US2758025A - High temperature stainless steel - Google Patents

Description

z,75s,02s f 7 HIGH TEMPERATURE srAm Ess STE L] William Charles Clarke, Jr., -Dundalk, Md., to. Armco Steel Corporation, a corporation of Ohio No Drawing. Application May 23,1952, a

Serial No.289,657 I l 3 Claims. (c1. 75-5128 1 This application for patent is a" continuation-impart and durable high temperature austenitic stainless steel,

and products and articles of the steel, which are capable of resisting the" development of sigma phase while under load at high temperatures, which are resistant to stress-rupture and creep under load at 'thehigh'tempera- M tures encountered, and which are well suited for resisting attack by hot corrosive matter and avoiding the formation of heat scale.

Another object of my invention is the provision of high temperature stainless steel of thecharacter indi cated which is well protected against carbide precipitation and intergranular corrosion.

A further object of the present invention is that of has good hot working properties. Other objects of the invention in part will be obvious and in part pointed out more fully hereinafter. The invention accordingly consists in' the combination of elements, features of the steel, and in the features 40 of articles, products and manufactures of the steel, as

described herein, the scope of the application of which is indicated in the following claims. I

As conducive to a clearer'understanding' of certain aspects of my invention, it may be noted at this point that stainless steel in more general terms is defined as a steel which contains about 10% to "35% chromium, with or Without nickel, and sometimes, additions of such elements as manganese, silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium, titanium, sulphur,

or the like, for special purposes; the remainder of the steel being substantially all iron except for incidental impurities. Among these steels are the austenitic' chromium-nickel steels, which have many favorable properties. In this connection, though, high temperature prodnets and articles made of any of a variety of the austenitic steels are extremely susceptible to creep and stressrupture when under load for extended periods of time While hot. Many of-the products tend to form a constituent at high temperatures called the sigma phase.

This phase is definitely related to the composition of the product or article and the high temperature use. It is hard and brittle at room temperature, and is a non-magnetic intermetallic compound which is weak and ductile at high temperatures. phase slowly develops and when present in quantities above about 2%, impairs creep strength and stress-rupture properties. More than small quantities of the phase, therefore, are hurtful under high temperature conditions of use of the steel, if for no other reason than introducing a weakening effect with regard to stressrupture and creep.

tion required for normal corrosion resistance.

tially all iron.

While the steel is heated, this 35 2,758,025 i ear n e u Many austenitic stainless steels are susceptible to intergranular corrosion particularly after exposure to temperatures around and approaching 1500 F., and thus such a carbide draws heavily upon the chromium content of metal adjacent to carbide particles, thereby impoverishing the metal in chromium below the concentra- Accordingly, a vulnerable pathway through the metal is opened up along the grain boundaries whenever carbide precipitation occurs. When corrosion sets in along this pathway, the steel becomes unreliable for use under stress at either 'low or elevated temperatures.

The addition of one or more of the elements of the group consisting of titanium and columbium to certain austenitic stainless steels prevents intergranular corrosion, where the additional elements are present in sufficient quantity to tie up the carbon and thus arrest the formation of chromium carbides at the grain boundaries. In many of the steels which have either or both of the elements titanium and. columbium present, though, for arresting intergranular corrosion, there still remains a difficulty with sigma phase, and the steels accordingly are susceptible to creep and stress'rupture at temperatures in the approximate range of 1200 F. to 1500 R, these ingredients apparently intensifying the inclination to sigma phase formation. An appreciable loss of creep strength and stressto rupture strength occurs at these temperatures immediately with the formation of sigma phase. As "time progresses, deterioration of the high temperature providing stainless steel of the character indicated which 36 properties is progressive and accelerated until eventual failure. Someembrittlement at room temperature also occurs after long exposure of the steel to room conditions, though this perhaps is not too serious.

ticles of the steel which are strong and capable of withstanding sigma phase development and the development of carbide precipitation and intergranular corrosion, thus to prevent rupture of the metal from weakness or corrosion when in use.

Referring now more particularly to the practice of my invention, I provide austenitic stainless steel products and articles of the steel, which in including elements such as chromium and nickel in critical amounts along with proper quantities of one or both of the elements titanium and columbium, are for the intents and purposes free of sigma phase and have excellent high temperature prop-- erties inclusive of high resistance to creep and stressrupture and substantial freedom from carbide precipitation. More specifically, I provide stainless steels containing approximately 15% to 17% chromium, 9% to 12% nickel, manganese preferably ranging up to about 2.0%, one or both of the group consisting of titanium and columbium in minimum amount of (Ti+% Cb) :5 times the carbon content and in maximum amount of (TH- A Cb)'=0 .8 0% and the remainder substan- Wherecolumbium is absent from the steel, it will be understood that the titanium is present in minimum amount of 5 times the carbon and in maximum amount of 0.80%. Where the element titanium is absent from the steel, columbium is present in minimum amount of 8 times the carbon and in maximum amount of 1.30%.

My stainless steels are substantially wholly austenitic in structure. Ferrite, if present at all, is only in traces. This I find is essential to the required stress-rupture properties. Where appreciable amounts of ferrite are present (say above about 2%) the'stress-rupture values fall off; also the working properties of the metal suffer.

The composition limits given are in fact considered to be critical in every senseforj I find that where they are departedfrom one or more of' the desired qualities suffer. The chromium content of the steel where in excess of about 17% too readily promotes sigma phase and where below about 15% is apt to be so small as to leave the steel susceptible to heat scaling. Nickel also is critically defined for when below about 9%, sigma formation is detrimental, and if the nickel is above approximately 12% a loss in the creep and stress-rupture properties is experienced.

The carbon component of the steel not only is preferably restricted to amounts ranging up to about 0.10%, but is related'to the amount of titanium and columbium or both when both are present. Should one or both of the latter elements be used to excess, the excess will promote sigma formation. Where the titanium and/or columbium are present 'in accordance with the ranges indicated, however, the steel not only is stable with reto tie up the carbon and prevent carbide precipitation and impoverishment of the chromium at the metal'grain boundaries. The steels accordingly are stable with regard to intergranular corrosion and thus" are not likely gard to sigma phase formation but these elements serve,

to develop weaknesses at the metal grain boundaries,

temperatures. Titanium, moreover, is a nitrogen inhibitor and therefore is particularly useful should enough 'which will impair the load carrying capacity'at high I limit the quantity of silicon in my high temperature steel to amounts ranging up to about 1% because this is a commercially practical and acceptable limit and even more importantly because excess silicon tends to promote sigma phase. ferred primarily for assuring hot workability of the steel and because of being commercially practical. In certain instances, however, 'I use larger quantities of manganese and adjust the chromium-nickel balance for any excess above 2%, this by regarding the excess manganese as a nickel substitute.

Among the products or articles which 'I make of the steel are chemical apparatus and equipment for operating at elevated temperatures and under high pressures. Thus,

for example, I often provide containers as in the form ofvessels, tanks, tubing, orthe' like, of the steel for use in oil cracking units or in other chemical handling systems such'as where incandescent gases 'or other fiery chemicals are encountered. I find that the steel is amenable to welding'and to any of a host of other fabricating operations such as cutting and'punching, and therefore,'

I often resort to one or more of these operations in'producing certain products, as for example seam-welded tubes of the steel. In use, the products and articles are The manganese limit of 2% is prethoroughly capable of withstanding the effects of high temperatures up to 1500 F. or more without hurtful sigma formation or carbide precipitation. High pressure and elevatedtemperature operating conditions such as in the pressure fluid containers which I make of the steel are satisfactorily accommodated. In this, the metal reliably resists creep and stress-rupture and is not susceptible tof intergranular corrosion such. as might. lead to weaknesses and rupture under the temperatures and pressures.

Thus it will be seen that in this invention there are provide'd'high temperature austenitic stainless steels and products and articles of the steels in which the various objects noted, together with many thoroughly practical advantages, are successfully achieved. It will be seen that the products are strong and durable and are well adapted to withstand high temperatures while under load even over long periods of time. Also, it will be appreciated that the steels are corrosion resistant in hot corrosive atmospheres or when exposed to chemicals which corrode ordinary low-carbon steels. My products, it will be seen, have the remarkable ability to resist sigma phase development, and are all the more strong, durable and corrosion resistant in view of their stability against carbide precipitation and intergranular corrosion.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

' I claim:

1. High temperature duty austenitic stainless steel characterized by good resistance to stress-rupture and to creep at 1200 to 1500 F. or more containing 15% to 17% .chromium, 9% to 12% nickel, up to 1% silicon, carbon, metal of the group consisting of titanium and columbium in minimum amount of (Ti+% Cb) :5 times the carbon content and in maximum amount of (Ti+% Cb) 0.80%, and the remainder substantially all iron, said chromium, nickel, silicon, carbon, titanium and columbium being present in such relative amounts that the steel is fully austenitic with ferrite content not exceeding 2% and free of sigma phase and carbide precipitation at elevated temperatures.

2. High temperature duty austenitic stainless steel characterized by good resistance to stress-rupture and to creep at 1200 to 1500 F. or more containing 15% to 17% chromium, 9% to 12% nickel, up to 1% silicon, carbon, titanium from 5 times the carbon content up to 0.80%, and the remainder substantially all iron, said chromium, nickel, silicon, carbon and titanium being present. in such relative amounts that the steel is fully austenitic with ferrite content not exceeding 2% and free I of sigma phase and carbide precipitation at elevated temperatures.

3. High temperature duty austenitic stainless steel characterized by good resistance to stress-rupture and to creep at 1200 to 1500" F. or more containing 15% to 17% chromium, 9%; to 12% nickel, up to 1% silicon,

carbon, columbium from 8. times the carbon content up to 1.30%, and the remainder substantially all iron, said chromium, nickel, silicon, carbon and columbium being present in such relative amounts that the steel is fully austenitic with ferrite content. not exceeding 2% and free of sigma phase and carbide precipitation at elevated temperatures.

References Cited in the file of this patent

Claims (1)

1. HIGH TEMPERATURE DUTY AUSTENITIC STAINLESS STEEL CHARACTERIZED BY GOOD RESISTANCE TO STRESS-RUPTURE AND TO CREEP AT 1200 TO 1500* F. OR MORE CONTAINING 15% TO 17% CHROMIUM, 9% TO 12% NICKEL, UP TO 1% SILICON, CARBON, METAL OF THE GROUP CONSISTING OF TITANIUM AND COLUMBIUM IN MINIMUM AMOUNT OF (TI+5/8 CB)=5 TIMES THE CARBON CONTENT AND IN MAXIMUM AMOUNT OF (TI+8/13 CB=0.80%, AND THE REMAINDER SUBSTANTIALLY ALL IRON, SAID CHROMIUM, NICKEL, SILICON, CARBON, TITANIUM AND COLUMBIUM BEING PRESENT IN SUCH RELATIVE AMOUNTS THAT THE STEEL IS FULLY AUSTENITIC WITH FERRITE CONTENT NOT EXCEEDING 2% AND FREE OF SIGMA PHASE AND CARBIDE PRECIPITATION AT ELEVATED TEMPERATURES.