This procedure covers the repair welding of pearlitic and ferritic ductile (or nodular) iron. The corresponding base material specifications are VIWI MS1 or equivalent.
2.0 DEFECT REMOVAL
The defect may be removed by chipping, grinding or machining. While carbon-arc may be used on ductile iron, it is not recommended due to thermal shock and the formation of carbide/martensite structure on the surface of the cavity.
It is important that the weld cavity consist of sound, clean metal. In critical areas, the use of magnetic-particle or dye-penetrant inspection is strongly recommended.
3.0 PREHEAT TEMPERATURE
The preheat temperature is 550° F (290°C). Preheating should be done in a temperature-controlled furnace. Begin heating the part at 100° F (40° C), holding one (1) hour/inch (25 minutes/cm) of metal section for six (6) hours maximum. Raise the temperature 100° F (40° C) per hour to 550° F (290° C), hold one hour/inch (25 min/cm) or six (6) hours maximum.
Localized preheating may be used when furnaces are not available. The following precautions should be observed:
1) The area preheated must be at least two (2) feet (60cm) from the edge of the weld cavity. Castings more than 6 inches (15cm) thick should be preheated at least three (3) feet (90cm) from the cavity.
2) Heating must be done slowly and evenly. Temperature gradients of more than 100° F (40° C) should be avoided.
3) The temperature must be uniform throughout the section preheated. Holding times of one hour/in (25 min/cm) of metal section should be observed.
4.0 INTERPASS TEMPERATURE
The interpass temperature is 700° F (370° C) maximum. When practical 600° F (320° C) should be used, but in no case exceed 700° F (320° C).
5.0 WELDING PARAMETERS
The “shielded metal arc welding” process (SMAW) should be used for all welding.
- Welding Current: Direct-current, reverse-polarity (DCRP) is required for the electrodes recommended. The voltage and amperage settings should be based on the recommendations of the electrode manufacturer.
- Electrodes: Two (2) electrodes are recommended. The first produces a machinable weld deposit and a lower hardness in the heat- affected-zone (HAZ). The second electrode produces a higher strength weld but may be difficult to machine. Greater care is also required in welding with the second electrode to prevent cracking in the fusion zone.
- AWS ENiFe-CI. This is a 55% nickel electrode having a weld strength of about 50,000 psi (345 MPa) tensile strength. It requires a lower amperage and less heat input. It melts at lower temperature than carbon-steel electrodes, therefore, creates less carbide-martensite in the HAZ.
- AWS E-7018 (E7016 or E7015). This is a carbon steel electrode with a higher strength as-deposited, 70,000 Psi (485 MPa) maximum tensile. It requires a higher amperage, more heat input, and is more susceptible to cracking.
As with all low-hydrogen electrodes, they should be stored in a warm, dry compartment after the cans are opened.
The weld bead should be of stringer type. The arc should be struck on the weld deposit whenever possible. The slag should be removed from the weld deposit between each pass by chipping, peening and wire brushing.
7.0 POST WELDING HEAT TREATMENT
All ductile iron castings should receive a post-welding heat treatment. Material poured to specification VIWI MS1 Grade 1 need only be stress-relieved while VIWI MS1 Grades 2 and 3 material should be normalized and tempered. The recommended heat treatments are as follows:
(A) Grade 1. Place in a furnace at 200° F (95° C) and soak for one (1) hour per inch (25 min/cm), six (6) hours maximum. Heat slowly at 100° F/hour (40° C/hour) to 1100° F (600° C) and hold one (1) hour per inch (25 min/cm), six (6) hours maximum. Cool in still air.
(B) Grades 2 and 3. Place in a furnace at 200° F (95° C) and soak for one (1) hour per inch (25 min/cm), six (6) hours maximum. Heat slowly at 100° F (40° C/hr) to 1000° F (540° C) and 200°F/95° C/hr to 1650° F (900° C); hold one hour per inch (25 min/cm six (6) hours maximum. Cool in still air. When the temperature of the casting is below 600° F (320° C), it may be placed into a tempering furnace, soaked at 600° F (320° C) for one hour per inch (25 min/cm) six hours maximum, and heated slowly 200° F per hour/95°/hr) to 1200° F (650° C). Hold at temperature for one (1) hour/inch (25 min/cm) six (6) hours maximum and cool in still air.
The welding of ductile iron presents two (2) problems:
- The base metal when melted at the fusion zone will not resolidify as ductile iron — the graphite will precipitate as vermicular or quasi-nodular. The ductility and impact resistance will be reduced about 50%, and some carbides are likely to form, particularly in the pearlitic grade.
- The HAZ will produce a martensite, especially in the pearlitic grade, which must be tempered to restore any ductility. Ductile iron is more susceptible to welding stresses, and more likely to crack while welding or during cooling.
Therefore, ductile iron should never be welded in highly stressed parts or areas. Welds should never be made when the weld is more than 20% of the metal thickness.