Creep of Pb–2·5Sb–0·2Sn alloy at low stresses

Abstract

The creep of a Pb–2·5Sb–0·2Sn alloy has been studied at stresses up to 6·5 MN m^?2 in the temperature range 318–348 K (0·53–0·58T_m) using helical specimens. At 333 K, a transition in the stress exponent from ~1 to 3 occurred at ~3 MN m^?2. The observed good agreements below the transition stress, both for experimental dE/do and predictions for Coble diffusional creep of lead, and for measured activation energy for creep and the activation energy for grain boundary self-diffusion in lead, suggest that grain boundary diffusional creep is the dominant mechanism. at low stresses. The presence of antimony does not seem to affect the magnitude of dE/do appreciably, and the results suggest that the grain boundary self-diffusivity of lead is not influenced by the presence of segregated antimony on the grain boundaries. The diffusional creep occurred above a threshold stress of magnitude ~0·5 MN m^?2, and its temperature dependence was characterised by an activation energy of ~20 kJ mol^?1, similar to the value of 23 ± 7 kJ mol^?1 typical of pure metals in the temperature range investigated. The stress exponent of ~3 observed for the power law regime suggests control by viscous glide of dislocations constrained by dragging of solute atmospheres. Preliminary tests on sagging beam specimens of as-worked material at an applied stress of 2·5 MN m^?2 and a test temperature of 333 K has provided the first direct evidence that anisotropic grain shape affects Coble creep. The specimen with the longest grain dimension along the stress axis underwent slower creep than the specimen with the longest grain dimension perpendicular to the stress axis. This observation is in qualitative agreement with theoretical predictions.

MST/1139     

Effects of carbon content on mechanical properties of 5%Mn steels exhibiting transformation induced plasticity

Abstract

A series of highly ductile, high strength steels exhibiting transformation induced plasticity due to retained austenite was developed by varying the carbon content in the range 0·01–0·4 wt-% in 5 wt-%Mn based steel. For up to 0·l%C the mechanical properties are insensitive to cooling rate after intercritical heating, but afurther increase in carbon content causes a large sensitivity to the cooling rate, owing to carbide precipitation occurring during slow cooling. By suppressing this carbide precipitation with water quenching after the intercritical holding, an excellent combination of tensile strength (1580 MN m^?2) and uniform elongation (21%) was attained at 0·3%C in this series.

MST/1964     

Investigation of mechanical properties of advanced Al–Zn–Mg–Cu alloy

Abstract

The mechanical properties of the rapidly solidified 7000 series powder alloy CW 67 were investigated for various extrusion and heat treatment conditions. The principal aim of the work was to ascertain the optimum processing route for peak aged (T6) material. The highest proof stress in the T6 condition was found to be 572 MN m^?2 for material extruded at 325°C and aged for 13·5 h at 120°C after solutionising. The ductility of this material was found to be 13·5%. The fracture toughness was measured in two orientations and found to be approximately 21 MN m^?3/2 in the short transverse direction and 44 MN m^?3/2 in the longitudinal direction. Degassing and hot compaction was found to improve the fracture toughness of the material substantially.

MST/1504     

Diffusion welding of Ti/Ti and Ti/stainless steel rods under phase transformation in air

Abstract

Titanium (Ti/Ti) rods and titanium and stainless steel (Ti/SS) rods have been joined by diffusion welding under phase transformation in an air atmosphere. Specimens were heated to above the transformation temperature of titanium and cooled below that temperature one, two, or three times by alternately applying and breaking an electric current. The welding process was finished in a few minutes. In Ti/Ti welds no joint interface was observed inside the specimen. With a maximum temperature of 1200–1300 K in the thermal cycling, the bulging ratio was less than 30%, the yield stress was 280 MN m^?2 – exceeding 90% of that of the base metal, and the ultimate tensile strength was 380 MN m^?2, exceeding 80% of that of the base metal. In Ti/SS welds a joint interface was observed inside the specimen, but there was no gross void at the interface. With a maximum temperature of 1150–1200 K in the thermal cycling, the bulging ratio was less than 30%, the yield stress was 250 MN m^?2 – exceeding 90% of that of the stainless steel, and the ultimate tensile strength was 260 MN m^?2 – exceeding 60% of that of titanium.

MST/278     

Microstructure and mechanical properties of SiC whisker reinforced ZrO2–6 mol.-%Y2O3 composites

Abstract

Microstructure, mechanical properties, fracture behaviour, and toughening mechanisms of hot pressed SiC whisker (SiC_w)

reinforced ZrO₂–6 mol.-%Y₂O₃ composites were investigated via transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and mechanical testing. The experimental results show that there is a continuous increase in the Vickers hardness, elastic modulus, and fracture toughness of the composites with increasing SiC_w content, and an addition of 30 vol.-%SiC_w increases the fracture toughness from 3·42 MN m^?3/2 for the unreinforced matrix to 5·83 MN m^?3/2. The flexural strength is increased from 293 MN m^?2 for the unreinforced matrix to a maximum of 372 MN m^?2 by an addition of 10 vol.-%SiC_w, then it is significantly decreased by further increasing the SiC_w content. Observations via transmission electron microscopy show that no distinct second phase or intermediate layers form at the SiC_w/ZrO₂ interface. Diffusional separation of tetragonal phase from the cubic matrix occurred during cooling after hot pressing. Whisker bridging and crack deflection are the main toughening mechanisms, but whisker pull-out, crack branching, and refinement of the matrix particles also contribute to the improvement in the fracture toughness.

MST/1747     

Influence of silver on structure and properties of low-carbon steel

Abstract

Silver has only slight solubility in low–carbon steel at elevated temperatures (~ 0·08 wt-% at the solidus) and precipitates on cooling as fine particles (< 10 nm) in the interphase mode. When present in as-cast steel at an estimated volume fraction of ~ 0·0006, silver can cause precipitation strengthening of up to 50 MN m^?2. In as–rolled and in normalized steels, silver (0·02 wt-%) improves both strength and toughness mainly through grain refinement, with only a small influence from precipitation hardening. An important observation is the improvement to the heat affected zone toughness of high heat input welds which can result from small silver additions (0·02 wt-%) to C–Mn–AI–V steels. However, because of its relatively high cost, it remains to be demonstrated whether silver will impart sufficient benefit to steel properties to be commercially acceptable as an alloying addition.

MST/44     

Effects of shot peening on bending fatigue strength of spring steel SS 2090

Abstract

The influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m^?2 being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m^?2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40 μm in size, were analysed and found to be Al₂O₃. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.

MST/1392     

Stepped heat treatment for austempering of high manganese alloyed ductile iron

Abstract

A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C. The change in the microstructure and phase characteristics with time during the second austempering step are described. Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m^?2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 J.

MST/3119     

Joining of particulate silicon carbide reinforced 2124 aluminiumalloy by diffusion bonding

Abstract

The diffusion bonding of 2124 aluminium alloy reinforced with particulate silicon carbide has been investigated. The shear strength of the bonded composite decreased with increasing volume fraction of particulate V_p owing to the reduced area of metal to metal contact across the joint, from 60 MN m^?2 for V_p=0·25 to 35 MN m^?2 for V_p=0·40. The use of an interlayer of 2124 alloy increased the area of metal to metal contact to give improved bond strength. Microstructural characterisation of the bonds showed that the magnesium present in the 2124 alloy was a crucial factor in forming a joint, since it disrupted the surface oxide to allow metal interdiffusion.

MST/3189     

Creep rupture properties of nickel-base transition joints after long-term service

Abstract

Three superheater transition joints, between 2·25Cr–1 Mo and 316 stainless steel, welded with nickel–base weld metal, removed from service after 72337 h, have been examined using optical and scanning electron microscopy. In addition, microhardness measurements have been made and local chemical compositions have been analysed using the energy dispersive X-ray attachment on a scanning electron microscope. Temperature accelerated creep rupture tests have been carried out between 590 and 625°C at stresses of 31–62 MN m^?2 on cross–weld tensile specimens machined longitudinally from the walls of the joints. Detailed metallographic examinations showed the same failure mode as that found in long–term service failures. Therefore, the use of post-exposure temperature accelerated testing of uniaxial cross-weld specimens appears to be a viable method of assessing the remanent life of nickel-base transition joints operating at elevated temperatures. The applicability of various multiaxial stress rupture equations to transition joint failures is considered. The present rupture data are compared with previous data generated from initially as-welded specimens to provide upper and lower estimates of the long-term failure lives.

MST/403     

Improving tensile strength and bend ductility of titanium/AlSI 304L stainless steel friction welds

Abstract

The metallurgical and mechanical properties of friction welds between titanium and AISI 304L stainless steel were examined. Joint tensile strength increased when high friction pressure (>196 MN m^?2) and high upsetting pressure (294 MN m^?2) were used during welding. Although the surface roughness of the titanium substrate had no effect on joint strength, decreasing the surface roughness of the AISI 304L material did increase the tensile strength of completed joints. As welded dissimilar joints had poor bend test ductility and failed in the interface region. Detailed microscopy and X-ray diffraction analysis confirmed that the poor bend ductility was caused by a combination of high hardness of the titanium material immediately adjacent to the joint interface, the presence of unrelieved residual strain at the joint interface, and intermetallic phases formed during the welding operation. Detailed transmission electron microscopy and X-ray analysis confirmed that a thin layer rich in intermetallics was present in the as welded joints. (FeNiCr)Ti phases were formed during seizure formation and disruption; this provided the necessary conditions for anomalously high rates of diffusion of titanium in stainless steel, and of iron, chromium, and nickel in titanium. Low temperature post-weld heat treatment (PWHT), involving heating to 500–600°C followed by immediate air cooling, reduced intermetallic precipitation, promoted stress relaxation, and facilitated complete bonding across the whole joint interface. This treatment markedly improved bend ductility and had a negligible effect on joint tensile strength. High PWHT temperatures (≥900°C) and long holding times at temperature markedly reduced joint tensile strength and bend ductility, owing to excessive formation of intermetallic phases at the joint interface.

MST/1521     

Improved low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel through modified heat treatments

Abstract

The low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel can be improved significantly by thefollowing treatments. Modified oil quenching (MOQ): interrupt quenching at 573 K just below the martensitic transformation temperature followed by short time tempering at 673 K (up-quenching) before oil quenching and subsequently 473 K tempering (after conventional 1133 K austenitisation). Modified austempering (MA): the same up-quenching treatment followed by austempering at 673 K and subsequently water cooling. Each modified treatment was compared with its corresponding conventional treatment. The MOQ treatment significantly improved the notched tensile strength of the steel with slightly increased 0·2%PS and UTS, owing to an increase infracture ductility over the temperature range 123–203 K and also improved the Charpy impact energy of the steel over the temperature range 203–373 K. As a result of the MA treatment, the 0·2%PS and UTS and the notched tensile strength were developed remarkably with little change of fracture ductility over the temperature range 123–293 K. This treatment also improved the Charpy impact energy of the steel over the temperature range 203–293 K. The beneficial effect of the modified heat treatments on the mechanical properties is briefly discussed in terms of a modified law of mixtures, fibre loading theory, and fracture profiles.

MST/1157     

Overheating behaviour of a grain-refined low-sulphur steel

Abstract

The effect of cooling from temperatures in the range 1100-1400°C at 2 K min^?1 and air cooling on the overheating behaviour of a low-alloy Ni–Cr–Mo steel (A 508 Class 2) containing ~ 10 ppm S at two levels of manganese, i.e. 0.01 and 0·2 wt-% is described. A third steel which contained 80 ppm S and 0·2 wt-%Mn was also investigated. The results show that by decreasing the levels of manganese and sulphur the tendency to overheating can be reduced. However, unless the levels of aluminium and nitrogen are also controlled in the high-purity steel, high-temperature treatments lead to an even greater decrease in toughness as a result of the precipitation of AIN on the prior austenite grain boundaries. By increasing the manganese content, which has been found to lower the solubility temperature of A IN, and/or by air cooling from the high preheat temperatures, any deleterious effects caused by the precipitation of AIN may be reduced. In the steel of high sulphur content, intergranular fracture was observed after heat treating at temperatures above 1250°C. Manganese sulphides were identified on the intergranular facets. A decrease in toughness for this steel occurred after cooling at 2 K min^?1 from the highest preheat temperatures; this decrease appears to be related to the rod-shaped morphology of the sulphides.

MST/137     

Effect of consolidation temperature on mechanical properties of rapidly solidified Al–7Mg–1 Zr alloy

Abstract

The mechanical properties achieved via the extrusion of non-degassed billets prepared from an inert gas atomised powder of nominal composition Al–7Mg–lZr are reported. The alloy was extruded over the temperature range 350–550°C and the tensile mechanical properties and plane strain fracture toughness were evaluated. It was found that the yield strength remained fairly constant over the entire temperature range, with only a small decrease in strength observed at the highest extrusion temperature. The strength could be related to microstructure using standard models for solid solution, dispersoid, and substructural strengthening mechanisms, and the last was found to make the greatest contribution. The sensitivity of strength to subgrain size was found to be nearly three times higher than that for pure Al. The optimum combination of strength and fracture toughness was obtained for extrusion at 500°C (yield strength 400 MN m^?2; T–L K_Iv 21 MN m^?3; elongation 20%). The poor values of K_lv obtained at other temperatures were attributed to coarse dispersoids (highest extrusion temperature), undeformed powder particles (lowest extrusion temperature), and inhomogeneous dispersoid distributions (intermediate temperatures). It is concluded that extrusion process control plays an important role in determining the mechanical properties of consolidated rapidly solidified powders. Considering the excellent ductility and toughness obtained, vacuum degassing before extrusion may not be essential in the processing of inert gas atomised powders of a non heat treatable composition.

MST/1721     

Solidification structures and mechanical properties of Zn–27Al alloy cast in metal moulds

Abstract

Rectangular and round cross-section test bars of Zn–Al alloy containing (wt-%) 27·67%Al; 2·30%Cu; 0·13%Mg; 0·04%Fe; <0·0025%Pb, Cd, and Sn; balance Zn were cast in metal moulds. The variables studied were: bar cross-section 3–25 mm thickness and 14·5–28 mm dia., pouring temperature 550–700°C, feeder location at bar shoulders or over full gauge length, and cast iron mould preheating temperature 100 or 200°C. The results show that the above variables can lead to variations of 0·14–1·72% bar porosity, 18–0·5% elongation, 428–305 MN m^?2 tensile strength, and 108–128 Brinell hardness. Metallographic examination leads to the conclusion that these properties are influenced by the volume, distribution, and morphology of shrinkage porosity and of eutectic constituent. The observed variations in the microstructure and their effects on the mechanical properties are related to the cooling and feeding conditions of the bars.

MST/549     

Low temperature mechanical properties of quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling

Abstract

The mechanical properties of a quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling (CRP steel) have been studied over the temperature range 77–293 K with the aim of developing a CRP steel for low temperature ultrahigh strength applications. The results obtained were compared with those of a conventional quenched and tempered 0·4C–Ni–Cr–Mo steel (CHT steel). The CRP process was found to improve greatly the strength, ductility, and fracture and impact toughness for tempers at and below 473 K, independent of test temperature, but there was some concomitant deterioration in the transverse properties. It is postulated that the fine subcell structure, introduced during the CRP, is mainly responsible for the improved mechanical properties. However, there is an abrupt reduction in fracture energy of fatigue precracked steels for tempers above 473 K, so above this temperature there is little difference in the properties of the CRP and CHT steels. This is attributed to fine carbide precipitation, which promotes shear localisation and dimple fracture. Despite this, it is demonstrated by the present work that the CRP steel is attractive for low temperature ultrahigh strength steel applications.

MST/734     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Susceptibility to hydrogen attack of a thick-section 3Cr–1 Mo–1 Ni pressure-vessel steel-role of cooling rate

Abstract

A study has been made of the susceptibility to hydrogen attack of a newly developed 3Cr–1 Mo–1Ni pressure-vessel steel, intended for use in coal-conversion vessels, following long-term exposure to high-pressure (14-17 MPa) gaseous hydrogen at 550 and 600°C; the results are compared to the behaviour of 2·25Cr–1Mo steel. To simulate the condition of both surface and mid-section locations of thick-section (400 mm) plate during commercial normalizing, oil quenched and slowly cooled (8 K min^?1) structures were examined after tempering at 650 and 700°C, with respect to their strength, ductility, and impact toughness properties. Compared to unexposed samples, structures exposed to hydrogen were observed to show some softening (up to 20% reduction in yield stress) and ‘embrittlement’ (up to 22% reduction in upper-shelf Charpy energies and increases in the transition temperatures by 65–100 K), although there were no visible signs of major microstructural damage. Moreover, the behaviour of the oil-quenched and slow-cooled structures was largely similar. In comparison to 2·25Cr–1 Mo steel, which can become highly susceptible to hydrogen damage in the very slowly cooled state, the present 3Cr–1Mo–1Ni steel was found to have far superior resistance to hydrogen attack. This was attributed primarily to two factors: namely, the increased hardenability, which resulted from the absence of pro-eutectoid ferrite in as-cooled microstructures, and the accelerated kinetics in the carbide precipitation sequence, which resulted in the more rapid replacement of M₃C by more stable, higher-alloyed carbides, such as M₂₃C₆.

MST/95     

Serrated yielding in Nb–V dual phase steel

Abstract

The characteristics of serrated yielding (the Portevin–Le Chatelier effect) in a Nb–V dual phase steel have been studied in the temperature range 85–210°C at strain rates between 1·2 × 10^?5 and 1·2 × 10^?2 s^?1. Serrated yielding was found to initiate only after a critical strain ?_c was reached. The strain between two successive serrations ??_s increases almost linearly with strain, while the stress drop ?σ_c increases with strain up to ?σ_max, then decreases. The exponent β in the mobile dislocation density–plastic strain relationship (ρ_m= ?^β) is 1·09 in the temperature range 85–140°C and 1·34 in the temperature range 140–210°C. The results also indicate that in the same temperature ranges there are two values of activation energy for type A serrations, i.e. 79 and 119 kJ mol^?1 respectively. The results are discussed in terms of substitutional–interstitial solute atom interaction and changes of concentration of interstitial atoms.

MST/934     

Effect of telmperature and strain rate on tensile behaviour of M250 maraging steel

Abstract

The effect of temperature and strain rate on the 0·2% yield strength, ultimate tensile strength, and percentage elongation of M250 maraging steel was investigated under uniaxial tensile conditions in the temperature range from 25 (room temperature) to 550°C and strain rate range 10^?4–10^?1 S^?l. Up to 400°C the steel shows essentially strain rate insensitive behaviour with a gradual decrease in the 0·2% yield strength and ultimate tensile strength. The elongation remains constant at all strain rates up to 300°C. Fractographic analysis indicates that the increasing strain rate induces strain constraint resulting in an increased dimple size. An elongated structure was observed at temperatures above 400°C. X-ray diffraction reveals the presence of reverted austenite in the specimens tested at 550°C.

MST/3263