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Fwd: Re: Lincoln Arc

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Tom-

In response:

The "Lincoln Rod Problem" is not specifically a "Lincoln" 
problem.  The "problem" is that a specific electrode used in FCAW, 
specifically, the E70-T4 electrode, produces low toughness weld 
metal, with typical Charpy V-Notch values on the order of 5ft-lb at 
70deg. F or less.  Low toughness metal (weld or base) is quite 
vulnerable to brittle fracture.  Fracture initiates as a result of 
stress exceeeding the ultimate tensile strength of the material.  
Usually, steel and weld metal behave in a ductile manner, meaning 
that fracture occurs after large deformation.  In brittle fracture, 
failure occurs without large deformation and progresses rapidly 
through and across the section.  Brittle fracture is common in low 
toughness materials and can be initiated by stress concentrations at 
notches.  The stress concentration essentially raises the stress 
very locally, above the ultimate tensile strength.  This produces a 
small fracture.  The small fracture, in turn, increases the notch 
effect.  This increases the stress concentration - producing more 
fracture - essentially resulting in a runaway reaction.

Stress concentrations (notches) can occur from many causes.  - 
Geometry of a joint - for example- the basic "T" joint produces 
stress concentrations, unless the joint is rounded at the fillet.  
Metallurgical changes in the material can produce notch effects - 
e.g. an arc strike on the steel can prdouce local deposits of 
martensite, which because of its different properties, can be a 
fracture initiation point.  Also - almost all welds have slight 
imperfections (if they are too large they are called defects) that in 
low toughness material - can lead to initiation of brittle fracture 
at elastic stress levels.

Now back to the "Lincoln" problem.

The E70-T4 electrode was developed by Lincoln in the 1960s and 
they started marketing it in the late 1960s.  Because of the speed of 
welding with this electrode, compared to other available consumables, 
it rapidly caught on and by the early 1970s, nearly all field welding 
of large volume welds were performed using this electrode.  Obviously 
a great product like this is going to be imitated and other 
manufacturers of electrodes also produced and sold T4.

In addition to the low toughness problem, the T4 electrode can be 
easily abused by allowing welders to place metal too rapidly, in too 
large a bead size.  These welds are susceptible to slag inclusions, 
poor fusion and other defects.  This is all accentuated by the low 
toughness.

You can find out much more information - First obtain a copy of 
FEMA-267 Interim Guidelines - Welded Steel moment Frames, produced by 
FEMA in 1995.  It is available free from FEMA - You can get the phone 
number to order this at 1-800-480-2520.  FEMA is also producing a 
publication FEMA-288 that has more than 400 pages of information on 
Fracture Mechanics and welding processes (as well as other items of 
interest).  You may find it useful.  You can order this at the same 
number.

The L.A. Times is not a particularly good place to research this 
information.  The articles that appeared on this topic were not 
technical in nature and were highly slanted by one side in litigation 
to present facts only that favor a particular position in the 
litigation.

Ronald O. Hamburger
Project Director, Product Development
SAC Joint Venture;
Member - AWS D1.1 Subcommittee on Welding for Seismic Applications
Member - AISC TC-115 on Structural Shapes




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Richard Lewis, P.E.
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