Texture and anisotropy of low-interstitial 18 Pct Cr-2 Pct Mo ferritic stainless steel

The effects of various processing schedules on the formability of low-interstitial 18 pct Cr-2 pct Mo ferritic stainless steel sheet were evaluated by utilizing 1) plastic strain ratios and work-hardening exponents derived from tensile tests; 2) Swift and Olsen cup tests; and 3) X-ray pole figure analysis. Deep drawability was shown to increase with increasing total cold reduction for processing schedules with either one or two cycles of cold rolling and annealing. A condition of very low planar anisotropy was shown to be associated with 85 pct cold reduction, either in a single cycle or in the second cycle of a two-cycle schedule. The measured plastic strain ratios were correlated with the observed crystallographic textures.     

Mechanical properties of high purity Fe-26 Cr-1 Mo ferritic stainless steel

The physical metallurgical characteristics of high purity Fe-26 Cr and Fe-26 Cr-1 Mo ferritic stainless steels, containing low carbon and nitrogen contents, were studied. The studies demonstrate that a commercially produced Fe-26 Cr-1 Mo high purity alloy containing a maximum of 50 to 150 ppm of carbon and nitrogen is capable of good mechanical properties and fabricability. The alloy was previously demonstrated to exhibit good corrosion resistance. Because of these properties, the high purity Fe-26 Cr-1 Mo alloy is a viable alternate to 300-series stainless steels as well as more highly alloyed materials for many applications. This paper is based on a presentation made at a symposium on “New Developments in Ferritic and Duplex Stainless Steels,” held at the Fall Meeting in Cleveland, Ohio, on October 19, 1972, under the sponsorship of the Corrosion Resistant Metals Committee of TMS-IMD and the Corrosion and Oxidation Activity of the ASM.     

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H₂SO₄ solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10⁻⁵ mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.     

Formability of stainless steel

The forming behavior of austenitic stainless steels (types 201, 301, and 304) and ferritic stainless steels (types 437, 439, 444, and 468) was investigated. The tensile behavior and the forming-limit diagrams (FLDs) for these grades were determined. The ferritic alloys behave similarly to plain carbon steels and are relatively insensitive to small variations of strain rate and temperature. The formability of the austenitic alloys is influenced greatly by martensitic transformation during straining. The fraction of martensite transformed as a function of strain was found to be very sensitive to temperature, which, in turn, depends on the strain rate at typical testing rates (10⁻³ to 10⁻¹/s). At low rates (when the specimen remains near room temperature), the formability of the austenitic alloys is markedly improved by transformation strengthening. The enhancement of formability is largest on the biaxial side of the FLD, because the fraction martensite transformed was found to depend on the absolute thickness strain, which is maximized in the balanced biaxial strain state.     

Mechanical properties of a 29 Pct Cr-4 Pct Mo-2 Pct Ni ferritic stainless steel

The mechanical properties of a new ferritic stainless steel consisting essentially of 29 pct Cr, 4 pct Mo, 2 pct Ni (29-4-2) have been evaluated. The mechanical properties of the alloy are dependent on the thermomechanical processing and the final heat treatment conditionsi.e., both annealing temperature and cooling rate from the anneal. The alloy has excellent toughness, ductility and strength at room temperature when fast cooled from elevated temperatures. Slow cooling from elevated temperatures results in a degradation of impact resistance and an increase in strength. The alloy is subject to two major forms of embrittlement. One form results from the precipitation of intermetallic compounds in the temperature range 704°C (1300°F) to 954°C (1750°F) while the other results from the classical phenomenon called 475°C (885°F) embrittlement in the temperature range 399°C (750°F) to 510°C (950°F). Degradation of room temperature impact resistance occurs faster after the high temperature type of embrittlement and failure is characterized by an intergranular fracture mode. Embrittlement after exposure to 475°C (885°F) results in a slower degradation in toughness and results in failure by a transgranular cleavage mode. Impact resistance and tensile ductility are also decreased by exposure to 593°C (1100°F); however, to a lesser degree than 475°C (885°F) or 760°C (1400°F) exposure. The alloy deforms by slip or twinning depending on the metallurgical condition of the material. Deformation by twinning rather than slip is not manifested by a reduction in either toughness or ductility. Exposure to 482°C (900°F) promotes deformation by twinning whereas exposure to 760°C (1400°F) does not.     

The influence of substructure on the elevated and room temperature strength of a 26 Cr-1 Mo ferritic stainless steel

The high temperature flow stress behavior of an electron beam melted 26 Cr-1 Mo ferritic stainless steel was determined for large torsion strains (e ~ 15) over a temperature range from 400 °C (750 °F) to 1000 °C (1830 °F) and a strain rate range from 6 × 10^-3 to 6.3 s^-1. The room temperature compressive yield strength measured after torsional warm working was also investigated. It was found that the high temperature flow strength and the room temperature compressive yield strength were strong functions of the subgrain size. Strain softening was observed during warm working while the room temperature compressive yield strength was found to increase with prior torsional strain. The increase in the subgrain misorientation angle and, to a lesser extent, the subgrain shape change that occurs with increased warm working strain appear to be responsible for the strain dependence of the flow stress at both elevated and low temperatures. At the time this investigation was performed, all authors were affiliated with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305.     

Creep-rupture behavior of forged, thick section 9Cr-1Mo ferritic steel

Detailed investigations have been performed to examine the creep-rupture behavior of a 1000-mm diameter and 300-mm-thick tube plate forging of 9Cr-1Mo ferritic steel in quenched and tempered (Q + T), simulated postweld heat treatment (SPWHT), and thermally aged (TA) conditions. Creep tests were conducted over a wide stress range (50 to 275 MPa) at 793 and 873 K. The alloy exhibited well-defined primary, steady-state, and extended tertiary creep stages at all test conditions. At 793 K, no significant difference in the creep-rupture properties was noted between Q + T, SPWHT, and TA conditions. On the other hand, SPWHT specimens exhibited lower creep-rupture strength than that of Q + T specimens at 873 K. Applied stress (σ _a ) dependence of rupture life (t _r ) exhibited two-slope behavior. Both the Monkman-Grant (ε _s .t _r = C _MG) and modified Monkman-Grant (ε _s .t _r /ε _f = C _MMG) relationships were found to be valid for 9Cr-1Mo steel, where ε _s is the steady-state creep rate and ε _f is the strain to failure. The two-slope behavior was also reflected as two constants in the Monkman-Grant relationship (MGR) and modified Monkman-Grant relationship (MMGR) in the two stress regimes. Further, two creep damage tolerance factors (λ = 1/C _MMG) of 5 and 10 were also observed in the high and low stress regimes, respectively. The alloy exhibited high creep ductility, which was retained for longer rupture lives at low stresses, and the creep ductility increased with increase in test temperature. The failure mode remained trangranular under all test conditions. The extensive tertiary creep in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure. The creep-rupture strength of the forging was found to be lower than that of thin section bars and tubes.     

High- temperature fatigue property of 18CrNbTi ferritic stainless steel

In order to determine the influence of dwell time and atmosphere environment on high- temperature fatigue properties of 18CrNbTi ferritic stainless steel, the high- temperature fatigue behaviors of 18CrNbTi steel with 10s dwell time at maximum stress in argon and simulated automobile exhaust gas were investigated under axial tension- tension stress- controlled mode at 800??, and compared with its high- temperature fatigue behavior in air without dwell time. The fracture surfaces of fatigued specimens were observed with scanning electron microscope. The results show that: the fatigue limit of 18CrNbTi steel in air is 25MPa; the fatigue resistance and fatigue life of 18CrNbTi steel significantly reduce after introducing 10s dwell time; in comparison with those in argon, the fatigue resistance and fatigue life have a further decrease in the simulated automobile exhaust gas; the fatigue cracks both initiate and propagate in a transgranular mode under all testing conditions.     

Intermetallic phase formation in 25Cr-3Mo-4Ni ferritic stainless steel

Two ferritic stainless steels, containing nominally (by weight) 25Cr-3Mo-4Ni and either Nb and Al or Nb and Ti as stabilizing elements, have been solution treated and subjected to annealing between 600 and 1000 °C for times ranging between 1 and 600 minutes. Selective etching and light microscopy, X-ray diffraction of extracted residues, and transmission electron microscopy have been used to identify intermetallic phases and to construct temperature-time-precipitation diagrams for sigma, chi, and Laves phase formation. In the Nb-Al stabilized alloy, sigma phase forms prior to the Laves and chi phases but over a somewhat lower temperature range. In the Nb-Ti stabilized alloy, Laves phase forms prior to chi and sigma, and the temperature range of formation of all the phases is about the same. Laves phase forms at ferrite grain boundaries, but its growth is limited by sigma phase envelopment. Once established, sigma allotriomorphs grow with a coarse dendritic morphology. Austenite, some of which transforms to martensite, forms between the dendritic branches of sigma phase.     

Kinetics and hardening mechanism of the 475°C embrittlement in 18Cr-2Mo ferritic steels

The kinetics of the 475°C embrittlement of a Ti-stabilized 18 Cr-2 Mo ferritic steel have been studied by the measurement of hardness and tensile properties. The results are in accordance with the Cottrell-Bilby-Harper theory for precipitation on dislocations. The high increase in proof stress (～400 MN/m²) is suggested to depend largely on the precipitation of chromium-rich ?-ferrite in subgrain boundaries, in that this increases the subgrain contribution to the stress by a Hall-Petch type of mechanism.     

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo,and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M₂₃C₆ carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.     

铁素体不锈钢的冲压性

铁素体不锈钢的冲压性能劣于奥氏体不锈钢,所以极大地限制了它的广泛应用.介绍了铁素体不锈钢的优缺点,阐述了铁素体不锈钢冲压性的两个主要研究方向是深冲性能和抗起皱性.然后,介绍了影响铁素体不锈钢冲压性能的各个因素,包括微观组织、织构、合金成分、凝固组织、轧制工艺、退火制度、第二相粒子、晶粒尺寸等,各个影响因素对冲压性能的影响也进行了详细的分析.最后,对今后重点研究的方向提出了建议.     

Comprehensive microstructural characterization in modified 9Cr-1Mo ferritic steel by ultrasonic measurements

Modified 9Cr-1Mo ferritic steel (T91/P91) has been subjected to a series of heat treatments consisting of soaking for 5 minutes at the selected temperatures, starting from the α-phase region (1073 K) to the γ + δ-phase region (1623 K), followed by oil quenching. Hardness measurements, microstructural features, and grain-size measurements by the linear-intercept method have been used for correlating them with the ultrasonic parameters. Ultrasonic velocity and attenuation measurements, and spectral analysis of the first backwall echo have been used for characterization of the microstructures obtained by various heat treatments. As the soaking temperature increased above Ac ₁, the ultrasonic velocity decreased because of the increase in the volume fraction of martensite in the structure. There were sharp changes in the ultrasonic velocities corresponding to the two critical temperatures, Ac ₁ and Ac ₃. Ultrasonic longitudinal- and shear-wave velocities were found to be useful in identifying the Ac ₁ and Ac ₃ temperatures and for the determination of hardness in the intercritical region. However, ultrasonic attenuation and spectral analysis of the first backwall echo were found to be useful to characterize the variation in the prior-austenitic grain size and formation of δ ferrite above the Ac ₄ temperature. The scattering coefficients have been experimentally determined for various microstructures and compared with the theoretically calculated value of the scattering coefficients for iron reported in literature.     

The thermal activation energy for fatigue of Fe- 1 pct Cr- 0.5 pct Mo

The temperature dependence of fatigue crack propagation is considered in an Fe-1 pct Cr-0.5 pct Mo alloy steel. This material was tested at temperatures between 425 and 550 °C, a frequency of 1 Hz, and anR-ratio of 0.1. It is shown that the effect of temperature can be explained in terms of a thermal activation energy for fatigue. The magnitude of this activation energy is a function of ΔK and varies from more than 150 kJ/mole at 15 MPa√m to 30 kJ/mole above 30 MPa√m. The magnitude of these activation energies supports the idea that oxidation, and not creep, is the rate-controlling time-dependent process for the test conditions studied.     

Recrystallization of stabilized ferritic stainless steel sheet

An experimental study of the microstructural evolution attending recrystallization of cold-rolled Ti-stabilized ferritic stainless steel is presented. Particular attention is paid to the slow approach to full recrystallization. It is shown that this “sluggish” recrystallization can be attributed to the heterogeneity of microstructure and texture resulting from the processing of the material. In particular, it is shown that the presence of fine Ti(C,N) precipitates acts to significantly hinder the final approach to full recrystallization.     

Ductility of stabilized ferritic stainless steel welds

An investigation was made into the mechanism of ductility loss in low interstitial 18 Cr-2Mo ferritic stainless steel welds stabilized with Ti and Nb. It was found that stabilizing TiN or Nb(C,N) precipitates are dissolved during the welding process, resulting in a finer distribution of precipitates in the weld metal than in the base metal. Furthermore, the FATT was found to increase by more than 200°C, leading to decreased room temperature ductility. Such an increase in FATT may not be explained solely in terms of grain growth. Internal friction measurements indicate that no free nitrogen is present in the weld metal, yet wet chemical analysis reveals that the nitrogen is present in a soluble form. Kinetic arguments suggest that the stabilized nitrogen dissolved during welding tends to reprecipitate during solidification in the form of a chromium rich nitride phase.     

Interesting relationships for creep deformation and damage and their applicability for 9Cr-1Mo ferritic steel

The paper presents the validity of several interesting relationships examined for better understanding of creep behaviour of 9Cr-1Mo ferritic steel. Creep rate-rupture time relationships of Monkman-Grant type have been found to be valid. Like stress dependence of creep rate and rupture life, both Monkman-Grant and modified Monkman-Grant relations (MGR and MMGR) exhibited distinct constant values of C _MG and C _MMG , respectively for low and high stress regimes. The validity of MGR and MMGR is a consequence of the creep deformation behaviour of 9Cr-1Mo ferritic steel obeying first order kinetics. On the basis of creep rate-rupture time relationships of Monkman-Grant type, several other relationships involving transient and tertiary creep parameters have been evolved and their applicability have been examined for the steel. Analogous to MGR and MMGR, a relationship involving transient creep parameters and the other involving tertiary creep parameters were found to be valid. Further, 9Cr-1Mo steel obeyed a recently introduced critical damage criterion interrelating time to reach Monkman-Grant ductility with rupture life, and the criterion depends only on creep damage tolerance factor. This unique relationship is evolved based on the seminal concept of time to reach Monkman-Grant ductility as the time at which the useful safe creep life is exhausted and damage attains a critical level. The important implications of this concept have been discussed.