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Know the Important Welding-properties
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Welding-properties are strength, hardness, ductility, fracture toughness.
However, in specific cases, properties may change: depending on welding history, and on the presence of certain factors, catastrophic failures may occur because of brittleness, stress raisers, triaxial stresses, nil ductility.
Below transition temperature, and lacking crack arrestors, welded structures may not stand the stresses.
Therefore it is important to know the joining questions needing answers. These are some of the items developed in this page for the benefit of interested readers.
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In a previous page on WELDING INFORMATION, it was recommended that all necessary information be clearly indicated in the binding drawings involving welding.
Here we are going to deal in some more detail with important Welding-properties affecting the survival of welded structures in cold weather and in other dangerous situations.
Tip!: Even if the problem is not met every day in the average welding shop, it is probably useful and helpful to know that there may be extreme situations where common wisdom and experience are not sufficient to prevent catastrophic failures.
If the content of this page on Welding-properties will lit a warning red lamp when you meet such a case, and save you from a painful experience, it will have accomplished its task.
The Welding-properties of joints considered here are those which have been studied most because of their tragic consequences in important welded structures like ships and bridges.
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Historic catastrophic failures...
One of the first problems in Welding-properties that needed thorough investigation originated from spectacular setbacks.
The most famous example in Arc Welding History is the occurrence of horrific failures which plagued eight all-welded ships of the Liberty class, built during World War II, which suddenly broke in two as a consequence of exposure to high seas under particularly cold weather.
We presented a short account of this story in a page on Arc Welding Process.
Tip!: For a short overview of these events, the interested reader is referred to an article that can be found by clicking here.
The lessons learned during the thorough investigations that followed to clarify the underlying causes of inadequate Welding-properties, were essential to the progress of knowledge in this particular metallurgical matter.
It was found that it was not welding as such to blame: rather it was an unknown before characteristic of certain steels to become susceptible to brittle fracture at low temperatures, together with a design that did not anticipate the need to include "crack arrestors" to avoid the catastrophic instant spread of cracks through the whole structure.
All important material property...
That material property has since been identified and named. It is now called fracture toughness and it is one of the most important characteristics of the material that has to be taken into account while designing welded structures for cold weather exposure.
Ductility is always needed
What are the factors reducing ductility, the most wanted property of tough steel? It was found that composition is important, because certain elements are detrimental to this property.
Further studies of Welding-properties clarified the fateful influence of internal constraints and of stress raisers, that is of geometric discontinuities where locally the stress reaches much higher levels than in the surroundings, that sudden load by impact is dangerous, that low temperature can be fatal.
It was found that the Welding-properties of a normally ductile steel, welded into a structure, are not guaranteed under adverse conditions.
In presence of one or more of the above factors, the welded steel structure could reach the state of nil-ductility or of total brittleness, and then break suddenly without warning, that is without any appreciable deformation.
The universal requirement that cracks or linear cracklike features be absolutely absent from a welded structure derives from the proven fact that even tiny cracks can grow under repeated stresses and reach in time a critical dimension where they propagate rapidly, in conditions of nil-ductility, causing a catastrophic failure of the whole structure.
Dangerous brittleness at low temperatures
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These studies were conducted using, among others, an impact test of a notched specimen, called Charpy V-notch test, with different materials and conditions at decreasing temperatures.
Tip!: For every steel one can find the temperature of safe usage, above which the fracture is ductile and below which the fracture gradually becomes more and more brittle with absorption of minimal energy. That is the transition temperature.
What to do...
Whenever there is a suspicion that operational conditions might be extreme, one should take the following precautions:
- Material should be selected so that it presents adequate fracture toughness even in anticipated extreme operational conditions.
- Design should limit stress raisers by devoting much attention to details.
- Welding procedures should provide the best workmanship accomplishments.
Another field of scientific inquiry to which considerable investigation efforts were devoted is that of fatigue strength of welded joints.
That is because there was a justified suspicion that fatigue, that is the progressive development of a crack, stood at the origin of certain failures.
The results of the main Welding-properties investigations were translated into Codes that change the permissible stress in the members as a function of their geometry. By this procedure one can simplify the design process.
An Article on Fatigue Failures was included in our Practical Welding Letter No. 04 of December 2003. To see the article click on PWL#004.
In general one should always be aware of the influences of local welding on the Welding-properties of the immediate surroundings. Especially when attempting to repair a damaged structure.
One should remember that welding heat propagates to nearby areas tending to re-temper the material to lower hardness: is this allowed? Is it safe?
If the carbon content is sufficient, re-hardening could occur. This means that the affected material underwent uncontrolled metallurgical transformations making it hard and brittle. This is a dangerous situation to avoid even if cracks were not developed.
One further element that should be considered regarding the Welding-properties of structures is their resistance to corrosive attack.
One such occasion when welding may impair the corrosion resistance of the base material has been addressed fully in another page of this Site, on welding of Stainless steel.
This means in general that even a material known to present adequate corrosion resistance in a certain corrosive environment, may loose some or all of its properties if welded with procedures that do not take into account the influence of welding on corrosion resistance.
Tip!: Susceptibility to corrosive attack is higher when different materials are in contact in a corrosive environment, when the corrosive agent may remain trapped in cracks or between opposite surfaces, when residual stresses are present, when flux remainders that were not eliminated sit on or near the welded joint.
If a protective layer (i.e. paint, plating etc.) was locally removed during welding, it should be restored by the same or other means as applicable.
Or High Temperature...
High temperature oxidation and scaling resistance are material characteristics which may or may not be adequate for certain exposures.
To counter the tendency, if needed, one must select materials presenting some measure of corrosion resistance at their design service temperature. Filler materials should be compatible with base metals and the heat of welding should not generate harmful structural transformations.
It is not within the scope of this short overview to cover in any details these important issues. If the problems were not addressed in advance they will usually pop up with failures in service. Repeating old errors will bring new failures.
The complete and real cost...
People are generally reluctant to consider expensive material selection for applications where they would be most advisable, because of the initial high cost involved.
The correct calculation for the plant should include also, in case of failure, shut down costs, maintenance and repair costs, lost productivity, and dangers to people and property. An initial elevated cost may well be the most economic if the whole life cycle is considered.
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Watch the following TWI Video
The Big Bang - The facts behind brittle fracture
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