Characteristics And Mechanism Of Grain Boundary Oxidation For TP347H Alloy Under Service Environment

TP347H,as a classic 18-8 austenitic heat-resistant stainless steel,is often used as a superheater in firepower units,heater and other high temperature and high pressure components,because of its good resistance to intergranular corrosion,high creep strength and stable oxidation resistance,.After failure analysis of a batch of service TP347H pipes,the grain boundary oxidation of the pipe and cracking along the grain boundary was found,which reduced the mechanical properties of the alloy.On the one hand,the grain boundary oxidation is different from the conventionally studied high-temperature oxidation.On the other hand,the occurrence of grain boundary oxidation will destroy the integrity of the grain boundary and impair the grain boundary effect.Therefore,the behavior and mechanism of grain boundary oxidation on TP347H and the effect of grain boundary oxidation on metal crack initiation and propagation were studied.In this paper,grain boundary oxidation tests of TP347H considered as the research object are performed.Optical microscopy,scanning electron microscopy,electron probes and other equipment are used to characterize microstructures,analyze components,and perform semi-quantitative analysis.The morphology characteristics of the grain boundary oxidation behavior under service,compressive and tensile stresses are discussed.Combined with the thermodynamic basis of oxidation,the mechanism of grain boundary oxidation was studied.At the same time,the micro-morphology of the impact fracture at room temperature was analyzed to explore the relationship between grain boundary oxidation,crack initiation and crack propagation.The results of the study indicate that chromium oxides appear in the tip region of crack propagation in the TP347H pipe under the service environment;two layers of Cr-rich Fe and Cr oxides and a layer of Fe-rich non-crac oxides appear in the middle region of crack propagation.The phenomenon of oxide layer is"sandwich"structure;no"sandwich"phenomenon was found at the initial end of crack initiation,and two layers of Cr-rich Fe and Cr oxides were intercalated with a Fe₂O₃ oxide layer.The sample was opened along the grain boundary oxidation zone and subjected to EDS analysis.The non-oxide layer of Fe-rich sandwich structure was an irregular rhombohedral crystal.It was a ferrite phase with an average diameter of about 3m;the iron-chromium oxide layer is dense and non-porous,closely attached to the substrate.Iron oxide at the initial end of the crack is spherical,about 1.5?m in diameter.At the 650°C reoxidation test,the production and growth of iron-rich non-oxidized layers in the"sandwich"structure was not observed;it was observed that with the prolonged oxidation time,the thickness of the oxide layer increased.In a stress-free,high-temperature oxidation test at 700°C,it was found that the surface of the sample produced a double-layer oxide structure with an inner layer of chromium-rich oxide and an outer layer of iron-rich oxide;the inner layer of oxidized region partially extended along the grain boundary into the matrix about 3?m.Simulation of grain boundary oxidation under compressive stress results in two layers of chromium oxide-rich oxide-rich triple-oxide structures near the simulated grain boundaries.The experiment of grain boundary oxidation under tensile stress was carried out by creep experiment.The chromium in the crack tip area was preferentially oxidized,and two layers of oxide structure with rich chromium oxides and iron-enriched oxides were produced in the middle region of the crack.The grain boundary oxidation mechanism of TP347H is preliminarily explored:in the early stage of grain boundary oxidation,chromium is preferentially oxidized into Cr₂O₃;in the second stage of grain boundary oxidation,iron,nickel and chromium react on the surface of the grain to form iron-chromium-nickel oxide,and iron element will be diffused from the matrix to the grain boundary and oxidized to Fe₂O₃ under the impetus of the concentration difference.Finally,the special morphology of two iron-chromium oxide layers sandwiching the iron-rich oxide layer is formed.In special circumstances,if there is less oxygen diffusion into the grain boundary,the second phase of grain boundary oxidation can be divided into two steps:In the first step,because the oxygen partial pressure is too small to form iron oxide,the ferrite phase could be formed near the grain boundary;second step,as the crack propagates,the oxygen partial pressure at the grain boundary gradually increases,at last the ferrite is oxidized to iron oxide.At room temperature,the as-received TP347H matrix exhibits ductile fracture characteristics,while the TP347H region with grain boundary oxidation undergoes significant intergranular fracture,and the transition from ductile fracture to intergranular brittle fracture is observed at the crack tip.The grain boundary oxidation causes the intergranular cracking of the alloy,which reduces the mechanical properties of the alloy.