Weld-cracking:
Origins and Avoidance.
Solutions with Effective, Powerful Advice
What is Weld-cracking?
Weld-cracking is obviously a most dangerous situation that compromises the
ability of a fusion welded structure or machine to perform the design
functions in a safe and consistent way.
Cracks are discontinuities presenting fracture. They have two dimensions quite
large relative to their opening, and a sharp tip that tends to propagate
under stress. Therefore cracks must always be avoided or eliminated.
In previous pages we briefly considered Weld Cracking as a danger and cracks as
unacceptable defects (Click on
Welding Defects) and discussed some welding conditions that may promote the appearance
of cracks in Alloy Steels (Click here).
Here we wish to present the different types of Weld-cracking discussing the
conditions for their appearance and the measures that can be employed to avoid
them.
Four main groups of Weld-cracking are described, known as hot cracks,
microfissures of Heat Affected Zones, cold cracks and lamellar tearing.
Hot cracks
Of the four types of Weld-cracking discussed here, hot or solidification
cracks appear near the end of the solidification process in the fusion zone.
They result because of the incapacity of the molten and semisolid material to
absorb without tearing the thermal shrinkage strains due to weld solidification
and cooling.
Weld-cracking, formed in the weakest places like grain boundaries, relieves the
associated stresses. Some microstructures are more prone than others to the
formation of solidification Weld-cracking.
The presence in the metal composition of low melting elements like
sulfur, lead, bismuth etc. is deleterious. The tendency of certain metals to
exhibit hot Weld-cracking is called hot shortness, caused by low-melting
constituents segregated at grain boundaries. For welding, these susceptible
compositions should be avoided. (Example: free machining steels or stainless
steels).
Welding parameters should be selected with care, because they can influence the
formation of hot Weld-cracking by the rate of strain application (lower
rate = lower danger), which is higher for welding processes that cause rapid
solidification and cooling.
Joint design should strive to limit the molten mass, joint gap should be
contained and good fit-up should be preferred, in order to reduce shrinkage
strains and Weld-cracking. Weld speed, an important productivity factor, may
have to be contained at such levels that do not cause solidification Weld-
cracking, other data being constant.
For any given alloy there may be a possible range of welding parameters
that permits avoiding hot Weld-cracking. This range is narrow for alloys with a
large melting interval, that are more prone to hot shortness.
On the contrary, alloys with a narrower melt interval enjoy a larger range of
possible welding parameters. It so happens because cumulative shrinkage strains
are proportional to the magnitude of that melting temperature range, that
depends essentially on chemical composition.
Crater cracks are a common occurrence at the end of a weld if the welding
current is interrupted abruptly. It should instead be reduced gradually,
possibly directing the arc on top of an already welded beam.
Solidification cracking susceptibility can be measured in different alloys by
performing a series of tests (varestraint testing) and measuring the
maximum length of the induced cracks. The results permit to rate the different
compositions as to their hot Weld-cracking susceptibility.
Heat Affected Zone Cracks
Microfissures are cracks that develop in a place of partial melting
occurring near the Heat Affected Zone, bordering on the fusion line. As partial
melting occurs below the melting point of the alloy, especially if secondary
elements segregation is present along grain boundaries, local thermal strains
that manifest themselves can induce grain boundary separation similar to what
explained above.
HAZ cracks, may occur with or without the presence of liquid. In the presence of
brittle structures associated to intermetallics, failures happen if the thermal
stresses associated with the weld thermal cycle exceed the local tensile
strength at the actual temperature.
Some alloys undergo microstructural transformations in the Heat Affected Zone as
a consequence of thermal cycles brought about by welding. Typical is the
martensitic transformation, producing a hard and brittle crack-susceptible
structure, which is accompanied by a volumetric change inducing noticeable
stresses.
Cold cracks
Hydrogen induced delayed cold Weld-cracking occurs as a consequence of
contamination with this gas being absorbed in the molten metal while welding.
Hydrogen sources should be avoided and removed: not only water and humidity must
be controlled by drying and preheating. Organic matter contamination has to be
removed by cleaning and good housekeeping.
Three conditions are necessary for the formation of cold cracks: the
presence of a certain stress, of a suitable microstructure and at least a
critical level of hydrogen.
A long standing theory claimed that atomic hydrogen moving interstitially within
the solid metal aggregated to molecular form with pressure increase sufficient
to tear apart metallic bonds.
This has been recently challenged and replaced by a competing model involving
the presence of preexisting defect sites in the metal, where, under stress,
hydrogen preferentially diffuses, reducing the local cohesive strength.
Fracture occurs when the remaining strength falls below the intensified
stress level. Hydrogen would then accumulate in the newly generated voids and
the process would repeat itself.
Hydrogen induced Weld-cracking is a serious cause for concern especially with
high strength steels. Preheat and postheat procedures, depending on
material type but also on thickness and joint constraints, are commonly employed
to reduce the danger of hydrogen Weld-cracking.
The purposes are to eliminate water, to reduce cooling rate in order to avoid
dangerous structures (untempered martensite), to temper and soften this hard
structure if formed, to reduce and relieve thermal stresses, and to allow for
hydrogen gas to escape if entrapped.
An Article on Preheating, a technique useful for reducing the risk of Weld-cracking, was published in Section 2 of Issue 37 of Practical Welding Letter for September 2006. To read the article click on PWL#037.
Lamellar Tearing
Lamellar Tearing is a kind of Weld-cracking that forms beneath a weld. It is a
dangerous condition occurring when certain plate materials presenting low
ductility in the thickness (or through) direction are welded to a perpendicular
element.
Tearing always lies within the base metal, generally outside the HAZ and
parallel to the weld fusion boundary. The problem is caused by welds that
subject the base metal to high shrinkage stresses in the thickness direction.
Tearing is not visible on the outside, generally, but it can be found by
ultrasonic testing.
The problem can be avoided by selecting base material having adequate ductility
in the thickness direction, that has been screened by ultrasonic testing for
absence of laminations or other internal defects.
It can also be managed by paying due attention to joint details and avoiding
massive welds that develop significant shrinkage stresses. Although appearing
sometimes long after welding, a structure stress relieved and ultrasonically
tested for lamellar tearing after welding should be considered safe.
Special design details were developed for corner joints, to put the bulk of
shrinkage stresses in the direction of rolling of one of the elements, and
freeing the other element from stresses in the through direction.
An Article on Lamellar Tearing was published (7) in Issue 46 of Practical Welding Letter for June 2007. Click on PWL#046 to read it.
* * *
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Checking the hardness of an unknown material before welding may help avoid Weld-cracking.
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Weld-cracking is a major concern for any welding job. It is important to know
causes and prevention. See here...
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