Formation of a submicrocrystalline structure in austenitic 08Kh18N10T steel during equal-channel angular pressing followed by heating

The structure and mechanical properties of austenitic 08KhN10T steel subjected to equal-channel angular pressing (ECAP) at room temperature (? = 3.2) and subsequent heating are studied. In the course of ECAP, the steel undergoes a martensitic transformation; the martensite content reaches 45%. Upon heating, martensite (ferrite) transforms into austenite. The partly submicrocrystalline oriented structure of the 08Kh18N10T steel in the austenitic (55%)-martensitic (45%) state (formed upon ECAP) provides its high strain hardening (σ_0.2 = 1315 N/mm²), as compared to the initial state (σ_0.2 = 250 N/mm²), and high plasticity δ = 11%. After heating to 550°C, the steel has a predominantly submicrocrystalline austenitic (80%)-ferritic (20%) structure, σ_0.2 = 1090 N/mm², and δ = 11%.     

Phase and structural transformations in corrosion-resistant steels upon high-pressure torsion and heating

High-pressure torsion (HPT) at a pressure of 6 GPa and room temperature is found to form a nanocrystalline structure in corrosion-resistant austenitic 05Kh15N9D2TAMF and 08Kh18N10T steels and a submicrocrystalline structure in corrosion-resistant ferritic 08Kh18T1 steel and armco iron. X-ray diffraction analysis of both austenitic steels reveals the γ → α and γ→ ?→ α martensitic transformations during HPT at room temperature. After HPT, the strain hardening in the austenitic and ferritic steels is approximately the same and mainly determined by nano- and submicrocrystalline structures, and the role of alloying and phase composition weakens. The thermal stability of the hardening in the austenitic and ferritic steels is almost the same, ～400°C. As a result of HPT, the austenitic 08Kh18N10T and ferritic 08Kh18T1 steels acquire an axial texture with the predominant 〈211〉_γ direction in austenite and the 〈110〉_α and 〈311〉_α directions in martensite and ferrite, respectively. The axial texture is retained in both steels up to a heating temperature of 750°C.     

Behavior of an Austenitic–Martensitic VNS9-Sh Sheet TRIP Steel during Static and Cyclic Deformation

The properties of austenitic–martensitic VNS9-Sh (23Kh15N5AM3-Sh) sheet TRIP steel during static and cyclic loading are studied. The specific features of the mechanical behavior of the steel during static tension that are related to shearing, twinning, and martensite formation processes are detected. The static stress–strain curve of the steel has a developed microyield stage, a long yield plateau, and a serrated stage of strain hardening (Portevin–Le Chatelier effect). The shear mechanisms at the initial stages of cyclic deformation and fatigue crack propagation mechanisms are investigated.     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

Influence of deformation on the structure and mechanical and corrosion properties of high-nitrogen austenitic 07Kh16AG13M3 steel

The correlation has been studied between the structure of a high-nitrogen austenitic Cr-Mn-N steel formed in the process of combined hardening treatment, including cold plastic deformation (CPD), and its mechanical and corrosion properties. The structure and properties of commercial high-nitrogen (0.8% N) 07Kh16AG13M3 steel is analyzed after rolling by CPD and aging at 500 and 800°C. It is shown that CPD of the steel occurs by dislocation slip and deformation twinning. Deformation twinning and also high resistance of austenite to martensitic transformations at true strains of 0.2 and 0.4 determine the high plasticity of the steel. The contribution of the structure imperfection parameters to the broadening of the austenite lines during CPD is estimated by X-ray diffraction. The main hardening factor is stated to be lattice microdistortions. Transmission electron microscopy study shows that heating of the deformed steel to 500°C leads to the formation of the intermediate CrN phase by a homogeneous mechanism, and the intermtallic χ phase forms along the austenite grain boundaries in the case of heating at 800°C. After hardening by all investigated technological schemes, exception for aging at 800°C, the steel does not undergo pitting corrosion and is slightly prone to a stress corrosion cracking during static bending tests, while aging at 800°C causes pitting corrosion at a pitting formation potential E _pf = ?0.25 V.     

Machinability of the high-strength corrosion-resistant high-ductility austenitic steel 06Kh22AG15N8M2F

The machinability of the high-nitrogen corrosion-resistant austenitic steel 06Kh22AG15N8M2F during turning is studied. The specific features of the structure of the surface layers in steel workpieces after turning are revealed. The cutting conditions that provide the lowest wear of VK8 alloy cutting tools upon turning are found: the cutting speed is 21–74 m/min, the feed is 0.15–0.60 mm/rev, and the cutting depth is 0.15–0.75 mm. The presence of a large amount of Cr₂N-type chromium nitrides in the structure of the steel annealed at 800°C for 2 h and a high nitrogen content in the austenite of the steel quenched from 1100°C increase the wear of the cutting tools. As to turning of the forged steel, the wear resistance of the cutting tools upon turning of the 06Kh22AG15N8M2F steel is higher than that upon turning of 08Kh18N10T steel, in which deformation martensite forms (in surface layers) during turning.     

Influence of heat treatment on the structure and mechanical properties of chrome steel with unstable austenite

The structure and mechanical properties of 35Kh12G3MVFDR steel are investigated. After normalization or quenching, the steel contains up to 35 vol % austenite and may be assigned to the martensitic–austenitic class. On heat treatment—tempering, isothermal holding, or isothermal quenching—the austenite is converted to martensite within 2 h. The martensite in 35Kh12G3MVFDR steel is more thermally stable: the first signs of its conversion to sorbitic structure are observed after 25-h isothermal quenching at 640°C, and its complete decomposition requires 50 h. The breakdown of martensite is accompanied by decrease in the high-temperature strength and hardness. Aging of the quenched and tempered 35Kh12G3MVFDR steel at 670–720°C lowers the hardness from 61–65 HRA to 55–60 HRA after 1600–3200 h and the yield point at 20°C from 1350 MPa to 750–850 MPa. Likewise, the yield point at 720°C declines from 310 MPa to 160–230 MPa after 600 h and then stops. The state of the martensitic structure of 35Kh12G3MVFDR steel determines its creep resistance at 700°C. For example, the martensite remains in the steel structure after relatively brief isothermal quenching (up to 24 h at 640°C), and consequently the creep limit σ^700°C _0.1%/h is no lower than after simple quenching with subsequent high tempering: 86.2 ± 9.4 MPa and 89.3 ± 8.8 MPa, respectively. At the same time, in response to the decomposition of martensitic structure as a result of prolonged aging (1600 h at 670°C), σ^700°C _0.1%/h declines to 63.9 ± 7.1 MPa. In contrast to martensite, the austenite in 35Kh12G3MVFDR steel is thermally unstable and is converted to martensite after only 1–2 h of heating, depending on the temperature.     

冷轧变形量对高氮奥氏体不锈钢Mnl7Crl9N0.6组织和性能的影响

对感应炉-电渣重熔冶炼的节镍型高氮奥氏体不锈钢Mn17Cr19N0.6的3mm热轧板进行变形量10%～60%的冷轧及拉伸实验。结合金相组织观察及XRD物相分析,研究高氮奥氏体不锈钢冷变形过程中微观组织变化规律,得出结论:在冷轧过程中,随着变形量的增加,实验钢中晶粒的形状由块状到压扁拉长状,滑移从单滑移为主到交滑移,孪晶最终被分割破碎。实验钢在不同冷轧变形量后的组织均为单一的奥氏体相,并没有出现其他相,实验钢在冷变形过程中没有发生马氏体转变,因此,实验钢在冷轧过程中没有通过相变强化,以形变强化为主,抗拉强度从冷轧变形量为10%时的1045 MPa升高至变形量为60%时的1880MPa,因此通过冷变形可以制备出不同强度级别且组织为单一奥氏体的特种材料。     

Niti and NiTi-TiC composites: Part II. compressive mechanical properties

The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC participates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stressinduced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology D. MARI, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

等径角挤压变形奥氏体不锈钢的组织演变

 用实验方法研究了奥氏体不锈钢在等径角挤压冷变形(路径RC)过程中组织变化。实验结果表明：当剪切方向与孪晶带方向成一定角度时，在剪切力的作用下，孪晶逐渐由大块孪晶→由剪切带分割的孪晶(楼梯状)→小块状→奥氏体亚晶或马氏体晶粒；部分孪晶在剪切力作用下，剪切带可直接碎化成具有大角度位向差的细小晶粒(奥氏体亚晶+马氏体晶粒)，可发生马氏体相变；当剪切方向与孪晶带方向相同时，孪晶带区域也可发生马氏体转变；3道次变形后，具有明显特征的孪晶已很少，此后继续进行剪切变形，孪晶碎化组织(含马氏体)和奥氏体剪切滑移带(含碎化晶粒)的变形以剪切滑移方式进行，当奥氏体的滑移遇到阻力时，可局部形成局部形变孪晶来协调变形；随变形道次的增加，马氏体转变也越多，在多次剪切以及道次中的交叉滑移作用下，马氏体板条逐渐被高密度位错墙分割而碎化成细小的晶粒；8道次变形后，可获得60~230 nm的等轴晶粒。     

Viscoplastic model for the strain resistance of 08Kh18N10T steel at a hot-deformation temperature

The viscoplastic model of the strain resistance of a metal developed earlier is shown to be applied to austenitic corrosion-resistant 08Kh18N10T steel with the fcc lattice at a hot-deformation temperature of 1150°C. Metallographic examination supports dynamic recrystallization occurring at this deformation temperature.     

Low cycle fatigue of a high strength metastable austenitic steel

The low cycle fatigue (LCF) behavior of a high strength, metastable austenitic steel called TRIP steel has been studied. High strain LCF experiments on cylindrical, well-polished specimens under diametral strain control were carried out. To study the effect of a mixed austenite-martensite matrix, LCF tests were also done on the TRIP steel after inducing significant amounts of martensite in the austenite matrix by means of a very high unidirectional prestrain. To establish the role played by the martensite transformation, tests were also run above the M_D. The amount of martensite induced was magnetically measured by means of a “permeameter” built specifically for this purpose. It was found that the LCF life of the TRIP steel, both at room temperature (in the presence of martensitic transformation) and at 200°C (in the absence of the transformation), was related to the plastic strain range, ε_PR, by the Manson-Coffin law. Either cyclic hardening or softening occurred at room temperature, depending primarily upon the plastic strain range used in cycling. Hardening was observed below 3 pct plastic strain range. For LCF tests at 200°C, cyclic softening was observed in all cases. The hardening and softening behavior has been found to depend on the martensitic transformation taking place in these steels during cycling. However, the LCF life correlated best to the percent reduction in area, independent of the extent of the martensite transformation.     

Nanograined/Ultrafine-Grained Structure and Tensile Deformation Behavior of Shear Phase Reversion-Induced 301 Austenitic Stainless Steel

We describe here an electron microscopy study of shear reversion-induced nanograined/ultrafine-grained (NG/UFG) structure and evolution of tensile strained microstructure in metastable type 301 austenitic stainless steel. The NG/UFG structure with grain size in the range of 200 to 500 nm was obtained by severe cold deformation and controlled annealing in the narrow temperature range of 973 to 1073 K (700 to 800 °C). The different stages of annealing involve the following: (a) transformation of strain-induced martensite to highly dislocated lath-type austenite, (b) formation of dislocation-cell structure and transformation to recovered austenite structure with defect-free subgrains, and (c) coalescence of subgrains to form a NG/UFG structure concomitant with a completely recrystallized structure, and consistent with martensitic shear-type phase reversion mechanism. The optimized cold working and annealing treatment resulted in NG/UFG material with a high yield strength (~1000 MPa) and high ductility (~30 pct) combination. Multiple deformation mechanisms were identified from postmortem electron microscopy examination of tensile strained NG/UFG 301 austenitic stainless steel and include dislocation glide and twinning. The evidence of heterogeneous nucleation of overlapping stacking faults and partial dislocations points toward deformation     

SEM Investigation of High‐Alloyed Austenitic Stainless Cast Steels With Varying Austenite Stability at Room Temperature and 100°C

The deformation mechanisms of high‐alloyed cast austenitic steels with 16% of chromium, 6% of manganese, and a nickel content of 3–9% were investigated by in situ and ex situ scanning electron microscopy. The austenite stability and the stacking fault energy were influenced by variation of the chemical composition as well as by changing deformation temperature (room temperature; RT and 100°C). The study shows that both an increase in austenite stability and stacking fault energy yield a significant change in the deformation mechanisms. Both increase of nickel content and increase in deformation temperature reduce the intensity of the martensitic phase transformation. Thus, the steel with low nickel content shows at RT pronounced formation of α′‐martensite. The steel with the highest nickel content, however, shows pronounced twinning.     

Strain Rate Dependent Flow Stress and Energy Absorption Behaviour of Cast CrMnNi TRIP/TWIP Steels

Modern steel developments often use additional deformation mechanisms like the deformation induced martensitic transformation (TRIP‐effect) and mechanical twinning (TWIP‐effect) to enhance elongation and strength. Three high‐alloyed cast CrMnNi‐steels with different austenite stabilities were examined. Dependent on the austenite stability, TRIP‐effect and TWIP‐effect were found. A low austenite stability causes a distinctive formation of deformation induced α'‐martensite and therefore a strong strain hardening. The increase of strain rate leads to an increase in yield strength and flow stress, but also to a counteractive adiabatic heating of the specimen. Dependent on the degree of deformation, low austenite stabilities and high strain rates lead to excellent values in specific energy absorption.     

Changes in transformation behaviour and microstructure of a CrV-spring steel by thermomechanical treatment

The effect of austenite deformation on the transformation behaviour was investigated on a CrV-spring steel with the major attention put on the martensitic transformation. In the first part, a small review is given on the relation between the state of austenite after hot deformation and its influence on the formation of martensite. In the laboratory tests, the second part of the paper, a conventional heat treatment (CHT) was compared with two types of austenite conditioning by thermomechanical treatment (TMT): TMT_R - with deformation above the recrystallization temperature ?_R leading to a fully recrystallized austenite and TMT_N- with deformation below ?_R with a not recrystallized but possibly polygonized austenite. For the laboratory tests, the hot deformation simulator Wumsi was employed. After quenching In oil, the martensite after TMT consisted of associations of many fine fragments with a smaller number of large acicular martensitic units than observed after CHT. In both TMT-variants small ferritic areas (< 1 μ m) could be revealed. Different behaviour of martensite during tempering at low temperatures was observed after CHT and TMT. It can be explained by reduced inherent stresses generated during martensitic transformation after TMT, presumably as a result of a better ability of deformed austenite to withstand the accommodation strain during martensitic transformation. This may have considerable consequences for the toughness properties of tempered martensite.     

High-plasticity heat-resistant 03Kh14G16N6Yu-type steels with heat-and deformation-resistant austenite

The structure and mechanical properties of 03Kh14G16N6Yu-type austenitic steels alloyed by molybdenum, tungsten, vanadium, and zirconium are studied after normalization at 1075°C and long-term holding at 500–700°C. The chemical composition of these steels ensures the resistance of their austenite to the martensitic transformation in the temperature range from 1200 to ?196°C and during cold plastic deformation at a reduction of up to 60%. The best combination of the mechanical and technological properties is achieved in a 03Kh15G17N6YuVF steel with 0.08% W and 0.12% V. Long-term (up to 1000 h) holdings at 550–750°C do not cause the precipitation of carbide, nitride, and intermetallic phases in this steel. The long-term strength of the 03Kh15G17N6YuVF steel at temperatures up to 650°C is comparable with and its plasticity and impact toughness are higher than those of high-nickel Kh16N9M2 and Kh16N12M2 steels, which are applied in the main parts of electric power installations.     

Structure and mechanical properties of corrosion-resistant high-nitrogen 04Kh22AG15N8M2F and 05Kh19AG10N7MFB steels after hot deformation

The structure and mechanical properties of corrosion-resistant high-nitrogen austenitic 04Kh22AG15N8M2F and 05Kh19AG10N7MFB steels are studied after hot rolling at 950 and 1100°C. The following specific features of the structure of hot-rolled 04Kh22AG15N8M2F steel are revealed: the presence of coarse grain-boundary precipitates of the molybdenum-rich ?? phase and its nonuniform distribution over the volume of austenite grains. The 05Kh19AG10N7MFB steel hot rolled at 950°C contains ultrafine carbonitrides particles and has the best combination of a high strength and a sufficient elasticity and impact toughness. The structures of the hot-rolled steels have no ferrite, martensite, and traces of recrystallized austenite grains.     

Effect of heat treatment on the structure and properties of a high-nitrogen austenitic corrosion-resistant 03Kh20AG11N7M2 steel

The structure and mechanical and corrosion properties of a high-strength austenitic 03Kh20AG11N7M2 steel after quenching and aging at 500 and 800°C are analyzed. The phase composition of the steel and the mechanism of the decomposition of austenite during heat treatment are studied by electron-probe microanalysis and transmission electron microscopy. This steel is thermally stable upon heating to 800°C for 1 h and is stable to the γ → α and γ → ɛ martensitic transformation during deformation up to tensile strains leading to fracture. The homogeneous decomposition of a supersaturated γ solid solution at 500°C leads to the formation of disperse CrN nitrides, which increase the strength of the steel and insignificantly decrease its plasticity. In this case, the stress corrosion cracking resistance slightly decreases and the passivation of the steel increases in an corrosive medium without loading.     

Effect of cold rolling on the structure and mechanical properties of austenitic corrosion-resistant 10Kh18N8D3BR steel

The effect of section rolling of austenitic corrosion-resistant 10Kh18N8D3BR steel at room temperature on its structure and mechanical properties is studied. During section rolling, the steel acquires a lamellar-type structure consisting of α′-martensite lamellas and retained austenite, and the fraction of α′ martensite increases to 70% at a true strain ? ≈ 4. In the initial state, the yield strength of the steel is 285MPa and the relative elongation is 60%. Cold plastic deformation to ? ≈ 0.4 increases the yield strength to 1010 MPa. Further deformation is accompanied by higher hardening of the steel: the yield strength increases to 2050 MPa at ? ≈ 4, and the relative elongation decreases to 2%.