CEMP-E
TI 809-26
1 March 2000
CHAPTER 3
WELDING PROCESSES AND MATERIALS
1. WELDING AND RELATED PROCESSES.
a. General - Welding. The proper selection of welding processes, materials, and procedures is vital to
achieving the strength and quality necessary for adequate performance in the structure. The contract
documents, prepared by the Engineer, should specify any special requirements for materials, inspection,
or testing beyond that required by the codes and standards.
b. General - Heating and Thermal Cutting. The application of heat, whether for straightening, cutting,
or welding, may have a significant effect upon the mechanical properties of the steel, weld, and heat-
affected zones. Should any limitations in the use of heat be needed beyond those specified in the codes,
the contract documents prepared by the Engineer should so state.
c. General - Weld Heat-Affected Zone. The heat-affected zone (HAZ) is the portion of steel
immediately adjacent to the weld that has been metallurgically modified by the heat of the welding. The
microstructure has been changed, and the mechanical properties typically have been degraded with
reduced ductility and toughness, but with increased strength. Also, hydrogen from the welding operation
will have migrated into the hot HAZ, then subsequently been trapped within the metallurgical structure,
embrittling the steel. The hydrogen will eventually migrate out the HAZ, at rates dependent upon initial
hydrogen levels, thickness and temperature. The HAZ is typically about 3 mm (1/8 in.) thick for common
size welds, primarily depending upon welding heat input.
d. Project Specifications. In most cases, it is adequate to simply require compliance with the codes.
The contractor may be allowed the full choice of welding processes and materials. The use of "matching"
prequalified filler metals is encouraged. When SMAW is performed, the use of low-hydrogen electrodes
is encouraged. Recently, the use of specified toughness levels for filler metals in specific seismic
building applications has been added to standard practice. For further guidance in the use and selection
of welding processes and materials, see Appendix C.
2. APPLICATION OF HEAT FOR WELDING.
a. Cooling Rate Control. Preheat is used primarily to slow the cooling rate of the heat-affected zone
(HAZ). Because preheating slows the cooling rate, the steel remains at an elevated temperature longer,
increasing the rate and time of hydrogen diffusion and reducing the risk of hydrogen-assisted cracking.
Preheat also aids in the removal of surface moisture and organic compounds, if present, from the
surface to be welded, reducing porosity and other discontinuities. Preheating may also reduce residual
stresses and improve the toughness of the completed joint.
(1) High Cooling Rates. A high cooling rate may cause a hard, martensitic HAZ microstructure with
a higher risk of cracking during cooling. The HAZ will also contain higher levels of hydrogen, also
embrittling the steel and increasing the risk of cracking.
(2) Low Cooling Rates. Conversely, a very low cooling rate can detrimentally affect toughness
because of grain growth. When preheat above approximately 300oC (550oF) is used, weld metal
properties may be degraded as well. If the steel is manufactured using heat treatment processes, such as
3-1
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