Evaluation of Microstructural and Mechanical Behaviours of Tempered 2.25Cr–1Mo Steel Through Electromagnetic Characterization

The effect of tempering treatment has been investigated on water quenched P22 steel with the chemical composition of 0.13C, 0.24Si, 0.47Mn, 0.012P, 0.005S, 2.19Cr, 0.93Mo and balance Fe (all in wt%) within the temperature ranges of 650–900 °C. The microstructural, mechanical and magnetic properties of as-quenched and tempered steels have been investigated through optical and scanning electron microscopy, hardness and universal tensile testing, electromagnetic sensor (Magstar), respectively. The water quenched sample consists of fine martensitic structure with a hardness of 381 HV. With the progress of tempering, the martensite becomes coarse till 800 °C, decreasing the hardness of steel samples. The tempering at 700 °C results in martensite coarsening and precipitation of rod and globular shaped carbides; while a fraction of globular carbide is observed to increase in the matrix after 750 °C of tempering. Beyond 800 °C, the ferrite and bainite phases gradually form by replacing martensite, and the ferrite structure is prevalent after 900 °C. Due to microstructural changes, the magnetic properties are also affected as a function of tempering temperature. The coarsening of martensite causes the decrease in coercivity with increasing tempering temperature, leading to magnetic softening.     

Examination of Microstructure and Mechanical Properties of Austempered Ductile Iron (ADI) As Per Austempering Temperature and Time

Austempered ductile iron is a heat treated form of as-cast ductile iron. The heat treatment process-austempering, was developed with the intent of improving the strength and toughness of ferrous alloys. It offers a range of mechanical properties superior to those of other cast iron, and shows excellent economic competitiveness with steels and aluminum alloys. The main aim is to analyze the mechanical properties and microstructural characteristics of as-cast ductile iron austenitized at 900 °C for 90 min and afterward austempered over a range of temperatures to obtain distinctive microstructures. The samples were austempered for durations of 60, 90 and 180 min at each austempering temperature of 340, 360, 380, and 400 °C. The influence of these austempering temperatures and times on the microstructure and tensile properties were investigated at room temperature.     

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.     

Response of Phase Transformation Inducing Heat Treatments on Microstructure and Mechanical Properties of Reduced Activation Ferritic-Martensitic Steels of Varying Tungsten Contents

Microstructure and mechanical properties of 9Cr-W-0.06Ta Reduced Activation Ferritic-Martensitic (RAFM) steels having various tungsten contents ranging from 1 to 2 wt pct have been investigated on subjecting the steels to isothermal heat treatments for 5 minutes at temperatures ranging from 973 K to 1473 K (700 °C to 1200 °C) (below Ac₁ to above Ac₃) followed by oil quenching and tempering at 1033 K (760 °C) for 60 minutes. The steels possessed tempered martensite structure at all the heat-treated conditions. Prior-austenitic grain size of the steels was found to decrease on heating in the intercritical temperature range (between Ac₁ and Ac₃) and at temperatures just above the Ac₃ followed by increase at higher heating temperatures. All the steels suffered significant reduction in hardness, tensile, and creep strength on heating in the intercritical temperature range, and the reduction was less for steel having higher tungsten content. Strength of the steels increased on heating above Ac₃ and was higher for higher tungsten content. Transmission Electron Microscopy (TEM) investigations of the steels revealed coarsening of martensitic substructure and precipitates on heating in the intercritical temperature range, and the coarsening was relatively less for higher tungsten content steel, resulting in less reduction in tensile and creep strength on intercritical heating. Tensile and creep strengths of the steels at different microstructural conditions have been rationalized based on the estimated inter-barrier spacing to dislocation motion. The study revealed the uniqueness of inter-barrier spacing to dislocation motion in determining the strength of tempered martensitic steels subjected to different heat treatments.     

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Effects of Tungsten Addition on the Microstructure and Mechanical Properties of Microalloyed Forging Steels

In the current study, the effects of tungsten (W) addition on the microstructure, hardness, and room/low [223 K and 173 K (?50 °C and ?100 °C)] temperature tensile properties of microalloyed forging steels were systematically investigated. Four kinds of steel specimens were produced by varying the W additions (0, 0.1, 0.5, and 1 wt pct). The microstructure showed that the addition of W does not have any noticeable effect on the amount of precipitates. The precipitates in W-containing steels were all rich in W, and the W concentration in the precipitates increased with the increasing W content. The mean sizes of both austenite grains and precipitates decreased with the increasing W content. When the W content was equal to or less than 0.5 pct, the addition of W favored the formation of allotriomorphic ferrite, which subsequently promoted the development of acicular ferrite in the microalloyed forging steels. The results of mechanical tests indicated that W plays an important role in increasing the hardness and tensile strength. When the testing temperature was decreased, the tensile strength showed an increasing trend. Both the yield strength and the ultimate tensile strength obeyed an Arrhenius type of relation with respect to temperature. When the temperature was decreased from 223 K to 173 K (from ?50 °C to ?100 °C), a ductile-to-brittle transition (DBT) of the specimen with 1 pct W occurred. The addition of W favored a higher DBT temperature. From the microstructural and mechanical test results, it is demonstrated that the addition of 0.5 pct W results in the best combination of excellent room/low-temperature tensile strength and ductility.     

Reciprocating Sliding Wear Behavior of Newly Developed Bainitic Steels

The present work discusses the mechanical properties and wear behavior of newly developed bainitic rail steels with nominal composition of 0.71 pct C, 0.35 pct Si, 1.15 pct Mn, 0.59 pct Cr, 0.40 pct Cu, and 0.20 pct Ni (all in wt pct)). Isothermal transformation has been carried out at different time and temperatures for obtaining different bainitic morphologies. Linearly reciprocating sliding wear behavior of the steels has been studied and compared with that of the conventional pearlitic rail steel. Considerable improvement in mechanical properties of the bainitic steels has been noticed. The hardness of the bainitic steels increases with decrease in isothermal transformation temperature. It leads to enhancement of wear resistance of the bainitic steels compared to the conventional rail steel. Finally, it clearly draws correlation between mechanical properties, wear resistance, and microstructural variation of a series of bainitic rail steels.     

Influence of Warm Tempforming on Microstructure and Mechanical Properties in an Ultrahigh-Strength Medium-Carbon Low-Alloy Steel

A 0.4 pct C-2 pct Si-1 pct Cr-1 pct Mo steel was quenched and tempered at 773 K (500 °C) and deformed by multi-pass caliber rolling (i.e., warm tempforming). The microstructures and the mechanical properties of the warm tempformed steels were investigated as a function of the rolling reduction. At rolling reductions of more than 28 pct, not only extension of the martensite blocks and/or the packets in the rolling direction (RD) but also a grain subdivision became more significant, and an ultrafine elongated grain (UFEG) structure with a strong ??110??//RD fiber deformation texture was formed after 78 pct rolling. The tensile deformation behavior became significantly anisotropic in response to the evolution of UFEG structure. The longitudinal yield strength (??_y) of the quenched and tempered sample increased from 1480 to 1860 MPa through the 78 pct rolling, while the transverse ??_y leveled off at around 1600 MPa up to 28 pct rolling. The transverse true fracture stress was also markedly degraded in contrast to the longitudinal one. Charpy impact properties were enhanced at a rolling reduction of 52 pct or more. The 52 pct-rolled sample underwent a ductile-to-brittle transition in the temperature range from 333 K to 213 K (60 °C to ?60 °C), while the 78 pct-rolled sample showed an inverse temperature dependence of the impact toughness because of brittle delamination. The tensile and Charpy impact properties are discussed in association with the microstructural evolution.     

Variation in Mechanical Properties and Heterogeneity in Microstructure of High-Strength Ferritic Steel During Mill Trial

HS600 and HS800 are two new generation, high-strength advanced ferritic steels that find widespread application in automobiles. During commercial production of the same grades with different thicknesses, it has been found that mechanical properties like tensile strength and stretchability varied widely and became inconsistent. In the current endeavor, two different thicknesses have been chosen from a mill trial sample of HS600 and HS800. An in-depth structural characterization was carried out for all four alloys to explain the variation in their respective mechanical and shear punch properties. The carbon content was smaller and Ti + Mo quantity was higher in case of HS800 with respect to HS600. The microstructure of both steels consisted of the dispersion of (Ti,Mo)C in a ferrite matrix. The grain size of HS800 was little larger than HS600 due to an increased coiling temperature (CT) of the former in comparison to the latter. It was found that in case of same grade of steel with a different thickness, a variation in microstructure occurred due to change in strain, CT, and cooling rate. The strength and stretch formability of these two alloys were predominantly governed by a microalloyed carbide. In this respect, carbides with a size range above 5 nm were responsible for loosing coherency with ferrite matrix. In case of HS600, both ≤5 and >5-nm size (Ti,Mo)C precipitates shared a nearly equal fraction of microalloyed precipitates. However, for HS800, >5-nm size (Ti,Mo)C carbide was substantially higher than ≤5-nm size alloy carbides. The ultimate tensile strength and yield strength of HS800 was superior to that of HS600 owing to a higher quantity of microalloyed carbide with a decreased column width and interparticle distance. A higher degree of in-coherency of HS800 made the alloy prone to crack formation with low stretchability.     

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Effect of normalized cooling treatment on banded structure of gear steel 18CrNiMo7-6 was analyzed through metallographic microscope. Effects of banded structures with different degree on micro hardness uniformity, composition segregation, mechanical properties of test steels after quenching and tempering were analyzed through microhardness tester, scanning electron microscope, tensile testing machine, impact testing machine and metallographic microscope. The results show that, banded structure can be reduced to 1.5 degree after austenizing heat preservation at 930??, Strong wind cooling to 610??, furnace cooling to 400?? and then air cooling. But continuous rapid cooling can result in the formation of a large amount of bainite; Micro hardness range of test steels with different banded structure degree is between 30~35 HB after quenching at 850?? and tempering at 180??. Hardness uniformity and anisotropy of mechanical properties are comparable.     

冷却速度对高强度低合金钢组织和性能的影响

采用热模拟试验机、拉伸试验机以及光学显微镜等设备,根据CCT曲线和拉伸实验的结果,研究了冷却速度对700MPa级超级钢组织和性能的影响。在实验室热轧实验的基础上,比较了500MPa和700MPa两个强度级别HSLA钢的组织和性能随冷却速度的变化情况。结果表明,冷却速度对两种钢的影响不同：冷却速度增大时,500MPa级钢的晶粒明显变细,而700MPa级钢的晶粒尺寸变化不大。     

Transformation strengthening by thermomechanical treatments in C-Mn-Ni-Nb steels

The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation mechanism associated with the austenite → ferrite transformation caused by heavy deformation in the austenite range. Formerly Research Engineer with the Steel Products Division, Research Institute of Industrial Science and Technology     

Evolution of Phases and Mechanical Properties of Thermomechanically Processed Ultra High Strength Steels

The present study concerns the development of two low carbon microalloyed ultra high strength steels on a pilot scale. This recent endeavour has been made towards the reduction of weight by achieving high strength to weight ratio together with improved weldability for the various prospective high performance defence applications such as explosive ammunition, gun barrel, missile skins, light-weight military bridges etc. These steels were thermomechanically processed and finished at different finish rolling temperatures followed by water quenching. Variation in microstructure and mechanically properties at different finished rolling temperatures was studied. The experimentally determined continuous cooling transformation diagrams have revealed that adequate hardenability is achievable in these steels usually at a cooling rate >5 °C/s. Lath martensite along with the microalloy (Ti, Nb) CN precipitate particles are the characteristic microstructural feature of the investigated steels. The high strength value obtained in the present steels is due to the accumulated contribution of fine grained pan-caked austenite, highly dislocated lath martensite along with the presence of tiny precipitates of microalloy carbide/carbonitride and Cu rich precipitates. The good combination of strength (1,364–1,538 MPa) and ductility (11–16 %) has been achieved for the selected range of finish rolling temperature. The Charpy impact toughness values (30–80 J) reveal approximately consistent fall with the lowering of testing temperature.     

Effect of Annealing Temperature on Microstructure and Mechanical Properties of Bulk 316L Stainless Steel with Nano- and Micro-crystalline Dual Phases

Microstructures and mechanical properties of 316L stainless steels with dual phases austenite prepared by an aluminothermic reaction casting were explored. It is found that the steels consist of nano- and micro-crystalline austenite phases, a little δ ferrite and contaminations. Before and after annealing at 1073 K and 1273 K (800 °C and 1000 °C), average grain sizes of the nanocrystalline austenite phase are about 32, 31, 38 nm, respectively. Tensile strength increases first from 371 to 640 MPa and then decreases to 454 MPa. However, elongation ratio increases gradually from 16 to 23 and then 31 pct after annealing. The results illustrate that the steel after annealing at 1073 K (800 °C) has better properties, also indicating that combination of dual nano- and micro-crystalline austenite phase is conductive to improving tensile properties of materials.     

Microstructure Evolution and Age-Hardening Behavior of Microalloyed Austenitic Fe-30Mn-9Al-0.9C Light-Weight Steels

The aging behavior and mechanical properties of microalloyed austenitic Fe-30Mn-9Al-0.9C light-weight steels were investigated through transmission electron microscopy analysis, electron backscatter diffraction analysis, tensile tests, and Vickers hardness tests. The base steels were aged at 823 K (550 °C) for up to 10,000 minutes. The true stress–strain responses of solution-treated samples before aging showed that the addition of Nb and/or V improved the strength by grain refinement and precipitation hardening. During the process of tensile deformation, the strain-hardening rate of Fe-30Mn-9Al-0.9C steel steadily increased due to the microband-induced plasticity (MBIP) from the onset of plastic deformation to ε = 25 pct, while such behavior was weakened and not observed in Nb- and/or V-added steels despite MBIP. In the early stage of aging, the Vickers hardness gradually increased with an increase in the aging time due to the precipitation of κ-carbide of (Fe,Mn)₃AlC and remained stagnant between the aging times of 1000 and 3000 minutes. The hardness increased again after 3000 minutes due to the formation of ferrite and brittle β-Mn.     

High Temperature Strengthening in 12Cr-W-Mo Steels by Controlling the Formation of Delta Ferrite

Novel 12Cr-W-Mo-Co heat resistance steels (HRSs) with excellent mechanical properties have been developed for ultra-supercritical (USC) applications above 923 K (650 °C). The thermal analysis of the present steels indicates that the remelting temperature of secondary phases is increased by Co alloying, resulting in the improvement of microstructural stability. Delta ferrite in these HRSs is completely suppressed as the content of Co is increased up to 5 pct. The room temperature tensile strength (TS), yield strength (YS), and the elongation (EL) of the HRS with 5 pct Co reach 887.9, 652.6 MPa, and 21.07 pct, respectively. At 948 K (675 °C), the TS and YS of the HRS with 5 pct Co attain 360 and 290 MPa, respectively, which are higher than those of T/P122 steel by 27.4 and 22.1 pct, respectively. TEM study of the microstructure confirmed that the strengthening effects for these 12Cr-W-Mo-Co HRSs are attributed to the suppression of delta ferrite, the formation of fine martensitic laths with substructure, dislocation networks and walls, and the precipitation of second nanoscale phases.     

The Effect of Cooling Rate,and Cool Deformation Through Strain-Induced Transformation,on Microstructural Evolution and Mechanical Properties of Microalloyed Steels

In this article, a detailed study was conducted to evaluate the microstructural evolution and mechanical properties of microalloyed steels processed by thermomechanical schedules incorporating cool deformation. Cool deformation was incorporated into a full scale simulation of hot rolling, and the effect of prior austenite conditioning on the cool deformability of microalloyed steels was investigated. As well, the effect of varying cooling rate, from the end of the finishing stage to the cool deformation temperature, 673 K (400 °C), on mechanical properties and microstructural evolution was studied. Transmission electron microscopy (TEM) analysis, in particular for Nb containing steels, was also conducted for the precipitation evaluation. Results show that cool deformation greatly improves the strength of microalloyed steels. Of the several mechanisms identified, such as work hardening, precipitation, grain refinement, and strain-induced transformation (SIT) of retained austenite, SIT was proposed, for the first time in microalloyed steels, to be a significant factor for strengthening due to the deformation in ferrite. Results also show that the effect of precipitation in ferrite for the Nb bearing steels is greatly overshadowed by SIT at room temperature.     

高锰奥氏体TRIP/TWIP钢的组织和力学性能

研究了两种不同锰含量的高锰奥氏体钢在室温拉伸变形过程中力学性能和组织的变化.结果表明,随着钢中锰含量的变化,实验钢在流变应力的作用下出现相变诱导塑性的TRIP效应和孪晶诱导塑性的TWIP效应.在1×10-3 s-1的初始应变速率条件下,锰的质量分数为23.8%的实验钢可达到666 MPa的抗拉强度和67%的伸长率,而锰的质量分数为33%的实验钢可达到540 MPa的抗拉强度和97%的伸长率.并且在10-3～10-1 s-1的初始应变速率范围内,实验钢的抗拉强度对于流变应力不敏感,而实验钢的塑性则表现出一定的应变速率敏感性.由于该钢具有较好的综合力学性能,有望作为新一代高强度、高塑性汽车用钢.     

Microstructure and Tensile Properties in Dual Phase and Trip Steels

Advanced high‐strength steels, like dual phase and TRIP steels, have gained much interest for automotive application. The complex microstructures in dual phase steels, and even more critical, the metastable microstructure in TRIP steels, do not follow the well‐established traditional microstructure‐property relationships for deep drawing steels. The volume fraction of the different phases, the phase distribution, and the stability of metastable phases influence significantly the forming potential. This paper discusses the correlation between different microstructural features and the mechanical properties. The tensile test properties of dual phase steels are governed by the martensite volume fraction, the martensite hardness and to a much smaller extent the martensite island diameter. Both in dual phase and more pronounced in TRIP steels the retained austenite content plays a vital role in determining the formability. The stability of the retained austenite can be described by different methods, it needs to be adjusted according to the forming temperature and the type and amount of strain. In general, multiphase steels require a very strict microstructure control in order to develop predictable forming behaviour.     

Effect of Welding Parameters on Microstructure,Thermal, and Mechanical Properties of Friction-Stir Welded Joints of AA7075-T6 Aluminum Alloy

A high-strength Al-Zn-Mg-Cu alloy AA7075-T6 was friction-stir welded with various process parameter combinations incorporating the design of the experiment to investigate the effect of welding parameters on the microstructure and mechanical properties. A three-factors, five-level central composition design (CCD) has been used to minimize the number of experimental conditions. The friction-stir welding parameters have significant influence on the heat input and temperature profile, which in turn regulates the microstructural and mechanical properties of the joints. The weld thermal cycles and transverse distribution of microhardness of the weld joints were measured, and the tensile properties were tested. The fracture surfaces of tensile specimens were observed by a scanning electron microscope (SEM), and the formation of friction-stir processing zone has been analyzed macroscopically. Also, an equation was derived to predict the final microhardness and tensile properties of the joints, and statistical tools are used to develop the relationships. The results show that the peak temperature during welding of all the joints was up to 713 K (440 °C), which indicates the key role of the tool shoulder diameter in deciding the maximum temperature. From this investigation, it was found that the joint fabricated at a rotational speed of 1050 rpm, welding speed of 100 mm/min, and shoulder diameter of 14 mm exhibited higher mechanical properties compared to the other fabricated joints.