Dimple Zone Research Articles

Effect of α‐Ferrite on Plasticity and Fracture Mechanisms of Dual‐Phase Ultrastrong Heterolamellar Steels

Lamellar heterostructure design is widely recognized as a potential method to improve strength and ductility simultaneously of high‐performance steel. However, the high mechanical contrast among soft/hard phase and inharmonic plastic deformation lead to complex fracture mechanisms and limit the improvement of plasticity. Herein, the plasticity and tensile fracture mechanisms of two types of δ‐ferrite and lath martensite heterolamellar steels with and without α‐ferrite are investigated. The sample without α‐ferrite shows a combination of cleavage in δ‐ferrite and dimple zones in the martensite. The microcracks initiate in the martensite early and the then the cleavage fracture occurs in the δ‐ferrite due to the stress concentration. In contrast, the sample with α‐ferrite has lower plasticity. Failure of the samples with α‐ferrite is govern by the microcracks initiating in the interface of martensite/α‐ferrite. Eliminating the impact of the effect of α‐ferrite, the total elongation increases from 8.9% to 11.6%, and the ultimate tensile strength (UTS) increases from 1567 to 1652 MPa. This study investigates the critical factor in plasticity control of dual‐phase heterostructure and promotes the development and application of lamellar structure.

A generalized thermal deviation factor to evaluate the comprehensive stress of tubes under non-uniform heating

The heaters working with supercritical fluids normally suffer from high temperature, high pressure and non-uniform heating, which would lead to thermal fragile and stress failure in practical engineering. However, efficient and accurate methods to predict the comprehensive stress in non-uniformly heating tubes are missing. Besides, to reduce the comprehensive stress, a criterion is required to provide optimization guidance of non-uniformly heating tubes. In present study, the thermal and mechanical performances of three typical non-uniformly heating tubes are numerically investigated using the FVM-FEM method. A generalized thermal deviation factor (GTDF) is then proposed. The results showed that the GTDF can accurately predict the ratio of comprehensive stress to yield strength and GTDF > 1 indicates the plastic deformation, which is much more convenient and efficient than the conventional structural analysis. Based on the GTDF, two methods of reducing heat flux non-uniformity and enhancing tube-inside heat transfer are proposed to reduce the comprehensive stress. The results show that the unilaterally elliptic dimpled tube reduces the maximum comprehensive stress by 30.8% compared with the smooth tube and also prevents the stress concentration in the dimple zone, which can be employed to reduce the comprehensive stress in various tubes under non-uniformly heating conditions.

Microstructure and mechanical properties of two-stage aged Al–Cu–Mg–Ag–Sm alloy

High-strength Al–Cu–Mg–Ag–Sm alloy was fabricated and subjected to single-stage aging and pre-aging (two-stage aging). Effect of pre-aging on microstructure and mechanical properties of the alloy was investigated. It is found that the alloy is mainly composed of α-Al, Al2Cu, Al2CuMg and AlCu4Sm. The number of plate-like Ω Al2Cu precipitates is comparable to that of rod-like S Al2CuMg precipitates in the single-stage aged alloy, whereas, in the two-stage aged alloy, it is much higher than that of S precipitates. Ω precipitates have a smaller plate thickness and distribute more uniformly in the two-stage aged alloy than in the single-stage aged alloy. Ultimate tensile strength (UTS) and yield strength of the two-stage aged alloy are 12% higher than those of the single-stage aged one, indicating a better aging hardening caused by the two-stage aging. The increased tensile properties mainly come from both stronger precipitation strengthening caused by more Ω precipitates in the two-stage aged alloy and stronger solution strengthening from Mg atoms. The fracture surfaces consist of both dimple zones composed of microscale dimples and platform zones composed of nanoscale dimples. The total area of dimple zones for single-stage aged alloy is much higher than that for two-stage aged alloy, which can be attributed to different numbers of Ω precipitates in the two alloys.

Microstructural and crystallographic study of hydrogen-assisted cracking in high strength PSB1080 steel

The microstructural and crystallographic study of hydrogen-assisted cracking in high strength PSB1080 steel was conducted. The results indicate that the mechanical properties of PSB1080 steel are seriously deteriorated in the presence of hydrogen, which is ascribed to the coupling effect of hydrogen-assisted cracking from the O–Al–Si–Ca inclusion and accelerated phase transformation from the austenite to the martensite due to hydrogen. The hydrogen uncharged sample exhibits dimple fracture pattern, whereas the fracture surface of hydrogen charged sample consists of three zones, i.e., quasi-cleavage zone, a mixed zone of quasi-cleavage and dimple as well as dimple zone. Crystallographic orientation analysis beneath the three zones demonstrates that the proportion of low angle grain boundary is the highest, followed by high angle grain boundary and then medium angle grain boundary, and the high Kernel Average Misorientation region facilitates hydrogen-assisted crack propagation. Additionally, the grains oriented with {001}//ND, {110}//ND, {123}//ND exhibit the high possibility of hydrogen-assisted cracking. This suggests that these oriented grain textures should be reduced to design the resistance-hydrogen embrittlement alloys.

Effect of thermal ageing on microstructure, tensile and impact properties of reduced activated ferritic-martensitic steel

Effects of thermal ageing on tensile and impact properties of India-specific Reduced Activated Ferritic-Martensitic (INRAFM) steel (9Cr-1.4W-0.06Ta-0.22V) have been studied by correlating with microstructure. The tempered martensitic steel received in the normalized and tempered (N&T) condition was thermally aged at 873K for 5000h. Thermal ageing was found to decrease hardness and tensile strength and the decrease was accompanied with increase in ductility. The steel displayed plateaus/humps in the temperature variation of yield stress and ultimate tensile strength and dip in ductility over the range 473–673K, which is considered due to the occurrence of dynamic strain ageing (DSA). The prevalence of DSA activity was found more on thermal ageing. Increase in ductile to brittle transition temperature (DBTT) and decrease in upper shelf energy (USE) of the steel were observed on thermal ageing. Detailed microscopic investigation of the steel revealed coarsening of martensitic sub-boundaries and M23C6 carbides along with the precipitation of Laves phase on thermal ageing. Coarsening of microstructural features on thermal ageing is considered to influence the reduction in strength and USE of the steel; whereas coarsening of M23C6 and precipitation of Laves phase is considered to cause the increase in DBTT. SEM fractographic study of the impact specimens revealed three distinct zones as shear lip, fibrous dimple and cleavage in an ordered away from the notch tip. With decrease in test temperature, the widths of the shear lip and fibrous dimple zones deceased whereas width of the cleavage zone increased progressively. Thermal ageing increased the widths of the shear lip and fibrous dimple zones and decreased the cleavage zone in the N&T steel at relatively higher temperatures but at lower test temperatures cleavage zone became more predominant. TEM study of the specimens extracted from the impact fractured samples at the vicinity of fracture surface, revealed substantial activity of dislocations manifested as dislocation debris in N&T steel even for impact testing at −80°C against individual dislocations for the aged steel, implying enhancement of DBTT on thermal ageing.

Heat transfer and friction factor performance in a pin fin wedge duct with different dimple arrangements

ABSTRACTIn this study, numerical simulations are conducted to investigate the effects of dimple positions on the endwall heat transfer and friction factor in a pin fin wedge duct. The dimple diameter is the same as the pin fin diameter, while the ratio between dimple depth and dimple diameter is 0.2. Three different dimple positions are investigated (i.e., directly upstream of the pin fins, in a staggered manner relative to the pin fins, or in line with the pin fins. The Reynolds number ranges from 10,000 to 50,000. Results for endwall Nusselt number, friction factor, and flow structure are included. For convenience of comparison, the pin fin wedge duct without dimples is studied as baseline. It is found that dimples can effectively enhance endwall heat transfer. Among the tested parameters, the dimple position in line with the pin fins provides the best heat transfer enhancement, with low friction factor penalty. However, the various dimple positions have distinct effects on the friction factor depending on the flow structure near the dimple zone. For the first position, the friction factor is markedly increased due to flow impingement, recirculation, and mixing, while for the second and third positions, the friction factor is changed slightly due to different flow behaviors.

Dynamic Deformation Response of G33 Steel under High Temperature

The high temperature deformation and fracture behavior of ultra-high strength G33 steel under high strain rate compression are investigated by means of a split Hopkinson p ressure bar. Impact tests are performed at strain rates of 1000/s and 2200/s and at temperatures ranging from 25°C to 700°C. The SEM and TEM techniques are also used to analyze the microstructure evolution of the adiabatic shear band (ASB) and fracture characteristics of the deformed specimens at high temperature. The experimental results indicate that the flow stress of G33 steel is significantly dependent on temperatures and strain rates. The flow stress of G33 steel increases with the increase of strain rates, but decreases with the increase of temperatures. The strain rate sensitivity is more pronounced at the low temperature of 25°C. In addition, G33 steel is more liable to fracture at high temperatures than at 25°C. Observations of microstructure show two well-developed symmetric parabolic adiabatic shear bands on the longitudinal cross-section of the cylindrical specimen deformed at the temperature of 700°C and at the strain rate of 2200/s. Within the ASB, the width of the fine equiaxed grain structure is about 7μm. The size of those equiaxed grains is approximately 100nm. The fracture analysis results indicate that the ASBs are the predominant deformation and the specimens fracture along adiabatic shear bands. The fracture surfaces of the deformed G33 steel specimens are characterized by two alternating zones: rough dimple zone and relatively smooth shear zone. Further observations reveal that smooth shear zones consist of severely sheared dimples.

Normalizing effect on fatigue crack propagation at the heat-affected zone of AISI 4140 steel shielded metal arc weldings

The fractography and mechanical behaviour of fatigue crack propagation in the heat-affected zone (HAZ) of AISI 4140 steel welded using the shielded metal arc process was analysed. Different austenitic grain size was obtained by normalizing performed at 1200 °C for 5 and 10 hours after welding. Three point bending fatigue tests on pre-cracked specimens along the HAZ revealed that coarse grains promoted an increase in fatigue crack growth rate, hence causing a reduction in both fracture toughness and critical crack length, and a transgranular brittle final fracture with an area fraction of dimple zones connecting cleavage facets. A fractographic analysis proved that as the normalizing time increased the crack length decreased. The increase in the river patterns on the fatigue crack propagation in zone II was also evidenced and final brittle fracture because of transgranular quasicleavage was observed. Larger grains induced a deterioration of the fatigue resistance of the HAZ.

In situ pressure measurements in dimpled elastohydrodynamic sliding contacts by Raman microspectroscopy

In situ pressure measurements within dimples formed in glass–steel point contacts under various sliding conditions were performed using Raman microspectroscopy. Experiments were conducted using a ball-on-disc type apparatus in which a circular contact is formed between a rotating glass disc and a rotating steel ball. Film thickness distributions were measured by duochromatic optical interferometry. Polyphenyl ether oil (5P4E) which has a high pressure–viscosity coefficient was used to produce a dimple in the contact area. Experimental results show that the pressure increases locally in the dimple zone and the pressure profile changes, accompanying the changes of the dimple location as a function of the slide-to-roll ratio. The maximum pressure is located downstream the maximum height of the dimple. The rheological response of the oil film under dimple occurrence conditions is discussed.

Relationship Between Temperature Distribution in EHL Film and Dimple Formation

This paper describes an experimental study on dimple formation under elastohydrodynamic lubrication (EHL) conditions. The oil film thickness between a ball surface and a glass disk was measured using optical interferometry, and the temperatures of both the surfaces and of the oil film averaged across it were measured using an infrared emission technique. It was found that the temperature profile across the oil film varies abruptly along the sliding direction, and the Couette flow decreases due to the viscosity wedge action as the oil flow is close to the dimple zone. The maximum temperature rise in the dimple zone sometimes reached over 400 K, thus, the phase transition of the oil from liquid to glass may not occur.

높은 미끄럼 비의 점 접촉 EHL 하에서 발생하는 딤플 영역의 온도 분포와 유막 두께 분포의 측정

In this paper, film thickness and temperature distribution are measured in EHL point contact at high slip ratios. Infrared temperature mapping with two band pass fillers. proposed by Ausherman (1976). is used to measure temperature distribution. And the optical interferornetric method with two filters (red and green filters) is used to measure film thickness. Result of experiment showed that temperature rising at film and ball surface occurred very dramatically in Dimple zone. As slip velocity, slip ratio and load increased, size of Dimple and temperature rising became more large In addition, Position and shape of Dimple we changed by slip ratios, and increasing of Dimple size decreased traction coefficient. In short, it is appointed that the Dimple phenomenon be developed by the effect of viscosity wedge.

Fracture mechanism and toughness of the welding heat-affected zone in structural steel under static and dynamic loading

Due to the influence of the welding thermal cycle, the toughness of structural steel generally degenerates. Recently, the intercritically reheated coarse-grained heat-affected zone (IC CG HAZ) was found to demonstrate the worst toughness in welded joint, which was associated with its fracture mechanism. In this article, two IC CG HAZs of a structural steel were prepared by welding thermal-cycle simulation techniques. For the two IC CG HAZs, the static and dynamic fracture toughness were evaluated; the fracture mechanism was also studied. Under both static and dynamic loading, cracks in the IC CG HAZ were found to initiate at the intersection of bainitic ferrite α /0 packets with different orientations, followed by propagation in cleavage. In some crack propagation regions, adjacent cleavage facets are connected by shear, thus producing dimple zones. Though the brittle fracture initiation mechanism remains unchanged, the cleavage facet size, the proportion of the dimple zones between facets, and the distance from the cracking initiation site to the crack tip vary with loading speed and welding conditions. These changes were found to be related to the variations caused by strain rate and welding conditions in fracture toughness for the IC CG HAZs.

Evaluation of dynamic fracture toughness of welding heat-affected zone of structural steel

Due to the influence of welding thermal cycle toughness of structural steel degenerates. Recently, the intercritically reheated coarse grained zone (IC CG HAZ) was found display the worst toughness in welded joint, which was associated with its fracture mechanism. In this work, two IC CG HAZs of a structural steel were prepared by welding thermal-cycle simulation techniques. For the two IC CG HAZs the dynamic fracture toughness and the fracture mechanism were studied, and the correlation between fracture toughness and fracture mechanism was also discussed. Under both static and rapidly loading, cracks in the IC CG HAZ were found to initiate at the intersection of αB0 packets with different orientations, followed by propagating in cleavage. In some places of crack propagation, adjacent cleavage facets are connected by shear, producing dimple zones. Though the brittle fracture initiation mechanism remains unchanged, the cleavage facet size and the proportion of the dimple zones between facets vary with loading speed. These changes are shown being associated with the effect of strain rate on fracture toughness. In this work, the fracture behavior of IC CG HAZ was also compared with that of base metal and prestrained steel.

Fracture Mechanism of Inconel 718 at 4K.

An elastic plastic fracture toughness test was carried out at 4K and 290K to clarify the fracture mechanism of Inconel 718. It was revealed, at 4K, that pop-in behavior was dominant until final fracture and that this material was quite brittle. The elastic plastic fracture toughness corresponds well with plane strain fracture toughness. The tearing modulus is comparatively low if the energy level just before final pop-in is employed to obtain the J-R curve. On the other hand, at 290K, this material is ductile although the fracture toughness is less than that at 4K. The material strength contributes to the difference in fracture toughness between 4K and 290K. Microscopic examination shows that, at 4K, a small dimple zone is often located at the initial crack front although a cleavage facet or twinning is dominant at the fracture surface. However, at 290K, only a dimple zone is found on the fracture surface.

Sulphur-induced ductility-dip cracking in low carbon steel weld metal

Abstract Two types of hot cracks in low carbon steel weld metal, namely, solidification cracks and ductility-dip cracks have been verified using hot cracking tests and hot ductility tests. The formation temperature ranges of the two types of hot cracks are 1480–1450°C, and 1100–850°C, respectively. The surface morphology of ductility-dip cracks is intergranular fracture with dimple zones and smooth facets containing voids which are associated with (Mn, Fe)S inclusions. This type of cracking can be prevented by increasing the manganese content in the weld metal.