Patent Application
20080178767
The invention relates to a product in the form of powder, of the dry-spray type, intended for protecting the moulds used for the centrifugal casting of cast iron pipes, combined with a sublayer product of the wet-spray type; the casting moulds used are commonly referred to by the name “shells”.
Unless otherwise indicated, all the values relating to chemical compositions are expressed in percentages by weight.
The coatings used for protecting centrifugal casting shells for cast iron pipes may consist of inoculants and refractories in powder form, and also mixtures of silica and bentonite, these being put into place by spraying an aqueous suspension. Such coatings are described for example in patent U.S. Pat. No. 4,058,153 (Pont-A-Mousson) and are known as wet-spray coatings.
It is also usual to employ powders sprayed dry onto the shell before the iron is cast, these powders then being referred to as dry-spray powders.
In the case of pipes with a diameter typically greater than 400 mm, it is common practice to combine the two types of product, that is to say:
Whatever the technique employed for depositing them, these products are used for several purposes, in particular:
It is well known that insufficient inoculation in the iron results in the formation of carbides, considerable shrinkage upon cooling and rapid demoulding, a gauge of high productivity. However, the castings thus obtained require a subsequent heat treatment, which may prove to be expensive.
It may, depending on the case, be preferable to inoculate further, even if this entails a reduction in the production rate, in order to avoid the final heat treatment, or on the contrary to inoculate less, in order to raise the productivity, and to subject the casting to heat treatment downstream.
The inoculability of the product may therefore be positioned within quite broad limits; in contrast, the other required effects are subject to more constant requirements.
In the specific case of dry-spray products deposited on a wet-spray first layer, these must also allow the wet-spray layer to remain bonded to the pipe while it is being demoulded, so as to prevent its accumulation and the formation of dross in the shell with the risk of this being entrained into the iron during casting, thus forming inclusion defects in the pipe, or creating surface defects on said pipe.
Dry-spray products therefore generally consist of a blend of several components, including:
In the specific case of dry-spray products deposited on a wet-spray first layer, such blends do not allow one of the desired objectives to be achieved, namely that of allowing the wet-spray product to remain bonded to the pipe while it is being demoulded.
For this purpose, it is common practice to use, as dry-spray product, a powder generally consisting of the “CaSi” alloy as mentioned above.
However, although this type of powder does indeed ensure that the wet-spray layer remains bonded to the pipe, it is not really satisfactory as regards the other aforementioned effects, including in particular the inoculating effect and more generally the effects relating to control of the metallurgical structure of the cast iron pipe.
The subject of the invention is a powder product for the protection of moulds or shells, used for the centrifugal casting of cast iron pipes, in a process comprising the following steps:
According to one advantageous embodiment, the additive is an alkali metal carbonate; according to another embodiment, it is an alkali metal silicate.
The additive may also be a blend in any proportion of one or more alkali metal carbonates and/or one or more alkali metal silicates.
Preferably, the alkali metal silicate is sodium silicate in which the ratio of the mass content of the SiO2 component to that of the Na2O component varies from 1 to 3.
According to another preferred embodiment, the additive is anhydrous sodium metasilicate.
Finally, according to one advantageous embodiment, the mass fraction of additive in said product is between 3 and 25%.
The powder products of the prior art, used as dry-spray products for protecting the centrifugal casting moulds for cast iron pipes and sprayed dry onto a wet-spray first layer obtained beforehand by spraying an aqueous suspension of a mixture of silica and bentonite onto the hot shell, have a number of drawbacks. Specifically, if they consist, like the dry-spray products that are sprayed dry directly onto the shell, of several components, in particular including:
The inert mineral filler when present in an excessively large amount has the same risk.
One solution consists in increasing the “CaSi” alloy content of the dry-spray product, generally even up to 100%.
However, although the desired effect of keeping the wet-spray product bonded to the pipe while it is being demoulded is substantially improved, the other desirable effects are degraded thereby. This is because, although the “CaSi” alloy is itself slightly inoculating, it does not by itself control the metallographic structure of the cast iron constituting the pipe as effectively as the blends described above. Increasing its amount, in order to increase its effect, results in the same drawback as mentioned above, namely the formation of dross in the shells and more particularly in those regions of its impression that correspond to the bell ends of the pipes. This phenomenon results in inclusion defects in the pipe or surface defects thereon, generally resulting in the pipe being scrapped.
To alleviate these drawbacks, the Applicant has therefore sought to obtain a dry-spray powder product having a composite effect, that is to say one giving both the effect of keeping the wet-spray product bonded to the pipe while it is being demoulded and the other effects, namely the anti-pinhole effect, the inoculation effect and the effect of controlling the metallographic structure of the iron.
This result can be obtained by a dry-spray product consisting:
This additive may advantageously be an alkali metal carbonate or an alkali metal silicate, especially sodium silicate in which the ratio of the mass content of SiO2 component to that of the Na2O component varies from 1 to 3, or else a blend in any proportion of one or more of these additives, namely one or more alkali metal carbonates and/or one or more alkali metal silicates, or, finally, anhydrous sodium metasilicate, in all cases with a particle size of less than 350 μm. The particle size of the powder product according to the invention is less than 580 μm and preferably less than 250 μm.
The effectiveness of wet-spray product bonding may be characterized by the percentage area of the external surface of the pipe where the wet-spray product does not remain bonded. In all the examples below, a wet-spray product was firstly deposited on the hot shell, this product containing, after drying, 95% silica (in diatomite form), 1% CaO and 3% alumina, provided by bentonite. The dry-spray product was then deposited on the wet-spray product, after it had dried merely due to the heat of the shell, using conventional techniques for depositing this type of product.
As a control test, a dry-spray product consisting of 100% of a “CaSi” alloy containing 61.1% Si, 30.4% Ca and 1.21% Al was used, the particle size of the product being characterized by a 63 μm undersize of 25% and a 200 μm undersize of 98%.
This product gave satisfactory results: the pipes were practically free of pinholes; the few pinholes present were shallow and allowed the specifications to be met; the carbide content was 8% and a ferritic iron thickness of 35 μm was noted on the external surface of the pipe. When demoulding the pipe, it was observed that only over 2% of its surface did the wet-spray product not remain bonded. The dross was concentrated in the bell, forming a band 15 mm in thickness therein.
A blend was prepared from the following constituents:
Particle size analysis showed that it had a 63 μm undersize of 28% and a 200 μm undersize of 97%.
Used under the same conditions as in Example 1, this product gave the following results: the pipes were practically free of pinholes; the few pinholes present were shallow and allowed the specifications to be met; the carbide content was 10% and a ferritic iron thickness of 30 μm was noted on the external surface of the pipe. These results are satisfactory. The thickness of the dross band in the bell was only 5 mm. However, the percentage area of the surface of the pipe without wet-spray product rose to 60%. It is clearly apparent that the dry-spray product of the conventional type (Example 2) has a metallurgical effect very similar to that of a dry-spray product consisting of a “CaSi” alloy alone; however, it is much less subject to the formation of dross, but is also substantially less effective as regards the wet-spray product remaining bonded to the pipe.
A blend was prepared from the following constituents:
Particle size analysis showed that it had a 63 μm undersize of 24% and a 200 μm undersize of 95%.
Used under the same conditions as in Example 1, this product gave satisfactory results for all the parameters: the pipes were practically free of pinholes; the few pinholes present were shallow and allowed the specifications to be met; the carbide content was 10% and a ferritic iron thickness of 35 μm was noted on the external surface of the pipe. The thickness of the dross band in the bell was 7 mm. The percentage area of the surface of the pipe without wet-spray product was only 3%.
A metallurgical effect similar to that of a conventional dry-spray product (Example 2), but with excellent effectiveness in respect of wet-spray product bonding to the pipe, was therefore observed, together with a very low tendency to the formation of dross (much less than for a dry-spray product consisting of “CaSi” alloy alone and similar to that of a dry-spray product of conventional type on a point test, but with a substantially reduced risk during continuous use owing to the substantially improved bonding of the wet-spray product to the pipe).
| Number | Date | Country | Kind |
|---|---|---|---|
| 0503949 | Apr 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/000676 | 3/29/2006 | WO | 00 | 10/18/2007 |
