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.     

The ductility of continuously-cast steel near the melting point—hot tearing

At temperatures near the melting point steels fail in a brittle manner. This brittle failure can lead to the formation of surface and internal cracks in continuously cast steel, during casting, if the steel is subject to a tensile strain. In this investigation the nature of the brittle failure has been considered for a wide range of continuously-cast steels, by examining the strength and ductility of the steels as a function of temperature, composition, and cast structure. The results show that for steels containing 0.05 to 0.12 pct C, brittle failure is due to incipient melting at grain boundaries at temperatures between approximately 40°C below the solidus and the solidus. The incipient melting is ascribed to solute or residual segregation, at the grain boundaries following extensive boundary migration. For steel containing approximately 0.16 pct C, with increasing test temperature brittle failure starts 70°C below the solidus. For steels containing 0.25 to 1.0 pct C brittle failure starts 40°C below the solidus over the entire carbon range. Failure due to melting alone occurs interdendritically at temperatures above the solidus. In general the melting or ductile-brittle transition temperatures are independent of the initial cast structure, or large increases in the solute or residual levels, other than carbon.     

Development of High Strength Plates with Low Yield Ratio by the Combination of TMCP and Inter‐Critical Quenching and Tempering

A new processing route of thermo‐mechanical processing (TMCP) followed by inter‐critical quenching and tempering (L‐T) was developed to produce 590MPa grade high strength plates based on a relatively lean composition of plain carbon manganese steels microalloyed with Nb, V and Ti. The effect of quenching temperatures on the evolution of microstructure and mechanical properties were investigated. The nano‐hardness measurements of martensite were performed with a nano‐indenter, which indicated that the fractions of as quenched and tempered martensite increased and their hardness values decreased with increasing quenching temperatures in the range from 760 °C to 810 °C. For both as quenched and tempered samples, ferrite grain sizes decreased with increasing quenching temperature in almost linear relationships. The yield strength increased with increasing the fraction of martensite while the tensile strength remained almost unchanged, leading to the increase of yielding ratio with increasing quenching temperatures. The optimum quenching temperature was determined to be around 760 °C in terms of strengths and yield ratio.     

Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler

The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).     

Investigation of Austenite-to-Ferrite Transformation in Ultralow and Low-Carbon Steel Using High-Speed Quenching Dilatometry and Thermokinetic Simulation

The isothermal austenite decomposition kinetics is studied in 0.004 wt pct C ultralow carbon (ULC) and 0.11 wt pct C low-carbon (LC) steel using high-speed quenching dilatometry. Standard samples of these steels are heated to austenitization temperatures of 1223 K and 1373 K (950 °C and 1100 °C) and then quenched to testing temperatures between 1163 K and 933 K (890 °C and 660 °C). The measured and calculated austenite-to-ferrite phase fractions are compared with dilatation values to analyze the ferrite nucleation and growth conditions during austenite decomposition. Ferrite evolution profiles are assessed to investigate the underlying growth kinetics. The analysis in ULC steel shows regimes of partitionless, partitioning, and two-stage transformation kinetics. In contrast, LC steel shows only diffusion-controlled transformation kinetics. The experimental results are well reproduced with thermokinetic calculations, thus supporting our interpretation of governing mechanisms during transformation.     

Scavenging additions of boron in low C – low Al steels

Scavenging of boron on nitrogen is almost complete on the hot strip for B/N ≥ 0.7–0.8. Nitrogen ageing does not appear and low coiling can be used (650°C). BN precipitates as polycrystalline aggregates (spherulites) and in association with other precipitates or inclusions. Solute carbon is somewhat reduced in B steels coiled at 650°C but a more important reduction is achieved when coiling at 750°C due to an Fe₂₃ (C, B)₆ precipitation in the optimum range 1.0 ≤ B/N ≤ 1.5. Yield stresses (YS) lower than 190 MPa are obtained for optimized compositions of low Al-B steels coiled at 650°C and continuously annealed (825°C – 60 s – cooling rate: 50°C/s). Further softening is achieved when coiling at 750°C (YS < 160 MPa). Lower ageing (≤ 20 MPa) of B steels coiled at 650°C is also for c.a. steels when compared to low Al grades; bake hardening remains at a high level (> 50 MPa). No ageing is observed in B steels coiled at 750°C of which the bake hardening is reduced (25 MPa). Deep drawability of B steels is improved at reduced C, Al, Mn contents and at higher coiling temperatures; a detrimental effect of a B excess is observed. Low nitrogen contents and reduced Al additions are needed to reach best B steel properties.     

Hot rolling and heat treatment of Ni−Cu−Cb(Nb) steel

Copper-containing steels can be susceptible to hot shortness and the properties of columbium (Nb) grain-refined steels are sensitive to processing conditions. Ni?Cu?Cb steel embodies both copper age-hardening and columbium grain refining; therefore, the effect of nickel on hot shortness and the effects of hot rolling and heat treatment on mechanical properties were examined with the 0.85 pct Ni-1.2 pct Cu-0.03 pct Cb, age hardenable steel. The nickel level of 0.7 to 1.0 pct in Ni?Cu?Cb steel is sufficient to prevent hot shortness due to copper. A suitable aging treatment is 1 hr at 1050°F while comparable properties can be obtained by aging for 4 hr at 975°F. Laboratory processing has shown that lower hot rolling finishing temperatures and greater reductions at the finishing temperature improve laboratory-made heats of this steel by lowering the ductile-to-brittle transition temperature and raising the yield strength. Lower soaking temperatures also result in lower impact transition temperatures but have no effect on strength.     

Microstructure Evolution during Recrystallization of Newly Developed Low Ni High Mn‐N Stainless Steels

Low cost stainless steels where nickel is replaced in a conventional Fe‐Cr‐Ni stainless steel by manganese and nitrogen were studied. In this work, three new steels based on the system (mass %) Fe‐18Cr‐15Mn‐2Ni‐2Mo‐XN were prepared and their microstructure after each treatment was evaluated by optical and scanning electron microscopy, and X‐ray diffraction. A good correlation between texture and microstructure evolution during annealing was established. A randomization of the texture during recrystallization of the austenite was observed. Recrystallization starts at temperatures above 850°C, and after annealing for 0.5 h at 900°C, the austenite is completely recrystallized, reaching the orientation density a value near 1. Precipitation of σ ‐ phase was observed in the samples annealed at temperatures ranging from 700 to 950°C.     

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.     

Correlation of microstructure and fracture toughness in two 4340 steels

A correlation is made of plane strain fracture toughness and microstructure in two steels corresponding to AISI 4340 composition. The steels were deoxidized with aluminum and titanium-aluminum additions, respectively. In the case of the aluminum killed steel, austenitizing at temperatures above 950 °C led to large austenite grain sizes, whereas in the titanium steel grain sizes were maintained below about 70 μm even after austenitizing at temperatures approaching 1200 °C. This allowed a comparison of variations in plane strain fracture toughness with austenitizing temperature between microstructures that underwent large increases in grain size and those that did not. The results are interpreted using a simple fracture model which indicated that particle spacing is of primary importance in controlling toughness. The overall observed phenomenology, however, is not explainable using simple models that essentially require that either critical stresses or critical strains be achieved over distances scaling with microstructure. This finding suggests that more detailed crack tip models than presently exist are required if the full effects of heat treatment are to be understood and explained. Formerly Graduate Student at Brown University     

Influence of thermomechanical treatments on the microstructure and mechanical properties of HSLA-100 steel plates

The influence of thermomechanical treatment (TMT), i.e., controlled rolling and direct quenching, as a function of rolling temperature and deformation on the microstructure and mechanical properties of HSLA-100 steel have been studied. The optical microstructure of the direct quenched (DQ) and tempered steel rooled at lower temperatures (800 °C and 900 °C) showed elongated and deformed grains, whereas complete equiaxed grains were visible after rolling at 1000 °C. The transmission electron microscope (TEM) microstructure of the 800 °C rooled DQ steel showed shorter, irregular, and closer martensite laths with extremely fine Cu and Nb(C,N) precipitates after tempering at 450 °C. The precipitates coarsened somewhat after tempering at 650 °C; the degree of coarsening was, however, less compared to that of the reheat-quenched (RQ) and tempered steel, indicating that the DQ steel was slightly more resistant to tempering. Similar to the RQ steel, at a 450 °C tempering condition, the DQ steel exhibited peak strength with extremely poor impact toughness. After tempering at 650 °C, the toughness of the DQ steel improved significantly, but at the expense of its strength. In general, the strength of the DQ and tempered steel was good and comparable to that of the RQ and tempered steel, although, its impact toughness was marginally less than the latter. The optimum combination of strength and toughness in the DQ steels was achieved after 900 °C rolling with 50 pct deformation, followed by direct quenching and tempering at 650 °C (yield strength (YS)=903 MPa, ultimate tensile strength (UTS)=928 MPa, and Charpy V-notch (CVN) strength=143 J at −85 °C).     

Dependence of Accelerated Creep Rate at 580 °C and Room Temperature Yield Stress for Two Creep Resistant Steels

Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 °C or 8760 h at 750 °C to study the growth and redistribution of carbide precipitates in martensite. On specimens annealed for a different time, yield stress at room temperature and accelerated creep rate at 580 °C were determined. With increasing yield stress in the range from 350 to 650 MPa the accelerated creep rate decreased continuously by about 2 orders of magnitude from 8·10^?7 s^?1 to 5·10^?9 s^?1. For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20.     

Effect of Annealing and Hot Rolling on Grain Boundary Segregation of Arsenic in an Mn-Steel Microalloyed by Ti,Cr and Nb

Steel samples with size of 10 mm×10 mm×5 mm were cut down from a hot-rolled Mn-steel microalloyed by Ti, Cr and Nb and produced by compact strip production (CSP) technology. The samples were annealed at 950 °C for different time firstly, and then hot rolled or cooled in the air, in water and in furnace, respectively. Auger electron spectroscopy (AES) was used to study the effects of annealing and hot rolling on the segregation of arsenic at grain boundary (GB) in the steel. The results indicated that a higher content of arsenic was found at grain boundaries than in the matrix when the steel was annealed at 950 °C for 2 h and then cooled to room temperature by water quenching. But the content of arsenic at grain boundaries was similar to that in the matrix when the steel was annealed at 950 °C for 2 h and then cooled to room temperature by furnace cooling. A longer holding time, such as 12 h and 36 h at 950 °C, resulted in a similar arsenic content at grain boundaries to that in the matrix of the steels. Hot rolling led to a similar content of arsenic at grain boundaries and within grains in the steels as well.     

Hot Toughness of Continuously Cast Steel Slabs during Reheating

Laboratory hot tensile tests on specimens taken from continuously cast steel slabs are executed by means of a Gleeble apparatus to investigate the ductility and hot toughness of steels during reheating of as cast conditions. The reduction in area at fracture and the fracture energy for 10 different heats with the carbon content in the range from 0.003 to 0.521 mass% and some variations in Nb, Cr and N are presented in the temperature range from room temperature up to 1025°C. Ductility minima regarding toughness are identified around 300°C and 650°C. Relationships of the reduction in area at fracture and of the specific fracture energy with the relevant elements of the steel composition are established. Equivalent carbon concentrations are defined which take the N concentration into account for the 300°C embrittlement and additionally the micro‐alloying element Nb for the 650°C ductility minimum. Limits for the reduction in area values and for the specific fracture energy are proposed to validate the crack sensitivity of as cast carbon steel slabs.     

Effect of alloying elements on metadynamic recrystallization in HSLA steels

By means of interrupted torsion tests, the kinetics of metadynamic recrystallization (MDRX) were studied in a Mo, a Nb, and a Ti microalloyed steel at temperatures ranging from 850 °C to 1000 °C and strain rates from 0.02 to 2 s¹. Quenches were also performed after full MDRX. In contrast to the case of static recrystallization (SRX), the kinetics of MDRX are shown to be highly sensitive to a change of an order of magnitude in strain rate and are relatively insensitive to temperature changes within the range of values applicable to industrial hot-rolling practice. A similar algebraic dependence of the MDRX grain size on strain rate and temperature was found in the three steels. The kinetics of MDRX were slower in the Nb than in the Mo steel, and those of the Ti steel were slower than in the Nb and Mo steels. Above 900 °C and 950 °C, the retardation of MDRX in the Nb and Ti steels, respectively, is due to solute drag. Models predicting the start time for Nb and Ti carbonitride precipitation showed that MDRX is delayed below these temperatures by this mechanism. Comparison of the MDRX and precipitation start times in the Nb steel indicated that a temperature of “no-MDRX” could not be defined, in contrast to the well-definedT _nr (no recrystallization temperature) of SRX. By means of torsion simulations composed of multiple interruptions, it is shown that MDRX is retarded decreasingly as the accumulated strain is increased. This appears to be due to the promotion of precipitate coarsening by the continuing deformation.     

Effect of silicon on the wear resistance of high-carbon steels with the structures of isothermal decomposition of austenite during friction and abrasive action

The hardness and wear resistance during sliding and abrasive friction of 80S2 (0.83% C, 1.66% Si) and U8 (0.83% C) steels subjected to the isothermal γ → α decomposition in the temperature range 330–650°C and additional 5-min annealing at 650°C are compared. The optimum decomposition temperature is found to be 550°C. At this temperature, fine lamellar pearlite with the maximum hardness and wear resistance as compared to other pearlitic and bainitic structures forms in the silicon steel. The silicon-alloyed fine lamellar pearlite of 80S2 steel is found to have high hardness and abrasive wear resistance as compared to the similar structure in plain U8 steel; however, this pearlite has no advantages in the wear resistance under conditions of sliding friction on a steel plate. Silicon alloying of the bainitic structures in the eutectoid steel leads to a noticeable decrease in the wear resistance during sliding friction and abrasive action. Friction oxidation is shown to negatively affect the abrasive wear resistance of the silicon steel.     

Mechanical properties of new stainless steels based on the system Fe‐30Mn‐5A1‐XCr‐0.5C

New stainless steels based on the system Fe‐30Mn‐5AI‐XCr‐0.5C (Cr mass contents of ≤ 9 %) were developed and evaluated as a replacement of conventional AISI 304 steel. The alloys were produced by induction melting and thermomechanically processed to obtain a fine equiaxed microstructure. A typical thermomechanical processing for AISI 300 austenitic stainless steels was used and included forging at 1200°C, rolling at 850 °C and final recrystallization at 1050 °C. A final fully austenitic microstructure with grains of about 150 μm in size was obtained in all the steels. Tensile tests at temperatures ranging from ‐196 to 400 °C showed similar results for the various alloys tested. In accordance with the values for the elongation to fracture, this temperature range was subdivided into three regions. In the temperature range of ‐196 °C to room temperature, elongation to fracture increases with decreasing temperature. At temperatures ranging from 100 to 300 °C, elongation to fracture increases with testing temperature and serrations on the stress‐strain curve were observed. Finally, higher testing temperatures were accompanied by a decrease in ductility. Examination of the microstructures after deformation led to the conclusion that mechanical twinning was the dominant mechanism of deformation at the tested temperatures.     

Low strain rate plasticity of continuously cast carbon steels

By a special plastometric method, the straightening operation of a vertically cast strand of two carbon steels was simulated. The temperature dependence of the low strain rate plasticity of both the steels examined is analogous although caused by very different reasons. As to the plain carbon steel, the amount of ferrite and its location is of prime importance, whereas in the case of eutectoid steel various modes of fracture and the influence of temperature on the relative plasticity of type II MnS inclusions are influencial. To avoid transverse cracking, it seems to be suitable to keep the strand at a rather high temperature (above 900 °C). Temperature cycling seems to be fundamental as to the structural changes taking place in the strand (grain size refining, ferrite fraction if any), whereas analogous influences of straining are more or less inexpressive. Of course, the results obtained are of quality only since the practical and laboratory conditions differ significantly. The knowledge obtained could be applied in regulating the intensity of strand cooling with the aim of lowering the waste portion and improving the surface quality of worked products.     

The workability of commercial and experimental 0.6 pct carbon low alloy steels in the temperature range of 650 °C to 870 °C

The torsional strength and ductility of commercial AlSI 4063 steel along with four 0.6 pct carbon, low alloy experimental steel compositions were determined with temperature. These parameters were then related to the workability of the steels. The influence of initial lamellar or spheroidal microstructures, as well as of vacuum or air-melt practices, were studied in the deformation temperature range of 650 °C to 870 °C and strain-rate range of 0.71 to 2.13 s⁻¹ The experimental steels showed increased ductility and lower peak flow stresses over the entire temperature range when compared to the commercial alloy. Lamellar microstructures resulted in higher maximum flow stresses and subsequent work softening in the ferritic regime. Initial carbide morphology did not influence the maximum flow stresses in the austenitic range. Improved ductility of the experimental steels over the entire working temperatures could possibly be attributed to the combination of a reduced amount of oxides and sulfides, reduced particleto-matrix decohesion, improved grain-boundary cohesion, or the ability to annihilate or heal microcracks which may form during deformation. Constitutive equations were developed for the ferritic and austenitic conditions with both spheroidized and lamellar carbides.     

The strength and ductility of continuouslycast steels above 800°C

The tensile strength and ductility of continuously cast steels have been determined for temperatures above 800°C. The results show that the maximum stress decreases progressively until close to the melting temperature, then drops to zero. The maximum stress is essentially independent of residual and solute concentrations, cast structure and prior heat treatment. Above 1250°C the steels are ductile to near the melting temperature. Below 1250°C the ductility decreases, the amount of decrease, and the temperature range over which the change occurs depending on composition, cast structure and heat treatment. The ductility decreases at higher temperatures with high sulfur and phosphorus levels in the steel. Increasing sulfur from 0.010 to 0.025 pct in laboratory cast steels markedly decreases the ductility. Small grain size also tends to reduce ductility. Preheating the steel to near its melting point prior to testing markedly reduces the ductility below 1250°C in most steels. It is postulated that this is due to local remelting of solute rich pockets of material which extend along the grain boundaries. The extended material is brittle at lower temperatures reducing the ductility of the steel.