NITROGEN-CONTAINING MARTENSITIC STAINLESS STEEL ALLOY

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT Not Applicable

BACKGROUND

[0001] This invention relates to the field of stainless steel alloys and particularly to martensitic stainless steel alloys containing nitrogen.

[0002] Those skilled in the art recognize three basic types of stainless steel: Ferritic, Austenitic and Martensitic. Martensitic stainless steel is generally considered the highest strength and most fatigue resistant of the three basic types of stainless steel. Martensitic stainless steels are typically in the lower range of chromium for stainless steels and, therefore, lower in corrosion resistance compared to other stainless steels. Martensitic stainless steels can be heat treated to a wide range of strengths and have good machinability. Martensitic steels are generally easy to heat treat and relatively easy to hot and cold work.

[0003] High strength and high cycle fatigue resistance are very desirable properties for propellers and other products formed from stainless steel. Higher strength and fatigue resistance allows parts to be made lighter and/more reliable. To achieve such properties, 15-5 martensitic stainless steel is commonly used.

[0004] 15-5 martensitic stainless steel has a composition of

0.07% by weight maximum carbon, 14.0 - 15.5% by weight chromium, 2.5 - 4.5% by weight copper, 1.0% by weight maximum manganese, 3.5 - 5.5% by weight nickel, 0.5 - 0.45% by weight niobium, 0.04% by weight maximum phosphorous, 1.0% by

weight maximum silicon, 0.03% by weight maximum sulfur and the balance iron. The 15-5 martensitic stainless steel is a precipitation hardening stainless steel that combines good strength, toughness and ductility along with hardness capability and corrosion resistance. 15-5 stainless steel may be used in the solution treated condition or heat treated to obtain a variety of properties. The strength of 15-5 martensitic stainless steel in the as cast condition is about 116 ksi yield strength (YTS) and 136 ksi ultimate tensile strength (UTS). While 15-5 martensitic stainless steel may be further strengthened by heat treating, heat treating adds cost and may warp and distort if the 15-5 martensitic stainless steel cast in thin sections, such as those used in propeller blades. Accordingly, it is desirable to increase the strength and fatigue resistance of 15-5 martensitic stainless steel without heat treating or decreasing the corrosion resistance.

[0005] U.S. Patent No. 4,314,863 to McCormick describes a stainless steel alloy consisting of 13 - 19% by weight chromium, 2.0 - 3.6% by weight nickel, 2.0 - 3.5% by weight copper, 0.20 - 1.4% by weight manganese, 0.5 - 1.0% by weight silicon, 0.1 - 0.8% by weight carbon, 0.10% by weight maximum nitrogen, less than 0.10% by weight molybdenum, less than 0.10% by weight aluminum, less than 0.10% niobium, 0.035% by weight maximum sulfur, 0.035% by weight maximum phosphorous and the balance essentially iron. The McCormick patent indicates that the sum of nickel and copper must be at least 5% by weight. The stainless steel is described as being designed to be economical and substantially hard without supplemental heat treatments. However, the preferred microstracture is substantially austenite in combination with some martensite or delta ferrite.

[0006] U.S. Patent No. 5,089,067 to Schumacher describes a substantially martensitic stainless steel having good castability, ductility and capability of being hardened to a wide range of hardness. The alloy composition consists essentially of, in weight percent, up to about 0.08% carbon, above 1.0% to about 4.0% manganese, about 1.0% maximum silicon, less than 1.0% nickel, less than 1.0% molybdenum, about 1.5% to about 4.0% copper, up to about 0.12% nitrogen, about 13.0 - 17.0% chromium, boron up to 0.005%, sulfur up to about

0.5%, phosphorous up to about 0.03% and the balance essentially iron with normally occurring residuals. This stainless steel composition of the Schumacher patent is used for investment cast and forged golf club heads and boat propellers. The high manganese content decreases the fluidity of the alloy and impedes its castability. Furthermore, high manganese content is known to those skilled in the art to have considerable segregation in castings and thereby results and property variation throughout a giving casting. The high manganese level also interacts with the sulfur present in the alloy performing manganese sulfides in the material. Such manganese sulfides are not desirable because they reduce fatigue resistance. Furthermore, silicon is preferably present at a level below about 0.75%. The Schumacher patent indicates that silicon contents above 1.0% may tend to cause low ductility, contributing to fracture.

[0007] U.S. Patent No. 5,514,329 to McCaul, et al describes a castable metastable austenitic steel alloy that contains nitrogen. The alloy contains 17.0 - 18.5% by weight chromium, 1.0% by weight maximum nickel, 0.3 - 1.0% by weight silicon, 0.45% by weight maximum nitrogen, 14.0 - 16.0% by weight manganese, 0.08 - 0.12% carbon, and the balance being iron and impurities. As with the Schumacher patent described above, the exceedingly high manganese content of 14.0 - 16.0% by weight significantly decreases the fluidity of the alloy and hinders the alloy's castability. Furthermore, the high manganese content causes substantial segregation in casting resulting in property variation throughout the casting.

[0008] Accordingly, it is desired to provide an improved stainless steel alloy that is castable, fatigue resistant, has a uniform casting, and provides increased strength and fatigue resistance without heat treating decreasing corrosion resistance.

SUMMARY OF THE INVENTION

[0009] In one aspect of the present disclosure, a substantially martensitic stainless steel alloy is provided. The substantially martensitic stainless steel alloy consists essentially of 12.0 - 16.0% by weight chromium, 1.10 - 2.00% by

weight silicon, 3.00 - 4.50% by weight nickel, 3.50% by weight maximum copper, 0.065% by weight maximum carbon, 1.00% by weight maximum manganese, 0.11 - 0.20% by weight nitrogen, 0.20% by weight maximum molybdenum, and the balance essentially iron. [0010] In another aspect, the alloy according to the present disclosure further consists of 0.03% by weight maximum phosphorous, 0.20% by weight maximum molybdenum, 0.005% by weight maximum sulfur, 0.10% by weight maximum niobium, 0.05% by weight maximum tantalum, 0.05% by weight maximum aluminum, 0.20% by weight maximum vanadium, 0.20% by weight maximum tungsten, and/or 0.25% by weight maximum cobalt.

[0011] An alloy according to the present disclosure preferably has a hardness of about 39 Rockwell C hardness in the as cast condition. The alloy further preferably has an ultimate tensile strength (UTS) of about 146 ksi in the as cast condition and a yield tensile strength (YTS) of about 129 ksi in as cast condition. Furthermore, the alloy may be heat treated to provide a UTS of about 180 ksi and a YTS ofabout l50 ksi.

[0012] The alloy of the present invention may be used to cast propeller blades, steering arms for outboard marine motors, and a multitude of other wrought, forged and cast articles, including, but not limited to, steering components, midsection components, clamp brackets, mounting hardware, knuckles, suspension devices, forks, brackets, and connecting rods.

BRIEF DESCRIPTION OF THE DRAWINGS [0013] Fig. 1 is a graph demonstrating the meantime to failure for the alloy of the present disclosure as compared to 15-5 stainless steel.

DETAILED DESCRIPTION

[0014] The alloy described below is a substantially martensitic stainless steel alloy with increased strength and resistance to fatigue failures. The alloy is harder and stronger than the conventionally used 15-5 stainless steel alloy and

also exhibits greater fatigue resistance.

[0015] The alloy of the present disclosure has a preferred composition, in weight percent, as follows:

ELEMENT RANGE

Cr 12.0 - 16.0%

Si 1.10 - 2.00%

Ni 3.00 - 4.50%

Cu 3.50% maximum

C 0.065% maximum

Mn 1.00% maximum

N 0.11 - 0.20%

Fe Balance

[0016] In another embodiment, a substantially martensitic stainless steel alloy in accordance with the present disclosure consists essentially of, in weight percent:

ELEMENT RANGE

Cr 12.0 - 16.0%

Si 1.10 - 2.00%

Ni 3.00 - 4.50%

Cu 3.50% maximum

C 0.065% maximum

Mn 1.00% maximum

N 0.11 - 0.20%

Mo 0.20% maximum

P 0.03% maximum

Nb 0.10% maximum

Co 0.25% maximum

Fe Balance

[0017] The alloys of the present disclosure may further include

0.005% by weight maximum sulfur, 0.05% by weight maximum tantalum, 0.05% by weight maximum aluminum, 0.20% by weight maximum vanadium, and 0.20% by weight maximum tungsten.

[0018] The alloys of the present disclosure may further have chromium content of about 14 - 16% by weight chromium, a copper content of about 2.00 - 3.50% by weight, a maximum manganese content of about 0.75% by weight.

[0019] The martensitic stainless steel alloy of the present disclosure, in the as cast condition, has an ultimate tensile strength (UTS) of about 146 ksi, a yield tensile strength (YTS) of about 129 ksi, and a hardness of about 39 Rockwell C hardness. This is a substantial improvement over the typical 15-5 stainless steel alloy where the UTS is about 129 ksi, the YTS is about 116 ksi and the hardness is approximately 33 Rockwell C Hardness. Furthermore, the alloy of the present disclosure may be heat treated to provide additional strength benefits. When the alloy of the present invention is heat treated, UTS improves to approximately 180 ksi and YTS improves to approximately 150 ksi.

[0020] The above noted compositions may be particularly applied to cast propellers, steering arms, and other components for marine applications. In this circumstance, the alloy of the present disclosure has the most preferred composition as follows, in weight percent:

ELEMENT RANGE

Si 1.30 - 1.80%

Cr 15.15 - 15.85%

Cu 2.90 - 3.50%

Ni 3.40 - 3.80%

C 0.035 - 0.065%

N 0.12 - 0.16%

Mn 0.60% maximum

ELEMENT RANGE

P 0.03% maximum

S 0.005% maximum

Nb 0.10% maximum

Ta 0.05% maximum

V 0.20% maximum

Mo 0.2% maximum

Al 0.05% maximum

Co 0.25% maximum

W 0.20% maximum

Fe Balance

[0021] The alloy of the present invention also provides cost savings because of the relatively low nickel content. Furthermore, the surface of the castings is less prone to damage and provides the advantage of being heat treatable.

[0022] Fig. 1 is a Weibull curve demonstrating the meantime to failure of the present alloy relative to 15-5 stainless steel. Two embodiments of the alloy of the present disclosure are represented, one with high Cu content, and one with low Cu content. As recognized by one of ordinary skill in the art, the meantime to failure for the high copper alloy of the present invention is approximately 557,000 cycles and the meantime to failure for the low copper alloy of the present disclosure is approximately 312,000 cycles. This is in substantial contrast to the 15-5 control alloy wherein the meantime to failure is approximately 145,000 cycles.

[0023] The data that contributes to Fig. 1 was obtained by casting seven 15-5 control alloy propellers, ten propellers according to the present disclosure with low copper, and five propellers according to the present disclosure with high copper. The cast propellers for run until failure, with the high values from each set removed. The results demonstrate that the alloy of the present invention, particularly with high copper, shows a substantial fatigue resistance as compared to

the traditional 15-5 stainless steel alloy.

[0024] Furthermore, experiments were conducted to measure the relative strength and ductility of the alloy of the present disclosure as compared with 15-5 stainless steel. Four samples of propellers were cast with the alloy of the present disclosure and were measured for UTS, YTS and percent elongation in the as cast condition. The results are as follows:

SAMPLE 1 2 3 4

UTS (ksi) 146 150 142 152

YTS (ksi) 127 119 129 134

El% 6 6 5 3

[0025] Additionally, three sample propellers were cast from the alloy of the present disclosure and were subsequently heat treated and measured for UTS, YTS and percent elongation. The results are as follows:

SAMPLE 1 2 3

UTS (ksi) 180.9 181.1 180.9

YTS (ksi) 155.1 153.7 151.7

El% 15.2 15.7 17.5 [0026] The collected data demonstrates that the alloy of the present disclosure, in the as cast condition, has an average UTS of 147.5 ksi, an average YTS of 127.25, and an average percent elongation of 5. When the alloy of the present invention was heat treated, the samples had an average UTS of 180.97, and an average YTS of 153.5 and an average percent elongation of 16.13.

[0027] Accordingly, the alloy of the present disclosure demonstrates a substantially improved fatigue strength over typical 15-5 stainless steel. This is unexpected and it is believed that microstructural features controlled by the composition specified in the present disclosure are responsible for this result. Furthermore, it is recognized by those of skill in the art that nitrogen has a high solubility in stainless steel and has a solid solution hardening effect substantially greater than that of carbon. Nitrogen also retards the diffusion of carbon and,

therefore, is beneficial because it hinders the formation and coalescence of carbides that decrease fatigue properties.

[0028] It should be apparent to those skilled in the art that the present invention as described herein contains several features, and that variations to the preferred embodiment disclosed herein may be made which embody only some of the features disclosed herein. Various other combinations, and modifications or alternatives may also be apparent to those skilled in the art. Such various alternatives and other embodiments are contemplated as being within the scope of the following claims which particularly point out and distinctly claim the subject matter regarded as the invention.