Wednesday, 10 May 2017

WELDING METALLURGY BY SINDO KOU FREE DOWNLOAD PDF

WELDING METALLURGY

BY SINDO KOU

FREE DOWNLOAD PDF




CONTENTS
  1. Fusion Welding Processes
  2. Heat Flow in Welding
  3. Chemical Reactions in Welding
  4. Fluid Flow and Metal Evaporation in Welding
  5. Residual Stresses, Distortion, and Fatigue
  6. Basic Solidification Concepts
  7. Weld Metal Solidification I: Grain Structure
  8. Weld Metal Solidification II: Microstructure within Grains
  9. Post-Solidification Phase Transformations
  10. Weld Metal Chemical Inhomogeneities
  11. Weld Metal Solidification Cracking
  12. Formation of the Partially Melted Zone
  13. Difficulties Associated with the Partially Melted Zone
  14. Work-Hardened Materials
  15. Precipitation-Hardening Materials I: Aluminum Alloys
  16. Precipitation-Hardening Materials II: Nickel-Base Alloys
  17. Transformation-Hardening Materials: Carbon and Alloy Steels
  18. Corrosion-Resistant Materials: Stainless Steels



Fusion Welding Processes - Introduction : Fusion welding processes will be described in this chapter, including gas welding, arc welding, and high-energy beam welding. The advantages and disadvantages of each process will be discussed.

OVERVIEW : Fusion Welding Processes - Fusion welding is a joining process that uses fusion of the base metal to make the weld. The three major types of fusion welding processes are as follows:

1. Gas welding:
  • Oxyacetylene welding (OAW)


2. Arc welding:
  • Shielded metal arc welding (SMAW)
  • Gas–tungsten arc welding (GTAW)
  • Plasma arc welding (PAW)
  • Gas–metal arc welding (GMAW)
  • Flux-cored arc welding (FCAW)
  • Submerged arc welding (SAW)
  • Electroslag welding (ESW)


3. High-energy beam welding:
  • Electron beam welding (EBW)
  • Laser beam welding (LBW)


Since there is no arc involved in the electroslag welding process, it is not exactly an arc welding process. For convenience of discussion, it is grouped with arc welding processes.

Power Density of Heat Source : Consider directing a 1.5-kW hair drier very closely to a 304 stainless steel sheet 1.6mm (1/16 in.) thick. Obviously, the power spreads out over an area of roughly 


OXYACETYLENE WELDING 

The Process : 
Gas welding is a welding process that melts and joins metals by heating them with a flame caused by the reaction between a fuel gas and oxygen. Oxyacetylene welding (OAW), shown in Figure 1.7, is the most commonly used gas welding process because of its high flame temperature. A flux may be used to deoxidize and cleanse the weld metal. The flux melts, solidifies, and forms a slag skin on the resultant weld metal. Figure 1.8 shows three different types of flames in oxyacetylene welding: neutral, reducing, and oxidizing (4), which are described next.

Three Types of Flames 

A. Neutral Flame :  This refers to the case where oxygen (O2) and acetylene (C2H2) are mixed in equal amounts and burned at the tip of the welding torch. A short inner cone and a longer outer envelope characterize a neutral flame The inner cone is the area where the primary combustion takes place through the chemical reaction between O2 and C2H2, as shown in Figure 1.9. The heat of this reaction accounts for about two-thirds of the total heat generated.The products of the primary combustion, CO and H2, react with O2 from the surrounding air and form CO2 and H2O. This is the secondary combustion, which accounts for about one-third of the total heat generated. The
area where this secondary combustion takes place is called the outer envelope. It is also called the protection envelope since CO and H2 here consume the O2 entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral flame is used. 

B. Reducing Flame : When excess acetylene is used, the resulting flame is called a reducing flame.The combustion of acetylene is incomplete.As a result, a greenish acetylene feather between the inert cone and the outer envelope characterizes a reducing flame (Figure 1.8b). This flame is reducing in nature and is desirable for welding aluminum alloys because aluminum oxidize easily. It is also good for welding high-carbon steels (also called carburizing flame in this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld metal. 

C. Oxidizing Flame : When excess oxygen is used, the flame becomes oxidizing because of the presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing flame (Figure 1.8c). This flame is preferred when welding brass because copper oxide covers the weld pool and thus prevents zinc from evaporating from the weld pool. 


SHIELDED METAL ARC WELDING : The Process

Shielded metal arc welding (SMAW) is a process that melts and joins metals by heating them with an arc established between a sticklike covered electrode and the metals, as shown in Figure 1.10. It is often called stick welding. The electrode holder is connected through a welding cable to one terminal of the power source and the workpiece is connected through a second cable to the other terminal of the power source. The core of the covered electrode, the core wire, conducts the electric
current to the arc and provides filler metal for the joint. For electrical contact, the top 1.5 cm of the core wire is bare and held by the electrode holder. The
electrode holder is essentially a metal clamp with an electrically insulated outside shell for the welder to hold safely. The heat of the arc causes both the core wire and the flux covering at the electrode tip to melt off as droplets. The molten metal collects in the weld pool and solidifies into the weld metal.The lighter molten flux, on the other hand, floats on the pool surface and solidifies into a slag layer at the top of the weld metal.

Functions of Electrode Covering: 

The covering of the electrode contains various chemicals and even metal powder in order to perform one or more of the functions described below. 

A. Protection It provides a gaseous shield to protect the molten metal from air. For a cellulose-type electrode, the covering contains cellulose, (C6H10O5)x. A large volume of gas mixture of H2, CO, H2O, and CO2 is produced when cellulose in the electrode covering is heated and decomposes. For a limestone- (CaCO3 ) type electrode, on the other hand,CO2 gas and CaO slag form when the limestone decomposes. The limestone-type electrode is a low-hydrogentype electrode because it produces a gaseous shield low in hydrogen. It is often used for welding metals that are susceptible to hydrogen cracking, such as high-strength steels. 
B. Deoxidation It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld metal. The solid slag formed also protects the already solidified but still hot weld metal from oxidation. 
C. Arc Stabilization It provides arc stabilizers to help maintain a stable arc. The arc is an ionic gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose readily into ions in the arc, such as potassium oxalate and lithium carbonate. They increase the electrical conductivity of the arc and help the arc conduct the electric current more smoothly.
D. Metal Addition It provides alloying elements and/or metal powder to the weld pool. The former helps control the composition of the weld metal while the latter helps increase the deposition rate. 

Advantages and Disadvantages : 

The welding equipment is relatively simple, portable, and inexpensive as compared to other arc welding processes. For this reason, SMAW is often used for maintenance, repair, and field construction. However, the gas shield in SMAW is not clean enough for reactive metals such as aluminum and titanium. The deposition rate is limited by the fact that the electrode covering tends to overheat and fall off when excessively high welding currents are used.The limited length of the electrode (about 35 cm) requires electrode changing, and this further reduces the overall production rate 


Residual Stresses, Distortion, and Fatigue : 

RESIDUAL STRESSES : Residual stresses are stresses that would exist in a body if all external loads were removed. They are sometimes called internal stresses. Residual stresses that exist in a body that has previously been subjected to nonuniform temperature changes, such as those during welding, are often called thermal stresses

Development of Residual Stresses

A. Three-Bar Arrangement : The development of residual stresses can be explained by considering heating and cooling under constraint . Figure 5.1 shows three identical metal bars connected to two rigid blocks. All three bars are initially at room temperature. The middle bar alone is heated up, but its thermal expansion is restrained by the side bars (Figure 5.1a). Consequently, compressive stresses are produced in the middle bar, and they increase with increasing temperature until the yield stress in compression is reached. The yield stress represents the upper limit of stresses in a material, at which plastic deformation occurs.When heating stops and the middle bar is allowed to cool off, its thermal contraction is restrained by the side bars . Consequently, the compressive stresses in the middle bar drop rapidly, change to
tensile stresses, and increase with decreasing temperature until the yield stress in tension is reached. Therefore, a residual tensile stress equal to the yield stress at room temperature is set up in the middle bar when it cools down to room temperature. The residual stresses in the side bars are compressive stresses and equal to one-half of the tensile stress in the middle bar. 

B. Welding : Roughly speaking, the weld metal and the adjacent base metal are analogous to the middle bar, and the areas farther away from the weld metal are analogous to the two side bars (Figure 5.1c). This is because the expansion and contraction of the weld metal and the adjacent base metal are restrained by the areas farther away from the weld metal. Consequently, after cooling to the room temperature, residual tensile stresses exist in the weld metal and the adjacent base metal, while residual compressive stresses exist in the areas farther away from the weld metal. Further explanations are given as follows.

A schematic representation of the temperature change (DT) and stress in the welding direction (sx) during welding (2). The crosshatched area M–M¢ is the region where plastic deformation occurs. Section A–A is ahead of the heat source and is not yet significantly affected by the heat input; the temperature change due to welding, DT, is essentially zero. Along section  B–B intersecting the heat source, the temperature distribution is rather steep. Along section C–C at some distance behind the heat source, the temperature distribution becomes less steep and is eventually uniform along section D–D far away behind the heat source. Consider now the thermally induced stress along the longitudinal direction, sx. Since section A–A is not affected by the heat input, sx is zero.Along section B–B, sx is close to zero in the region underneath the heat source, since the weld pool does not have any strength to support any loads. In the regions somewhat away from the heat source, stresses are compressive (sx is negative) because the expansion of these areas is restrained by the surrounding metal of lower temperatures. Due to the low yield strength of the high-temperature metal in these areas, sx reaches the yield strength of the base metal at corresponding temperatures. In the areas farther away from the weld sx is tensile, and sx is balanced with compressive stresses in areas near the weld. Along section C–C the weld metal and the adjacent base metal have cooled
and hence have a tendency to contract, thus producing tensile stresses (sx is positive). In the nearby areas sx is compressive. Finally, along section D–D the weld metal and the adjacent base metal have cooled and contracted further, thus producing higher tensile stresses in regions near the weld and compressive stresses in regions away from the weld. Since section D–D is well behind the heat source, the stress distribution does not change significantly beyond it, and this stress distribution is thus the residual stress distribution. 





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