Contribution of Z-Phase Precipitation to Recovery of Martensitic Structure in High Chromium Creep Resistant Steel

2007 ◽  
Vol 539-543 ◽  
pp. 3000-3005 ◽  
Author(s):  
Kota Sawada ◽  
Hideaki Kushima ◽  
Kazuhiro Kimura
Keyword(s):  
Phase Formation ◽  
Prior Austenite ◽  
Martensitic Structure ◽  
Number Density ◽  
Austenite Grain ◽  
Nb Content ◽  
Lath Structure ◽  
Austenite Grain Boundary ◽  
Martensitic Lath ◽  
Prior Austenite Grain

The precipitation site, main metallic composition and number density of Z phase have been investigated in T91 in order to clarify the influence of Z phase formation on recovery of martensitic structure and creep strength degradation. The Z phase particles were mainly present around prior austenite grain boundaries and/or packet boundaries in the steels crept at 550oC and 600oC. The Z phase particles were found in specimens crept at 550oC to 650oC. There was no indication of Z phase formation up to about 62475.0 h at 500oC and 14106.5 h at 700oC. The Nb content of Z phase observed at 550oC was lower than that at 600oC. The number density of Z phase measured at 550oC was lower that that at 600oC, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550oC in contrast with 600oC.

Cavitation control in steels of high residual element content

1980 ◽  
Vol 295 (1413) ◽  
pp. 305-305 ◽  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Prior Austenite ◽  
Low Alloy Steels ◽  
Boundary Zone ◽  
Austenite Grain ◽  
Alloy Steels ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain ◽  
Transverse Boundary

The cavitational mode of failure of prior austenite grain boundaries in bainitic creep-resisting low alloy steels is now well established as a principal factor in the high incidence of cracking problems which has developed on modern power plant in recent years. The microstructural features dominating the cavitation process at the reheat temperature in a ½CMV bainitic steel of high classical residual level have been determined. The prior austenite grain boundaries become zones of comparative weakness ca . 1 pm thick at 700 °C and are incapable of sustaining significant shear loads. Deformation is therefore initiated by a relaxation of load, through a process of prior austenite grain boundary zone shear, from inclined to transverse boundaries such that a concentration of normal stress develops across the latter. The overall deformation is thereafter determined by cavitation of the transverse boundary zones, the necessary inclined boundary displacements being accommodated by further grain boundary zone shear. Transverse boundary cavitation is shown to be an essentially time-independent process of localized ductile microvoid coalescence resulting from the plastic deformation of the boundary zone.

Prior austenite grain boundary embrittlement of low alloy steel by boron

1980 ◽  
Vol 295 (1413) ◽  
pp. 298-298
Keyword(s):  
Grain Boundary ◽  
Prior Austenite ◽  
Alloying Element ◽  
Reheat Cracking ◽  
Austenite Grain ◽  
Segregation Effect ◽  
Commercial Steel ◽  
Grain Boundary Embrittlement ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain

A previous study of reheat cracking in a CrMoV steel (Ducol W-30), in which the fracture toughness of the coarse grain size region of the h.a.z. was measured as a function of temperature, revealed a large decrease in toughness at 600 °C accompanied by prior austenite grain boundary (p.a.g.b.) fracture (Ritter & McPherson 1974). This severe embrittlement was eliminated if the steel was heated to 680 °C before testing at 600 °C suggesting that the effect may have been associated with a p.a.g.b. segregation effect. This hypothesis has been examined by comparing the degree of embrittlement at 600 °C of specimens with stimulated h.a.z. microstructures prepared from laboratory heats, with the same alloying element composition as the commercial steel used previously, but doped with the trace elements, S, P, As, Sb, Sn, Gu, A1 and B, singly or in various combinations.

Effects of V and Carbides on the Temper Embrittlement of the 2.25Cr-1Mo Steel

2006 ◽  
Author(s):  
Jeong Tae Kim ◽  
Byung Hoon Kim ◽  
Byeong Ook Kong ◽  
Dong Jin Kim
Keyword(s):  
Transition Temperature ◽  
Grain Boundary ◽  
Prior Austenite ◽  
Temper Embrittlement ◽  
Aging Treatment ◽  
Atomic Ratio ◽  
Fracture Appearance Transition Temperature ◽  
Austenite Grain ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain

The characteristics of temper embrittlement and carbides precipitated in 2.25Cr-1Mo and 2.25Cr-1Mo-V steels which were aged up to 50,000h at 454∼515°C were investigated. The temper embrittlement susceptibility was evaluated as the shift of 54 Joule transition temperature (vTr54) and 50% fracture appearance transition temperature (FATT50) by the Charpy v-notch impact tests. The shift of vTr54 and FATT50 in the 2.25Cr-1Mo steel rapidly increased with the aging time up to 10,000h and then slowly saturated. Peak ΔvTr54 and ΔFATT50 in the 2.25Cr-1Mo-V steel were obtained up to 30,000h aging at 454°C. The distribution of phosphorus in the 2.25Cr-1Mo-V steel and 2.25Cr-1Mo steel after aging treatment was different. Phosphorus in the conventional 2.25Cr-1Mo steel was mainly observed at the prior austenite grain boundary, however, phosphorus in the 2.25Cr-1Mo-V steel was observed at the prior austenite grain boundary and the interfaces between carbide and matrix. The type and composition of carbides were changed to the stable Mo-rich ones, the carbide of M6C type was manifestly precipitated in both steels, and the new type of M4C3 carbides in the 2.25Cr-1Mo-V steel were finely distributed within grain and the atomic ratio of M4C3 was changed from (Fe0.08Cr0.21Mo2.61V1.10)C3 in the PWHT state to (Fe0.22Cr0.20Mo2.78V0.80)C3 with aging.

Temperature Dependence of Austenite Nucleation Site on Reversion of Lath Martensite

2010 ◽  
Vol 638-642 ◽  
pp. 3424-3429 ◽  
Author(s):  
Nobuo Nakada ◽  
Toshihiro Tsuchiyama ◽  
Setsuo Takaki ◽  
Naoki Miyano
Keyword(s):  
Grain Boundary ◽  
Temperature Dependence ◽  
Elastic Strain ◽  
Prior Austenite ◽  
Nucleation Site ◽  
Lath Boundary ◽  
Austenite Grain ◽  
Austenite Grains ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain

The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

A Study on the Characteristics of the Boundaries in Bainitic Low Alloy Steels Using Electron Back-Scatter Diffraction (EBSD) Technique

1999 ◽  
Vol 5 (S2) ◽  
pp. 238-239
Author(s):  
Yong-Jun Oh ◽  
Min-Chul Kim ◽  
Jun Hwa Hong
Keyword(s):  
Prior Austenite ◽  
Dislocation Cell ◽  
Electron Back Scatter Diffraction ◽  
Chemical Compositions ◽  
Low Alloy Steels ◽  
Austenite Grain Size ◽  
Austenite Grain ◽  
Alloy Steels ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain

Bainitic low alloy steel has a complex microstructure exhibiting several types of boundaries. The boundaries in bainitic steel, although certain boundaries are absent with respect to the alloy composition and the manufacturing process, could be typically divided into 4 types; dislocation cell boundary, lath boundary, packet boundary, and prior austenite grain boundary, in increasing order of size. The size and distribution of the respective boundaries are an important factor which controls the mechanical properties of the steels, including brittle fracture. In the present research, the characteristics of the boundaries in the bainitic low alloy steels were investigated in view of misorientation between grains enclosed by the respective boundaries.The alloys investigated were Mn-Mo-Ni low alloy forging steels having chemical compositions shown in TABLE 1. Steel-A was manufactured by the Vacuum Carbon Deoxidation(VCD) process. For the finer prior austenite grain size, Steel-B was produced by the aluminium addition and the silicon killing process. Before EBSD analysis, the microstructures of the alloys were observed using SEM and TEM. EBSD measurements were obtained using a Link OPAL system(Oxford) linked to a JEOL JSM 6300 SEM operating at 15KeV with the sample tilted at 70°.

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