SAN-OM-29
Microstructure of the Base Metal after Welding
LITERATURE REVIEW:
The present study arose out of a perceived need to understand the relationship between the microstructure characteristics and mechanical properties of steel-welded joint and to document the influence of the multi-pass welding process on these characteristics. This project necessitates a review of advanced information on the microstructural features of the steel multi-pass weld under consideration: the 13%Cr-4%Ni steel (13Cr4Ni).
The literature review presented in this chapter starts with a general overview of 13Cr4Ni martensitic stainless steel classification and characteristics. 13Cr4Ni weld metallurgy are then discussed, followed by a review of the specific 13Cr4Ni steel weld features. In the final section of the chapter, some specific austenite revealing techniques used in this study are introduced.
13Cr4Ni Metallurgy :
A review of 13Cr4Ni steel metallurgy is presented in this section and the microstructural characteristics of the steel are outlined.
General :
The 13Cr4Ni alloy was developed in the 1960s as part of a research project that sought to develop new types of stainless steel suitable for hydroelectric turbine rotors [3]. Conventional martensitic stainless steels were previously used for this purpose, but they carried a high risk of hot cracking and required numerous precautions be followed carefully during the welding procedure. To decrease the risk of hot cracking during welding, low carbon content martensitic stainless steels were used, while nickel was added to the composition of the steel to maintain its martensitic structure, producing a steel containing 13% Cr and 4% Ni [4]. Although 13Cr4Ni steel is known to have a fully martensitic microstructure, the actual steel making process may create a complex microstructure consisting of martensite, δ-ferrite, and austenite. Previous research has shown that the makeup of the final microstructure is related to the composition, solidification, and heat treatment history of the steel [5-7]. In order to understand the mechanisms leading to this complexity, a concise review of the steel microstructure formation is offered below.
Austenite Revealing Techniques :
Austenite phase is an important aspect of 13Cr4Ni martensitic stainless steels, either as the origin of its final martensite phase or as the reformed austenite after tempering. Research showed that austenite characteristics affect the microstructure and mechanical properties of the final martensitic stainless steel . Many techniques are available to reveal and study the characteristics of austenite phase in both cases. The following sections introduce briefly the austenite grain recalculation and austenite revealing electropolishing techniques. The former is used to reveal austenite parents grains and texture from the martensite phase, and the latter is the technique to reveal austenite particles formed by tempering the steel.
Austenite Grain Recalculation :
Austenite grain recalculation provides means to study the former austenite phase before undergoing martensitic transformation. Unfortunately, it is rarely possible to characterize austenite microstructure at high temperatures, and it is also hard to trace it at room temperature, since it fades inside the martensite matrix. However, many methods have been developed to characterize austenite microstructure by its remaining traces or its effects on subsequent martensite phase . The most common ones are based on metallographic techniques that reveal the austenite grain boundaries but they are often hard to interpret and use .
Recently, reconstruction methods have been proposed and developed by using Electron Backscattered Diffraction analysis (EBSD) that recalculate the parent austenite grains from the martensite orientation maps. Some of these methods involve manual grouping of variants while others involve automated reconstruction [63, 65]. Although some of these reconstruction methods are promising in the case of steels, the research in this field is very active and on-going .
Austenite Revealing Electropolishing Technique :
Electropolishing technique is very useful to reveal austenite particles formed by tempering. The selective corrosion of phases with this technique provides the opportunity to study austenite phase while other metallography techniques may transform the austenite or they are unable to reveal austenite on martensitic stainless steels. Electropolishing technique also provides a polished surface and can remove traces of deformation formed by prior surface preparations.
The electropolishing process includes smoothing and brightening of the metal surface. By running the electrolytic cell consisting of a pre-ground sample, electrolytic solution and a cathode, the sample is treated by the solution with a proper combination of parameters . The metal surface becomes the anode of the cell and a thin viscous liquid layer is formed by the reaction between the metal and the electrolyte. This layer of solution, called the “polishing film,” controls the steady flow of ions to the metal surface. More rapid ionic and molecular movement through the thinner polishing film at peaks may also be responsible for smoothing action.
Introduction:
13Cr4Ni belongs to the low carbon martensitic stainless steels. They have lots of applications in hydroelectric, power generation, offshore and petrochemical industries. Multipass welding processes are common for the fabrication and repair of this steel as the carbon content is low enough to avoid loss of toughness and compressive residual stresses built in the weld after each pass . Generally, the composition of the electrode is similar to the base metal in order to produce weld metals with similar properties . 410NiMo filler metal family is the best choice among available electrodes.
13Cr4Ni steel solidifies to δ-ferrite, then starts to transforms into austenite at around 1300 ˚C and ends, in a thermodynamically equilibrium conditions, at around 1200 ˚C . At temperatures lower than 1200 ˚C austenite decomposes and if a thermodynamically equilibrium is achieved; ferrite and carbides are expected to be the stable phases at room temperature. However in cooling conditions which are typical of production, the very slow rate of ferrite-carbides formation maintains the austenite existence at low temperature and then austenite is subjected to the martensitic transformation.
The fully martensitic microstructure expected after cooling to room temperature, may be very complex. Alloying elements segregation in-between dendrites at the final stages of solidification can stabilize δ-ferrite phase which can remain in the microstructure even at room temperature . Furthermore, the transformation of austenite to martensite can be incomplete and small amounts of retained austenite may remain between martensite laths .
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Table des matières
INTRODUCTION
CHAPTER 1 LITERATURE REVIEW
1.1. 13Cr4Ni Metallurgy
1.1.1. General
1.1.2. 13Cr4Ni Classification
1.1.3. Effect of Alloying Elements on Microstructure of Martensitic Steels
1.1.3.1. General Effects of Alloying Elements
1.1.3.2. Effects of Alloying Elements on 13Cr4Ni Microstructure
1.1.4. Transformation during cooling of 13Cr4Ni Martensitic Stainless Steels
1.1.4.1. Solidification and cooling of 13Cr4Ni
1.1.4.2. 13Cr4Ni Martensitic Transformation
1.1.4.3. Martensite Transformation Start Temperature
1.1.5. 13Cr4Ni Martensite Characteristics
1.1.5.1. 13Cr4Ni Martensitic Microstructur
1.1.5.2. Retained Austenite
1.2. Welding Aspects of 13Cr4Ni Martensitic Stainless Steels
1.2.1. Welding Metallurgy of Martensitic Stainless Steels
1.2.2. δ-ferrite Formation
1.2.3. Heat Affected Zones
1.2.4. Multi-Pass Welding
1.2.5. Post-Weld Heat Treatment
1.2.5.1. Reformed Austenite
1.2.5.2. Effect of Tempering Temperature
1.2.5.3. Effect of Holding Time
1.2.5.4. Carbide Formation
1.2.5.5. Widmanstätten Austenite
1.3. Austenite Revealing Techniques
1.3.1. Austenite Grain Recalculation
1.3.2. Austenite Revealing Electropolishing Technique
1.4. Conclusions
CHAPTER 2 ARTICLE NO1- MICROSTRUCTURE CHARACTERIZATION OF SINGLE AND MULTI-PASS 13CR4NI STEEL WELDED JOINTS
2.1. Introduction
2.2. Experimental Conditions
2.3. Chemical Compositions
2.4. Microstructure of the Base Metal after Welding
2.5. Microstructure of the Single-pass Weld Metal
2.6. Microstructure of the Double-pass Weld Sample
2.7. Hardness Maps
2.8. Conclusions
CHAPTER 3 ARTICLE NO2 – MICROSTRUCTURE CHARACTERIZATION AND HARDNESS DISTRIBUTION OF 13CR4NI MULTIPASS WELD METAL
3.1. Introduction
3.2. Materials and Characterization Methods
3.3. Results and Discussions
3.3.1. Chemical Composition
3.3.2. Microstructure of the As-Welded Multipass Sample
3.3.3. EBSD Analysis of Multipass Weld Sample
3.3.4. Inhomogeneity in the Weld
3.3.5. Hardness
3.4. Conclusions
CHAPTER 4 ARTICLE NO3 – EFFECTS OF VARIOUS POST WELD HEAT TREATMENTS ON AUSTENITE AND CARBIDES FORMATION IN A 13CR4NI MULTIPASS WELD
4.1. Introduction
4.2. Experimental Conditions
4.3. Microstructure of the As-Welded Multipass Sample
4.4. Carbides and Austenite Formations during Single Heat Treatments
4.5. Effects of Double Tempering Heat Treatments
4.6. Effect of Heat Treatments on Hardness and Austenite Percentage
4.7. Conclusions
CHAPTER 5 DISCUSSION
5.1. Microstructure and Texture of a 13Cr4Ni Single-Pass Weld
5.2. Microstructure and Texture of a 13Cr4Ni Double-Pass Weld
5.3. Microstructure and Texture of a 13Cr4Ni Multi-Pass Weld
5.4. Effects of Tempering Heat Treatments on Microstructure
5.4.1. Reformed Austenite Formation
5.4.2. Effect of Tempering Temperature
5.4.3. Effect of Tempering Holding Time
5.4.4. Carbide Formation
5.4.5. Effect of Heat Treatment on Hardness
5.4.6. Double Tempering
5.5. Contributions
5.6. Recommendations for Future Work
CONCLUSIONS
LIST OF REFERENCES
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