Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1990-08-22
1992-07-14
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
420 79, 4205841, C22C 3806, C22C 3840
Patent
active
051300850
DESCRIPTION:
BRIEF SUMMARY
1. Field of the Invention
The present invention relates to a high Al austenitic heat-resistant steel having a superior resistance to oxidation at high temperatures and resistance to corrosion at high temperatures and further an excellent hot workability.
2. Background of the Relatevent Art
It Al is added in an alloy and an oxide film comprised mostly of Al.sub.2 O.sub.3 is formed on the surface in a high temperature oxidizing atmospher, extremely excellent oxidation resistance is displayed, it is known. For example, Fe-Cr-Al alloy steels are used as members for sintering equipment and other members exposed to atmospheres of up to 1200.degree. C. However, the above steels are basically low in strenght at the high temperature due to the ferrite phase and have therefore been limited in range of application as they could not be used at positions requiring strength at high temperatures.
On the other hand, Fe-Ni-Cr or Ni-Cr and other austenitic heat-resistant steels are superior in high temperature strength and mechanical properties at ordinary temperatures, so have been widely used as high temperature members, but these steels have Cr.sub.2 O.sub.3 formed on their surfaces at high temperatures and this film is used to maintain excellent oxidation resistance, so at 1000.degree. to 1100.degree. C. or more, where the film begins to varporize as CrO.sub.3, the oxidation resistance rapidly deteriorates. Further, the spalling resistance of the oxide film is also poor and in the case of continued heating or erosion, there is a large tendency of weight decrease of the material due to oxidation.
Numerous attempts have been made up to now to add Al to the above steels so as to improve the austenitic heat-resistant steels. However, if the amount of Al added is small, no Al.sub.2 O.sub.3 oxide film is formed on the alloy surface and the film which is formed is mainly composed of a spinel oxide film of Fe, Ni, and Cr. This oxide film is porous and relatively easily permeated by oxygen and nitrogen, so the speed of oxidation of the matrix just under the oxide film is high and further AlN precipitates below the same in a block form, so the Al is consumed and there is little effect of the addition of the same. To form a uniform Al.sub.2 O.sub.3 film on the surface of an austenitic alloy and bring out a superior oxidation resistance, it is necessary to added a minimum of 4.0 percent in terms of weight in the alloy. This is described, for example, in Japanese Examined Patent Publication (Kokoku) No. 55-43498, etc.
However, if Al is added in an austenitic steel, the hot workability rapidly deteriorates and serious cracking occurs during not rolling, hot forgoing, hot extrusion, and other working. Further, there are cases where working is impossible. This cracking occurs at the grain boundaries near the surface and propagates along the grain boundaries to develop into large cracks. This is because the Al is in solid solution in the austenite phase, so the integranular deformation resistance in the hot state significantly rises and the intergranular strenght falls relatively, to increase the susceptibility to cracking, and further the NiAl intermetallic compounds precipitate in the grains and at the grain boundaries during solidification or hot deformation, so the intergranular ductility falls.
To improve the hot workability of such an austenitic stainless steel containing a high concentration of Al, Jpanese Examined Patent Publication No. 55-43498 and Japanese Exampled Patent Publication No. 56-11302 discloses, based on the way of thinking of conventional conventional stainless steels, to precipitate some .delta.-ferrite in the austenite phase during solidification and to add La, Ce, and other rare earth elements so as to improve the hot workability, but high Al austenitic stainless steel, as mentioned above, is fundamentally much more susceptible to cracking under hot working compared with conventional stainless steel and with just the precipitation of .delta.-ferrite or addition of rare earth elements, sufficient hot workability
Tendo Masayuki
Tsuboi Harumi
Tsuchinaga Masamitsu
Yamanaka Mikio
Nippon Steel Corporation
Yee Deborah
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