Oxidation and carburization behaviour of two type 316H stainless steel casts in simulated AGR gas environment at 550 and 600 °C

Creep performance of carburized 316H stainless steel at 550 °C

“…Thus, hereafter the steels will be labelled as 'Low Mn' (0.98 wt.% Mn) and 'High Mn' (1.52 wt.% Mn). The grain size was determined previously [19] using the linear line intercept method on Electron Backscattered Diffraction (EBSD) maps [20,21]. Both the 'Low Mn' and 'High Mn' steels were in the form of pipes with a wall thickness of ~63 mm, which had been in AGR operation within the secondary coolant loop at ~525 °C for 910 4 h and 110 5 h, respectively, but had not been exposed to AGR primary gas coolant.…”

“…The gauge length of the specimens were mechanically ground and polished to 0.25 m colloidal silica surface finish (Fig. 1b) producing a deformation free surface with a surface roughness of 0.01 Ra ± 0.002 Ra [19]. After surface preparation, the creep specimens were inserted into an autoclave for pre-conditioning carburization at either 550 or 600 °C and a pressure of 600 psi (4.14 MPa), in simulated AGR primary gas coolant containing 500 vppm H2O, 100 vppm H2, 300 vppm CH4, and 1 vol.% CO, balanced with CO2.…”

“…1c). These coupons were removed at selected time intervals, enabling the evaluation of surface oxide formation and the development of the sub-surface carburized layer which have been previously reported [19]. After either 3000 h at 600 °C or 8000 h at 550 °C, the 'pre-conditioned' 'Low Mn' and 'High Mn' creep specimens from were removed from the autoclave and tested mechanically (Fig.…”

“…Therefore, it is expected that from an early stage during creep testing the oxide regions are not load-bearing [18,26]. Thus, to determine the applied stress, the load-bearing cross-sectional area of the specimens was assumed to not include the surface oxide layer [19,26,27]. A total of 25 creep specimens were tested to a target engineering creep strain in the range of 0.00-1.50%.…”

“…The main parameters that characterise the as-received microstructure of both steels are shown in Table 2 together with the average thickness of the surface oxide and sub-surface carburized layers, as well as the bulk and peak surface hardness after pre-conditioning summarised from [19]. The carburized depth was characterised using nano hardness and defined as the location in the microstructure where the local hardness value is at 120% of the bulk hardness [19]. The two materials present an equiaxed austenite grain structure in the asreceived condition, with 'Low Mn' steel having a larger average grain size of 142  110 µm compared to 81  67 µm for the 'High Mn' steel.…”