Alloys or metallic compositions – Ferrous – Chromium containing – but less than 9 percent
Reexamination Certificate
2000-02-08
2001-08-28
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Chromium containing, but less than 9 percent
C420S109000, C420S111000, C148S663000, C148S660000, C148S334000
Reexamination Certificate
active
06280685
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a steel of the “3% to 5% by weight chromium” family as used for making tools that withstand heat and that work under high levels of stress, such as dies for stamping and forging, dies for wire drawing, molds for static casting or for casting under pressure, using various alloys such as alloys of aluminum, copper, or titanium.
Such steels are alloyed with chromium, molybdenum, and vanadium, elements which give them the required hot strength properties. More precisely, they are subdivided into three families of compositions having properties that are similar, such that these three families are used in the same applications. These are compositions that comprise the following alloying elements, with percentages expressed by weight:
5% chromium, 1.3% molybdenum, 0.5% to 1.3% vanadium, approximately; or
3% chromium, 3% molybdenum, 0.5% vanadium, approximately; or else
5% chromium, 3% molybdenum, 0.8% vanadium, approximately.
Some of those steels are specified in the AISI nomenclature in the United States under the terms H11, H12, and H13, or in the DIN nomenclature in Germany under the names W1.2343, W1.2606, and W1.2344, and they are mentioned in French standard NF A 35-590.
In use, the surface of the tooling comes into contact with materials that are heated to high temperature, for example liquid aluminum at 600° C.-750° C. or steel that is to be forged and that has been preheated to 1200° C.
Consequently, the surface of the tooling is itself raised to high temperature. As a result, temperature conditions are established within the tooling between its working portion which is subjected to heating and the remainder of the part which is cooled either by natural conditions or by forced cooling.
Under severe conditions of use implementing high surface temperatures and high levels of mechanical stress, a tool is destroyed quickly by two processes:
the mechanical strength of material decreases smoothly with increasing temperature; and
the material loses its initial properties which were imparted thereto by preliminary heat treatment because of the metallurgical transformations that take place under the combined effects of stresses and temperature giving rise to its mechanical strength weakening and then collapsing.
Thus, rapid or even catastrophic deterioration is observed of such tooling employed under severe conditions because the working surface softens, creeps, deforms plastically, and is subject to thermal fatigue.
SUMMARY OF THE INVENTION
The tool steel of the present invention overcomes these deficiencies and includes by weight percent 0.3-0.4 C, 2.0-4.0 Cr, 0.8-3.0 Mo, 0.4-1.0 V, 1.5-3.0W, 1.0-5.0 Co, 0-1.0 Si, 0-1.0 Mn and 0-1.0 Ni with the balance being mainly iron and inevitable impurities. The present invention further includes a method of preparing a tool steel with the aforesaid composition including steps of heating the steel to a temperature of 1020° C. to 1100° C. followed by staged quenching at temperatures of 250° C. to 320° C.
DETAILED DESCRIPTION OF THE INVENTION
In a first aspect, the present invention provides a steel composition that withstands said severe operating conditions well.
The composition of the invention comprises, in weight percentage:
C 0.3%-0.4%
Cr 2.0%-4.0%
Mo 0.8%-3.0%
V 0.4%-1.0%
W 1.5%-3.0%
Co 1.0%-5.0%
Si 0-1.0%
Mn 0-1.0%
Ni 0-1.0%
the balance being mainly constituted by iron and inevitable impurities.
Preferably, the composition lies within the following ranges:
0.33%-0.37%
Cr 2.58%-3.50%
Mo 1.20%-2.20%
V 0.6%-0.9%
W 1.8%-2.6%
Co 1.5%-3.0%
Si 0.2%-0.5%
Mn 0.2%-0.5%
Ni 0-0.3%
In more particularly preferred manned, the composition of the invention has concentrations of P, Sb, Sn, and As, expressed in weight percentages, which satisfy the following relationships:
P≦0.008%
Sb≦0.002%
Sn≦0.003%
As≦0.005%
while the value given by Bruscato's relationship:
B=(10P+5Sb+4Sn+As)×0.01
is not greater than 0.10%.
The set of alloying elements whose actions complement one another is balanced so as to provide sufficient quenchability as is required for obtaining uniform properties throughout the thickness of parts of large size.
Carbon is the basic hardening element, and its level is adjusted so as to obtain sufficient mechanical strength while ensuring that eutectic carbides do not form during solidification because carbon concentration is too high. Its concentration in the alloy of the invention lies in the range 0.3% to 0.4% by weight, and preferably in the range 0.33% to 0.37% by weight.
Chromium and molybdenum contribute to quenchability and to hardening after quenching and tempering by forming alloyed carbides during tempering heat treatment. The concentrations of these elements must not be excessive so as to avoid excessively encouraging the formation of chromium-molybdenum carbides to the detriment of vanadium and tungsten carbides. The concentration of chromium in the alloy of the invention is 2.0% to 4.0% by weight, preferably 2.50% to 3.50% by weight, while the concentration of molybdenum is 0.8% to 3.0% by weight, and preferably 1.20% to 2.20% by weight.
Vanadium contributes to hardening during tempering treatment by forming specific carbides, thereby making it possible to increase structural resistance to heating, and thus to raise the highest acceptable operating temperatures. An excess of this element is prejudicial to toughness because eutectic carbides are formed on solidification, and because of the segregating nature of this element. Its concentration in the alloy of the invention is 0.4% to 1.0% by weight, and preferably 0.6% to 0.9% by weight.
Similarly, tungsten complements the action of vanadium by mechanisms of the same type and thereby contribute to raising the temperatures which are compatible with use, and in the same manner, excess tungsten is prejudicial to toughness and to structural uniformity. Its concentration in the alloy of the invention is 1.5% to 3.0% by weight, and preferably 1.8% to 2.6% by weight.
It is the complementary and appropriately balanced effects of these four carbide-generating elements Cr, Mo, V, and W that impart new properties to the alloy of the invention.
Cobalt improves mechanical strength when hot. Its concentration in the alloy of the invention is 1.0% to 5.0% by weight, and preferably 1.5% to 3.0% by weight.
The concentrations of silicon and of manganese in the alloy of the invention are each 0% to 1.0% by weight, and preferably 0.20% to 0.50% by weight. The concentration of nickel in the alloy of the invention is 0% to 1.0% by weight, and preferably 0% to 0.30% by weight.
More generally, although there is no desire to be tied to any particular theory, it is believed that the obtention of good characteristics for such steels depends on balancing the elements of the alloy; it is the result of the individual properties of each of the elements, and also of the way they interact.
The effect of tungsten stems from the formation of carbides, with this element contributing to the composition thereof. It is in competition with chromium and molybdenum, given that a predominance of chromium carbides is harmful for stability in operation.
Nevertheless, the crystallographic nature of the carbides formed depending on the steel is still poorly known at present, and
the effects of these carbides on the properties and the structural stability are known only in broad outline.
The steel of the invention is made using the methods applicable to the usual materials referred to.
The invention also provides a method of preparing tool steel having the above-defined composition, and in which, in a particular implementation, an appropriate tempering treatment is performed prior to the heat treatment of use, so as to obtain a metallographic structure that presents carbides which are fine and well distributed.
In a particular implementation, quenching is performed by heating the part to a temperature lying in the range 1020° C. to 1100° C., and preferably in the range 1040° C. to 1070° C., and then cooling by stepped quenching
Grellier André
Siaut Michel
Aubert & Duval
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
Yee Deborah
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