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Participant

Join Date: Jul 2010
Posts: 1

Thread Laps & Cracks

07/22/2010 2:19 AM

What is the difference between thread laps & cracks?

What are the acceptable limits of laps & cracks?

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Power-User

Join Date: Jan 2010
Location: Mineral wells Tx
Posts: 371
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#1

Re: Thread Laps & Cracks

07/22/2010 3:17 AM

According to AWS seams and laps are linear discontinuities found in rolled products that result from improper steel making practices. When they are parallel to the principal stress, seams and laps are generally not considered to be a critical defect. When perpendicular to the applied or residual stress, they willoften propagate as a crack. Welding over seams and lapscan cause cracking.

Cracks may occur in the weld or base metal, or both, when localizate stresses exceed the strength of the material. Craks can be classified as either hot craks or cold craks.

Hot craks, normally not acceptable) develop at high temperatures. They commonly from on preferential solidification of alloy of the metal near the melting point.Hot cracks propagate between the grains when the preferential solidification occurs. Cold caracks develop after solidification is complete and are often service -related.

Regards WP

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Guru

Join Date: Jul 2006
Location: "Dancing over the abyss."
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#2
In reply to #1

Re: Thread Laps & Cracks

07/22/2010 11:35 PM

A lap is a portion of metal which is folded over the main product. Think of how lap is used in the word Overlap and you will get it. A "combover" is a Non metallurgical "Lap"

A crack is an absence of material or void in a linear orientation.Usually cracks are jagged rather than smoothly straight,A crack which is smooth and straight is usually determined to be a SEAM which was created in hot rolling by the presence of either entrapped gas or sulfur which excaped at the high temperature and mechanical forces of rolling.

Cracks can be longitudinal or transverse. Meaning no disrespect to the AWS source cited, above, But seams are normal occurrence in Steel products, and in the case of coldfinshed steel bars, are rejectable only when they exceed the seam allowance per side.

Laps are always rejectable as "poor workmanship, " and non seam cracks are generally held to be rejectable.

ASTM A 108 would be the controlling authority for cold drawn steel bar products, The blue print or engineering drawing for the threaded part exhibiting the crack or lap would be the authority in a contract relation.

Good luck..

Milo

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