Ti Stainless Steel Research Articles

Use of Ion Microanalyzer in the Analysis of Passive Layers on Stainless Steel

Abstract Passive layers on various stainless steels were studied with an ion microanalyzer. An argon ion beam was directed on the specimen surface in an oxygen atmosphere. The use of oxygen atmosphere effectively minimized the difference in ion yields of the elements in the oxidized layers and matrices, making it possible to determine these elements with good reproducibility. 18% Cr–8% Ni and 18% Cr–8% Ni–0.4% Ti stainless steel was exposed to neutral water containing 8 ppm oxygen at 289 °C. The passive layers contained larger amounts of iron and smaller amounts of chromium than the matrices.

The effect of iodine vapour on creep rupture properties of nitrided 20% Cr/25% Ni/Nb/1.5 Ti stainless steel

The creep-rupture behaviour of nitrided 20Cr/25Ni/Nb/1.5Ti stainless steel has been studied in environments of iodine vapour, air and vacuum under initial applied stresses 138–310 MN/m 2 at 750°C. Iodine vapour reduced the times to failure compared with vacuum or air over this stress range, but only reduced the failure strain in tests lasting > 100 h. The secondary creep rate remained unaffected by the test environment. The results are compared with creep-rupture data obtained previously on 20Cr/25Ni/Nb stainless steel in the same test environments.

Postirradiation Mechanical Properties of Types 304 and 304 +0.15% Titanium Stainless Steel

The tensile and creep-rupture properties of Types 304 and 304 + 0.15% Ti stainless steels have been determined after irradiation at temperatures in the range 400 to 820°C to maximum neutron fluences of 6.1 × 1022 n/cm2 (>0.1 MeV). Changes in mechanical properties were related to the microscopic observations of irradiation-produced defects.When irradiated in the annealed condition in the neighborhood of 450°C, Type 304 stainless steel exhibited an increased yield stress, reduced strain hardening coefficient, and reduced uniform and total elongation. The increased yield stress could be correlated with the strengthening expected from irradiation-produced voids and dislocations. With increasing irradiation temperature the concentration of these defects decreased and thus the magnitude of the yield stress increase became less. At 500 to ∼600°C irradiation and test temperatures a pronounced reduction in creep-rupture ductility was observed. Fractures were intergranular. It is suggested that in this temperature range the void-dislocation structure together with the transmutation-produced helium were responsible for the intergranular fractures and low ductilities. At higher temperatures no void-dislocation structures were formed. For these conditions the ductilities were higher than in the 500 to 600°C range but still significantly below the unirradiated value.Variation in alloy composition and pre-irradiation microstructure had a strong influence on the postirradiation properties. Type 304 + 0.15% Ti stainless steel exhibited significantly higher tensile and creep-rupture ductilities than the standard alloy when irradiated and tested above 450°C. The strength properties of specimens irradiated in the 10% cold-worked condition were similar to those of material irradiated in the annealed condition, but the total elongation and reduction in area were slightly lower.

18-8Ti鋼のクリープおよびクリープ破断特性におよぼす初期析出状態の影響

18-8Ti鋼のクリープおよびクリープ破断特性におよぼす初期析出状態の影響

18-8 Tiステンレス鋼の高温強度

A comprehensive study was carried out to investigate the effects of the chemical compositions, heat treatments and grain size on the creep rupture properties of the 18-8 Ti stainless steels, in wide use for superheater tubes and reheat tubes of power boilers. The conclusions are as follows:(1) Effect of chemical compositionsIt was found that the ferrite-free austenitic steels had the creep rupture strength superior to that with 14% ferrite at 550°C and 650°C, both of which had different compositions even though within the range of the specification (AISI 321 or JIS STB 42 D) and were obtainable to single or duplex structure by selection of proper composition also within the range of the specification.That is speculated to be attributable to occurrence of the precipitation of enormous chromium carbide around the ferrite in duplex austenite-ferrite structure at testing temperature.From these results, it is desirable as for the chemical compositions to offer fully austenitic structure, if possible, for elevated temperature applications. The results of measuring of the creep rupture strength of the 18-8 Ti steels within the range of 3 to 19 of Ti/C ratio, keeping the constant contents of carbon and other alloying elements, revealed that it increased with decrease of Ti/C ratio. It is assumed that increase of Ti/C ratio follows additional insoluble TiC at the prevalent solution treatment temperature (1100°C) and consequently decrease of TiC effective to creep rupture on the contrary, which precipitates finely and dispersedly into the austenite structure at service temperature. Thus it is recommendable to adopt low Ti/C ratio within the range of 3 to 7, to obtain high strength at elevated temperature.(2) Effect of heat treatmentsArising of the solution treatment temperature in the range of 1000°C to 1300°C brought about increase of the creep rupture strength of the steels. We might conclude, however, that to the temperature range of 1100°C to 1150°C is practically preferable, because too high temperature heat treatment bears low toughness.(3) Effects of grain sizeIt has generally been said that the coarsely grained steel has good creep rupture strengtth in comparison with that of the finely grained. However, in case of austenitic stainless steels, as creep rupture strength markedly depends on the condition of the precipitation of carbide along the grain boundaries at the service temperature range of 550°C to 650°C, it was found that the coarsely grained steel does not always show good creep rupture strength. Thus we may consider that the fact described in (2) is due not to the coarse grain but to the fine and uniform distribution of the precipitated carbide at service temperature, which shows the higher solubility to the matrix at higher solution treatment temperature.(4) Long-time creep rupture strengthCreep rupture tests up to 13000 hours are practised for advantageously heat-treated 18-8 Ti stainless steel with desirable chemical compositions. The extraporated 100000 hours rupture strength was 9.3kg/mm2. That is superior to the average value, 5.5kg/mm2, of ASTM STP No. 124. We may probably attribute that to underestimation of 18-8 Ti stainless steel, because the data of ASTM had been obtained before the relations between metallographic behaviors and creep rupture strength in this steel were satisfactorily clarified.

AISI 321型18-12 Tiステンレス鋼鍛造材のクリープ破断性質について

AISI 321型18-12 Tiステンレス鋼鍛造材のクリープ破断性質について

AISI 321型18-12 Tiステンレス鋼のクリープ破断性質と組織変化との関連について

AISI 321型18-12 Tiステンレス鋼のクリープ破断性質と組織変化との関連について

18-8Ti型ステンレス鋼の破断強度について

Recently, AISI 321 type 18-8 Ti stainless steel has widely been used as the material of superheater or reheater tubes for large power plants, and it is generally recognized that the creep rupture strength of this type of steel varies greatly with the grain size in connection with heat treatment. However, in view of the fact that both of the grain size and condition of precipitation are affected by the heat treatment, we consider that the grain size is not a sole factor to have an effect on the creep rupture strength, but also the condition of carbides or σ phase precipitation has a great effect on it. To confirm this we carried out 650°C creep rupture tests of the material treated at various heat treatments after working. As a result of the tests, we have reached the following conclusions:It makes a great difference in the creep rupture strength whether the heat treatment after working is carried out at a comparatively low temperature in the precipitation range, (less than about 1000°C), or at a high temperature in the solution range.During recrystallization, precipitation and coagulation develop very quickly, and the precipitation hardening effect that prevents creep deformation is lost due to over-aging. Therefore, the strength of the material subjected only to working or treatment at a low temperature is small at the long period side. On the other hand, the strength of the material treated at a high temperature after working is not reduced, because the precipitation never occurs, and precipitated carbides or σ phase are dissolved in matrix during heat treatment.There will be some relation between the creep rupture strength and the grain size. However, when the material is treated at a comparatively low temperature and has fine grains, it will also have an undue condition of precipitation. This coincidence might often cause a misconception that the creep rupture strength was affected only by the grain size, but we believe that the greater part of the reduction of the rupture strength is due to undue condition of precipitation. We shall make further studies on this point.

18-8Ti型ステンレス鋼管材のクリープ破断特性について

18-8Ti型ステンレス鋼管材のクリープ破断特性について

18-8Ti型ステンレス鋼のクリープ破断特性と組織変化について

18-8Ti型ステンレス鋼のクリープ破断特性と組織変化について

THE FORMATION OF σ-PHASE IN A MODIFIED 18/8 STAINLESS STEEL

The formation of σ-phase in a 18% Cr 11% Ni 2.6% Mo 0.8% Ti stainless steel has been investigated. Microscopic and X-ray examinations showed that the formationσf σ-phase from δ-ferrite was accompanied by the formation of austenite, the reaction being most rapid at 850℃The formation of a caused a marked decrease in Izod value and a slight increase in hardness. The σ-phase and its detrimental effects can be removed by a re-solution treatment at 1100℃.