Thermomechanical behavior of Fe–Mn–Si–Cr–Ni shape memory alloys modified with samarium

The deformation and training behavior of Fe–14Mn–3Si–10Cr–5Ni (wt.%) shape memory alloys containing samarium addition has been studied in the iron-based shape memory alloys. It is noticed that thermomechanical treatment (training) has significant influence on proof stress, critical stress and shape memory behavior of the alloys. The improvement in shape memory behavior can be attributed to the decrease in the proof stress and critical stress which facilitates the formation of ? (hcp martensite). It is also observed that alloy 2 containing samarium undergoes less softening as compared to alloy 1 with training which inhibits the formation of ? (bcc martensite) and thus enhances the shape memory behavior. The excessive thermomechanical treatment with increase in the training cycle has led to the formation of ? (bcc martensite) along with ? (hcp martensite) in the alloy 1 which appeared to have decline in the shape memory effect. This has been demonstrated by the examination of microstructure and identification of ? (bcc martensite) martensite in the alloy 1 as compared to alloy 2.     

Load sequence effects in fatigue crack growth of thick-walled welded C–Mn steel members

Many welded steel structures in marine, offshore, and infrastructural industries are subjected to variable amplitude (VA) fatigue loads. It is well known that the level and sequence of the load cycles can cause crack growth retardation or acceleration and thus influence the fatigue life. An important sequence effect is generated by a large stress cycle followed by smaller stress cycles. Whereas the effect of single large stress cycles in a further constant amplitude (CA) load on central through cracks in thin-walled aluminium sheet is well established, studies into the effects of practical VA loads on cracks in thick-walled welded steel structures are less common. This paper presents the results of CA tests with large stress peaks and VA tests on 70 mm C–Mn steel butt welded 4-point bending specimens with crack growth in thickness direction. It is demonstrated that loading by a sequence of accelerating and subsequent decelerating stress cycles cause significant retardation of the crack growth and that the same stress cycles but placed in random sequence hardly result in retarded crack growth. The obtained crack growth versus number of cycles for as-welded and stress relieved specimens have been simulated using two relatively simple crack rate retardation models, being the well-known Willenborg model and the Space-state model developed by Ray and Patankar. The latter model is also used to simulate crack growth of semi-elliptical surface cracks in welded steel structures tested by others. The Space-state model is able to predict experimental results with reasonable to good accuracy. A proposal is put forward for future improvement of the model.     

Influence of aluminium on solidification structure and initial deformability of continuously cast C–Mn–Si–N steel

Abstract

The solidification structure and the initial deformability of continuously cast steel were investigated by assessment of cracks on billets and on rolled product. Some billets were rolled directly off the casting machine and some cooled to ambient temperature, then reheated to rolling temperature. On direct rolled steels, the number of defects increases with increasing aluminium content, while virtually no defects are found on steel rolled after reheating. By increasing the aluminium content, the solidification structure of steel is highly modified and a columnar structure obtained over the entire section of the billet. It was shown by chemical analysis and fracture examination that the increased hot shortness is not related to the effect of AIN. It is concluded that the hot shortness is related to the effect of aluminium on the solidification structure.

MST/761     

Prediction of creep crack initiation in Cr–Mo–V steel specimens with different geometries

By using stress dependent creep ductility and strain rate model in a ductility exhaustion based damage model, the creep crack initiation (CCI) behaviour in Cr–Mo–V steel specimens with different geometries and dimensions (different constraints) over a wide range of C^* has been predicted by finite element simulations. The predicted creep crack initiation time agree well with the existing experimental data. In low and transition C^* regions, the constraint induced by specimen geometries and dimensions has obvious influence on CCI time. With increasing constraint level of specimens, the CCI time decreases due to the increase of stress triaxiality ahead of crack tip. Different CCI trends and constraint effects on CCI behaviour in a wide range of C^* result from the interaction of crack-tip stress state and stress dependent creep ductility of the steel. It is suggested that in CCI life assessments of high temperature components, the long-term CCI time data at low C^* region should be obtained and used, and the constraint effects need to be considered by using constraint dependent CCI data.     

Microstructural evolution and mechanical properties of friction stir welded joint of Fe–Cr–Mn–Mo–N austenite stainless steel

2 mm thick Fe–18.4Cr–15.8Mn–2.1Mo–0.66N high nitrogen austenite stainless steel plate was successfully joined by friction stir welding (FSW) at 800 rpm and 100 mm/min. FSW did not result in the loss of nitrogen in the nugget zone. The arc-shaped band structure, consisting of a small amount of discontinuous ferrite aligning in the bands and fine austenite grains, was a prominent microstructure feature in the nugget zone. The discontinuous ferrite resulted from newly formed ferrite during welding and the remained ferrite, whereas the fine austenite grains were formed due to dynamic recrystallization of the initial austenite during FSW. The fine dynamically recrystallized grains in the nugget zone significantly increased the hardness compared to that of the base material. The strength of the joint was similar to that of the base material, with the joint failing in the base material zone.     

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.     

Modifying high Cr–Mn cast iron with boron and rare earth–Si alloy

Abstract

By modifying 13Cr–4Mn (wt-%) white cast iron with boron and rare earth (RE)–Si complex, the carbide morphology of the iron can be changed from interconnected, coarse clusters of rods into a parallel distribution of isolated, fine rods, and the impact toughness of the iron can reach 6–7 × 10⁴ J m^?2. In a pin wear test, the relative abrasion resistance of the iron is 1·01 and in a repeated impact abrasive wear test it is 0·95, in comparison with 15Cr–3Mo cast iron. Thus, it is stated that modifying high Cr–Mn cast iron with boron and RE–Si complex is very cost effective, and has almost the same abrasion resistance, when compared with 15Cr–3Mo cast iron.

MST/957     

Effects of solidification structure on fatigue crack growth in rheocast and thixocast Al–Si–Mg alloys

Fatigue crack growth test was performed for rheocast and thixocast Al–Si–Mg aluminum alloys. At small stress intensity factor range (ΔK), fatigue crack growth (FCG) rate of sample with coarse acicular Si particles decreased slightly compared with specimen with small acicular Si particles. However, at large ΔK, fatigue crack growth rate of specimen with coarse acicular Si particles drastically increased. This is because large acicular Si particles induce high strain hardening at small ΔK, but such particles are easily cracked with the increase in ΔK. Morphology of the Si particles strongly affects striation formation.     

Influence of case-carburizing and micro-defect on competing failure behaviors of Ni–Cr–W steel under gigacycle fatigue

Axial loading test was performed to investigate the influence of case-carburizing and micro-defect on competing failure behaviors of Ni–Cr–W Steel under gigacycle fatigue. The interior failures induced from inclusion and microstructural inhomogeneity become the predominant failure mode in the life regime beyond 10⁵ cycles. The case-carburizing has no effect on the fatigue strength with interior failure. Compared with the lower limit values of experimental S–N data, the predicted results by using GP distribution is relatively suitable. From the viewpoint of reliability, the modeling method of interior S–N curve with the maximum defect size at a given probability is satisfactory.     

Improving impact toughness of high-C–Cr bearing steel by Si–Mo alloying and low-temperature austempering

Nanostructured bainite and dispersed carbide particles were formed in Si–Mo-alloyed high-C–Cr bearing steels by low-temperature austempering after partial austenitizing in the intercritical gamma + carbide region. Comparing with conventional quenched and tempered high-C–Cr bearing steel, the impact toughness is remarkably enhanced; the hardness is still adequate for the bearings even though it is slightly decreased.     

Influences of Nb-microalloying on microstructure and mechanical properties of Fe–25Mn–3Si–3Al TWIP steel

The Fe–25Mn–3Si–3Al TWIP steel was microalloyed by niobium in this paper, and the appropriate heat treatment and cold rolling processes were drafted in order to improve the poor yield strength of the steel. The results show that the yield strength of the steel increases from 320 MPa to 445 MPa, and the tensile strength increases from 680 MPa to 795 MPa, but the uniform elongation decreases from 65% to 55%. Nb addition can strongly hinder the growth of recrystallized grains, moreover Nb atoms react with C atoms to form nanoscale NbC precipitations, and these precipitations can block the dislocation motion, and then the yield strength and initial work hardening ability of Fe–25Mn–3Si–3Al steel is clearly improved. Furthermore, the strain-induced twinning is still a major deformation mechanism for the Nb-microalloying TWIP steel, and the twinning induced plasticity (TWIP) effect ensures a satisfactory ductility for the steel. Finally, the modified TWIP steel obtains a better match between the strength and plasticity by the joint action of precipitation strengthening and TWIP effect.     

Cyclic fatigue crack growth behaviour in β-(Si–Al–O–N) at ambient and elevated temperatures

Four-point bending fatigue tests on a hot-pressed sintered Sm–-(Si–Al–O–N) ceramic were conducted at room temperature, 900 °C and 1000 °C in air under different load ratios and cyclic frequencies. The growth of indentation cracks was measured during the fatigue tests. The results indicate that the cyclic fatigue crack growth threshold is lower and crack growth rates are higher, for given values of K_max, at 1000 °C than those at room temperature. The cyclic fatigue crack growth behaviour at 900 °C is similar to that at room temperature. It was found that the crack growth retardation due to cyclic fatigue loading is much more pronounced at higher frequencies. An increase in cyclic frequency from 1 to 10 Hz cause a reduction of up to two orders of magnitude in crack propagation rates. High-temperature cyclic fatigue crack growth rates increased and threshold stress intensity factor ranges decreased with increasing load ratio. Possible mechanisms for cyclic crack growth are discussed.     

Cyclic behavior of 9–12% Cr steel under different control modes in low cycle regime: A comparative study

Cyclic behavior of 9–12% Cr steel under both stress and strain control modes was investigated at 873 K. Significant asymmetric deformation and cyclic softening were observed for both modes. Under the strain-controlled fatigue, a strain level-independent softening factor (SF) was observed. The SF was dependent on applied stress under the stress-controlled fatigue. For the strain-controlled fatigue, the magnitude of cyclic asymmetry decreases with the increase of strain amplitude, while under the stress-controlled fatigue, the asymmetry increases with the increase of stress amplitude. The anomalous ratcheting strain resulted from the asymmetry under the stress-controlled mode has a detrimental effect on fatigue life.     

Effect of orientation and presence of hydride on the fatigue crack growth behavior of Zr–2.5 wt% Nb

Fatigue crack growth (fcg) behavior of cold-worked and stress relieved Zr–2.5 Nb was studied in the longitudinal (with and without hydrides) and transverse direction at ambient temperature and load ratio of 0.1 using compact tension samples. Fatigue loading in the transverse direction (distribution of both hard and soft grains) showed facet formation on the fracture surface and the highest ΔK_th whereas loading in the longitudinal direction (distribution of primarily soft grains) showed no facet formation and a lower ΔK_th. Hydrided Zr–2.5 Nb loaded in the transverse direction showed large facets with the lowest ΔK_th. Texture influenced fcg at low ΔK but not at higher ΔK.     

Effect of deformation of austenite and cooling rates on transformation microstructures in a Mn–Cr gear steel

The effect of austenite deformation and cooling rates on continuous cooling transformation microstructures for a Mn–Cr gear steel were investigated using a Gleeble 1500 thermomechanical test system. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, leading to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The deformation enhances the stability of austenite against bainite transformation, which results in an increase in amount of martensite/austenite (M/A) constituent with deformation at some cooling rates studied. Moreover, cooling rate also affects amount of M/A constituent. With decrease of cooling rate, amount of M/A constituent increases at first, but decreases subsequently till disappears eventually.     

Effect of increasing cooling rate from normalising temperature on structure and properties of a Mn–Cr–Mo–V steel

Abstract

The work was undertaken to study the effect of increasing the cooling rate from the normalising temperature on the microstructure and mechanical properties of a Mn–Cr–Mo–V steel. The steel was to a chemical composition suitable for grade 271 in BS 1501 Part 2 and as such would be used in the normalised and tempered condition. Three linear rates of cooling (12, 36, and 120 K min ^?1) from the normalising temperature were used. With the tenfold increase in cooling rate, yield strength increases of about 20% and tensile strength increases of about 15% were obtained in the final tempered steel. Although these strength increases resulted in a loss in ductility and toughness, the values of these properties were still relatively high. The improvements in strength with increases in cooling rate have been related to an increase in the proportion of bainite and a decrease in the amount of ferrite in the resulting microstructure.

MST/683     

Properties and application of Fe–6Si–14Mn–9Cr–5Ni shape memory alloy

Abstract

Three iron based shape memory alloys were studied and Fe–6Si–14Mn–9Cr–5Ni alloy showed the best shape memory effect. By thermomechanical training, the shape memory effect was improved and an absolute recovery strain of 6·2% was obtained. To promote the ε→γ transformation, which is not complete even after heating the alloy to 1000 K, the As and Af temperatures are decreased and the transformation enthalpy is increased by thermal cycling and increasing prestrain. The alloy also shows good creep and stress relaxation resistance. In addition, under a tensile force of 20 kN and a sealing test pressure of 6 MPa pipe joints made using the alloy remain effective and can satisfy the requirements for possible industrial applications.     

Monotonic and cyclic deformation behavior of the Al–Si–Cu–Mg cast alloy with micro-additions of Ti,V and Zr

Monotonic and cyclic tests were used to assess the influence of micro-additions of Ti, V and Zr on the deformation behavior of the Al–7Si–1Cu–0.5Mg (wt.%) alloy in as-cast and T6 heat treated conditions and to compare the results with alloys of similar chemistry described in the literature. The microstructure of the as-cast alloy consisted of α-Al, eutectic Si, and Cu, Mg and Fe based phases Al_2.1Cu, Al_8.5Si_2.4Cu, Al_7.2Si_8.3Cu₂Mg_6.9 and Al₁₄Si_7.1FeMg_3.3. In addition, the micro-size Zr–Ti–V-rich phases Al_21.4Si_4.1Ti_3.5VZr_3.9, Al_6.7Si_1.2TiZr_1.8, Al_2.8Si_3.8V_1.6Zr and Al_5.1Si_35.4Ti_1.6Zr_5.7Fe were present in the as-cast state. During solution treatment, Cu based phases were completely dissolved, while the eutectic silicon, Fe- and Zr–Ti–V-rich intermetallics experienced only partial dissolution. The monotonic test results showed that the T6 heat treated alloy achieved a tensile strength of 343 MPa and a compressive strength of 418 MPa. Also, the cyclic yield stress of the studied alloy in the T6 temper condition was higher than the monotonic value and reached 335 MPa. The fatigue life of the studied alloy was substantially longer than that of the reference alloy with the same base but lower additions of V, Zr and Ti, reported in the literature. The fractography revealed the tensile crack propagation through the eutectic Si and primary phases, exhibiting intergranular fracture along with some cleavage-like features of the plate-shape Zr–Ti–V-rich intermetallics with a presence of fatigue striations on the latter, indicating their ductile nature. It is believed that the intermetallic precipitates containing Zr, Ti and V improve the fatigue life of the studied alloy in the T6 condition.     

Tensile and compressive deformation behavior of the Al–Si–Cu–Mg cast alloy with additions of Zr,V and Ti

The deformation behavior of the Al–Si–Cu–Mg cast alloy with micro-additions of Zr, V, and Ti was investigated under uniaxial tension and compression. It was found that after T6 heat treatment the change of the load from tension to compression caused an increase in strength from 348 MPa to 417 MPa and in fracture strain from 1.3% to 37.0%. As calculated based on Mott’s theory of strain hardening, the dislocation slip distance in compression was twice of that in tension. The observed differences in alloy fracture strain were explained by changes in re-orientation and fracturing of the eutectic silicon particles. Due to deformation, fracturing of the silicon particles occurred with major cracks being parallel to the compression axis but perpendicular to the tensile load axis. An influence of deformation mode on change in orientation of the silicon particles was revealed. While for tensile load, the silicon particles were stationary during deformation and exhibited an orientation practically the same as in unstrained structure, for compression there was a substantial change in the particle orientation, especially for an angle between the load axis and the particle axis in the range from 0° to 30°.