CORROSION STUDY ON WELDMENTS OF AUSTENITIC STAINLESS STEEL

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1 897 CORROSION STUDY ON WELDMENTS OF AUSTENITIC STAINLESS STEEL V. Gopalakrishnan 1, V.BaluSamy 2, S.O. Mohammed Rafi 3 1 Associate Professor, Department of Mechanical Engineering, Dhanalakshmi Srinivasan engineering college, Perambalur 2 Associate Professor, Department of Mechanical Engineering, PSG college of Technology, Coimbatore 3 Student, Department of Mechanical Engineering, Dhanalakshmi Srinivasan engineering college, Perambalur shaikhrafi100@gmail.com ABSTRACT 304 (Fe-Cr-Mn-Ni) stainless steel is the most widely used for general corrosive resistant tubing and pipe applications in boiler; it is used in chemical plants, refineries, paper mills, and food processing industries. Stress corrosion cracking (SCC) is a common mode of failure encountered in boiler components especially in austenitic stainless steel tubes at high temperature and in chloride-rich water environment. Welding in stainless steel forms the corrosion due to carbide precipitation in heat affected zone. The SCC behaviour of 304 stainless steel in welding joint has been widely described in this work. Keywords- 304 stainless steel, Stress corrosion cracking, carbide precipitation, chloride-rich water. I. I N T R O D U C T I O N Stainless steels or, more precisely, corrosion-resisting steels are a family of iron-base alloys having excellent resistance to corrosion. These steels do not rust and strongly resist attack by a great many liquids, gases, and chemicals. Many of the stainless steels have good low-temperature toughness and ductility. Most of them exhibit good strength properties and resistance to scaling at high temperatures. All stainless steels contain iron as the main element and chromium in amounts ranging from about 11% to 30%. Chromium provides the basic corrosion resistance to stainless steels. Manganese steels are not harden able by heat treatment and are nonmagnetic in the annealed condition. They may become slightly magnetic when cold worked or welded. This helps to identify this class of stainless steels. All of the austenitic stainless steels are weld able with most of the welding processes, with the exception of Type 303, which contains high sulphur and Type 303Se, which contains selenium to improve much inability. The austenitic stainless steels have about 45% higher thermal coefficient of expansion, higher electrical resistance, and lower thermal conductivity than mildcarbon steels. High travel speed welding is recommended, which will reduce heat input and carbide precipitation, and minimize distortion. The melting point of austenitic stainless steels is slightly lower than melting point of mild-carbon steel. Because of lower melting temperature and lower thermal conductivity, welding current is usually lower. The higher thermal expansion dictates that special precautions should be taken with regard to warping and distortion. Tack welds should be twice as often as normal. Any of the distortion reducing techniques such as back-step welding, skip welding, and wandering sequence should be used. On thin materials it is very difficult to completely avoid buckling and distortion. Stainless Steel (type 304) is one of three materials taken for the experiment. Type 304 stainless is the most widely used analysis for general corrosive resistant tubing and pipe applications; it is used in chemical plants, refineries, paper mills, and food processing industries. Type 304 has a maximum carbon content of.08%. It is not recommended for use in the temperature range between 800 F and 1650 F due to carbide precipitation at the grain boundaries which can result in inter-granular corrosion and early failure under certain conditions. Stainless steel 304 type has very has very low thermal conductivity in the order of 16 w/m.k. Welding of Stainless Steel Due to the low thermal conductivity, the heat transfer rate is also very low, which will results low tensile strength. Experiments are conducted to increase the strength the welding process selected for stainless is shield metal ARC welding and TIG welding. Austenitic stainless steels (which contain 18% Cr 8% Ni) are engineering materials widely used in many branches of industry, especially in the food and beverage manufacturing and processing sector, due to their attractive combination of good mechanical properties, formability, and corrosion resistance. Their corrosion resistance is afforded by a thin Cr 2 O 3 surface film (typically 1 3 nm thick), known as passive film, which has self-healing capability in a wide variety of environments. However, when austenitic stainless steels are exposed to the critical temperature range of _C for a given period of time, chromium (Cr) carbides are formed at the grain boundaries and Cr depletion occurs adjacent to these carbides, affecting their corrosion resistance.corrosion occur mainly due to dust inclusion, moisture present in the gas and heat affected in the weld zone.

2 898 STRESS CORROSION CRACKING: Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected sudden failure of normally ductile metals subjected to a tensile stress, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure. I I. M A T E R I A L A N D M E T H O D O L O G Y MATERIAL FORMATION: 304 Austenitic stainless steel is the family of iron based alloys having excellent resistance to corrosion. It is taken in the form plate and it is formed to the standard dimension for testing. Without welding two specimen are formed. With weld two specimen are formed. F i g. 1 S t a n d a r d d i m e n s i o n o f s p e c i m e n Fig. 2. Dimensionof weldspecimen F i g. 3. A f t e r m a c h i n i n g Fig.1 represents the dimensions of standard specimen without welding and the specimen which are to be welded are as shown in fig.2, the two specimen pieces are welded to form the length of standard specimen, the welding is carried out with the help of TIG welding with standard settings, the final view of the test specimen after carrying out the machining as per the dimension shown in fig. 3 are as follows F i g. 4 f i n a l v i e w o f t e s t s p e c i m e n C O R R O S I O N A N A L Y S I S : Corrosion is the gradual destruction of materials by chemical reaction with its environment. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. Corrosion can also occur in materials other than metals, such as ceramics or polymer. H I G H T E M P E R A T U R E C O R R O S I O N : High-temperature corrosion is chemical deterioration of a material (typically a metal) as a result of heating. This non-galvanic form of corrosion can occur when a metal is subjected to a hot atmosphere containing oxygen, sulphur or other compounds capable of oxidizing (or assisting the oxidation of) the material concerned. C O R R O S I O N I N S T A I N L E S S S T E E L : Stainless steel can pose special Corrosion challenges, since its passivation behaviour relies on the presence of a major alloying component (chromium, at least 11.5%). Because of the elevated temperatures of welding and heat treatment, chromium carbide can form in the grain boundary of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries in the heat affected zones) in highly corrosive environments. A stainless steel is said to be sensitized if chromium carbides are formed in the microstructure. A typical

3 899 microstructure of a normalized type-304 stainless steel shows no signs of sensitization while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries. loss of base metal and weld metal graphically represented and which as shown below. WEIGHT LOSS MEOD: Preparation of corrosion media: 1. NaCl- 3.5grams+96.5ml distilled H2O 2. Oxalic acid- 5grams+95ml distilled H2O 3.Oxalic acid -10grams+90ml distilledh2o 4. Nitric acid -5grams+95ml distilled H2O 5.Nitric acid-10grams+90ml distilledh2o Fig.5. Corrosion media Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected sudden failure of normally ductile metals subjected to a tensile stress, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. Fig.6. NaCl - 3.5grams+96.5ml distilled water III. RESULTS AND TABLES Five sample of base metal weight is note down and similarly five sample of weld metal weight is note down. Sample is kept in corrosion medium under observation at normal laboratory condition for 30 days. Weight of each specimen after taken from the corrosion medium is noted down. Finally the weight

4 900 T a b l e : 1 W e i g h t l o s s c o m p a r i s o n T a b l e : 3 T e n s i l e v a l u e o f w e l d m e t a l S r. N o. C o r r o s i o n m e d i a I n i t i a l w e i g h t ( g m s. ) F i n a l w e i g h t ( g m s. ) 1 N a C l MATERIA L/ CONDITI ON YIELD TENSILE % OF ELONG ATION 2 O x a l i c a c i d 5% 3 O x a l i c a c i d 10% 4 N i t r i c a c i d 5% STAINLE SS STEEL STAINLE SS STEEL30 4Withoutc orrosionm N i t r i c a c i d 10% MATERIA L/CONDITI ON YIELD T E N S I L E T E S T : The specimen is machined to Indian standard for transverse tensile in universal testing machines. The specimen is tested and the ultimate tensile strength is observed. The values of UTS are listed in the table. Material Specification: SS 304 Welding Process: TIG Welding Description: Butt Joint T a b l e : 2 T e n s i l e v a l u e o f b a s e m e t a l TENSILE %OF ELONGAT ION H A R D N E S S T E S T : Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behaviour of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness. Hardness is dependent on ductility, elastic, stiffness, plasticity, strain, strength, toughness, viscoelasticity and viscosity. T a b l e : 4 H a r d n e s s t e s t S r. n o. m a t e r i a l A v e r a g e v a l u e o f H R B 1 B a s e m e t a l 90.3 STAINLES S STEEL304 with corrosion media Base STAINLES S STEEL304 Withoutcor rosionmedi a base plate W e l d m e t a l 87.7 O P T I C A L M I C R O S C O P E T E S T : E m b e d d i n g, materials after embedding in resin, the specimen is usually ground and polished to a mirrorlike finish using ultra-fine abrasives. The polishing process must be performed carefully to minimize scratches and other polishing artefacts that reduce image quality. Sectioning produces thin slices of specimen, semitransparent to electrons. These can be cut on diamond knife to produce ultra-thin slices about nm

5 901 thick. Disposable glass knives are also used because they can be made in the lab and are much cheaper. Staining uses heavy metals such as lead, uranium or tungsten to scatter imaging electrons and thus give contrast between different structures, since many (especially biological) materials are nearly "transparent" to electrons. OPTICAL MICROSCOPE RESULT: strength, hardness, ductility and other physical properties. ACKNOWLEDGEMENT We give all the glory and thanks to our ALMIGHTY GOD for showering upon, the necessary wisdom and grace for accomplishing this work. We express our gratitude and thank our PARENTS first for giving health and sound mind for completing this work. We extend our gratitude to all our friends and family member who have helped physically and morally for the development of this work. REFERENCES [1].N.B.Mostafa, M.N.Khajavi, Optimization of welding parameters for weld penetration in FCAW Journal of achievements in materials and manufacturing Engineering, Volume16, issue 1-2 MAY-june Fig.7. Before etching [2] The effect of annealing on properties of AISI 316L base and weld metals Stjepan Kožuh1, Mirko Gojić1, Ladislav Kosec2, RMZ Materials and Geoenvironment, Vol. 54, No. 3, pp , [3] Emerging Markets and Trends in the use of Stainless Steels, Nickel-Based Alloys, and Titanium presented at the Stainless Steel world 2007 conference in Maastricht, The Netherlands, November7, world.net. Fig.8. After etching IV. PROBLEM IDENTIFICATION: After testing we are identified some problems in weld metal compare to the base metal. Problems are Corrosion is occur in both metal because of chlorine rich environment and high temperature. On comparison weld metal having little bit corrosion than the base metal. Compare to the base metal the weight of weld metal, strength and hardness also get decreases.stress corrosion cracking is occurred in the heat affected zone in weld metal. [4] Leif karisson, ESAB AB, Gothenburg, Stainless steels past present and future,svetsaren no [5] Welding of Stainless Steel Technical and Trade information-cigweld. V. CONCLUSION In boiler manufacturing industries high capacity boiler 800 mw and 1000 mw are under design and in the boiler the pressure and temperature is more so that water to steam and steam to water through the pipe for this purpose, In austenite stainless steel pipe which is welded. Because of welding stress corrosion cracking is occur which results reduce in material property like