Degree Of Cold Work Research Articles

Effect of cold work and aging on mechanical properties of a copper bearing microalloyed HSLA-100 (GPT) steel

Investigations have been carried out on the effect of cold work and subsequent aging on mechanical properties of a Cu-bearing HSLA-100 steel microalloyed with Nb and Ti. Aging at 400°C after various degrees of cold work (25–70 pct) exhibits multiple hardness peaks. The treatments cause significant improvement in hardness and tensile strength, but at the cost of impact strength. Cold work also causes deterioration in ductility, which again improves on subsequent aging. The C70A treatment involving 70 pct deformation exhibits maximum response to age hardening giving a hardness of 465 VHN and a UTS of 1344 MPa, but with low values of ductility (5 pct) and impact energy (24 J). C50A treatment involving 50 pct cold work and aging results in an optimum combination of mechanical properties. This treatment in the second hardness peak stage yields a hardness of 373 VHN, UTS of 1186 MPa together with a ductility value of 11 pct and impact energy of 109 J. Scanning electron microscopic studies of fracture surfaces reveal that the impact fracture occurs by formation of dimples and nucleation and growth of voids and cracks. Fracture in tensile specimens is caused by formation of voids and cracks at high density striations. Formation of voids and cracks is also assisted by the presence of precipitated carbide particles.

Some aspects of corrosion and film formation of austenitic stainless steel type 316LN using electrochemical impedance spectroscopy (EIS)

Corrosion and passive film characteristics of 316LN stainless steel with different degrees of cold work (0–25%) were studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques in deaerated acidic and alkaline (pH = 8) media. EIS measurements were conducted at open circuit potential (OCP) as well as after passivation in these media. Using the capacitance data from EIS measurements, film thickness was calculated. A definite correlation was observed between film thickness and corrosion rate after passivation. Analysis of the series capacitance and series resistance data from the EIS spectra showed that the films formed at OCP and after passivation were of semiconducting type. The nature of the semiconducting type of films was determined to be n-type using the ratios of anodic and cathodic transfer coefficients (αa/αc).

Development of fabrication technology for low activation vanadium alloys as fusion blanket structural materials

Vanadium alloys, candidate fusion reactor blanket materials, consist of intrinsic low activation elements for 14 MeV fusion neutrons (e.g. vanadium, chromium and titanium). From 500 to 1000 K, the tensile strength of vanadium alloys is independent of temperature, enabling the fusion blanket to be operated at least 100 K higher than a blanket made from ferritic steel. However, most of the data have come from laboratory-scale fabricated vanadium alloy, so a feasibility study on large scale production of vanadium alloy products is required. In the present study, high-purity vanadium alloy products (e.g. plates, wires and tubes) were fabricated from reference high-purity V-4Cr-4Ti ingots designated as NIFS–HEAT, with the use of technologies applicable to industrial scale fabrication. A critical issue for NIFS–HEAT large-scale melting was to reduce the levels of interstitial impurities (e.g. C, N and O) that are known to deteriorate mechanical properties before and after neutron irradiation. Impurity behaviour during working and annealing processes and its effect on mechanical properties were investigated. Mechanical properties of the products were significantly improved by the control of Ti-C, N and O precipitation induced during the processes. Good mechanical properties comparable to laboratory-scale alloys were obtained at above 96% in cold working degree.

Improvement of Vanadium Alloys by Precipitate Control for Structural Components of Fusion Reactors

Potential enhancement of mechanical properties of V-4Cr-4Ti by controlling precipitates of interstitial impurities (C, O and N) was investigated by means of various thermal and mechanical treatments. The increase in the cold working degree resulted in band structure of coarse Ti-CON precipitates having enhanced impact properties. Solid solution heat treatment followed by re-heating resulted in the formation of high density Ti-O-C precipitates enhancing high temperature strength. Combination of re-heating and cold rolling enhanced further the strength. Improvements in thermal and mechanical treatments are discussed for application of V-4Cr-4Ti to fusion structural materials.

Electromigration and Diffusion of Hydrogen in Palladium Defected by Cold Rolling

Influence of defects induced by cold rolling on hydrogen electromigration in palladium was investigated. It was found that the obtained results can not be explained by means of the occurrence of only one (interstitial) diffusion mechanism. However, accepting the assumption that in the process of hydrogen electromigration in defected palladium there appear two mechanisms of diffusion, that is interstitial (i) and interstitial-defective (i-d), that describe the experimentally obtained dependences well enough. In order to determine the influence of kinds of defects and their concentration on hydrogen electromigration, examination of changes in positron lifetimes and electric resistance in dependence on the degree of cold working on palladium were additionally conducted.

Intergranular stress corrosion cracking of copper in nitrite solutions

Semi-hard tubes of deoxidized high phosphorous copper with different levels of tangential residual stresses have been exposed to nitrite solutions in a laboratory heating circuit. After characterization of investigated materials influence of temperature, location of heating, concentration of solution, electrochemical potential, and atmosphere on stress corrosion cracking susceptibility of those copper tubes has been investigated. Threshold stress for crack initiation has been determined. Maximum duration of experiments was 1 month. Breakthrough time of tubes has been measured as criterion for susceptibility to SCC. To vary stress level in a wider range constant load tests on tubes with different degrees of cold working (annealed, semi-hard, hard) were done by mounting them in a steel frame. Stress corrosion cracks were always intergranular. A tenorite layer covered surface of cracked copper tubes. A reaction scheme for ammonia formation is presented. Necessary conditions for formation of stress corrosion are shown and critically discussed. Results show that intergranular cracking takes place at much lower stress levels below yield strength when compared to literature data on transgranular cracking above yield strength. For transgranular cracking cross slipping and cleavage formation as cracking mechanism is confirmed while for intergranular cracking chemical dissolution of grain boundaries plays a more important role.

The effect of cold work on the precipitation kinetics of AA6111 aluminum

In the automotive industry, the drive to meet Corporate Average Fuel Efficiency (CAFE) standards has resulted in the need to reduce vehicular weight in new automobile designs. One effective method has been to use lighter materials such as aluminum alloys in outer body panels. Heat treatable AA6111 is one of the materials being employed in outer body panels due to its unique combination of formability, paint bake strengthening and superior corrosion resistance characteristics. For a formed and painted automobile panel, the final mechanical properties involve a combination of strength components arising from cold work, strain aging, precipitation and recovery. The challenge is to find a method of quantifying each of these strength components. In this study, the effect of cold work on the precipitation kinetics of AA6111 has been evaluated, by means of tensile testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results show a considerable improvement in yield and tensile strength with increasing level of cold work. DSC showed acceleration in the precipitation kinetics of the alloy with increasing level of cold work. The Avrami-Johnson-Mehl approach, further developed by Mittemeijer and co workers was employed to determine the activation energies for dissolution of GPI zones and β″ formation. The activation energy of dissolution of GPI zones is observed to increase with increasing level of cold work while that for formation of β″ decreases with increasing level of cold work. As expected, TEM showed strong interaction of strengthening precipitates with dislocations. The density of dislocation tangles is shown to increase with increasing degree of cold work.

Effect of thermomechanical treatment on spray formed Cu – Ni – Si alloy

The heat treatment response of a spray formed Cu – 2.4Ni – 0.6Si (wt-%) alloy has been investigated. The spray formed alloy was given various thermomechanical treatments prior to isothermal aging. These treatments included solutionising and/or cold rolling with different reductions in original thickness. The variation in hardness and electrical conductivity of the alloys was measured as a function of the aging time. The results indicated the highest peak hardness value of ~250 kg mm-2 for the alloy aged after solution treatment and cold rolling to 40% reduction in thickness, compared with the maximum hardness of 220 kg mm-2 for specimens aged directly in the as spray formed condition. However, the electrical conductivity after aging was observed to be a maximum of 65%IACS (International Annealed Copper Standard) in specimens cold rolled to 80% reduction in thickness before aging. The aging response was observed to accelerate with the degree of cold working. Optical, scanning electron and transmission electron microscopy were used for microstructural characterisation of the materials. Precipitation of the second phase was observed to dominate in deformation bands. The alloy showed evidence of discontinuous precipitation, particularly when the alloys were cold rolled before aging. The onset of discontinuous precipitation led to a drastic deterioration in hardness of the alloys. The precipitation behaviour of the alloy is discussed in the light of microstructural characteristics associated with various processing conditions of the alloy.

Effect of epsilon martensite on the damping of high strength Fe–Cr–Mn alloys

Abstract The damping property of Fe–12 wt.% Cr–(6–30) wt.% Mn alloys has been investigated intending to develop high damping and high strength alloys. Particularly, the effects of the phases of alpha, gamma and epsilon martensite, which constitute the structure of the alloys Fe–12 wt.% Cr–(6–30) wt.% Mn alloys, on the damping capacity at room temperature has been investigated. Various fraction of these phases were formed depending on the Mn content and cold work degree. The combination of 22 wt.% Mn and 50% cold rolling formed the largest volume fraction of epsilon martensite phase and showed the highest damping capacity. The other phases of alpha and gamma had actually no effect on the damping capacity. Fifty percent cold worked Fe–12 wt.% Cr–22 wt.% Mn alloy indicated the largest tensile strength of more than 1000 MPa.

Influence of thermomechanical treatments on the production of rheocast slurries by partial melting

This work deals with the influence of thermomechanical treatments on the production, by partial melting, of rheocast structures of Al–3.35wt.%Cu alloy. Two types of treatments are performed: recrystallization and partial melting (RAP), in which the alloy is submitted to cold work at room temperature prior to heating to the semi-solid state; and overaging treatment (OAT), where the alloy is submitted to homogenisation and controlled precipitation of second phase followed by deformation at room temperature and heated to temperatures above Tsolidus. The influence of initial grain size, degree of cold work and overaging conditions in the resulting microstructure is investigated. Results are presented in terms of Cu distribution, solid fraction, globule diameter and shape factor. For both processes there are substantial reductions in the globular primary phase size compared to initial grain sizes, this reduction being more significant for rheocast produced via the OAT than the RAP route. The degree of previous deformation only influences the final globule diameter of the rheocast structure obtained by the OAT process, while initial grain size influences globule diameter in both processes. Changes in shape factor with process parameters show that different mechanisms are operative during rheocasting, such as recrystallization coarsening and coalescence; the predominance of one of them depending on the initial microstructure of the material. In general, OAT produces more rounded and smaller globules than RAP, mainly from higher deformed and coarser raw material, due to the predominance of recrystallization.

The Influence of Heat Treatment and Mechanical Work on the Lifetimes of Positrons in Fe, Pd and Pd–H

The influence of annealing temperature and stretching with external forces on lifetimes of positrons in Fe, Pd and Pd-H0.05 was investigated. It was found that in all of the examined cases the best fitting of the obtained spectrum of lifetimes of positrons to the theoretical model can be achieved with the assumption that the spectra consist of two components of lifetimes τ1 and τ2 and intensities I1 and I2, respectively. In rolled Fe and Pd, at a high degree of cold work, all the positrons annihilate from the trapping state in two types of defects of clearly different lifetimes. It was observed that dislocations in Fe become active in temperatures exceeding 923 K and in Pd—above 623 K. Deformation through one-axis stretching of Fe, Pd and PdH0.05 within the range of proportional elastic stresses, causes a decrease in lifetimes of positrons in defects of larger sizes. A hysteresis of component τ2 was observed at returning from the stressed state to the free one. Within the range of plastic deformations, changes in the long life component of the spectrum of positron lifetimes correlate with the changes in the length resulting from stretching with external forces.

Improvement of Intergranular Corrosion Resistance of Type 316 Stainless Steel by Laser Surface Melting

Laser surface melting (LSM) on sensitised austenitic stainless steel with various degrees of cold work (5, 10, 15, 20, and 25%) was carried out by using a continuous wave CO2 laser with a maximum power 450 W. The stainless steel was sensitised at 898 K for 10, 20, and 50 h durations. The ASTM A262 Practice A test was used to examine the microstructures of the specimens before and after LSM, while the single loop electrochemical potentiokinetic reactivation (SL-EPR) technique was used to quantify the degree of sensitisation (DOS). A trend is noted between the level of cold work in the stainless steel and the degree of desensitisation by LSM and it is inferred that laser surface melting can be effectively used as a method to improve the intergranular corrosion resistance of austenitic stainless steel.

Effects of prior cold work and sensitization heat treatment on chloride stress corrosion cracking in type 304 stainless steels

The effects of prior cold work (CW) and sensitization treatment on the stress corrosion cracking (SCC) behaviour of Type 304 stainless steels have been studied in chloride solutions using electrochemical tests and magnesium chloride tests with U-bend specimen. The results indicated that the SCC behaviour of the CW steel was essentially different from that of the solution-annealed steel. Intergranular stress corrosion cracking (IGSCC) of solution-annealed material changed into a mixed mode or dominant transgranular stress corrosion cracking (TGSCC) when the degree of CW was increased. The sensitization treatment enhanced IGSCC susceptibility by shortening failure time and accelerating crack initiation and propagation rates, but this effect was different for several other degrees of CW. These effects were revised taking into consideration the electrochemical and microstructural phenomena. The most dangerous degrees of deformation for different sensitization conditions for the development of IGSCC and TGSCC processes have been determined.

Elimination of Intergranular Corrosion Susceptibility of Cold-Worked and Sensitized AISI 316 SS by Laser Surface Melting

Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO2 laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M23C6 due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.

Investigation of residual stress relaxation under cyclic load

Compressive residual stresses induced by mechanical surface treatment such as shot peening, autofretage, hole expansion, laser shock peening, and low-plasticity burnishing can be highly beneficial to fatigue resistance. Cyclic relaxation of compressive residual stress, however, reduces the benefit. An analytical model is proposed for estimation of residual stress relaxation. Parameters considered by the model include the magnitude and distribution of the residual stress, the degree of cold working required, the applied alternating and mean stresses, and the number of applied loading cycles. An elasto–plastic finite element model was used to demonstrate the model.

The non-destructive estimation of the superficial mechanical properties of components in the INCONEL 600 alloy by X-ray diffraction peak width

INCONEL 600 alloy is largely used as nuclear power center components. The non-destructive determination of the superficial mechanical proprieties is indispensable for the qualification of the fabrication process and for the service duration estimation in aggressive environment. X-ray diffraction (XRD) technique is non-destructive and can be used to determine microscopic internal stresses from the broadening of the XRD peaks. The average magnitude of XRD peak broadening is related directly to the hardness, the elastic limit or degree of cold work of the material which can be used as a means of quantifying the surface mechanical properties. With an empirical relationship between the {311} XRD peak width and the conventional mechanical proprieties (hardness, elastic limit, hard work degree) for INCONEL 600 alloy, the superficial mechanical characteristics can be estimated from diffraction peak width for just finished parts or for maintenance pieces. Several TEM observations are also realized for microstructure study.

Nondestructive method of checking the quality of a semifinished product for tungsten wire

It is recommended that nondestructive methods be used to check the semifinished product in the production of tungsten wire. In particular, the elastic modulus of the bars should be determined. The clastic modulus of the semifinished product depends to a considerable extent on the presence of microcracks and the degree of cold-working of the metal: the degree of cold-working can be used to evaluate the processing characteristics of the semifinished product and predict the yield of useable products during subsequent deformation. During the initial stages of deformation of tungsten compacts, the wire semifinished product should either be placed in at least one of three groups based on elastic modulus or should be discarded as being unprofitable for further processing. Wire from grade-VA tungsten compacts with a cross section of 10.5 × 10.5 mm is most easily obtained from 7.2-mm-diam, bars with an elastic modulus in the range (410–510)·103 MPa. Such bars are best used as the semifinished products for making filament wire with a diameter 1.5 mm).

The Effect of Nitrogen on the Mechanical Behaviour of Cold-Worked Austenitic Stainless Steel Rod

The effect of nitrogen in solid solution on the work hardening rate of austenitic stainless steel is well known and forms the basis for production of many engineering components. An obvious application is in rod and wire production while in bioengineering, for example, stainless steel prostheses derive strength from the combined effects of nitrogen in solid solution and cold forging, while improved corrosion resistance is also gained from nitrogen in solid solution. In the present work a requirement was identified for stainless steel rod with improved surface hardness supported by a strengthened rod core. This geometry is easily utilised in both theoretical modelling and experimental investigation of a solid-state processing route. A method of fabrication was proposed comprising of nitriding of the surface of austenitic steel rod to a controlled depth and subsequent swaging or wire drawing to provide cold deformation. These two steps can be assimilated into a continuous industrial process to produce a composite rod structure with high surface hardness and a strong core. The evolution of microstructure and properties in a pilot-scale process are described together with the variation of properties of the nitrided rod with different degrees of cold work. The composite rods are tested for mechanical behaviour and a model is developed to describe the response to loading. The objective is to devise a predictive model which will allow process development without the need for detailed experimental work at every stage. The model combines a diffusion calculation to predict the penetration of nitrogen during nitriding with a mechanical model to give the response to loading.

Microstructural refinement of an As-cast Al-12.6 wt pct Si alloy by repeated thermomechanical treatment to produce a heavily deformable material

A certain degree of cold working is advantageous in developing a fine microstructure with minute silicon crystals for eutectic and/or hypereutectic Al-Si cast alloys. A novel process, repeated thermomechanical treatment (RTMT), was applied to an Al-12.6 wt pct Si cast alloy. The process involves multiple-pass cold working (less than a 20 pct reduction in section area) and heat treatment at 793 K for 3.6 ks. Cold-work annealing was repeated up to about an 80 pct reduction in section from the beginning. The RTMT material showed a refined microstructure with high ductility. Most silicon crystals were fragmented to only a few micrometers and were spheroidized. The RTMT material showed such marked plasticity that it could be wrought up to a 99 pct reduction in section at room temperature. The Cold-worked RTMT materials exhibited an excellent balance between tensile strength and elongation and a higher strain hardening than the cast material.

Growth of recrystallising grains in boron doped Ni76 Al24

Cold rolled boron doped Ni76 Al24 was partially recrystallised at 850, 875, and 900°C. The recrystallised volume fraction and the interfacial area between recrystallised and unrecrystallised parts were determined as functions of time. The analysis of results showed the growth rates of recrystallised grains to be dependent on temperature, but independent of time. The activation energy for the transformation was 168 kJ mol-1 and that for growth was 87 kJ mol-1. The activation energy for growth was much less than that for diffusion and was dependent on the degree of cold working. This has been attributed to the strong interaction of boron with the structural defects produced by cold working.