CA1191039A - Powder metallurgy tool steel article - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A powder metallurgy article, which is particularly adapted to the manufacture of hot working rolls and tooling as well as high toughness cold work tooling such as shear blades and slitter knives, formed from compacted prealloyed powder of an alloy consisting essentially of, in weight percent, manganese 0.2 to 1.5, silicon 2 max., chromium 1.5 to 6, molybdenum 0.50 to 6, sulfur 0.30 max., vanadium 7 to 10, carbon expressed by the formula (.25 minimum, .40 maximum + .16 x percent vanadium), optional carbide forming elements such as tungsten and columbium m amounts up to 5 percent (with the corresponding stoichiometric carbon required for balance) may partially replace vanadium, optional cobalt additions may be included for heat resistance and balance iron and incidental impurities; the article is characterized by a fully martensitic structure with essentially no carbon in the steel matrix in excess of the carbon necessary to combine with the vanadium present to form vanadium carbides and to insure said fully martensitic structure.

Description

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In applications for tool s~eels, such as in the manufacture o~ Llot working rolls and tooling used in rolling hot metal, the tooling is subjected to condition~ of extreme wear as a result of contact with the workpiece, thermal shoc~, as a result 5 of being subjected to high temperatures when in contact with the hot workpiece and then rapid cooling when ou~ of contact with the wor~piece, and high compressive stresses, as a result of the roll separating ~orces encountered during rolling. In view of these service conditions it is desirable that tool steels from which 10 wor~rolls and other similar hot work tooling are made be charac~erized by good wear resistance, toughness, strength and resistance to thermal fatigue and shock. In tool steels o~ this type it is known to provide vanadium and sufficient carbon to combine therewith to produce vanadium carbides, which impart wear 15 resistance to the alloy. For purposes of strength it is typical to provide excess carbon over that necessary to combine with vanadium so tilat there is carbon in the alloy matrix to contribute significantly to the strength. It is generally believed bv those skilled in the art that carbon may be stoichiometrically balanced 20 with vanadium to produce vanadium carbides by having 0. 270 carbon for each 1% vanadium present-. Although alloys of this ~ype provide good strength and wear resistance they have been deficient in ~ertain applications such as for the manufacture of hot work rolls and tooling in that they tend to crack due to thermal fatigu~
25 and shock when subjected to drastic temperature cyclPs during use.
It is accordingly a primary object of the present invention to provide a powder metallurgy tool steel article having in combination wear resistance, toughness, strength and resistance to thermal fatigue and shock, and thus particularly adapted for hot 30 working applications.

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In accordance with the invention the composition limits for the alloy would be as follows, in weight percen~;
Manganese 0.2 to 1.5 Silicon 2 max.
Chromium 1.5 to 6 Molybdenum 0.50 to 6 Sulfur 0.30 max.
Vanadium 7 to iO
Carbon .25 min., .40 max. + .16 x %
v nadium Tungsten, Columbium, Up to 5 Cobalt Iron Balance and incidental elements and impurities characteristic of steelmaking practice The alloy would be processed by powder metallurgy techniques in the conventional manner and would be characterized by a fully martensitic structure with essentially no carbon in the steel matrix in excess of carbon necessary to combine with vanadium present to form vanadium carbides and to insure a fully mar~ensitic structure. After quenching from austenitizing temperature tne hardness may be at least 50 Rc for hot working applications; lower hardnesses may be provided for cold wor~
tooling requirements. At the relatively high vanadium carbide content of tne alloy powder metallurgy processing is required to ensure a fine, even carbide distrlbution necessary for toughness and grindability.
The term "powder metallurgy article" as used herein is used to designate a compacted prealloyed particle charge that has been formed by a combination of heat and pressure to a coheren~
mass having a density in final form, in excess of 99% of theoretical density; this includes intermediate products, such as ~ 3~

billets, blooms, rod and bar and the like, as well as final products, sucn as tool steel articles including rollq, punches, dies, wear plates, slitter knives, shear blades and the like~
which ar~icles may be fabricated from intermediate product forms from the initial prealloy particle charge. The particle charge may be produced by conventional gas atomization.
Tne term '~C-type vanadium carbides" as used herein refers to the carbide characterized by the face-centered cubic crystal structure with M representing the carbide-forming element essentially vanadium; this also includes M4C3-type vanadium car~ides and includes the partial replacement of vanadium by other carbide forming elements such as iron, molybdenum, chromium and of carbon by nitrogen to encompass what are termed carbonitrides.

Although the powder me~allurgy article of this invention is defined herein as containing all MC-type and M4C3 vanadium-carbides, it is understood that other types of carbides, such as M6C, M2C, and M23C6 carbldes, may also be present in minor amounts, but are not significant from the standpoint of achieving the objects of the invention specifically from the standpoint of wear resistance.
To determine the optimum composition for the alloy of this invention, experimental compositions were prepared by powder metallurgical technology and microstructural studies were conducted on heat treated specimens to determine the compositional balance with respect to van~dium and carbon which is requir.ed to develop a fully martensitic structure. A summary of the relation between the microstructural observations and compositions is shown in lable 1.

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hemical Composition (Wt. ~O) Steel C Mn Si _r Mo V Heat Treated Microstructure Alloy 1 2.46 .50 0.95 5.00 1.33 9O75 Fully Martensitic ~lloy 2 2.00 .67 lo 39 4.82 1.36 10.23 " "
Alloy 3 1.92 .49 1.14 4.89 1.34 9.78 " "

Alloy 4 1 r 80 .49 1,14 4.89 1.34 9O78 Minor amounts of ferrite otherwise martensite Alloy 5 1.60 .49 1.14 4.89 1.34 9.78 Notable amounts of ferrite remainder martensite CPM 9V* 1.78 .49 0.81 5.33 1.20 8.80 Fully Martensite *invention alloy ~ study of these results shows that the 1.78 carbon-8.80 vanadium alloy is the leanest composition which develops the fully martensitic structure desired in our invention. At least about 0.25% carbon is required in the matrix to develop a fully martensitic structure with the remainder present in the form of MC or M4C3 carbides and also that a matrix carbon con-tent of over about 0.40% may be detrimental to toughness. To further assess the effect of the same compositional variables on a key property for the alloy of this invention, C-notch impact tests were conducted on specimens heat treated to the HRC ~8 to 50 hardness range. The results in Table II show that a signi-ficant toughness advantage was presented by the 1.78 carbon -8.80 vanadium alloy (CPM 9V) of the invention. Specifically, the 1.78 carbon - 8.80 vanadium alloy in accordance with the invention exhibited a C-notch impact strength value (ft-lbs) of 74 at an HRC of 49.5 which demonstrates a drastic improvement in toughness at a hardness level comparable to the ha~dnesses of the conventional alloys set forth in Table II.

~ 3 TABLE II
CHARPY C-NOTCH IMPACT TOUGHNESS VALUES
Hardness C-Notch Impact C~ V HRC Strength (ft-lbs)

2.46 9.75 48 29 2.00 10.23 48 23.5 1.92 9.78 49.5 29.5 1.80 9.78 48.5 26 . 1.7& ~.80 49.5 74 The results of the Charpy C-notch impact tests are shown in Tabie III for the CPM 9V alloy of the invention a~
various heat treatments and hardnesses.

TABLE lII
CHARPY C-NOTCH ~PACT TEST RESULTS FOR CPM 9V
Impact Heat Treatment ~.RC Temp.(F) Strength (ft-lbs) 52050F~10 min, AC, 1025F/2+2 hr. 56 RT 26 ~2050F/30 min, AC, 1050F/2+2 hr. 53 RT 54 2050F/30 min, AC, 1125F/2+2 hr. 49 RT 53 1950F/1 hr,AC, 1100F/2+2 hr. 49.5 RT 74 " 300 81 10 ~ " " 500 94 " " 800 82 1950FIl hr, AC, 1140F/2+2 hr. .46.5 RT 61 " " 300 78 - " " 500 ~4 - " " 800 ~0 1850F/1 hr, AC, 1090F/2+2 hr. 4~.0 RT 75 " " 300 87 " " 500 103 " " 800 1~0 201850F/1 hr, AC, 1125~ 2+2 hr. 44.0 RT 70 " " 500 84 " " . 800 78 For comparison purposes Table IV shows the C-notch impact results, as well as hardness (Rockwell C scale), for a conventional powder metallurgy produced tool steel with a nominal composition, in weight percent, carbon 2.4, manganese .45, silicon .89, chromium 5.25, vanadium 9.85, molybdenum 1.25 and balance iron. ~ne distinguishing featurP between this composition and the 30 above-reported CPM 9V composition is that with this latter conventional composition there is excess carbon present in the _7_ .

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matrix wnlch is intended for strengthening. As may be seen from the comparison of the toughness values for the ma~erial in accordance witn the invention presented in Table III at similar heat treated conditions the material of the invention exhibits far superior Charpy C-notch impact test values than the conventional material, the ~est results of which are presented on Table IV
along witn comparison values for CPM 9~. At the relatively lower vanadium content of the invention alloy compared to the vanadium content of the conventional alloy, it has been determined, in accordance with the invention, that if carbon is present in an amount equaling 0.2% carbon for each 1% vanadium this can result in carbon being present in the matrix in an amount in excess of that necessary to ensure a fully martensi~ic structure and thus toughness is impaired. Hence, in accordance with the invention lS carbon = .25% minimum, .40% maximum ~ .16 x percent vanadium.

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As may be seen from Table V, which gives the hardness values for the material in accordance with the invention its hardness is comparable to that of the conventional hot work tool materials after elevated temperature exposures slightly above the expected ~aximum temperature range of application for ~he steel article o~ this invention.

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1 As may be seen from the data presented in Tables I to V, by controlling carbon at a level expressed by the formula C = .25 min., .40 max. + .16 x ~V
one is able to achieve a significant improvement with respect to toughness, as demonstrated by the Charpy C-notch impact test results for the material of the invention without sacrificing the required strength and hardness. In addition, by the presence of vanadium and sufficient carbon to combine there~ith to pro-duce vanadium carbides the material has excellent wear resistance.
Table VI compares, after heat treatment, the wear re-sistance of the CPM 9V material of the invention which conven-tional high alloy hot work tool steels of conventional cast and wrou~ht production. As may be seen from Table VI the CPM 9V
material of the invention shows drastically improved wear re-sistance over the AISI H13, AISI Hl9 and AISI H21 steels even in instances wherein the hardenss of the CPM 9V material is sig-nificantly lower than that of the conventional steels.
TABLE VI

WEAR RESISTANCE OF HOT WORK STEELS
Hardness Wear Resistance Grade Heat Treatment (HRC) (xlol psi) -CPM 9V 2150F/1 hr, AC, 1025F/2-~2 hr 56 71 CPM gV 2050F/1 hr, AC, 1050F/2+2 hr 53 61 CPM 9V 1850F/1 hr, AC, 1095F/2+2 hr 48 22 CPM 9V 1850F/1 hr, AC, 1125F/2+2 hr 45 21 AISI H13 1850F/1 hr, AC, 1050F/2+2 hr 52 3.6 AISI Hl9 2150F/1 hr, AC, 1025F/2+2 hr 56 3.7 AISI H21 2150F/1 hr, AC, 1025F/2+2 hr 56.5 2.1 For evaluation of wear resistance, the cross-cylinder wear test was used. In this test, a cylindrical specimen (5/8 in.

diameter) of the respective cold-work or warm-work tool material - ]2 -and a cylindrical specimen (1/2 in. diameter) of tungsten carbide (with 6% cobalt binder) are positioned perpendicularly to one another. A fifteen-pound load is applied through weight on a leve~
arm. Then the tungsten carbide cylinder specimen is rotated at a speed of 667 revolutions per minute. No lubrication is applied.
As the test progresses, a wear spot develops on the specimen of the tool material. From time to time, the extent of wear is determined by measuring the depth of the wear spot on the specimen and converting it into wear volume by aid of a relationship specifi~ally derived for this purpose. The wear resistance, or the reciprocal of the wear rate, is then computed according to the following formula:
Wear resistance = - 1 = L~s = L~ d~N
wear rate ~ v ~ v where v = the wear volume, (in.3) L = the applied load, (lb.) s = the sliding distance, (in.) d = the diameter of the tungsten carbide cylinder, (in.) and -1~ = the number of revolutions made by the tungsten carbide cylinder, (rpm) This test has provided excellent correlations with wear situations encountered in prac~ice.
The thermal fatigue properties of the steel of the invention when compared with conventional powder metallurgy produced cold wor~ tool steels and conventional cast and wrought steels of this type are shown in Table VII; in this Table, the steel of the invention, CPM 9V, is compared with a conventional powder 30 metallurgy produced tool steel containing 2.46% carbon and 9.75%

vanadium and a conventional cast and wrought steel of this type, which is identified as AISI H13.
TABLE VII
RESIST~CE T0 THE~*~L FATIGUE
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Thermal Fatigue Grade Cycles ~o Failure AISI H13 27,000 .46C - 9.75V 4,500 CPl~i 9V Greater than 60,000 - As may be seen from Table VII the thermal fatigue resistance of the CPM 9V material of the invention is drastically greater than that of both of the other conventional steels tested, including the 2.46 carbon - 9.75 vanadium material which is a powder metallurgy produced steel designed for cold or warm work Itooling.
'ihe thermal fatigue test involves the use of an electrically heated lead pot, a hot water quenching bath and a solenoid valve operated, pneumatic-operated mechanical transfer for transferring the specimens between the lead pot and the bath.
Specimens are transferred into the lead bath for a 4-second heating period: They are then transferred quickly to a position abovP the water bath wherein they are quenched for 2 s~conds at a water bath temprature of 180F. This cycle is repeated at a rate of 3 cycles per minute. Each specimen during each cycle is dried above the lead pot for a period of 5 seconds. Including transfer time each cycle takes approximately 20 seconds. ~uring each cycle dif-ferential heating occurs in the rim and hub of each specimen and nence from the thermal expansion, the rim periphery is mechanically strained to set up compressive stresses in this region. ~pon quenching the reverse of the phenomenon takes place.

During this por~ion of the cycle, the hub opposes the thermal con~raction of the rim causing residual (peripheral) tensile stresses to be set up. Typically, fatigue is demonstrated by the beginning of cracks in the rim periphery of ~he samples which propagate toward the hub with the rate of cracking being determined by the thermal fatigue resistance of the steel being tested.
With reference to the tou~hness and wear resistance advantages offered by CPM 9V over adominant cold work ~ool steel grade namely AISI D2, Table VIII shows the Charpy C-notch impact and the wear resistance comparisons of these steels.
! TABLE VIII
CHARPY C-NOTCH IMPACT STRENGTH OF WEAR RESISTANCE C02~PARISONS
` OF CPM 9V AND AISI D2 L5 Hardness C-Notch Impact Wear Resistance Steel HRC Strength (ft-lb) (xlol0 psi) ~O AISI D2 56 19

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A powder metallurgy tool steel particularly adapted for the manufacture of hot working rolls and tooling, said article formed from compacted prealloyed powder of an alloy consisting essentially of, in weight percent, manganese 0.2 to 1.5, silicon 2 max., chromium 1.5 to 6, molybdenum 0.50 to 6, sulfur 0.30 max. vanadium 7 to 10, carbon = .25%
min., .40% max. + .16 x % vanadium and balance iron and incidental elements and impurities characteristic of steelmaking practice, said article being characterized by a fully martensitic structure with essentially no carbon in the steel matrix in excess of the carbon necessary to combine with the vanadium present to form vanadium carbides and to ensure said fully martensitic structure and having a minimum Charpy C-notch impact strength value in foot pounds of greater than 29.5 when austenitized at 1950F and tempered at 1100F.

2. The powder metallurgy article of claim 1 in the form of a workroll.

3. The powder metallurgy article of claims 1 and 2 having a hardness of at least 50 Rc after quenching from the austenitizing temperature.