US3044872A

US3044872A - Steel alloy composition

Info

Publication number: US3044872A
Application number: US850117A
Authority: US
Inventors: Walter C Hayes; Robert W Dickinson
Original assignee: North American Aviation Corp
Current assignee: North American Aviation Corp
Priority date: 1959-11-02
Filing date: 1959-11-02
Publication date: 1962-07-17
Anticipated expiration: 1979-07-17
Also published as: BE596717A; CH415068A; GB921838A

Description

United States Patent @fifice 3,044,872 Patented July 17, 1

This invention relates to an improved high temperature alloy. More particularly, this invention relates to an improved high temperature steel alloy with low thermal coeflicient of expansion, high thermal conductivity, and good high temperature mechanical properties.

Austenitic stainless steels are well known in the art. An example of a stainless steel is AISI type No. 304 which contains from 18% to 20% chromium and 8% to 11% nickel with 2% max. of manganese. It has a low carbon content and has many uses. However, it has some draw-backs, one of which is its relatively high cost because of the high chromium and nickel content. Also, its thermal coeflicient of expansion is a little high for certain applications while its thermal conductivity is too low. On the other hand, ferritic steels have low coetficient of expansion anda high thermal conductivity as well as low cost but they are weak at elevated temperatures. Consequently, it would be advantageous to obtain a ferritic alloy which would combine the conductivity and expansion properties of ferritic steels with the excellent strength properties of austenitic steels for use at high temperatures. For such applications, high thermal. conductivity, low thermal coeflicient of expansion and, therefore, low thermal stresses are important.

It is, therefore, an object of this invention to provide a new steel alloy having high creep and stress rupture properties at elevated temperatures. It is also an object of this invention to provide a steel alloy having a high corrosion resistance in molten metal service. Another object of this invention is to provide a steel alloy having adequate strength and ductility in the heat-affected zone of weldments without costly post weld heat treatments. Another object of this invention is to provide a ferritic steel alloy having high temperature properties equal to or surpassing those of the austenitic steels. Still another object is to provide a steel alloy which is obtainable at a much lower cost than stainless steel. It is also an object to provide a steel alloy which has a low coefficient of expansion, a high thermal conductivity, and a high resistance to thermal, stresses at elevated temperatures. Still other objects of this invention will become apparent from the discussion which follows.

The above and other objects of this invention are obtained by providing a steel alloy having improved high temperature properties containing from about 0.4% to about 10% chromium, from about 0.4% to about 4% molybdenum, from about 0.05% to about 0.4% carbon, from about 0.1% to about 1.5% manganese, from about 0.1% to about 1% niobium, from about 0% to about 1.4% titanium, from about 0% to about 4% nickel, and the remainder substantially iron. In addition, the alloy can contain about 0.1% (max.) boron, about 0.02%

piping systems, heat exchangers, and pressure vessels manufactured of these alloys for use in refineries Where the. added strength over known stainless steels is beneficial.

The steel composition or alloys of this invention contain from about 0.4 to about 10 weight percent chromium in order to give the steel adequate corrosion and oxida tion resistance. The steel alloys of this invention are particularly useful for service above about 1050 F. In order to assure oxidation resistance at temperatures of this magnitude it is preferred to include at least about 1.5 weight percent chromium. This constitutes a preferred minimum chromium concentration. One of the properties of the steel alloy of this invention is good notched impact properties at high temperatures. It is found that these are not enhanced to any appreciable degree by chromium concentrations above about 7.5 weight percent and, therefore, the latter amount constitutes a preferred upper limit to the chromium concentration. Hence, the preferred range of chromium is from about 1.5 weight percent to about 7 weight percent.

The molybdenum concentration in the steel alloys of this invention varies from about 0.4 to about 4 weight percent. An amount of molybdenum equivalent to about 0.4 weight percent together with the amount of chromium indicated above is sufiicient to impart a perceptible degree of resistance to oxidation and a fair degree ofstrengthening at elevated temperatures. The 0.4 weight percent, therefore, constitutes a preferred lower limit for the molybdenum content. No additional benefit is obtained from amounts of molybdenum above about;4 weight percent and the latter, therefore, constitutes an upper limit of the molybdenum concentration.

The carbon is present in the novel steel alloys inamounts ranging from about 0.05 to about 0.4 weight percent in order to impart strengthening characteristics to the steel. At least about 0.05 weight percent carbon is required in order to impart a discernible increase in the strength of the steel. Above about 0.4 weight percent carbon there is a danger of making the steel too brittle. The broad range of carbon concentrations, there fore, varies in the range of from about 0.05 to about 'to about 1 Weight percent niobium to the above described steels containing chromium, molybdenum, and carbon in the amounts indicated, imparts ahigh degree of strength increase at elevated temperatures. 'With the added 'niobium, the resulting steel has a low notch sensitivity in stress rupture tests at elevated temperatures. It also exhibits good notched impact properties at ambient (room) temperature. columbium becomes eifectively significant at a concentration of about 0.1 weight percent niobium. The beneficial strengthening effect decreases rapidly per increment of niobium add-ed above about l weight percent. Therefore, the preferred range of niobium concentration in the steel alloy of this invention varies from about 0.1 to about 1 weight percent.

Many steels contain titanium which functions as a grain refining element and astrengthening agent. It is found that the addition of niobium to titanium-containing steels in which the amounts of chromium, molybdenum, and carbon are as stated hereinabove, greatly increases the high strength property of the steel. Usually about 0.1 weight percent titanium is added to the steel in order to achieve a significant effect in the elevated temperature The degree of strengthening due to the 3 mechanical properties. Above about 1.4 weight percent titanium it is found that the quality of the steel is not improved to any significant degree and, therefore, the titanium content varies from about 0.1 to about 1.4 weight By elevated temperature strength in the above discussion is meant the strength at temperatures above about 1050 F. In addition to the high temperature strength, the alloys of this composition also have the added adpercent in one embodiment of the steel alloy of this in- 5 vantage of low cost, good general corrosion resistance, vention. To the titanium-containing steel alloy can be resistance to chloride stress corrosion, good weldability, added from about 0.1 to about 1 weight percent niobium and good resistance to thermal stresses. to increase the elevated temperature strength of the al- In general, a heat of steel of this ihvehtloh is melted loy as stated hereinabove. It is, however, found that poud Cast y Conventional thethodsh cast metal can timum strengthening properties are obtained when the 10 b hot Worked y Convohtloual Praotlces to a ll/{ought total amount of the added niobium and titanium varies product. Heat treatment is accomplished by heat ng to from about 0.2 to about 1.5 weight percent and wherein om bout 1300" to about 2200 for a Peflod of the amount of niobium is from about 0.1 weight percent from to about hours- Following thls, the to about 1 weight percent, and this constitutes a preferred metal alloy alt Cooled of quenched to a tempofatufe of b di of hi i i 15 from about 1300 F. to about 0 F. This is followed Nickel is added to steel in order to lower the minimum y aging of tomPefing at a tempel'hthro of from about temperature required for solution heat treating, and to 1100 to about a Poflod of h' about improve low temperature ductility and impact strength. to about 24 hours. Th1s treatment results in a steel alloy Amounts of nickel ranging f o about 0 1 to about 4 whrch is more ductile and less notchsensitive than ordiweight percent give satisfactory results in the solution heat y high -o o Steelstreating of the steel alloy compositions of this invention. The f0ll0W1ng non-llmltlng e p r h Illustrate A least about O 1 weight pol-Cont of nickel is required the var1ous steel alloy compositions of this invention. in order to give a significantly perceptible effect, while EXAMPLE amounts above about 4 weight percent of nickel decrease the response to heat treatment. The addition of niobium 25 A11 induction heat was air melted and cast into a hot to a nickel-containing steel alloy having chromium, mopp Square tapered iugot- A chemical ahall'sis lybdenum, and carbon in the amounts outlined hereinof the ingot Showed Substantially the following p above, substantially increases the elevated temperature tion in Percentage y g 232% Chromium, 199% strength of the steel. The amount of niobium added in molybdenum, 047% titanium. 013% Carbon, 0.49% this instance is the same as that added to the other g 7b 0.002% P p 10227 sulniobium-containing steel embodiments of this invention. fill, 011% 51110011, (X00623 t g falumll'lumi In other words, the chromium, molybdenum, carbon, and and the balance substantlally Iron The 1hgot nickel-containing steel as hereinabove specified can conto form a Plato Wlth a Toduotlou Tatlo of P- tain from about 0.1 to about 1 weight percent niobium. p f y T e steel was heated to 2100 F. for This, then, constitutes another embodiment of the instant a Perlod of 1 hour followed y coollhg to a p invention ture of substantially 70 F. It was then heated to a tem- Still another embodiment of this invention is a steel Perathre of Substantially 1300 for a Phrtod of 8 alloy composition containing the chromium, molybdenum, hours: and then Cooled to carbon, and nickel in the amounts specified hereinabove XAM together with titanium and niobium in an amount vary- E PLE H ing from about 0.2 to about 1.5 weight percent wherein Followlhg the p o h of Example I, a heat having the amount of niobium varies from about 0.1 to about 1 fohoWlug oomposltloll Was Processodl 242% chroweight percent. This composition then has the properties 111111111, 194% o yb o of a similar composition minus nickel described herein- 3311656, 012% hloblum, tltahlum, P above as well as the added advantage of having a low- Phofus, Sulfur, Silicon, O-006% nitrogen, ered solution heat treating temperature. The nickel-, aluminum, and the balance Substantially iron. titanium-, and niobium-containing alloy, therefore, con- Still other steel alloys of this invention and their comstitutes still another embodiment of this invention. position are given in the following Table I. The balance Table I Heat No. Or Mo 0 Mn Nb Ti Ni B, N, .41, Si, P, 3,

Max. Max. Max. Max. Max. Max.

of the composition in each heat is substantially iron. Tables II and III show the various properties of a steel alloy of this invention together with the properties of ferritic steel and 304 stainless steel for comparison purposes.

fllable II Tensile Properties (at Room Temperature) Alloy (Wt. Percent) 0. 2% Ofiset Ultimate Yield Tensile Strength Strength (10 p.s 1) (10 p.s.i.)

Fe+2.25 Cr+1 Mo+0.4 Ti+0.4 Nb 93. 108.0 Fe+2.25 Cr-l-l M0 41. 5 79. 9 Type 304 Stainless Steel 37. 2 85. 2

Short Time Tensile Properties (at 1,050 F.565 G.)

Fe+2.25 Cr+1 Mo+0.4 'li-l-OA Nb Fe+2.25 Cr+1 M0 Type 304 Stainless Steel Short Time Tensile Properties (at 1,200 F.650 O.)

Fe+2.25 Cr+1 Mo+0.4 Ti+0.4 Nb. 56.0 60.0 Fe+2.25 Cr+1 M0 18. 3 32. 9 Type 304 Stainless Steel 12.0 48.0

Table III Min. Creep Rates (at Stress to Rup- 1,050 F.565 O.) ture (at 1,100 F.593 0.) Alloy (Wt. Percent) Stress Min. Creep, Time 10 10 Rate Percent in hours p.s.l. p.s.i. Per El.

7. 5 48 Fe+2.25 Or+1 Mo+0.4 Ti+ 12.5 40 0.4 Nb 17. 5 33 25.0 26 7. 5 45 Type 304 Stainless Steel 25.0 17 7. 5 27 Fe+2.25 Cr+1 M0 f? 10.3

The chemical analysis of the steel alloy of this invention given in Table II and III as Fe+2.25 Cr+l Mo+0.4 Ti+0.4 Nb is that given for heat No. 27 in Table I, supra. It will be noticed from Table III that the alloy of this invention requires from about 6.7% to about 53% higher stress to rupture in the same time as that required for the rupture of 304 stainless steel, and from about 78% to about 152% higher stress than required for the rupture of terrific steel containing 2.25 weight percent chromium and 1 weight percent molybdenum. That is, by merely adding a small amount of niobium and titanium, the composition of the steel is improved with respect to stress to rupture by 152%. From Table II it is seen that the offset yield strength of the niobium-containing alloy of this invention is 124% higher than that of 2.25 chromium and 1% molybdenum fenritic steel and 150% higher than that of 304 stainless steel at room temperature; 147% higher than that of ferritic steel and 378% higher than that of stainless steel at 1050 F. (565 C.) and 206% higher than ferritic steel and 366% higher than stainless steel at 1200 F. (650 C.). It is also seen that the ultimate tensile strength of the niobium-containing steel is 35% higher than that of ferritic steel and 26% higher than that of stainless steel at room temperature; 41% higher than 6 that of ferritic steel and 29% higher than that of stainless steel at 1050 F.; and 82% higher than that of ferri-tic steel and 25% higher than that of stainless steel at 1200 F. From Table III it is also noted that the minimum creep rate of the niobium-containing steel of this invention is from about a factor of 2 to about a factor of 10 lower than for ferritic steel and firom about a factor of 5 The steel alloy specimens of this invention used for the tests given in Tables II and III were 4 inches long with a 1.5 inch gauge length and a gauge diameter of 0.375 inch. The specimen steel alloys of this invention described in the Examples and in Table I, like the specimen used to illustrate the improvement in the properties listed in Tables II and III, all have properties which are an improvement over the ferritic and stainless steel compositions.

While the invention has been described with reference to specific steel alloy compositions, it is to be understood that this is by way of example only and no-tby way of limitation. The spirit and scope of the invention is to be limited only by the appended claims.

We claim:

1. An alloy having improved high temperature properties consisting essentially offrom about 0.4% to about 7.5% chromium from about 0.4% to about 4.0% molybdenum from about 0.05% to about 0.4% carbon from about 0.1% to about 1.5% manganese from about 0.1 to about 1.0% niobium from about 0.1% to about 1.4% titanium and the remainder essentially iron.

2. An alloy having improved high temperature properties consisting essentially offrom about 2.0% to less than 4.0% chromium from about 1.0% to about 2.0% molybdenum from about 0.05% to about 0.4% carbon from about 0.1% to about 1.5% manganese from about 0.1% to about 1.0% niobium from about 0.11% to about 1.4% titanium and the remainder essentially iron.

3. An alloy having improved high temperature properties consisting essentially ofabout 2.25% chromium about 1.0% molybdenum about 0.15% carbon about 0.5% manganese about 0.4% niobium about 0.4% titanium about 0.1% (max.) boron about 0.02% (max.) nitrogen about 0.2% (max) aluminum about 1.5% (max.) silicon about 0.03% (max.) phosphorus about 0.03% (max.) sulfur and the remainder essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,159,723 Franks May 23, 1939 2,194,178 Becket et al Mar. 19, 1940 2,513,935 7 Harris July. 4, 1950 2,590,835 Kirkby et al. Apr. 1, 1952 2,905,577 Harris et a1 Sept. 22, 1959

Claims

1. AN ALLOY HAVING IMPROVED HIGH TEMPERATURE PROPERTIES CONSISTING ESSENTIALLY OF-