Weldability of Ferritic Stainless Steel

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CHAPTER TWO LITERATURE REVIEW 

INTRODUCTION

Stainless steels are ironbase alloys containing a minimum of 11wt.% chromium content for adequate corrosion resistance. This chromium content is the minimum that prevents the formation of “rust” in air or in polluted atmospheres by forming a very thin surface film of chromium oxides known as the “passive film”, which is selfhealing in a wide variety of environments. Today, the chromium content in stainless steels approaches 30wt.% in some alloys and other elements are often added to provide specific properties or ease of fabrication. Some of these elements are nickel (Ni), nitrogen (N) and molybdenum (Mo) which are added for corrosion resistance; carbon (C), Mo, N, titanium (Ti), aluminium (Al) and copper (Cu) which are added for strength; sulphur (S) and selenium (Se) are added for machinability; and Ni is added for formability and toughness, particularly to obtain an austenitic microstructure which is far less prone to the loss of toughness from a large grain size.

CLASSIFICATION OF STAINLESS STEELS

Stainless steels are divided into three groups according to their crystal structures: austenitic (facecentred cubic, fcc), ferritic (bodycentred cubic, bcc) and martensitic (bodycentred tetragonal or cubic, bct). Stainless steels containing both austenite and ferrite usually in roughly equal amounts are known as “duplex stainless steels”.
The general considerations for the choice of the base metal are that it should have the following properties: (a) since these alloys are used at high temperatures or under demanding conditions, they should have adequate corrosion resistance. This implies that either Cr or Al at a level of about 15% or higher, should be added to the alloy. (b) The room temperature structure should be austenitic, primarily to avoid the formation of martensite during cooling to room temperature and secondly, to prevent a ferritic structure which has a lower solubility for carbon and favours the formation of intermetallic precipitates rather than carbides. The addition of austenite formers (mainly Ni, Mn and N) is, therefore, necessary in austenitic stainless steels.
The FeCrNi system as the base alloy is by far the most suitable as large quantities of Cr can be taken into solution and maintained in solution down to room temperature. Secondly, Ni is also a strong austenite former and Cr lowers the martensite start (M_s) temperature sufficiently to avoid the formation of martensite, see Figure 2.1.
Both nitrogen and carbon are strong austenite formers, with nitrogen being increasingly used to provide certain attractive properties such as good fracture strength and it also improves the corrosion resistance [13,14]. From Figure 2.2, it is noted that carbon is a very strong austenite former, and if the carbon content is very low, slightly more Ni may have to be added to compensate for the loss of the austenitic properties of the carbon.

FERRITIC STAINLESS STEEL

This stainless steel derives its name from the bcc crystallographic structure that is generally stable from room temperature up to the liquidus temperature. Typically, ferritic stainless steel contains approximately 11 to 30wt.% chromium and small amounts of other alloying elements. The chromium additions give the steel its corrosion resistance and it further stabilises the bcc crystal structure. Recently, a very low (C+N) content has been specified in the socalled superferritic stainless steels. The higher alloy compositions can also include up to 4%Ni, provided this does not alter their fully ferritic structure. Due to their adequate corrosion resistance and lower cost, ferritic stainless steels are chosen over austenitic stainless steels in less severe applications such as a replacement to mild carbon steels in automobile exhaust systems. However, poor weldability, which leads to low toughness and is also associated with grain growth in the HAZ, limits their use even with very low carbon levels.

AUSTENITIC STAINLESS STEEL

As with ferritic steel, the austenitic stainless steel’s name originates from its fcc crystallographic structure. These steels contain 16 to 25wt% chromium and 7 to 10% nickel. Austenitic stainless steel has a high nickel content to stabilise the austenite fcc structure at room temperature. The increase in alloy content creates a higher cost of production but the fcc structure exhibits very high ductility, resulting in material with good formability and very good corrosion resistance. Another advantage of these steels is the relative ease of recrystallisation, which allows for better control of the mechanical properties.

MARTENSITIC STAINLESS STEEL

Martensitic stainless steels contain 12 to 17% chromium for good corrosion resistance. However, since chromium is a strong ferritic stabiliser, austenite stabilisers are added so that the necessary austenite can be formed during solution treatment for the subsequent martensite formation. Therefore, these steels have a high carbon content to stabilise the austenite at higher temperatures. The high carbon content will increase the strength through solid solution strengthening and the precipitation of a large number of (Fe, Cr) carbides. These steels use the quench and temper process to achieve a very high strength with reasonable ductility. Because of the high alloy content, these steel have a superior hardenability. The disadvantage of the high hardenability often leads to degradation of the corrosion resistance when compared with ferritic and austenitic stainless steels.

 DUPLEX STAINLESS STEEL

These steels contain a mixture of ferrite and austenite phases at room temperature in order to combine the beneficial properties of both components. These steels typically contain 18 to 30% chromium and an intermediate amount of nickel (39%) that is not enough for the formation of a fully austenitic structure at room temperature. Duplex stainless steels have an intermediate level of high mechanical strength and corrosion resistance properties lying between those of austenitic and ferritic products.

COMPOSITION OF STAINLESS STEELS

The composition of stainless steel can be related to its nonequilibrium metallurgical structure by means of a Schaeffler diagram [16], which shows the microstructure obtained after a rapid cooling from 1050°C to room temperature. It is, therefore, not an equilibrium diagram and is often used in welding phase analysis. This diagram was originally established to estimate the amount of delta ferrite (that is, ferrite formed on solidification, as opposed to alpha ferrite, which is a transformation product of austenite or martensite) content of welds in austenitic stainless steels. The alloying elements commonly found in stainless steels are regarded either as austenite stabilisers or as delta ferrite stabilisers. The relative “potency” of each element is conveniently expressed in terms of an empirical equivalence to either nickel (austenite stabiliser) or chromium (ferrite stabiliser) on a weight percentage basis. The nickel and chromium equivalents, which form the two axes of the Schaeffler diagram, can be calculated as follows:
%Ni equivalent = %Ni + %Co + 30%C + 25%N + 0.5%Mn +0.3%Cu
%Cr equivalent = %Cr + 2%Si + 1.5%Mo + 5%V + 5.5%Al + 1.75%Nb + 1.5%Ti + 0.75%W

STRUCTURE OF FERRITIC STAINLESS STEEL

Ferritic stainless steels at room temperature consist of alpha (α) solid solution having a body centred cubic (bcc) crystal structure. The alloy contains very little interstitial carbon and nitrogen in solution; most of the interstitial elements appear as finely distributed carbides and nitrides. A typical phase diagram of the ironchromium system is shown in Figure 2.3.
Chromium is a ferrite stabiliser and it extends the alphaphase field and suppresses the gammaphase field. This results in the formation of the socalled gamma loop as seen in Figure 2.3, which, in the absence of carbon and nitrogen, extends to chromium contents of about 12 – 13wt % [17]. At the higher chromium contents, transformation to austenite is no longer possible and the metal will remain ferritic up to its melting temperature. This constitutes an entirely different class of stainless steels in which grain refinement can no longer be brought about by transformation through heat treatment.
With carbon and nitrogen present in these alloys the diagram is modified in certain respects. The effect of carbon and nitrogen is to shift the limits of the gamma loop to higher chromium contents and widens the duplex (α + γ) phase area [18]. Figure 2.4 shows the changes in this part of the diagram. However, the solubility levels of the interstitials in the ferrite matrix are sufficiently low so that it is rarely possible to distinguish between solute embrittling effects and the effects of secondphase precipitates. The precipitates, in fact, become more important than the solute when the amount of interstitial elements significantly exceeds the solubility limit. The presences of carbon or nitrogen, in amounts in excess of the solubility limit, serve to increase the ductile to brittle transition temperature (DBTT). This embrittling effect is closely linked to the amount or the number and size of carbides and nitrides formed on the grain boundaries but also to the ferrite grain size. Precipitate films act as strong barriers to slip propagation across the grain boundaries and are also often inherently brittle by themselves. Grain boundary precipitates are suppressed by quenching from above the solution temperature when the interstitial content is low enough.

PREFACE 
ACKNOWLEDGEMENT 
TABLE OF CONTENT 
TABLE OF FIGURES 
NOMENCLATURE 
CHAPTER ONE  GENERAL INTRODUCTION 
1.1 Introduction
1.2 Problem Statement
1.3 Objectives
CHAPTER TWO LITERATURE REVIEW 
2.1 Introduction
2.2 Classification of Stainless Steels
2.2.1 Ferritic Stainless Steel
2.2.2 Austenitic Stainless Steel
2.2.3 Martensitic Stainless Steel
2.2.4 Duplex Stainless Steel
2.3 Composition of Stainless Steels
2.3.1 Structure of Ferritic Stainless Steel
2.4 Toughness of Ferritic Stainless Steels
2.4.1 Effect of Grain Size on Brittle Behaviour
2.4.2 Embrittlement at 475°C
2.4.3 Precipitation of the Secondary Phases in Stainless Steels
2.4.4 Notch Sensitivity
2.4.5 Weldability of Ferritic Stainless Steel
2.4.6 Effect of Niobium and Titanium Additions to Ferritic Stainless Steels
2.5 Theories of Brittle Fracture
2.5.1 Zener’s/Stroh’s Theory
2.5.2 Cottrell’s Theory
2.5.3 Smith’s Theory
2.5.4 Cleavage Fracture Resistance
2.6 Thermomechanical Processing
2.6.1 Cold-Rolling
2.6.2 Hot-Rolling
2.6.3 Cooling Rate
2.6.4 Heat Treatment
2.7 Applications of Stainless Steels in Automobile Exhaust System
2.8 Heat Resistant Ferritic Stainless Steels
2.9 Stabilisation
2.9.1 Stabilisation with Titanium
2.9.2 Stabilisation with Niobium
2.9.3 Solid Solution Hardening and Solute Drag by Niobium
2.9.4 Effects of Temperature on Solute Drag
2.9.5 Dual Stabilisation with Titanium and Niobium
2.10 AISI Type 441 Stainless Steels
2.11 Calphad Methods
2.11.1 Thermodynamic Softwares
2.12 Intermetallic Laves Phase
2.12.1 Crystallographic Structure
2.12.2 Occurrence
2.12.3 Orientation Relationship
CHAPTER THREE THEORY OF PRECIPITATION REACTIONS IN STEELS 
3.1 Introduction
3.2 Classical Theory of Nucleation
3.3 Growth by Supersaturation
3.4 Transformation Kinetics
3.5 Overall Transformation Kinetics
3.6 Capillarity
3.7 Dissolution of the Metastable Phase
3.8 Particle Coarsening
3.9 Summary
CHAPTER FOUR EXPERIMENTAL PROCEDURES 
4.1 Materials
4.2 Thermodynamic Modelling
4.3 Heat Treatments
4.4 Mechanical Testing
4.5 Microanalysis of Specimens
4.6 Identification of Precipitates
4.7 The Orientation Relationship Between the Laves Phase and the Matrix
CHAPTER FIVE THERMODYNAMIC MODELLING 
5.1 Introduction
5.2 Description of Thermo-Calc® Software
5.3 Experimental Alloys
5.4 Possible Stable Phases at Equilibrium
5.5 Phase Diagrams
5.6 Property Diagrams
5.7 Relative Phase Stabilities
5.8 Equilibrium Chemical Composition of the Laves Phase
5.9 Driving Force for Nucleation
5.10 Summary
CHAPTER SIX EXPERIMENTAL RESULTS 
6.1 Introduction
6.2 Microstructural Analysis of an AISI Type 441 Ferritic Stainless Steel
6.3 Effect of Annealing Treatment on the Microstructural and Mechanical Properties
6.4 Effect of Annealing Treatment on the Charpy Impact Energy and DBTT
6.5 Effect of Re –embrittlement treatment on The Room Temperature Charpy Impact Energy
CHAPTER SEVEN EXPERIMENTAL RESULTS EFFECT OF THE STEEL’S COMPOSITION 
7.1 Effect of Annealing Treatment on Steel B
7.2 Effect of the Equilibrium Laves Phase Volume Fraction on the Room Temperature Charpy Impact Energy
7.3 Effect of Annealing Treatment on the Embrittlement of the Experimental Stainless Steels C to E
CHAPTER EIGHT EXPERIMENTAL RESULTS LAVES PHASE KINETICS STUDY 
8.1 Introduction
8.2 Equilibrium Laves Phase Fraction
8.3 Laves Phase Transformation Kinetics
8.4 Temperature Effect on Isothermal Transformations
8.5 Effect of the Grain Size on the Transformation Kinetics of Laves Phase
8.6 Effect of the Steel’s Composition on the Laves Phase’s Transformation Kinetics
8.7 Microstructural Analysis of the Transformation Kinetics
8.8 Orientation Relationship Between the Laves Phase and the Ferrite Matrix
CHAPTER NINE DISCUSSIONS LAVES PHASE EMBRITTLEMENT 
9.1 Introduction
9.2 Precipitates Found in AISI 441 Ferritic Stainless Steel
9.3 Embrittlement of Type 441 Ferritic Stainless Steel
9.4 Recrystallisation and Grain Growth
CHAPTER TEN DISCUSSIONS TRANSFORMATION KINETICS MODELLING 
10.1 Introduction
10.2 Modelling in Kinetics of Laves Phase Precipitation
10.3 Parameters Required for Calculations
10.4 Calculations
10.5 Summary
CHAPTER ELEVEN CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK 
11.1 Conclusions
APPENDIX A
APPENDIX B
REFERENCE
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