post galvanizing toe cracks in welds - tsdos

SEPTEMBER 5 - 7, 2018

Post Galvanizing Toe Cracks in WeldsCan We Continue to Rely on a “First Aid” Approach to this Problem?

Wesley J Oliphant, PE, AWS-CWI, F.SEI, F.ASCEPrincipal, Chief Technical Officer, Exo

Zachary J OliphantPrincipal, President, Exo


First: What is a post galvanizing “toe crack”?

• “Toe cracks”are a delayed weld cracking in the “toe” of the pole shaft to base plate (or flange plate) weld.

• Post galvanizing “toe cracks”are those that occurimmediately following the hot-dip galvanizing process.

• Typically occur at the “toe” of the reinforcing fillet weld on the shaft side.

• Typically occur at the “corners” of multisided formedshafts


AWS D1.1 Structural WeldingCode ASCE/SEI 48-11 Design of SteelTransmission Pole Structures

Two important documents:


ASCE/SEI 48-11 Design of Steel Transmission Pole Structures

…..” For galvanized members with large T-Joint connections, such as base plates, flange plates, etc., ultrasonic nondestructive weld testing shall be performed on 100% of all such joints, not only before , but after galvanizing to ensure that no cracks have developed.”

Paragraph 10.3.5 Weld Inspection.


AWS D1.1 establishes numerous requirements, that if diligently followed, will result in sound welds, both before and aftergalvanizing.

AWS D1.1 Structural WeldingCode


AWS D1.1 Structural WeldingCode ASCE/SEI 48-11 Design of SteelTransmission Pole Structures


Contributing Factors • Base Plate to Pole Shaft Wall Volume Ratio

Source: Aichinger, Richard, Higgins, Warren, (2006). “Toe Cracks in Base Plate Welds, - 30 Years Later”, Proc., ASCE Electrical Transmission Line and Substation Structures Conference, ASCE, Reston VA, 274-285.


Contributing Factors • High Residual Tensile Stress in the deposited

weld during solidification

As a weld cools, it contracts providing residual tensile stress

Higher heat input during the deposit of a weld is more prone to higher residual tensile stress

Increased restraint of the joint will increase residual tensile stress




Monitor Pre-Heat and Interpass Temperatures carefully.

The Aichinger & Higgins study inferred that the AWS D1.1 minimum pre-heat requirements were insufficient in the effort to prevent toe cracks, and should be increased.


Contributing Factors • Heat Input during welding

Heat Input (Kjoules/inch) = Amps x Volts x 60

1000 x Travel Speed (inches/min)

or

Amps x Volts x 60Travel Speed (inches/min) =

1000 x Heat Input (Kjoules/inch)

Recommended: limited to less than 65 kJoules/inch



So within the recommended limit of less than 65 kJoules/inch

Volts = 28

Amps = 550 amps

Travel speed = 16 in/min

28 volts x 550 amps x 601000 x 16 inches/min

= 57 kJoules/in

But if travel speed changes to 12”/minThe heat input goes to 77 kJoules/in., above the recommended limit of heat input.


Contributing Factors • Highly restrained joint characteristic


Contributing Factors • Stress concentrations / stress risers due to weld

profile being deposited.

Abrupt change increases potential to crack under tensile load


Contributing Factors • Stress concentrations / stress risers due to

welding processes followed

Starts and stops of the weld beads

Toe Crack


Contributing Factors • High Carbon Equivalency of the Base Metal


Contributing Factors • High Tensile Strength of the Base Metals

Recommended to limit the Tensile Strength to a maximum of 20% increase over its specified minimum value.

Example A572-65 has a minimum 80 ksi TS, it would be limited to 100 ksi TS as delivered.


Contributing Factors • Exposure to thermal gradients during the

galvanizing process

Stress is an important factor, and thermal stresses may be very high where extremely large parts are being gradually immersed into the 840 deg. F. molten bath. This creates a thermal gradient rise at the point of immersion in the zinc.


Contributing Factors • Steel and Weld Chemistries that include high

potential for Liquid Metal Embrittlement (LME)

Galvanized-induced cracking (GIC) is generally considered a form of liquid metal embrittlement (LME) or liquid metal assisted cracking (LMAC).

A contributing factor to LME can be excess silicon in the base or weld metals

Large amounts of silicon in the weld filler metal will produce a large and undesirable buildup of zinc on the welds. This zinc will form several post-galvanization compounds of Fe + Zn on the surface of the weld, changing the transverse stress component of the weld.


Proactive Approach (Prevention)

vs. Reactive Approach

(Resort to Detection & Repair)


Proactive Approach (Prevention)1. Insure all WPS’s and PQR’s are properly prepared and documented,

including the requirement for Charpy testing in the deposited weld metal and the HAZ.

2. Make sure all welders are properly certified per the requirements of AWS D1.1 Structural WeldingCode.

3. Make sure all welders are properly trained on the requirements of the WPS’s– preheat, interpass temperature, volts, amps, travel speed, joint prep,etc.

4. Make sure all materials are what they are supposed to be – Charpy’s, chemistries, mechanicals,etc.


5. Calculate Carbon Equivalencies of the specific steels being welded. Make adjustments of preheat as needed.

6. Review chemistries of the specific steels and weld materials being used, particularly silicon content.

7. Review Tensile Strength of materials being welded.

8. Pay attention to the sequence of weld passes. It is always preferable to finishthe weld with the final passes occurring in the middle of the weld.

9. Pay attention to the size and type of weld passes (stringer beads are preferable to weaving.



And finally:

10. Make absolutely certain that preheat and interpass temperatures are maintained for the entire plate through the entire welding sequence (start to finish).

11. Make sure the weld profile is proper- no excessive concave or convex surface to the fillet weld.



Questions?

Wesley J. Oliphant, [email protected]

mailto:[email protected]

post galvanizing toe cracks in welds - tsdos

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