| Reheat cracking failure of weld joint during service process,such as 321-type and 347-type stabilised austenitic stainless steel and CrMo ferritic heat-resistant steel,has always been a key problem affecting engineering structure life and security.Reheat cracking always occurs during the post-weld heat treatment or long-term service at high temperatures.Due to the metallurgical reactions of the heating and cooling processes during the welding thermal cycle,the heat-affected zone is generally considered to have higher reheat cracking susceptibility,thus many studies on reheat cracking are mainly focused on the reheat cracking generated in the heat-affected zone.In contrast,there are relatively few studies on the reheat cracking of weld metal.However,the weld metal still has the conditions to generate reheat cracks.Relevant cases have reported the loss of alloy service performance due to the reheat cracking failure in the weld metal.Super304H austenitic stainless steel has been widely used in superheater and reheater pipes of the ultra-supercritical thermal power unit boilers.The reheat cracking failure in pipe welded joints under long-term service has great uncertainty for the decades of service requirements in thermal power units.In this dissertation,Nbstabilised Super304H austenitic stainless steel was selected to study the reheat cracking of weld metal during long-term thermal exposure.The advanced method by precompressing the compact tensile specimen was used,and the reheat cracking susceptibility of the weld metal was successfully and quantitatively studied.The Nb content in the welding consumables was regulated,and we systematically studied the effects of microstructure evolution,residual stress,temperature,and Nb element on the reheat cracking of Nb-containing stabilised austenitic stainless steel weld metal and Nbfree non-stabilised austenitic stainless steel weld metal during long-term thermal exposure.The main research contents and conclusions are as follow:The distribution and change of the primary and secondary nanoscale precipitates of the Nb-free and Nb-containing Super304H weld metal prepared by argon tungsten arc welding were analyzed before and after aging.The strength change caused by the evolution of the precipitates and the influence of Nb on the strength of the weld metal were discussed according to the change of microhardness with time.The results show that adding Nb element in the as-welded weld metal changes the precipitation distribution between the interdendritic region and the grain boundaries.The grain boundary is tortuous in the Nb-containing weld metal,and the grain size is small.During long-term thermal exposure,the Nb element in the weld metal inhibited the precipitation of the M23C6 phase at the initial stage of aging and changed the precipitation behavior of the M23C6 phase in the interdendritic region.The size of the nanoscale M23C6 phase,nanoscale Nb(C,N)phase,and nanoscale Cu-rich phase in the weld metal all increased with aging time.The nanoscale Cu-rich phase plays a major role in the strength of the weld metal.Nb element mainly improves the strength of weld metal through grain refinement,solid solution strengthening,and precipitation strengthening of nanoscale Nb(C,N)phase.However,during long-term thermal exposure,the addition of Nb has limited improvement on the strength of weld metal.The mechanisms of reheat cracking generation were expounded for Super304H weld metal under short-term and long-term aging conditions,and the one model for cavity growth was constructed under the influence of the intergranular M23C6 phase.The nanoscale Cu-rich phase particles and the nano-scale M23C6 phase in the Nb-free weld metal at the initial aging stage will lead to intragranular strengthening,which promotes the strain to concentrate on the intergranular M23C6 phase and induces cracks.In the later aging stage,the coarse intergranular M23C6 phase further aggravates the crack propagation.For cracks generated within short-term aging in the Nb-containing weld metal,the strong grain interior induced by the nanoscale Cu-rich particles and the grain boundary weakening of the Nb(C,N)phase are the main cause.In the later aging stage,the large-sized intergranular M23C6 phase will seriously weaken the grain boundaries and promote cracks.The 45° misorientation grain boundaries have the highest interfacial energy,the M23C6 phase is easy to nucleate and grow,and the probability of reheat cracking is high.The incoherent interface between the M23C6 phase and the matrix has an apparent chromium-depletion zone,higher interfacial energy and strain concentration,making it easier to form holes and cause cracks.The effect of residual stress and test temperature on the reheat cracking susceptibility of Super304H weld metals were discussed.The results show that high residual stress and test temperature increase the reheat cracking susceptibility of the weld metals.Since high residual stress weld metals have a higher degree of crack damage,they have a lower high-temperature tensile strength.The area close to the Ushaped mouth has higher residual stress and dislocation density,which accelerates the precipitation of intragranular small particle phases at high temperatures and induces intragranular hardening.Thereby the reheat cracking susceptibility of the weld metals increases.Since the Nb-free weld metal is susceptible to reheat cracking,cracks can occur in a very short time,and most of the residual stress can be released.At this moment,the residual stress has little effect on the intergranular precipitates.The high residual stress in the Nb-containing weld metal will accelerate the precipitation of the intergranular M23C6 phase,aggravate the weakening of the grain boundary,and increase the reheat cracking susceptibility.The effect of temperature on the reheat cracking susceptibility of weld metal can be explained as follows:During long-term thermal exposure at 650℃,the rapid growth of intergranular precipitates at the initial stage of aging causes the grain boundary strength to decrease faster.In addition,the stress relaxation rate is higher at this temperature,resulting in the residual stress at the local grain boundary can exceed the grain boundary strength in a shorter time.Therefore,the crack generation time is shorter than 600℃.The effect of the Nb element on the reheat cracking susceptibility was revealed for Super304H weld metal.The results show that the addition of Nb element reduces the reheat cracking susceptibility of the weld metal under the same plastic strain by the digital image correlation method.The main reasons for this phenomenon are as follow:Firstly,the Nb-containing weld metal has a more uniform intragranular and intergranular strain distribution.Secondly,the Nb-containing weld metal has a larger grain boundary area,reducing the strain concentration of the grain boundary per unit length.Thirdly,the tortuous grain boundary morphology of the Nb-containing weld metal reduces the tensile stress component perpendicular to the grain boundary and the strain concentration around the intergranular precipitates.In addition,within 0-1.5%Nb content,the higher the Nb content,the less prone to reheat cracking generation in the weld metal. |
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