CA1206023A

CA1206023A - Wear-resistant stainless steel

Info

Publication number: CA1206023A
Application number: CA000436670A
Authority: CA
Inventors: Richard D. Zordan; Paul Crook
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 1982-10-25
Filing date: 1983-09-14
Publication date: 1986-06-17
Also published as: FI75604C; AT387791B; JPS6238427B2; SE8305242D0; AU555365B2; CH658672A5; FI833604A0; FR2534931B1; ZA837168B; KR840006376A; FR2534931A1; DE3338503A1; US4487630A; IT8323427A0; SE457884B; AU2052683A; GB8328413D0; IN161777B; GB2128633B; SE8305242L

Abstract

ABSTRACT OF THE DISCLOSURE
WEAR-RESISTANT STAINLESS STEEL

A high chromium stainless steel especially suited for use as wear (galling) resisting components, for example, valve parts. A typical alloy generally contains chromium, nickel, silicon, carbon, an effective cobalt content and the balance iron plus normal impurities. The alloy may be produced in the form of castings, P/M products, hardfacing and welding materials and wrought mill products.

Description

WEAR-RESISTANT STAINLESS STEEL

This invention relates to iron-base alloys and, more particularly, to a high chromium stainless steel suitable for seVeTe service wear-Tesistant applications, such as valve components.
Stainless steel has been in the state of constant development and-improvement since its invention as an Fe-Cr-Ni corrosion Tesistant steel. There are hundreds of varieties lD of stainless steels. Many have been designed for specific uses. The prior art is replete with modifications in steel compositions to provide desired speciic properties as required.
I`here is a critical need for a low cost alloy resistant to corrosion and mechanical wear as now p~o~ided by cobalt-base alloys. A well-kno~rn a;Lloy of this class is marketed as STELLITE~ alloy No.6 containing typically 28 chromium~ 4.5 tungsten, 1.2 carbon, balance cobalt. Because of the low cost and availability of iron, some iron base alloys have been proposed for wear applications~ For example, U. S.
Patent 2,635,044 discloses the basic 18-8 stainless steel with additions of molybdenum, beryllium and silicon as a hardenable stainless steel resistant to galling and erosion-corrosion when hea~ trea~ed.

PRIOR ART
U. S. Patent 1,790,177 discloses a wear resistant steel alloy suitable for use as drilling tools and welding rods.
This steel contains only chromium, nickel, siiicon and ~ -5 carbon as the essential elements with chromium 25 to 35~ as the principal feature. U. S. Patent 2,750,283 discloses the addition of boron to en}lance the hot rolling characteristics of nearly every known chromium-iron alloys with or without nickel, carbonS silicon, manganese, molybdenum, tungsten, 10 cobalt or other optional elements. U. S. Patent 4,002,510 discloses the addition of silicon to 18-8 stainless steels to promotc the formation of delta f3rrite~ thus enhancing resistance to st~es~ corrosion cracking.
As used herein~ all compositions are given in percent by weight.
U. S. Patents 3~912,503 and 4,039,356 relate to a modiied 13-8 stainless stee:l with critical contents of manganese and silicon. Known also in the art is an analogous commercial steel under A~MCO Inc.'s, trademark NITRONIC 60 containing typically, in weight percent, .10 max. carbon, 8 manganese, 4 silicon, 17 chromium~ 8.5 nickel and .13 nitrogen. Data show these steels have good wear properties, especially in galling tests.
~etal wear in industrial and consumer mechanical ~5 2 ~ .

operationS continues to be an expensive and, at times, hazardous problem. Conditions of wear environment are so diverse tha$ theTe can be no optimum or parfect wear-resist-ant alloy to solve all problems. Furthermore, cost and availability of elements to produce certain wear-resistant 5 alloys become an important consideration. The art is con-stantly searching for new and improved alloys ~o satisfy these needs.
For example, valve components subjected in service to chemically aggressivs media are constructed either from the stainless steels or high nickel alloys. Typically, the stainlecis 304 is selected by the food processing indus~ry and ~or other systems which involve mild corrodants, 316 is well used by the chemical processing industry, and thc high nickel alloys are selec-ted when sev~rely aggressive media are present.
A major drawback o the ~500 Type s-tainless steels and the high nickel alloys, however, is their tendency to gall (i.e., suffer fTom severe surface damage) when they are subjected to relative motion under the high loads inheren$
in valve operation. Of particular concern, in this respect, are the valve seat faces~ which must retain their integrity foT sealing purposes.

~Lz~;c?z3 Generally speakirlg, the 300 Series S~eels are the basic corrosion resistant stainless steels~ As a means to reduce the use of nickel, the 2QO Series Stainless steels were developed wherein manganese and nitrogen were substituted for a portion of the nickel. These 200 Series Steels were found to,have improved mechanical strengths over the 300 Series Steels o~ some uses. To improve the galling resi.st-ances of these alloys, higher silicon contents were added resulting in the alloys of NITRONIC 60 ~ype. NITRONIC 60 has improved galling resistance when compared to the 200 and 300 Series Steels.
Experimellts have shown NITRONIC 60 to have a high degree oE resistance to galling wh~n the alloy is coupled to itself. However, theTe is on:Ly limited resistance to galling when coupled with other counter~ace materials, in particular the 300-Series Steels and high nickel alloys. Thus, there is a limitation in the use of these alloys in the art.
Furthermore, in the general production of nitrogen containing alloys, experience has shown that nitrogen 20 content is difficult to control. Nitrogen tends to promote gas problems during welding. Manganese appears to be t,he source of serious deterioration of certain furnace lining materials.

~Z~ 23 OBJECTS
Therefore it is a principal object of this invention to provide an alloy that has a higher degree of wear resistance than is now available.
I~ is another principal object of ~his invention to provide ~n alloy that is more wear resistant under a ~ariety of wear conditions.
Other objects of this inventic)n may be discerned by those skilled in the art from the alloy of this invention as disclosed in Table 1.
THF: INVENTION
Table 1 presents the ranges of composition that define various embodiments of ~he alloy o~ this invention. The Broad range in Table 1 define~ the scope wherein some advantage o the in~ention may be obtained und~ certain circumstances. The Preerred range in Table 1 deines the scope wherein a higher clegree of advantages may be obtained.
Data shol~ that many propertieC; are impro~ed with compositions - within this range. The More Preferred range in Table 1 ! 20 defines the scope wherein a more desirable combination of engineering pToperties are obtained.
The Typical alloy defined in Table 1 is the optimum composition of one embodiment of the invent.ion. The typical alloy has an efective working scope essentially as defined in the Typical Range as shown in Table 1.

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Chromium is present in the 2110y of tl~s invention to provide corrosion resistance and to promote ~he formation of chromium carbides, chromium borides and the like. Less than 10% chromium will not provide sufficient corrosion ~esistance while over 40~ chromium content will tend to reduce ductility of the alloy.
Nickel must be present to promote an austenitic structure in the alloy. At least 5% nickel îs re~uired to be effective;
but over 15% does not provide additional benefits. Test results show that with nickel at only 5.12% there is a high degree of galling damage with the alloy coupled with a high nickel alloy. With nickel at 14.11~ there is also poor galling resistance with a hi,gh nickel alloy and when the 14~11% nickel alloy is coupl~sd with itsel~.
Silicon must be present in the alloy to enh~nce the anti-galling c~aracteristics o~ the alloy. Less than 3% is not sufficient while over 7% will embrittle the alloy.
The alloy of this inven*ion is enhanced with the folma-tion of carbides and borides o a group of elements including molybdenum, t~ngsten, vanadium, tantalum, columbium, titanium, chromium, zirconium, hafnium and others known in the a~tO
Carbides and borides of iron, o course, may be formed. To obtain these carbides and borides in ~he alloy in effec~ive ~5 amounts, carbon and boron must be present to~alin~ not less ~han .25%. Over 3.5% total carbon and boron will tend to reduce the ductility of the alloy. The total content of carbide or boride formers (other than iron~ listed above must be present not less than 10% to be efective; but, over 40~ will tend to reduce ductility and further add to costs.
It is understood that the carbides and borides may be ! in the form of complex st~uctures with three or more elements, for example, a chromium i~on carbide, Of course, at least a ]0 portion of the carbide-boride former elements may be found in the matrix.
Nitrogen may be beneficial in the alloy of this inven-tion Eor some applications a;nd may be present in an effective amount not more than .2~ to ,avoid the formation of excessive nitrid~s and avoid problems related to gas in weldments.
Cobalt is especially critical in the co~pos,ition of the alloy. Subsequent data will show a controlled content of cobal~ provides essential features of the alloy, and, in particular, impact strength. Cobalt content must be at leas~
5% to provide an effective increased impact strengthO Over about 30% cobalt the beneficial effects of cobalt are lost and no addit,ional improvement is provided in view of the additional costs. Actual test results show the optimum ~2~ 3~;~

cobalt content is about 12%. Thus, a p-referred range of cobalt at 5 to 20% is suggested for best advantage of the invention.
In a series of tests the criticality of cobalt was tested in two îron base alioys. Alloy A is essentially alloy 6781 in Table 2 except for cobalt. Alloy B contained 20.37 chromium, ~.83 nîc~el, 4.74 silicon~ 2.~ carbon and 7.93~
vanadium. Cobalt additions were made in the basic Alloys A
and B. The resulting alloys were tested for impact strength.
Tests were performed on ~he standard Charpy imp~ct testing unit and values were obtained in joules rom unnotched specimlens. Data ar~ presented in Table 3 and ~raphically in the attached figure .
The data and the fi~ure clearly show that a controlled content of cobalk dramatically affects impact strengths. The data show that about 12~ is the optimum cobalt content. The effect of cobalt continues to be beneficial up to about 30%
; cobalt content for Alloy A and 20~ cobalt c~n~ent for Al:Loy B.
The data also shol~ that basic Alloy A generally h~s higher impact strength; however, the influence o cobalt in basic Alloy B is similar.

lZ~3~?~3 Considering all of the material combinations tested, da~a as shown in Table 4 show when cobalt is presen~ at only 4~86% (Alloy A-l~, general resistance to galling is less than the alloy con~aining 11.95% ~Alloy A-Zl. However, in-creased cobalt content ~o 26.92~ (Alloy A-31 results in little improvement in resistance to galling. As a means to make di~ect comparison with Xnown prior art alloys, Table 4 also presen~s data for STELLITE alloy 67 NITRONIC 60 and HASTELLOY alloy C-276 the well known nickel base alloy. The galling test procedure will be described hereina~ter.
These wear data show the alloy of this invention to be comparable to or better than typical commercially available alloys.
In view o~ these data, it is suggested the maximum cobalt content should be 30QO and~ preerably, at 20% in view o~ cobalt costs.
y Manganese is no~ essential in the alloy o this inven-' tion but may be present in the alloy together with nickel in ; a total amount not exceeding 20%.

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TAB LE 2 .
Exampl e A1 :L oys o f this invention in wt . %

ALLOY ALLOY
6781 67Bl-W

Cr 29. 54 29 . 07 Ni 9 . 72 11. 08 Ni ~ ~In - 11. 58 Si 4.73 4.23 C 1.07 1.~7 N2 . 06 . 01 Fe plus impurities .Bal Bal Co 11 . 95 1~ . ~2 ;23 TABLE 3.
EFFECTS_OF COBALT
Unnotched ~ Cobalt Impact Strength, Basic Alloy ACon~ent Joules ft. lbf.
A - 1 4.86% 7.1 5.2 A - 2 11.95% 18.6 13.7 A - 3 26.92% 8.1 6.0 Basic Alloy B
~ - 1 0 1.7 1.3 B - 2 12~33 7.1 5.2 B - 3 19.37 2.4 1.8 :~Z~6i~Z~

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~6~3 EXA~IPLES
A series of experimental alloys was prepared for testin~.
The alloy examples or testing were induction melted ; and aspiration cast into glass tubes yielding 4.8mm (.188 inch) dia~eter weld rods. Depositions of the weld rods were made by gas tungsten arc melting. The deposits were fashioned into test specimens.
Alloy 6781-I~ of this invention was prepared in the ~orm of wrought product. Table 2 shows the analysis of the alloy.
The alloy was vacuum induction melted, then electroslag remelted (ESR). The ESR ~ars were fo~ged at about 2150F
~1177C) then hot rolled at the same temperature into plate and ~inally to about 1.59mm ~1/16 inch~ thick sheet ~or testing~ Galling test data show the alloy of this inrention, in wrought form, to have outstandîng anti-galling properties similar to the properties o~E the alloy in the form o~ hard-facing deposits.
The wrought alloy was impact tested by the standard test method well ~nown in the art. Data are presented in Table 5.
Powder products may also be produoed from the alloy of this invention. A composite product may be formed by the mixture of the alloy of this invention l~ith hard particles, ~2~

TABLE 5~
CHARPY IMPACT DATA

- Impact Strength - Joules ~t . lbf . ) .: Alloy Notched IJnnotched , ~ . . .
6781-W 5 . 4 ~4 . O) 88 . 8 (65 . 5) .~ .
I

, ~Z~ 23 such as tungsten caTbide, titanium diboride and the like.
The mixture is then further processed into a useful shape.
In addition9 components of the mixture may be added separ-ately to a welding torch and the end product is a composite deposit.
The galling test used to generate the data in Table 4 involved:
a. the twisting back and forth (ten times through

2.1 rad [120] arc) of a cylindrical pin (of diameter 15.9mm) (~625 inch) against a counterface block under load.
b. study oE the tes-t faces twhich were ini~ially in surface ground condîtion~ by profilometry to determine the degree o~ damage incurred during sliding.
Tests were performed or each test couple at three loads; 1350.8 kg (3000 1~), 2721.6 kg (6000 lb.) and 408Z~3 kg (9000 lb). The pins were twisted manually by means of a wrench and the load transmitted by means of a ball bearing.
The neck portion of the pins was designed to accommodate both the wrench and ball ~earing.
Since metallic faces, subjected to sliding under high loads, tend to have irregular profiles, often featuring one or tl~o deep grooves, it was deemed appropriate to measure degree of damage in terms of the change in maximum peak to valley amplitude (of the profile), rather than the change in average rou,hness (~hich would tend to mask the presence of any badly damaged regions~.
In visual terms, the cylindrical pin and the block appear to suffer the same degree of damage in a given test.
Only the blocks were used in the quantitative determination of damage, therefore, since they are more amenable to profilometry, allowing travel of the stylus to and beyond the circumference of the sGar. For accuracy, the stylus was passed twice over each 'scar (one pass along the diameter parallel to the sides o~ the block; the other along the diameter perpe~dicular to it~. Appreciable overlap of the adjacent unworn surace regions was afected to enable calculation of the initial peak to valley ampli~ude.
By considering each radius as a distinct region of the scar, four val~les of ~inal peak to valley ampli$ude were measured per scar. The average of these four values was used to determine the degree of damage incurred, subtracting the ave1age of four values o~ initial peak to valley amplitude.
The galling test procedure used to obtain galling evaluations described above was developed and modified from known test methods to provide more severe and more meaningful test results. Thus, the test data reported herein do not necessarily correspond directly with published data ob~ained ' by other ~es~ methods.
' 18 Unless otnerwise stated ! all galling tests reported herein were made under identical test conditions and the resulting test data are, therefore, valid in making direct comparisons among th various alloys tested herein.

~5 19

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An alloy consisting essentially of t in weight percent, 10 to 40 chromium, 5 to 15 nickel, 20 maximum nickel plus manganese, 3 to 7 silicon, .25 to 3.5 carbon plus boron, .2 maximum nitrogen, 10 to 40 one or more of molybdenum, tungsten, vanadium, tantalum, columbium, titan-ium, chromium, zirconium, and hafnium, 5 to 30 cobalt and the balance iron plus impurities.

2. The alloy of claim 1 containing 15 to 40 chromium, 7 to 13 nickel, 15 maximum nickel plus manganese, 3.5 to 6 silicon, .75 to 3.0 carbon plus boron, .15 maximum nitrogen, 5 to 20 cobalt 9 15 to 40 one or more of molybdenum, tungsten, vanadium, tantalum, columbium, titanium, chromium, zirconium and hafnium.

3. The alloy of claim 1 containing 25 to 40 chromium, 7 to 13 nickel, 15 maximum nickel plus manganese, 4 to 5.5 silicon, .75 to 2.5 carbon plus boron, 0.10 maximum nitrogen, 9 to 15 cobalt, and 25 to 40 one or more of molybdenum, tungsten, vanadium, tantalum, columbium, titanium, chromium, zirconium and hafnium.

4. The alloy of claim 1 containing about 30 chromium, about 10 nickel, about 4.7 silicon, about 1 carbon, about 12 cobalt.

5. The alloy of claim 1, containing 28.5 to 31.5 chromium, 9 to 11 nickel, 4.4 to 5.2 silicon, .85 to 1.15 carbon, 11 to 13 cobalt.

6. The alloy of claim 1, wherein said cobalt is present in an effective amount to provide combined good impact strength and good wear, especially galling, resistance.

7. The alloy of claim 1, in the form of a casting or a wrought product or hardfacing material or as a sintered powder metallurgy product.

8. The alloy of claim 1, as a component of a composite material wherein said alloy is the matrix with dispersions of hard particles.

9. The alloy of claim 8, wherein said hard particles are of tungsten carbide or titanium diboride.