Abstract:
A high purity martensitic stainless steel alloy containing only critically limited amounts of minor elements including manganese, silicon, tin, phosphorus, aluminum, antimony and arsenic has high strength and toughness and unique resistance to both reversible and irreversible embrittlement.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to martensitic alloys and is more particularly concerned with new high purity stainless steel with high strength and toughness and unique resistance to both reversible and irreversible embrittlement. 
     BACKGROUND OF THE INVENTION 
     Martensitic stainless steels having excellent strength, low brittle to ductile transition temperature and good hardening characteristics in thick sections have long been used as gas turbine wheel materials. They are, however, subject to embrittlement on exposure to elevated temperatures due to formation of detrimental phases within the alloy grains (irreversible embrittlement) or due to segregation of some harmful elements to the grain boundaries (reversible embrittlement). Recognizing this problem, others have added molybdenum, cobalt and other strong carbide formers which limit the tendency toward irreversible embrittlement. While a degree of success has thus been gained, the problem of reversible embrittlement remains, as heat treatments to relieve the condition may degrade desired properties and dimensional integrity of the products. Also, changes in alloy chemistry, particularly phosphorus content, yielded results indifferent enough to discourage special measures for phosphorus removal. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, based on our discoveries set forth below, a new stainless steel alloy called High Purity M152 (HP M152) is provided which has all the desired properties of those of the prior art, but has unique resistance to embrittlement. Further, this new alloy imposes no mechanical- or corrosion- resistant property penalty and involves only a modest increase in cost. Consequently, this alloy can be used to special advantage in steam turbine and jet engine applications, as well as in gas turbines. 
     In making this invention we discovered that the shortcomings of the prior art described above can be overcome by reducing the relatively small amounts of some minor constituents of stainless steel alloys. Limiting phosphorus, tin, antimony and arsenic to a little more than trace amounts, provides tremendous reduction in the amount of embrittlement which occurs. The importance of phosphorus in this system is striking in view of the earlier experience noted above. Additionally, reduction of the manganese and silicon contents from 0.7 and 0.3, respectively, to about 0.050% provides further benefit. 
     We have also found that the new results and advantages of this invention can consistently be obtained with such alloy in which the manganese, silicon and the other minor constituents are in amounts varying from those stated above. Thus, while the ideal alloy of this invention contains essentially none of these various elements just mentioned, as a practical matter in commercial use or production, all of them will be present in some detectable quantity without significant detriment to the desired properties provided no minor element is present in excess of the maxima set out below. 
     Briefly stated, alloy compositions within the purview of this invention and therefore within the scope of the appended claims include the following: 
     
         ______________________________________Base Material  Element         Wt %______________________________________  C      0.08-0.15  S      0.004 max  Cr     11.00-12.50  V      0.25-0.40  Mo     1.50-2.00  Ni      2.5-3.10  Al     0.001-0.027  Mn     0.03-0.13  Fe     Balance  P      0.01 max  Si     0.010-0.10______________________________________ 
    
     As a matter of our present preference, an alloy of this invention does not contain more than about 0.050% manganese, 0.050% silicon, 0.0020% phosphorus, 0.0010% tin, 0.0005% antimony, 0.0030% arsenic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings accompanying and forming a part of this specification, 
     FIG. 1 is a chart on which fracture appearance transition temperature (FATT) is plotted against aging time in thousands of hours for data gathered in tests of a prior art stainless steel alloy of this general type as described below, and 
     FIG. 2 is a chart like that of FIG. 1 showing aging time data gathered in tests on an alloy of this invention as described below. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Because as indicated above and described in detail below, particularly in reference to FIGS. 1 and 2, the composition of these new alloys of this invention is critical in that small changes can result in major differences in desirable properties, the formulation of these alloys and production thereof are carried out with special care. Thus, in the best melting and casting practice these alloys are made by bringing together the alloy constituents in a state of refinement or purity such that the ultimate alloy content of minor constituents is carefully controlled and limited. While chemically pure alloy constituents would be desirable, for reasons of economy they are not used. Instead the selection of the major elements is made so that the aggregate content of the alloy minor elements does not exceed the limits described above and set forth in the appended claims. 
     A consequence of failure to exercise such control is the loss of major advantages of this invention to a significant extent. The embrittlement characteristics of the resulting alloys can, for example, be substantially adversely affected if the limits of the minor elements are exceeded. As a practical matter, an excess of any one or more of the minor elements could not be corrected without remelting the alloy and adjusting the melt chemistry in accordance with the present invention. 
     The differences in property levels of major importance between the alloys of this invention and the prior art alloys of basically similar chemistry are graphically illustrated in FIGS. 1 and 2. The change in FATT is used as the primary measurement of embrittlement and is a method of estimating the fracture toughness of an alloy by measuring the Fracture Appearance Transition Temperature (FATT). The FATT is the temperature at which a Charpy V-Notch impact specimen will break and exhibit 50% brittle fracture. The higher the FATT, the less ductile the material is, and the lower the fracture toughness. 
     Embrittlement is quantified by measuring the change in FATT which results from aging at elevated temperatures. The FATT of a material is measured prior to temperature exposure, when first produced. This value is called the As-Received FATT. To age material for studies, test blocks are placed in a furnace at the desired aging temperature. After a period of aging time at temperature, the test block is removed and the FATT is measured. If embrittlement has occurred, the aged FATT will be substantially higher than the As-Received FATT. The difference in values of the two measurements (Aged FATT)-(As-Received FATT) is referred to as the Delta FATT. The higher the Delta FATT, the more embrittlement. 
     As illustrated by the data points and the extreme variations between them in the two cases, particularly in the 10,000-hour region, the alloys of this invention show excellent resistance to embrittlement relative to prior art alloys. 
     Cast and fabricated bodies of alloys of this invention, in contrast to those made of the 12-chromium stainless steels of the prior art, can as a result of their resistance to embrittlement illustrated in the drawings, be used for much longer times at temperatures above 600° F. without suffering from excessive reduction in toughness due to embrittlement. Gaining this advantage without sacrificing other desirable properties and at only a moderate increase in cost of production constitutes an important advance in the art. 
     Products made using these new alloys of this invention are suitably produced in accordance with the practice in art. Gas turbine wheels thus are cast and forged to shape and size by technique presently in general use. 
     Those skilled in the art will gain a further and better understanding of this invention and its important new advantages and results from the following illustrative, but not limiting, detailed accounts of actual experimental operations. 
     EXAMPLE I 
     Gas turbine sized disks were made from a commercial prior art 12- chromium martensitic stainless steel alloy (JETHETE M 152) of the following nominal analysis: 
     
         ______________________________________Carbon         0.10Chromium       12.0Manganese      0.7Silicon        0.3Molybdenum     1.8Nickel         2.4Phosphorus     0.025Vanadium       0.35Sulphur        0.025Iron           Balance______________________________________ 
    
     Test pieces from these disks were subjected to the FATT embrittlement test procedure described above at temperatures and times as set forth the following table: 
     
                       TABLE I______________________________________   Age          Age Time  Embrittled   Temperature  x1E-3     DELTA FATTSample  (°F.) (hrs)     (°F.)______________________________________A       500          1.0       -12A       500          2.5       1B       500          1.0       -2B       500          2.5       9A       600          1.0       5A       600          2.5       17B       600          1.0       11B       600          2.5       15A       650          1.0       11A       650          2.5       18B       650          1.0       14B       650          2.5       26C       675          17.5      88D       675          17.5      114A       700          1.0       15A       700          2.5       22B       700          1.0       10B       700          2.5       33B       750          3.576     44B       750          28.0      121E       750          3.576     56E       750          28.0      154F       750          28.0      168G       750          3.576     80G       750          28.0      163H       750          24.1      105H       750          56.9      130I       750          28.0      144A       750          1.0       12A       750          2.5       23B       750          1.0       12B       750          2.5       19J       800          3.576     81J       800          28.0      198K       800          28.0      147L       800          28.0      161M       800          3.576     76M       800          17.5      211N       800          28.0      167O       850          18.7      349O       850          50.4      520P       850          17.6      350P       850          50.4      458Q       850          56.0      250Q       850          90.0      293R       850          25.0      306S       850          21.0      449T       850          21.0      550T       850          50.4      597U       850          18.7      253U       850          50.4      514V       850          18.7      331V       850          50.4      488W       850          10.6      447X       850          3.0       45X       850          8.8       80X       850          15.0      120X       850          40.00     160Y       850          14.6      365Z       850          9.6       495AA      850          10.6      555AB      850          10.6      495AC      850          1.0       16AC      850          3.0       55______________________________________ 
    
     Representative data from this TABLE I appears on the chart of FIG. 1. Only test data obtained in less 15,000 hours are shown. 
     EXAMPLE II 
     Two gas turbine sized disks and one trial forging of the allow HP 152 of the present invention were prepared to provide test specimens for use in the manner described in Example I. The data resulting is set out in the following table, representative data items being entered on FIG. 2. 
     
                       TABLE II______________________________________     Age        Age Time    EmbrittledSample    Temperature                x1E-3       DELTA FATTID        (°F.)                (hrs)       (°F.)______________________________________Trial Forging     750        1.0         20Trial Forging     750        3.0         13Trial Forging     750        10.0        43Trial Forging     850        1.0         -9Trial Forging     850        3.0         13Trial Forging     850        10.0        56Disk #1   500        1.0         -7Disk #1   500        2.5         -17Disk #1   600        1.0         -5Disk #1   600        2.5         10Disk #1   600        8.0         9Disk #1   650        1.0         4Disk #1   650        2.5         16Disk #1   650        8.0         7Disk #1   700        1.0         -14Disk #1   700        2.5         25Disk #1   700        8.0         -1Disk #1   750        1.0         18Disk #1   750        2.5         5Disk #1   750        8.0         48Disk #1   800        1.0         6Disk #1   800        2.5         49Disk #1   800        8.0         71Disk #1   850        1.0         18Disk #1   850        2.5         29Disk #1   850        8.0         72Disk #2   750        2.5         27Disk #2   800        2.5         33Disk #2   850        2.5         5______________________________________ 
    
     As is evident from the tables and from the data shown on FIGS. 1 and 2, the new alloys of this invention are far superior to the comparable prior art alloys in respect to resistance to embrittlement and thus in terms of useful service life in gas turbine, steam turbine and jet engine environments. 
     In the specification and the appended claims, wherever percentage or proportion is stated, reference is to the weight basis unless otherwise expressly noted.