Thermal shock behaviour in ferritic steels: laboratory tests and comparison with service conditions

Abstract

‘Down’ and ‘up’ thermal shock experiments on cast and forged forms of 9/12Cr steels and their weldments at a maximum temperature of 565°C have demonstrated a marked reluctance to form traditional craze cracking and propagation into the body. However, in specimens of ex-service cast 1CrMoV material tested to the same number of cycles (1000), initiation and growth of (intergranular) cracks is much more likely in layers near the inner surface of the component than in layers deep within the section. This is due to the effect of prior thermal cycling of the inner material during start up and shut down events.

Thermal shock and isothermal tests at a maximum temperature of 550°C on 1 weld features have indicated that crack propagation is generally along the fusion boundary (though for some cases, in the weld metal itself) and that cyclic growth rates lie close to an upper bound law valid for ferritic steels. The results from laboratory tests are compared with service experience as regards stresses generated, likely crack path and possibility of arrest.

Keywords:

• downshock
• upshock
• cylindrical shell
• residual stress
• temperatureystress gradient
• fusion boundary cracking
• stress intensity factor
• transgranular/intergranular cracking