Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.     

Austenitic Nickel- and Manganese-Free Fe-15Cr-1Mo-0.4N-0.3C Steel: Tensile Behavior and Deformation-Induced Processes between 298 K and 503 K (25 °C and 230 °C)

The high-temperature austenite phase of a high-interstitial Mn- and Ni-free stainless steel was stabilized at room temperature by the full dissolution of precipitates after solution annealing at 1523 K (1250 °C). The austenitic steel was subsequently tensile-tested in the temperature range of 298 K to 503 K (25 °C to 230 °C). Tensile elongation progressively enhanced at higher tensile test temperatures and reached 79 pct at 503 K (230 °C). The enhancement at higher temperatures of tensile ductility was attributed to the increased mechanical stability of austenite and the delayed formation of deformation-induced martensite. Microstructural examinations after tensile deformation at 433 K (160 °C) and 503 K (230 °C) revealed the presence of a high density of planar glide features, most noticeably deformation twins. Furthermore, the deformation twin to deformation-induced martensite transformation was observed at these temperatures. The results confirm that the high tensile ductility of conventional Fe-Cr-Ni and Fe-Cr-Ni-Mn austenitic stainless steels may be similarly reproduced in Ni- and Mn-free high-interstitial stainless steels solution annealed at sufficiently high temperatures. The tensile ductility of the alloy was found to deteriorate with decarburization and denitriding processes during heat treatment which contributed to the formation of martensite in an outermost rim of tensile specimens.     

Recrystallization Behavior of a Heavily Deformed Austenitic Stainless Steel During Iterative Type Annealing

The study describes evolution of the recrystallization microstructure in an austenitic stainless steel during iterative or repetitive type annealing process. The starting heavily cold deformed microstructure consisted of a dual phase structure i.e., strain-induced martensite (SIM) (43 pct in volume) and heavily deformed large grained retained austenite. Recrystallization behavior was compared with Johnson Mehl Avrami and Kolmogorov model. Early annealing iterations led to reversion of SIM to reversed austenite. The microstructure changes observed in the retained austenite and in the reverted austenite were mapped by electron backscatter diffraction technique and transmission electron microscope. The reversed austenite was characterized by a fine polygonal substructure consisting of low-angle grain boundaries. With an increasing number of annealing repetitions, these boundaries were gradually replaced by high-angle grain boundaries and recrystallized into ultrafine-grained microstructure. On the other hand, recrystallization of retained austenite grains was sluggish in nature. Progress of recrystallization in these grains was found to take place by a gradual evolution of subgrains and their subsequent transformation into fine grains. The observed recrystallization characteristics suggest continuous recrystallization type process. The analysis provided basic insight into the recrystallization mechanisms that enable the processing of ultrafine-grained fcc steels by iterative type annealing. Tensile properties of the processed material showed a good combination of strength and ductility.     

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     

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.     

Heat Treatment of Thixo-Formed Hypereutectic X210CrW12 Tool Steel

Steel is a particularly challenging material to semisolid process because of the high temperatures involved and the potential for surface oxidation. Hot-rolled X210CrW12 tool steel was applied as a feedstock for thixoforming. The samples were heated up to 1525?K (1250?°C) to obtain 30?pct of the liquid phase. They were pressed in the semisolid state into a die preheated up to 473?K (200?°C) using a device based on a high-pressure die casting machine. As a result, a series of main bucket tooth thixo-casts for a mining combine was obtained. The microstructure of the thixo-cast consisted of austenite globular grains (average grain size 46 ??m) surrounded by a eutectic mixture (ferrite, austenite, and M₇C₃ carbides). The average hardness of primary austenite grains was 470?HV_0.02 and that of eutectic 551?HV_0.02. The X-ray analysis confirmed the presence of 11.8?pct ??-Fe, 82.4?pct ??-Fe, and 5.8?pct M₇C₃ carbides in the thixo-cast samples. Thermal and dilatometric effects were registered in the solid state, and the analysis of curves enabled the determination of characteristic temperatures of heat treatment: 503?K, 598?K, 693?K, 798?K, 828?K, 903?K, and 953?K (230?°C, 325?°C, 420?°C, 525?°C, 555?°C, 630?°C, 680?°C). The thixo-casts were annealed at these temperatures for 2?hours. During annealing in the temperature range 503?K to 693?K (230?°C to 420?°C), the hardness of primary globular grains continuously decreased down to 385HV_0.02. The X-ray diffraction showed a slight shift of peaks responsible for the tension release. Moreover, after the treatment at 693?K (420?°C), an additional peak from precipitated carbides was observed in the X-ray diffraction. Thin plates of perlite (average hardness 820?HV_0.02) with carbide precipitates appeared at the boundaries of globular grains at 798?K (525?°C). They occupied 17?pct of the grain area. Plates of martensite were found in the center of grains, while the retained austenite was observed among them (average hardness of center grains was 512?HV_0.02). A nearly complete decomposition of metastable austenite was achieved after tempering at 828?K (555?°C) due to prevailing lamellar pearlite structure starting at grain boundaries and the martensite located in the center of the grains. The X-ray analysis confirmed the presence of 3.4?pct ??-Fe, 84.6?pct ??-Fe, and 12?pct M₇C₃ carbides. The dilatometric analysis showed that the transformation of metastable austenite into martensite took place during cooling from 828?K (555?°C). The additional annealing at 523?K (250?°C) for 2?hours after heat treatment at 828?K (555?°C) caused the precipitation of carbides from the martensite. After tempering at 903?K (630?°C), the thixo-cast microstructure showed globular grains consisting mainly of thick lamellar perlite of the average hardness 555?HV_0.02.     

Effect of Nitrogen Content on Grain Refinement and Mechanical Properties of a Reversion-Treated Ni-Free 18Cr-12Mn Austenitic Stainless Steel

Martensite reversion treatment was utilized to obtain ultrafine grain size in Fe-18Cr-12Mn-N stainless steels containing 0 to 0.44 wt pct N. This was achieved by cold rolling to 80 pct reduction followed by reversion annealing at temperatures between 973 K and 1173 K (700 °C and 900 °C) for 1 to 10⁴ seconds. The microstructural evolution was characterized using both transmission and scanning electron microscopes, and mechanical properties were evaluated using hardness and tensile tests. The steel without nitrogen had a duplex ferritic-austenitic structure and the grain size refinement remained inefficient. The finest austenitic microstructure was achieved in the steels with 0.25 and 0.36 wt pct N following annealing at 1173 K (900 °C) for 100 seconds, resulting in average grain sizes of about 0.240 ± 0.117 and 0.217 ± 0.73 µm, respectively. Nano-size Cr₂N precipitates observed in the microstructure were responsible for retarding the grain growth. The reversion mechanism was found to be diffusion controlled in the N-free steel and shear controlled in the N-containing steels. Due to a low fraction of strain-induced martensite in cold rolled condition, the 0.44 wt pct N steel displayed relatively non-uniform, micron-scale grain structure after the same reversion treatment, but it still exhibited superior mechanical properties with a yield strength of 1324 MPa, tensile strength of 1467 MPa, and total elongation of 17 pct. While the high yield strength can be attributed to strengthening by nitrogen alloying, dislocation hardening, and slight grain refinement, the moderate strain-induced martensitic transformation taking place during tensile straining was responsible for enhancement in tensile strength and elongation.     

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo,and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M₂₃C₆ carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.     

Interrelations of cooling rate,microstructure, and mechanical properties in four HSLA steels

The present study was carried out on four steels containing 0.1 pct C-1.5 pct Mn-0.003 pct B* in common, with additions of 1 pct Cr, 0.5 pct Mo, 0.25 pct Mo + 1 pct Cr, 0.2 pct Ti + 1 pct Cr. They were designated, accordingly, as Cr, Mo, Mo-Cr, and Cr-Ti steels. All the steels exhibited a complete lath martensite microstructure with thin interlaths of retained austenite (≈0.05 pct) in the quenched condition. The normalized microstructures, granular bainite, contained massive areas of ferrite and granules of bainite laths. Both microconstituents contained a fine dispersion of cementite particles (size ≈50 Å) together with high dislocation densities. A mechanism explaining their for-mation has been given. The Cr steel, due to its low hardenability, showed in addition polygonal ferrite in the neighborhood of the so-called M-A constituent (twinned martensite and/or austenite). The annealed microstructure (using a cooling rate of 0.033 °C s^?1) of the Cr steel consisted of coarse ferrite-pearlite. Addition of 0.2 pct Ti to the Cr steel markedly refined the structure, whereas an addition of 0.25 pct Mo altered the microstructure to ferrite-lower bainite. In the 0.5 pct Mo steel, polygonal ferrite was found to be completely missing. The mechanical properties of the four steels after quenching, normalizing, and annealing were investigatedvia hardness and tensile test mea-surements. An empirical equation, relating the ultimate tensile strength to the steel composition, for steels that had granular bainite microstructures in the normalized condition, was proposed. The fracture surfaces exhibited cleavage and variable-size dimples depending on the microstructure and steel composition.     

Crack paths,microstructure, and fatigue crack growth in annealed and cold-rolled AISI 304 stainless steels

To assist in the understanding of micromechanisms for corrosion fatigue crack growth in metastable austenitic steels, the relationships between the crack paths and the underlying microstructure were investigated for annealed and cold-rolled (CR) 304 stainless steels that had been tested in a deaerated 3.5 pct NaCl solution, air, and vacuum. Corrosion fatigue in the deleterious environments (3.5 pct NaCl and air) was brittle and occurred primarily by {001}_γ and other unidentified, quasi-cleavage (QC), accompanied by preferential cracking along {111}_γ twin and grain boundaries. In contrast, fatigue cracking in vacuum was ductile, fully transgranular, and noncrystallographic. Transformation to alpha prime (α′-) martensite by fatigue was found to be essentially complete in the CR steel, which contained ε-martensite, and in the annealed steel tested in vacuum, but was substantially less in the annealed steel tested in air and 3.5 pct NaCl solution. These results, taken in conjunction with the crack growth and electrochemical reaction data, support hydrogen embrittlement (HE) as the mechanism for corrosion fatigue crack growth in 304 stainless steels in 3.5 pct NaCl solution. Martensitic transformation appears not to be the only responsible factor for embrittlement. Other microstructural components, such as twin and grain boundaries, slip bands, and cold work-induced lattice defects, may play more important roles in enhancing crack growth rates.     

Influence of Temperature and Grain Size on Austenite Stability in Medium Manganese Steels

With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation.     

Effect of High-Pressure Torsion Processing and Annealing on Hydrogen Embrittlement of Type 304 Metastable Austenitic Stainless Steel

The effect of high-pressure torsion (HPT) and annealing on hydrogen embrittlement (HE) of a type 304 stainless steel was studied by metallographic characterization and tensile test after hydrogen gas charging. A volume fraction of ~78 pct of the austenite transformed to α′ martensite by the HPT processing at an equivalent strain of ~30. Annealing the HPT-processed specimen at a temperature of 873 K (600 °C) for 0.5 hours decreased the α′ martensite to ~31 pct with the average grain size reduced to ~0.43 μm through the reverse austenitic transformation. Hydrogen charge into the HPT-processed and the HPT+annealed specimens in the hydrogen content of ~10 to 20 ppm led to no severe HE but appeared in the solution-treated specimen. Especially the 873 K (600 °C) annealed specimen had the ~1.4 GPa tensile strength and the ~50 pct reduction of area (RA) despite the hydrogenation.     

节镍型奥氏体不锈钢14.0Cr-1.1Ni-9.2Mn-0.30Cu组织及性能

在13.36Cr-1.12Ni-11.2Mn节镍型奥氏体不锈钢基础上降Mn加Cu,优化开发14.0Cr-1.1Ni-9.2Mn-0.30Cu不锈钢,并研究不同化学成分奥氏体不锈钢固溶处理、冷轧退火后的金相组织、显微织构、力学性能及成形性能,分析了奥氏体稳定性和冷轧形变诱导马氏体相变的控制规律。结果表明：14.0Cr-1.1Ni-9.2Mn-0.30Cu与13.36Cr-1.12Ni-11.2Mn不锈钢力学性能相当,固溶处理、冷轧退火后奥氏体组织再结晶充分,冷轧平均晶粒尺寸分别为12.6μm、14.0μm,显微织构为铜织构{112}<111>;14.00Cr-1.1Ni-9.2Mn-0.30Cu不锈钢的杯突值和极限拉深比分别为16.15和0.46,优于13.36Cr-1.12Ni-11.2Mn不锈钢;14.0Cr-1.1Ni-9.2Mn-0.30Cu与13.36Cr-1.12Ni-11.2Mn钢Md30/50分别为87.02℃和83.55℃,Md30/50高,则奥氏体稳定性差,形变诱导马氏体量和冷轧变形抗力大,退火后硬度高。通过将冷轧退火工艺速度由10 m/min降低至9 m...     

Response to thermal treatment of a powder metallurgy Fe-0.8 % B-1.3 % C-1.6 %Cr alloy

Argon atomized powders of the Fe-0.8%B-1.3%C-1.6%Cr alloy in the rapid solidification condition as well as the consolidated material were characterized. Consolidation was carried out by extrusion at 1050°C and by hot isostatic pressing at 700°C. The microstructure of powder particles in the rapid solidification condition is dendritic and remains unchanged after hot isostatic pressing. This microstructure is gradually lost after annealing at temperatures above 900°C. The extruded material presents a fine and homogeneous microstructure constituted by small grains of laminar pearlite and spherical M₂₃(B,C)₆ borocarbides of 1.8 μm in size. A martensite microstructure with a uniform dispersion of M₃(B,C) borocementite particles is obtained after annealing at 1095°C 10 min and water quenching. For long austenitization time at 1095°C, a coalescence of borocementite as well as decarburization is observed in the steel. Dilatometric studies, between 20 and 400°C, of samples quenched from 1095°C show the presence of two contraction volume points at 160 and 310°C. At 160°C, the high carbon martensite decomposes into a two phase mixture of metastable carbides and low carbon martensite. At 310°C simultaneous transformation of austenite to ferrite and metastable carbides into cementite occurs.     

Influence of strain-induced phase transformation on the surface crystallographic texture in cold-rolled-and-aged austenitic stainless steel

Stainless steels (SSs) having a stable and metastable austenitic phase were studied to see the influence of strain-induced phase transformation in the metastable austenitic stainless steel on the evolution of texture during cold rolling and aging. AISI 304L and 316L SS plates were unidirectionally cold rolled up to a 90 pct reduction and aged at different aging temperatures. The strain-induced transformation of austenite to α′-martensite phase and the evolution of texture in both the phases were studied as a function of rolling reduction as well as aging temperature in the metastable 304L austenitic stainless steel. The X-ray diffraction (XRD) technique was employed to quantify the volume fractions and characterize the texture of austenite and martensite phases in the rolled and aged conditions. Results are compared with the texture evolution in the stable austenitic 316L SS.     

On the Significance of Nature of Strain-Induced Martensite on Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d < 100 nm), ultrafine (d ≈ 100 to 500 nm), and submicron (d ≈ 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)–high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocation-cell–type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell–type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell–type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperature-time annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell–type structure in lieu of lath-type facilitates NG/UFG structure with a high strength–high ductility combination. Interestingly, the yield strength follows the Hall–Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.