region surrounding the joint, to a specific desired temperature, called the
preheat temperature, prior to welding.
Heating may be continued during the welding process, but frequently the
heat from welding is sufficient to maintain the desired temperature without a
continuation of the external heatsource.
The interpass temperature,
defined as the base metal temperature at the time when welding is to be
performed between the first and last welding passes, cannot be permitted
to fall below the preheat temperature.
Interpass temperature will not be discussed further here; however, it will be
the subject of a future column.Preheating can produce many beneficial
effects; however, without a working knowledge of the fundamentals
involved, one risks wasting money, or even worse, degrading the integrity of
the weldment.
Why Preheat?
There are four primary reasons to utilize
preheat:
(1) it slows the cooling rate in the weld metal and base metal,
producing a more ductile metallurgical structure with greater resistance to
cracking;
(2) the slower cooling rate provides an opportunity for hydrogen
that may be present to diffuse out harmlessly, reducing the potential for
cracking;
(3) it reduces the shrinkage stresses in the weld and adjacent
base metal, which is especially important in highly restrained joints;
(4) it raises some steels above the temperature
at which brittle fracture wouldoccur in fabrication.
Additionally, preheatcan be used to help ensure specific
mechanical properties, such as weld metal notch toughness.
When Should Preheat Be Used?
In determining whether or not to preheat, the following should be considered:
- Code requirements
- Section thickness
- Base metal chemistry
- Restraint
- Ambient temperature
- Fillermetal hydrogen content and previouscracking problems.
If a welding code must be followed, then the code generally
will specify the minimum preheat temperature for a given base metal,
welding process and section thickness.
This minimum value must be attained regardless of the restraint or
variation in base metal chemistry; however, the minimum value may be
increased if necessary.When there are no codes governing
the welding, one must determinewhether preheat is required, and if so,
what preheat temperature will be appropriate. In general, preheat usually
is not required on low carbon steels less than 1 in (25 mm) thick.
However, as the chemistry, diffusible hydrogen level of the weld metal,
restraint or section thickness increases, the need for preheat also increases.
How does one determine an appropriate preheat temperature?
The two methods outlined in Annex XI of AWS D1.1-96 are:
(1) Heat affectedzone (HAZ) hardness control.
(2) Hydrogen control.
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The hydrogen control method is based on the assumption that cracking will not
occur if the amount of hydrogen remaining in the joint after it has cooled
down to about 120°F (50°C) does not exceed a critical value dependent on
the composition of the steel and the restraint. This procedure is extremely
useful for high strength, low-alloy steelsthat have high hardenability. However,
the calculated preheat may be somewhat conservative for carbon steels.
The three basic steps of the hydrogen
control method are:
(1) Calculate a composition parameter similar to the
carbon equivalent;
(2) Calculate a susceptibility index as a function of the
composition parameter and the filler metal diffusible hydrogen content;
(3) Determine the minimum preheat temperature from the restraint level,
material thickness, and susceptibility index.
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The HAZ hardness control method, which is restricted to
fillet welds, is based on the assumption that cracking will not occur if the
hardness of the HAZ is kept below some critical value. This is achieved
by controlling the cooling rate. Thecritical cooling rate for a given hardness
can be related to the carbon equivalent of the steel, which is given in preheat
formula article.
From the critical cooling rate, a minimum preheat temperature can then be
calculated. AWS D1.1-96 states that “Although the method can be used to
determine a preheat level, its main value is in determining the minimum
heat input (and hence minimum weld size) that prevents excessive hardening”
(Annex XI, paragraph 3.4).
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