Alloys or metallic compositions – Ferrous – Chromium containing – but less than 9 percent
Patent
1995-04-03
1997-04-22
Yee, Deborah
Alloys or metallic compositions
Ferrous
Chromium containing, but less than 9 percent
420111, 148334, 148335, 148547, C22C 3812, C21D 600
Patent
active
056226748
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a family of steels known as 3%-5% chromium steels (% by weight) and that are used for manufacturing tooling that withstands heat under high stresses, such as dies for stamping and forging, and dies for casting under pressure or for static casting of various alloys such as alloys of aluminum or of titanium.
In general, such steels contain 3% to 5% by weight of chromium, even though contents in the range 2% to 6% are to be observed. More precisely, they comprise essentially three families of compositions which, although slightly different from one another, all confer physical properties that are similar such that these steel compositions are used for the same applications. These families are compositions that comprise, expressed by weight:
Over the last few decades, the use of such steels has become widespread in workshops for making forged or stamped parts on presses and on stampers, and also in light alloy foundries, e.g. for making dies for parts that are cast in steels or light alloys for the automotive industry, such as sumps, clutch casings, or gear box casings.
Some of these steels are designated by the names H 11, H 12, and H 13 in the AISI nomenclature of the United States of America, or by the names W-1.2343, W-1.2606, and W-1.2344 in the DIN nomenclature. French standard NFA 35590 likewise defines analogous compositions.
Silicon is a hardening element, and a content of about 1% by weight confers high strength of about 1800 MPa or more to mechanical parts. This strength is not required in the intended forging uses, except for parts that are very flat, and is never required in pressure dies for aluminum, where a Rockwell C hardness (HRC) no greater than 48 suffices.
It is known, in particular for 3%-5% chromium steels, that successful annealing heat treatment is a necessary prerequisite for successful quality heat treatment. Thus, the fineness and the homogeneity of the microstructure in the finished product after treatment for final use are derived from those observed after annealing. That is why professionals commonly use a chart of microphotographs of structures in the annealed state showing microstructures that are within specification and microstructures that are not.
This practice, which is widespread at present, has progressively "frozen" conditions of manufacture, of thermomechanical transformation, and of annealing. In addition, it has been observed that the fining of the annealed structure is conditioned by the homogeneity of the structure in the austenite range, which makes it necessary to avoid the presence of primary carbides, and by the coherent dispersion of the precipitates of secondary carbides M.sub.23 C.sub.6 (M=Cr, Fe, Mo, . . . ) during subsequent heat treatments.
The Applicant has also been able to show that certain zones that appear to be rougher, and that sometimes appear in the form of needles of Bainite-like appearance, particularly in pieces of large section, have higher concentrations of silicon.
On the basis of these fundamental considerations, steels have been developed that have acceptable homogeneous annealed structures.
To this end, the invention provides two types of tool steel composition.
The first type of tool steel composition of the invention comprises, expressed by weight:
The second type of tool steel composition of the invention comprises, expressed by weight: impurities.
Such compositions give rise to homogenization of the annealed structure, which becomes more difficult to achieve with increasing section of the parts, by eliminating the formation of silicon-enriched ferritic zones and also the formation of primary carbides which are always difficult to put into solution.
In addition, these two modifications do not give rise to significant changes to the range of heat treatments in fields of utilization: the differences that may be observed for the values of strength and elastic limit can easily be compensated by adjusting the temperature of the second annealing, which is within the competence of any person skilled in the ar
REFERENCES:
patent: 3272622 (1966-09-01), Bengtsson
patent: 5011656 (1991-04-01), Ohori et al.
patent: 5437742 (1995-08-01), Siga et al.
Aubert et Duval SA
Yee Deborah
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