The Effect of Nitrogen Addition and Heat Input on Microstructure and Mechanical Properties of GTAW of Stainless Steel 304

Transcription

1 A Study on The Effect of Nitrogen Addition and Heat Input on Microstructure and Mechanical Properties of GTAW of Stainless Steel 304 By Eng. Hossam Ahmed Ali Mansour B.Sc. in Metallurgical Engineering Faculty of Petroleum and Mining Engineering Suez Canal University A Thesis Submitted to the Faculty of Engineering at Cairo University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In METALLURGICAL ENGINEERING Faculty of Engineering, Cairo University Giza, Egypt 2012

2 A Study on The Effect of Nitrogen Addition and Heat Input on Microstructure and Mechanical Properties of GTAW of Stainless Steel 304 by Eng. Hossam Ahmed Ali Mansour B.Sc. in Metallurgical Engineering Faculty of Petroleum and Mining Engineering Suez Canal University A Thesis Submitted to the Faculty of Engineering at Cairo University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In METALLURGICAL ENGINEERING Under the Supervision of Prof. Dr. Mohamed Raafat El Koussy Prof. Dr. Nahed Ahmed Abdel Raheem Prof. Dr. Alber A. Sadek. Faculty of Engineering, Cairo University Giza, Egypt 2012

3 A Study on The Effect of Nitrogen Addition and Heat Input on Microstructure and Mechanical Properties of GTAW of Stainless Steel 304 By Eng. Hossam Ahmed Ali Mansour B.Sc. in Metallurgical Engineering Faculty of Petroleum and Mining Engineering Suez Canal University A Thesis Submitted to the Faculty of Engineering at Cairo University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In METALLURGICAL ENGINEERING Approved by the Examining Committee: Prof. Dr. M. Raafat El Koussy, Main Supervisor (FECU) Prof. Dr. Nahed Ahmed Abdel Raheem, Supervisor (FECU) Prof. Dr. Ahmed Mohamed El Sheikh, Member (FECU) Prof. Dr. Abdel Monem Mohamed ElBatahgy, Member (CMDRI) Faculty of Engineering, Cairo University Giza, Egypt 2012

4 ABSTRACT The effect of nitrogen content in shielding gas during GTAW of 304 austenitic stainless steel on mechanical properties, microstructure, and corrosion resistance has been studied. Nitrogen was introduced to the shielding gas with 2%, 3.5%, and 5% by volume. Welding with pure argon was used for comparison. The welding performance was evaluated by recording voltage, current and travel speed, measuring the heat input, and visual examination of surface appearance of the weldments. The effect of heat inputs on weld metal and HAZ was also studied. Tensile and hardness tests were carried out study the effect of welding parameters on mechanical properties. Ferrite percent measurements and optical microscope were used to investigate the microstructure and hot cracking susceptibility. Potentiodynamic polarization tests were carried out study the effect of welding parameters on corrosion resistance. The results of the present work showed that adding nitrogen to shielding gas changed solidification mode from ferrite-austenite (FA) to austenite-ferrite (AF) and then to austenite (A) mode with increasing nitrogen content. The optimum content of nitrogen in welding gas was found to be 2.0%. The content of delta ferrite in the weld metal decreased with increasing nitrogen content. At 5% N in shielding gas the corrosion resistance increased, the tendency for hot cracking also increased due to elimination of delta ferrite in the weld metal and due to austenite-ferrite / austenite (AF/A) modes of solidification. I

5 Acknowledgment I would like to express my gratitude and appreciation to my parents and my wife for their patience, Love, help and support along all my life time. I wish to express my sincere thank and deep gratitude to my supervisors, Prof. Dr. Eng. M. Raafat El Koussy (FECU), Prof. Dr. Eng. Nahed A. Abdel Raheem (FECU), and Prof.Dr. Eng. Alber A.Sadek Head of Welding & Inspection Department Central Metallurgical Research & Development Institute (CMRDI) for their continuous support, their valuable guidance, and their helpful advices. I would like also to express my deep appreciation for their interest and patience for me. Many thanks to Prof. Dr. Eng. Randa Abdel Karim (FECU) for her help and valuable advices in the corrosion part. I can t forget the helpful spirit that I found from the staff of Mechanical Testing Lab (FECU) I wish also to thank all professors and teaching staff in the Metallurgical Department in Cairo University for their Continuous Support. I gratefully acknowledge the support of Central Metallurgical Research and Development Institute (CMRDI), especially Eng. Osama Khader & Eng. Ahmed Sayah in Department of Welding Technology for providing laboratory facilities, their help and encouragement. II

6 Table of Contents Title Page Abstract... I Acknowledgment... II Table of Contents... III List of Tables... VII List of Figures... VIII Introduction... 1 Chapter One: Literature Survey 1.1-General Definition Classification of stainless steel and their application Properties General welding characteristics Welding processes Arc welding Shielded metal arc welding Gas metal arc welding Flux cored arc welding Gas tungsten arc welding Filler metal selection Corrosion and corrosion properties Definition of Austenitic Stainless Steel Standard Alloys and Consumables Composition Filler metals Mechanical Properties Welding Metallurgy.26 III

7 1.11 Fusion Zone Microstructure Evolution Type A: Fully Austenitic Solidification Type AF Solidification Type FA Solidification Type F Solidification Metallurgical characteristics Base metals Weld metal Corrosion of austenitic stainless steel Weld Decay Stress Corrosion Cracking Ferrite Formation Measurement Importance Effects of welding conditions Effects of Nitrogen on Steel properties Austenite stability Mechanical properties Corrosion resistance Effects of Nitrogen in Shielding Gas on Weld Metal Properties Nitrogen contents in weld metals Retained ferrite content Weld metal hardness Angular distortion Microstructure Residual stresses IV

8 Chapter Two: Materials and Experimental Work 2.1 Base Metal Welding Machine Welding Procedure Preparations Samples Shielding Gas Filler metals Welding Electrodes Recording of Heat Input Experimental details Examination of Weld Profile Tensile test Hardness test Microstructure investigation Scanning Electron Microscope (SEM) Metallographic (Macro) Examination Ferrite measurement Chemical analysis of weld metal Corrosion Resistance Tests Pitting Resistance According to ASTM G48 Method A Test Conditions Test Evaluation Polarization test 69 Chapter Three: Results and Discussion 3.1 Effect of Nitrogen on Mechanical Prosperities Tensile Measurement Hardness Measurement Chemical Analysis of Weld Metal Effect of Nitrogen additions to shielding gas on weld delta-ferrite V

9 3.4 Effect of nitrogen on weld morphology (Weld Metal Area) Effect of adding nitrogen to the shielding gas on microstructure Microstructure features of hot cracking Corrosion test results Pitting Tests Results Effect of Nitrogen content on corrosion rate of weld metal..98 Chapter Four: Conclusion References VI

10 List of Tables Table No. Title Page 1-1 Typical Welding Problems in Stainless Steels. 1-2 Composition of typical wrought austenitic stainless steel. 1-3 Composition of Typical Cast Austenitic Stainless Steels Chemical Composition of all-weld Metal Deposited from Stainless Steel Covered. Chemical composition of bare, metal cored and standard stainless steel welding electrodes and rods. Chemical Composition of all Welds metal Deposited from Flux Cored Stainless Steel Electrodes. Typical Physical Properties of wrought stainless steels in the annealed condition. 1-8 Solidification Types, Reactions, and Resultant Microstructures 1-9 Effects of alloying elements in austenitic stainless steels. 2-1 The Standard Chemical Composition ASTM A240 Gr The Actual Chemical Composition ASTM A240 Gr.304 The Mechanical Properties of ASTM A240 Gr.304 Typical Chemical Composition of ER309L According to AWS/SFA The Various Heat Inputs Used in this Work Conditions. 3-1 Failure location in tensile test for all samples in low heat input. 3-2 Failure location in tensile test for all samples in High heat input. 3-3 Areas covered by hardness test positions VII

11 List of Figures Fig. No Title Page No. 1-1 Stress-strain curves for some stainless steels Impact toughness for different types of stainless steels Phase diagrams:(a) Fe C; (b) Fe Cr; (c) Fe Ni Cr at 70% Fe Relationship of solidification type to the pseudo binary a) Type A solidification, fully austenitic b) Fusion zone 28 microstructure resulting from fully austenitic. 1-6 (a & b) Fusion zone microstructure resulting from Type AF solidification Type FA Solidification:(a) Skeletal Ferrite; (b) Lathy 30 Morphology 1-8 Fusion Zone Microstructure Resulting from FA Solidification: (a) Skeletal Ferrite; (b) Lathy Morphology Fusion Zone Microstructure Resulting from F Solidification: 33 Widmanstatten Austenite Nucleates from Austenite Along Ferrite Grain Boundaries Effect of the combined carbon and silicon contents on the 35 ductility of welds in in Austenitic Stainless Steel 1-11 Schaeffer diagram for estimating the microstructure of stainless steel weld metal Delong constitution diagram for austenitic stainless steel weld metal Intergranular Corrosion in HAZ of a 304 Stainless Steel 38 Containing 0.05%C Grain Boundary microstructure in Sensitized Austenitic 39 Stainless Steel Welds in Austenitic Stainless Steel: (a)weld Decay in 304 Stainless Steel,(b)No Weld Decay in 321 Stainless Steel 40 VIII

12 Fig. No Title Page No Stress corrosion cracking in a 304 stainless steel pipe gas 41 tungsten arc welded and exposed to a corrosive liquid Pseudobinary Section of the Fe-Cr-Ni System at 70% iron Ferrite along the austenite grain boundaries in the HAZ of type L Stainless Steel Solubility of nitrogen in austenite Cr-Ni Steels with low carbon content in the temperature range between 600 and 1000 C Influence of nickel content on carbon solubility and 47 precipitation boundary or mixed carbide M 23 C 6 In ferrous alloys with 18%chromium Concentration Profiles in the ternary iron-chromium-nitrogen 48 diagram at 18% and26%chromium Solubility of nitrogen in austenitic Cr-Ni steels at solidification, 51 represented by the porosity free solidification limits of ingots Shows the influence of nitrogen on the reduction of delta ferrite 52 content in the iron-chromium-nickel system Influence of carbon content on the content of delta ferrite in iron- 52 chromium-alloys with about 25%chromium and 7%nickel Measured hardness of GTA weld metal as a function of nitrogen 53 added in argon shielding gas Microstructure of type 316L weld metal (a) Without nitrogen 55 addition; (b) with 0.14%N and (c) with 0.19%N Effect of nitrogen content on maximum principal residual stress 57 with: (a) High heat input; (b) low heat input Welding machine setting Welding machine Plate of 304 stainless steel for Fit Up and Preparing for welding Welder During Welding of the plates by GTAW process. 60 IX

13 Fig. No Title Page No. 2-5 Welder during Clearing between Passes with stainless steel pusher Cylindrical of Pure Argon & Nitrogen Specimens ready to make tensile test Universal Testing Machine Used for Tension Test Vickers Hardness Machine Cutting Machine Grinding Machine Polisher Machine Electrochemical Etching Optical Microscope JEOL 5410 Scanning Electron Microscope Magnifying Optical Lens (Stereoscope) Ferrite Number Devices Schematic Diagram of an Apparatus to Measure Electrode 70 potential of a Specimen 2-19 The Experimental Apparatus Used for Polarization Tests (Volta lab) Relation between Ultimate tensile strength and nitrogen in 73 argon at high & low heat input. 3-2 Location of hardness distribution measurement shown the hardness distribution of two samples (A) at low & (B) 75 high heat input 3-4 Weld metal nitrogen content as a function of nitrogen addition 76 to the shielding gas, at high heat input 3-5 Weld metal nitrogen Content as a Function of Nitrogen 77 Addition to the Shielding Gas, At Low & High Heat Inputs. 3.6 Combined Curve for Weld Metal Nitrogen Content as a function of 77 nitrogen addition to the shielding gas, at low & high heat input. 3.7 Relation between ferrite number and nitrogen in argon at high & low heat input. 78 X

14 Fig. No Title Page No. 3-8 Relation between width of weld metal and nitrogen content Relation between depth of weld metal and nitrogen content Relation between width to depth ratio of weld metal and 80 nitrogen content 3-11 Macro-etch of specimens welded by different conditions Microstructure of Base Metal FA and AF modes of solidification in austenitic stainless steel Effect of Nitrogen % in shielding gas on the microstructure 87 HAZ, fusion zone and weld. Low heat input. Original magnifications 200X Effect of Nitrogen percentage in shielding gas on the 88 microstructure HAZ, fusion zone and weld. High heat input. Original magnifications 200X Effect of Nitrogen percentages in shielding gas on the 89 microstructure of weld. Low heat input. Original magnifications 200X Effect of Nitrogen percentages in shielding gas on the 90 microstructure of weld. High Heat Input. Original magnifications 200X 3-18 Effect of Nitrogen percentages in shielding gas on the 91 microstructure weld. Low high & heat input. Original magnifications 100X SEM microstructure of weld metal showed the effect of 92 Nitrogen percentages in shielding gas on the (A) Low heat input & (B) High heat input. FA/AF mode solidification 3-20 SEM showing the effect of Nitrogen percentage in shielding gas on the microstructure of the weld at Low heat input. AF/FA mode of solidification. 93 XI

15 Fig. No Title Page No SEM showing the effect of Nitrogen percentage in shielding gas on the microstructure welds at low high heat input. AF mode of solidification Effect of Nitrogen Percentages in Shielding gas on the Weld Metal After Pitting Test, at Low high & heat input Mass losses after pitting test milligram/cm2 for all samples at High Heat Input Mass losses after pitting test milligram/cm2 for all samples at Low Heat Input Corrosion rate after Potentiodynamic Polarization test for all Samples at High Heat Input Corrosion rate after Potentiodynamic Polarization test for all Samples at Low Heat Input Combined Curve for Corrosion rate after Potentiodynamic polarization test for all samples at low and high heat input XII

16 Introduction INTRODUCTION Austenitic stainless steels have been used widely by the fabrication industry owing to their excellent high temperature and corrosion resistance properties. Some of the typical applications of these steels include their use as nuclear structural material for reactor coolant piping, valve bodies, vessel internals, chemical and process industries, dairy industries, petrochemical industries etc. Out of 300 series grade of these steels type 304 SS is extensively used in industries due to its superior low temperature toughness and corrosion resistance. One of the typical applications of type 304 SS include storing and transportation of liquefied natural gas (LNG),whose boiling point is -162 C under 1 atmosphere. Another typical application of this material includes its use as bellows used as conduit for liquid fuel and oxidizer in propellant tank of satellite launch vehicle. (37) As an alloying element in austenitic stainless steel nitrogen has gained a remarkable attention. In part, this has stemmed from the desire to use nitrogen as a substitute for nickel, thereby reducing alloying element costs. In addition to the fact that the consumption of an expensive strategic metal is reduced, nitrogen is considered to be as much as thirty times as powerful as nickel as an austeniteformer. Nitrogen also imparts a number of other beneficial properties to austenitic stainless steels. It is an excellent solid solution strengthening element in stainless steel, 1

17 Introduction increasing the yield strength at room temperature with no significant decrease in toughness or ductility. As a result, nitrogen-alloyed austenitic stainless steels offer a unique combination of strength and toughness. Nitrogen is also reported to improve the passivation characteristics of stainless steels. It increases resistance to localized corrosion, and reduces sensitization effects during welding. The aim of the present work was to study the effect of nitrogen content in shielding gas during GTAW of 304 austenitic stainless steel on mechanical properties, microstructure, ferrite content in the weld, and corrosion resistance. Nitrogen was introduced to the shielding gas with 2%, 3.5%, and 5% by volume. Welding with pure argon was used for comparison. The welding performance was evaluated by recording voltage, current and travel speed, measuring the heat input, and making visual examination of surface appearance of the weldments. The effect of heat inputs on weld metal and HAZ was also studied. Tensile, hardness, and impact toughness tests were carried out study the effect of welding parameters on mechanical properties. Ferrite percent measurements and optical microscope were used to investigate the microstructure and hot cracking susceptibility. Also the solidification mode at different nitrogen contents in the 2

18 Introduction shielding gas was studied. Potentiodynamic polarization tests were used to study the effect of welding parameters and nitrogen content on corrosion resistance. 3

19 Chapter (1) Literature Survey 1.1 General Definition CHAPTER (1) LITERATURE Survey Stainless steels constitute a group of high-alloy steels based on the Ironchromium, iron-chromium-carbon, and iron-chromium-nickel systems. To be stainless steel, these steels must contain a minimum of 10.5 wt. % chromium. This is level of chromium allows formation of a passive surface oxide that prevents oxidation and corrosion of the underlying metal under ambient, noncorrosive conditions. (1) Stainless steels may contain as little as 9 wt. % Cr and be subject to general corrosion ("rusting") at ambient temperatures. Few stainless steels contain more than 30 wt. % Cr or less than 50 wt. % Fe. (2) 1.2 Classification of stainless steel and their application Unlike other material system, where classification is usually by composition, stainless steel are categorized in five distinct families according to their crystal structure and strengthening precipitates. Each family exhibits its own general characteristics in terms of mechanical properties and corrosion resistance. Within each family, there is a range of grades that varies in composition, corrosion resistance, and cost. (3) Stainless steels are commonly divided into the following general groups: (1) Martensitic(4XX) (2) Ferritic(4XX) (3) Austenitic(2XX,3XX) (4)Duplex(austenite and ferrite) (5) Precipitation-hardening(PH) 4

20 Chapter (1) Literature Survey The first three groups are characterized by the predominant metallurgical phase present when the stainless steel is placed in service. The fourth group contains approximately 50% austenite and 50%ferrite taking advantage of the desirable properties of each phase. The fifth group those stainless steels that can be strengthened by an aging heat treatment. (4) In addition, there are a number of wrought stainless steels that are not classified by AISI. Some of these are designated by the same numbering system as the AISI types, and others are known by trade names. There are minor variations to many of the stainless steels. These variations include special carbon control for corrosion or high temperature applications; chemical stabilization with Al, Cb, or Ti; high S or Se for better machinability, and other alloy additions for special characteristics. (5) Corrosion resistant (stainless) steel castings are standardized by the Alloy Casting Institute (ACI). They are designated by a two-letter system, HX, or a letter-number system, CX-XXX, such as CF-8 or CF-12M. Many cast types are similar to counterparts in the AISI wrought stainless steel system. (5) Most stainless steels have been assigned numbers under the SAE-ASTM Unified Numbering System (UNS). Wrought and cast stainless steels are identified by the letters S and J respectively followed by five digits. (5) Magnetic prosperities can be used to identify some stainless steel. The Austenite types are essentially nonmagnetic. A small amount of residual ferrite or cold working may introduce a slight ferromagnetic condition, but it is notably weaker than a magnetic material. The ferritic and martensitic types are ferromagnetic. Duplex stainless steels are relatively strongly magnetic, due to their high ferrite content. (6) 5