Microstructure and Mechanical Properties of a Nitride-Strengthened Reduced Activation Ferritic/Martensitic Steel

Nitride-strengthened reduced activation ferritic/martensitic (RAFM) steels are developed taking advantage of the high thermal stability of nitrides. In the current study, the microstructure and mechanical properties of a nitride-strengthened RAFM steel with improved composition were investigated. Fully martensitic microstructure with fine nitrides dispersion was achieved in the steel. In all, 1.4?pct Mn is sufficient to suppress delta ferrite and assure the steel of the full martensitic microstructure. Compared to Eurofer97, the steel showed similar strength at room temperature but higher strength at 873?K (600?°C). The steel exhibited very high impact toughness and a low ductile-to-brittle transition temperature (DBTT) of 243?K (?C30?°C), which could be further reduced by purification.     

Studies on Nb Microalloying of 13Cr Super Martensitic Stainless Steel

The effect of Nb microalloying on microstructure, mechanical properties, and pitting corrosion properties of quenched and tempered 13?pct Cr-5?pct Ni-0.02?pct C martensitic stainless steels with different Mo and N contents was investigated. The microstructure, density, and dispersion of high-angle boundaries, nanoscale precipitates, and amount of retained austenite were characterized by using electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction to correlate with properties. The results show that the combined effects of lowering nitrogen content in 13?pct Cr-5?pct Ni-1~2?pct Mo-0.02?pct C steels to 0.01?wt pct, and adding 0.1?pct Nb are to decrease the amount of Cr-rich precipitates, as Nb preferentially combines with residual carbon and nitrogen to form carbonitrides, suppressing the formation of Cr₂N and Cr₂₃C₆. Austenite grain refinement can be achieved by Nb microalloying through proper heat treatment. If the nitrogen content is kept high, then Cr-rich precipitates would occur irrespective of microalloying addition. The NbN would also occur at high temperature, which will act as substrate for nucleation of coarse precipitates during subsequent tempering, impairing the toughness of the steel. It was shown that the addition of Nb to low interstitial super martensitic stainless steel retards the formation of reversed austenite and results in the formation of nanoscale precipitates (5 to 15?nm), which contribute to a significant increase in strength. More importantly, the pitting corrosion resistance was found to increase with Nb addition. This is attributed to suppression of Cr-rich precipitates, which can cause local depletion of Cr in the matrix and the initiation of pitting corrosion.     

Fracture toughness of aisi M2 high-speed steel and corresponding matrix tool steel

The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4 pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated. In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit 2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples is a maximum. The above described results were explained through changes in the microstructure and the fracture modes. CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI.     

Influence of Martensitic Structure and Carbide Precipitation Behavior on Mechanical Properties of 25CrMoNbB Alloy Steels with Mo Contents of 0.25 and 0.5 Pct

In this article, the effects of Mo contents of 0.25 and 0.50 pct on the martensitic structure and carbide precipitation behavior of the experimental steels were investigated and their effects on strength, toughness, and fatigue strength were studied. The results showed that the martensitic blocks and laths were refined and the dislocation density increased with the addition of Mo contents of 0.25 and 0.50 pct. Meanwhile, the amount of carbides increased and the size of carbides decreased in tempered steels. The refinement of carbides is due to the increment of nucleation sites resulting from martensitic structure refinement and the dislocation density increment. Besides, the improvement of thermal stability of M₂₃C₆ carbides enriched with Mo also contributes to carbide refinement. With the addition of 0.50 pct Mo, the strength was improved and the toughness did not deteriorate compared with the steel 0.25 pct Mo. Meanwhile, the fatigue strength was also significantly improved with the addition of 0.50 pct Mo.

     

Effect of carbon concentration on precipitation behavior of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M₂₃C₆ carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M₂₃C₆ carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M₂₃C₆ carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M₂₃C₆ carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades

Acicular ferrite nucleates intragranularly on nonmetallic inclusions, forming a microstructure with excellent fracture toughness. The formation of acicular ferrite is strongly affected by the size, content, and composition of nonmetallic inclusions, but also by the composition of the steel matrix. The potential of inclusions in medium carbon HSLA (high-strength low-alloyed) steels has been the main focus in the literature so far. The current study evaluates the acicular ferrite capability of various inclusions types in four different steel grades with carbon contents varying between 0.04 and 0.65 wt pct. The investigated steels are produced by melting experiments on a laboratory scale and subsequent heat treatment in a High-Temperature Laser Scanning Confocal Microscope. Inclusions are exclusively formed by deoxidation and desulfurization reactions. No synthetic particles are added to the melt. The inclusion landscape is analyzed by Scanning Electron Microscopy. Final ductility of the samples is evaluated based on performed tensile tests. Inclusion types in every steel grade are assessed regarding their nucleation potential always considering the interaction with the steel composition, especially focusing on the role of manganese. The effects of (Ti,Al)O_x-, MnS-, and MgO-containing inclusions are discussed in detail.     

Thermal treatment improving creep properties of nitrogen-added Mod.9Cr-1Mo steels

Generation IV reactors are being developed to produce a reliable energy safely and with an economic benefit, because nuclear energy is being seriously considered to meet the increasing demand for a world-wide energy supply without environmental effects. Ferritic/martensitic steels are attracting attention as candidate materials for the Gen-IV reactors due to their high strength and thermal conductivity, low thermal expansion, and good resistance to corrosion. In recent years, new ferritic/martensitic steels have been developed for ultra supercritical fossil power plants through advanced technologies for steel fabrication. The microstructural stability of these materials for the pressure vessel, cladding and core structure of the VHTR and SFR is very important. Nitrogen is a precipitation hardening element, and the thermal stability of nitrides is superior to that of carbides. So the formation of nitrides may improve the thermal stability of the microstructure and eventually increase the creep rupture strength of high Cr steels. The effect of nitrogen on the creep rupture strength and microstructure evolution of nitrogen-added Mod.9Cr-1Mo steels has been studied. Creep testing was carried out at 873 and 923 K under constant load conditions. The optimum controlled Cr₂X precipitates were developed by special heat treatment, and they were not dissolved after a creep deformation. These fine and stable Cr₂X precipitates contributed to the increase of the creep rupture strength. The prior austenite grain size and martensite lath width were decreased by the resultant stable nitrides.     

The effect of tungsten on creep

The effect of tungsten on creep behavior and microstructural evolution was investigated for tempered martensitic 9Cr steels with various W concentrations from 0 to 4 wt pct. The creep rupture testing was carried out at 823, 873, and 923 K for up to 54 Ms (15,000 hours). The creep and creep rupture strength increased linearly with W concentration up to about 3 wt pct, where the steels consisted of the single constituent of the tempered martensite. It increased only slightly above 3 wt pct, where the matrix consisted of the tempered martensite and δ-ferrite. The minimum creep rate was described by a power law. The apparent activation energy for the minimum creep rate showed a tendency similar to the W concentration dependence of the creep-rupture strength and was larger than the activation energy for self-diffusion at high W concentrations above 1 wt pct. The martensite lath microstructure with fine carbides along lath boundaries was responsible for a high resistance to creep deformation. With increasing W con- centration, the martensite lath microstructure became stabilized, which decreased the minimum creep rate and increased the apparent activation energy for the minimum creep rate.     

Strength and toughness of Fe-10ni alloys containing C,Cr, Mo,and Co

The effects of C (0.10 to 0.20 pct), Cr (0 to 3 pct), Mo (0 to 2 pct), and Co (0 to 8 pct) on the yield strength, toughness (Charpy shelf energy), and tempering behavior of martensitic lONiCr-Mo-Co steels have been investigated. Variations in the carbon content between 0.10 and 0.20 pct result in yield strengths between 160 and 210 ksi (1.1 and 1.45 GN/m²) when these steels are tempered at 900° to 1000°F (480° to 540°C) for times of 1 to 100 h. These steels exhibit a secondary-hardening peak at 900° to 1000° F (480° to 540°C) where coarse Fe₃C carbides are gradually replaced by a fine, dislocation-nucleated dispersion of (Mo, Cr)₂C carbides. Maximum toughness at a given yield strength in these steels is only obtained when they are tempered for sufficiently long times so that the coarse Fe₃C carbides are completely dissolved. Molybdenum is primarily responsible for the secondary-hardening peak observed in these steels. However, chromium additions do result in lower secondaryhardening temperatures and promote coarsening of the secondary-hardening carbide. Best combinations of strength and toughness are obtained with steels containing 2 pct Cr and 1 pct Mo. Cobalt increases the yield strength of these steels over the entire tempering range and results in a higher secondary-hardening peak. This effect of cobalt is attributed to 1) a retardation in the rate of recovery of the dislocation substructure of the martensite, 2) the formation of a finer dispersion of secondary-hardening carbides, and 3) solid-solution strengthening. The finer dispersion of secondary-hardening carbides in steels containing cobalt is favored by the finer dislocation substructure in these steels since the (Mo, Cr)₂C carbide is dislocation-nucleated. This fine dispersion of (Mo, Cr)₂C carbide combined with the high nickel content accounts for the excellent combination of strength and toughness exhibited by these steels.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M_s but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

细晶强化和位错强化对中锰马氏体钢的强化作用

 研究了碳和锰含量对淬火中锰马氏体钢的位错密度、残余奥氏体含量、晶粒尺寸等组织结构以及室温力学性能的影响。借助于SEM、EBSD、TEM和XRD表征了材料的微观组织,探讨了马氏体钢的强化机制。结果表明:随着碳含量增加,淬火中锰钢的位错密度和残余奥氏体体积分数逐渐增加,板条束和板条块尺寸逐渐细化,大角晶界百分数逐渐增加,强度逐渐升高;增加锰含量能够提高马氏体钢的位错密度和抗拉强度。分析认为,位错强化和细晶强化是淬火中锰马氏体钢的主要强化机制。马氏体板条尺寸是马氏体抗拉强度的结构控制单元,而原奥氏体晶粒尺寸则是马氏体屈服强度的结构控制单元。     

On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels

Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.     

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.     

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.     

淬火工艺对马氏体高强钢组织和性能的影响

摘要：采用拉伸、冲击、金相、电子背散射衍射、透射电镜、X射线衍射等试验手段,研究了在线直接淬火+回火（DQT）与离线再加热淬火+回火（RQT）工艺对马氏体高强钢组织性能的影响。结果表明,2种试验钢组织均为板条马氏体,RQT试验钢原奥氏体晶粒及板条束呈等轴状,板条块较短,板条较宽,DQT试验钢原奥氏体晶粒呈扁平状,板条束贯穿整个晶粒,板条块呈细长状,板条宽度较小;位错强化是DQT试验钢强度较RQT高的主要原因;板条束为控制DQT和RQT试验钢韧性的最小单元;DQT试验钢大角晶界比例较低,其具有较大的马氏体板条束尺寸以及更高的位错密度,断裂应力较低,低温韧性较差。     

Evolution of Microstructure and Mechanical Properties of Thermomechanically Processed Ultrahigh-Strength Steel

In the present study, low carbon microalloyed ultrahigh-strength steel was manufactured on a pilot scale. Transformation of the aforesaid steel during continuous cooling was assessed. The steel sample was thermomechanically processed followed by air cooling and water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures (FRTs) was studied. A mixture of granular bainite and bainitic ferrite along with interlath and intralath precipitation of (Ti, Nb)CN particles is the characteristic microstructural feature of air-cooled steel. On the other hand, lath martensitic structure along with a similar type of microalloying precipitates of air-cooled steels is obtained in the case of water-quenched steel also. The best combination of strength (1440 to 1538 MPa) and ductility (11 to 16 pct) was achieved for the selected range of FRTs of water-quenched steel.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM _S but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Thermal Desorption Analysis of Hydrogen in High Strength Martensitic Steels

Thermal desorption analyses (TDA) were conducted in high strength martensitic steels containing carbon from 0.33 to 1.0 mass pct, which were charged with hydrogen at 1223 K (950 °C) under hydrogen of one atmospheric pressure and quenched to room temperature. In 0.33C steel, which had the highest M _s temperature, only one desorption peak was observed around 373 K (100 °C), whereas two peaks, one at a similar temperature and the other around and above 573 K (300 °C), were observed in the other steels, the height of the second peak increasing with carbon content. In 0.82C steel, both peaks disappeared during exposure at room temperature in 1 week, whereas the peak heights decreased gradually over 2 weeks in specimens electrolytically charged with hydrogen and aged for varying times at room temperature. From computer simulation, by means of the McNabb–Foster theory coupled with theories of carbon segregation, these peaks are likely to be due to trapping of hydrogen in the strain fields and cores of dislocations, and presumably to a lesser extent in prior austenite grain boundaries. The results also indicate that carbon atoms prevent and even expel hydrogen from trapping sites during quenching and aging in these steels.