ABSTRACT
The resistance and susceptibility of austenitic stainless steel (ASS) type 304, exposed to sodium hydroxide (NaOH) and hydrochloric acid (HCl) media (0.5M concentrations) at ambient temperatures was investigated using design expert software 6.0.6 and scanning electron microscope (SEM). The austenitic stainless steel flat bar of 3mm thickness was cut into length of 100mm; they were further cut into two equal parts with the two faces plain to have a 90° angle, leaving a root and face gap of 2mm. The flat bars were then joined together using gas metal arc welding (GMAW) process using stainless steel electrode wire of 0.9mm diameter (G 19.9 L). After welding, some specimens were immersed in the two media (sodium hydroxide and hydrochloric acid) for forty (40) days in an interval of eight (8) days and some samples served as standards for comparison and analysis. Tensile, Izod impact and hardness tests were carried out. The results of the studies show that welding parameters and corrosion really affect the mechanical properties of the alloy, the control strength (without welding) was 225MN/m2 while that of the welded without immersion (C4) was 133MN/m2. The control impact energy (without welding) was 11.5J, while that of the welded without immersion (C3) was 21.7J. Also, the welded control sample without immersion (C3) for hardness tests were FZ = 30.2HRA and HAZ = 35.1HRA. The design expert was used to determine the surface responses and interactions between the parameters while SEM was used to examine the test specimen‘s surface morphology after immersion in the corrosive media. It was found that increase in welding current and speed at constant voltage gave the optimum performance of the ASS weldment in NaOH and HCl environments obtained at speed of 30mm/sec to 40mm/sec and current of 100A to 110A. This shows a corresponding minimal material deterioration. Surface corrosion deposit composition was analyzed with the SEM paired with energy dispersive spectrometer (EDS). Mechanical destructive tests (hardness, impact and tensile tests) were also used to examine the materials optimum performance in sodium hydroxide and hydrochloric acid media and it was found that hardness, impact and tensile strength increased with increasing weld parameters. It is concluded from the research that relatively high speed and current at a constant voltage gives a satisfactory weldment with a better integrity. This research work showed the observed susceptibility of ASS type 304 to stress corrosion cracking and the aggressiveness of chloride ion (Cl-) in the corrosive medium.
TABLE OF CONTENTS
Title Page ii Declaration iii Certification iv Dedication v Acknowledgement vi Table of Contents viii List of Figures xiv List of Tables xix List of Plates xx List of Appendices xxi List of Abbreviations and Symbols xxiii Abstract xxvii CHAPTER ONE 1 1.0 INTRODUCTION 1 1.1 Background 1 1.2 Statement of the Problem 4 1.3 Present Research 5 1.4 Aim and Objectives 5 1.5 Significance of the Study 6 1.6 Scope of the Research 6 CHAPTER TWO 8 2.0 LITERATURE REVIEW 8 2.1 Introduction to Stainless Steel 8 2.2 Stainless Steels Classification 9
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2.2.1 Ferritic stainless steels 9 2.2.2 Martensitic stainless steels 10 2.2.3 Austenitic stainless steels 11 2.2.4 Duplex stainless steels 12 2.2.5 Precipitation hardening stainless steels 13 2.2.6 Super alloy of stainless steels 14 2.3 Microstructure of Stainless Steel 14 2.3.1 Physical properties of stainless steels 15 2.3.2 Mechanical properties of stainless steels 16 2.3.3 Corrosion resistance of stainless steel 17 2.4 Welding and Characteristics of Welding Process 19 2.5 Welding Methods for Austenitic Stainless Steels 26 2.6 Welding Processes 27 2.6.1 Gas tungsten arc welding process 27 2.6.2 Gas metal arc welding 28 2.6.3 Shielded metal arc welding 30 2.6.4 Selecting shielding gas for welding of stainless steels 32 2.7 Welding Parameters 33 2.7.1 Welding current 33 2.7.2 Welding speed 34 2.7.3 Welding voltage 34 2.7.4 Electrode size 34 2.7.5 Electrode work angle 34 2.7.6 Polarity 35 2.7.7 Electrode stick-out and melting rate 35 2.8. Metallurgical Aspects of Welding 35
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2.8.1 Welding metallurgy 35 2.8.2 Heat affected zone 37 2.8.3 Weld fusion zone 38 2.9 Welding procedure 38 2.10 Weldability 40 2.11 Filler metal 41 2.12 Heat input 41 2.13 Preheat and multi-pass 42 2.14 Factorial Design 43 2.14.1 Factorial design of experiment in process optimization 43 2.15 Review of Related Works 44 CHAPTER THREE 52 3.0 MATERIALS, EQUIPMENT AND METHOD 52 3.1 Introduction 52 3.2 Materials 52 3.3 Equipment 52 3.4 Methodology 53 3.4.1 Pre-welding sample preparation 54 3.4.2 Welding procedure 55 3.5 Welding Parameters Analysis 55 3.5.1 The welding speed 55 3.5.2 The welding current 55 3.5.3 The welding voltage 55 3.5.4 Full factorial design of experiment for the welding parameters ASS 55 3.5.5 Preparation of stock solution 57
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3.5.6 Weight loss measurement 58 3.6 Mechanical (Destructive) Test Procedure 58 3.6.1 Tensile test 58 3.6.2 Impact Test 60 3.6.3 Hardness Test 61 3.7 Metallography of Austenitic Stainless Steel 62
3.7.1. Scanning Electron Microscopy of austenitic stainless steel 62
3.8 Sample Labeling 63 CHAPTER FOUR 64 4.0 RESULTS 64 4.1 Introduction 64 4.2 Chemical Composition of Research Material 64 4.3 Results of Factorial Design 65 4.3.1 Interactions between two welding variables 68 4.3.2 Interaction between two welding variables and a constant 76 4.4 The Mechanical Properties of the Samples 82 4.4.1 Hardness values of the samples 82 4.4.2 Tensile properties of the samples 87 4.4.3 Impact properties of the samples 97 4.5 Scanning Electron Photo-Micrograph of the Samples 102 CHAPTER FIVE 108 5.0 DISCUSSION OF RESULTS 108 5.1 Elemental Analysis of Austenitic Stainless Steel 108 5.2 Responses and ANOVA for the Factorial Model for Speed and Current at a Constant Voltage in NaOH Environment 108 5.3 Responses and ANOVA for the Factorial Model for Speed and
Current at a Constant Voltage for HCl Medium 110
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5.4 Interaction between Two Welding Variables and the Effect on ASS Immersed in Corrosive Media 112 5.4.1 Effects of the interaction of two welding variables on the ASS immersed in NaOH medium 112 5.4.2 Effects of the interaction of two welding variables on the ASS immersed in HClmedium 113 5.5 Interaction between Three Welding Variables and the Effect on ASS Immersed in Corrosive Media 114 5.5.1 Effects of the interaction of three welding variables on ASS immersed in NaOH medium 114 5.5.2 Effects of the interaction of three welding variables on ASS immersed in HClmedium 115 5.6 Comparative Effects of Sodium Hydroxide and Hydrochloric Acid Media on ASS 116 5.7 Hardness Properties of the Samples 117 5.8 Tensile Properties of the Samples 118 5.9 Impact Properties of the Samples 119 5.10 Correlation of Hardness, Tensile and Impact Strength Tests 120 5.11 Discussion onScanning Electron Micrograph of Samples 120 CHAPTER SIX 124 6.0 CONCLUSIONS AND RECOMMENDATIONS 124 6.1 Conclusions 124 6.2 Recommendations 125 6.3 Contribution to Knowledge 125 REFERENCES 127 APPENDICES 135
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CHAPTER ONE
1.0 INTRODUCTION 1.1 Background The importance of austenitic stainless steel (ASS) in industrial applications and development cannot be over-emphasized. Its excellent properties which range from high tensile strength, good impact strength, corrosion and wear resistances have found various applications in many engineering industries today. In addition, ASS sheets have gained wide acceptance in the fabrication of components, requiring high temperature resistance and corrosion resistance such as metal bellows used in expansion joints in aircraft, aerospace and petroleum industries. This material is used in almost all environments that require an optimization of these properties, some of which are low and high pressure boilers and vessels, fossil-fired power plant, flue gas desulphurization equipment, evaporator tubing, super heater reheating tubing and steam heaters and pipes to mention but a few (Streicher, 1977; Munoz et al., 2004;Galal et al., 2005 and Kondapalli et al., 2013). The austenitic stainless steel has been dominating the manufacturing and metallurgical field since the time of its first commercial production of stainless steels. Superior properties combined with the comparable ease of production and fabrications due to their excellent weldability, make this steel a most favoured one (Sathiya et al., 2012).
Austenitic stainless steels have many advantages from a metallurgical point of view. They can be made soft enough (i.e., with a yield strength of about 200 MPa) to be easily formed by the same tools that work with carbon steel, but they can also be made
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incredibly strong by cold work, up to yield strengths of over 2000 MPa (290 ksi)(www.asminternational.org). Their austenitic (face centered cubic-fcc) structure is very tough and ductile. They also do not lose their strength at elevated temperatures as rapidly as ferritic (body centered cubic-bcc) iron base alloys. The least corrosion-resistant versions can withstand the normal corrosive attack of the everyday environment that people experience, while the most corrosion-resistant grades can even withstand boiling seawater (www.asminternational.org). This steel (ASS), has a nickel content of at least 7%, which makes the steel structure fully austenitic and gives it ductility, a large scale of service temperature, non-magnetic properties and good weldability (Suresh et al., 2011). Austenitic stainless steels are among the most widely used types of stainless steel. The most commonly used grades are the American Iron and Steel Institute (AISI) 300 series of alloys (Sedriks, 1996). Starting from the basic 304 alloy (Fe-19Cr-10Ni), Mo is added to improve resistance to pitting (2-3 wt. % in the case of type 316 and 3-4 wt. % in type 317). Sensitization due to Chromium depletion during welding and other heat treatments, and the possible resultant intergranular corrosion, can be avoided through the use of low-carbon grades (304L, 316L, 317L, in which C is limited to 0.03 wt. % max.). The addition of Cr also imparts greater oxidation resistance, whilst Ni improves the ductility and workability of the material at room temperature (Fraser, 2009).
Austenitic stainless steels offer excellent resistance to corrosion. These high chromium steels are ductile and strong. They are non-magnetic and can be readily formed and welded. Higher strengths can be obtained by cold working, although this makes the alloy slightly magnetic and may reduce its corrosion resistance(www.asminternational.org).
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Because of its inherent corrosion resistance, austenitic stainless steels, known as 300 series (AISI standard), have become cost-effective, staple materials for long-term applications in many industrial sectors including gas, petroleum, petrochemicals, fertilizers, food processing, and pulp industries as well as power generating plants. They have found also widespread use for the manufacturing of chemical installations including stationary pressure tanks and tanks for transport of liquid and compressed gases, pipelines of high diameter in hydraulic power plants, for manufacturing of ships, for transport of chemicals and installations of drilling rigs, etc (Abdel-Monem, 2012).
As the name implies the microstructure of austenitic stainless steel consists entirely of fine grains of austenite in the wrought condition. When subjected to welding, however, a secondary ferrite phase may be formed on the austenite grain boundaries, in the heat affected zone and in the weld metal. The extent of the formation of this secondary phase may depend on the composition of the steel or filler material and the heat input during welding (www.boc.com.au).
Generally, when metals are exposed to an environment containing water molecules, they can give up electron, becoming themselves positively charged ions. The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons that are consumed by a secondary process (cathodic reaction); these two processes have to balance their charges (www.stoprust.com).
In another development, welding parameters are developed to achieve a specific weld quality and production output. However, a change in any parameter will have an effect on the final weld quality, so the welding variables normally are written down or stored in the welding equipment memory. Therefore, to determine the welding parameters, the
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national and international welding standards and also welding experience in application are taken into consideration for gas metal arc welding method (Ugur et al., 2011).
Generally, duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure; addition of 5% Nickel to ferritic stainless steel gives a duplex stainless steel, and addition of 8% Nickel to ferritic stainless steel gives a fully austenitic stainless steel(www.keytometals.com).
1.2 Statement of the Problem Many research findings have proved that improper techniques employed in welding austenitic stainless steels may lead to serious consequences of the welded structures (Avery, 1963; Parijslaan, 2002). Failure as a result of poor mechanical properties and poor corrosion resistance have also found their places in annals of times, from household equipment to industrial structures such as railways, road bridges, storage tanks and ocean liners. One of such failures is the corrosion cracking of a grade 304 stainless steel pipe improperly seam welded and meant for the conveying of glucose solution in Illinois USA (James, 2000). The Point Pleasant Bridge Disaster in Ohio in USA was traced to stress corrosion cracking initiated during welding (Chamberlain and Trethewey, 1988). Many other failures have proved to be welding prone or propagated.
However, a vast majority of repairs of failed components in industries are carried out using one of the welding processes and the success depends on many factors such as weldability of the material, type of damage, availability of suitable welding technique, possibility of carrying out pre- heating or post-weld heat treatment and post repair inspection by non-destructive techniques (NDT).
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But with all the aforementioned favourable properties of ASS, they still fail mostly at weld points which can be as a result of the welding process, process variables used and the welding environmental conditions. Hence, it is therefore pertinent to investigate the influence of some welding parameters on the mechanical and corrosion behavior of austenitic stainless steel in hydrochloric acid (HCl) and sodium hydroxide (NaOH) environments. 1.3 Present Research Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The process of arc welding is widely used because of its low capital and running costs(http://www.jansinc.com/welding.html). In this research, the effects of Gas Metal Arc Welding (GMAW) parameters on the mechanical and corrosion behaviour of austenitic stainless steel in some environments (HCl and NaOH) were investigated. 1.4 Aim and Objectives The aim of this research is to investigate the effects of Gas Metal Arc Welding parameters on the mechanical and corrosion behaviour of austenitic stainless steel in hydrochloric acid and sodium hydroxide media. The specific objectives are:
1. to study the effects of GMAW parameters on the mechanical behaviour of austenitic stainless steels.
2. to determine the effects of GMAW parameters on the corrosion behaviour of austenitic stainless steel in acidic ax= b ccfzfcmnd basic media.
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3. to determine the optimum GMAW parameters of austenitic stainless steel in acidic and basic media.
4. to use factorial design (Design-Expert 6.0.6 software) to determine the responses of the corrosion effects on the austenitic stainless steel.
5. to evaluate corrosion susceptibility of austenitic stainless steel in the different media mentioned above.
1.5Significance of the Study Essentially, gas metal arc welding is one of the most widely used among the various arc welding processes today. There has been considerable interest in the investigation of the effects of various welding parameters on the mechanical and corrosion behaviour of weldments in different environments. During welding process, the fusion and heat affected zone (HAZ) normally undergo metallurgical transformations due to the weld heat, thus consequently affecting the mechanical properties of the materials leading to varying mechanical and microstructural properties of the material (Adebayo and Odepidan, 2002). Welding of austenitic stainless steels with high demand of sound mechanical properties require a high degree of control of welding parameters, consumable and thermo mechanical condition with regard to their effect on mechanical and metallurgical properties. 1.6 Scope of the Research This research work is restricted to the effects of gas metal arc welding parameters on mechanical and corrosion behaviour of austenitic stainless steel plates when immersed in HCl and NaOH media. The study only covered the following areas:
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1. The use of destructive mechanical testing machines to determine the respective mechanical properties of the samples.
2. The use of scanning electron microscope (SEM) to determine the surface morphology of the ASS before and after immersion in the corrosive media.
3. The use of gas metal arc welding process in the welding operation.
4. The use of grinding and polishing machines to grind and polish the samples before and after welding.
5. The use of Design-Expert 6.0.6 software to determine theeffect(s) and interactions between the parameters.
6. The use of digital weight balance to determine the weight loss of the samples after immersion in the corrosive environments (hydrochloric acid and sodium hydroxide).
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