This procedure covers the repair welding of gray iron (flake- graphite iron) components. The appropriate base material specification is VIWI MS4 or equivalent.
2.0 DEFECT REMOVAL
The defect may be removed by chipping, grinding or machining. Flame cutting is not recommended due to the excessive heat input, which could cause cracking.
It is important to remove all of the defects prior to welding. On critical components the use of magnetic-particle or dye-penetrant inspection is strongly recommended.
3.0 PREHEAT TEMPERATURE
The minimum preheat temperature is 400° F (205° C). The component should be preheated in a temperature-controlled furnace, in order to achieve uniform heating. Localized heating should be used only in emergencies.
The casting should be heated slowly, 100° F/hour (40° C/hr) C/hr) to 400° F (205° C) and held at temperature for one hour/inch (25 min/cm) or not more than six (6) hours.
4.0 INTERPASS TEMPERATURE
The interpass temperature should be held to a maximum of 600°F (320°C). Higher temperatures could cause underbead cracking and will certainly create stresses in the component.
5.0 WELDING PARAMETERS
The “shielded metal arc welding” process (SMAW) is used in all welding.
(A) Welding Current
Direct-current, reverse-polarity (DCRP) is required for the electrodes recommended. The voltage and amperage settings should be based upon the recommendations of the electrode manufacturer. In the absence of specific instructions the following guide may be used:
Electrode Amperage Voltage Diameter Flat Position Flat Position 1/8″ (3mm) 120 22/26 5/32″ (4mm) 160 23/27 3/16″ (5mm) 240 24/29
NOTE: The use of smaller electrodes, and the corresponding lower amperage is recommended.
Two (2) electrodes are recommended:
(1) AWS ENiFe-CI (2) AWS E7018 (E7016 or E7015)
The first electrode is a 55% Ni Iron electrode that deposits a machinable weld of about 50,000 PSI (345 MPa) tensile strength. This electrode melts at a lower temperature than the steel electrode and consequently generates less heat.
The second electrode is an unalloyed carbon steel electrode of 70,000 PSI (482 MPa) tensile strength. In using this electrode there is a pick-up of carbon from the base metal into the weld deposit. This may cause the formation of carbides in the fusion zone and heat-affected-zone (HAZ). The higher heat input can also cause martensite formation in the HAZ. Both of these conditions cause excessive hardness, poor machinability and possibility of cracking.
The first electrode ENiFe-CI is preferred for all repair welding of all gray iron. The steel electrode (E7018) should be used only when the 55% nickel electrode is not available.
The weld bead should be the stringer type. Use smaller diameter electrodes with a low amperage setting, commensurate with good welding practice. Do not arc directly on the casting except on the root pass.
7.0 POST-WELDING HEAT TREATMENT
Stress-relieving after welding is required. Gray cast iron, having virtually no ductility is susceptible to massive stress build-up during welding. The entire casting should be heated in a temperature-controlled furnace at a rate of about 100° F/hour (40° C/hr) to 1100° F (600° C). Holding times at temperature are one (1) hour per inch (25 min/cm), but not more than six (6) hours. Localized heating is inadequate since it is almost impossible to reach 1100° F (600° C). The temperature gradient would be too high, inducing more stresses during cooling, and there is a real danger of cracking the casting.
Gray Iron is difficult to weld; requiring strict adherence to the prescribed welding parameters. Even under ideal conditions, the welds will be only moderately successful. The HAZ will be mostly tempered martensite of a much higher hardness than the base or weld metal. The fusion zone will have varying amounts of primary carbides, which are hard, brittle and unmachinable. Thus, repair welding of gray iron must be held to an absolute minimum. In no case should welding be done in a highly stressed area. The weld should not extend more than 20% of the wall thickness even in lightly stressed areas.