Frequently Asked Questions
The exact post-weld heat treatment of welded cast iron depends on the degree of stress relief or softening required.
In grey irons, for a minimal stress relief and partial softening of the hardened HAZ, heat slowly to 480°C and cool in still air. For greater softening and stress reduction (to approximately one half of the original residual stress level), heat to 590°C and cool in still air. Ideally it is wise to perform heat treatment without allowing the casting to cool after welding.
Maximum softening can be achieved by heating to 900°C followed by furnace cooling. This produces a ferritic matrix, and the slow cooling ensures a structure free from residual stress.
In ductile irons, it is often desirable to restore the ductility of welded joints to levels near those of the original casting. For moderate stress relief and softening, heating to 480°C and slow air cooling are sufficient. Greater improvements in properties are achieved by treatments designed to dissolve any carbides that have formed in the welded region. A heat treatment which produces a structure similar to that of the original casting is usually chosen, i.e. a treatment which ferritises the heat affected zone matrix would be suitable for a ductile iron with a ferritic matrix, but not for a stronger iron with a matrix containing pearlite.
The following treatment will also transform martensite to austenite, which will form ferrite and pearlite on cooling: Heat to 900°C at less than 55°C per hour and hold for 40 minutes per centimetre of thickness. Furnace cool to 260°C, again at a rate less than 55°C per hour, then cool to room temperature in still air.
A softer ferrite matrix can be achieved as follows: Heat to 840-900°C, (>55°C/h) and hold at this temperature for 40 minutes per centimetre of thickness. Furnace cool to 675°C (>55°C/h) and hold for 5-6 hours, then furnace cool to 260°C (>55°C/h) before cooling to room temperature in still air.
The heating and cooling rates are restricted, to minimise development of thermal stresses in the components. This is especially necessary for castings of complex shape.
See also Welding of cast irons - a guide to best practice , from which this FAQ is extracted.