Ultrafine-grained Steel Research Articles

Effect of Crystal Grain Size in Stainless Steel on Cutting Process in Micromilling

The crystal grain size of the work material is relatively large compared to the removal depth in micro-scale cutting. Therefore, the micromilling requires the small crystal grains in the material to machine accurate products in a stable manner. The study investigates the effect of crystal grain size on the cutting process in micromilling. The crystal grains of stainless steel in this study are downsized to an average size of 1.5 μm by repetition of material forming and phase transformation. The milling processes of ultra fine-grained steels were compared with those of normal grain steels. The milling tests were performed to measure the cutting force and the surface quality. The force component ratio of the ultra fine-grained steel is higher than that of the normal grain steel. The shearing force decreases in cutting of the ultra fine-grained steel; meanwhile, the friction and/or the indentation forces increase. Burr formation can be reduced with the crystal grain size. In cutting of the normal grain steel, thrust component in the cutting force suddenly drops near the end of the grooves and a large burr is left on the edge of the groove.

Fatigue Properties of an Ultrafine-Grained Steel Processed by Warm Tempforming

Giga-cycle fatigue properties were investigated for an ultrafine-grained steel processed by warm tempforming. The fatigue properties of the tempformed (TF) steel were compared with conventional tempered martensite (QT) steel. Fatigue tests were carried out at frequencies of 20Hz and 20kHz, using servo-hydraulic and ultrasonic fatigue testing machines, respectively. Both theTF and the QT steels showed surface fracture in short life regions and fish-eye fracture in long life regions. The fish-eye fracture origins were oxide inclusions comprised of Si, Mg, Ca other than Al. Difference of fatigue strength was not notable between the TF and the QT steels, i.e. the fatigue strength of the TF steel was the same as that of the QT steel.

Hydrogen Effects on Ultrafine-Grained Steels Processed by High-Pressure Torsion

This article reviews the interactions between hydrogen and lattice defects produced by high-pressure torsion (HPT) process in ferritic iron and austenitic steels, and it shows the direction of development of hydrogen-resistant high-strength steels. In ferritic iron, grain boundaries as well as dislocations participate in hydrogen trapping. For austenitic steels, the contribution of grain boundaries to hydrogen trapping is negligible, while short-circuit diffusion occurs through them. Hydrogen trapping on precipitates caused by annealing after HPT processing depends on the affinity of hydrogen with constituting elements. In metastable austenitic steels, the presence of hydrogen suppresses deformation-induced martensitic transformation through enhanced slip planarity. If hydrogen-containing austenite transforms to martensite, excess hydrogen corresponding to a difference in the solubility between both phases is generated and diffuses out of the martensite, resulting in hydrogen concentration at the interphase. Deformation-induced martensite generated from ultrafine-grained austenite is used to simultaneously enhance strength and ductility without suffering from hydrogen embrittlement.

Estimations of the True Stress and True Strain until Just before Fracture by the Stepwise Tensile Test and Bridgman Equation for Various Metals and Alloys

True stress (·)­true strain (¾) relationships until just before fracture, i.e., the plastic deformation limit, were estimated by the stepwise tensile test and the Bridgman equation for various metals and alloys with different crystal structures. The estimated ·­¾ relationships were different from the nominal stress­strain curves including the conventional tensile properties. In the relationships between the true stress (·pdl) and true strain (¾pdl) at the plastic deformation limit, SUS304 and SUS329J4L indicated a better ·pdl­¾pdl balance. On the other hand, SUS329J4L, tempered martensite, and an ultrafine-grained steel showed superior results in the yield strength­¾pdl balance. The estimated ·­¾ relationship for the ultrafine-grained steel suggests that grain refinement strengthening can improve · and ¾ up until the plastic deformation limit. The value of ¾pdl became larger with increasing the reduction in area and a decrease in the fracture stress. The products of ·pdl and ¾pdl became larger with increasing work-hardening rate at the plastic deformation limit. [doi:10.2320/matertrans.MD201112]

Surface integrity analysis when milling ultrafine-grained steels

This paper quantifies the effects of milling conditions on surface integrity of ultrafine-grained steels. Cutting speed, feed rate and depth of cut were related to microhardness and microstructure of the workpiece beneath machined surface. Low-carbon alloyed steel with 10.8 µm (as-received) and 1.7 µm (ultrafine) grain sizes were end milled using the down-milling and dry condition in a CNC machining center. The results show ultrafine-grained workpiece preserves its surface integrity against cutting parameters more than the as-received material. Cutting speed increases the microhardness while depth of cut deepens the hardened layer of the as-received material. Also, deformations of microstructure following feed rate direction were observed in workpiece subsurface.

Prediction of the work-hardening exponent for ultrafine-grained steels

The work-hardening exponent is used to quantify the work-hardening of ultrafine-grained steels. The exponent for the types of steel with the carbon content below 0.22 wt% is proposed to be directly predicted by either yield strength or ferrite grain size. The exponent's value linearly decreases with increasing yield strength or D−1/2 (where D is the ferrite grain size), and becomes zero when yield strength is higher than 620 MPa (or D is less than 1 μm).

Study on the Microstructure and Properties of Resistance Spot Welding Joint for Ultra Fine-Grained Steel

In this study, the effect of welding parameter on the microstructure and properties of resistance spot welding joint for 400MPa ultra fine-grained steel is investigated. The results indicated that the microstructure of HAZ of ultra fine-grained steel joint consists of hardening organization: bainite, martensite, M-A constituent, i.e. while the fusion zone mainly consists of the coarse martensite. By applied forge-delay force during resistance spot welding will increase the nugget diameter, eliminate the shrinkage defect in nugget zone and improve the shearing strength of joints.

Effects of Al Addition on the Microstructure of Fe-Mn-Si Ultra-Fine Grain Steel Welded Heat Affected Zone

In order to improve toughness of the weld heat affected zone (HAZ) of ultra-fine grained (UFG) steel, a thermal simulation procedure of Fe-Mn-Si UFG steels with different Al addition was conducted and the corresponding microstructure was characterized and investigated. The results indicate that the maximum value on driving force of nucleation is ultra-fine grain steel with wtAl=0.6%, it has been explained in Nucleation kinetics that the grains are fine in UFG2. On the other hand, the bainite ferrite growth orientation has been increased. The toughness was remarkably enhanced in the steel welded heat affected zone. The formation of strie retained austenite and grain refinement of bainite ferrite in Fe-Mn-Si UFG steels with Al additions results in the increasing toughness of weld HAZ.

Processing, microstructure and corrosion behaviour of multi-axially forged low carbon steels

Increasing attention is being paid to processing bulk ultrafine grained (UFG) materials for structural applications. Along with mechanical properties, study of corrosion behaviour of these materials is important for them to find wider applications. In this work, low carbon steels that are extensively used for general purpose structural applications are studied. Low carbon steels were processed by using multi-axial forging, a technique of severe plastic deformation. Steel specimens were processed using six passes of warm multi-axial forging operations, to produce UFG structures. The present work further investigated the corrosion behaviour of these severely deformed UFG steels. Potentiodynamic tests, immersion tests, and salt spray tests were performed in chloride environments. In most environments employed in this study, UFG steels showed improved uniform corrosion behaviour compared to their coarse grained counterparts. The improvement in corrosion properties is explained based on microstructural evolution with respect to grain size, dislocation population and substructure development.

Physical Simulation of Controlled Rolling and Accelerated Cooling for Ultrafine-Grained Steel Plates

Physical Simulation of Controlled Rolling and Accelerated Cooling for Ultrafine-Grained Steel Plates The work shows the possibilities of obtaining ultrafine-grained ferrite-pearlite and ferrite-bainite structures in the process of controlled rolling of sheet metal using immediate accelerated cooling after the final pass. Low-carbon steel without micro-alloy additives was analyzed. The analysis was conducted using the Gleeble 3800 device with Hydrawedge II MCU module which enabled a multiple cycle of fast compression of the material. During the test, 10×15×20 mm rectangular parallelepiped specimens were deformed in flat anvils gaining the flat state of deformation in the zone of compression. Then the influence of the used scheme of deformation, cooling rate, time of break between the last deformation and the beginning of the accelerated cooling was analyzed as well as the temperature at the end of accelerated cooling of the structure and the mechanical properties of the final item.

Process Design Concepts for the Production of Ultrafine Grained Steels through Multi-Pass Warm Rolling: Bridging Science and Technology

Steel bars having a cross section of 18mm square with uniform distribution of ultrafine ferrite grains were produced through a multi-pass warm caliber rolling process in a 0.15%C-0.3%Si-1.5%Mn steel. The average ferrite grain sizes of 0.43μ m, 0.70μ m and 1.2 μ m were obtained in the isothermal warm caliber rolling processes at 773K, 823K and 873K respectively. Even though caliber rolling results in inhomogeneous strain distribution, multi-pass caliber rolling to large cumulated strains of 2 or 3 can be uniformly introduced in to the bar samples. Strain accumulation due to the multi-pass warm deformations was confirmed by comparing microstructural evolution through the multi-pass deformations with that of single pass deformation. The size of ultrafine grains formed through warm deformation was found to depend on the Zener-Hollomon parameter. The similarity of the microstructural evolution with single pass deformation reveals that the multi-pass warm deformation is an effective method to obtain ultrafine grained ferrite structure in bulk materials. It is proposed that compressive strain-Z parameter plots along with grain size-Z parameter plots help in establishing the processing conditions for obtaining products with a desired microstructure and grain size. Finally, such “processing maps” developed for a variety of materials serve useful purpose in bridging the science and technology of developing bulk ultrafine grained materials in semi-finished / finished products.

Microstructure and Properties of Ultrafine-Grained Steel Produced by Warm Compression of Martensite

Specimens of medium carbon steel were quenched and warm-compressed on a Gleeble 3500 Machine. The microstructure of the specimens was studied by using an optical microscope and a transmission electron microscopy. And the properties were investigated by using tensile tests and hardness tests. Results show that the starting microstructure is lath martensite with a small amount of flake martensite. After 50% compression at 550-650°C, ultrafine grains can be observed in the specimens. The microstructure of the specimens compressed at 600°C is equiaxed ultrafine ferrite grains + nano-carbides and a good combination of strength and ductility is obtained. The tensile strength and total elongation are 861MPa and 19.1%, respectively. The hardness is 233.81Hv.

하이브리드 용접에 의한 세립강 용접부의 기계적 성질에 관한 연구

The effect of Mn and Ti contents in filler wire on the microstructure and mechanical property of weld metal has been investigated after hybrid welding with ultra fine grained (UFG) steel. The microstructure and distribution of alloy compositions at the top region of weld zone were quite different with those at the bottom region after hybrid welding. The bottom region of weld zone contained higher Mn and Ti contents, and consequently the hardness of bottom region was higher than that of top region. With the increase of Mn and Ti contents in filler wire, the volume percent of acicular ferrite in weld metal decreased, and the weld zone showed higher hardness and better impact property.

Fatigue behavior of ultrafine-grained and coarse-grained Cr–Ni austenitic stainless steels

High-cycle bending fatigue behavior of an ultrafine-grained Cr–Ni Type 301LN austenitic stainless steel, obtained by reversion annealing, was investigated and compared to that of the conventional coarse-grained counterpart. The fatigue limit was significantly increased from 350 MPa to 630 MPa, reaching 59% of the tensile strength, a behavior attributed to grain refinement. Fatigue cycling resulted in hardness increments that were very different between these structures; in the coarse-grained steel 47%, but only 6–10% in the ultrafine-grained steel. The fatigue damage was observed to occur by grain boundary cracking in the ultrafine-grained steel, while mostly by slip band formation and crack propagation along these slip bands and grain boundaries in the coarse-grained counterpart. Dislocation structures observed reflected these pronounced differences in the cyclic behavior.

Pearlitic Transformations in an Ultrafine-Grained Hypereutectoid Steel

Common steels of both hypo- and hypereutectoid compositions form pearlitic or martensitic constituents when cooled from austenite temperature in air or in water. Here, we provide evidence that this is not the case in an ultrafine-grained hypereutectoid steel. In this system, when the grain size was reduced to a scale of 2 to 4 μm, normal pearlite could not be obtained when the steel was cooled in air; instead, nanometer-sized granular cementite and ferrite were formed in the eutectoid transformation. When the cooling rate was increased by quenching in saltwater, martensite was no longer formed; instead, fine lamellar pearlite was formed. This research indicates that these abnormal phase transformations were related to the rapid diffusion present in the ultrafine-grained steel, which changed the diffusive transformations.

Mechanical Behavior on Micro-compression Test in Ultra-low Carbon Steel Produced by High Pressure Torsion

ABSTRACTMechanical behavior of ultra-fine grained (UFG) steel fabricated by high pressure torsion (HPT) was investigated by micro-sized compression test of a micro-sized pillar with uniform dimensions (non-tapered, non-filleted) carefully fabricated by FIB milling. After HPT process, grains were elongated to shear direction and its grain size was decreased down to 300 nm in diameter with increasing strain amount. Compression test confirmed that the uniform elongations of ultra-fine grained materials were lower than 3% and do not depend on the grain size.

OS0614 温間テンプフォーミング処理を施した超微細粒鋼のギガサイクル疲労特性(OS6-3 材質および熱処理の影響,OS-6 金属材料の超高サイクル疲労と信頼性評価)

Giga-cycle fatigue properties were investigated for an ultrafine-grained steel processed by warm tempfonning. The fatigue properties were compared with conventional tempered martensite steel. Fatigue tests were carried out at frequencies of 30Hz and 20kHz, using servo-hydraulic and ultrasonic fatigue testing machines, respectively. Both tempformed and conventional steels showed surface fracture in short life regions and fish-eye fracture in long life regions. The fatigue strength at 10^7 cycles was higher in the tempformed steel than in the conventional, while it was not in case of the 10^<10> cycles fatigue strength. The fish-eye fracture origins showed different aspects between the two types of the steel. In case of the conventional steel, Al_2O_3 inclusions caused the fish-eye fracture, while in case of the tempformed steel, Mo carbides surrounded by Mo-rich layers were observed at the origins. Sizes of the Mo carbides were smaller, while those of the Mo-rich layers were identical to the Al_2O_3 inclusions of the conventional steel. Therefore, the Mo-rich layers, as well as the carbides, would probably cause the fish-eye fracture. This means that suppression of the Mo-rich layers could lead to more improvement of the giga-cycle fatigue strength of the tempformed steel.

Comparison of radiation-induced segregation in ultrafine-grained and conventional 316 austenitic stainless steels

Due to a high number density of grain boundaries acting as point defect sinks, ultrafine-grained materials are expected to be more resistant to irradiation damage. In this context, ultrafine-grained 316 austenitic stainless steel samples have been fabricated by high pressure torsion. Their behavior under ion irradiation has been studied using atom probe tomography. Results are compared with those obtained in an ion irradiated conventional coarse-grained steel. The comparison shows that the effects of irradiation are limited and that intragranular and intergranular features are smaller in the ultrafine-grained alloy. Using cluster dynamic modeling, results are interpreted by a higher annihilation of point defects at grain boundaries in the ultrafine-grained steel.

Ultrafine-Grained Steel Produced by Warm Rolling and Annealing of Lath Martensite

Steel plates with lath martensite microstructure were rolled up to 68% reduction at 673 K and then annealed at 473-973 K. The microstructure evolution was studied by using an optical microscope and a transmission electron microscopy. And the properties were investigated by using tensile tests and hardness tests. Results show that ultrafine grains + nano-carbides are obtained in the steel plates. The specimen annealed at 823 K has a good combination of strength and ductility. The tensile strength and total elongation are 1028 MPa and 7.2%, respectively. And the hardness is 338 Hv.

Microstructural Evolution of Ultrafine-Grained Steel during Tandem Rolling Process

Titanium microalloyed steel whose grain size is around 3μm has been developed by CSP process. Samples after different plastic deformation were obtained by suddenly stopping the six-stand finishing train during tandem rolling. Experimental results and analysis show that plastic deformation during tandem rolling plays a major role for grain refinement. With the successive severe strain, microstructure of the same stock during tandem rolling is further refined because of repeated recrystallization at higher temperature and strain accumulation at lower temperature. Technological features of CSP process, such as large reduction per pass, rapid cooling after rolling and high solidification and of thin slab, are important reasons for obtaining ultafine-grained structure. Besides these, chemical composition of titanium microalloyed steel has beneficial effects on grain refinement.