Occurrence Of Strain Aging Research Articles

On the fatigue and dwell-fatigue behavior of a low-density steel and the correlated microstructure origin of damage mechanism

The present work deals with revealing the fatigue and dwell-fatigue behavior and correlated damage mechanisms of Fe–Mn–Al–C lightweight steel. Surprisingly, alteration in loading mode from monotonic to cyclic induces reversible dislocation movement and facilitates the occurrence of dynamic strain aging. Additionally, applying dwell time by an acceleration of strain aging intensified stress asymmetry during dwell fatigue. The occurrence of strain aging has a bilateral effect on the crack initiation and growth. On one hand, strain aging stimulates twin formation and retards fatigue crack initiation, however, on the other hand, reduces hardening capacity, restricts the plastic deformation and facilitates crack propagation.

Secondary Hardening Behavior in Super Duplex Stainless Steels during LCF in Dynamic Strain Ageing Regime

Cyclic deformation behaviors in five modified duplex stainless steel S32705 grades have been studied at 20 °C, 200 °C, 250° and 350 °C. The influence of temperature and nitrogen concentration on the occurrence of the second hardening phenomenon, in the stress response curve was focused. An increase in nitrogen concentration can have a positive effect on dynamic strain ageing by increasing the first hardening and also the second hardening behavior during cyclic deformation. Furthermore, an increase in nitrogen concentration in the super duplex stainless steel increases the fatigue life in the strain ageing temperature range. The occurrence of strain ageing in duplex stainless steel has greatly changed dislocation structures. The formation of irreversible dislocation structures and stacking faults can contribute to the formation of second hardening in the stress response curve.

Effect of rolling speed on the occurrence of strain aging during and after warm rolling of a low-carbon steel

In this study, effect of rolling speed on strain aging phenomena in warm rolling of a carbon steel has been investigated. For this purpose, by using a mathematical model and predicting temperature and strain rate fields, the possibility of occurrence of dynamic strain aging during the warm rolling was first evaluated. In the next stage, warm-rolled samples were aged up to 11 months at room temperature for studying the kinetics of static strain aging, while mechanical tests as well as microstructural evolutions have been performed to determine the effect of strain aging on material behavior. The results indicate that dynamic strain aging may not occur for the employed rolling program; however, static strain aging takes place after warm rolling leading to changes in the mechanical behavior of the warm-deformed samples. In addition, by increasing rolling speed, the aging time required to achieve the maximum hardness increases.

An Assessment of Strain-Aging Effect on Weldable Offshore Structural Steel

Summary During the fabrication of offshore structures, the steel plate condition changes from the as-received normalized state to a strain-aged state. This paper assesses the effect of 5% prestrain and aging at 250C [482F] for 1 hour on the mechanical properties of normalized low-alloy BS 4360–50D steel plates obtained from two different suppliers. For both plates, strain aging led to an increase in yield strength, ultimate strength, Charpy and crack opening displacement (COD) transition temperatures, and fatigue resistance. The results of fracture toughness tests show that the degree of sensitivity to strain aging may depend on the type of test used; therefore, a test that more closely simulates offshore loading conditions, such as a COD test, is most appropriate. The paper also shows that the impact of strain aging on the integrity of offshore structures is minimized by the competing effects of lower fracture toughness and slower crack-growth rates. Introduction The selection of steel for offshore structures is governed by several factors, including strength, notch toughness, and fatigue strength. These properties are, in general, established on the basis of the as-received, normalized steel condition. Since offshore structures are generally fabricated by forming steel plates into tubular members and connecting these members by welding, a change in the properties would be expected as a result of strain aging. The strain occurs during forming, and aging is accelerated by the temperature rise during welding. Strain aging manifests itself mainly by an increase in yield strength on aging after straining. Strain aging may also change other properties, such as ultimate tensile strength, ductility, ductile/brittle transition temperature, fatigue strength, and electrical and magnetic properties. Nevertheless, the increase in yield stress on unloading and aging is the most universal indication for the occurrence of strain aging. Therefore, strain aging can be detected simply by the tensile test (Fig. 1). The increase in yield stress after aging, is often used as a measure of the strain-aging susceptibility. This change in properties is attributed to the pinning of the free dislocations that are formed during straining by the carbon and nitrogen atoms that are dissolved in ferrite. It is now universally accepted that strain aging occurs in the presence of interstitial elements such as carbon, nitrogen, and boron. Since virtually all offshore structural steels contain carbon and nitrogen and do not contain boron, it is the first two elements that are of greatest interest. It is therefore suggested that the effect of strain aging can be restricted by lowering the concentration of carbon and nitrogen atoms that are dissolved interstitially in the ferrite. The effect of free nitrogen can be reduced adding nitride-forming elements, such as aluminum. To bind the free carbon atoms, elements that form stable carbonitrides and carbides, such as niobium, vanadium, molybdenum, or titanium, are added. Available evidence suggests that a nitrogen content of 0.0001% produces detectable aging, while 0.001 to 0.002% produces severe aging. Increases above this level probably have little further effect. This suggests that the uncombined nitrogen must be reduced to about 0.0001% or less to eliminate strain aging. Wilson and Russell suggested that in moderately deformed steels, the segregation of about 0.0005 to 0.002 wt% carbon or nitrogen solute (depending on the exact dislocation density) is sufficient to complete dislocation locking. Since the solubility of nitrogen in ferrite is about 100 times that of carbon, it is always suggested that strain aging at low temperatures (less than 100C [212F]) is attributed to nitrogen. At high aging temperature >100C [212F]), the solubility of carbon is increased, and strain aging is produced even when nitrogen has been completely removed from solution. Aging at higher temperatures >350C [662F]) enables substitutional solutes, such as phosphorus, niobium, nickel, silicon, and titanium, to cause strain aging. Strain aging is known to affect toughness adversely. Baird recently has shown that the increase in impact transition temperature after 10% prestrain and aging at 250C [482F] for 30 minutes is not very sensitive to composition, prior heat treatment, or type of impact specimen. The overall increase (caused by both straining and aging) lies in the range of 40 to 70C [104 to 158F] for nearly all steels. Recognizing the potential effect of strain aging on fracture toughness of offshore structural steel, some technical standards restrict the use of Charpy V-notch impact test results from the parent plate to the conditions where the forming strain does not exceed 5% and the material temperature does not exceed 650C [1,202F]. Otherwise, Charpy tests shall be done with specimens machined from fabricated plates. The present study is aimed at establishing the effect of strain aging on one class of steel plates, BS 4360–50D, which is used extensively in the fabrication of offshore structures. JPT P. 141^

Relation between strain aging and Portevin-LeChatelier effect in Al-1at%Mg alloys

The relation between strain aging and Portevin-LeChatelier effect (dynamic strain aging) was studied for clarifying the mechanism of P-L effect.Experiments for strain aging were conducted on Al-1at.%Mg alloy which had been stretched by 2%at-60°-20°C, and then, aged at the deformation temperature under interrupted load.The strain aging process was divided into two stages. The 1st stage of less than 7sec. in aging time approximately conformed to the time law of t2/3 and the activation energy was found to be 0.21eV. The time law in the 2nd stage, more than 10sec. in aging time, was approximately to t1/3 and the value of activation energy was 0.45eV. The activation energy in the 2nd stage was interpreted as the migration energy of pairs of Mg atom-single vacancy. The activation energy of 0.60eV previcusly found in the process of P-L effect of this alloy was assumed to be the migration energy of Mg atom-single vacancy pairs, part of which move by cutting the binding between the Mg atom and the vacancy, becuase P-L effect occurred at higher temperature than that for strain aging.It was noted that the vacancies induced by strain played an important role in appearance of yield point due to strain aging form the results of experiments for eliminating the vacancies which had existed before the occurrence of strain aging. As the results of above studies, it may be concluded that P-L effect is the phenomenon in which strain aging occurs during plastic deformation.

Yield Phenomena in Copper-Arsenic Alloys

The occurrence of strain ageing in single crystals of arsenical copper (0.36% As) has been established by simultaneous determinations of stress-strain curves and changes in electrical resistivity. During yielding the electrical resistance increased more rapidly than normally. The change in resistance during yielding is explained in terms of Cottrell's model of an impurity atmosphere around dislocations.