Understanding the Sources and Remedies of Hydrogen-Assisted Cracking

8/11/2019 Understanding the Sources and Remedies of Hydrogen-Assisted Cracking

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Understanding the Sources and

Remedies of Hydrogen-Assisted Cracking

in Transmission Pipeline Applications

Derick Railling

Product Manager Global Onshore Pipeline


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Presentation Outline

Introduction What is hydrogen?

Hydrogen in welding Poor elongation

Cracking

Sources of hydrogen Ambient moisture

Residual oils on plate (from machining/cutting) Lubricants on wire

Raw materials

Filler metals

Filler metal storage

Importance of pre- and post-weld heating Fisheyes

Summary


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What is Hydrogen?

#1 on the periodic table

Lightest element

Most abundant element in the universe Highly flammable reacts violently with oxygen

to form water

So What?
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=MYNb0w001KKyxM&tbnid=dic_3epvlv00aM:&ved=0CAUQjRw&url=http://www.chemistryexplained.com/elements/C-K/Hydrogen.html&ei=xHObUbPTBcTtygHl5YHIDA&bvm=bv.46751780,d.aWc&psig=AFQjCNH9SbBXJxk3Q4Hk9l3no5Z6fU8G7Q&ust=1369228528192466http://../upload.wikimedia.org/wikipedia/commons/e/ee/Electron_shell_001_Hydrogen_-_no_label.svg


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Hydrogen in Welding

Can squeeze into small spaces in the steellattice due to small size.

Very mobile - especially at higher temperatures.

The solubility of hydrogen in iron (or steel) ismuch higher in the gamma () phase, which

exists at higher temperatures than in the alpha

phase (). The alpha phase exists at room

temperature.


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Sources of Hydrogen

Moisture in the air Oil, grease, paint, coatings on the base plate

Welding consumables

Lubricant in seam Raw materials

Moisture in coating (stick electrodes)

Packaging/storage

Shielding gas


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Hydrogen Solubility


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Hydrogen-Assisted Cracking (HAC)

Also referred to as delayed cracking, coldcracking, toe cracking, underbead cracking.

When steel cools and transforms from to ,any hydrogen that is present becomes trapped

in the matrix. Over time, the hydrogen diffuses and collects asH2gas near defects and grain boundaries in thematrix.

As the hydrogen gas builds up, the stress levelsincrease until cracks form.

HAC compromises the integrity of the weld.


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http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=s0wjbBO-27tZJM&tbnid=A2_HXDyLfF2F5M:&ved=0CAUQjRw&url=http://subhankar4students.blogspot.com/2011/12/welding-defects-cracks.html&ei=AnObUaDGAu2gyQH-8IDoCw&bvm=bv.46751780,d.aWc&psig=AFQjCNG0MQinh3QqlRj2sXXHTkmT5sctUg&ust=1369227956892294


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Video Showing Hydrogen Cracking

http://www.youtube.com/watch?v=bv9ApdzalHM
http://www.youtube.com/watch?v=bv9ApdzalHMhttp://www.youtube.com/watch?v=bv9ApdzalHM


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Requirements for HAC

High strength steelCrack Sensitive

Microstructure

Inherent to weld metalStress

Controllable element

for HobartHydrogen

As a producer, we can help reduce HAC via controlling hydrogen in our products.


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Reducing the Chances of HAC

Low-hydrogen filler metals

Improving pre- and post-weld heating

Maintaining interpass temperatures

Welding process change

Material handling

Storage methods

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Filler Metals

How to Choose

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Filler Metal Selection

All major processes have an optional hydrogendesignator (GMAW, FCAW, SAW, SMAW).

Filler metal is tested to ANSI/AWS A4.3

(Standard Methods for Determination of theDiffusible Hydrogen Content).

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How to Tell if a Filler Metal is Low

Hydrogen

Look for a hydrogen designator in the AWSclassification, e.g.:

E7018-1 H4R (SMAW)

E70C-6M H4 (Metal-Cored) E71T-1C H8 (FCAW)

E71T8-Ni1 J H8 (Self-Shielded FCAW)

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What to Use?

Cellulosic SMAW electrodes (XX10)

Commonly used for root pass, fill pass and cap

pass welding.

Good penetration and performance characteristics

(vertical down for pipe welding).

However, these electrodes provide hydrogen levels

far exceeding 16ml/100g (25-60ml/100g).

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Not recommended for steel with a UTS exceeding 60ksi.

Nota good choice for high strength pipe!


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Low-hydrogen SMAW electrodes (EXX16 /

EXX18)

Typically used as fill and cap passes.

Hydrogen designations of H4 or H8. Less penetration than EXX10.

Different performance characteristics (vertical-up

for pipe).

Not acceptable for root pass welding on pipe.

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What to Use?


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Hobart metal-cored wires for root pass welding Typically low levels of hydrogen < 4ml/100g.

Used with Miller RMD (modified short circuit)in the

vertical-down position.

Typically only prone to surface moisture on the

metal powders.

Metal-cored wires are typically prone to surface

moisture on the metal powders, whereas FCAWproducts can contain non-metallic raw materials

that may absorb moisture to a greater degree.

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What to Use?


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FCAW self-shielded Typically < 8ml/100g of weld metal.

Special self-shielded products are designed to run

vertical-down on pipe.

FCAW gas-shielded Typically < 4ml/100g or < 8ml/100g of weld metal.

Like low-hydrogen SMAW (EXX18) these products

are designed for vertical-up welding.

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What to use?

FCAW wires feature a number of hydrogen scavengers like fluoride compounds,

which combine with hydrogen to remove it from the weld!


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20

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Metalloy Metal-Cored Wires Fabshield FCAW-S Cellulosic EXX10

3.66.4

34.8

HydrogenLevels

inml/100g

Diffusible Hydrogen Levels


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Hydrogen Difference 10 Min. After Being in Mineral Oil

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E6010

E71T8-Ni1 J H8 E70C-6M H4


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Storage of Filler Metals

All filler metals should be stored in a clean, dryarea and should remain in the original packaging

until ready to be used.

Keep the filler metals sealed.

Minimize or eliminate the transfer of filler metals

from cold to hot environments.

Prevents condensation.

If moved, filler metals should be protected andallowed time to normalize to ambient temperature.

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Welding Practices to Reduce Hydrogen

Metal-cored, flux-cored and solid wire Use a longer contact tip-to-work distance (CTWD).

Keep within the recommended range.

Welding outside of the recommended range can lead to:

Loss of shielding gas Reduced penetration

Poor operabili ty (rough arc, spatter, kick off)

Increased CTWD allows for greater pre-heating of

the wire.

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The Importance of Heat

Pre-heat, Post-Heat and Interpass

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What is Happening?

Heating and rapid cooling during welding.

Puts stresses into the weld.

Creates hard, highly-stressed microstructures that

are more susceptible to HAC. Rapid cooling = less opportunity for hydrogen to

diffuse out of the weld and heat affected zone

(HAZ).

Rapid cooling = greater opportunity for cracking.

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Traditional Heating Methods

Propane Torches Hydrocarbon (compound of carbon and hydrogen).

The process of igniting and applying a flame to the

pipe introduces hydrogen into the weld joint.

Does not ensure uniform heating.

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Drives and keeps moisture out of the weld

Reduces the chances of hydrogen-assisted cracking.

Heats Uniformly

Ensures heating consistency.

Improves Safety Eliminates open flames & exposed heating elements.

Provides faster time-to-temperature

Increases productivity.

Induction Heating (Miller ProHeat 35)


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Fisheyes

What They Mean to Pipeline

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Fisheyes

Fisheye:A discontinuity, attributed to the

presence of hydrogen in the weld, observed on

the fracture surface of a weld in steel consisting

of a small pore or inclusion surrounded by anapproximately round, bright area.

(AWS A3.0M/A3.0:2010)

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The center is known as the pupil and the areaaround the pupil is the iris.

Fisheyes

Pupil Iris


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The pupil is the only original imperfection

associated with the fisheye.

The iris has fractured due to local hydrogen

embrittlement. Hydrogen collects in pores or near inclusions.

When a slow strain rate is applied fisheyes

may occur (example: nick-breaks or tensilepulls).

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Fisheyes


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Fisheyes have no effect on mechanicalproperties (tensile and yield strength, or impact

toughness).

However, they do affect elongation and

reduction of area.

Fisheyes are artifacts of testing a steel that

contains hydrogen and not defects in and of

themselves.

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Fisheyes

API 1104 (Section 5.6.3.3) states Fisheyes, as defined in

AWS A3.0, are not cause for rejection.


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Things to Focus On

Proper handling of the pipe

Watching for outside sources of hydrogen

Filler metals

Selecting a low-hydrogen filler metal when weldinghigher strength pipe.

Pre-heat, interpass, post-heat

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Summary

Focusing on low hydrogen welding practicesshould be a priority to ensure a high integrityweld.

All factors must be considered and only

changing one variable is not sufficient. Changing to a low hydrogen filler metal does not

guarantee a reduction in HAC, however, itgreatly reduces the risks.

Proper heating methods mustbecome anintegral part of the welding procedure.

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Questions?

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Understanding the Sources and Remedies of Hydrogen-Assisted Cracking

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