EP0076027A2

EP0076027A2 - Powder metallurgy articles

Info

Publication number: EP0076027A2
Application number: EP82304064A
Authority: EP
Inventors: Walter T. Haswell; William Stasko
Original assignee: Crucible Materials Corp; Colt Industries Operating Corp
Current assignee: Crucible Materials Corp
Priority date: 1981-09-28
Filing date: 1982-08-02
Publication date: 1983-04-06
Also published as: IN158518B; DK158795C; JPS5858255A; CA1191039A; MX159525A; DK158795B; ES8305424A1; KR840001456A; DE3274261D1; EP0076027B1; JPH0140904B2; ES513486A0; EP0076027A3; ATE23567T1; DK231882A

Abstract

A powder metallurgy article, e.g., a hot working roll or tool or a high toughness cold work tool such as a shear blade or slitter knife, formed from compacted prealloyed powder of an alloy consiting 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), optical carbide forming elements such as tungsten and niobium in 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 characterised 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.

Description

This invention relates to powder metallurgy articles.
In applications for tool steels, such as in the manufacture of hot working rolls and tooling used in rolling hot metal, the tooling is subjected to conditions of extreme wear as a result of contact with the workpiece, thermal shock, as a result of being subjected to high temperatures when in contact with the hot workpiece and then rapid cooling when out of contact with the workpiece, and high compressive stresses, as a result of the roll separating forces encountered during rolling. In view of these service conditions it is desirable that tool steels from which work- rolls and other similar hot work tooling are made be characterised by good wear resistance, toughness, strength and resistance to thermal fatigue and shock. In tool steels of this type it is known to provide vanadium and sufficient carbon to combine therewith to produce vanadium carbides, which impart wear resistance to the alloy. For purposes of srength it is typical to provide excess carbon over that necessary to combine with vanadium so that there is carbon in the alloy matrix to contribute significantly to the strength. It is generally believed by those skilled in the art that carbon may be stoichiometrically balanced with vanadium to produce vanadium carbides by having 0.2% carbon for each 1% vanadium present. Although alloys of this type provide good strength and wear resistance thay have been deficient in certain applications such as for the manufacture of hot work rolls and tooling in that they tend to crack due to thermal fatigue and shock when subjected to drastic temperature cycles 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 working applications.
An additional object of the invention is to provide for cold work tooling requirements by having a combination of high toughness and excellent wear resistance for critically demanding applications, such as slitter knives and shear blades..
The present invention provides a powder metallurgy formed from compacted prealloyed powder, in which the powder is of an alloy having the composition, 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, optionally partially replaced by up to 5% tungsten and up to 5% niobium.
Carbon : 0.25 min., 0.40 max., plus 0.16x% vanadium plus the stoich- metric amount required to balance any tungsten and niobium present
Cobalt; Up to 5
Balance, iron and incidental elements and impurities characteristic of steelmaking practice,

Broadly, in accordance with the present invention, a tool steel powder metallurgy article, particularly adapted for the manufacture of hot work rolls and tooling, is provided wherein the carbon content is controlled in relation to vanadium so that there is sufficient carbon to combine with vanadium to form vanadium carbides, which provide the required wear resistance. In addition, there is sufficient carbon to form and provide for a fully martensitic structure upon cooling from austenitising temperatures, but otherwise there is essentially no excess carbon in the alloy matrix. Typically, the alloy is heat treated by austenitising and air cooling or oil quenching during which cooling a fully martensitic structure is obtained. More specifically, it has been found that tool steels in accordance with the invention are characterised by excellent wear resistance because of the vanadium carbide content; good toughness because of the absence of excess carbon in the matrix over that required to ensure a fully martensitic structure; strength, because of the strengthening effect of the martensite; and resistance to thermal fatigue and shock,-resulting from the absence of carbon in the matrix other than that needed to form martensite.
The alloy would be processed by powder metallurgy techniques in the conventional manner and would be characterised by a fully martensitic structure with essentially no carbon in the steel matrix in excess of carbon necessary to combine with vanadium to form vanadium carbide and to ensure a fully martensitic structure. After quenching from austenitising temperature the hardness may be at least 50 R for hot working applications; lower hardnesses may be provided for cold work tooling requirements. At the relatively high vanadium carbide content of the alloy powder metallurgy processing is required to ensure a fine, even carbide distribution necessary for toughness and grindability.
The term "powder metallurgy article" as used herein is used to designate a compacted prealloyed particle charge which has been formedby a combination of heat and pressure to a-coherent mass having a density in final form, in excess of 99% of theoretical density; this includes intermediate products, such as billets, blooms, rod and bar and the like, as well as final products, such as tool steel articles including rolls, punches, dies, wear plates, slitter knives, shear blades and the like, which articles may be fabricated from intermediate product forms from the initial prealloy particle charge. The particle charge may be produced by conventional gas atomisation.
The term "MC-type vanadium carbides" as used herein refers to the carbide characterised by the face-centred cubic crystal structure with M representing the carbide-forming element essentially vanadium this also includes P4₄C₃ttype vanadium carbides 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 metallurgy 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 carbides, 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 vanadium and carbon which is required to develop a fully martensitic structure.
A summary of the relation between the microstructural observations and compositions is shown in Table I.
A 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 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 a matrix carbon content of over 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'48 to 50 hardness range. The results in Table II show that a significant 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 of 100 joules (74 ft.-lbs) at an HRC of 49.5 which demonstrates a drastic improvement in toughness at a hardness level comparable to the hardnesses of the conventional alloys set forth in Table II.
The results of the Charpy C-notch impact tests are shown in Table III for the CPM 9V alloy of the invention at various heat treatments and hardnesses.
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. The distinguishing feature between this composition and the above-reported CPM 9V composition is that with this latter conventional composition there is excess carbon present in the matrix which is intended for strengthening. As may be seen from the comparison of the toughness values for the material in accordance with 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 test results of which are presented on Table IV along with comparison values for CPM 9V. 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 equalling 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 martensitic structure and thus toughness is impaired. Hence, in accordance with the invention carbon .25% minimum, .40% maximum +.16 x percent vanadium
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 maximum temperature range of application for the steel article of this invention.
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 therewith to produce vanadium carbides the material has excellent wear resistance.
Table VI compares, after heat treatment, the wear resistance of the CPM 9V material of the invention with conventional high alloy hot work tool steels of conventional cast and wrought production. As may be seen from Table VI the CPM 9V material of the invention shows drastically improved wear resistance over the AISI H13, AISI H19 and AISI H21 steels even in instances wherein the hardness of the CPM 9V material is significantly lower than that of the conventional steels.
For evaluation of wear resistance, the cross-cylinder wear test was used. In this test, a cylindrical specimen of 15.9mm( 5/8in)diameter of the respective cold work or warm-work tool material and a cylindrical specimen of 12.7mm (1/2 in.) diameter of tungsten carbide (with 6% cobalt binder) are positioned perpendicularly to one another. A 6.8kg(fifteen-pound) load is applied through weight on a lever 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 specifically derived for this purpose. The wear resistance, or the reciprocal of the wear rate, is then computed according to the following formula:

Wear resistance

v =the wear volume (cm³or in3)
L =the applied load (kg or lb)
s =the sliding distance, (cm or in)
d =the diameter of the tungsten carbide cylinder (cm or in.)

N =the number of revolutions made by the tungsten carbide cylinder (rpm)

This test has provided excellent correlations with wear situations encountered in practice.
The thermal fatigue properties of the steel of the invention when compared with conventional powder metallurgy produced cold work 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 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 asAISI H13..
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 tooling.
The 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 above the water bath wherein they are quenched for 2 seconds at a water bath temperature of 82 C(180 F). 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. During each cycle differential heating occurs in the rim and hub of each specimen and hence from the thermal expansion, the rim periphery is mechanically strained to set up compressive stresses in this region. Upon quenching the reverse of the phenomenon takes place. During this portion of the cycle, the hub opposes the thermal contraction 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 the samples which propogate toward the hub with the rate of cracking- being determined by the thermal fatigue resistance of the steel being tested.
with reference to the toughness and wear resistance advantages offered by CPM 9V over a dominant cold work tool steel grade namely AISI D2, Table VIII shows the Charpy C-notch impact and the wear resistance comparisons of these steels,

Claims

1. A powder metallurgy article formed from compacted prealloyed powder, characterised in that the powder is of an alloy having the composition, 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, optionally partially replaced by up to 5% tungsten and up to 5% niobium

Carbon : 0.25 min., 0.40 max., plus 0.16x% vanadium plus the stiochiometric amount required to balance any tungsten and niobium present

Cobalt : up to 5

Balance, iron and incidental elements and impurities characteristic of steelmaking practice,
and in that the article has a fully martensitic structure with essentially no carbon in the steel matrix in excess of the carbon necessary to combine with the vanadium and any tungsten and niobium present to form vanadium, tungsten and niobium carbides and to ensure the fully martensitic structure.

2. A powder metallurgy article as claimed in claim 1, further characterised in that the powder is of an alloy having the composition, 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 0.25%min., 0.40% max.+0.16 x % vanadium and balance iron and incidental elements and impurities characteristic of steelmaking practice, the article being characterised 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 the said fully martensitic structure.

3. A powder metallurgy article as claimed in claim 1 or claim 2, further characterised in that it is in the form of a workroll.

4. A powder metallurgy article as claimed in any one of the preceding claims, further characterised by a hardness of at least 50 R after quenching from the austenitising temperature.