Strong, Tough, and Hard Stainless Steel and Article Made Therefrom

Abstract

An iron-base, fine-grained, martensitic stainless steel alloy is disclosed. The alloy is essentially free of delta ferrite and provides very high hardness and good corrosion resistance. The alloy consists essentially of the following composition in weight percent. C about 0.20 to about 0.70 Mn about 5 max. Si about 1 max. P about 0.1 max. S about 0.03 max. Cr about 7.5 to about 15 Ni about 2 to about 5 Mo about 4 max. Co about 4 max. Cu about 1.2 max. Ti about 0.01 to about 0.75 Al about 0.2 max Nb about 1 max. about 2 max. N about 0.02 max. B about 0.1 max.

The balance of the alloy is iron and the usual impurities. A composite article of manufacture is also disclosed that includes a case portion formed of the foregoing alloy.

Claims

1. A martensitic stainless steel alloy consisting essentially of, in weight percent: C about 0.20 to about 0.70 Mn about 5 max. Si about 1 max. P about 0.1 max. S about 0.03 max. Cr about 7.5 to about 15 Ni about 2 to about 5 Mo about 4 max. Co about 4 max. Cu about 1.2 max. Ti about 0.01 to about 0.75 Al about 0.2 max Nb about 1 max. about 2 max. N about 0.02 max. B about 0.1 max. the balance is iron and usual impurities, and wherein the alloy optionally contains Zr, Ta, and Hf such that the sum 1.17×% Ti+0.62×% Zr+0.31×% Ta+0.31×% Hf is about 0.135% to about 1%.

2. The alloy as claimed in claim 1 which contains at least about 0.30% carbon.

3. The alloy as claimed in claim 1 which contains at least about 10.5% chromium.

4. The alloy as claimed in claim 3 which contains not more than about 13% chromium.

5. An article of manufacture comprising a core portion and a case portion that is formed on said core portion, wherein the case portion consists essentially of a martensitic stainless steel case alloy having the following composition in weight percent: C about 0.20 to about 0.70 Mn about 5 max. Si about 1 max. P about 0.1 max. S about 0.03 max. Cr about 7.5 to about 15 Ni about 2 to about 5 Mo about 4 max. Co about 4 max. Cu about 1.2 max. Ti about 0.01 to about 0.75 Al about 0.2 max Nb about 1 max. about 2 max. N about 0.02 max. B about 0.1 max. the balance is iron and usual impurities, and the core portion consists essentially of a martensitic stainless steel core alloy having the following composition in weight percent: C about 0.05 to about 0.15 Mn about 5 max. Si about 1.5 max. P about 0.1 max. S about 0.03 max. Cr about 7.5 to about 15 Ni about 1 to about 5 Mo about 4 max. Co about 10 max. Cu about 5 max. Ti about 0.01 to about 0.75 Al about 0.2 max Nb about 1 max. about 2 max. N about 0.02 max. B about 0.1 max. and the balance of the core alloy is iron and the usual impurities.

6. The article as claimed in claim 5 wherein the case alloy contains at least about 0.30% carbon.

7. The article as claimed in claim 5 wherein the case alloy contains at least about 10.5% chromium.

8. The article as claimed in claim 7 wherein the case alloy contains not more than about 13% chromium.

9. The article as claimed in claim 5 wherein the article is a bar and the case portion surrounds the core portion.

10. The article as claimed in claim 9 wherein the case has an effective case depth of about 0.025 in. to about 0.100 in.

11. The article of manufacture as claimed in claim 5 wherein the article is a thin-gauge article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 is a photomicrograph of a sample of a composite article in accordance with the present invention;

[0048] FIG. 2 is a photograph of a sample of the alloy according to the present invention after testing in accordance with ASTM Standard Test Procedure B117;

[0049] FIG. 3 is a photograph of a sample of comparative Example A after testing in accordance with ASTM Standard Test Procedure B117;

[0050] FIG. 4 is a photograph of a sample of comparative Example B after testing in accordance with ASTM Standard Test Procedure B117;

[0051] FIG. 5 is a photograph of a sample of comparative Example C after testing in accordance with ASTM Standard Test Procedure B117; and

[0052] FIG. 6 is a photograph of a sample of comparative Example D after testing in accordance with ASTM Standard Test Procedure B117.

DETAILED DESCRIPTION

[0053] The alloy and articles according to the present invention are produced from a base alloy that is described in U.S. Pat. Nos. 6,890,393 and 6,899,773, the entire disclosures of which are incorporated herein by reference. As described in the referenced patents, the known alloy is produced by using a thermal-mechanical treatment to refine the grains and precipitate a relatively uniform dispersion of fine, coarsening-resistant, MX-type precipitates. The steel alloy of the present invention incorporates the metallurgical attributes of the previously disclosed steel and includes additional carbon to achieve very high hardness and strength relative to the known steel. The additional carbon is provided by diffusing carbon into the steel in a carburizing process. After the carburizing process, the resulting alloy contains significantly more carbon in the diffused region than the base alloy.

[0054] The alloy in accordance with the present invention contains at least about 0.20% carbon to benefit the higher hardness provided by the alloy such that a hardness value of at least about 50 Rc can be provided without a significant loss of corrosion resistance. It is further noted that when the alloy contains at least about 0.30% carbon, the alloy can provide a hardness value greater than about 56 Rc. A very high hardness, i.e., greater than about 60 Rc, can be provided when the alloy contains between about 0.35% carbon and about 0.63% carbon. As carbon levels increase from about 0.63% to about 0.70%, the hardness decreases from about 60 Rc to about 50 Rc, respectively. As carbon levels continue to increase from about 0.70% to about 0.83%, the hardness continues to decrease from about 50 Rc to about 32 Rc, respectively. The reason the hardness decreases as the carbon level increases from about 0.63% to about 0.83% is that increased carbon levels stabilize greater amounts of relatively soft, retained austenite in the steel, which does not transform to hard martensite even after the as-quenched carburized alloy is subjected to cryogenic treatment. At a carbon level greater than about 0.83%, the volume fraction of hard, chromium-rich particles increases in the alloy, which gradually increases the overall hardness of the steel from about 32 Rc to about 59 Rc. However, when more than about 0.83% carbon is present in the alloy, the corrosion resistance provided by the alloy is adversely affected because the increased formation of chromium-rich particles depletes chromium from the matrix material. Preferably, the alloy contains not more than about 0.70% carbon, and for the best combination of hardness and corrosion resistance, the alloy contains not more than about 0.63% carbon.

[0055] The carbon content of the alloy according to the present invention cannot be obtained conventionally, such as by adding carbon during melting because such an alloying addition would result in a relatively large volume fraction of primary MC particles with a large interparticle spacing that would be ineffective at pinning grains during subsequent thermal mechanical treatment or hot working to result or maintain a fine grain microstructure. The preferred method for the steel of the present invention to be produced with such high carbon levels is for the carbon to be diffused into the steel after the base alloy has been thermal-mechanically processed.

[0056] The alloy according to this invention is produced by first melting and casting the base alloy described above. No special techniques are needed to melt and cast the alloy. The alloy can be melted using a conventional air melting process such as arc melting or the alloy can be vacuum melted such as by vacuum induction melting. When the alloy is air melted, it is preferred that the chemistry of the alloy be controlled such the alloy contains at least about 0.01% of aluminum and silicon combined as residual elements from deoxidizing additions during melting. The base alloy is cast into an ingot or may be cast using a continuous casting technique.

[0057] The alloy ingot or billet is subsequently processed using a thermal mechanical treatment as described in the aforesaid patents. The purpose of the thermal mechanical treatment is to recrystallize the microstructure during hot working and to precipitate a uniform dispersion of fine MX particles to pin the boundaries of the newly recrystallized grains such that a fine-grained, equiaxed microstructure is obtained after the alloy is cooled to room temperature.

[0058] The carbon is diffused into the hot-worked alloy form by a carburization process. The carburization process is carried out under conditions of temperature, time, and carbon atmosphere that are selected to provide the desired carbon content and depth of the carburized layer. After the desired amount of carbon is diffused into the hot worked steel, the steel is cryogenically treated to convert retained austenite that may have formed during the carbon diffusion process to martensite. The cryogenic treatment is carried out by chilling the alloy at about −321° F. to about −100° F. for a time selected to substantially completely transform any retained austenite to martensite. The alloy is then warmed to room temperature in air. The steel is then tempered to increase the toughness and ductility of the core and case when the article according to the invention is embodied as a composite article having a very hard, corrosion resistant case or surface layer and a strong, tough core within the case. The tempering temperature for the alloy or article according to the invention is about 200° F. to about 600° F. Tempering at higher temperatures reduces corrosion resistance, while tempering at lower temperatures reduces toughness and ductility. Preferably, the tempering temperature for the steel of the present invention or a composite article made therefrom is about 300° F. to about 400° F. It will be apparent to one skilled in the art that it is possible to temper the alloy material outside the aforesaid temperature ranges and still obtain desirable properties for a particular application. When processed as described above, a composite steel article according to the present invention can have a core hardness of about 38.5 Rc to about 41 Rc and a Charpy V-notch impact energy of about 40 to about 49 ft-lbs. in combination with a carburized case hardness of about 59 Rc to about 61 Rc.

[0059] The steel alloy of the present invention can be used as part of a composite article that is formed from the lower carbon stainless steel “base” alloy before carbon is diffused into it. The higher carbon surface layer of the composite article can extend to a depth of about 0.025 in. to about 0.100 in. The boundary can be defined as the point where the hardness of the material decreases to below about 50 Rc. FIG. 1 shows a portion of a composite article in the form of a partial cross section of a bar 10 in accordance with this embodiment. The bar 10 includes an inner core portion 20 that is characterized by high strength and good toughness. The bar 10 also has an outer case portion 30 that is characterized by very high strength and hardness. The core portion 20 and the case portion 30 are also characterized by having good corrosion resistance.

[0060] Although there are applications that require a composite article having a combination of high surface hardness, good corrosion resistance, and good core toughness and ductility (e.g., bearings and gears), there are other applications, such as knives and cutlery, that could benefit from a steel that has a combination of high hardness and good corrosion resistance throughout the part, i.e., through the cross section of the article. For such applications, another embodiment of the steel of the present invention is a thin-gauge article made from the base alloy and in which carbon has been diffused throughout, or substantially throughout, the part, thereby rendering it hard throughout, while maintaining excellent corrosion resistance. A through-hardened knife blade that utilizes the technology and methods described herein, for example, could be repeatedly sharpened without adverse consequences. This aspect of the invention has been successfully realized with thicknesses of about 0.09 inches to about 0.1 inches. However, it is believed that the invention can be extended to thicknesses up to about 0.125 inches or more.

Working Examples

[0061] In order to demonstrate the novel combination of hardness and corrosion resistance provided by the alloy according to this invention, comparative testing of an example of the alloy was carried out. More specifically, the hardness and corrosion resistance provided by the alloy of this invention was compared to samples of four known, martensitic, corrosion resistant alloys, namely, the PYROWEAR 675 alloy (Ex. A), the BG42 alloy (Ex. B), AISI Type 440C alloy (Ex. C), and the CPM S90V alloy (Ex. D) (CPM and S90V are registered trademarks). The weight percent compositions of the tested alloys are presented in the table below.

TABLE-US-00001 Element Invention Ex. A Ex. B Ex. C Ex. D C 0.103* 0.07* 1.15 1 2.30 Mn 0.43 0.65 0.5 0.5 Si 0.42 0.3 0.3 P 0.014 S 0.0005 Cr 10.86 13 14.5 17 14.0 Ni 2.95 2.6 Mo 0.49 1.8 4 0.5 Co 0.03 5.4 Cu 0.14 Ti 0.29 Al 0.015 Nb 0.06 V 0.10 0.60 1.2 9.0 N 0.01 B 0.002 Sn 0.004 *Carbon concentration before carburization.
The balance of each alloy is essentially iron.

[0062] The sample of the alloy according to the invention was carburized, quenched, cryogenically treated, and them tempered as described above. The carburized case depth was about 0.055 inches. The sample of Example A was carburized and then heat treated using a known process for that alloy. The carburized case depth was about 0.040 inches. The samples of Examples B, C, and D were heat treated in accordance with the known heat treatments for those alloys. The samples of each alloy were then tested for surface hardness and corrosion resistance.

[0063] The sample of the alloy of the present invention had a measured hardness of 61 Rc. The sample of Example A had a measured hardness of 63 Rc. The measured hardness of the sample of Example B was 61 Rc. The sample of Example C had a measured hardness of 59 Rc and the sample of Example D had a measured hardness of 58 Rc.

[0064] Corrosion testing was performed on samples of each alloy in accordance with ASTM Standard Test Procedure B117 (salt fog test). Shown in FIGS. 2-6 are photographs of the samples of each alloy after a 200-hour exposure to the salt fog. FIG. 2 shows the sample of the alloy according to the present invention. FIG. 3 shows the sample of comparative Example A. FIG. 4 shows the sample of comparative Example B. FIG. 5 shows the sample of comparative Example C and FIG. 6 shows the sample of comparative Example D.

[0065] The terms and expressions which are employed in this specification are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized that various modifications are possible within the invention described and claimed herein.