Metal treatment – Stock – Ferrous
Patent
1993-03-09
1994-05-03
Yee, Deborah
Metal treatment
Stock
Ferrous
148329, 148619, C22C 3804, C21D 602
Patent
active
053084080
DESCRIPTION:
BRIEF SUMMARY
This invention concerns a high alloyed wear resisting manganese steel of Hadfield-type and its production method.
Hadfield steels have been known since the 1880's. They are used mainly as cast products e.g. as wear parts of stone crushers, excavator buckets and loader shovels. In these operating conditions the steel pieces are exposed to very strong impact and abrasive wear and to heavy impact stresses.
Hadfield steels are suitable for the types of wear conditions described above, because after the heat treatment their microstructure is austenitic and thus very ductile. In this condition the hardness is relatively low--approx. 200 . . . 250 BHN--and the wear resistance is not very good. The most important feature of the Hadfield steels is the strong work hardening ability as a result of impacts and pressure against the steel surface. The surface hardness of the steel can in such a case increase up to 550 BHN. This hardening is limited, however, into a thin surface layer of the steel whereas the inner part remains soft and ductile and the whole steel shows a ductile behaviour. The prerequisite for this kind of behaviour is that the microstructure of the steel is fully austenitic without continuous band of carbides at the grain boundaries. In the as cast condition all the grain boundaries in the microstructure are filled with brittle mixed carbides--mainly iron/manganese carbides and the whole behaviour of the steel is brittle. Under impacts and other mechanical stresses the steel breaks along the brittle grain boundaries. The grain boundary carbides can be eliminated by a solution heat treatment at temperatures of over 1000.degree. C. and by an immediate rapid cooling after the soaking, by a quenching. During the high temperature soaking the grain boundary carbides dissolve into the steel matrix and the rapid quenching prevents the reprecipitation of the carbides.
A fully austenitic, carbide-free, ductile Hadfield steel serves very well in the wear parts of traditional jaw and cone crushers and also in the front plates of buckets in quarry conditions under heavy impact loads. The crushers described above break the stones by impact and compression and also in the quarry loading the impact stresses are heavy. The crushing efficiency of the modern jaw and cone crushers has been raised by increasing the stroke length and by transforming the crushing by compression alone into a combined effect of compression and shear. In these types of crushing processes the formerly impact load has largely been replaced by an abrasive wear with a result that the impact loads against the wear parts have not been strong enough to cause the maximum work hardening of the Hadfield steel and the relative service life of the wear parts has shortened. The situation is the same in the excavator buckets and loader shovels when loading fine grain material, where the impact and compression loads are not always sufficient for the work hardening of Hadfield steels. The wear resistance of this kind of non-work hardened steel without any hard components in the microstructure has not proved to be sufficient in the operating conditions of the modern crushers nor in the loading of fine grain material.
Attempts have been made to improve the work hardening ability of the Hadfield steel whose original chemical composition is:
______________________________________ Carbon 1.0 . . . 1.4%
Manganese 10.0 . . . 15.0%
Silicon 0.3 . . . 1.5%
Phosphorus max 0,07%
Sulphur max 0.07%
______________________________________
molybdenum, vanadium and tungsten--have proved to have the best effect on
the work hardening ability. These alloying elements are also very strong
carbide formers and in addition to the improvement of work hardening
ability the carbide network at the grain boundaries is stabilized and
thickened--it is difficult to eliminate it by the heat treatment. These
grain boundary carbides improve the wear resistance of the steel in
abrasive wear--it is true, but as fully brittle components in the
microstructure they cause the brea
REFERENCES:
patent: 4394168 (1983-07-01), Hartvig et al.
Lokomo Oy
Yee Deborah
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