US3259528A - High strength stainless steels - Google Patents

Description

July 5, 1966 K. M. CARLSEN 3,259,528

HIGH STRENGTH STAINLESS STEELS Filed May 2, 1962 "/0 CARBON 2 3 2843 2B4l 2842 X x x x e Q Q 2835 2836 2837 2838 2840 2844 l I l l l T l l l I 3.0 4.0 5.0 6.0 ADJUSTED NICKEL IN'ALLOY Flg. l.

320 U) a. o o 9 300 284 m 2 0 28 3 8 gem o 6 2 43 x2846 l l l l 3.0 4.0 5.0 6.0

ADJUSTED NICKEL IN ALLOY INVENTOR.

KURT M. CARLSEN F' .2. BY

' fibm;

his ATTORNEY United States Patent 3,259,528 HIGH STRENGTH STAlNLESS STEELS Kurt M. Carlsen, Bergen, Norway, assignor to Jones &

Laughlin Steel Corporation, Pittsburgh, Pa., 21 corporation of Pennsylvania Filed May 2, 1962, Ser. No. 191,889 2 Claims. (Cl. 14837) This invention relates to stainless steels which are austenitic in the annealed condition. It is more particularly concerned with steels of that type in which the preponderance of the austenite is transformed to martensite by cold-Working so as to raise the yield and tensile strengths of the steels to extremely high values.

A standard grade of austenitic steel is AISI type 301, which is also known as 17-7 because its nominal chromium content is 17% and its nominal nickel content is 7%. The section entitled Wrought Stainless Steels of Metals Handbook for 1961, published by The American Society for Metals, lists on page 416 the properties of type 301 stainless steel in the annealed and in the cold-worked conditions. Yield strength at .2% offset of conventional 301 type steel is about 33,000 p.s.i., which is typical of an austenitic steel. However, cold-working this grade greatly increases its mechanical properties. If the steel is cold-reduced about 45% to 50%, its yield strength is raised to as much as 200,000 p.s.i. The cold work transforms a portion of the austenite into martensite, and it is considered that the pronounced strengthening is caused by martensite precipitating along the slip planes of the alloy.

While it is possible to treat certain known austenitic stainless steel so as to obtain yield strengths somewhat higher than 200,000 p.s.i., that high performance either requires expensive modifications of the steel composition, or expensive treatments. It would be desirable to have available a steel no more complicated chemically than type 301, but having a yield strength on the order of 250,000 to 300,000 p.s.i.

Accordingly, it is an object of my invention to provide a simple chromium-nickel stainless steel having high yield and tensile strength in the cold-worked condition. It is another object to provide such a steel which has normal properties in the annealed condition. Other objects will appear in the course of the following description of my invention.

I have found that very high yield and tensile strengths are exhibited by cold-reduced steels of a composition similar to that of type 301 if the carbon and nickel contents are adjusted to each other in a manner to be described. In the accompanying drawings, FIGURE 1 illustrates graphically the relation between carbon and nickel in steels of my invention, and FIGURE 2 illustrates the effect of changes in nickel content on the yield strength of a steel of my invention.

The steels of my invention generally contain nickel from about 3.5% to about 6.6% and carbon from about .05% to about .15%, the nickel content being in the lower end of its range when the carbon content is in the higher end of its range, and vice versa, so that those elements may be said to be adjusted to be inversely proportional to each other. The steel contains manganese in the amounts generally found in austenitic stainless steels of the A181 300 series, that is, not more than 2% and preferably in the range from about 90% to about 1.10%. Silicon is likewise present in amounts generally found in conventional steels of that series, that is, not more than about 1% and preferably between about .50% and .70%. The nitrogen content ranges from about to about .10%. The metalloids phosphorus and sulphur and other incidental impurities are present in normal amounts. Enough chromium must be present to render "ice the steel austenitic in the annealed state, but the chromium content does not exceed about 18%. The minimum chromium content is about 16%.

It is known that in austenitic steels of the A181 300 series, chromium and nickel, within limits, are interchangeable, and they are interchangeable in my steels to some extent. Within the limits of those elements which I have mentioned, I find that chromium and nickel can be interchanged in the proportion of about 1.4% chromium to 1% nickel. Because of this interchangeability, I find it convenient in the paragraphs to follow to discuss the dependence of physical properties on the chemistry of my steel in terms of adjusted nickel or adjusted percentage nickel. The adjusted nickel is actual nickel content of each steel mentioned lowered or raised an amount depending on the difference between the actual chromium content of the steel and the value 17.4%, the proportion of chromium to nickel being that above stated.

As the carbon content of the steels herein described is raised, and the nickel content correspondingly lowered, the steels become increasingly difiicult to work in the annealed state, so that the practical upper limit of carbon content is about .15 the corresponding practical lower limlt of nickel content being about 3.5

TABLE 1 HeatNo. 0 Mn P 8 81 Cr Ni Ni N *Adjusted.

In Table 1 are listed the designations and compositions of a number of heats of steel of my invention, as well as several heats outside the scope of my invention. In Table 2 are listed the pertinent physical properties of the steels of Table 1, both in the annealed state and after being cold reduced 55%. FIGURE 1 is a plot of carbon content against adjusted nickel content for those steels. Steels of my invention, which are identified by circles, fall within the area a-b-c-d-e marked out on FIGURE 1. Steels outside the scope of my invention are indicated by crosses. It will be understood by those skilled in the art that the lines marking out the area abcd-e in FIGURE 1 cannot be precisely located, because a physical property such as the yield strength of steel can be measured only with a fair degree of reproducibility. Furthermore, the change of yield strength with change of composition of the steel is not abrupt, as may be observed from FIGURE 2. That figure is a plot of yield strength against adjusted nickel content for steels of Tables 1 and 2 containing .07% carbon, and cold worked by reducing them 55%.

Table 2 shows that the steels falling within the area abcde of FIGURE 1 had yield strengths in the annealed condition of less than about 40,000 p.s.i., that is to say, the steels were all austenitic. The same steels after being cold-reduced about 50% all exhibited yield Patented July 5, 1966 3 strengths greater than about 250,000 p.s.i. The steels falling outside the area a-b-c-d-e either had yield strengths higher than about 40,000 p.s.i. in the annealed condition, thus showing that they were not austenitic,

4 I claim: 1. A cold reduced austenitic-martensitic steel having a yield strength not less than about 250,000 p.s.i., said martensite being produced by cold reducing the steel about or yield strengths lower than 250,000 p.s.i. in the cold- 5 55%, consisting of about 3.5 to about 6.5% nickel, about worked condition, or both. .05 to about .15% carbon, not more than 2% manganese, FIGURE 2 illustrates the high strengths obtained from not more than 1% silicon, sufilcient chromium to render an .07% carbon steel if its nickel content is critically the steel austenitic in the annealed condition but not more adjusted to its carbon content in accordance with my than about 18%, and the balance iron and incidental iminvention. The steels plotted in FIGURE 2 which are purities in normal amounts, the carbon content and the austenitic in the annealed condition are indicated by adjusted nickel content, adjusted to a chromium content circles, While those which are not austenit-ic in the anof 17.4% on the basis of 1% nickel being equivalent to nealed condition are indicated by crosses. That figure 1.4% chromium, defining a point falling Within the area shows that an .07% carbon steel displays maximum yield abcde of FIGURE 1. strength at an adjusted nickel content of about 5.4%. 2. A steel as in claim 1, in which the carbon and ad- That yield strength is in the neighborhood of 275,000 justed nickel contents are related to each other by the p.s.i. for the cold-worked steel, but in the annealed state equation: the steel has a yield strength of less than about 40,000 P 4 ercent n1ckel=8.3%-31 (Percent carbon p.s.1. FIGURE 2 shows that the y1e1d strength of the steel in the cold-worked condition drops off as the ad- 0 References Cited by the Examiner justed nickel content is increased or reduced from its UNITED STATES PATENTS critical value for the given carbon content. The yield strength of the steel in the annealed condition likewise 3,903,386 9/1959 WaXWelleI' 3 8 increases as the adjusted nickel content is reduced from OTHER REFERENCES its critical value. Similar curves could be plotted for steels of other carbon contents within the range of my Stamless f Pagfis 210 and Ed 1t9d by invention. An empirically derived equation relating car- Zapfie- Published In 1949 by the American Soclety for bon and nickel for maximum yield strength in steels of Metals: Cleveland: OblomY 13: HYLAND BIZOT, Primary Examiner.

Percent (Percent DAVID L. RECK, Examiner.

That equation is plotted as a broken line in FIGURE 1. P. WEINSTEIN, Assistant Examiner.

TABLE 2 Annealed Cold Rolled Heat N o. 0.2% Ultimate Elonga- 0.2% Ultimate Elonga- Yield Tensile tion, Yield Tensile tion, Strength, Strength, Percent Strength, Strength, Percent p.s.i. p.s.i. p.s.i. p.s.i.

Claims (1)

1. A COOL REDUCED AUSTENITIC-MARTENSITIC STEEL HAVING A YIELD STRENGTH NOT LESS THAN ABOUT 250,000 P.S.I., SAID MARTENSITE BEING PRODUCED BY COLD REDUCING THE STEEL ABOUT 55%, CONSISTING OF ABOUT 3.5 TO ABOUT 6.5% NICKEL, ABOUT .05 TO ABOUT .15% CARBON, NOT MORE THAN 2% MANGANESE, NOT MORE THAN 1% SILICON, SUFFICIENT CHROMIUM TO RENDER THE STEEL AUSTENITIC IN THE ANNEALED CONDITION BUT NOT MORE THAN ABOUT 18%, AND THE BALANCE IRON AND INCIDENTAL IMPURITIES IN NORMAL AMOUNTS, THE CARBON CONTENT AND THE ADJUSTED NICKEL CONTENT, ADJSTED TO A CHROMIUM CONTENT OF 17.4% ON THE BASIS OF 1% NICKEL BEING EQUIVALENT TO 1.4% CHROMIUM, DEFINING A POINT FALLING WITHIN THE AREA A-B-C-D-E OF FIGURE 1.

Images