GENERAL CONSIDERATIONS
1.0 SCOPE
The purpose of this procedure is to establish a general
welding procedure for the repair of some casting components of Vulcan
Foundation Equipment Inc. hammers. It will cover only the fundamental
parameters of the welding process. The welding techniques are well
established and generally standardized.
2.0 QUALIFICATION
It is recommended that welding operators be certified in
accordance with the requirements of Section IX of the ASME Boiler Code or
ASTM A-488. Performance Qualification of Welders under these documents is
simple, testing the ability of the welder to produce a "sound"
weld in small carbon-steel plate.
3.0 THE WELDING PROCESS
There are many welding processes available today, from
completely automatic "electroslag" processes to semi- automatic
"Metal-Inert-Gas" (MIG) process. In this procedure only one
process will be considered; the "Shielded-Metal-Arc- Welding" (SMAW)
process. This is the oldest, simplest and most versatile process available
today.
3.1 Welding Current
The current for the SMAW process is usually DC Positive
(Reverse Polarity). Any type of welding current generators may be used.
Most welding machines have "rectifiers" with a capacity of at
least 400 amps and a 100% duty-cycle.
3.2 Electrodes
Only low-hydrogen electrodes should be used for this
process. In the AWS-ASTM Classification the approved electrodes would be:
EXX15, EXX16, and EXX18. It is important to keep these electrodes
moisture-free after removing from the container. They should be stored in
an oven at 250° F (120° C) and used within one (1) hour after removal.
The absorption of moisture in the electrode coating will result in
hydrogen pick-up in the weld deposit and likely produce cracking.
3.3 Techniques
Standard welding techniques are used in this procedure.
The bead should be of the "stringer" type. Heat input should
always be kept as low as is practical. The welding pass sequence should be
selected with the idea of keeping welding stresses to a minimum. This can
be done by controlling the heating and cooling rate of the process. The
welding sequence passes should be separated as much as is practical; the
interpass should be kept low by the use of small-diameter electrodes and
low welding current.
4.0 WELDING PARAMETERS
The keys to high quality welding can be summarized in
three (3) factors: (1) the selection of the proper electrode for the
particular base metal to be repaired, (2) the establishment of the correct
preheat temperature to (a) maintain a low hardness in the
Heat-Affected-Zone (HAZ) and (b) minimize welding stresses, (3) the
selection and use of a suitable interpass temperature to reduce the
effects of heating and cooling on the components. This is important since
post-welding heat treatment is not always practical and thus, excessive
hardness and stresses must be minimized by the correct selection of the
welding parameters.
4.1 Electrode Selection
The selection of the proper electrode is based on the
composition and mechanical properties of the metal to be welded.
The composition of welding electrodes contains the same
alloys as the casting; i.e. manganese, nickel, chromium, molybdenum and
vanadium. However, the carbon and silicon contents are lower than the base
metal. Therefore, the alloy content of the rod must be higher to achieve
the same mechanical properties.
Ideally, the composition and mechanical properties of
the electrode should match the casting. When this is not possible, it is
more important to coordinate the mechanical properties than the
composition. This is true of carbon/low alloy steels, but not of corrosion
and heat-resisting high- alloy steels.
4.2 Preheat Temperature
The selection of the preheat temperature should be based
on three (3) factors, listed in order of importance:
1. Composition and hardenability of the base or parent
metal. 2. The feasibility of post welding heat treatment 3. The size and
configuration of the part to be welded.
All of these factors require a higher preheat
temperature. The preheat can range from 100° F (40° C) for unalloyed
steel to 400° F (200° C) for steels of high hardenability. The
temperature is always listed as "minimum" and may be higher, if
desired. If possible, the entire part should be preheated in a
temperature-controlled furnace. Otherwise, localized heating with torches
may be used. There are several precautions that must be observed: (1) the
heating must be done slowly (100° F) (38° C/hr), (2) the part must be
heated throughout the section, (3) the area preheated should be at least
12" (30 cm) from the edge of the cavity.
The temperature of the part can be checked by use of
"Temp- Sticks", a temperature-sensitive marker that melts at the
designated temperature. The temperature of both sides of the part should
be checked to insure complete heating throughout the section.
4.3 Interpass Temperature
The interpass temperature is the temperature of the base
metal measured 2" (5cm) from the weld, between weld passes in a
multi-pass welding operation. It is always listed as a maximum, and is
usually less than 200° F (95°C) above the preheat temperature.
The primary purpose of controlling the interpass
temperature is to minimize the welding induced stresses. A reasonable
interpass temperature will also prevent the parent metal from becoming so
hot that it "anneals" the weld deposit, causing lower hardness
and strength.
5.0 POST WELDING HEAT TREATMENT
Heat treatment after welding accomplishes two (2)
things: (1) relieves the stresses incurred during welding; (2) reduces the
hardness in the HAZ. For these reasons it is advantageous to heat-treat
all parts after welding. Unfortunately, this is not always possible.
There have been many attempts to stress-relieve parts
without using a heat treating furnace. Most of these methods are
ineffective. The amount of stresses relieved is dependent on the
temperature of the part and the time at that temperature. Since the
temperature of these make-shift operations is usually in the range of
700°-800°F (370-430°C) the holding times would have to be in the 10-12
hour range. These methods are not recommended.
6.0 CHOOSING A WELDING PROCEDURE
The choice of a particular welding procedure is strictly
dictated by the material composition of the piece being repaired. As
VULCAN FOUNDATION EQUIPMENT INC. is constantly improving its material
specifications, it is necessary that the end user of the product or the
repair facility doing the welding contact
the factory to determine the exact material composition of the
particular casting involved.
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