| Ferritic stainless steel mainly refers to the stainless steels with ferrite microstructure containing 11%～30% chrominum, and the crystal structure is body-centered. This kind of steel usually does not contain nickel, sometimes contains a small amount of Mo, Ti, Nb and other elements. Refining technology (AOD or VOD) made carbon, nitrogen and other interstitial element reduce greatly, so this kind of steel is widely used. In recent years, as the shortage of nickel resources in the world, austenitic stainless steel production has declined, and the ferritic stainless steel has been used in a large proportion because it does not contain nickel or only contain a small amount of nickel and low price, especially in the automotive field, therefore, researches and development of ferritic stainless steel will become the future trend.Ferritic stainless steel can be divided into low, medium and high ferritic stainless according to chromium content. The low chromium ferritic stainless steel refers to ferritic stainless steel containing 11%～15% chromium, is widely used in areas such as container, automobile exhaust system and gondola. Ferritic stainless steel is very prone to intergranular corrosion in using process with heat welding. Intergranular corrosion, is a corrosion form between the boundary of metal grain for internal expansion. After the occurrence of surface corrosion of metals and alloys, it remains metallic luster certain, without any signs of damage, but the binding force between grains decreased significantly, with mechanical properties deterioration, making metal could not withstand the percussion, so is the a very dangerous corrosion. At present, there are only standards of evaluating intergranular corrosion of medium and high chromium ferritic stainless steel, but for low chromium ferritic stainless steel, it does not exist.In this article, the key parameters of the existing sulphuric acid-copper sulfate method to evaluate high chromium ferritic stainless steel were adjusted to find out the optimum condition to evaluate low chromium ferritic stainless steel in sulfuric acid and copper sulfate method, and the following results has been got:(1) Studied 409L,410L, T4003 and 429 welding specimen by sulfuric acid copper sulfate immersion test, and got the best condition of 10%H2SO4+6%CuSO4+ copper, boiling 10h. Among them,429(14Cr) stainless steel, both base metal and heat affected zone and fusion zone, showed very good resistance to intergranular corrosion during the whole experimental, which determines the importance of Cr to improve the resistance to intergranular corrosion.410L stainless steel, whose Cr content is slightly higher than that of 409L and T4003, but C, N content twice more than 409L and T4003, and does not contain Ti, Nb stable element, exhibited very poor corrosion resistance to intergranular corrosion. Therefore, the ferritic stainless steel should strictly control the content of C and N, and add the stabilizing elements Ti, Nb and so on, to improve the ferritic stainless steel intergranular corrosion resistance.(2) Got the best condition of 0.5% H2SO4+6%CuSO4+copper (boiling 6h) immersion method to evaluate the intergranular corrosion resistance of 409L and 410L after heat treatment, and drew the preliminary TTS curve, and got the tip temperature of 409L and 410L is both around 600℃.(3) Tried to get the testing conditions of DL-EPR method to evaluate of the intergranular corrosion resistance of 409L, and the finally test conditions determined were:0.5% H2SO4+0.002%KSCN, under the scan rate of 1.667mV/s, and the solution temperature is 30℃, besides, got the most serious sensitized condition for 409L is 2h at 550℃, among the conditions of annealing temperature 550℃ for 1 h,2 h and 600℃ for 1h,2h. |
