Materials Science Forum Vols. 475-479

Abstract: Hydrogen absorption of incoherent TiC particles that were once reported to be strong hydrogen traps in iron at room temperature was investigated by means of thermal desorption spectrometry (TDS). The results indicated that incoherent TiC particles in iron do not trap hydrogen at all at room temperature even they are cathodically charged for a long time. Only at high temperatures and in atmosphere containing hydrogen source, incoherent TiC particles can trap hydrogen. The origin of hydrogen trapped by incoherent TiC particles was justified to be water vapor in the atmosphere during heat treatment.

Abstract: We have developed an improved HVOF spray process called “Gas-shrouded HVOF” (GS-HVOF) over the past several years. By using an extension nozzle at the exit of a commercial HVOF spray gun, GS-HVOF is capable of controlling the oxidation of sprayed materials during flight as well as achieving higher velocity of sprayed particles. These features result in extremely dense and clean microstructure of the sprayed coatings. The process has been successfully applied to corrosion resistant alloys such as SUS316L, Hastelloy C, and alloy 625 as well as cermets such as WC-Cr3C2-Ni. The spray process, coatings microstructure and property evaluation will be discussed with potential industrial applications in the near future.

Abstract: The influence of trace boron and cooling rate on the thermal embrittlement of 2.25Cr -1Mo steel was investigated by thermo-mechanical simulator and SEM analysis. It was proved that the trough in the thermo-ductility curve located between 1000°C and 700°Cand shifted to lower temperature and the temperature range of the trough became shallow and narrow as the boron content increased. When the cooling rate decreased, the ductility trough became shallow and narrow too. This made the steel free of surface crack in CC process.

Abstract: The development of an ultrafine grained carbon steel during repetitive shear deformation of side extrusion and the properties after heat treatment were investigated. Side extrusions were carried out at room temperature and the used material was 0.50% carbon steel. The repetitive side extrusions with a constant lateral pressure were carried out up to 3 passes without rotation. The specimens of these steels after 3 passes were annealed at a constant temperature of 600°C changing the treatment time. After side extrusion and heat treatment, the fatigue property was better than that of the as-received material.

Abstract: With the Uddeholm self-restricted thermal fatigue method, the transformation of phase constitutions and morphology of boride layer after 3000 cycles of thermal fatigue test was studied by XRD and SEM and the surface residual stress in the process of thermal fatigue test was researched by X-ray Stress Analyzer; the formation of the heat checking of boride layer was also analyzed. The results show that with the high strength and excellent thermal stability of boride layer, morphology of heat checking alters. Therefore, boronizing treatment of H13 steel improves its thermal fatigue behavior.

Abstract: Low cycle fatigue tests were conducted in high temperature water for A533B pressure vessel steels with sulfur contents of 0.013, 0.025 and 0.038 wt.% respectively. Cyclic stress amplitude response and fatigue resistance as well as influence of strain rate, temperature and dissolved oxygen concentration in water were investigated. Fatigue cracking/fractograhpic features were examined. Sulfide-related environmentally assisted cracking mechanism is discussed.

Abstract: It was attempted to assess nondestructively the degree of isothermal degradation of 2.25Cr-1Mo steel by using high frequency longitudinal ultrasonic wave. Microstructural parameter (mean size of carbides), mechanical property (Vickers hardness) and ultrasonic attenuation coefficient were measured for the 2.25Cr-1Mo steel isothermally degraded at 630°C for up to 4800 hours in order to find the correlation among these parameters. The ultrasonic attenuation coefficients at high frequencies (over 35MHz) were observed to increase rapidly in the initial 1000 hours of degradation time and then slowly thereafter, while the ones at low frequencies showed no noticeable increase. Ultrasonic attenuation at high frequencies increased as a function of mean size of carbides. Ultrasonic attenuation coefficient was found to have a linear correlation with the hardness, and suggested accordingly as a potential nondestructive evaluation parameter for assessing the mechanical strength reduction of the isothermally degraded 2.25Cr-1Mo steel.

Abstract: Based on the damping alloy Fe-13Cr-6Al, five types of alloys are prepared in the present study with 1 at.% of Cr replaced by Mo, Mn or Nb, and 0.5 at.% of Al replaced by Cu. The effects of annealing temperature, vibrating frequency, strain-amplitude and static load on the damping capacity of these alloys are studied through damping capacity measurement and optical microscope observation. The following results were obtained: (1) The damping capacity of Fe-13Cr-6Al based alloys increases with increasing annealing temperature. (2) The replacing elements demonstrate different effects on the damping capacity of Fe-13Cr-6Al based alloys. The damping capacity of the alloys with Cu or Mn substitution increases, while that of alloys with Nb substitution decreases significantly as compared with Fe-13Cr-6Al. (3) The damping capacity of Fe-13Cr-6Al based alloys is not sensitive to frequency. It decreases with increasing in static load, and increases rapidly with strain before reaching a steady state.

Abstract: A structural steel, 35CrMoV steel, has been attempted firstly by explosive powder compaction followed by sintering (EPC-sintering). The nitrogen content of the steel was 0.15wt%, which was accordant with the definition of high nitrogen steel (HNS). The final density of the EPC-sintering steel was only about 6.9g/cm3, which indicated that the processing parameters must be modulated further. In the sample of this steel, some radial cracks were found around the center of the cross-section of the steel, resulting in no mechanical tests carrying out. Observing the majority of the rim region of the sample of this steel, the microstructures were very tight, suggesting that it was possible and successful to manufacture HNS through EPC-sintering. The characteristics of the EPC-sintering high nitrogen 35CrMoV steel were that the cementites in the pearlites were found to be extremely fine. There were many (Cr,Mo)23(C,N)6 carbonitrides precipitates in the matrix. Some precipitates were round and others were needle-like. Some were distributing orderly in matrix and crossing over the dislocations. The dislocation density in the EPC-sintering steel remained high.

Abstract: The wire for high strength and toughness TMCP steels of submerged arc welding was developed. The low carbon and micro-alloying with Ti-B system was adopted to obtain the acicular ferrite dominated deposited metals. Experimental results show that the carbon equivalent (Pcm) should be higher than 0.17, which can ensure the high strength and high toughness of the deposited metals. In the alloy system, Oxygen and Nitrogen contents, micro-alloyed elements (C, Mn) and its mixture ratio are the key factors that affect the deposited metals toughness. With increasing C, Mn content, the acicular ferrite is increased and toughness is improved. Oxygen and Nitrogen are deleterious to the toughness of deposited metals.