Zheng Yangzeng

Resistance of a high nitrogen austenitic steel to cavitation erosion

Abstract The resistance to cavitation erosion (CE) was measured using a rotating disk device for a high-nitrogen CrMn austenitic steel. The damaged morphology and microstructure characteristics in the surface layer exposed to CE were analyzed by the use of SEM and TEM. The results show that the resistance to CE of 18Mn18Cr0.5N (HNS) steel is 3.3 and 2.3 times higher than that of 0Cr13Ni6Mo (CrNi1) and 0Cr16Ni5Mo (CrNi2) steels, respectively, which are in commonly used for hydraulic turbine runners. The excellent resistance to CE of HNS steel is related to its good mechanical properties and the following microstructure changes: strengthening due to changes of dislocation configurations and the formation of substructures during CE, consumption of CE impact energy by the processes of CE-induced mechanical twins or stacking faults etc. as well as the strengthening due to the CE-induced products itself, the strengthening due to the block of strong N(C)Me covalent bond on dislocation motion and the reconfiguration of crystal lattices.

Structural changes after cavitation erosion for a CrMnN stainless steel

Abstract The structural changes after cavitation erosion were investigated by the use of backscattering Mossbauer spectra for a CrMnN dual-phase (metastable austenite and martensite) stainless steel. The atomic configurations around iron atoms in martensite before and after cavitation erosion were analyzed and discussed. Because of cavitation erosion, the weight fraction of α-martensite in the surface layer being exposed to cavitation erosion increases gradually, and a new kind of configuration containing more alloying elements around iron atoms in α-martensite is induced whose hyperfine effective magnetic field value is 23.2 T.

Mechanical properties of 32Mn-7Cr-0.6Mo-0.3N austenitic steel for cryogenic applications

The tensile, impact, and fracture toughness tests from ambient temperature to 77 K were carried out on 32Mn-7Cr-0.6Mo-0.3N austenitic steel. The fracture surfaces and the phase constitution were analyzed using scanning electron microscopy and x-ray diffraction. The results show that the relation between yield strength and temperature is σ0.2 · 300 + 1392.4 exp (−0.0106T). The 77 K yield strength is 883 MPa · m1/2 and the KJ0.05 value is about 236 MPa · m1/2. The cryogenic intergranular fracture is fully suppressed. The 77 K fracture surfaces exhibit a tough character composed of many dimples and few small quasi-cleavage facets. The results of x-ray analysis show that the austenite phase of the steel is stable even under cryogenic deformation conditions.

Influence of welding parameters on nitrogen content in welding metal of 32Mn-7Cr-1Mo-0.3N austenitic steel

The transfer behavior of nitrogen into the welding metal during gas tungsten arc welding process of 32Mn-7Cr-1Mo-0.3N steel was investigated. The effects of gas tungsten arc welding process variables, such as the volume fraction of nitrogen in shielding gas, arc holding time and arc current on the nitrogen content in the welding metal were also evaluated. The results show that the volume fraction of nitrogen in gas mixture plays a major role in controlling the nitrogen content in the welding metal. It seems that there exhibits a maximum nitrogen content depending on the arc current and arc holding time. The optimum volume fraction of nitrogen in shielding gas is 4% or so. The role of gas tungsten arc welding processing parameters in controlling the transfer of nitrogen is further confirmed by the experimental results of gas tungsten arc welding process with feeding metal.