Cr Heat Resistant Steel Research Articles

Quantification of the Laves phase in advanced 9–12% Cr steels using a standard SEM

The precipitates in 9–12% Cr steels are usually quantified using the TEM/EFTEM method in combination with an image analysis routine. Because of the limited spatial area of investigation, a complete statistical quantification and description of Laves-phase precipitation, based on this method, is not possible. To compensate the limited magnification range, a technique has been developed to measure larger particles also using a standard scanning electron microscope (SEM). To enable the scanning of a large specimen area, a special preparation technique, in combination with a self-developed quantification routine, was applied. The growth and coarsening of the Laves phase was investigated for several advanced 9–12% Cr heat-resistant steels at 600 and 650 °C. The results are compared with published values obtained from (partly) different measuring techniques as well as to thermodynamic equilibrium calculations.

Analysis of factors affecting type IV cracking in welded joints of high chromium ferritic heat resistant steels

In the present work, creep tests on single pass welded joints were carried out to investigate the factors affecting the deterioration of the creep strength of weldments of high Cr heat resistant steels. In creep tests at a temperature of 923 K and stresses of 90 and 120 MPa, creep fracture occurred in the fine grained heat affected zone (FGHAZ) and was identified as type IV cracking. It was found that a peak temperature between Ac1 and Ac3 in the FGHAZ caused the observed creep property deterioration. The factors identified as leading to the deterioration are a fine grain structure, low hardness, and large precipitates. Further investigations confirmed the influence of the fine grain structure as a critical factor decreasing the creep rupture time. The hardness difference between the FGHAZ and the base metal is insignificant, especially at 923 K. However, SEM investigations revealed that many large precipitates are distributed on the boundaries of the martensite laths and prior austenite grain boundaries. It is considered that the large precipitates also have a significant influence on the creep strength deterioration.

Mechanical response of 9% Cr heat-resistant martensitic steels to abrupt stress loading at high temperature

Microscopic deformation behavior upon abrupt stress loading at high temperature and the effects of microstructural factors on the behavior were investigated in 9% chromium martensitic steels with a variety of microstructures. Anelastic deformation occurs upon abrupt stress loading in steels with a martensitic lath structure. The anelastic deformation originates from the lath structure; that is, the cell structure independent of the presence of a high dislocation density, precipitates, or solute atoms of W or Mo. The lath boundaries, which correspond to sub-boundaries, elastically bend under applied stress, and the motion of the lath boundary requires time due to its complicated structure. This is the reason for the anelastic deformation in the martensitic lath structure. The anelastic strain of extremely low-carbon 9% Cr steel is smaller than that of conventional 9% Cr steels, indicating the difficulty of lath boundary motion in the low-carbon steel. The many MX precipitates present at the lath boundaries effectively retard the elastic bending of the lath boundary in the extremely low-carbon 9% Cr steel. The difficulty of lath boundary motion expected from the analysis of anelastic strain can reasonably explain the improvement of creep strength in extremely low-carbon 9% Cr steel reported in the literature.

Solubility Product of VN in Austenite of High Cr Heat Resistant Steel

Dissolved V in austenite of 7%Cr-0.4%V-0.09%N steel (hereinafter, % means mass%) was measured and the solubility product of VN was determined as follows:log([%V][%N]) = 4.38−(8436/T)where T is the absolute temperature in K. The solubility product obtained is 3 to 5 times larger than those in the literature and about one order larger than that for iron. The decrease in the activity coefficient of nitrogen due to the presence of Cr and V in the experimental steel is responsible for the increase in the solubility product of VN.

Compositional Change of Refractory Elements in Solution during Aging in High Cr Heat Resistant Ferritic Steels.

Compositional Change of Refractory Elements in Solution during Aging in High Cr Heat Resistant Ferritic Steels.

高Crフェライト系耐熱鋼における長時間クリープ強度に注目したCr含有量の最適化

高Crフェライト系耐熱鋼における長時間クリープ強度に注目したCr含有量の最適化

Effect of Mo and W on the Phase Stability of Precipitates in Low Cr Heat Resistant Steels.

Effects of Mo and W on creep strength of low Cr heat resistant steels have been investigated from the stand points of the phase stability of precipitates during long-term exposure at the elevated temperatures. The study on 2.25%Cr-Mo-V-Nb (Mo-steel) and 2.25%Cr-W-V-Nb (W-steel) with a same amount of Mo-equivalent has shown that the substituting W for Mo retards the evolution of microstructure, and thus remarkably improves the creep rupture strength. TEM observations have suggested that the most important precipitates strongly affecting the stability of microstructure are M 6 C type carbide and MC type carbide in both Mo-steel and W-steel. It is found that the M 6 C carbide precipitates with concentration of Mo and W during long-term aging. Therefore, the amounts of solute Mo and W supersaturated in matrix have reduced to the thermal equilibrium levels with changing the substructure from bainite lath structure to sub-grain liked structure. The kinetics of Mo- and W-partitioning between matrix and M 6 C carbide has been successfully expressed by a Johnson-Mehl-Avrami type equation and estimated the activation energy for diffusion of solutes; 125 kJ/mol in Mo-steel and 219 kJ/mol in W-steel. Thus the diffusion rate of W is suggested to be definitely lower than that of Mo. Furthermore, Mo- and W-partitioning to MC carbides have also been confirmed to affect the coherency and the growth rate of MC carbides. TEM observations on the strain image have shown that the MC carbide in Mo-steel has already lost the coherency with matrix. On the other hand, the finer MC carbide in W-steel has kept the coherent relationship with matrix even after long-term aging. It is concluded that W-steel is superior to Mo-steel in the stabilization of bainite lath structure and precipitates at elevated temperatures, resulting in the higher creep strength.

Creep Strengthening Mechanism of Mo and W in 9% Cr Heat Resistant Steels

Creep Strengthening Mechanism of Mo and W in 9% Cr Heat Resistant Steels

Atom-probe investigation of precipitation in 12% Cr steel weld metals

The microstructure of two types of 12% Cr steel weld metals, one with the composition of a common 12% Cr steel and the other with a higher nitrogen content, was studied using TEM (transmission electron microscopy) and APFIM (atom-probe field-ion microscopy) in post-weld heat-treated condition. The microstructure of the 12% Cr weld metals consisted of tempered martensite, retained δ-ferrite, an irregular low-dislocation α-ferrite and precipitates. Precipitates in the weld metals were dominantly M 23C 6 on different boundaries. Plate-like and fine cubic MN and M 2N were found inside the α-ferrite. APFIM analysis showed that M 23C 6 was almost a pure carbide and MN was almost a pure nitride. Carbon and nitrogen in the weld metals mainly existed in the precipitates. High nitrogen content did not change the composition of the precipitates, but increased the quantity of nitrides. Therefore, in the high nitrogen weld metal, the content of strong nitride-forming elements in the matrix decreased. These results are important in order to understand the strengthening mechanism of the high Cr steel weld metals, as well as of other high Cr heat-resistant steels.

Effects of Alloying Elements on Microstructure and Elevated Temperature Strength of 18 Cr Heat-Resistant Cast Steels

Effects of Alloying Elements on Microstructure and Elevated Temperature Strength of 18 Cr Heat-Resistant Cast Steels

Current Progress in Advanced High Cr Ferritic Steels for High-temperature Applications.

The article presents recent developments and future trends in high Cr ferritic heat resistant steels. Research programs are underway worldwide to improve the performance of 8 to 13% Cr ferritic steels for high temperature applications of up to 650°C. We developed the super 12% Cr heat resistant steel called TAF steel in 1956. The creep rupture strength of TAF steel is two or three times higher than those of H46 and AISI422 at 600 and 650°C. Our research in this area was well ahead of work being conducted in other countries.Recently the author succeeded in developing TR1100, TR1200, TB9 and TB12 with excellent high temperature properties and room temperature toughness through the improvement of TAF steel. In the near future, these new steels will be applied to turbine rotors, blades, and boiler tubes for advanced supercritical power plants, to fuel cladding, wrapper and steam generator tubing for fast breeder reactors, and to first wall materials for conceptual fusion reactors. These steels are a key to making these plants and reactors practical.

10Cr耐熱鋼のクリープ破断強度及び常温靱性に及ぼすMoとW量の影響

10Cr耐熱鋼のクリープ破断強度及び常温靱性に及ぼすMoとW量の影響