Improvement of Mechanical Properties of CrNiMoCuTi Stainless Steel With Addition of Vanadium

 The effect of vanadium (V) addition on the mechanical properties of a Cr Ni Mo Cu Ti stainless steel was studied and its influence on microstructural changes was also investigated. Results indicate that the structure of the solution treated specimens mainly comprises of austenite martensite, and adding V leads to the formation of a considerable amount of ferrite. Under this condition, austenite phase is not mechanically stable, and transforms to martensite by plastic deformation. The addition of 05% - 10% (in mass percent) of V increases the hardness and the strength of the 80% cold rolled and aged steel, without any effect on ductility. Improvement in mechanical properties is presumably attributed to the formation of a small amount of ferrite in the primary structure, and the formation of certain precipitates is accelerated by the addition of V during aging. By contrast, excessive V decreases the strength and ductility simultaneously. This is due to the strong effect of ferrite formation compared to the beneficial effect of precipitation. The loss of ductility caused by adding higher amount of V is due to the formation of ferrite phases which in turn are suitable nucleation sites for crack propagation.     

Co-Al二元系中的fcc/hcp马氏体相变和形状记忆效应

It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp martensitic transformation and the SM effect were investigated in Co-Al binary alloys(mole fraction of Al=0～16%).The γ/ε martensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60 ℃ at x(Al)=0 to 150 ℃at x(Al)= 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4%(mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcp →fcc reverse transformation above 200 ℃. The SM effect was significantly improved by precipitation ofβ (B2) phase and the maximal shape recovery strain of 2. 2% was obtained, which can be explained by precipitation hardening. The crystallographic orientations between theβ, ε and γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690 ℃and 120 emu/g, respectively. It is concluded that the Co-Al alloys hold promise as new high-temperature and ferromagnetic SM alloys.     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

Mechanical Property and Microstructural Characterization of C-Mn-Al-Si Hot Dip Galvanizing TRIP Steel

 Mechanical properties and microstructure in high strength hot dip galvanizing TRIP steel were investigated by optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD), dilatometry and mechanical testing. On the heat treatment process of different intercritical annealing (IA) temperatures, isothermal bainitic transformation (IBT) temperatures and IBT time, this steel shows excellent mechanical properties with tensile strength over 780 MPa and elongation more than 22%. IBT time is a crucial factor in determining the mechanical properties as it confirms the bainite transformation process, as well as the microstructure of the steel. The microstructure of the hot dip galvanizing TRIP steel consisted of ferrite, bainite, retained austenite and martensite during the short IBT time. The contents of ferrite, bainite, retained austenite and martensite with different IBT time were calculated. The results showed that when IBT time increased from 20 to 60 s, the volume of bainite increased from 14.31% to 16.95% and the volume of retained austenite increased from 13.64% to 16.28%; meanwhile, the volume of martensite decreased from 7.18% to 1.89%. Both the transformation induced plasticity of retained austenite and the hardening of martensite are effective, especially, the latter plays a dominant role in the steel containing 7.18% martensite which shows similar strength characteristics as dual-phase steel, but a better elongation. When martensite volume decreases to 1.89%, the steel shows typical mechanical properties of TRIP, as so small amount of martensite has no obvious effect on the mechanical properties.     

Microstructure evolution and mechanical properties influenced by austenitizing temperature in aluminum-alloyed TRIP-aided steel

The Fe-0.21 C-2.2 Mn-0.49 Si-1.77 Al transformation induced plasticity(TRIP)-aided steel was heat treated at various austenitizing temperatures under both TRIP-aided polygonal ferrite type(TPF)and annealed martensite matrix(TAM)processes.The microstructure evolution and their effects on mechanical properties were systematically investigated through the microstructure observation and dilatometric analysis.The microstructure homogeneity is improved in TPF steel heated at a high temperature due to the reduced banded martensite and the increased bainite.Compared with the mechanical properties of the TPF steels,the yield strength and elongation of the TAM steels are much higher,while the tensile strength is lower than that of TPF steels.The stability of intercritical austenite is affected by the heating temperature,and thus the following phase transformation influences the mechanical properties,such as the bainite transformation and the precipitation of polygonal ferrite.Obvious dynamic bainite transformation occurs at TAM850,TAM900 and TAM950.More proportion of polygonal ferrite is found in the sample heated at 950°C.The bainite transformation beginning at a higher temperature results in the wider bainitic ferrite laths.The more proportion of polygonal ferrite and wide bainitic ferrite laths commonly contribute to the lower strength and better elongation.The uniform microstructure with lath-like morphology and retained austenite with high average carbon content ensures a good mechanical property in TAM850 with the product of strength and elongation of about 28 GPa·%.     

A multi-scale grained microstructure of the surface nanocrystallized 304 stainless steel sheets after warm-rolling

An ultrafine grained microstructure was obtained for 304 stainless steel(304SS)sheets by using surface nanocrystallization and warm-rolling.The microstructure and mechanical properties were determined by X-ray diffraction(XRD),transmission electron microscope(TEM)and a test on microhardness.Experimental results were shown that the microstructure was featured by a continuous distribution from the nanocrystalline on the surface to micro-grains in the center,in which the volume fraction of the micro-sized grains is about 40% in the surface layer.This multi-scale grained microstructure was composed of austenite and martensite phases with a gradient increasing volume fraction of austenite from the surface to the centre.The microhardness of the resultant steel was higher than 150% of that as received,due to the refined grains and strain-induced martensitic transformation.The hardness distribution was consistent with the microstructural variation,suggesting a good combination of high strength and improved ductility.     

A New Series of Mo-free 215Cr-35Ni-xW-02N Economical Duplex Stainless Steels

A new series of economical Mo-free duplex stainless steels 21.5Cr-3.5Ni-xW-0.2N(x=1.8-3.0,mass%) have been developed.The effects of W on mechanical properties and corrosion resistance were investigated,and the microstructures were analyzed by optical microscopy,X-ray diffraction,transmission electron microscopy and electron backscatter diffraction.The designed steels have a balanced ferrite-austenite relation and are free of sigma phase after solution treatment at 750-1 300℃for 30min followed by water-quenching,whereas a small number of Cr23 C6 precipitates were found after solution treatment at 750℃.After solution treatment at 1 050℃,the steel with 1.8%(mass percent)W exhibits the highest room temperature tensile strength due to the strongest work hardening effect, while the steel with 3.0%(mass percent)W exhibits the highest fracture elongation owing to the transformation-induced plasticity(TRIP)effect.The ductile-brittle transition(DBT)and martensite transformation are respectively found in the ferrite and austenite,which deteriorates the impact properties of the steels with the increase of W content.The corrosion resistance of the designed steels is improved with the increase of W content.The pitting resistance of austenite is obviously better than that of ferrite for the designed alloys.Among the designed steels,the steel with 1.8%(mass percent)W is found to be an optimum steel with excellent comprehensive properties and lowest production cost.     

Phase Transformation and Its Effect on Mechanical Properties of C300 Weld Metal after Aging Treatment at Different Temperatures

The influence of aging temperature on phase transformation and mechanical properties of weld metal of maraging steel(grade C300)was studied.Microstructure was analyzed by means of optical microscopy,transmission electron microscopy,scanning electron microscopy and energy dispersive spectrum analysis.Gibbs free energy of Ni3 Ti and Fe2 Mo at different temperature was calculated by Thermal-calc software.The microstructure of weld metal in aswelded state is martensite.The yield strength of weld metal after 430 ℃ aging process may increase to 1 561 MPa from 890 MPa in as-welded state,which is ascribed to the formation of spinodal constitute and GP zones.After 480 ℃aging process,there are great deal of Ni3 Ti precipitates in the martensite matrix and 10%reverted austenite phase in the cellular grain boundary,and the yield strength increases to 1 801 MPa.After aging process at 580 ℃,there are many Fe2 Mo precipitates in the martensite matrix and 30% reverted austenite phase in the cellular grain boundary,and the yield strength is 1 329 MPa,which is the lowest among the three cases.The phase transformation may also influence the toughness.It is found that precipitates make the toughness decrease and reverted austenite increases it.The mechanism of phase transformation on strength and toughness is discussed.     

Effect of Heat Treatment on Microstructure and Property of Cr13 Super Martensitic Stainless Steel

 The microstructures and mechanical properties of Cr13 super martensitic stainless steel after different heat treatments were studied. The results show that the structures of the steel after quenching are of lath martensite mixed with a small amount of retained austenite. With the raising quenching temperature, the original austenite grain size increases and the lath martensite gradually becomes thicker. The structures of the tempered steel are mixtures of tempered martensite and reversed austenite dispersed in the martensite matrix. The amount of reversed austenite is from 754% to 2249%. After different heat treatments, the tensile strength, the elongation and the HRC hardness of the steel are in the range of 813-1070 MPa, 101%-212% and 2133-3237, respectively. The steel displays the best comprehensive mechanical properties after the sample is quenched at 1050 ℃ followed by tempering at 650 ℃.     

Effect of Niobium on Transformations From Austenite to Ferrite in Low Carbon Steels

Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite.However,the effects of niobium on transformation behaviour have not been fully characterised and understood to date.This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents,together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature.It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content.When austenite grain sizes are consistent,the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.     

Microstructure and texture development during the phase transformation in an Fe-Ni-Co-Ti shape memory alloy

In an Fe-Ni-Co-Ti shape memory alloy the austenite to martensite phase transformation was studied by means of microstructure investigation as well as micro- and macrotexture measurements. To obtain a crystallographically reversible martensitic transformation the samples were ausaged. Phase transformation into martensite was performed both thermally and stress induced. The transformation temperatures allowed investigation of both the austenite and martensite at room temperature. From the microtexture investigation the Nishiyama-Wassermann orientation relation was experimentally confirmed to explain the austenite to martensite transformation texture. Variant selection was shown to take place during the transformation and analyzed on an individual grain level.     

Fractality and reversibility of ferrous martensite

Martensite formation is characterized by a diffusionless structural phase transformation from austenite to martensite, associated with a considerable amount of lattice variant shear γ_γα ? 0.2. Ferrous martensite shows all possible features connected with the transformation. Different modes of initiation of the martensite formation are possible. The reasons for the burst phenomenon can be considered as an analogy to discontinuous yielding. The transformation only procedes if further thermodynamical driving force is provided by cooling or shear stress. In some cases fractal microstructures are formed in which several fragmentations can be recognized. In contrast to shape memory alloys, steels usually do not show reversibility of the reverse α → γ transformation. The factors which favour reversibility have been defined. Knowledge of these is necessary for the development of iron-base shape-memory alloys.     

Shape memory behavior of FeNiCoTi single and polycrystals

We present experimental and theoretical evidence of thermoelastic martensites in Fe29Ni18Co4Ti alloys. In this class of alloys, the high strength in the austenite domains limits the slip deformation as verified with transmission electron microscopy. The restriction of slip permits a higher degree of recoverability of the transformation. Using both single crystals with [123] orientation and polycrystals, the appearance of martensite plates upon deformation, and their reversion back to austenite upon heating (the shape memory effect), is revealed with in-situ optical microscopy. Theoretical results for the transformation strains and the detwinning of martensite are presented, which demonstrate convincingly the potential of these classes of alloys. Electrical resistance measurements identified the stress and temperature levels at the onset of forward and reverse transformations in isothermal deformation and thermal cycling experiments, respectively. The return of the electrical resistance to its reference value, upon austenite to martensite followed by martensite to austenite transformation, verified the recovery in the transformation strains measured in the experiments.     

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveA _s, with microcracks forming during theM→γ→M transformation.     

Stress-Assisted and strain-induced martensites in FE-NI-C alloys

A metallographic study was made of the martensite formed during plastic straining of metastable, austenitic Fe-Ni-C alloys withM _s temperatures below 0°C. A comparison was made between this martensite and that formed during the deformation of two TRIP steels. In the Fe-Ni-C alloys two distinctly different types of martensite formed concurrently with plastic deformation. The large differences in morphology, distribution, temperature dependence, and other characteristics indicate that the two martensites form by different transformation mechanisms. The first type, stress-assisted martensite, is simply the same plate martensite that forms spontaneously belowM _s except that it is somewhat finer and less regularly shaped than that formed by a temperature drop alone. This difference is due to the stress-assisted martensite forming from cold-worked austenite. The second type, strain-induced martensite, formed along the slip bands of the austenite as sheaves of fine parallel laths less than 0.5μm wide strung out on the {111}_γ planes of the austenite. Electron diffraction indicated a Kurdjumov-Sachs orientation for the strain-induced martensite relative to the parent austenite. No stress-assisted, plate martensite formed in the TRIP steels; all of the martensite caused by deformation of the TRIP steels appeared identical to the strain-induced martensite of the Fe-Ni-C alloys. It is concluded that the transformation-induced ductility of the TRIP steels is a consequence of the formation of strain-induced martensite. Formerly a graduate student at Stanford University     

Ti-V-Al轻质记忆合金的研究进展

Ti-V-Al合金基于热弹性马氏体相变而呈现出形状记忆效应。同时,Ti-V-Al合金不仅呈现出良好的冷热加工性能,还具有较低的密度,这可满足当今航空航天领域对轻量化制造的需求。文中主要综述国内外研究学者在Ti-V-Al轻质记忆合金研究方面的重要工作和进展,其中重点阐述了Ti-V-Al轻质记忆合金热循环稳定性、力学性能与功能特性方面的研究。最后,简单阐述了Ti-V-Al轻质记忆合金功能特性的演化规律与机制,并对后续Ti-V-Al轻质记忆合金的发展方向进行了展望。      

Effect of Co Addition on the Microstructure, Martensitic Transformation and Shape Memory Behavior of Fe-Mn-Si Alloys

The effect of Co addition has been studied in Fe-30Mn-6Si-xCo (x = 0 to 9 wt pct) shape memory alloys in terms of their microstructure, martensitic transformation and shape recovery. Microstructural investigations reveal that in Fe-Mn-Si-Co alloys, the microstructure remains single-phase austenite (??) up to 5 pct Co and beyond that becomes two-phase comprising ?? and off-stoichiometric (Fe,Co)₅Mn₃Si₂ intermetallic ??-phases. The forward ??-?? martensite transformation start temperature (M _S) decreases with the addition of Co up to 5 pct, and alloys containing more than 5 pct Co, show slightly higher M _S possibly on account of two-phase microstructure. Unlike M _S, the ??-?? reverse transformation start temperature (A _S) has been found to remain almost unaltered by Co addition. In general, addition of Co to Fe-Mn-Si alloys deteriorates shape recovery due to decreasing resistance to plastic yielding concomitant with the formation of stress induced ?? martensite. However, there is an improvement in shape recovery beyond 5 pct Co addition, possibly due to the strengthening effect arising from the presence of (Fe,Co)₅Mn₃Si₂ precipitates within the two-phase microstructure and due to higher amount of stress induced ?? martensite.     

The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: I. Morphology and transformation characteristics

Fe-Pt alloys near the composition Fe₃Pt transform from fee austenite to bcc martensite at near ambient temperatures. The effect of austenite ordering in depressing theM _s temperature has been reported previously, but more importantly the present work shows that ordering leads to a reversible martensitic transformation. The characteristics of this reversible transformation have been investigated by optical metallography, cinematography, and electrical resistivity measurements. It is concluded that in austenite ordered to an appropriate degree, the transformation to martensite possesses all of the characteristics of a thermoelastic martensite transformation. This transformation in ordered Fe~25 at. pct Pt alloys is the first thermoelastic martensite transformation reported for an iron-base alloy. The present experiments indicate that martensite “nuclei” are not destroyed by the transformation, and are reactivated on each cooling cycle at approximately the same temperature. D. P. DUNNE, formerly with the University of Illinois at Urbana-Champaign, Urbana, 111. 61801     

Effect of transformation substructure on the strength and toughness of Fe−Mn alloys

Phase transformations in Fe?Mn alloys containing up to 9 pct Mn were studied by optical and electron transmission microscopy. Either equiaxed ferrite, massive ferrite, or massive martensite can form on cooling from austenite. The particular type of transformation product formed was found to depend on the alloy content, austenite grain size, and cooling rate. The mechanical properties of all the transformation products were evaluated using tensile and impact testing and are discussed in terms of the observed microstructural features. Yield strength and impact transition temperature were found to be relatively insensitive to manganese content but were strongly influenced by the transformation substructure and grain size of the transformed phase. In martensite it has been shown that the structural unit analogous to grain size in ferrite is the martensite packet size, which in turn is controlled by the prior austenite grain size. The fracture surface of broken impact specimens and the fracture profile were examined by means of electron and optical microscopy techniques. These fractographic observations were correlated with impact test data and microstructural observations of the various transformation products.