Fracture toughness of a welded duplex stainless steel

The present work includes fracture toughness testing on 30 and 50 mm thick duplex stainless steel 2205 (22% Cr, 5.5% Ni, 3% Mo, 0.15% N). Base metal and submerged arc weldments (SAW) at subzero temperatures have been tested using full size three point bending. The evaluation of the results has been carried out using J-integral calculations and correlations to Charpy impact values. It is shown that the temperature dependence of the fracture toughness can be described by a transition temperature curve, the master curve. The reference temperature was evaluated to −143 and −101 °C for base and weld material respectively.     

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb-Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.     

Microstructure-fracture toughness correlation in weld joints of Cr-Mo steel

The strength-toughness-microstructure relationship in relation to the micromechanics of a fracture process has been investigated in the weld joints of two alloys: 0.5 Mo and 2.25 Cr-1 Mo steels. These alloys are extensively used to fabricate super-heater tubes, boilers, piping, gas lines, etc., by welding. The applications require high temperature and pressure to be maintained during service. The crack initiation toughness and tearing resistance were evaluated using crack tip opening displacement/J-integral parameters at different temperatures. Quantitative analysis of micro-structure and fracture surfaces was used to study the micromechanics of fracture process in the heat-affected zone (HAZ) of the alloys. Molybdenum steel exhibited a higher percentage of ferrite and lower martensite content, while the other steel showed aligned carbide as the major constituent. The higher hardness and strength values in the HAZ and welding zone (WZ) of Cr-Mo steel, compared to molybdenum steel, may be attributed to the higher amount of martensite phase in the alloy. The higher initiation toughness at 200° C in both the alloys was reflected in the larger dimple size, compared to the size observed at room temperature. A tendency for void sheet formation was noticed in both alloys. Acicular ferrite and martensite appeared to be the most influential constituents affecting tearing resistance and initiation toughness.     

Effects of preheating temperature on cold cracks,microstructures and properties of high power laser hybrid welded 10Ni3CrMoV steel

Laser hybrid welding has become one of the most promising welding methods for high strength low alloy steels due to combining the advantage of the laser and arc. A novel Y-groove cold cracking test adapted to laser hybrid welding is designed to assess the weldability of 10Ni3CrMoV steels at room temperature and different preheating temperatures. The experimental results show that the orientation of the predominant root cracks generally follows the contour of the fusion line. As the temperature increases from 25 °C to 150 °C, at first the root crack rate decreases and then slightly increases at 150 °C. The root crack rate obtained at 120 °C is the lowest. The fracture model changes from a brittle cleavage fracture to a mixture fracture with quasi-cleavage facets and dimples. The thermal cycle curves of laser hybrid welding obtained by temperature measurement systems are used to evaluate the crack resistance and microstructure transformation. The microstructures of welded joints obtained at different temperatures are analyzed by optical microscope (OM). The results reveal that the microstructures of the coarse grained region and the fusion zone at 120 °C have higher cold crack resistance and good impact toughness. Mechanical properties of the welded joint obtained at 120 °C and 150 °C are comprehensively evaluated by microhardness test, uniaxial tensile test and charpy V-notch impact test with side notches. Fractographs of the impact specimens are studied by scanning electron microscopy (SEM). The test results show that the welded joints obtained at 120 °C have satisfactory mechanical properties that can meet the technical requirements for shipbuilding industry.     

Effects of short- and long-term thermal exposures on the stability of a Ni–Co–Cr–Si alloy

Long-term thermal stability is often needed for high temperature alloys used in a variety of industrial applications for extended operating lifetimes. In this paper, the effects of thermal exposures or aging on the mechanical properties and microstructure of a Ni–Co–Cr–Si alloy (HAYNES® HR-160® alloy) were studied. It includes both short- and long-term elevated temperature exposures ranging from 649 °C to 1093 °C (1200–2000 F) for duration of 6 min (0.1 h) to 6 years (50,000 h). The residual room temperature (RT) tensile and Charpy-V impact toughness properties were evaluated and correlated to microstructural changes as well as to fracture surfaces of the tensile tested samples. It was found that the RT ductility and impact toughness of the HR-160 alloy decreased continuously with time. A significant percentage of reduction in the ductility occurred in the initial 1000 h of exposure and the subsequent exposure led only to a minimal loss of ductility and impact toughness values. The concomitant microstructural changes were studied using optical metallography, SEM/EDS and X-ray diffraction of extracted residues. The results presented in this paper demonstrated that the HR-160 alloy exhibits good thermal stability characterized by >16% RT elongation after 50,000 h exposures at 649 °C, 760 °C, and 871 °C.     

Microstructure,mechanical properties and high stress abrasive wear behavior of air-cooled MnCrB cast steels

Three medium carbon low alloyed MnCrB cast steels containing different Cr contents (0.3%, 0.6%, and 1.2%) were designed and the effect of Cr contents on the microstructure, mechanical properties and high stress abrasive wear behavior of the cast steels after 850 °C air-cooling and 220 °C tempering was studied. The results show that the hardenability of the MnCrB cast steels was excellent. The microstructure of the cast steels with low Cr contents (0.3% or 0.6%) consists of granular bainite and lower bainite/martensite multiphase. With increasing of Cr content, the formability of martensite was improved, the hardness and wear-resistance increased, but the impact toughness decreased in that more bainite was replaced by martensite. The air-cooled MnCrB cast steel containing 0.6% Cr, with granular bainite and lower bainite/martensite multiphase, exhibited excellent combination of strength, hardness, ductility, and impact toughness. In addition, its abrasive wear-resistance was 30% more than that of Hadfield cast steel in the high stress abrasive wear condition. This air-cooled MnCrB cast steel by simple alloying scheme and heat treatment has the advantages of high-performance, low cost, and environmentally friendly. It is a potential advanced wear-resistant cast steel for low- or even medium-impact abrasive conditions.     

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, – 50, – 100 and – 150 °C. It was found that plane-stress,K _C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestK _C value at all temperatures among the three alloys. TheK _C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,K _IC values of the F2 alloy at – 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at –150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aK _C value of 106 MPa m^1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.     

Crack arrest testing of high strength structural steels for naval applications

The crack arrest fracture toughness of two high strength steel alloys used in naval construction, HSLA-100, Composition 3 and HY-100, was characterized in this investigation. A greatly scaled-down version of the wide-plate crack arrest test was developed to characterize the crack arrest performance of these tough steel alloys in the upper region of the ductile-brittle transition. The specimen is a single edge-notched, 152 mm wide by 19 mm thick by 910 mm long plate subjected to a strong thermal gradient and a tensile loading. The thermal gradient is required to arrest the crack at temperatures high in the transition region, close to the expected service temperature for crack arrest applications in surface ships. Strain gages were placed along the crack path to obtain crack position and crack velocity data, and this data, along with the applied loading is combined in a “generation mode” analysis using finite element analysis to obtain a dynamic analysis of the crack arrest event. Detailed finite element analyses were conducted to understand the effect of various modeling assumptions on the results and to validate the methodology compared with more conventional crack arrest tests.Brittle cracks initiation, significant cleavage crack propagation and subsequent crack arrest was achieved in all 15 of the tests conducted in this investigation. A crack arrest master curve approach was used to characterize and compare the crack arrest fracture toughness. The HSLA-100, Comp. 3 steel alloy had superior performance to the HY-100 steel alloy. The crack arrest reference temperature was T_KIA = −136 °C for the HSLA-100 plate and T_KIA = −64 °C for the HY-100 plate.     

Bimetallic low thermal-expansion panels of Co-base and silicide-coated Nb-base alloys for high-temperature structural applications

The fabrication and high temperature performance of low thermal expansion bimetallic lattices composed of Co-base and Nb-base alloys have been investigated. A 2D sheet lattice with a coefficient of thermal expansion (CTE) lower than the constituent materials of construction was designed for thermal cycling to 1000 °C with the use of elastic-plastic finite element analyses. The low CTE lattice consisted of a continuous network of the Nb-base alloy C-103 with inserts of high CTE Co-base alloy Haynes 188. A new coating approach wherein submicron alumina particles were incorporated into (Nb, Cr, Fe) silicide coatings was employed for oxidation protection of the Nb-base alloy. Thermal gravimetric analysis results indicate that the addition of submicron alumina particles reduced the oxidative mass gain by a factor of four during thermal cycling, increasing lifetime. Bimetallic cells with net expansion of 6 × 10⁻⁶/°C and 1 × 10⁻⁶/°C at 1000 °C were demonstrated and their measured thermal expansion characteristics were consistent with analytical models and finite element analysis predictions.     

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress-strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175-300 °C at a nominal strain rate of 3 × 10⁻³ s⁻¹. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.     

Relationship between stress-induced martensitic transformation and impact toughness in low carbon austenitic steels

The effect of test temperature, which controls the stability of austenite, on the impact toughness of a low carbon Fe-Ni-Mn-C austenitic steel and 304 stainless steel, has been investigated. Under impact conditions, stress-induced martensitic transformation occurred, in a region near the fracture surface, at test temperatures below 80°C for the Fe-Ni-Mn-C steel and below –25°C for 304 stainless steel. The former shows significant transformation toughening and the highest impact toughness was obtained at 10°C, which corresponds to the maximum amount of martensite formed by stress-induced transformation above the Ms temperature. The stress-induced martensitic transformation contributes negatively to the impact toughness in the 304 stainless steel. Increasing the amount of stress-induced transformation to martensite, lowered the impact toughness. The experimental results can be well explained by the Antolovich theory through the analysis of metallography and fractography. The different effect of stress-induced transformation on the impact toughness in Fe-Ni-Mn-C steel and 304 stainless steel has been further understood by applying the crystallographic model for stress-induced martensitic transformation to these two steels.     

The tempering behavior of granular structure in a Mn-series low carbon steel

The granular structure in a Mn-series low carbon steel composed of ferrite matrix and martensite-austenite islands does not exhibit temper brittleness which is quite different from common microstructures in steels. This characteristic facilitates the performance optimization through adjusting tempering temperature. A good combination of tensile strength (750-1000 MPa) and impact toughness (Aku, 138-154 J) can be obtained after quenching and tempering at 400 °C for a round billet with 250 mm in diameter.     

Effects of temperature on tensile and impact behavior of dissimilar welds of rotor steels

Tensile and impact behavior of dissimilar weld joints of newly developed rotor steels 23CrMoNiWV88 and 26NiCrMoV145 were conducted at various temperatures below 350 °C. Inhomogeneous microstructures and asymmetrical micro-hardness along the dissimilar welding joint were observed. With the increase of temperature, strength decreased which was associated with the increased plasticity, and fracture location changed from weld metal (WM) to intermediate pressure (IP) base metal (BM) at around 300 °C. Compared to the homogeneous impact specimen with two fracture zones at fracture surface, a combined quasi-cleavage and ductile fracture mode with three zones is observed at the fracture surface of the dissimilar weld joint when the testing temperature is in the range of 0–40 °C. The occurrence of separated zones are mainly ascribed to the multi-layer welding process and thus improved the impact toughness of the welding joint.     

Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

Effects of alloying elements on microstructural evolution and mechanical properties of induction quenched-and-tempered steels

The effects of Cr and/or Mo additions and tempering temperatures on mechanical properties in relation to the microstructural evolution during tempering were investigated in induction-tempered steels. The additions of Cr and/or Mo result in the finer distribution of cementite particles due to the decrease in the coarsening rates of cementite particles above tempering temperature of 400°C, while their influence is less effective at low tempering temperatures. Accordingly, the increments of tensile strength and yield strength by the addition of alloying elements become more pronounced at high temperatures above 400°C. The occurrence of maximum peak of yield strength at 400°C would be related to further precipitation of the cementite at low temperatures, and the subsequent spheroidization and coarsening process of the cementite at high temperatures. The addition of alloying elements does not change the minima in Charpy impact values, related to tempered martensite embrittlement, since alloying elements do not have an influence on the decomposition of retained austenite and the formation of the cementite at boundaries. The strain-hardening exponent, n, decreases up to 400°C and then continuously increases with tempering temperature. This abrupt increase of n at 300°C is related to the transformation of retained austenite during straining in induction-tempered 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.     

Experiments and predictions of the effects of load history on cleavage fracture in steel

A series of experiments conducted on two steels, A533B and A508, are summarised. Tests were conducted to explore the influence of different room temperature pre-loading cycles on subsequent low temperature (−150 °C and −170 °C) cleavage fracture. In all cases the low temperature fracture toughness was modified, with tensile pre-loading increasing the toughness and precompression reducing the toughness.Results from finite element simulation of the pre-loading cycles are illustrated. Tensile pre-loading created compressive residual stresses and precompression generated tensile residual stresses. The residual stresses were adopted in a stress based local approach to fracture model using Weibull statistics and applied to the experimental results. The parameters in the Weibull model were calibrated for the virgin steels prior to its application to prior loading cases. The model is found to be successful in predicting the change in toughness relative to the virgin material for pre-loading in tension of A533B steel. The model underestimated the change in toughness for tensile pre-loading of A508 steel and overestimated the toughness change for precompression of both steels.     

A comparative analysis of tensile and impact-toughness behavior of cold-worked and annealed 7075 aluminum alloy

The present research reports comparative analysis of effects of cold working (CW) and annealing on tensile and impact-toughness behavior of 7075 Al alloy. Cold-rolled samples were annealed at various temperatures in the range of 225–345 °C for 5 min. A remarkable increase in ductility and impact toughness was observed when specimens were annealed at temperatures above 265 °C for 5 min. It was also found that cold rolling has a profound effect on strength anisotropy that enhances with amount of % CW. The maximum strengths were observed in the transverse direction in the investigated alloy. Cold rolling has been found to impart a significant effect on decreasing the impact toughness of alloy that enhance with amount of % CW; this loss in impact energy could not be compensated by recrystallization process. It has also been shown that impact test can be considered as a simple method for measurement of toughness and plastic anisotropy in sheet and plate. The analysis of the fracture surfaces with the scanning electron microscope presented dimpled morphology for the failure ductile mechanism in starting material and fibrous structure with some quasi-cleavage regions in cold-rolled samples, corresponding to the ductile to brittle fracture mechanism.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.