Blue Brittleness Research Articles

熱間加工状態における極軟鋼の変形挙動

Equipment was constructed which enabled metalic materials to be quenched by hydrogen gas immediately after dynamic tension (\dotε=18 sec−1) at high temperatures. With this equipment the deformation behaviours of mild steel (C=0.036 wt%) were studied over the temperature range of 25°C to 1200°C. Measurements of mechanical properties and slip line observations revealed the presence of five temperature regions where mechanisms seemed to be different; (1) the low temperature deformation region between 25 and 300°C, (2) the blue brittleness region between 300 and 600°C, (3) the high temperature deformation region of the α-phase between 600°C and A1 point (723°C), (4) the α-γ mixed phase region between A1 and A3 point (\simeq895°C), and (5) the γ-phase region above A3 point.True stress vs strain curves in region (3) showed steady state flow in the range of high strain above 40∼50%. The size of subgrains which were formed in the original grains by deformation was nearly constant in the range of high strains and also the values of microhardness and coercive force on specimens which were hot deformed and then immediately quenched were nearly constant in this range. The recrystallized-grains did not appear during hot deformation. They indicate that deformation of α-iron in region (3) is controlled by dynamic recovery.The structural change due to dynamic recovery is considered to retard or prevent crack initiation, there by leading to increase in hot ductility. The remarkable decrease of ductility with increasing temperature in region (4) may be explained by the fact that most of deformation was concentrated in the region of the α-phase in which flow stress was lower and ductility is larger than that of the γ-phase and consequently non-uniform deformation occurred considerably.

On Fatigue Strength of Nodular Cast Iron at Elevated Temperatures

The rotating bending fatigue tests were perfomed on the specimens of nodular cast iron with unnotch, semicircular notch and 60°V-notch at room temperature, 150°C, 300°C, 400°C, 550°C, and 800°C with the frequency of 3600rpm. Their values of fatigue strength were obtained at 107 cycles.The results are as follows:(1) The fatigue strength of the unnotched specimen decreases with the rising test temperatures. At about 400°C, however, the fatigue strength takes the slightly high value due to the blue brittleness. Beyond the range of 500°C to 550°C, the fatigue strength decreases rapidly.(2) The fatigue strength of the notched specimens decreases with the rising test temperatures. At about 300°C to 400°C, however, the fatigue strength shows a near constant value. Beyond 400°C, the fatigue strength decreases rapidly.(3) The value of fatigue notch factor gradually increases with the elevating temperatures above room temperature and then increases rapidly beyond about 400°C to 500°C.

The nature of hardening of carbon steels deformed in the temperature interval of blue brittleness

The nature of hardening of carbon steels deformed in the temperature interval of blue brittleness

On the Yielding Behavior in Tension of Sintered Iron

Yielding behaviors of sintered iron compacts were investigated through the microscopic observations and the tensile tests at both room and high temperatures with different strain rates.Yield drop, named as pseudo yield point, in a stress-strain curve is observed in various iron compacts, pressed at 3-7 t/cm2 and sintered in hydrogen for 1 hr at 1200°C. This behavior, which differs from that of mild steel as to the existence of plastic strain before yield point, may be due to the yielding of matrix and subsequently to the stress relaxation by nucleation and propagation of the crack from residual pores. The pseudo yield point and tensile strength of sintered compacts may depend en the strain rate (10-5-10-2/sec) as in the case of mild steel.The strength increment due to strain aging and the serrated stress-strain curves known as blue brittleness are clearly observed in iron compacts sintered even in hydrogen. Furthermore, this phenomenon is remarkable in the high nitrogen iron compacts sintered in dissociated ammonia.

低炭素-鉄合金の変形応力ならびに組織におよぼす変形温度の影響

The relation between dislocation structures and tensile properties have been investigated in the temperature range from room temperature to 400°C using four kind of a-irons with different carbon concentration. The grain size dependence of the yield and flow stress have also been studied in the same temperature range.The specimens showed serrated flow curves with high flow stress and high work-hardening rate. The higher the solute carbon content was the higher the flow stress and work-hardening rate became, and the temperatures of serrated flow being observable moved to a slightly higher temperature by decreasing the carbon content of the specimens.The flow stresses of the specimens were analyzed in term of the Petch parameters k and σi. It was found that kf at the temperatures of blue brittleness region was identical with ky, but σ showed higher value than the value at room temperature. These high values of frictional stress well corresponded to high workhardening rates. The change of flow stress component, therefore, was almost related to the frictional stress (σi). In the temperature range of serrated flow, deformed structures showed high dislocation densities and very small cell structures. Rapid increase in flow stress and high work-hardening rates were closely related with high rate of dislocation-multiplication.

The mechanical properties and failure of mild steel in tension

1. The temperature range of blue brittleness is 100° wider in fine-grained mild steel than in coarsegrained, and extends approximately from +100 to +300°C. 2. With all three grain sizes investigated maximum ductility occurs in the temperature range from −100 to +20°C. Maximum permanent elongation occurs at −50°C, and maximum relative transverse contraction after failure occurs in the range from −100 to +50°C. In this range transverse contraction is slightly sensitive to temperature. 3. The methods of optical and electron fractography have revealed, even in tests at very low temperatures, traces of plastic deformation in the fracture, with patterns typical of slip and twinning. The appearance of cleavage tongues is evidently due to the intersection of a cleavage crack with deformation twins generated by a high-stress zone at the tip of a moving crack.

Investigation of blue brittleness by the internal friction method

Investigation of blue brittleness by the internal friction method

The effect of dynamic strain-ageing on the ductile fracture process in mild steel

A study has been made of the effect of dynamic strain-ageing on the initiation and propagation of ductile fracture in mild steel. Grain boundary carbide cracking has been found to contribute to void formation in the temperature range 77°K to 573°K. Using scanning electron microscopy it has been shown that this crack density increases with strain. The rate of increase of crack density with strain is greatest at 77°K but deformation under dynamic strain-ageing conditions also leads to an increased rate of crack initiation over that observed at room temperature. These results have been interpreted in terms of the effectiveness of slip lines as stress concentrators increasing at low temperatures and under conditions of dynamic strain-ageing. Examination of fracture surfaces has revealed no change in fracture mode associated with ‘blue brittleness’ although there is some indication that voids link up easily under dynamic strain-ageing conditions. It is suggested that the low ductilities observed in the ‘blue brittle’ region occur because of a change in the work hardening characteristics of the material leading to the onset of necking instability at an early stage in the stress-strain curve.

Elongation and reduction of carbon steels at blue brittleness temperatures

Elongation and reduction of carbon steels at blue brittleness temperatures

Warm Working of Metals and Alloys

The authors developed a warm working method of steels, i.e. the deformation in the blue brittleness temperature range. Firstly the strengthening mechanism of warm working was made clear, that is, the increase of dislocation density due to the dislocations locked by carbon and nitrogen atoms. Moreover, the Bauschinger effect in this temperature range decreases due to the same mechanism. Various deformation methods were newly investigated ; twisting, straightening, surface rolling and pressing of joint holes for railes. In warm twisting and straightening, high strength and relatively ductile materials were obtained, due to the decrease of Bauschinger effect and the increase of work hardening. In warm surface rolling and pressing, high fatigue strength materials are obtained due to the increase of compressional residual stress and work hardening. The authors applied the warm working to non-ferrous metals such as Al-alloys and Cu-alloys. Combining the deformation with precipitation hardening and low temperature annealing hardening, high electric conductivity, high strength and excellent machinability were obtained by warm working.

溶接熱サイクル過程において塑性予歪をうけた鋼の室温における延性

This report discusses mechanical properties, in particular ductility, at room temperature of steels subjected to prestrain during weld thermal cycles. Three kinds of steels, a plain carbon steel (SM-41A), 50 kg/mm2 high strength steel (SM 50) and a heat-treated high strength steel (HY 80) were used for experiments. (See Table 1.) Thermal cycle simulating weld heat-affected zone was applied to a round bar specimen shown in Fig. 1 and plastic strain was given at a temperature during cooling. Fig. 2 shows schematic explanation of the test procedure. High frequency induction heating was used to obtain a synthetic weld thermal cycle. Plastic strain was given for about 4 seconds at a temperature during cooling through the tensile load produced by compressed air. Tension test was done at room temperature on the specimensubjected to the thermal cycle and prestraining. Attention was paid to general elongation or strain at the maximum load in addition to usual mechanical properties such as reduction in area, yield stress, ultimate tensile strength and fracture stress.The mechanical properties at room temperature change with cooling conditions, prestraining temperature and magnitude of prestrain. Prestrainig at 400°C to room temperature, in particular in the temperature range of blue brittleness, increases flow stress and decreasees ductility at room temperature (See Figs. 10 and 11.) Serious effect was the decrease in general elongation at room temperature. (See Figs. 12 and 15.) General elongation of SM41A steel and HY80 steel decreased respectivity from 24% and 15% as received to 15% and 9.5% after thermal cycle and to only 8% and 1% after thermal cycle combined with 5% prestrain at 200-300°C during cooling. The general elongation decreased with the increase in ratio of yield stress to ultimate tensile strength. (See Fig. 24).

Brittleness of steel U7 with different structures

1. The temperature range of cold brittleness depends on the microstructure of the steel. 2. Of the high-strength structures, bainite has the best resistance to brittle fracture (isothermal quenching at 300°C); at the same static strength as bainite, temper troostite has a lower resistance to brittle fracture (tempered at 380°C after oil quenching). 3. The blue brittleness temperature range of steel U7 depends less on the structure. The microstructure affects the temperature at whichan begins to decrease (Tlowblue) but has almost no effect on Tupblue.

Influence of deformation rate on the blue brittleness temperature and dislocation density of carbon steel

The reduction of the plasticity of carbon steel under conditions of static and dynamic elongation obeys the fundamental kinetic law of strain aging and consequently is caused by the diffusion of impurity atoms (C and N2) to dislocations and the resulting pinning of dislocations by formation of Cottrell atmospheres. The dislocation density for dynamic loading in the temperature range of reduced plasticity proves to be one to one and one-half orders higher than for static loading.

Compressive Strength of Mild Steel at High Strain Rate and at High Temperature

This paper deals with the dynamic behaviour of 0.03% C steel at high temperature (up to 700°C) within the range of strain rates of 10∼103/sec. The effects of temperature and strain rate on the compressive strength were clarified experimentally and the particular behaviour at the temperature near blue brittleness was explained using the Cottrell's theory. The dynamic stress to cause a certain amount of strain rises with the increase of strain rate at room temperature, but the stress is scarcely affected by the strain rate at 200∼600°C, and then the stress increases again with the strain rate at 700°C. The blue brittleness occurs at higher temperature with the increase of strain rate. For the static test and the dynamic tests of 102/sec, 3×102/sec and 103/sec of strain rate, the blue brittleness temperature is about 200, 550, 600 and 700°C respectively.

軟鋼の引張性質におよぼす変形速度の影響

The effect of the strain rate on the blue brittleness temperature, lower yield stress and tensile strength of a killed steel containing 0.14%C was investigated under various strain rates: 1×10−3, 1.2×10−2, 1×10−1, 1.2 and 1.1×102 l/sec. Main results in the present paper are summarized as follows:(1) The blue brittleness temperature increases linearly with the logarithm of the strain rate in the range from 1×10−3 to about 1 l/sec. Namely, a ten-fold increase of the strain rate induces a rise of the brittleness temperature by 50°C. However, the increase of the temperature is steeper with the rate beyond 1 l/sec.(2) The yield stress and tensile strength have two strain rate sensitivities, respectively; i.e., smaller sensitivities below the rate of 10−1 l/sec and much larger ones beyond it. Both yield stress and tensile strength at 300°∼−78°C in the range of the rate larger than 10−1 l/sec, and yield stress at −78°C in the smaller range are described quite well as increasing linear functions with log (strain rate). The rate sensitivities for both yield and tensile strength increase with lowering of temperature. This phenomenon is attributed to the sensitivity for “friction stress” (resistance against the motion of free dislocations). It was also discussed that the yield stress was more sensitive to the strain rate than the tensile strength.(3) In the range of lower strain rates, strain ageing (blue brittleness) can give an account for the abnormal or negative strain rate sensitivities for yield and tensile strength at the temperatures higher than 200°C, but not for the absence or the slightness of sensitivity and slight one at room temperature and −78°C.

Fatigue Strength of Low Carbon Steels under the Variations of Stress and Temperature

Several fatigue tests were conducted on the low carbon steel at elevated temperatures. First of all, the fatigue strengths under rotating bending stress and the torsional fatigue stress were compared. Then the rotating bending fatigue tests were carried out with stepwise varying stress at 480°C, and the effect of the varying stress on the fatigue life was discussed. Finally, the torsional fatigue tests were carried out to see the effect of the variation of temperature on the fatigue strength.The mains conclusions from this study are as follows:(1) The fatigue strength of low carbon steel shows an approximately similar trend against the temperature ranging from room temperature to 500°C, irrespective of whether it is subjected to the rotating bending stress or to the torsional fatigue stress. But the temperature giving the peak of strength is somewhat different in each case.(2) When the low carbon steel is subjected to the stepwise varying stress at 480°C, Miner's rule is acceptable to hold as the first approximation. There is no significant difference in strengths obtained in the tests with increasing stress amplitude and with decreasing stress amplitude.(3) Miner's rule also holds in respect to temperature, when it varies cyclically between the two temperatures, in the range higher than 420°C, during the fatigue test. However, when the lower temperature approaches the temperature of blue brittleness, a large deviation from Miner's rule may occur, and a prolonged fatigue life will be obtained perhaps due to the strain aging.

Properties of ShKh15 steel remelted in vacuum

1. Remelting in vacuum (10−3–10−4 mm Hg) practically eliminates nonmetallic inclusions and gases from the steel. This decontamination decreases the susceptibility of the steel to blue brittleness and increases its wearability. 2. Remelting in vacuum decreases the amount of residual austenite and the tetragonality of martensite after heat treatment. This would improve the stability of the dimensions of bearings.

鋼の高温における引張諸性質の変形速度依存性

This paper describes of the influences of deformation rate of wide range on the tensile impact properties of several steels by using a high-speed impact-tensile testing machine with a large rotary disk. The deformation rate was varied from static region to 80m/s, and the testing temperature from room temperature to 800°C. The results obtained were summarized as follows:(1) Blue brittleness was observed clearly in mild steel and two alloy steels, and temperature range of blue brittleness shifted to higher temperature with increasing rate of deformation. This shift was conspiquous up to a speed of deformation of 10m/s, but was decreased beyond this speed. Moreover, at the speed of 40m/s the blue brittleness temperature was decreased on the contrary.(2) The similar dependence on deformation rate was also made clear, with regard to the brittleness of 18-8 stainless steel which was due to the effect similar to that causing blue brittleness as well as the brittleness of pure copper at elevated temperature.(3) At higher temperature over 600°C, the critical impact velocity was not observed so distinctly as obtained at room temperature. Tensile strength was lowered with a larger scattering as the deformation rate was increased, while, correspondingly, the value of elongation or reduction of area showed tendency to be rather larger. This phenomenon, named by the authors as “high-rate deformation softening”, was discussed in relation to the loadtime curves observed experimentally.

Structure of ferrite grain boundaries

It is difficult to explain, on the basis of the usual theory of the structure of solid solutions, the effect of foreign atoms on the solid solution matrix when~ ~or example, one atom of the dissolved substance is surrounded by hundreds or even thousands of solvent atoms. The reason for this effect becomes clearer when some refinements are introduced into the theory of the structure of solid solutions. It is known that there are considerable lattice distortions at grain boundaries, which are therefore sites of higher than average free energy. The special physical state of layers of atoms at the surfaces of grains is due to their asymmetrical position relative to the surrounding atoms. Gibbs was the first to establish a thermodynamic relation between surface tension and the change in concentration of the solution close to the surface. Qualitative and quantitative checks on the Gibbs equation, applied to liquid solutions, have given completely satisfactory results [i]. Gibbs ideas have been further developed by F~onobeevskii and others~who have shown that local changes in free energies of systems caused by tensile or compressive stresses lead to a redistribution of atoms within the solid solution. By developing and generalizing these ideas we conclude that the presence of any defects or lattice imperfections inside a real crystal (grain and block boundaries, inclusions, vacancies, distortions etc.) create the conditions for a redistribution of dissolved elements. This leads to the formation of heterogeneous structures, and under certain conditions, to heterogeneous equilibria: it should be emphasized that diffusion processes, leading to the formation of heterogeneous structures, proceed according to the laws of thermodynamics. From this viewpoint, the fact that under definite conditions a homogeneous solid solution, in the stable state, may become heterogeneous, should cause no surprise. The composition and state of the boundary layers exert a substantial effect on the properties of alloys and on their behavior at low and high temperatures. It has been found that in many cases atoms at grain boundaries are many times as mobile as those inside grains. This is of enormous importance, since diffusional processes at grain boundaries are responsible for aging, temper brittleness, blue brittleness and probably affect the mechanisms of phase transformations and recrystallization.

Slow strain-hardening of ingot iron

Results of experiments on the strain-hardening characteristics of Armco ingot iron during slow plastic deformation under tension (strain rate: 10 −4 sec 1̄) are presented. The observed anomalous effects are discussed. It is shown that the serrated stress—strain curves often observed with this type of material are a consequence of the anomalous velocity dependence of strain-hardening. In this connection a modification of the Nabarro theory of blue brittleness is tentatively proposed.