Stainless Steel

1. Technical Field

The present invention relates to an austentic, nonmagnetic stainless steel which has improved resistance to stress corrosion cracking.

2. Background Art

In austenitic stainless steels, chromium carbide often forms at the grain boundaries within the solid steel at temperatures in the range of about 800.degree.-1600.degree. F. Working the steel in the range from 1000.degree.-1300.degree. F. is generally considered the worst conditions for chromium carbide formation at the grain boundaries (second phase formation). Whenever chromium carbide is formed, chromium necessary to maintain the steel as a stainless steel is depleted. In the area immediately adjacent to the boundary, this depletion is particularly harmful because an electrochemical cell is established within each grain. The material next to the grain boundary (called "chromium-poor material") is eventually consumed because this chromium-poor material becomes anodic relative to the remainder of the grain material, initiating pitting-type corrosion. Further consumption can lead to both inter- and transgranular cracking, if degradation is allowed to progress.

The present invention relates to a fully austenitic, nonmagnetic stainless steel. The preferred steel has improved resistance to stress corrosion cracking because the steel chemistry is controlled to limit the concentration of carbon available in the steel and to include excess columbium in an amount sufficient to stabilize the steel by having the columbium preferentially scavenge carbon over chromium. In this manner, columbium carbide preferentially forms rather than chromium carbide (which would be detrimental to the resistance of the steel). Substantially all chromium carbide formation at grain boundaries is eliminated by the inclusion of excess columbium (niobium) and by maintaining a low carbon concentration. In a preferred, fully austenitic, nonmagnetic stainless steel, the carbon content of the final steel should be no greater than 0.035% by weight of the melt, and columbium should be added to a concentration of at least ten times the carbon concentration to form a fully austenitic, nitrogen-bearing, manganese-substituted, nonmagnetic stainless steel having the following composition:

16-20% Manganese,

12-15% Chromium,

5.0% Molybdenum max.,

2.5% Nickel max.,

1.0% Copper max.,

0.75% Silicon max.,

0.2-0.5% Nitrogen,

0.04% Phosphorus max.,

0.01% Sulfur max.,

0.035% Carbon max.,

Columbium at a concentration of at least ten times the Carbon concentration, and

the remainder being essentially Iron with incidental impurities.

The nominal mechanical properties of this preferred steel are 110 KSI yield strength, 125 KSI tensile strength, 30% elongation, and 60% reduction of area (with 60-100 ft. lb. CVN energy at room temperature). These properties are obtained by working the steel during the later stages in the temperature range between about 1100.degree.-1400.degree. F.

This steel uses manganese substitution for nickel in the basic composition and relies on nitrogen strengthening and carbon stabilization to achieve its overall mechanical/chemical properties. Because the steel is fully austenitic, it cannot be hardened by common heat treatment procedures, but must be hardened instead by "working" (forming). The ultimate strength of the alloy is principally determined by nitrogen strengthening (due to solid solubility), which is dependent upon the degree of work and the temperature of the material during working.

Maintaining the carbon content within the alloy as low as possible, plus the addition of columbium at a minimum of ten times the carbon content, effectively inhibits chromium carbide formation at grain boundaries. Improved corrosion resistance is achieved because the carbon content is near the solubility limit of carbon in the alloy, thereby reducing the tendency for second phase formation, and columbium is present to ensure that columbium rather than chromium carbide forms. The columbium carbide is distributed uniformly throughout the material, thereby minimizing second phase formation at the grain boundaries and eliminating the electrochemical problems of the prior art. This special carbon/columbium stainless steel greatly improves the chemical resistance of the material to corrosion, particularly corrosion associated with grain boundary networks of chromium carbide precipitates (commonly called "sensitized materials").

Stress corrosion cracking has been a long-standing and persistent problem of steels, particularly fully austenitic, nonmagnetic stainless steels of the type described in this invention. By carefully controlling the concentration of carbon within the melt and the ratio of columbium to carbon, it has been found that substantially improved resistance to stress corrosion cracking may be obtained. Therefore, it is particularly desirable to make fully austentic, nonmagnetic stainless steel consisting essentially of, by weight:

16-20% Manganese,

12-15% Chromium,

5.0% Molybdenum max.,

2.5% Nickel max.,

1.0% Copper max.,

0.75% Silicon max.,

0.2-0.5% Nitrogen,

0.04% Phosphorus max.,

0.01% Sulfur max.,

0.035% Carbon max.,

Columbium at a concentration of at least ten times the Carbon concentration, and

the remainder being essentially Iron with incidental impurities.

This steel avoids the formation of problematic chromium carbide at grain boundaries within the solid solution of the steel and may be worked in the temperature range of between 1100.degree.-1400.degree. F. to produce a steel with nominal mechanical properties of 110 KSI yield strength, 125 KSI tensile strength, 30% elongation, and 60% reduction of area (with 60-100 ft. lb. CVN energy at room temperature).

Manganese is added to the melt as a low-cost substitute for nickel and is necessary to provide a fully austenitic structure in the final stainless steel. Chromium is added to make the stainless steel. It is desirable to provide sufficient chromium to ensure that the final steel will be a stainless steel while minimizing the amount of chromium available for formation of chromium carbides. Therefore, the range of 12-15% chromium is particularly desirable in that it satisfies both constraints. Molybdenum, nickel, and copper are added to enhance corrosion resistance of the final steel. Silicon and nitrogen are added to improve the strength of the final product. Phosphorus and sulfur are rigidly controlled to enhance overall product quality.

The carbon concentration is quite low compared to typical, fully austenitic, nonmagnetic stainless steels and is limited so that the concentration of carbon in the final steel is near or substantially at the solubility limit of carbon in the final steel. At this concentration, the carbon tends to stay in solution rather than to combine with other metals in the steel. To substantially eliminate the formation of chromium carbide at grain boundaries in the solid solution of the final steel, columbium (niobium) is added to the melt in an amount sufficient to stabilize the steel by having columbium preferentially scavenge carbon over chromium. In this manner, columbium carbide forms and is distributed uniformly throughout the steel rather than chromium carbide, which is distributed essentially at the grain boundaries. The electrochemical cell phenomenon which leads to pitting-type corrosion and inter- and transgranular cracking is virtually eliminated by the addition of sufficient columbium. Thermodynamically, it is necessary to add between about five to eight times the carbon concentration of columbium, by weight, to provide the essential chromium carbide elimination. To ensure that adequate columbium is available for carbide formation and that extra columbium is available for enhancing the properties of the final steel, it is desirable and highly preferred to add a minimum of ten times the carbon concentration of columbium to the steel.

While others have tried to reduce the carbon concentration and have added columbium to stabilize stainless steels, to the knowledge of the inventors, no one has found the critical relationship between the concentrations of carbon and columbium in a fully austenitic, nitrogen-bearing, manganese-substituted nonmagnetic stainless steel. Therefore, the general concept of this invention is to maintain the carbon concentration of the steel near the solubility limit for carbon in the steel while adding columbium in an amount sufficient to stabilize the nitrogen-bearing steel by columbium's preferential scavenging of carbon over chromium in the stainless steel product. This preferential scavenging substantially eliminates chromium carbide formation at grain boundaries. Those skilled in the art will readily recognize the desirable methods for manufacturing steels of this quality, with such melt processing usually being conducted in an argon/oxygen decarburization vessel.

NMS-100 steel of Earle M. Jorgensen Co. having an analysis within the ranges indicated for the preferred steel of this invention was tested to show the susceptibility of intergranular corrosion through ASTM tests A262A and A262E under standard conditions. Each sample was initially sensitized with heat treatments at about 1200.degree. F. for 1-2 hours. When examined under the microscope, the samples passed both A262A and A262E, there being no cracks visible in the samples at low magnification. This steel had a low carbon concentration near the solubility of carbon in the solid solution of the alloy, and the columbium concentration was at least ten times the carbon concentration, by weight. The steel was fully austenitic, nonmagnetic, and fully stabilized.