STRENGTH of a welded joint

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STRENGTH of a welded joint

Strength of a welded joint depends on weld geometry and strength of materials. The minimum cross section of a weld is considered in strength calculations. Throat a is at a minimum. Throat is the distance from the root to the surface of a fillet weld. The throat of a fillet is a measure of the weld size. Critical force P for a lap joint using a 45 degree filet weld depends on allowable shear stress for the weld material t_allowable.


Bending stress is small if moment of inertia of the weld cross-section is large. The moment depends on the cross sectional area and it's distance from the neutral axis shown in the figure. The second example is three times stronger than the first. 

The tensile strength of a butt-joint is higher for the case with low stress concentration. Build-up welds are often produced to increase wear resistance, not tensile strength.


Spot resistance welding. The nugget is stronger under shear than in tension. Strength of nugget increases with thickness.


Contrary to uniform low-carbon steel plate, it's weld joint can have a brittle fracture mechanism: smaller critical stress at low temperature. Usually the temperature of brittle-ductile transition is below zero, ranging from -100^oC to -40^oC. Annealing increases the ductility of materials and prevents brittle fracture.


Crack resistance characteristic - the critical value of stress intensity factor depends on test temperature. Usually the brittle fracture for base metal, weld and heat-affected zone are similar, lower limits are equivalent. The characteristic of ductile fracture (upper limit) is usually smaller in the welded zone.


Presence of oxygen in weld material affects the embrittlement of the material. High oxygen content corresponds to embrittlement (relatively high transition temperature). 

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Residual Stress In Weld

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Residual Stress In Weld

Unhomogenous heating causes local thermal expansion of metals. This is reflected in residual stress after cooling. Residual stress is a tensile stress in the center of a weld. Tensile stress in a weld is compensated by compressive stress in base metal.


Weld metal is squeezed as it cools. During welding, edges move relative to each other, mostly perpendicular to the welding direction. Residual stress results in shrinkage of the structure.


The choice of welding sequence affects the distortion of the welded structure. If a welder uses opposite directions, the distortion is smaller.


If a weld is below the neutral axis the shape is concave up. If a weld is over the neutral axis the shape is concave down.


The angle b is small for small weld depths. The angle is not too large if the weld depth is equal to the thickness of the plate.


A similar effect can be observed in a tee-joint. For thin plates the displacement caused by residual stress is rather large, it decreases as thickness increases.


Residual stress is at a maximum for a rigid structure with a large number of welds and with closed loops. The structures are shown in order of increasing rigidity.


The residual stress decreases as annealing temperature increases. There are annealing procedures that can reduce residual stress to zero.

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DEFECTS IN WELD

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DEFECTS IN WELD

The quality of weld depends on many factors:

A. undercutting is caused by high amperage
B. porosity is caused by fast travel or dirty material surfaces
C. slag included in bead is caused by low amperage and short arc
D. lack of fusion is caused by low amperage and improper edge preparation
E. overlap is caused by electrode shaking

Surface defects perpendicular to tensile force are usually more dangerous than an inner defect of the same size. Lack of fusion, D is the sharpest and the most dangerous defect. 

The quality of manual welding is usually less than that for other methods. Some imperfections that are not dangerous:
A. Electroslag welding: 0.56 defects / 10 meters.
B. Automated welding under flux: 2.5 defects / 10 meters.
C. Electric arc manual welding: 35 defects / 10 meters. 


A. Incomplete penetration
B. Excess metal handing
C. Curved weld
D. Narrow weld at underside

Incomplete penetration means that tensile force lines meet obstacles on their path, causing high stress concentration. Other defects from the list do not cause high stress concentration.


Better melting takes place if there is a gap between the parts of Tee-weld. Residual stresses and cracking are smaller in this situation.


A. Cracking: toe, longitudinal, transverse, and underbead cracks
B. Incomplete fusion
C. Undercutting and underfilling
D. Surface damage: small droplets and arc strike (electrode touch)

Cleaning the weld area prior ro welding improves the fusion of weld and base parts. The operation can guard against incomplete fusion, B.

A surface defect E in the heat-affected zone can be considered the most dangerous defect in the Tee-joint. 


Residual tensile stresses in the vertical plate is less than in the first instance. The second instance is poor for heavy welded construction, showing lack of ductility through the thickness of the material.


Friction welding of steel bars. The bars are rotated relative to each other and squeezed together. 

A. Uniform weld is preferential
B. High pressure or low speed
C. Low pressure or high speed


Two defects can be considered as one if the distance between them is on the order of it's own size. The depth of the defect is more important than the length.

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STRESS CONCENTRATION IN WELD

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STRESS CONCENTRATION

Passing through welds, inner forces meet obstacles on their path. They concentrate at the ends of weld. The force lines bend smoothly as they pass through the welds, lines cannot bend sharply.


The figure shows a stress profile in the butt-weld. There is stress concentration in corners. Stress in the wider central section of weld doesn't exceed the nominal value.


This stress pattern is typical for a welded joint. Sum of the area (force) under the curve must be equal to the sum of the area under the line corresponding to nominal value.


Nominal shear stress is twice as large as the shorter welds. Stress concentration is higher if rigidities of connected parts are different. Stress is higher in the beginning of the short weld.


Stress concentration depends on the surface shape, not inside geometry of the weld. The larger the angle q, the smaller the stress concentration factor. In order to fulfill these requirements, a special cutting operation is made. Fatigue strength of a machined joint is higher than the first one.


A large fraction of inner force goes through the end nugget in the row. The numbers indicate approximate values of the parameter.


Stress concentration can be evaluated by stress concentration factor a_s which is equal to the ratio of maximum and nominal stresses.

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weld and welded joints in detail.

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WELDED JOINTS

Welding is a method of joining two parts by melting and/or pressing them together.
Welds are permanent joints of metals (iron, steels, aluminum alloys, titanium alloys) or plastic materials.
Aluminum and steel cannot be melted together since they have different melting points (temperatures).


There are the following types of welds: 
A. butt-weld 
B. corner weld
C. T-weld
D. lap weld


Static and fatigue strength is highest for a lap-weld in comparison with other joints from the list.

n forge welding, A, for steel chain manufacturing, two parts are heated and then hammered together.
Gas welding, B, uses an oxy-acetylene flame to heat the metal and a rod of metallic filler material.
In electric-arc welding, C the filler rod forms one electrode and the metal itself another. Electric current passes across the gap between the electrodes by arcing or sparking and melts the surfaces together. The electric current (ac or dc, alternating current or direct current) is stable with an amperage of 150 - 500 Amperes. Industrial power sources usually work with voltages between 22 - 36 Volts.
Contrary to gas welding, electric-arc welding is used for thick pieces of metal and high temperature.
If an electric current passes through two metal surfaces in close contact the temperature rises and melts the surfaces together known as spot welding or seam welding, D. This method is used in mass production.


There are the following types of butt-welds:
A. without a gap
B. with a gap
C. with one-sided bevel
D. with two-sided bevel

A butt-weld without a gap is used if there is a guarantee of full melting. A butt-weld with a gap is used for thin-walled structures. 

Edge preparation guarantees full melting and improved quality of the joint. There are Y-, U- and X-shaped edge preparation. U-shaped edge preparation is used instead of X-shaped edge preparation for thick parts if it is not possible to weld from two sides. Joints can be welded in a single pass or by few passes.


Weld joining of thick tubes also involves edge preparation, B in contrary to thin-walled tubes, A. Additional casing, C can be used.
Welds with a В«smoothВ» transition correspond to a stronger structure.
A great deal of skill is required to produce a reliable weld.


Arc heat is expended during the melting of metal electrodes as it is in the heating of base parts. Approximate values of arc heat expended in shielded metal-arc welding: 

A. Dissipation into the neighboring environment - 20%
B. Transition with molten drops - 26%
C. Vaporization of electrode metal - 24%
D. Absorption by base metal - 30%

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weld and MECHANICAL PROPERTIES

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MECHANICAL PROPERTIES

Mechanical properties are directly related to the material structure of weld and base metal. Weld metal is comprised of the metals of electrode and molten edges of base parts. High temperature affects the structure of base metal. Grain size enlarges at boundaries of the weld joint - in the heat affected zone (HAZ). Large grains have relatively poor mechanical properties. Outside the HAZ grain size is the same as in the base metal.

In molten weld metal the grains grow from colder parts of base metal.


Lower amounts of molten metal correspond to lower heat and smaller HAZ. HAZ is smaller for electron beam welding: 

A. Arc welding, butt-joint 
B. Electron beam welding, butt-joint 
C. Multi-layer arc welding, butt-joint 
D. Gas welding, build-up weld


For steel welds mechanical properties are usually highest at the weld. The hardness is lower in the weld.


Mechanical properties of a high-strength steel weld depend on carbon content in the material. Increase in strength corresponds to decrease in ductility (elongation is a measure of ductility).


Fast cooling, similar to quenching, could result in strength increase. Yield strength and ultimate tensile strength increase with higher cooling rate.


There are special tests for weld joints. The welded specimen is tested until a crack first starts. The weld is stronger if it lies for a long time before a test.


A large angle a characterizes the ductility of the weld joint. There are welded joints for which the angle could reach 180^o.

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Welding-Question and answers-for-Students

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Welding-Question and answers-for-Students

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