|Title of Invention||
"PRECIPITATION-HARDENABLE MARTENSITIC STAINLESS STEEL ALLOY COMPOSITION"
|Abstract||The present invention relates to a precipitation-hardenable, martensitic stainless steel alloy, comprising, in weight percent, C 0.030 max Mn 0.51 Si 1.00 max. P 0.030 max S 0.007-0.015 Cr 14.00-15.32 Ni 3.50-5.50 Mo 1.00 max. Cu 2.50-4.50 Nb+Ta (5xC)-0.25 Al 0.05 max. B 0.010 max N 0.030 max. the balance being and the usual impurities.|
|Full Text||AN ENHANCED MACHINABILITY PRECIPITATION-
HARDENABLE STAINLESS STEEL
FOR CRITICAL APPLICATIONS
FIELD OF THE INVENTION
This invention relates to high strength stainless steel alloys and, in particular, to a precipitation-hardenable, martensiiic stainless steel alloy having a unique combination of "strength, ductility, toughness, and machin ability.
BACKGROUND OF THE INVENTION
Aerospace material specification AMS 5659 describes a 15Cr-5Ni precipitation hardenable, corrosion resistant steel alloy for use in critical -aerospace components. AMS 5659 specifies minimum strength and ductility requirements which the alloy must meet after various age-hardening heat treatments. For example, in the H900 condition (heated at about 900F (482C) for 1 hour and then air cooled), a conforming alloy must provide a tensile strength of at least 190 ksi (1310 MPa) in both the longitudinal and transverse directions together with an elongation of at least 10% in the longitudinal direction and at least 6% in the transverse direction. However, products manufactured to meer that specification typically lack the ease of machinability desired by component fabricators.
As the alloy specified in AMS,5659 continues to be used in many structural components for aerospace applications, a need has arisen for an alloy that meets all of the mechanical requirements of AMS 5659, but which also provides superior machinability. It is generally known to add certain elements such as sulfur, selenium, tellurium, etc. to stainless steel alloys in order to
improve their machinability. However, the inclusion of such "free-machining additives", without more, will adversely affect the mechanical properties of uV. alloy, such as toughness and ductility, to the point where the alloy becomes unsuitable for the critical structural components for which it was designed. Consequently, a need exists for a precipiiation-hardenable martensitic stainless steel having good ductility, toughness, and notch tensile strength to be useful ioi critical applications and which also provides superior machinability compared with alloy compositions currently utilized for fracture-critical components.
SUMMARY OF THE INVENTION
The present invention is directed to ajprecipitation-hardenable mai'tensitic' stainless steel which provides mechanical properties (tensile and notch strength, ductility,-and toughness) that meet the requirements of AMS 5659 and which al.c provides sigmficantly^etter machinability compared to the known grades of 15Cr-5Ni precipitation-hardenable stainless steels. The.broad, intermediate, nnri preferred weight percent compositions of the alloy according to this invention are set forth in the following table.
The foregoing tabulation is provided as a convenient summary and is not mended thereby to restrict the lower and upper values of the ranges of the individual elements for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the ranges can be used with one or more of the other ranges for the remaining elements. In addition, a minimum or maximum for an element of a broad, intermediate, or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate compositor.. Here and throughout this specification the term "percent" or the symbol "%" means percent by weight unless otherwise specified.
DETAILED DESCRIPTION UK run tivvETSTioiv
The interstitial elements carbon and nitrogen are restricted to low levels in this alloy in order to benefit the rnachinahility of the alloy. Therefore, the alloy contains not moie-lhan-about 0.030% each of carbon and nitrogen and preferably not more than about 0.025% of each of those elements. Carbon and nitrogen arc strong austenite-stabilizing elements and limiting thenrto levels that are too lov. leads to the formation of undesirable amounts of fertile in this alloy. Therefore. at least about 0.010% each-of carborr and-nitrogen is preferably present in the alloy.
This .alloy contains a controlled amount of sulfur to benefit the machinability of the alloy without adversely affecting the ductility, toughness, and notch tensile strength of the alloy. To that end, the alloy contains ai leaM about 0.005% and preferably at least about 0.007% sulfur. Too much sulfm adversely affects the ductility, toughness, and notch tensile strength of this alloy Therefore, sulfur is restricted to not more than about 0.015%.and preferably to not more than about 0.013% in this alloy.
At least about 14.00% and preferably at least about 14.2,5% chromium y
present in &ie alloy to provide an adequate level of corrosion resistance. However, when chromium is present in excess of about 15.50% the formation of undesirable ferrite results. Therefore, chromium is restricted to not more than about 15.50% and preferably to not more than about 15.25% in this alloy.
At least about 3.50%, preferably at least about 4.00%, nickel is present m the alloy to maintain good toughness and ductility. Nickel also benefits the austenite phase stability of this alloy at the low levels of carbon and nitrogen u:;cd in the alloy. The strength capability of the alloy in the aged condition is advcn oly affected when-more man about 5.50% nickel is present because of -ine-ewpteie austenite-to-martensite transformation (i.e., retained austenite) at room temperature: Therefore, this alloy contains not more than about 5.50% nickel.
At least about 2.50%, preferably at least about 3.00%, copper is prescm in this alloy as the primary precipitation hardening.agent. "During the age hardening heat treatment, the alloy achieves substantial strengthening through the -precipitation of fine, copper-rich paiticlesTrrorn the martensitic matrix, Coppej h present in this alloy in amounts ranging from 2.50 to 4.50% to provide the desired precipitation hardening response. Too much copper adversely affects-the austenite phase stability of this alloy and can lead to formation of excessive austenite in the alloy after the age hardening heat treatment. Therefore, copper is restricted to not more than about 4.50% and preferably to not more than about 4.00% in this alloy.
A small amount of molybdenum is effective to benefit the corrosion resistance and toughness of this alloy. The minimum effective amount can be readily determined by those skilled in the art. Too much molybdenum incje.is.:s the potential for ferrite formation in this alloy and can adversely affect the alloy's phase stability by promoting retained austenite. Therefore, while this alloy m?y contain up to about 1.00% molybdenum, it preferably contains not more than ' about 0.50% molybdenum.
A small amount of niobium is present in this alloy primarily as ;stabilizing agent against the formation of chromium carbonitrides which arc deleterious to corrosion resistance. To that end the alloy contains niobium in an amount equivalent to at least about five times the amount of carbon in the alloy (5x%C). Too much niobium, particularly at the'low carbon and nitrogen levels present in this alloy, causes excessive formation of niobium carbides, niobium nitrides, and/or niobium carbonitrides and adversely affects the good machinability provided by this alloy. Too many niobium carbonitrides also adversely affect the alloy's toughness. Furthermore, excessive niobium result;, in the formation of an undesirable amount of ferrite in this alloy. Therefore, niobium is restricted to not more than about 0.30%, better yet to not more than about 0.25%, and preferably to not more than about 0.20%. Those skilled in the
art will recognize that tantalum may be substituted for some of the niobium 0.1.1 a weight percent basis. However, tantalum is preferably restricted to not more than about 0.05% in this alloy.
A small but effective amount of boron may be present in amounts up to about 0.010%, preferably up to about 0.005%, to benefit the hot workability of this alloy.
The balance of the alloy composition is iron except for the usual impurities found in commercial grades of precipitation hardening stainless steel.; intended-for similanase or service. For example, aluminum is restricted to not more, man about 0.05% and preferably to not more than about 0.025% in this alloy because aluminum can form aluminum nitrides and aluminum oxides which are detrimental to the good machinability provided by the alloy. Other elerm:ni:; such as manganese, silicon, and phosphorus are also maintained at low level;. because they adversely affect the good toughness provided by this alloy. The. composition of this alloy is balanced so that the microstructure of the steel undergoes substantially complete transformation from austenite to martcnsitr, during cooling from the annealing temperature to room temperature. Ar-described above, the constituent elements are balanced within their rcsp^.r.mv.
weight percent ranges such that the alloy contains not more than about 2 volume percent (vol.%) femte, preferably not more than about 1 vol%-ferrite, in the annealed condition.
The alloy according to this invention is preferably melted by vacuum induction melting (VIM), but can also be arc-melted in air (ARC). The alloy is refined by vacuum arc remelting (VAR) or electroslag remelting (ESR). The alloy may be produced in various product forms including billet, bar, rod, and wire. The alloy may also be used to fabricate a variety of machined, corrosion resistant pans that require high strength and good toughness. Among such end products are valve parts, fittings, fasteners, shafts, gears, combustion engine parts, components for chemical processing equipment and paper mill equipment, and components for aircraft and nuclear reactors.
The unique combination of properties provided by the alloy according to the present invention will be appreciated better in the light of the following examples.
In order to demonstrate the unique-combination of-properties provided by the alloy according to the present invention, examples of the alloy were prepared and tested relative to comparative alloys.
Four heats, each weighing approximately 400-pounds, were vacuum-induction melted and cast as single 7.5"-square ingots. The chemical analyses of the heats are shown in Table I in weight percent. Heat 1 is an example of the: steel according to this invention. Heats A, B, and C are comparative alloys.
The ingots were press-forged to 4" square billets, cogged to a 2.125" diam. round bars, and then hot rolled to 0.6875" diam. bar. All the bars were solution annealed by Seating them to a temperature of 1-040C, soaking for ont hour at that temperature, and then water quenching to room temperature. Fun ha processing consisted of straightening the annealed bars, turning to 0.637" dsam., restraightenmg, rough grinding to 0.627" diam., and then grinding the bars to a finish diameter of 0.625".
The microstnicture and mechanical properties of the bar products were evaluated and compared relative to the requirements of AMS 5659. Table IT shows that little or no ferrite was present in the microstructures of the solution-annealed 0.625" diam. bars.
(FERRITE CONTENT IN ANNEALED BARS)
* Measured from tint-etched longitudinal metallographic specimens via ima.
ultimate tensile strength (UTS) in ksi (MPa), the percent elongation in 4 diameters (% Elong.), the reduction in area (% RA), and the Rockwell C hardness (HRC).
TABLE III (LONGITUDINAL SMOOTH TENSILE PROPERTIES
AND HARDNESS OF ANNEALED BARS) Smooth Tensile Properties
(1) Average of duplicate specimens.
(2) Average of four measurements taken at midradius location-
(3) Convened from HB scale.
A comparison of room-temperature smooth tensile properties and hardness was-also developed forthe^aUoys in the various aged conditions specified in AMS 5659. Resulis are presented in Table IV including the 0.2% offset yield strength (.2% Y.S.) and ultimate tensile strength (UTS) in ksi (MP;i), the percent elongation in 4 diameters (Elong.), the reduction in area (RA), and i.hr. Rockwell C hardness (HRC).
AMS 5659105min. 135min. 16min.
(1) Average of duplicate specimens.
(2) Aging cycles are defined as follows:
H900: 900F/ 1 hour/ air cool
H925: 925F/ 4 hours/ air cool
H1025: 1025F/ 4 hours/ air cool
HI 150; 1150F/ 4 hours/ air cool
(3) Average of four measurements.
(4) Convened from HB scale.
The data presented in Tables HI and IV show that the hardness and mooth tensile properties of the four alloys are similar and that they all satisfy the ;quirements of AMS 5659 under the respective heat treating conditions.
The machinabilities of the annealed 0.625" diam. bars of each alloy /ere tested by employing a Brown and Sharpe Ultramatic (single, spindle) Screw Machine. Spindle speed was utilized as the variable test parameter. Three tcsl'; I'ere conducted on all four heats at speeds of 95.5 and 104.3 surface feet pe.r ninute (SFM). A given trial was terminated for one of two reasons a) part ;rowth exceeding 0.003" as a result of tool wear (Part Growth) or b) at least 400 •arts-were machined without 0.003" part growth (Discontinued). Catastrophic ool failure, a thirdreason for test termination, was not experienced in this esting. The screw machine test parameters and results are provided in Table V, ncluding the spindle speed (Spindle Speed) in SFM, the number of parts nachined (Total Parts) and thejeason for terminating each test (Reason for fe.sl Termination).
(SCREW MACHINE TEST RESULTS FOR ANNEALED BARS)
(1) A rough form tool feed-rate of 0:002 ipr (inchefrper revolution) was utilized for all tests.
Set forth in Table VI is a summary of the data, presented in Table V above, including the number of parts machined at each spindle speed (Parts Machined). The mean and standard deviation values for the comparative alloys are also shown.
(SCREW MACHINE TEST RESULT SUMMARY ANNEALED BARS)
* Test discontinued because of runout. .
When viewed together, the data in Tables II to VI show that Heat 1 provides a significantly better combination of properties relative to Heats A, K, and C, because it provides superior machinability while maintaining the mechanical and microstrucluraLproperty requirements of AMS 5659. ~
Sin "'lOO lb. heate were vacuum inHiirrinn mp.ltp.ri anri cast as 7Vi " ingots.
The chemical analyses of the heats are shown in Table VII in weight percent. Heats 2, 3. and 4 are examples of the steel according to this invention and Heats -D, E, and F are comparative alloys.
Heat 2 was prepared for comparison with Heat D, Heat 3 was prepared for comparison with Heat E, and Heat 4 was prepared for comparison with Heat F. The ingots were press forged to 4" square bars as described above in Example 1. The 4" square bars of Heats 2 and D were further processed to 5/8" djam round bars as described above in Example 1.
A comparison of the room-temperature, longitudinal smooth tensile
properties and hardness of Heats 2 and D in the annealed and Hi 150 conditions is given in Tables VTEA and VIUB. Prior to testing, the bars of each heat were annealed ai 1040C for 1 hour and then water quenched. Subsequently, the bars of each heat were age hardened by heating at-1150 F for 4 hours and then aii cooled The data presented in Tables VfflA and VIUB include the 0.2% offset yield strength (.2% Y.S.) and ultimate tensile strength (UTS) in ksi (MPa), the percent elongation in 4 diameters (% Elong.), the reduction in area (% RA), and The Rockwell C hardness (HRC). Also shown for reference are the tensile and hardness requirements specified in AMS 5659.
(SMOOTH TENSILE PROPERTIES AND HARDNESS OF ANNEALED BARS)
(1) Average of duplicate .250" diam. gage smooth tensile specimens
(2) Average hardness on cross section of bar at midradius-
(1) Average of duplicate .250" diam. gage smooth tensile specimens.
(2) Average-hardness on cross section of bar atmidradius.
Set forth in Tables IX and X-are the results of machinability testing oi the 5/8" bars-of Heats 2 and D in theHI150 age-hardened condition. Table IX shows the results for duplicate tests of each heat on the automatic screw machine as described in Example 1 above, including the relative amounts of C, S, and Nb, in weight percent, and the number of parts machined (Total Parts) until rest termination. In each case the spindle speed was 104.3 SFM and the too] feed rare was 0.002 inchesiper revolution (ipr).
(SCREW MACHINE TEST RESULTS FOR H1150 AGE-HARDENED BARS)
Set forth in Table X below are the results of duplicate tool life tests on each heat, including the relative amounts of C, S, and Nb, in weight percent, the tool failure limit (Tool Failure) expressed in inches (cm) to failure and time to failure (sec.), and the volume of material cut from the test bar (Cut Vol.) in in3 (cm3). In this test, lengths of bars of each heat were turned on a single point lathe employing cutting tool having a T15 high speed steel insert. Accelerated teed and machining speed parameters were selected to produce a catastrophic tool failure. All tests were conducted with a spindle speed of 200 SFM and ;i tool feed rate of .0132 ipr to achieve a material removal rate of 1.78 inVmmute.
(TOOL LIFE TEST RESULTS FOR HUSO AGE-HARDENED BARS)
The data in Tables DC and X show that Heat 2, representing an al lo v according to present invention, provides superior machinability relative, to Heat I) when the alloys are in the age-hardened condition (HI 150).
Set forth in Tables XIA and YTR sm>. the. results of smooth arid notch tensile, impact toughness, hardness, and fracture toughness testing ot the 4 h n,
of Heats 3, 4, E, and F in the Hi 150 age-hardened condition. Table XIA pi ss.i;ms data for longitudinally oriented specimens and Table' Xffi presents data for
transversely oriented specimens. The results shown in Tables X1A and XIB include the .2% offset yield strength (0.2% Y.S.) and ultimate tensile strength (UTS ) in ksi (MPa), the percent elongation in 4 diameters (% Elong.), the reduction in area (% RA), the notched tensile strength (NTS) in ksi (MPa.), ih TABLE XA
The data in Table XIA show that Heats 4 and 5, which are alloys according to the present invention, although providing similar smooth and notch tensile properties and hardness relative to Heats E and F, respectively, provide superior impact toughness and fracture toughness characteristics relative to those alloys. Similar results are demonstrated in Table XIB for the transversely oriented specimens, although at somewhat lower levels than the corresponding longitudinal properties. Good impact toughness and fracture toughness are especially important for materials used in critical structural components.
Considering the data presented in Tables VIIIA, VIIIB, IX, X, XIA. and XIB together, they clearly show the superior combination of strength, toughness, ductility, and machinability provided by the alloy according to the present invention.
The terms and expressions which have been employed herein are used as terms of description, not of limitation. There-is no-intention in. the use of such terms and expressions of excluding any equivalents of the elements or features shown and described or portions thereof. However, it is recognized that vanous modifications are possible within the scope of the invention claimed.
1. A precipitation-hardenable, martensitic stainless steel alloy composition,
comprising, in weight percent,
C 0.030 max
Mn 0.51 max.
Si 1.00 max.
P 0.030 max
Mo 1.00 max.
Al 0.05 max.
B 0.010 max
N 0.030 max.
the balance being and the usual impurities.
2. The precipitation-hardenable, martenistic stainless steel alloy composition
as claimed in claim 1 containing at least about 0.010% carbon.
4. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 1 containing not more than about 0.013% sulfur.
4. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 1 containing at least about 4.00% nickel.
5. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 1 containing not more than about 0.50% molybdenum.
6. The precipitation-hardenable, martenistic stainless steel composition alloy as claimed in claim 1 containing not more than about 0.025% nitrogen.
7. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 1 containing not more than about 4.00% copper.
8. A precipitation-hardenable, martenistic stainless steel alloy as claimed in
any preceding claim containing in weight percent, about
C 0.025 max
Si 0.50 max.
Mo 0.50 max.
Al 0.025 max.
B 0.005 max.
N 0.025 max.
9. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 8 containing not more than about 15.25% chromium.
10. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 8 containing not more than about 0.20% niobium-plus-tantalum.
11. The precipitation-hardenable, martenistic stainless steel alloy composition as claimed in claim 1 containing at least about 0.010% nitrogen
12. A precipitation-hardenable, martenistic stainless steel alloy composition as claimed in any preceding claim containing in weight percent, about
Si 0.50 max
P 0.025 max
Mo 0.50 max.
Nb+Ta (5XC) -0.20
Al 0.025 max.
B 0.005 max.
13. A steel article that provides a cunique combination of machinability, hardness, strength, ductility, and toughness, in the age-hardened condition, said article being formed of a precipitation-hardenable, martenistic stainless steel alloy as claimed in any of claims 1, 8, or 12.
|Indian Patent Application Number||IN/PCT/2001/00763/DEL|
|PG Journal Number||23/2011|
|Date of Filing||24-Aug-2001|
|Name of Patentee||CRS HOLDINGS, INC.|
|Applicant Address||209F BAYNARD BUILDING, 3411 SILVERSIDE ROAD, WILMINGTON, DELAWARE 19810, USA.|
|PCT International Classification Number||C22C 38/42|
|PCT International Application Number||PCT/US00/05916|
|PCT International Filing date||2000-03-08|