Austenite-Grain-Growth Kinetics and Mechanism in Type 347H Alloy Steel for Boiler Tubes

The research material (type 347H alloy steel) has been characterized using optical microscopy and an EDS/SEM system. Annealing experiments have been conducted at temperatures range of 600–1050°C for 30 min–20 h by using an atmosphere-controlled furnace. Normal grain growth with intermediate grain size has been related to the favouring of creep resistance to recommend the material suitable for boiler tubes at operating temperatures up to 750°C for long duration. The kinetics of grain growth in the 347H has been shown to behave similar to a pure metal in the initial stage of annealing in the range of 0–30 min, beyond which the grain-growth process was found to be suppressed due to second-phase (NbC) particle-pinning and solute drag effects. The grain-growth exponent n is computed to be in the range of 0.117–0.313; the deviation from ideal kinetic behavior (n=0.5) has been scientifically justified. The activation energy for grain growth Q_g, for the investigated alloy, has been graphically computed and validated.