Jai Won Byeon

Magnetic nondestructive evaluation of thermally degraded 2.25Cr–1Mo steel

Abstract Aging was performed to understand the microstructural degradation in 2.25Cr–1Mo steel. Microstructural parameters (mean equivalent carbide size, number of carbides per unit area), mechanical properties (UTS, Vickers hardness) and magnetic properties (coercivity, remanence) were measured to investigate the relationship among these parameters. The magnetic coercivity and remanence were observed to decrease rapidly in the initial 1000 h of aging time and then decrease slowly thereafter. Linear correlations between mechanical and magnetic properties were found.

Evaluation of Thermal Degradation of 2.25Cr-1Mo Steel by High Frequency Ultrasonic Attenuation Measurement

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.

Ultrasonic Evaluation of Creep Damage of High Temperature Metallic Materials with Different Microstructures

The creep damage levels of two metallic materials (PM1000 and IN738LC Ni based superalloy), which had different microstructures and creep damage mechanism, were evaluated by ultrasonic velocity and attenuation measurement. In case of PM1000 alloy, the ultrasonic velocity decreased with increasing creep time, which was mainly attributed to the decrease of Youngs modulus due to the increasing volume fraction of creep cavity. Ultrasonic attenuation coefficient increased with increasing creep time in IN738LC alloy, while ultrasonic velocity showed no distinct trend. This increase of ultrasonic attenuation was attributed to the directional coarsening (rafting) of γ′ precipitates. A linear correlation was found between attenuation coefficient and mean length of γ′ precipitates. Introduction It is increasingly important to evaluate the level of creep damage for securing the reliability of the equipments. It is desirable to establish the process evaluating the creep damage nondestructively, as it is difficult or sometimes impossible to remove part of the component for analysis [1-5]. In order to evaluate the damage to structure nondestructively using in-situ monitoring, ultrasonic [1,2], magnetic [3,4] and electrical resistivity [5] methods have been employed. In this investigation, the creep damage levels of two selected commercial Ni based superalloys (PM1000 and IN738LC alloy), which had differently evolved microstructures during creep (cavity formation/rafting of γ′ precipitates), were evaluated by ultrasonic velocity and attenuation measurement. Experimental Procedure Material. The PM1000 Ni based dispersion strengthened superalloy (Ni: balance, Cr: 20, Al: 0.3, Ti: 0.5 and Y2O3: 0.6 in weight %) was selected under the consideration that the major creep damage is grain boundary cavity formation not involving with complicated carbide or second phase formation [6]. The IN738LC precipitation strengthened Ni based superalloy (Ni: balance, , Cr: 16, Co: 8.6, Mo: 1.75, Ta: 1.8, Al: 3.1, Ti: 3.1, W: 2.7, C: 0.1, Si: 0.09, Mn: 0.03, Nb: 0.82 and Fe: 0.3 in weight %) shows superior high temperature strength due to large amount (volume fraction: approximately 40 %) of cuboidal γ′ precipitates (Ni3(Al,Ti)) [7]. This alloy was selected under the consideration that the most dominant microstructural change during creep deformation is rafting of γ′ precipitates rather than creep cavity formation [8]. Preparation of artificially creep damaged specimens and microstructural analysis. In case of PM1000 alloy, the creep temperature and stress ranges were determined from the previous results [6,8] so that the intergranular fracture dominated the creep fracture. The temperature was set at 1000°C, stress range was 110-123 MPa and the rupture time ranged 2-2000 hours. In case of IN738LC alloy, the creep temperatures and stress ranges were determined from the Larson-Miller parameter so that the rupture time ranged 10-1000 hours. Creep test was interrupted after some fraction of life time for microstructural examination and ultrasonic measurement. The amount of creep cavity in PM1000 alloy was estimated according to the Eq. (1) after measuring the density of Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 120-125 doi:10.4028/www.scientific.net/KEM.270-273.120

Assessment of Tensile and Fatigue Damage in 316L Stainless Steel by Acoustic Emission Technique

Acoustic emission (AE) technique was applied to evaluate both tensile deformation of 316L stainless steel and bending-fatigue damage of socket-welded pipe. AE activity was monitored during tensile deformation of plate-type specimens subjected to various heat treatments. Variation in RMS voltage of AE signal was correlated with the amount of strain-induced martensite phase. Secondly, actual size of socket-welded pipe specimen was bending-fatigued under various stresses. Crack initiation was determined by observing an abrupt increase in AE count, and confirmed by radiographic examination before and after the crack initiation cycle. Potentials of AE technique for monitoring fatigue crack initiation were discussed.

Transmission Electron Microscopy of Isothermally Oxidized EB-PVD Thermal Barrier Coating on (Ni,Pt)Al Bondcoat

The growth and microstructure of the thermally grown oxide (TGO) underneath the electron beam physical vapor deposited (EB-PVD) yttria stabilized zirconia (YSZ) topcoat were examined after short-term isothermal oxidation (1100 °C, up to 50 hours) for the as-coated and gritblasted (Ni,Pt)Al bondcoats. Microstructural analysis was carried out by a high resolution scanning transmission electron microscope equipped with bright/dark field imaging, high angle annular dark field imaging, and nano-spot energy dispersive spectroscopy. Presence of mixed oxide zone (MOZ) and a continuous Al2O3 oxide zone (COZ) was observed on the thermal barrier coating (TBC) with the as-coated (Ni,Pt)Al bondcoat. However, on the TBC with grit-blasted bondcoat, only the continuous-columnar Al2O3 scale was observed. For the as-coated type bondcoat, numerous voids were observed near the interface between MOZ and COZ after isothermal oxidation. On the other hand, COZ showed parabolic growth without any formation of voids for the grit-blasted specimens.

The effect of powder size on thermoelectric properties of 95%Bi2Te3-5%Bi2Se3 alloy

The n-type (95%Bi2Te3- 5%Bi2Se3) compound was newly fabricated by gas atomization and hot extrusion, which is considered to be a mass production technique of this alloy. The effect of powder size on thermoelectric properties of 0.04% SbI3 doped 95%Bi2Te3- 5%Bi2Se3 alloy were investigated. Seebeck coefficient (α) and Electrical resistivity (ρ) increased with increasing powder size due to the decrease in carrier concentration by oxygen content. With increasing powder size, the compressive strength of 95%Bi2Te3-5%Bi2Se3 alloy was increased due to the relative high density. The compound with ~300 μm size shows the highest power factor among the four different powder sizes. The rapidly solidified and hot extruded compound using 200~300 μm powder size shows the highest compressive strength.

Evaluation of Microstructure and Mechanical Properties of Metals by Non-Destructive Methods

Microstructural parameters (kinds of phases, inter-lamellar spacing of pearlite, precipitate size, number of precipitates), mechanical properties(UTS, Vickers hardness) and non-destructive parameters(ultrasonic velocity, ultrasonic attenuation coefficient, electrical resistivity, magnetic coercivity, magnetic remanence, Barkhausen noise) in various metallic materials were measured in order to investigate the mutual relationship among these parameters. The optimum non-destructive parameters were selected for particular purposes in order to evaluate the level of damage to the metallic structures, to differentiate the microstructures and to predict the mechanical properties of superalloy, low and eutectoid steel and Cu alloys non-destructively.

Microstructures and Tensile Properties of Fe-40Al Intermetallic with High Boron Content

The effects of B addition up to 0.4 wt% (i.e., 1.61 at%) to Fe-40Al alloy on microstructures and tensile properties were investigated. The vacancy-annealed specimens following casting and extrusion were examined by optical and scanning electron microscopy and thermal neutron-induced microradiography. The addition of a large amount of B resulted in grain refinement and changed the fracture mode from intergranular to transgranular to increase the tensile elongation. Especially, the alloy containing 0.3wt% B exhibited the elongation of 16.4% under the strain rate of 1x100s-1 at room temperature. The increase in elongation with increasing strain rate was discussed in terms of suppression of hydrogen embrittlement.

Microstructural Evaluation of Isothermally Aged 12Cr Steel by Magnetic Property Measurement

The magnetic coercivity of ferritic 12Cr steel was experimentally studied in order to characterize its microstructures and mechanical properties during isothermal aging. As the aging time increased, the M23C6 carbide coarsened and additional precipitation of Fe2W phase was induced. The width of martensite lath increased to about 0.4μm after 4000 hrs of aging. The coercivity decreased as the number of precipitate decreased and the width of martensite lath increased. The hardness was proportional to the magnetic coercivity. These empirical linear relations suggested that the change in the microstructures and strength of ferritic 12Cr steel during thermal aging could be evaluated by monitoring the magnetic coercivity.

Assessment of Mechanical Degradation in Pressure Vessel Steel by Morphological Analysis of Carbides

In this study, mechanical degradations in 2.25Cr-1Mo steel were evaluated by quantitative morphological analysis of carbides. Based on the morphology, carbides were classified as globular, fine acicular, rod, and grain boundary one. Mean size of carbides were determined as a function of morphology and thermal degradation time at 630°C. Area fraction of grain boundary carbides and fraction of grain boundary M6C carbides were observed to increase rapidly in the initial stage of degradation and then gradually afterwards. Both mean size of globular carbide and fraction of grain boundary M6C carbides were linearly correlated with strength. Potentials of carbide morphology analysis as a health monitoring technique were discussed, in term of correlation coefficient with strength.