AND TOUGHNESS OF THE STEELS OF TABLE I(a) IN THE SRH950 CONDITION Ult. Tens. 0.2% Y.S. Percent Rockwell Allison 3 Heat Str. p.s.i. p.s.i. E1. in 0. Parameter Hardness 1 Steel of the present invention.

2 Condition 1,700 F.1 hour and air cool; Refrigerate at 100 F. for 8 hours; Harden at 950 F. for 1 hour and air cool.

3 Transverse direction.

It will not noted that for the two induction melted steels of my invention (Heats R3819 and R4225) there is had in the age-hardened condition ultimate tensile strengths on the order of some 245,000 p.s.i. to 252,000 p.s.i. with toughness as measured by the Allison Bend Test of .58 and .91. About the same level of strength is had with the vacuum melted heat (Heat V88) with toughness, however, according to the Allison Bend Test of 1.38. The are furnace heat (Heat 31562) achieved a strength of some 232,000 p.s.i. with toughness of .55 Allison.

As contrasted with the illustrative steels of my invention (Heats R3819, R4225, V88 and 31562) with Allison Parameters of .58, .91, 1.38 and .55, the steels of the higher sulphur content, but otherwise of like chemical analysis and of about the same strength levels, had Allison Parameter of only some .26 to .34, the actual figures for the four samples being .30, .26, .34 and .32 in the order given. And the calculated toughness for Heat Table I (a) CHEMICAL ANALYSES OF 8 CHROMIUM-NICKEL-ALUMINUM STAINLESS STEELS Heat; 0 M11 P S Si Cr Ni MO Al N l Steels of the present invention.

A Bend Test for Toughness by Dean K. Hanink and George It. Sippel, Metal Progress for August 1960, DD. 89-92, Product Engineering Sept. 4, 1961, pp. 62 to 64.

-Report of a special ASTM Committee (American Somety for Testing of Metals) Fracture Testing of High-strength Sheet Materials, AS'lM Bulletin for January 1960, pp. 29 through 40.

31562, using the Center Notch fatigue-crack test (Kc), exceeded 235,000 p.s.i. /in. and net fracture stress exceeded 250,000 p.s.i.

As specific illustration of the steels of my invention of extremely low nitrogen content, the chemical analyses of three analyzing about 14% to 15 chromium, about 8% to 9% nickel, about 1% aluminum, about 2% to 3% molybdenum, manganese and silicon each not exceeding about .4%, with carbon not exceeding about .04%, sulphur and phosphorus each not exceeding about .004%, nitrogen not exceeding about .01% and actually not exceeding .001%, and remainder essentially iron, these to be compared with a steel of similar chemical composition except for nitrogen content, are set out in Table II(a):

Table II (a) CHEMICAL ANALYSES OF 8 CHROMIUM-NICKEL-ALUMINUM STAINLESS STEELS Heat C Mn P S Si C1 Ni M A1 I N O i H 3 1 Steels of present invention. 2 In parts per million.

Of the eight heats whose chemical analyses are given 15 gen content .091 the Allison Parameter only amounts to above, the Heats V87, V88 and V89 are melted in the vacuum induction furnace, the Heats R4223, R4224 and R4225, as well as the Heat R4235, presented for comparative purposes, are melted in the air induction furnace, and the Heat 31562 is melted in the air electric arc furnace.

The mechanical properties of the eight steels of Table II(a) are given below in Table II(b):

20 invention, there are given the chemical analyses of five steels, three at a molybdenum level of about 2% and another two at a molybdenum level of about 4%, in the Table III(a) below:

Table III (a) CHEMICAL ANALYSES OF FIVE CHROMIUM-NICKEL-ALUMINUM- MOLYBDENUM STAINLESS STEELS Table II( b) Heat 0 Mn P s 81 Cr N1 M0 Al N 35 The five heats identified above were melted in the induction furnace with resulting ingots forged, then sur- MECHANICAL PROPERTIES OF THE STEELS OF TABLE II(a) IN THE SRH950 CONDITION Ult. Tens. 0.2% Y.S. Percent Rockwell Allison 3 Heat Str. p.s.i. p.s.i. E1. in Parameter 2' Hardness l Steels of the present invention.

2 Condition 1,700 F.1 hour and air cool; Refrigerate at -100 F. for 8 hours; Harden at 950 F. for 1 hour and air cool.

3 Transverse direction.

In connection with the mechanical properties given above note particularly the increase in the Allison Parameter of the various steels as the nitrogen content of the steels is lowered. For the steels of the present invention, as the nitrogen content is lowered from a value of .033 for Heat 31562 down to a value of .001 for the Heats V87, V88 and V89, the Allison Parameter rises from .55 to a value of some 1.21 to 1.46. For the steel of nitrocontent on the order of 2% (R3840, R3843, R3847) have an ultimate tensele strength in the age-hardened condition of some 239,000 p.s.i. to 244,000 p.s.i., a toughness according to the Allison Parameter of some .48 to .70. The hardnesss of the steel is about Rockwell C50. In the an nealed condition the steel, of course, is soft and ductile and readily formable with hardness of Rockwell BSD-85 The steels with molybdenum content on the order of 4%, with correspondingly lower chromium contents than the steels of the 2% molybdenum content pointed to above, While of somewhat greater hardness and somewhat greater Table III (b) OPERTI S OF THE FIVE STEELS OF TABLE III(a) BOTH IN THE ANNEALED CONDITION MECHANICAL PR E AND IN THE AGE-HARDENED CONDITION Condition A Condition RH 950 1 Heat a U.T.S. 0.2% Y.S. Percent RB U.T.S. 0.2% Y.S. Percent R Allison p.s.i. p.s.i. E1. in 2" p.s.i. p.s.i. El. m 2" Parameter 1 C ndt'on A-Ar'meal 1 850 F. 1 minute and air cool. 2 C ndition' RH 950Au stenite-conditi0ning at 1,750 F. for 10 minutes and air-cool; refrigerate at F. for 6 to 8 hours; age-harden by reheat at 950 F. for 1 hour and water-quench.

3 Transverse direction.

In the matter of annealing or solution-treating the steel of my invention, then austenite-conditioning the same, refrigerating, and age-hardening, I find that the matters of refrigeration temperature and specific temperature of agehardening are not particularly critical. The particular temperature at which the steel is previously conditioned,

however, is critically important.

As generally illustrative of the effect of variable austenite-conditioning and refrigeration treatments, all at constant hardening treatment of 975 F. for 1 hour and air cool, see the mechanical properties for the Heat 31562 as -set out in Table IV below, all being taken in transverse direction:

By interpolating the data given in Table IV I find that the refrigeration temperature at which maximum strength is obtained, but with minimum toughness, varies from -100 F. to 160 F. as the conditioning temperature is varied from 1350 F. to 1750 F. And as to the conditioning temperature, for refrigeration, for example, at 75 F. for 8 hrs. and hardening at 975 F. for 1 hour, optimum strength, but with minimum toughness is had by previously conditioning the steel at a temperature on the order of some 1400 F. By conditioning the steel at higher temperatures, there is had a definite increase in the toughness of the metal although there is a decrease in the strength. And by increasing the duration of the time at conditioning temperature, there is something of a loss of toughness, although the strength is increased.

The effect of varying the temperatures and times of the age-hardening treatment, this for a preferred temperature of austenite-conditioning treatment of 1700 F. for 1 hour and a generally preferred refrigeration treatment of some 100 F. for 6 hours is shown by the mechanical properties of Heat 31562 as given in Table V below, all in transverse direction.

Table IV MECHANICAL PROPERTIES OF STEELS OF INVENTION FOR DIFFERING CONDITIONS OF REFRIGERATION UNDER DIFFERING AUSTENITE-CONDITIONING BUT CONSTANT HARDENING CONDITIONS (975 F. 1 HR.)

Refrigeration Ult. Ten. 0.2% Y.S. Percent Sample Str. p.s.i. p.s.i. E1. in 2 Hard. RA Allison Parameter Temp. Time, F. Hrs.

B"Conditioned at 1,350 F. for 1% hours DConditioned at 1,550 F. for 1% hours GCondltioned at 1,750 F. for 1% hours Table V MECHANICAL PROPERTIES OF STEEL OF INVENTION FOR DIFFERING CONDITIONS OI ACE-H AUSTENITE-CONDITIONING (1,700 F. 1 HR.) AND REFRIGERATION (-100 F FOfi 6 I I O T l I: S AT CONSTANT Hardening Sample Ult. Tens. 0.2% Y.S. Percent Hard Allison Parameter Str.p.s.1. p.s.i. E1. in 2 Bo Temp. F. Time, Hrs.

900 2% 229,200 210, 000 5 4s 4 900 5% 242, 500 222, 000 o 49 5 4 900 5 /1 245, 200 224, 900 5 49. 5 39 43 39 55 975 1 234, 700 215, 000 5 48.5 54 55 o4 63 975 4 229,000 214,900 6 48.5 .51 44 '25 '57 975 4 230,800 213, 300 4 41.5 .45 55 39 42 975 7 233, 400 213, 800 6 4s. 54 70 66 52 1, 050 192,200 177, 500 10 4s 1. 39 1. 23 1. 55 1I 3s With a preferred conditioning temperature of some 1700 F. for 1 hour followed by a particularly preferred refrigeration treatment at 100 F. for 8 hrs, an excellent combination of strength and toughness is had by finally hardening th emetal at 950 F. for 1 hour. Moreover, by decreasing the temperature of the age-hardening treatment to 900 F. and prolonging the time for 8 hours, both the ultimate tensile strength and the yield strengh of the metal are increased without, however, any significant loss of toughness, the tensile and yield strengths then respectively amounting to some 260,000 p.s.i., and 241,000 p.s.i., with toughness of about .55 Allison Parameter, as contrasted with ultimate tensile and yield strengths of 232,000 p.s.i. and 216,000 p.s.i., at like toughness, lfOI' the 950 F. age-hardening treatment.

In the cold-rolled condition, this for a reduction on the order of some 60%, my steel possesses great strength in the direction of rolling and in the transverse direction, as well. Although toughness is greatest in the longitudinal direction, yet the toughness in the transverse direction is substantial.

The eifect of hardening temperatures and times of treatment on the mechanical properties of cold-rolled sheet of Heat 31562 (analysis given in Table 1(a) above), both in longitudinal direction and in transverse direction, is illustrated below in Table VI:

Parameter toughness ranging from .98 to 2.06. The transverse direction sample, having an ultimate tensile strength of 283,500 p.s.i., in like hardened condition, has an Allison Parameter figure ranging from .02 to .11.

With hardening conducted at the higher temperature of 1050 F. for 1 to 5% hours, however, the toughness in transverse direction is subsantially increased, this amounting to some .10 to .40 Allison Parameter, although there is some loss in the ultimate tensile strength. The toughness in longitudinal direction also falls considerably, nevertheless this still is excellent, this ranging from some .58 to .89 Allison Parameter.

The ultimate tensile strength of the steels hardened at the higher temperature is to some extent sacrificed to gain the improved toughness in transverse direction, this strength amounting to some 224,200 to 244,400 p.s.i. in longitudinal direction and 233,500 to 237,000 p.s.i. in transverse direction for the steels hardened at 1050" F., as against 270,000 to 278,000 p.s.i. in longitudinal direction and 283,500 to 287,600 p.s.i. in transverse direction for steels hardened at the 900 F. temperature.

For a balance between strength and toughness in the longitudinal and transverse directions the higher agehardening temperatures are preferred. But for maximum toughness in longitudinal direction, with great strength, it

Table VI EFFECT OF VARIABLE HARDENIN G TREATMENT ON COLD-ROLLED SHEET OF HEAT 31562 Hardening Sample Ult. Tens. 0.2% Y.S. Percent Hard Allison Parameter Str. p.s.i. p.s.i. E1. in 2 R Temp. F. Time, Hrs.

Mechanical PropertiesLongitudinal Direction 000 2% 270, 000 261,200 2 51.5 2. 20 2. 39 1.67 1. 32 900 5% 271,000 260, 000 2 51.5 2. 21 2. 37 1. 61 1. 63 900 7 278, 000 275,000 2 52. 5 98 1.10 l. 14 2.06 975 1 275,000 263, 200 l. 5 51 2. 57 2. 27 2. 79 2. 975 4 253, 400 242, 200 2 2. 08 2. 08 2. 67 2. 02 975 4 252, 000 245, 200 2 50 1. 42 1. 39 2. 69 1. 02 975 7 268, 200 255,000 50. 5 2. 05 1. 81 2. 77 2, 48 1, 050 l 244, 400 233, 000 2 48. 5 1, 050 2% 227, 100 220, 000 4. 5 47. 5 58 1, 050 5% 224, 200 217, 400 4. 5 47 89 .81

Mechanical Properties-Transverse Direction From the information presented in Table VI it is noted that in longitudinal direction the sample hardened at a temperature of 900 F. for the time of 7 hours has an is the lower hardening temperature, a hardening temperature of 900 to 975 F., that is preferred.

Thus it will be seen that I provide in my invention ultimate tensile strength of 278,000 p.s.i., with Allison a semiaustenitic precipitation-hardenable chromimumnickel-aluminum stainless steel in which the various objects set forth above, together with many advantages are successfully achieved.

The steel of my invention is of such composition balance that it may be shipped from the mill in cold weather without fear of premature hardening. And when received by the customer-fabricator, is soft and ductile and readily lends itself to forming as by pressing, bending, shrinking, stretching, and the like; readily machined as by sawing, cutting, tapping, threading, etc.; and readily fabricated as by riveting, welding, brazing and other known fabricating operations. The steel and various fabricated articles and products then are hardened by simple heattreatment at comparatively low heat-treating temperatures to achieve great strength and toughness. Both strength and toughness are had in working direction of the metal as well as in transverse direction. The metal is resistant to tearing.

I also provide a method of heat-treating my steel in order to achieve the surprising combination of strength and toughness in longitudinal direction of working and in the transverse direction. The method is simple, direct and effective.

Since many embodiments of my invention may occur to those skilled in the art to which the invention relates, and many variations may be made in the embodiments herein disclosed, it will be understood that all subject matter described herein is illustrative and is not to be taken as limitative.

Having described my invention, I claim:

1. A semi-austenitic, precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 7.0% to 18.0% chromium, about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, manganese and silicon each not exceeding about 1.0%, with a carbon content not exceeding .05%, a phosphorus content not exceeding about .040%, a sulphur content not exceeding 0.010%, a nitrogen content not exceeding .05%, molybdenum up to about 8.0%, and remainder essentially iron.

2. A semi-austenitic, precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 7.0% to 18.0% chromium, about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, manganese and silicon each not exceeding about 1.0%, about .002% to .050% canbon, a phosphorus content not exceeding .040%, a sulphur content not exceeding 0.010%, a nitrogen content not exceeding .05 molybedenum up to about 8.0%, titanium up to about .10%, boron up to about .003%, and remanider essentially iron.

3. A semi-austenitic, precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 7.0% to 18.0% chromium, about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, manganese and silicon each not exceeding about 1.0%, with a carbon content not exceeding .05%, a phosphorus content not exceeding about .040%, a sulphur content not exceeding 0.015%, a nitrogen content not exceeding 0.01%, molybdenum up to about 8.0%, titanium up to about .10%, boron up to about .003%, and remainder essentially iron.

4. A precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 13.0% to 15.0% chromium, about 7.5% to 9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 3.0% molybdenum, manganese and silicon each not exceeding about 1.0%, carbon not exceeding .05%, phosphorus not exceeding .040%, sulphur not exceeding about 0.010%, nitrogen not exceeding .05%, and remainder essentially 11011.

5. A precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 14.2% to 14.7% chromium, about 8.1% to 8.6% nickel, about 2.0% to 2.5% molybdenum, about 1.05% to 1.30% aluminum, manganese and silicon each not exceeding about .7%, carbon not exceeding .05%, phosphorus not exceeding about .040%, sulphur not exceeding about .005%,

nitrogen not exceeding .05%, and remainder essentially iron.

6. Precipitation-hardenable chromium-nickel-aluminum stainless steel plate, sheet, strip, bars, rods, wire and like products which in hardened condition is strong and tough, which products essentially consist of about 13.0% to 15.0% chromium, about 7.5% to 9.5% nickel, about 2.0% to 3.0% molybdenum, about .75% to 1.50% aluminum, manganese and silicon each not exceeding 1.0%, with carbon .002% to .050%, phosphorus not exceeding about .040%, sulphur not exceeding about .007%, nitrogen not exceeding .05%, and remainder essentially iron.

7. Precipitation hardened chromium-nickel-aluminum stainless steel essentially consisting of about 13.0%' to 15.0% chromium, about 7.5% to 9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 3.0% molybdenum, manganese and silicon each not exceeding about 1.0%, about .02% to .045% carbon, phosphorus not exceding about 0.040%, sulphur not exceeding 0.015%, nitrogen not exceeding 0.05%, and remainder essentially iron.

8. Precipitation-hardenable chrimium-nickel-aluminum stainless steel essentially consisting of about 13.0% to 15.0% chromium, about 7.5% to 9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 3.0% molybdenum, manganese and silicon each not exceeding about 1.0%, carbon not exceeding .05%, phosphorus not exceeding about 0.040%, sulphur not exceeding about 0.015%, nitrogen not exceeding .01%, and remainder essentially iron.

9. Precipitation hardened chromium-nickel-aluminum stainless steel products having a tensile strength exceeding 225,000 p.s.i. in combination with a toughness exceeding .5 Allison Parameter, said products essentially consisting of about 7.0% to 18.0% chromium, about 6.0% to 12.0% nickel, about .5 to 2.5% aluminum, with carbon .002% to .05%, manganese and silicon each not exceeding 1.0%, phosphorus not exceeding about 0.040%, sulphur not exceeding about .007%, nitrogen not exceeding .05%, molybdenum up to 8.0%, and remainder essentially iron.

10. A precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 14% to 15% chromium, about 8% to 9% nickel, about 1% aluminum, about 2% to 3% molybdenum, manganese and silicon each not exceeding about .7%, carbon not exceeding about .04%, sulphur not exceeding about .008%, nitrogen not exceeding about .04%, and remainder essentially iron.

11. Precipitation hardenable chromium-nickel-aluminum stainless steel plate, sheet, strip, bars, rods, wire and like products essentially consisting of about 14% to 15 chromium, about 8% to 9% nickel, about 1% aluminum, about 2% to 3% molybdenum, manganese and silicon each not exceeding about .7%, with carbon not exceeding about .04%, sulphur not exceeding about .008% nitrogen not exceeding about .04%, and remainder essentially iron.

12. A precipit-ation-hardenable chromium-nickel aluminum stainless steel essentially consisting of about 7.0% to 18.0% chromium, about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, carbon not exceeding .05%, sulphur not exceeding .005%, nitrogen not exceeding .05 and remainder essentially iron.

13. A precipitation-hardenable chromium-nickel-aluminum stainless steel essentially consisting of about 10.25% to 15.0% chromium, about 7.5% to 9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 6.5% molybdenum, carbon not exceeding .05%, sulphur not exceeding .010%, nitrogen not exceeding .05%, and remainder essentially iron.

14. A precipitation-hardenable chromium-nickel aluminum stainless steel essentially consisting of about 10.25 to 15.0% chromium, about 7.5% to 9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 6.5% molybdenum, Carbon not exceeding .05%, sulphur not exceed- 1 7 ing .015 nitrogen not exceeding .01%, and remainder essentially iron.

15. In the production of precipitation-hardened chromium-nickel-aluminum stainless steel of great strength in combination with toughness, the method which comprises providing a steel essentially consisting of about 14% to 15% chromium, about 8% to 9% nickel, about 1% aluminum, about 2% to 3% molybdenum, carbon not exceeding about .04%, sulphur not exceeding 008%, nitrogen not exceeding about .04% and remainder essentially iron; austeni-te-conditioning the steel at a temperature of about 1700 F. for one hour or more; transforming the same by refrigerating at a temperature of about -100 F.;

and precipitation-hardening by reheating at a temperature of about 900 to 1050 F.

References Cited by the Examiner UNITED STATES PATENTS 2,505,764 5/1950 Goller 75-124 2,958,618 11/1960 Allen 75-124 3,117,861 1/1964 Linnert et al 75124 3,131,055 4/1964 Behar 75-124 10 3,151,978 10/1964 Perry et a1. 75-124 DAVID L. RECK, Primary Examiner.

H. W. TARRING, Examiner.