Investigation On The Effect Of QPQ Technology On Mechanic Property And Corrosion Resistance Of The Materials

QPQ technology contains mainly salt bath nitriding or salt bath nitrocarburising, salt bath oxidizing,polishing and second salt bath oxidizing.It has been used widely in automobile and die industries which demand good wear resistance.In recent years, because QPQ treated workpieces combine good wear resistance and high corrosion resistance,QPQ technology attracts attention of many other industries,such as piston pole and petroleum.In piston pole and other similar industries,chromeplating technology was used in the past.Now QPQ technology is widely used in these industries for environment protecting problem.In these industries,high corrosion resistance and high wear resistance are all needed.It provides a good stage for QPQ technology.Oil tubes were placed in atmosphere directly before they were used in oil well.When they were placed in oil well,the surface had rusted already.It will deteriorate the corrosion resistance.In this paper,for the above two problems in practice,two experiment methods were adopted to evaluate the corrosion resistance in atmosphere environment.The first method is immersion test.The immersion solution is 3‰H₂O₂+10%NaCl water solution by weight.Before and after the immersion test,the weight of specimens was quantified by electrical scale accurate to 0.0001 gram.The balance of them is the corrosion weight loss.The second method is neutral salt spray test.The solution used in this test is 5%NaCl water solution and the test temperature is 35℃.In the test,theΦ20×100mm cylinder specimens were placed 30°from the vertical direction.For the best corrosion resistance,an appropriate technics was attained by analyzing the relations of processing and the corrosion resistance.Firstly,wash and activate the workpieces during pretreatment.The second is to limit the roughness to 0.18μm.The third is to adjust the salt in the furnace and ensure the nitrided workpieces have compact compound layer to enhance the rust starting time in salt spray test.The fourth is to oxidize the workpieces after nitriding.The aim is to develope Fe₄O₃ oxidizing film to increase the corrosion resistance.The fifth is polishing and second oxidizing.During polishing,the porous compound layer should be eliminated completely.The sixth is immerse the workpieces in sealant to compensate the decreasing of corrosion resistance for the breakage of oxidizing film.After the sealant becomes dry,immerse the workpieces in antirust oil.At present,many oil fields in China come into middle-high hydrous exploitation period.With the increasing of water content,the corrosion of transportation system of oil,gas and water becomes more badly.Corrosion is the main cause of dependability and invalidation of oil tube.When oil tube is in use,it would endure corrosion and bear the stress.In this paper,considering circumstance of oil tube in oil well,autoclave test was used to evaluate the corrosion resistance of N80 steel in CO₂ solution.In the test,the solution is simulated oil field water.Before CO₂ was filled to scheduled pressure,oxygen was eliminated by pure nitrogen.To evaluate the corrosion resistance of oil tube steel in H₂S solution,constant force tensile test was adopted.Through the compare of untreated specimens and QPQ treated specimens,the corrosion resistance of oil tube steel in H₂S solution was evaluated.We manufacture specimens according to API 5CT standard.Though the tensile tests and impact tests,the tensile strength,yield strength,elongation and impact energy were attained.We analyzed the relations of QPQ processing parameter and mechanical properties too.Through the above tests,we attained the QPQ technics to treat N80 oil tube to meet the demands of high corrosion resistance and mechanical property at the same time.It demands the nitriding temperature should be above 650℃and nitriding duration should be about 90 minutes.After QPQ treatment,the workpieces should be immersed in sealant and antirust oil.