Roll-bonded nitrogenated 201 stainless steel

An alternative to high-pressure melting was used to produce sheets of high-nitrogen stainless steel. Hot isostatic pressure diffusion of nitrogen into thin sheets of 201 stainless steel resulted in enhanced nitrogen concentrations unobtainable in commercial grade stainless steels. Several different nitrogen enhancement techniques were attempted. Hot roll-bonding was then used to produce thick laminates. The resulting tensile strengths were dependent upon the nitrogen concentration of the laminates and were similar to the tensile strength obtained for high-pressure melted nitrogen stainless steels.     

On the Influence of Microstructure on the Fracture Behavior of Gamma-Based Titanium Aluminides. III. Effects of Interstitials and Microalloying with W

The results of a recent study of the effects of interstitial elements and microalloying with 0.2 at.% W on the tensile and fracture properties of Ti-48Al (compositions quoted in atomic % unless stated otherwise) base gamma alloys are presented in this paper. Lower interstitial oxygen levels are shown to promote higher levels of tensile ductility at the expense of yield/ultimate tensile strength and fracture toughness. Microalloying with W is also shown to result in microstructural instability and a concomitant degradation in tensile and fracture properties. Ductility and yield/ultimate tensile strengths in binary gamma alloys at room - and elevated-temperature are shown to exhibit a simple Hall-Petch dependence on the average grain/lamellar packet size. The observed Hall-Petch behavior is found to be independent of lamellar volume fraction and fracture mechanism at room- and elevated-temperature. The implications of the results are discussed for future alloy/process development.     

Alloying effects on the microstructure and phase stability of Fe–Cr–Mn steels

Austenitic Fe–Cr–Mn stainless steels interstitially alloyed with nitrogen have received considerable interest lately, due to their many property improvements over conventional Fe–Cr–Ni alloys. The addition of nitrogen to Fe–Cr–Mn stabilizes the fcc structure and increases the carbon solubility. The benefits of increased interstitial nitrogen and carbon content include: enhanced strength, hardness, and wear resistance. This study examines the effect of carbon, silicon, molybdenum, and nickel additions on the phase stability and tensile behavior of nitrogen-containing Fe–Cr–Mn alloys. Nitrogen and carbon concentrations exceeding 2.0 wt.% were added to the base Fe–18Cr–18Mn composition without the formation of nitride or carbide precipitates. Minor additions of molybdenum, silicon, and nickel did not affect nitrogen interstitial solubility, but did reduce carbon solubility resulting in the formation of M₂₃C₆ (M=Cr, Fe, Mo) carbides. Increasing the interstitial content increases the lattice distortion strain, which is directly correlated with an increase in yield stress.     

Free nitrogen effect on creep failure and creep crack growth of C-Mn steel

Different chemical compositions and heat-treatments of the C-Mn steels were investigated to characterise a susceptibility to cracking on cold bent tube. The C-Mn steels were categorised in terms of the content of free nitrogen. Ultimate tensile strengths, yield strengths and elongations were measured from the tensile testing at ambient, 250 and 360°C. Significant increase in ultimate tensile strength and decrease in elongation were observed in high free nitrogen material at 250°C the temperature at which free nitrogen is most active. From the uniaxial creep testing on high free nitrogen materials at 360°C increase in creep property, for example, lower minimum creep rate, was observed, however there was a decrease in creep ductility. This lower ductility of the high free nitrogen material has provided higher susceptibility to cracking in the creep crack growth tests at 360°C. Cracking in the high free nitrogen material A was approximately three times faster than the low free nitrogen material C at the same C* value. The creep cracking and rupture life in the high free nitrogen materials were more sensitive to the material condition, for example, pre-straining and/or heat treatment because of the role of the free nitrogen in the steels.     

High-Pressure Processing and Characterization of Fe–High-C/N Alloys

A new process has been developed that results in (i) enhanced nitrogen addition to ferritic iron–carbon alloys and (ii) melt-casting in a single operation. This new processing technique enables Fe–C alloys to retain high nitrogen interstitial concentrations and to reduce significantly, and possibly eliminate, carbide formation. In this study two commercial-grade, steel alloys were cast under elevated nitrogen pressures, resulting in solid solution (austenite, ferrite, and martensite) high-carbon and high-nitrogen iron alloys that were, within detection limits, carbide- and nitride-free. These alloys were subsequently thermally processed to transform part of the retained austenite to martensite. The microstructure and mechanical properties of the alloys were studied as a function of carbon and nitrogen composition and as a function of thermal processing. The retain high nitrogen concentrations in these cast and processed iron–carbon alloys resulted in a substantial improvement in compression strengths.     

Mechanical properties of In-based eutectic alloy solders used in Josephson packaging

InBiSn and InSn eutectic alloy solders are used in the packaging of the cryogenic Josephson processor. The ductile behaviour of these alloys is important to the application of joints which experience large thermal stresses. In order to characterize the mechanical behaviour at cryogenic temperatures, tensile and shear strengths of bulk solders were measured at and below room temperature. It is found that the ultimate tensile and shear strengths of bulk solders increase as temperature decreases. At low temperatures, the ultimate tensile strength of InBiSn alloy is about three times less than that of InSn alloy. The ductility of both alloys reduces at low temperatures. The fracture surfaces of both bulk solders and solder joints having an interface material (Pd and Au thin films) to the electrical contact pads (Nb thin film) were examined using SEM. Ductile fracture mode was observed for all the specimens down to liquid nitrogen temperature.     

High pressure nitrogen gas alloying of Fe-Cr-Ni alloys

The concentration of nitrogen in molten Fe-Cr-Ni alloys has been substantially increased by melting under nitrogen overpressures. Total nitrogen concentrations exceeding 26 at % were obtained when melted under 200 MPa. Nitrogen is present in solidified alloys as interstitial nitrogen and as metal nitrides. The nitrogen concentration depends upon the alloy composition; nickel decreases the nitrogen concentration, whereas chromium increases the concentration. When iron and Fe-Ni alloys were melted under high nitrogen pressures they produced iron nitrides and when Fe-Cr-Ni alloys were melted they produced ON dendrites and precipitates.     

Microstructures and mechanical properties of boride-dispersed precipitation-hardening stainless steels produced by RST

Two commercial precipitation-hardening (PH) stainless steels were modified with 2.64 to 2.86 wt% Ti and 1.2 to 1.3 wt% B via rapid solidification technology (RST) and powder metallurgy (PM). The resulting alloys exhibited improved tensile and yield strengths over their commercial PH stainless steel counterparts at room and elevated temperatures. Ductility improvements at elevated temperatures were also observed. The improved mechanical properties were due to extremely fine microstructures stabilized by a fine dispersion of boride phases.     

Vanadium high-strength low-alloy steels for low-temperature use

High-strength low-alloy (HSLA) steels having low impact transition temperature are possible substitutes for costlier 2 1/2% and 3 1/2% nickel steels. The effects of solid solution strengthening, grain size and precipitation in ferrite on the strength and toughness of low-carbon steels and the special advantages of vanadium as an alloying element in HSLA steels, are discussed. An investigation has been carried out with 1.5% manganese low-carbon steels containing vanadium in the range 0.12% to 0.29% and 0.013% to 0.017% nitrogen. Room temperature tensile and sub-zero temperature impact tests down to–100° C, and a metallographic study to determine the grain sizes and pearlite contents of the steels normalized at different temperatures, have been carried out. Calculations are made with empirical equations for yield and tensile strengths and the values obtained are compared with those experimentally observed. The solubility products of vanadium carbide and vanadium nitride are calculated and compared with available data to throw light on the mechanism of strengthening of the steels.     

Influence of hydrogen on mechanical properties of nitrogen supersaturated austenitic stainless steels

Abstract

Alloying austenitic stainless steels with nitrogen up to a concentration of 1 wt-% improves yield strength, tensile strength, and ductility. Further increase in the nitrogen concentration results in chromium nitride precipitation at the grain boundaries and a decrease in the ductility with a change in the fracture mode from ductile to intergranular. Hydrogen charging causes high reversible dilatation in the lattice and remarkable reduction in the ductility. The ductility losses caused by hydrogen are more pronounced at higher nitrogen concentrations and a change of the fracture mode from intergranular to transgranular is observed in steels with more than 1 wt-% nitrogen. Chromium nitride precipitates are shown to have an insignificant role in the hydrogen embrittlement. Hydrogen charging steels with nitrogen concentrations of below 1 wt-% enhances the strengthening effect of nitrogen but, at higher nitrogen concentrations, hydrogen is shown to be detrimental to the strength.     

Spinodal decomposition in Al/Zn alloys

The effect of spinodal decomposition on the mechanical behaviour of Al/Zn alloys was studied over the range of 30 to 60 wt % zinc. Two solution treatment temperatures, 365 and 435° C, were used; extensive ageing studies were carried out at 22 and 100° C, and limited tests were made on samples aged at other temperatures, 0, 55, and 200° C. The yield and tensile strengths were significantly increased by spinodal transformation, but ductility was seriously impaired. The tensile fracture was intergranular, with one exception, and was related to grain-boundary precipitation and a narrow denuded zone.Calculated yield strengths based on Cahn's analysis did not agree with those derived from the correct dislocation model. The wrong model did give fortuitous agreement. Spinodal hardening appears to offer a promising new hardening mechanism in aluminium alloys if the particular composition and treatment can be found to eliminate the serious lack of ductility.     

Mean stress effects in stress-life fatigue and the Walker equation

Mean stress effects in finite-life fatigue are studied for a number of sets of experimental data for steels, aluminium alloys and one titanium alloy. Specifically, the agreement with these data is examined for the Goodman, Morrow, Smith–Watson–Topper and Walker equations. The Goodman relationship is found to be highly inaccurate. Reasonable accuracy is provided by the Morrow and by the Smith–Watson–Topper equations. But the Morrow method should not be used for aluminium alloys unless the true fracture strength is employed, instead of the more usual use of the stress-life intercept constant. The Walker equation with its adjustable fitting parameter γ gives superior results. For steels, γ is found to correlate with the ultimate tensile strength, and a linear relationship permits γ to be estimated for cases where non-zero mean stress data are not available. Relatively high-strength aluminium alloys have γ≈ 0.5, which corresponds with the SWT method, but higher values of γ apply for relatively low-strength aluminium alloys. For both steels and aluminium alloys, there is a trend of decreasing γ with increasing strength, indicating an increasing sensitivity to mean stress.     

TRIP钢的动态拉伸变形行为研究

在气动式间接杆杆型冲击拉伸试验机上对二种TRIP钢的拉伸性能随应变率的变化进行了研究,对强度随应变率的变化以及断口形貌和孔洞与变形的关系进行了分析.研究结果表明:TRIP钢在所研究的应变率范围对应变率是敏感的,屈服强度和抗拉强度随应变率增加明显提高.SEM分析发现,TRIP钢断口体现为典型的延性断裂特征,而且残余奥氏体的变形诱发相变对孔洞形成位置的影响使得其具有较好的延性.     

Dualphasen-Stähle mit erhöhter Festigkeit und Verformbarkeit

Dual-Phase Steels with Improved Strength and Ductility. The term “dual-phase steel” refers to a new class of steels with a special ferritic-martensitic microstructure, which offers a superior combination of good formability, particularly concerning deep-drawing, and of high tensile strenght. This paper describes the alloys and the thermomechanical treatment to process a dual-phase microstructure. Comparing tensile tests of dual-phase structures and other ferritic-martensitic structures, factors are analysed which determine the typical mechanical properties of dual-phase structures. Their optimum mechanical properties of dual-phase structures. Their optimum mechanical properties, which are found to be better than those of conventional high-strength steels are due to the topology of the microstructure and to the addition of the unusual alloying element phosphorus. These new steels are of interest to all automobile manufactures because weight reduction of auto-body parts can be realized. The future perspectives for the application of dual-phase steels are discussed.     

LOW- AND HIGH-CYCLE FATIGUE PROPERTIES OF VARIOUS STEELS SPECIFIED IN JIS FOR MACHINE STRUCTURAL USE

Abstract —Low-cycle fatigue properties were investigated on four carbon steels and five low alloy steels specified in JIS (Japanese Industrial Standard) for machine structural use, which are the most commonly used in Japan. Several different heats from each of several representative manufacturers were sampled so as to represent the average fatigue characteristics of current materials. The cyclic deformation behaviour of material was denned by comparing the monotonie yield stress on the extrapolated tensile work hardening curve with the cyclic yield stress in the cyclic stress-strain curve determined by incremental step test. The normalized ferrite-pearlitic steels cyclically hardened, while the quench-tempered martensitic were cyclically stable or softened. The S–N relations derived from the strain-controlled low-cycle tests were compared with the results obtained by load-controlled high-cycle tests. The extrapolated S–N curves based on the cyclic stress-strain curve predicted the fatigue strength in the high-cycle range to be stronger for cyclic-hardening steels, but weaker for cyclic-softening steels. The predicted S–N curves for stable steels coincided with the high cycle test data. The fatigue limit had a proportional relationship with cyclic yield stress, slightly depending on the cyclic deformation behaviour. On the other hand, the cyclic yield stress was found to exhibit a very good linear correlation with the monotonie tensile strength, independent of cyclic deformation behaviour. This explains the empirical law that the fatigue limit is approximately proportional to the tensile strength.     

Contribution of nitrogen concentration to compressive elastic modulus of 18Cr–12Mn–xN austenitic stainless steels developed by powder metallurgy

It has been reported that nitrogen atoms affect the elastic constants of austenitic stainless steels differently. In this study, the effect of nitrogen content on the compressive Young’s modulus of nanostructured 18Cr–12Mn austenitic stainless steels is evaluated. The alloys are processed by mechanical alloying under a nitrogen gas atmosphere and subsequent sintering. The results show that by increasing the nitrogen concentration from 0.99 to 2.31 wt.%, the modulus progressively decreases. This is attributed to an increase in the volume of the unit cells and the weakening of the metallic bonds.     

Influence of nitrogen on tensile properties of 316LN SS

Abstract

The influence of nitrogen on tensile properties of 316L stainless steels has been studied for nitrogen levels of 0·07, 0·11, 0·14 and 0·22 wt-%. Tensile tests have been carried out at several temperatures in the range 300–1123 K. Nitrogen was found to be beneficial for tensile strength at all the test temperatures. Yield strength and ultimate tensile strength were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile ductility showed a non-monotonic variation with nitrogen content and test temperature. Equations have been developed to predict yield strength and ultimate tensile strength of 316L stainless steel as a function of nitrogen content and tensile test temperature.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

Superplasticity of Fine-Grained Fe–C Alloys Prepared by Ingot-And Powder-Processing Routes

Tensile elongation behavior of fine-grained Fe–C alloys has been investigated as a function of cementite volume fraction, degree of microstructural refinement, and the Zener-Hollomon parameter. The strain rate–stress relationships and creep strengths of Fe–C alloys with carbon contents from 1.3 to 5.25 wt. % C are found to be similar when grain size is similar. Superplastic ductility of ingot-processed alloys initially increases with carbon content but starts to decrease after 2.1% C. The increase of tensile ductility with carbon content below 2.1% C is attributed to a reduction in the case of dynamic grain growth associated with an increase in the number of fine cementite particles, whereas the decrease of tensile ductility above 2.1% C is due to an increase in the number of coarse cementite particles and an increase in the area of cementite/cementite grain boundaries. Superplastic ductility of Fe–C alloys with carbon contents higher than 2.1% C can be significantly enhanced when powder-processing routes are utilized instead of ingot-processing routes. Tensile elongation behavior of cementite-based alloys is revealed to be different from that of iron-based alloys when compared as a function of the Zener-Hollomon parameter.     

Mechanical stability and magnetic properties of austenite

Austenitic alloys have been produced by additional alloying in maraging steel grade 18 Ni at 2400 MPa. The concentration of Mo, Ni and Co was increased individually until the martensite start temperature M _s, was suppressed below ambient value. Charpy impact strength, tensile strength and magnetic properties were determined. The impact strength in the annealed condition ranged between 260 to 294 J. In alloys where martensitic transformation occurred following quenching in liquid nitrogen, the impact strength dropped appreciably and was found to be in the range 120–216 J. The tensile strengths of the austenite and martensite phases ranged between 680 to 890 and 1030 to 1100 MPa, respectively. It was observed that the austenite phase transformed to martensite in the region that under went plastic deformation during Charpy and tensile testing. The degree of transformation incorporated, varied as a function of composition. The magnetic properties of the austenite phases were typical of a very weak magnetic material. The coercive field and saturation magnetization values were in the range 1034–2387 Am^–1 and 1.6–2.9 T, respectively. In contrast to the general observation, the austenite phase containing high Co exhibited ferromagnetic behaviour. The coercive field and saturation magnetization of ferromagnetic austenite was 1034 Am^–1 and 11 T, respectively.