hot cracking during welding

7/27/2019 Hot Cracking During Welding

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HOT CRACKING DURING WELDING

byNabeel Ahmed Qureshi 21 Sep 2011, 04:27

What is this \Hot cracking phenomena during welding.Which alloys are susseptible to this

kind of issue during welding. How to control/avoid Hot cracking during welding ??

Regards

nabeel

Nabeel Ahmed Qureshi

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Re: HOT CRACKING DURING WELDING

byben 21 Sep 2011, 05:46

Hot cracking is a phenomena where weld starts cracking within the solidification range (at

higher temperature - > 1200 C). It can occur in all metals. Following are the main reasons:

a. Restrained piping (already under stress)

b. High welding current

c. Pre-heating non-compliance

d. Incompatible filler material

e. Travel speed (too fast)

ben

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byirish 22 Sep 2011, 07:46

"Hot Cracking" as the term depicts itself is formation of cracks when the metal is hot

(during welding just at the solidification state). Reasons have been correctly identified by

ben. I have seen formation of cracks normally when performing welds with incompatible /

wrong filler or when carbon steels are welded to stainless steels with a filler resulting in astructure prone to cracking. These cracks also appear when pipe is not adequately or over

supported causing stress at the joint location.

These cracks can appear in all metals but due to filler these are most commonly seen in

welding of stainless steels when martensite formation takes place.

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bymechcolor 22 Sep 2011, 18:14

Hot crackingis a welding defect that occurs with temperature higher than 1200C and its

due to the presence of impurities with lower melting point than the base metal. The

cooling is higher on the side of the weld and its from there that grain grows (dendrite)

oriented to the center of weld thats the last to become solid. On this stage, occurs a

formation of an impurity film, that promotes a reduction of melting point of the weld.

Meanwhile, weld doesnt sustain the tensions due to the material contractions during

solidification process, originating the center line crack on the weld.

Factors

Temperature > 1200C

Carbon (C), sulphur (S), phosphorus (P)

Prevention

Material with some percentage of delta ferrite in case of austenitic stainless steels (300

series)

Control on sulphur and phosphorous contents (impurities)

Manganese, chromium, molybdenum addition

mechcolor

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byoctane 23 Sep 2011, 02:09

As the name implies, these cracks form while the weld temperature is high and are usually

related to solidification; crack growth is typically assisted by cooling stresses induced in

the weld. Solidification cracking occurs when there is low solubility for alloying elementsin the primary phases that solidify during cooling. As the solidifying grains grow, they

exclude the impurities and/or minor alloying elements and push them towards the center

of the weld.

The impurities/alloying elements often do react with the dominant element to form low

melting phases that generally have poor strength. Typical

examples would be Iron-Sulfur compounds in steels and Aluminum-Copper compounds in

Aluminum alloys. Hot cracks typically occur in the throat of the weld and can extend along

the length of the weld, producing longitudinal cracks. In some fusion weld applications,

such cracks can also form at the end of the weld line and produce a star-shaped pattern of

cracks radiating outwards; such cracks are referred to as crater cracks.

In some Aluminum alloys, hot cracks form in the HAZ (heat affected zone) adjacent to the

fusion zone. Since the segregation of phases occurs along grain boundaries, hot cracks

grow intergranularly. Common alloy systems that suffer from hot cracking are steels and

aluminum alloys. Carbon steels with excessive sulfur are prone to hot cracking and such

tendencies can be reduced by using either steel with low sulfur or with high enough

Manganese that can combine with sulfur and render it inactive. Hot cracks are also found

in laser welded 316 austenitic stainless steels that solidify as primary austenite. Weld

cracking can be avoided by using a filler such 304 or 308 that provides enough ferrite

stabilizers; solidifying ferrite has a greater solubility for impurity sulfur and keeps it out of

trouble. Aluminum 6061 suffers similar problems that can be remedied in the fusion zone

by mixing with 4047/4043 Al alloys.

Hot cracks can also form when the weld is too weak to support the two components and

have been observed as root cracks. Analysis of the cracked region under and SEM to

identify elemental segregation is the first step in evaluating hot cracks.
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hot cracking during welding

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