A contribution to the understanding of the metallic coating of iron base alloys
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THE
wetting of s t e e l surfaces by liquid metals is necessary to obtain quality coated products such as aluminum coated steel, Type 1 and Type 2, and zinc coated steel. The literature1 and various patents (U.S. Patents No. 2,110,893 and No. 3,320,085) teach how excellent adherence may be consistently achieved with the continuous A1 or Zn coating of low carbon s t e e l products. By contrast, it has been observed that e r r a t i c coating adherence problems are often experienced when coating low carbon steels which are alloyed with even s m a l l amounts of strong oxideforming elements. A good example is the poor adherence of metallic coatings on alloy steels with higher A1 or Si contents which are coated under what are normally considered optimum coating conditions. A r m c o Steel has recently developed an alloy s t e e l (referred t o in this paper as 2SR) which, when a l u m i nized, possesses oxidation resistance superior to T409 stainless s t e e l (11 wt pct C r - 1 / 2 wt pct Ti). This alloy, which is covered under U.S. Patents No. 3,905,780 and No. 3,909,250, has a nominal composition of 2 wt pct Cr, 2 wt pct A1, 1 wt pct Si, and 1/2 wt pct Ti. In the e a r l y attempts t o aluminize this alloy in the laboratory, complete lack of adherence was experienced when n o r m a l coating practices were followed. T h e s e experiments were conducted on a 4 in. (0.1 m) wide continuous coating line. F i g u r e 1 is a schematic illustration of this line and the a p proximate strip temperatures for the experiments which will be described in a l a t e r section. The heating portion of this line is composed of two b a s i c sections: i) an initial high intensity, gas f i r e d preheater; and ii) a reducing, radiation heated furnace containing a N2-H2 gas mixture. Conventional coating p r a c JERRYL. ARNOLD, Senior Research Metallurgist, F. CURTIS DUNBAR, Senior Research Engineer, and CHARLES FLINCHUM, Engineer, are with ArmcoSteel Corporation, Research Center, Middletown, OH45043. Manuscript submitted November 23, 1976. METALLURGICAL TRANSACTIONS B
t i c e s call for keeping b o t h of these sections as c l e a n and reducing as possible such that a f r e s h metallic iron surface will always be produced on low carbon steel. This metallic iron surface appears t o be i d e a l for optimum wetting by the liquid m e t a l and excellent coating adherence is reliably obtained with low carbon steels. It was subsequently discovered (U.S. Patent No. 3,925,579) that excellent wetting of this new alloy s t e e l surface by liquid AI could also be consistently obtained if the gas f i r e d preheater were operated w i t h controlled amounts of e x c e s s air ("oxidizing preh e a t e r " ) as v e r s u s the n o r m a l operation with e x c e s s combustibles. This paper proposes a model explaining these observed phenomena and presents supplemental experimental data supporting i t . VARIABLE METALLIC COATING ADHERENCE DURING NORMAL OPERATION OF CONTINUOUS COATING LINE A. Model The model which is proposed to explain the v a r i
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