Kestas,
The steam mains are typically made from two materials. F22 (2-1/4% Cr, 1%Mo), and "CMV", or "half-half-quarter" (1/2% Cr, 1/2%Mo, 1/4 V).
The F22 was used in older stations, and newer. The CMV was used for a while as it's cheaper (less alloying elements) and stronger in its as delivered conditions, allowing thinner walls (steam mains can be 16" diameter, 2"+ thick.
As the CMV ages (the pipes run at 1000F/540C), the metalurgical properties change, pushing up the cold transition temperature and increasing the crack sensitivity.
One of the stations in Victoria can withstand a through wall crack 900mm long at operating temperature without catastrophic failure. At 15 C the same pipe can cope with a 30mm crack before unzipping.
Welding on this aged material is an amazingly complex process, put a bead down, peen it, grind half off. When finished, add another couple of passes and grind them off completely...then a day's worth of heat treatment (if you haven't cracked it yet).
Starting them up after a shut is a problem, as the steam heats the centre, and induces large tensile stresses in the still cold outer part of the pipes. Cracks can grow rapidly.
The steam mains are typically made from two materials. F22 (2-1/4% Cr, 1%Mo), and "CMV", or "half-half-quarter" (1/2% Cr, 1/2%Mo, 1/4 V).
The F22 was used in older stations, and newer. The CMV was used for a while as it's cheaper (less alloying elements) and stronger in its as delivered conditions, allowing thinner walls (steam mains can be 16" diameter, 2"+ thick.
As the CMV ages (the pipes run at 1000F/540C), the metalurgical properties change, pushing up the cold transition temperature and increasing the crack sensitivity.
One of the stations in Victoria can withstand a through wall crack 900mm long at operating temperature without catastrophic failure. At 15 C the same pipe can cope with a 30mm crack before unzipping.
Welding on this aged material is an amazingly complex process, put a bead down, peen it, grind half off. When finished, add another couple of passes and grind them off completely...then a day's worth of heat treatment (if you haven't cracked it yet).
Starting them up after a shut is a problem, as the steam heats the centre, and induces large tensile stresses in the still cold outer part of the pipes. Cracks can grow rapidly.
