CORROSION RESISTANT AUSTENITIC STAINLESS STEEL

Abstract: A free-machining, austenitic stainless steel having low carbon plus nitrogen contents of up to 0.070 weight percent in combination with manganese and sulfur additions. The steel may have silicon of 0.045 to 1.00 weight percent.

Description: This invention relates to corrosion resistant austenitic stainless steels. The austenitic chromium-nickel and chromium-nickel-molybdenum stainless steels are used in a variety of corrosion-resistant parts and fittings. The manufacture of many of these parts and fittings requires considerable machining, and thus the machinability as well as the corrosion resistance of these austenitic stainless steels is an important factor affecting their use in these applications. It is well known that the machinability of the chromium-nickel and chromium-nickel-molybdenum stainless steels can be improved by the addition of sulfur, selenium tellurium, bismuth, lead and phosphorus. However, the addition of sulfur and of these other elements adversely affects corrosion resistance and the ability of these stainless steels to be continuously cast or hot worked without undue difficulty. Efforts have been made to improve the machinability of the austenitic stainless steels without sacrificing corrosion resistance by adding small amounts of sulfur to achieve the greatest possible improvement in machinability without unduly reducing corrosion resistance. In this regard, U.S. Patent 3,563,729 discloses that austenitic stainless steels having improved machinability without a notable sacrifice in corrosion resistance can be achieved by the addition of 0.04 to 0.07 percent sulfur. While such austenitic stainless steels are very useful, many applications exist where the combination of machinability and corrosion resistance afforded by them is not satisfactory, and where still better machinability is desired without a decrease in corrosion resistance. Further, as with other sulfur-bearing austenitic stainless steels, they suffer the disadvantage that when continuously cast their machinability is adversely affected by the tendency of this casting technique to produce more numerous and smaller sulfide inclusions than achieved by conventional ingot casting. It is a primary object of the present invention to provide austenitic stainless steels having improved machining characteristics without adversely affecting corrosion resistance. A further object of the present invention is to provide wrought, continuously cast austenitic stainless steel products having improved machining characteristics without adversely affecting their corrosion resistance. The invention is based on the discovery that the machinability of the austenitic chromium-nickel and chromium-nickel-molybdenum stainless steels with either low or slightly elevated sulfur contents can be improved by maintaining carbon and nitrogen, in combination, at lower than conventional levels and by controlling silicon at an optimum level. An important advantage of this discovery is that machinability can be improved without a decrease in corrosion resistance. Further, in contrast to those austenitic stainless steels in which sulfur is the primary agent used to improve machinability, the steels of this invention can be continuously cast without difficulty and without significantly decreasing their machinability. Broadly in accordance with the present invention, the machinability of austenitic chromium-nickel and chromium-nickel-molybdenum stainless steels with either low or slightly elevated sulfur contents is improved by reducing their total carbon plus nitrogen contents below conventional levels and by optimizing the silicon content. In this regard, the total carbon plus nitrogen in combination at low levels in accordance with this invention is more effective in improving machinability than either low carbon or nitrogen alone. Further, the austenitic stainless steels of this invention have particular advantage as fontinuously cast and wrought products, since in contrast to prior art steels of this type, they can be continuously cast without difficulty and more importantly without a significant decrease in machinability. The chemical compositions of the austenitic stainless steels, and the continuously cast and wrought products of this invention are within the following limits, in weight percent: Carbon plus nitrogen total - up to 0.070, and preferably up to 0.052 or up to 0.040. Chromium 16 to 20, preferably 18 to 20 when up to 1.0 molybdenum is present or 16 to 18 when 2.0 to 3.0 molybdenum is present. Nickel - 8 to 14, preferably 8 to 12 when up to 1.0 molybdenum is present or 10 to 14 when 2.0 to 3.0 molybdenum is present. Sulfur - 0.02 to 0.07, preferably 0.02 to 0.04 for optimum corrosion resistance or 0.04 to 0.07 for optimum machinability. Manganese - up to 2.0. Silicon - up to 1.0, preferably 0.45 to 0.75. Phosphorus - up to 0.05. Molybdenum - up to 3.0, preferably up to 1.0 for lowest cost, or 2.0 to 3.0 for optimum corrosion resistance. Copper - up to 1.0 Boron - 0. to 0.01 Iron - balance, except for incidental impurities. The boron may be added to improve hot workability. To demonstrate the invention, and specifically the limits with respect to carbon plus nitrogen and silicon contents, ten 50-pound (23kg) vacuum induction heats were melted and cast into ingots. The ingots were heated to 2250°F (1232°C) forged to 103/16 inch (30mm) hexagonal bars, air cooled to ambient temperature, then annealed at 1950°F (1065°C) for 1/2 -hour, water quenched and lathe turned to 1-inch (25.4mm) rounds. The chemical compositions of the experimental heats are shown in Table 1. Metallographic evaluations were conducted on representative specimens taken from an annealed bar forged from each ingot. No ferrite was detected in any of the steels using metallographic or magnetic techniques. The sulfide inclusions in each heat were similar and were predominantly globular manganese sulfide inclusions, some of which were partially surrounded with a silicate type oxide. Some stringer type manganese sulfide inclusions associated with silicate type oxides were also observed in the heats with silicon contents of over 0.45%. In the low-silicon heats V475 (0.29% Si) and V476 (0.45% Si), both manganese chromium spinel and silicate type oxides were observed. Heat V476 contained primarily silicate type oxide inclusions, but heat V475 contained primarily spinels. In the high-silicon heat V606 (0.84% Si), both silicate and silica type oxide inclusions were observed. Machinability evaluations were conducted by subjecting annealed one-inch (25.4mm) round bars of the experimental heats to a lubricated plunge-cut lathe turning test at machining speeds from 160 to 180 surface feet per minute (sfm) (49 to 55 metres/min). In the plunge-cut test, the relative machining characteristics of the test materials are established by the number of approximately 1/4-inch (6.35mm) thick wafers that are cut from the test steel at various machining speeds prior to catastrophic failure of the cutting tool. The results of the plunge-cut testing of these experimental steels and the testing parameters are set forth in Table II. As can be seen in Table II, the number of wafers cut prior to tool failure varied widely with the carbon plus nitrogen and silicon contents of the experimental steels. At a cutting speed of 160 sfm (49 m/min), 8 to 11 wafers could be cut from heats V474 and V558, both having carbon plus nitrogen contents outside the limits of this invention. More wafers could be cut from the stainless steels having carbon plus nitrogen contents within the limits of this invention. The cut-off-tool-life test results also show that it is not necessary to have extremely low carbon plus nitrogen contents to achieve improved machinability. At 160 sfm (49m/min), heat V550 containing 0.005% carbon plus nitrogen produced 36 wafers; whereas, heat V472A having 0.040% carbon plus nitrogen produced 32 wafers. Manufacturing a 0.005% carbon pius nitrogen steel similar to heat V550 would require a special and expensive melting and refining process; whereas, the 0.040% carbon plus nitrogen content of heat V472A can be achieved by state-of-the-art melting and refining techniques. The effect of silicon content on machinability is clearly shown by the data in Table II for heats V475, V476,V477, and V606 which contain 0.29, 0.45, 0.62, and 0.84% silicon, respectively, and about the same sulfur and carbon plus nitrogen contents. At a cutting speed of 160 sfm (49m/min), the number of wafers that can be cut from these steels increases significantly with an increase in silicon content from 0.29 to 0.62% and then decreases as silicon content is further increased from 0.62 to 0.85%. Based on the number of wafers cut at this testing speed, the silicon contents making for best machinability range from about 0.45 to 0.75%. The variations in machinability with silicon content are believed to relate to the type of oxides present in the steel. The silicon-steel-oxygen equilibrium system in these steels is balanced such that at low silicon contents the manganese chromium spinel type of oxide is formed; whereas, at moderate silicon contents the silicate type oxide is formed; and at higher silicon contents the silica type oxide is formed, provided no other strong deoxidizing elements such as titanium or aluminum are present in the steel. At machining temperatures, the spinel type oxides maintain their angularity and are haider than the machining tool thus causing tool wear. Conversely, the rounded silicate type oxides exhibit decreased hardness and high plasticity at machining temperatures, thus causing less wear to the machining tool than do the spinel type oxides. The silica type oxides are also rounded, but like the spinel type oxides are harder than the machining tool at machining temperatures and thus cause more tool wear than the silicate type oxides. To further clarity the effects of carbon plus nitrogen and silicon content on the machinability of the steels of this invention, a multiple linear regression analysis was conducted on the lubricated lathe cut-off-tool-life test results at 160 sfm (49m/min) using the heats within the preerred range of silicon (0.45 to 0.75%). The resulting equation, wafer cuts at 160 sfm = 5-270 (% C+N) + 67 (% Si), indicates that on an equivalent weight percent basis, the carbon plus nitrogen content of the experimental steels has approximately 4 times greater influence on the number of wafers cut at a machining speed of 160 sfm (49m/min) than does the silicon content. To better clarify the effect of carbon plus nitrogen content on machinability, the lubricated lathe cut-off-tool-life results at a machining speed of 160 sfm (49m/min) were corrected for variations in the silicon contents of the experimental steels by using the silicon coefficient of the multiple linear regression equation, and using a nominal silicon content of 0.53% as the standard silicon content. As shown in Table III, the resulting corrected wafer cuts at a machining speed of 160 sfm (49 m/min) clearly indicate improved machinability with decreasing carbon plus nitrogen contents. For example, heat V473 with 0.070% carbon plus nitrogen provides a silicon corrected value of 23 wafer cuts, heat V476 with 0.053% carbon plus nitrogen provides a silicon corrected value of 25 wafer cuts, and heat V472A with 0.040% carbon plus nitrogen provides a silicon corrected value of 34 wafer cuts.

Claims: 1. A corrosion resistant austenitic stainless steel having improved machinability characterised in consisting of, in weight percent, carbon plus nitrogen up to 0.070, chromium 16 to 20, nickel 8 to 14, sulfur 0.02 to 0.07, manganese up to 2.0, silicon up to 1.0, phosphorus up to 0.05, molybdenum up to 3.0, copper up to 1.0, boron 0 to 0.01 and the balance iron with incidental impurities. 2. A steel according to claim 1, having silicon 0.45 to 0.75 and sulfur 0.02 to 0.04. 3. A steel according to claim 1, having silicon 0.45 to 0.75 and sulfur 0.04 to 0.07. 4. A steel according to claim 1, 2 or 3, having carbon plus nitrogen up to 0.052. 5. A steel according to claim 1, 2 or 3, having carbon plus nitrogen up to 0.040. 6. A steel according to any one of claims 1 to 5, having chromium 18 to 20, nickel 8 to 12 and molybdenum up to 1.0. 7. A steel according to any one of claims 1 to 5, having chromium 16 to 18, nickel 10 to 14 and molybdenum 2 to 3. 8. A continuously cast and wrought austenitic stainless steel product having improved machinability characterised in consisting of, in weight percent, carbon plus nitrogen up to 0.070, chromium 16 to 20, nickel 8 to 14, sulfur 0.02 to 0.07, manganese up to 2.0, silicon up to 1.0, phosphorus up to 0.05, molybdenum up to 3.0, copper up to 1.0, boron 0 to 0.01 and the balance iron and incidental impurities. 9. An austenitic stainless steel product according to claim 8, having silicon 0.45 to 0.75 and sulfur 0.02 to 0.04. 10. An austenitic stainless steel product according to claim 8 having silicon 0.45 to 0.75 and sulfur 0.04 to 0.07. 11. An austenitic stainless steel product according to claim 8, 9, or 10, having carbon plus nitrogen up to 0.052. 12. An austenitic stainless steel product according to claim 8, 9 or 10 having carbon plus nitrogen up to 0.040. 13. An austenitic stainless steel product according to any one of claims 8 to 12, having chromium 18 to 20, nickel 8 to 12 and molybdenum up to 1.0. 14. An austenitic stainless steel product according to any one of claims 8 to 12, having chromium 16 to 18, nickel 10 to 14 and molybdenum 2 to 3.

Inventors: ECKENROD, JOHN J, PINNOW, KENNETH E., RHODES, GEOFFREY O., ROYER, WILLIAM E.
Applicants: CRUCIBLE MATERIALS CORPORATION
Application year: 1987
Main classification: C22C38
IPC codes: C22C38