Effect of dynamic strain aging on mechanical properties of vanadium microalloyed steel

Abstract

In the present work, the dynamic strain aging behaviour in microalloyed steels has been examined using a C-Mn-Al-V-N steel, which occasionally exhibits low toughness in the subcritical heat affected zone (HAZ). This may be attributed to dynamic strain aging, whereby materials show lower ductility and higher yield strength owing to the interaction between mobile dislocations and diffusing solute atoms. The research has shown that the high temperature tensile behaviour of C-Mn-Al-V-N steel depends on the presence of clusters believed to be of vanadium and carbon atoms. The interaction between dislocations and clusters of vanadium and carbon atoms at 200-450°C changes the work hardening rate and contributes to dynamic strain aging as confirmed in the present work. These interactions may also decrease toughness in the subcritical HAZ and lead to the subcritical embrittlement observed in the C-Mn-Al-V-N microalloyed steel.     

Effect of finishing rolling temperature on fire resistance and dynamic strain aging behavior of a structural steel

The influence of finishing rolling temperature (FRT) on dynamic strain aging (DSA) behavior and high-temperature resistance of a fire resistant steel microalloyed with Mo and Nb was investigated by means of tensile tests performed at temperatures ranging from 25 to 600 °C and strain rates of 10⁻⁴ to 10⁻¹ s⁻¹. In these steels, DSA manifestations are less intense than those observed for carbon steels and they take place at higher temperatures. The precipitation behavior of the steels was also considered. Hardness of samples heat treated at 100–600 °C displayed a maximum at 400 °C. Samples treated at this temperature and tensile tested at 600 °C did not show a higher yield stress than the untreated specimens. Results obtained indicated that DSA in the fire resistant steel might have a contribution for its fire resistance. The empirical activation energies related to the appearance of serrations on the stress–strain curves and to the maxima on the variation of tensile strength with temperature suggested that the high-temperature strengthening associated with DSA in this steel is the dynamic interaction of interstitial-substitutional solute dipoles and dislocations. The steel with lower FRT is more susceptible to DSA because of its higher amount of carbon in solid solution and showed better results in terms of high-temperature resistance.     

Investigation on influence of dynamic strain ageing on fatigue crack growth behaviour of modified 9Cr–1Mo steel

Fatigue crack growth behaviour of modified 9Cr–1Mo steel is examined in the temperature range 300–823 K. An improvement in fatigue crack growth resistance is observed in the dynamic strain ageing regime. The activation energy for the process leading to this is estimated from the temperature-dependence of crack tip strain rate as 55–80 kJ/mole. This indicates that dynamic strain ageing due to interaction of dislocations with interstitial solute elements is responsible for the improved fatigue resistance in this range.     

Static strain ageing behaviour of dual phase steels

In this work an investigation was conducted into the cold deformation ageing susceptibility of a carbon steel and a microalloyed steel, both with dual phase micro-structure. Ageing experiments after different prestrains were carried out at temperatures ranging from 25 to 250 °C. It was found that yield strength (YS) and tensile strength (UTS) of the steels with different dual phase micro-structures exhibit maximum values at ageing temperature of 100 °C after different prestrains. It is assumed that the first rise is based on the formation of solute atom atmospheres around dislocations and the further strengthening in the second step is caused by the low-temperature carbide precipitation in ferrite. When the ageing temperature increased to 150, 200 or 250 °C, YS decreased due to tempering effect in martensite. It was also found that the ageing of the microalloyed steel occurred more slowly than that of the carbon steel. The slow occurrence of ageing was clearly observed at temperatures of 100, 150, 200 and 250 °C and was attributed to the chemical composition of the steels.     

Effect of ageing on the mechanical properties of directly quenched copper bearing microalloyed steels

The present work aims to study the ageing behaviour of directly quenched Cu-added microalloyed steels. Temperatures related to precipitation of Cu and recovery of dislocations retained in the microstructure after quenching of the steels from finish rolling temperature are determined by differential scanning calorimetric method. Ageing of the directly quenched steels has resulted in the reduction in hardness and strength with concomitant improvement of ductility. 1.5 wt% Cu-added Ti–B microalloyed steel has yielded the most attractive combination of strength and ductility. Presence of Ni in the 1.5 wt% Cu-added Ti–B microalloyed steel indicates sluggish kinetics of Cu precipitation. Ageing has generally deteriorated the impact toughness except for Ni containing Cu-added microalloyed steel above −25 °C temperature. Formation of recovered dislocation cells and fine ?-Cu precipitates during ageing have contributed to the microstructural softening and hardening, respectively.     

Influence of straining and ageing on the room temperature mechanical properties of dual phase steel

In this work, changes in mechanical properties in dual phase steel containing 20% martensite volume fractions were observed at various ageing temperatures. For example, ΔY (increase in yield strength due to strain ageing), YS and UTS exhibit maximum values at ageing temperature of 100 °C for the pre-strains of 2 and 4%. This is due to the formation of solute atom atmospheres around dislocation. When the ageing temperature increased to 200 °C, yield strength decreased due to overageing resulted from tempering that starts in martensite phase.     

Hot flow behavior of boron microalloyed steels

This research work studies the effect of boron contents on the hot flow behavior of boron microalloyed steels. For this purpose, uniaxial hot-compression tests were carried out in a low carbon steel microalloyed with four different amounts of boron over a wide range of temperatures (950, 1000, 1050 and 1100 °C) and constant true strain rates (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Experimental results revealed that both peak stress and peak strain tend to decrease as boron content increases, which indicates that boron additions have a solid solution softening effect. Likewise, the flow curves show a delaying effect on the kinetics of dynamic recrystallization (DRX) when increasing boron content. Deformed microstructures show a finer austenitic grain size in the steel with higher boron content (grain refinement effect). Results are discussed in terms of boron segregation towards austenitic grain boundaries during plastic deformation, which increases the movement of dislocations, enhances the grain boundary cohesion and modificates the grain boundary structure.     

Dissolving behaviour of second phase particles in Nb-Ti microalloyed steel

The dissolving behaviours of second phase particles in low carbon Nb-Ti microalloyed steel during isothermal holding at 1300 °C were investigated by carbon extraction replicas, TEM and EDX analysis. The experimental results indicate that there are two types of precipitates in as-forged steel: the coarsened Ti-rich precipitates originate from solidification, and the other finer Nb-rich particles attribute to strain-induced process. The strain-induced precipitates disappear after holding for 2 h at 1300 °C, while the coarsened particles from the solidification containing Nb-bearing (Nb,Ti)(C,N) still remain even holding for 48 h. With addition of Ti to the Nb-bearing microalloyed steel, the thermal stability of the carbonitrides would enhance remarkably. These results rationalize that formation and thermostability of precipitates are considerably influenced by interaction between Nb and Ti atoms.     

Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 °C, cooled at rates of 0.3, 1.2 and 3.0 °C s⁻¹ to temperatures in the range 750–1050 °C, and then strained to failure at initial strain rates of 1 × 10⁻⁴ or 1 × 10⁻³ s⁻¹. It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 °C s⁻¹ and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from 900 °C (near the Ae₃ temperature) to 1000 or 1050 °C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.     

Strain aging behaviour of Cu-containing microalloyed low carbon seamless pipeline steel

The effect of strain aging on the microstructure, including copper precipitation, and mechanical properties of an industrially produced Cu-containing microalloyed low carbon pipeline steel has been investigated. The precipitation of round bcc-Cu particles with diameters of ～7?nm has been clearly observed in the microstructure after strain aging. The strength of pipeline steel significantly increased while the elongation and impact toughness did not apparently decrease. Unlike welded pipe, few carbon atoms in supersaturated solid solution diffuse to the mobile dislocations, forming Cottrell atmospheres and producing strain aging phenomenon in seamless pipe. This difference is attributed to the different pipe making technique: thermo mechanically controlled processed for welded pipe and traditional heat-treatment for seamless pipe.     

The influence of dynamic strain aging on the low cycle fatigue behavior of near alpha titanium alloy IMI 834

Total strain controlled low cycle fatigue tests on IMI 834 have been conducted in air in the temperature range between 375 and 500 °C at a temperature interval of 25 °C at the nominal strain rate of 6.67 × 10⁻⁴ s⁻¹. The observed maximum peak stress ratio, minimum half-life plastic strain range and lower fatigue life at 425 °C indicates the occurrence of dynamic strain aging (DSA). Pronounced deformation bands, increased dislocation density and non-uniform dispersion of dislocations inside primary α grains observed by the study of transmission electron microscopy supports the occurrence of dynamic strain aging. Initial cyclic softening was attributed to shearing of Ti₃Al precipitates as revealed by TEM evidences.     

Effect of chemical composition, martensite volume fraction and tempering on tensile behaviour of dual phase steels

In this work an investigation was conducted into the tempering effects on mechanical properties of a carbon steel and a microalloyed steel, both with dual phase microstructure. The UTS and YS decreased and elongations increased with an increase of tempering temperature. However, the loss in UTS and YS at tempering temperatures of 100, 200, 300, 400, 500 and 600 °C was a minimum for microalloyed dual phase steel. This is thought to be due to tempering of the martensite and precipitation in the ferrite.     

Serrated flow behavior in a near alpha titanium alloy IMI 834

Serrated behavior of near alpha titanium alloy IMI 834 has been studied at elevated temperature from 400 °C to 475 °C. Serrations morphology was found as A type of locking serrations followed by B type serrations at 400 °C. E type of serrations has been observed at higher strains at 425 °C. B type and unlocking serrations of C_A type at 450 °C and again A and C_B type serrations were found at 475 °C. In strength parameters, anomalous tensile behavior was found in the variation of tensile strength and yield strength with test temperature in the temperature range between 400 °C and 475 °C. However, the variation of normalized flow stress showed regions I–III with test temperature. Regions I and III correspond to normal tensile behavior and region II corresponds to anomalous tensile behavior. Blue brittle temperature of IMI 834 was attributed at 450 °C by confirming minimum ductility of 8.2%. In present study, a different approach has been adopted to show the change in deformation behavior during serrated region called as abrupt change in strain path. Maximum irregularity in flow behavior has been observed at 450 °C and 475 °C. Room temperature fractographic features showed transgranular features whereas mixed ductile and cleavage fracture has been observed in the temperature range between 400 °C and 475 °C. However, reverse slope behavior has been observed in the plot of critical strain versus test temperature at 450 °C, which could be due to silicide precipitation. In the present study, interaction of dislocations with interstitial/substitutional solutes is responsible for dynamic strain aging in IMI 834.     

Influence of fusing temperature on microstructure, wear and corrosion resistance of induction melted bimetal of Co-based alloy and AISI 4140 steel

Two bimetals composed of a Co-based alloy and AISI 4140 steel were fabricated by induction fusing at 1200 °C and 1250 °C, respectively. Their microstructures were examined and their wear and corrosion resistances were investigated using a ball-on-disc system and immersion tests. The results show that the phases of the Co-based alloy of the bimetal, fused at 1200 °C, are inherited from its original powder and consist of a Co-rich phase, a Cr-rich phase and W₃CoB₃; however, the effect of iron dilution causes the formation of σ-CrFe and Co_0.72Fe_0.28. When fusing at 1250 °C, W₃CoB₃ decomposes to form Co₃B and increases the W content in the Co-rich phase thereby reducing the corrosion rate of the Co-rich phase and resulting in improved corrosion resistance of the bimetal. However, a higher fusing temperature causes a significant drop in hardness due to severe alloy dilution by iron diffusing from AISI 4140 steel and results in declined wear resistance. The associated wear behavior also changes from abrasion wear to oxidation wear.     

Determination of the critical conditions for the initiation of dynamic recrystallization in boron microalloyed steels

From the present research, the critical conditions associated with the onset of dynamic recrystallization (DRX) of hot deformed boron microalloyed steels were precisely determined based on changes in the strain hardening rate (θ) as a function of the flow stress. For this purpose, a low carbon steel microalloyed with four different amounts of boron (29, 49, 62 and 105 ppm) was deformed by uniaxial hot-compression tests at high temperature (950, 1000, 1050 and 1100 °C) and constant true strain rate (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Results indicate that the critical conditions for the initiation of dynamic recrystallization depend on the temperature and strain rate. In addition, both critical stress σ_c, and critical strain ?_c, were noticed to decrease as boron content increased. Such a behavior is attributed to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect. The critical ratios σ_c/σ_p and ?_c/?_p for all boron microalloyed steels remain fairly constant (≈0.82 and ≈0.53, respectively), such values are in agreement with those commonly reported for Al-killed, C-Mn, Nb, Nb-Ti, high carbon and stainless steels.     

Effect of dynamic strain ageing on the deformation behavior of Incoloy alloy MA956

Incoloy alloy MA956 is an oxide dispersion hardened ferritic stainless steel produced by powder metallurgy. It is used as a candidate material for the high temperature components of gas turbines. This material underwent dynamic strain ageing at 300–400°C and strain rate of 1.2 × 10⁻³ s⁻¹. The following features of dynamic strain aging were observed: serrated flow at 300 and 400°C, a peak in the ultimate tensile strength normalized by the elastic modules versus temperature curve at 400°C, a plateau in the 0.2% offset yield stress-temperature curve at 300–400°C, a peak in the deformation rate-temperature curve at 300°C and the elongation-temperature plot showed a minimum at 400°C associated with shear fracture and with a minimum in the reduction in area-temperature plot. These features of dynamic strain ageing were discussed in the view of the recent models of dynamic strain ageing. The effect of dynamic strain ageing on the deformation and fracture behavior of this material was discussed.     

Structure and wear resistance of the composite layers produced by glow discharge nitriding and PLD method on AISI 316L austenitic stainless steel

The rapid technical development enhances the demands on constructional materials in terms of their resistance to frictional wear, resistance to corrosion and erosion, high hardness, high tensile and fatigue strength. These demands can be satisfied by e.g. applying various surface engineering techniques that permit to modify the microstructure, phase and chemical composition of the surface layers of the treated parts. A prospective line of the development of surface engineering is the production of composite layers by combining various surface engineering methods. The paper presents the results of examinations of the phase composition and frictional wear resistance of the layers produced by hybrid processes, i.e. such that combined glow discharge assisted nitriding performed at 450 °C and 550 °C with a pulsed laser deposition of boron nitride coatings (PLD method). It has been shown that the boron nitride coatings formed on nitrided AISI 316L steel increase its frictional wear resistance.     

Sintering and characterization of YAG dispersed ferritic stainless steels

The present study investigates the effect of yttrium aluminium garnet (YAG) addition on the densification, mechanical, tribological and corrosion behaviour of ferritic (434L) stainless steels. The composites were sintered at both solid-state (1200 °C) and supersolidus (1400 °C) sintering conditions. Supersolidus sintering results in superior densification, hardness and corrosion resistance of both straight 434L stainless steel as well as YAG reinforced 434L stainless steels. The addition of YAG to 434L stainless steels at supersolidus sintered conditions improves the strength and wear resistance of 434L stainless steels without significantly degrading the corrosion performance.     

Influence of Carbides and Strain-induced Martensite on Wear Resistance of Austenitic Medium Manganese Steels

The abrasive wear behaviour of austeniticmedium manganese steels was studied under weakcorrosion-abrasive wear simulating the linerplate in wet metallic ore bail mill undernon-severe impact-loading working condition.Results show that the work-hardening mechanismand the wear resistance of high carbon austeniticmedium manganese steels differ from those ofmedium carbon austenitic medium manganese steel.Under non-severe impact and weakcorrosion-abrasive wear,the wear resistancesof high carbon and medium carbon austeniticmedium manganese steels are 50-90% and 20-40%higher than that of Hadfield steel respectively.     

Comparison of high temperature wear behaviour of plasma sprayed WC–Co coated and hard chromium plated AISI 304 austenitic stainless steel

The wear behaviour of plasma sprayed coating and hard chrome plating on AISI 304 austenitic stainless steel substrate is experimentally investigated in unlubricated conditions. Experiments were conducted at different temperatures (room temp, 100 °C, 200 °C and 300 °C) with 50 N load and 1 m/s sliding velocity. Wear tests were carried out by dry sliding contact of EN-24 medium carbon steel pin as counterpart on a pin-on-disc wear testing machine. In both coatings, specimens were characterised by hardness, microstructure, coating density and sliding wear resistance. Wear studies showed that the hard chromium coating exhibited improved tribological performance than that of the plasma sprayed WC–Co coating. X-ray diffraction analysis (XRD) of the coatings showed that the better wear resistance at high temperature has been attributed to the formation of a protective oxide layer at the surface during sliding. The wear mechanisms were investigated through scanning electron microscopy (SEM) and XRD. It was observed that the chromium coating provided higher hardness, good adhesion with the substrate and nearly five times the wear resistance than that obtained by uncoated AISI 304 austenitic stainless steel.