Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Reexamination Certificate
2002-07-02
2004-03-16
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
C428S557000, C428S683000, C428S685000, C148S514000, C148S529000, C148S612000, C148S621000
Reexamination Certificate
active
06706416
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the manufacture of corrosion resistant metal products and to products produced from the process. The invention has particular but not necessarily exclusive application to products comprising a core formed from recycled mild, carbon or stainless steel swarf and having a stainless steel cladding. For example, the invention may also be applicable to a product comprising a core formed from powdered iron ore and even from other metals and metalliferous ores in which the problems identified herein are encountered.
In this specification ‘swarf’ comprehends the off cuts from machining operations in general and is intended to include the off cuts from turning, boring, shaping and milling operations on engineering steels. The off cuts from a variety of other operations including some stamping and punching operations may also be suitable. For the purposes of this specification, the term includes not only such off cuts composed of raw swarf but also such off cuts from swarf which has been cleaned and/or otherwise treated, for example by the methods described herein, to make them more suitable for forming a billet from which the clad products are made.
The term “engineering steel” is intended to describe those low alloy steels which are commonly subjected to machining operations including mild steel (a term which itself includes carbon steel), forging steel and axle or shaft steel all of which contain significant amounts of carbon.
BACKGROUND OF THE INVENTION
The background of the present invention is set out in detail in the specification of international patent application #PCT/GB94/00091. In that specification reference is made to the specifications of several other patent applications which are discussed further below. One of the products of the process described in the aforementioned application PCT/GB94/00091 which is potentially of commercial and technical importance is a billet comprised of a stainless steel jacket filled with briquettes of mild steel swarf which can be heated and worked into a finished product having the desirable properties and low cost of mild steel but which has a stainless steel cladding for substantially increased corrosion resistance. Attempts to produce such products have not been as successful as was originally expected and it is an object of the present invention to address at least one of the problems which has contributed to this lack of success.
In the numerous experiments which have been conducted in attempts to produce such products, they have persistently exhibited a green chrome oxide layer occurring on the inner face of the stainless steel cladding and at the interface between the cladding and the core. This green layer has occurred despite the fact that metallographic examination of the core after the billet has been heated and rolled indicates substantially complete reduction of all surface oxides in the swarf and substantially complete fusion between the particles of swarf. Bonding between the cladding and the core cannot be relied on where this green layer occurs.
It is thought that chrome oxides on the stainless steel pipe form a barrier between the core of compressed swarf and the stainless steel. This barrier forms during heating and subsequent hot rolling and impedes bonding between the core and the cladding in the final product, To overcome this problem efforts have been directed at reducing or preventing the formation of chrome oxides on the stainless steel pipe. One technique which has been used is aimed at limiting the original oxide/oxygen content within the pipe, before heating commences. Application PCT/GB94/00091 discloses a technique aimed at eliminating surface oxides in the swarf by passing the swarf through a direct-reduction type kiln similar to the kilns used in the production of direct-reduced sponge iron in the production of steel. The equipment and plant required for this process are costly.
In another technique described in application PCT/GB94/00091, the Boudouard equation is suppressed by taking steps aimed at ensuring that reducing gases are present in the billet throughout heating. These steps include the addition of additives to the swarf which generate reducing gases in the billet when it is heated. The additives should not leave behind significant quantities of solid deposits which would later appear as inclusions which would affect the quality of the final product. The additives proposed include urea and ammonium chloride.
To date, the two aforementioned techniques have generally been used together. However, despite the use of these techniques, some degree of oxidation has continued to occur. Although the final product is often generally acceptable for some purposes, the level of rejects due to the unpredictable degree of bonding between the core and the jacket during rolling remains unacceptably high from a commercial point of view. The rejects exhibit excessive spreading of the cladding during the hot rolling process. This severely hinders efficient rolling of the product by limiting the reduction per rolling pass to only light draughts. This limitation causes excessive cooling of the product which in turn reduces bond strength and limits the number of sizes and shapes which can be rolled. Unpredictable bonding between the core and the stainless steel may also be manifested by elongation of the core which, in some cases, can protrude from the centre of the billet. When this happens, further rolling is prevented and the billet must be scrapped. This problem has been addressed by welding short lengths of mild steel pipe (about 100 mm long) to each end of the stainless steel pipe (which generally has been about 200 cm long). The mild steel ends arc crimped closed prior to loading the billet in the furnace. These mild steel ends are thought to act in two ways.
The coefficient of expansion of stainless steel is greater than that of mild steel which causes the pipe to separate from the core due to differential expansion. There is no significant such separation between the mild steel end portions and the core. The mild steel ends thus form with the core a type of “plug” at each end of the billet. The compressed mild steel core, furthermore welds very easily to the mild steel pipe ends during initial rolling, thus preventing the escape of the core from the billet during rolling. The use of these mild steel ends is described in detail in international patent application #PCT/GB90/101437. It is not known how effective these plugs are in preventing the ingress of oxidising gases further into the billet as the core is initially still porous. Perhaps only the end portions of the stainless steel pipe are oxidised due to atmospheric oxygen which penetrates the end portions of the billet.
Another advantage of using the mild steel pipe ends is that they facilitate entry of the billet into the rolls, particularly in the first pass.
It is time consuming to cut and weld the mild steel pipe ends to the stainless steel pipes. Furthermore, good quality welds are required to prevent the welds from breaking during hot rolling which would in turn cause oxidation and, at times, scrapping of the billet.
In summary the disadvantages of the techniques described above include:
a costly reduction kiln required to pre-reduce the swarf;
a commercially unacceptable level of rejects due to unpredictable bonding during rolling;
limitations in the sizes and shapes that can be rolled with the billets;
the added cost of welding the mild steel ends onto the stainless steel pipes.
The unpredictability of the described oxidation prevention techniques is thought to be due to the sequence of events which occurs during heating up of the billet.
In the initial phase of heating both NH
4
Cl and urea generate considerable volumes of reducing gases in the temperature range from 200° C. up to about 500° C. These gases are expelled from the billets as flames which are visible in the furnace in this temperature range. These flames usually cease abruptly when all of the NH
4
Cl or urea has evol
Savage Jason
Zimmerman John J.
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