Microstructural changes of flange welded joints at high temperature

When carbon steel and low-alloy heat-resistant steel welded components work at high temperature for a long time, their properties will change due to the unstable structure. Carbon steel and carbon-manganese steel flanges are most prone to pearlite spheroidization and graphitization after long-term operation at high temperatures, and Widmanstatten structures will also occur in the welding heat-affected zone.

(1) Pearlite spheroidization

The so-called pearlite spheroidization refers to the tendency of cementite (Fe, C) in lamellar pearlite to transform into spherical shape and aggregate into large pellets. The carbides precipitated in the surface ferrite also aggregate and grow at the same time, especially at the grain boundaries.

The shape and distribution of carbides in flange steel have a great influence on the thermal strength. Generally speaking, the thermal strength of flaky carbides is higher.

Spherical carbides, especially carbides that aggregate into large pieces, will significantly reduce the thermal strength of steel. Carbon steel flanges are the easiest to spheroidize, while molybdenum steel and chrome-molybdenum steel are more stable than carbon steel. However, if chrome-molybdenum steel is not operated properly (such as overheating, etc.), pearlite spheroidization will also occur. The pipe blasting test of l2CrlMoV steel at 180 °C shows that moderate spheroidization has little effect on the enduring strength, but complete spheroidization will reduce the enduring strength by 1/3. Spheroidized tissue can be restored to the original sheet-like tissue by normalizing.

(2) Graphitization

Graphitization is a more serious and harmful tissue change phenomenon than spheroidization. The reason for graphitization is mainly the self-decomposition of cementite at high temperature: Fe, C→3Fe+C (graphite). Stone is usually precipitated along the crystal and distributed in a chain shape. Due to the very low strength of graphite, it can be seen as voids or cracks in steel. For example, after working at 505°C for 5 and a half years, the pipeline of a power plant suddenly fractured along the entire cross-section at a distance of about 3-4mm from the weld metal, the reason being graphitization.

In order to avoid graphitization, elements with strong binding force with carbon, such as chromium and vanadium, are often added to steel. Generally speaking, 0.5% molybdenum steel has a greater graphitization tendency, so this steel has been replaced by Cr-Mo steel.

(3) Wei's organization

The reason for the Widmanstatten structure in the heat-affected zone of flange welding is due to overheating. The Widmanstatten structure has little effect on the strength at room temperature, but it can increase the high-temperature strength, but the plasticity has decreased. The biggest disadvantage of the Widmanstatten structure is that the impact toughness is too low, and Wangwang will cause brittle fracture of the joint.

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