Vanadium Modified Cr-Mo Steels 1

[Montana1]

Can anyone tell me why ASME IID limits design temperatures to 900F for 2 1/4Cr - 1Mo - 0.25V steels? (Non vanadium modified steels are good up to 1200F.) If you can give me a detailed metallurgical mechanism, that would be great. Also, I have read that PWHT for extended times (multiple repair cycles) for Vanadium modified Cr-Mo steels causes loss in ductility. What is the mechanism for this? Does WRC have any documents covering this area of research? (I am a WRC member.)

 

[metengr]

Here is a brief summary in dealing with Cr-Mo-V low alloy steels;

The limit of 900 deg F is because the Cr-Mo-V alloy steels cannot meet the creep rupture requirements under ASME Section VIII, Div 2 requirements. The mechanism for the loss in ductility in Cr-Mo-V low alloy steels is precipitation of vanadium carbides during a post weld or tempering cycle heat treatment after welding. Reheat cracking is also a common problem with vanadium modified low alloy steels and occurs during a post weld heat treatment cycle.

During tempering heat treatment cycle, the vanadium carbides precipitate and strengthen the alloy above a certain tempering temperature – this strengthening results in a loss of ductility.

Stress relief cracking occurs in the heat affected zone of the Cr-Mo-V base material, during a post weld heat treatment cycle. What happens is during welding vanadium carbides dissolve in the region next to the fusion zone of the weld. The heat applied during PWHT induces precipitation of the vanadium carbides in the grain interiors versus the grain boundaries. The vanadium carbides significantly strengthen the grain interiors but render the grain boundaries susceptible to intergranular fracture while relieving stresses during PWHT.

A paper that might be of interest on the background of these alloys is below;
“Advanced Vanadium Modified Steels for High Pressure Hydrogen Reactors”
by Hucinska, Gdansk University, Poland

Other literature sources;
“Physical Metallurgy Handbook” by Sinha

 

[davefitz]

Interesting. I know we have several steam turbines with main steam stop valves made of 1.25cr-1Mo- V, design temp = 1060F.

 

[metengr]

davefitz;
GE likes to use the cast version of 1Cr-1.25Mo-0.25V low alloy steel for stop valves, and HP steam chests and turbine inner cylinders. The 1Cr-1.25Mo-0.25V is also commonly used for HP and IP turbine rotor forgings.

The elevated temperature tensile strength of this material is excellent because of the vanadium and increased molybdenum additions. Creep strength is also very good.

I have done extensive condition assessments on our fleet of HP and IP turbine rotors (GE and W) over the years because some of them have over 250,000 operating hours at temperature. The creep rupture strength of these alloys have been well documented by turbine OEM’s. In most turbine design, a safety factor of 2.5:1 regarding creep strength (not deformation rate) is used by most turbine OEM's. Even after 250,000 operating hours, the creep deformation test results that I have obtained from our HP and IP turbine rotor samples indicates we are in the early to middle stage of secondary creep. Based on our analysis, the creep rupture life of these rotors is somewhere around 600,000 to 900,000 operating hours.

Keep in mind that we have backed off our HP turbine throttle temperatures from 1050 deg F (original design on six of our units) to 1025 deg F about 25 years ago, as a conscious decision to preserve the creep life of the boiler SH, the HP turbine blades, HP turbine stop valve bodies, inner shell, and piping. The remainder of our units are 1005 deg F throttle temperature by design.

 

[davefitz]

Meteng:
One advantage of backing down to 1025 F is certainly longer creep life, but I think that the main reason that coal fired units backed down was associated with liquid phase coal-ash corrosion of the final superheater and reheater tubes in the heated zone.