| The power industry has been trying to increase steam operating temperatures to enable steam turbine power plants to operate at higher thermal efficiency.The high-parameter ultra-supercritical unit is put into use,and it also puts forward higher requirements for the high-temperature oxidation resistance of the steel for boiler steam pipes.Most of the high-temperature heat-resistant steels used in boiler pipes today rely on foreign imports and are more expensive.Aluminum and aluminide have better resistance to steam oxidation and low cost.Therefore,in order to prolong the service life of steam pipeline,it is a more economical way to prepare aluminide coating on the steam side of boiler pipeline.9Cr-3W-3Co steel is the latest research for boiler steel with higher parameters above 600°C,which can be applied to the manufacture of various boiler components.In this paper,9Cr-3W-3Co steel was used as the substrate,and the preparation of iron-aluminum coating was carried out by pack cementation method.The preparation of the ductile FeAl coating with a certain thickness of the coating is achieved by adjusting the process parameters,such as the aluminizing temperature,the aluminizing time,the Al content in the pack,and the addition amount of the activator in the pack.The results show that the aluminizing temperature has the most obvious influence on the structure and the quality of aluminized coating.In the temperature range of 650-750°C,the thickness of the aluminized coating and the thickness of the outer Al-rich layer increase with increasing temperature.The aluminized coating obtained by aluminizing at 650°C is compact and no defects are found;the aluminized coating obtained by aluminizing at700°C has only discontinuous voids in the outer layer of the coating;the aluminized coating obtained by aluminizing at 750°C has more small voids near the substrate,and there are cracks penetrating the aluminized coating,so the density of the coating is poor.The aluminizing time and the aluminum content in the pack only affect the thickness of the coating.The prolongation of the aluminizing time or the increase of the Al content in the pack will increase the thickness of the coating;and the change of the activator content in the pack will not cause the change of coating structure and thickness,but it will affect the surface quality of the coating.In the high temperature steam environment,the quality of the surface of the aluminized coating determines the service life of the coating.In the 650-750℃aluminized temperature range,the relationship between the aluminized temperature(T),the aluminized time(t)and the Al content(W)in the pack and the thickness of the coating(h)was as follows:In this experiment,the steam oxidation resistance test of 9Cr-3W-3Co steel and T91 with or without coating was carried out in a steam environment of 660°C.The test results show that the uncoated 9Cr-3W-3Co steel oxide layer stores a higher content of Cr and has better oxidation resistance than that of the oxidized uncoated T91 sample.The coating of 9Cr-3W-3Co steel prepared by aluminizing temperature of 650°C slows the internal diffusion of Al atoms due to the presence of a large amount of W-Cr-Cu particles in the matrix.Compared with the T91 coating which is aluminized at 650°C,the 9Cr-3W-3Co steel coating has better oxidation resistance and thermal stability.The aluminized coating obtained on the surface of 9Cr-3W-3Co steel with aluminizing temperature of 620°C is thinner,but the surface of the coating is denser.After 3000 h of steam oxidation,no obvious oxide was formed in the thin aluminized coating.According to the oxidation kinetics of the samples,a distinct layer of alumina begins to form on the outer layer of the coating after 5250 h of oxidation.After the oxidation of steam for 3000 h,the Al content of the coating is about 8 at.%.With the prolongation of oxidation time,the Al content of the coating remains stable. |
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