A method of heat treating a nickel base superalloy disc comprising the steps of:- a) placing the disc in a furnace and solution heat treating the disc at a temperature below the gamma prime solvus temperature to produce a fine grain structure in the disc, b) cooling the disc to ambient temperature, c) placing insulation (52, 54) over at least one first predetermined area (36, 40) of the disc and leaving at least one second predetermined area of the disc without insulation to form an insulated assembly, placing insulation (52, 54) on the radially extending faces (24C, 24D) of the disc and such that the second predetermined area of the disc is the rim portion of the disc, placing a first disc shaped insulator on a predetermined area of a first radially extending face of the disc and placing a second disc shaped insulator on a predetermined area of a second radially extending face of the disc, the diameter of the first disc shaped insulator is less than the diameter of the disc and the diameter of the second disc shaped insulator is less than the diameter of the disc, such that a hub portion of the disc is covered by the insulation (52B, 54B) and a rim portion of the disc is not covered by insulation, the first disc shaped insulator has a greater diameter than the second disc shaped insulator, d) placing the insulated assembly of disc and insulation (52B, 54B) in a furnace at a temperature below the gamma prime solvus temperature, e) maintaining the insulated assembly at the temperature below the gamma prime solvus temperature for a predetermined time to achieve a uniform temperature in the disc, f) increasing the temperature in the furnace at a predetermined rate to a temperature above the gamma prime solvus temperature to maintain a fine grain structure substantially in a first region of the disc, to produce a coarse grain structure substantially in a second region of the disc and to produce a transitional structure in a third region positioned between the first region and the second region of the disc, and the third region is arranged at an angle relative to the axis (X-X) of the disc, the predetermined ramp rate is 110°C per hour to 280°C per hour, g) removing the insulated assembly from the furnace when the second region of the disc has been above the gamma prime solvus temperature for a predetermined time and/or the first region of the disc has reached a predetermined temperature and h) cooling the disc to ambient temperature, cooling the disc at a rate of 0.1°C per second to 5°C per second. a) placing the disc in a furnace and solution heat treating the disc at a temperature below the gamma prime solvus temperature to produce a fine grain structure in the disc, b) cooling the disc to ambient temperature, c) placing insulation (52, 54) over at least one first predetermined area (36, 40) of the disc and leaving at least one second predetermined area of the disc without insulation to form an insulated assembly, placing insulation (52, 54) on the radially extending faces (24C, 24D) of the disc and such that the second predetermined area of the disc is the rim portion of the disc, placing a first disc shaped insulator on a predetermined area of a first radially extending face of the disc and placing a second disc shaped insulator on a predetermined area of a second radially extending face of the disc, the diameter of the first disc shaped insulator is less than the diameter of the disc and the diameter of the second disc shaped insulator is less than the diameter of the disc, such that a hub portion of the disc is covered by the insulation (52B, 54B) and a rim portion of the disc is not covered by insulation, the first disc shaped insulator has a greater diameter than the second disc shaped insulator, d) placing the insulated assembly of disc and insulation (52B, 54B) in a furnace at a temperature below the gamma prime solvus temperature, e) maintaining the insulated assembly at the temperature below the gamma prime solvus temperature for a predetermined time to achieve a uniform temperature in the disc, f) increasing the temperature in the furnace at a predetermined rate to a temperature above the gamma prime solvus temperature to maintain a fine grain structure substantially in a first region of the disc, to produce a coarse grain structure substantially in a second region of the disc and to produce a transitional structure in a third region positioned between the first region and the second region of the disc, and the third region is arranged at an angle relative to the axis (X-X) of the disc, the predetermined ramp rate is 110°C per hour to 280°C per hour, g) removing the insulated assembly from the furnace when the second region of the disc has been above the gamma prime solvus temperature for a predetermined time and/or the first region of the disc has reached a predetermined temperature and h) cooling the disc to ambient temperature, cooling the disc at a rate of 0.1°C per second to 5°C per second.