Research On CVD Aluminizing Equipment And Technology

The turbine inlet temperature is constantly increased for advanced gas turbines inpursuit of higher "Ratio of Thrust and Mass". The working condition of gas turbineblades becomes more and more strict, and the performance requirement of turbine bladematerials is higher and higher. The hollow blade achieves the effect of air cooled to acertain extent,but it also makes the temperature in the hollow higher and higher, and thecorrosion on the hollow surface more and more serious. Therefore, it is particularlyimportant to choose a suitable coating and process for blade cooling channel protection.There are generally pack cementation aluminizing, gas phase aluminizing (VPA) andchemical vapor deposition (CVD) aluminizing in the blade channel protectiontechnology. However, it exists sintering, cleaning and blocking problems of powder inpack cementation.Powder is also used to pollute environmental and it is not easy tocontrol VPA process. HCl gas is used as the activator, without sintering, cleaning andblocking problems of powder in CVD process. CVD coating has the best oxidation/hotcorrosion resistance.Firstly, this topic studied the aluminizing agent and aluminizing reactionmechanism in CVD aluminizing system by means of thermodynamics with the help ofcomputer software HSC-chemistry5. Al and HCl gas reacted to formAlCl3(g) mainly，with a small amount of AlCl2(g) and AlCl(g).AlCl(g)cannot be formed still more than520K.With temperature increasing and pressure reducing, the content of AlCl2(g)andAlCl(g) were increased. Only AlCl2(g)and AlCl(g) could release Al on substrate surface,but AlCl3(g)is hard to react.Secondly, this topic achieved the CVD aluminizing equipment and process design，based on the knowledge of aluminized agent and aluminized reaction mechanism. CVDaluminizing equipment was designed into double chamber structures, AlClxgas furnaceand aluminizing furnace, with theN2-HCl-H2gas supply system.CVD aluminizingsystem contained reaction system, control system and exhaust gas treatmentsystem.AlClx(g)formation reaction and aluminizing reaction could happen in reactionsystem. Control system was divided into three sections—temperature, pressure andflow control. Exhaust gas treatment system adopts pump, cooling, dust and exhaust gastreatment. CVD aluminizing furnace consisted of reaction tank, heating element,insulation and sealing system. Reaction tank was made from high nickel-chromium alloys.The heating power was equal to30kw.300mm thick aluminum silicate fiber wasadopted to manufacture into thermal insulation body.Finally, this topic finished4h CVD aluminizing process for Inconel718and AISI304at1000℃and an oxidation test for aluminized samples at1100℃. Aluminidecoatings on the surface of two kinds of materials,were all divided into three layerstructures (outer layer, middle layer and diffusion layer).The coating thickness was117.8μm and560.2μm, respectively.Al content on the outer layer of the two aluminidecoatings were all high.Al content was46.84at.%,32at.%. After high temperatureoxidation at1100℃, Inconel718and AISI304aluminized samples had obviously betterhigh temperature oxidation resistance than matrix (un-aluminized samples).The hightemperature oxidation resistance of AISI304aluminized samples was superior to that ofInconel718un-aluminized samples. This made it possible to use AISI304stainless steelaluminized to take the place of high-grade stainless steels or nickel base alloys.