Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1995-08-14
1997-10-14
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148513, 294032, 428685, 428548, B22F 702, B32B 1516
Patent
active
056767757
DESCRIPTION:
BRIEF SUMMARY
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 mining, boring, shaping and milling operations on engineering steels. The off cuts from some stamping and punching operations may also be suitable. The term "engineering steel" is intended to describe those low alloy steels which am 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
A number of proposals have been made to rework scrap steel swarf into a useful product without remelt. Of these the only process known to the applicant which has been commercially worked is that disclosed in British patent #1313545. The commercial working of this process was carried out by the present applicants' predecessors in title. In this process steel swarf which has been cleaned and chipped is compacted and enclosed in a steel jacket to form a billet. The billet is heated to about 1200.degree. C. prior to being rolled in a substantially conventional rolling mill. The heating causes surface oxides on the swarf to be reduced by a reducing agent derived from carbon which is either premixed with the swarf or, more commonly as has turned out in practice, is evolved from decarburisation of the swarf. The onset of the reduction process is evinced by the presence of a blue flame at an orifice provided in the jacket to allow gases evolved inside the jacket to escape. When reduction is complete the blue flame disappears. In the rolling operation the chips weld together to form a substantially homogeneous mass.
The process is practically successful owing at least partially to the presence of the jacket which prevents atmospheric oxygen from getting to the swarf when it is hot. There have been other prior proposals in which a jacket is not used. In practice, no way has been found to prevent oxidation of a hot unjacketed billet while it is being worked.
One of the products of the process described in the aforementioned British patent #1313545 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 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 hoped and it is an object of the present invention to address 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 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 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 clear that the green layer is an indication of the formation of chrome oxides derived from the chromium in the stainless steel. It was originally thought that the chrome oxides were forming
REFERENCES:
patent: 3774289 (1973-11-01), Cacace et al.
Cacace Antonino Giorgio
Clark Allan Douglas
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