Microalloying effects on microstructure and mechanical properties of 18Cr–2Mo ferritic stainless steel heavy plates

The microstructure, including grain size and precipitation, tensile strength and Charpy impact toughness of (Nb + V) 18Cr–2Mo ferritic stainless steel heavy plates with/without Ti were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and standard tensile strength and Charpy impact toughness testing. It was found that for 18Cr–2Mo heavy plate, a good combination of Nb–V stabilized method without Ti induces refinement of grain sizes due to the precipitation of amounts of fine Nb carbonitrides and V nitrides. Meanwhile, the mechanical testing results indicate that optimal transformation of grain size, precipitation that Nb–V composition system brings to 18Cr–2Mo heavy plate is beneficial to improvement of strength and impact toughness.     

氮对钒微合金钢粗晶热影响区(CGHAZ)的组织和性能的影响

用热模拟方法研究了氮含量对钒微合金钢粗晶热影响区(CGHAZ)的组织和性能的影响。结果表明,氮含量为0.0031%或0.021%时,CGHAZ的韧性较差。氮含量0.0031%时CGHAZ中有少量的Ti(C,N),晶界铁素体(GBF)较少,晶内有大量尺寸较大的侧板条铁素体(FSP),解理裂纹沿FSP的直线扩展使其韧性较差。氮含量0.021%时在CGHAZ中生成了较为粗大的(Ti, V)(C, N)和GBF,解理裂纹沿GBF扩展使其韧性较差。氮含量为0.012%时低温韧性较好,在CGHAZ中生成了大量细小的(Ti, V)(C, N)粒子,且GBF尺寸相对较小,晶内有大量的针状铁素体(AF)。这些因素都有利于阻止裂纹扩展,使其低温韧性显著提高。     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

Fracture toughness of M2 and H13 alloy tool steels

Abstract

The influence of microstructural variations on the fracture toughness of two tool steels having compositions (wt-%) lC–4Cr–5Mo–2V–6W (AISI M2 high-speed steel) and 0·35C–5Cr–1·5Mo;amp;#x2013;1V (AISI H13 hot-work steel) was investigated. In the as-hardened condition, the H13 steel has a higher fracture toughness than M2 steel, and the latter steel is harder. In the tempered condition, the H13 steel is again softer and has a higher fracture toughness than M2. There is a decrease in fracture toughness and an increase in hardness when the austenitizing temperature is above I050°C for M2 steel and above 1100°C for H13 steel, in both the as hardened and hardened and tempered conditions. The fracture toughness of both steels was enhanced by reducing the grain size and increasing the overall carbide volume in the matrix. The steel samples of average grain diameter ≥40μm exhibit 2–3 MN m ^?3/2 lower fracture toughness than samples of average grain diameter ≤15 μm. A high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. Tempering improved the fracture toughness of M2 and H13 steels. The present results are explained using observations of changes in the microstructure and the modes of fracture.

MST/468     

钛氮合金的铸态组织与性能

采用熔铸工艺法制备了含氮量为0.045%～0.27%的原位自生氮化物增强钛基复合材料,分析并测试了合金的铸态组织和力学性能.研究结果表明:在Ti-N合金中,随着氮含量的增加,合金中氮化物的形态和相组成发生了明显的改变;当氮含量在0.045%～0.18%时,合金的基体为α-Ti,增强相为TiN0.3;氮含量增加到0.225%时,增强相转变为块状Ti2N;复合材料的硬度、抗压强度和弹性模量均高于纯钛基体且随着氮含量的增加而增加;当增强相由TiN0.3转变为Ti2N时,抗压强度显著增加;由压缩断口分析可知,基体为韧性断裂,随着氮含量增加合金由韧窝 解理断口向具有解理特征的脆性断裂转变.     

Microstructure and Fracture Morphology in the Welding Zone of Cr18Mo2 Ferritic Stainless Steel

Microstructure, precipitates and fracture morphology in the weld metal and the heat-affected zone (HAZ) of Cr18Mo2 ferritic stainless steel have been studied by means of metalloscope, SEM,TEM and X-ray diffractometer. Experimental results indicate that crystalline grain coarsening in HAZ is one of the reason resulting in the embrittlement fracture in the welding zone of the ferritic stainless steel. Some precipitates (TiC, TiN and Cr2N) in the steel promote production and development of the brittle cracks. In practical applications. the welding heat input should be as small as possible to prevent embrittlement caused by HAZ grain coarsening.     

Transparent polycrystalline ruby ceramic by spark plasma sintering

Fine-grained and transparent polycrystalline ruby ceramics (Cr₂O₃-doped Al₂O₃) were successfully prepared by spark plasma sintering (SPS). The effect of Cr₂O₃ concentration on the grain size, hardness, fracture toughness and thermal conductivity of ruby ceramics was investigated systematically. For 0.05 wt.% Cr₂O₃, high in-line transmittance of 85% at 2000 nm can be reached, further increase of Cr₂O₃ concentration leads to the decrease in transmittance. High hardness of 23.95-25.05 GPa can be achieved due to the fine grain size in all ruby ceramics. The fracture toughness of 1.9-2.29 MPa m^1/2 indicates that no improvement in fracture toughness over pure Al₂O₃ can be obtained by Cr₂O₃ doping in these submicron grained ruby ceramics. High thermal conductivity of 28-29.8 W/(m K) at room temperature, close to that of single crystal sapphire, can be achieved. The change in grain size for different Cr₂O₃ concentrations is the major reason for the change in mechanical and thermal properties, but not for the change in optical properties.     

正火型V-N-Ti和Nb-V-Ti海工钢焊接粗晶热影响组织和韧性研究

用焊接热模拟方法研究了V-N-Ti和Nb-V-Ti微合金化正火型海工钢模拟粗晶热影响区(CGHAZ)组织和韧性的变化规律。结果表明,组织的不同使V-N-Ti设计正火型海工钢的模拟CGHAZ韧性比Nb-V-Ti钢的好。对于V-N-Ti钢,较高的N含量提高了富Ti(Ti, V)(C, N)粒子析出温度和铁素体形核能力,使模拟CGHAZ原始奥氏体晶粒和(取向差角为15°)晶粒细化,并生成能阻止或使解理裂纹的偏转细小多边形铁素体,因此具有良好的低温韧性。而Nb-V-Ti钢模拟CGHAZ原奥氏体晶界上的链状M-A、粗大的原始奥氏体晶粒和有效晶粒尺寸,是模拟CGHAZ韧性差的原因。     

Upper shelf temperature dependence of fracture toughness for four low to intermediate strength ferritic steels

To examine the inadequacy and expose the potential consequences of utilizing transition temperature range fracture toughness values when performing fracture mechanics analyses on structures operating at upper shelf temperatures, the upper shelf fracture toughness behavior of three rotor forging steels (ASTM A471 NiCrMoV, ASTM A470 CrMoV and AISI 403 modified 12 Cr) plus an ASTM A217 $\text{2}\text{1}\text{4}\text{Cr}$-lMo cast steel was investigated. Elastic-plastic fracture toughness values were obtained via the $\text{J}{}_{\mathrm{IC}}$ resistance curve test technique. Except for the CrMoV rotor steel, upper shelf fracture toughness peaked near the temperature where zero percent brittle fracture first occurred in the Charpy V-notch impact tests after which decreases of 25–45% in toughness were realized. Fracture toughness tests were conducted up to 800°F.     

Realization of superhard chromium nitride-based films: A superlattice nanocrystalline-Cr2N/amorphous-WC film

Generally, it is a challenge for superhard nitride films to possess strong enough fracture toughness. In the present study, a superhard nanocrystal- (nc-) Cr₂N/amorphous- (a-) WC film was fabricated by arc ion plating and dc magnetron sputtering. The superlattice structure of nc-Cr₂N with 9 nm thickness and a-WC with 3 nm thickness was alternately grown on amorphous Cr/WC buffer layer. Accordingly, the multilayers nc-Cr₂N/a-WC nanocomposite showed a superhardness effect (~ 40 GPa) which presents an anomalous enhancement of hardness and elastic modulus. The adhesion strength of nc-Cr₂N/a-WC multilayers on the steel substrate exceeded 60 N. The tribological behavior of the nc-Cr₂N/a-WC film was proven that the superlattice nc-Cr₂N/a-WC nanocomposites have significant potential for high-speed dry machining and other wear-resistance precise parts.     

Experimental analysis of the practical LBZ effects on the brittle fracture performance of cryogenic steel HAZs with respect to crack arrest toughness near fusion line

Focusing on crack arrest behavior, this study investigates the practical influence of local brittle zones (LBZs) on the brittle fracture resistance of heat-affected zones (HAZs) in advanced 9% Ni cryogenic steel welds, and discusses whether the LBZs of this steel in practice have potentially deleterious effects as previously thought, or not. By analyzing the variations in brittle crack arrest toughness (K_a) and brittle crack initiation toughness (K_c) within actual HAZ, it is found that LBZs of this steel may not be harmful in consideration of crack arrest toughness near fusion line.     

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt _8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt _8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.     

Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.     

Improvement of room temperature fracture toughness in Cr2Nb-based alloys by heat treatment

The microstructure and room temperature fracture toughness of binary Cr/Cr₂Nb alloys annealed at 1653 K for 30 h were investigated at both the hypo- and hypereutectic compositions. The experimental results indicate that the high temperature heat treatment has a beneficial effect on the room temperature fracture toughness of the Cr₂Nb/Cr alloys. After the heat treatment, the room temperature fracture toughness of the hypo- and hypereutectic alloys are increased by about 212 and 203%, which are 15 and 8 times higher than that of as-cast Cr₂Nb Laves phase (1·2 MPa m^1/2). The fractographic analysis indicates that in the annealed condition, the strengthen of lamellar eutectic cohesive strength can provide significant toughening of the matrix by crack deflection, crack blunting and crack bridging mechanisms.     

Shape effect of ultrafine-grained structure on static fracture toughness in low-alloy steel

Abstract

A 0.4C-2Si-1Cr-1Mo steel with an ultrafine elongated grain (UFEG) structure and an ultrafine equiaxed grain (UFG) structure was fabricated by multipass caliber rolling at 773 K and subsequent annealing at 973 K. A static three-point bending test was conducted at ambient temperature and at 77 K. The strength–toughness balance of the developed steels was markedly better than that of conventionally quenched and tempered steel with a martensitic structure. In particular, the static fracture toughness of the UFEG steel, having a yield strength of 1.86 GPa at ambient temperature, was improved by more than 40 times compared with conventional steel having a yield strength of 1.51 GPa. Furthermore, even at 77 K, the fracture toughness of the UFEG steel was about eight times higher than that of the conventional and UFG steels, despite the high strength of the UFEG steel (2.26 GPa). The UFG steel exhibited brittle fracture behavior at 77 K, as did the conventional steel, and no dimple structure was observed on the fracture surface. Therefore, it is difficult to improve the low-temperature toughness of the UFG steel by grain refinement only. The shape of crystal grains plays an important role in delamination toughening, as do their refinement and orientation.     

双相不锈钢00Cr25Ni7Mo4N敏化过程中的脆化机理研究

研究了双相不锈钢00Cr25Ni7Mo4N在晶间腐蚀试验中发生的脆断现象,采用金相法、扫描电镜和透射电镜对材料的脆断机理进行了分析。研究发现双相不锈钢00Cr25Ni7Mo4N在敏化时有R相、Fe3Cr3Mo2Si2相等脆性相析出,导致材料脆化,冲击韧性下降。最后,给出了双相不锈钢00Cr25Ni7Mo4N热加工后的使用要求。     

Element distribution and phase constitution of Al2O3-TiC/W18Cr4V vacuum diffusion bonded joint

Vacuum diffusion bonding between Al₂O₃-TiC ceramic composites and W18Cr4V tungsten-based tool alloy has been carried out by using Ti/Cu/Ti multi-interlayer. Element distribution near the Al₂O₃-TiC/W18Cr4V interface was discussed and fracture morphology was analyzed using electron probe microanalysis. Additionally, phase constitutions of the Al₂O₃-TiC/W18Cr4V joint were determined by X-ray diffraction. The results indicate that Ti-rich layers are formed near both Al₂O₃-TiC and W18Cr4V. The Ti-rich layer near Al₂O₃-TiC helps to wet the Al₂O₃-TiC surface. The Ti-rich layer near W18Cr4V can restrain the formation of Fe-Ti intermetallic compounds in the diffusion transition zone. Residual Cu in the diffusion transition zone can act as a stress releasing zone. The structures of interfacial phases are identified as follows: Al₂O₃-TiC/TiO + Ti₃Al/Cu + CuTi/TiC layer/mixed layer of Fe₃W₃C, Cr₂₃C₆ and α-Fe/W18Cr4V. The fracture morphology of Al₂O₃-TiC/W18Cr4V joint appears brittle features and the failure occurs within the Al₂O₃-TiC ceramic.     

Microstructural thermostability of high nitrogen austenitic stainless steel

A high nitrogen austenitic stainless steel, Fe–24Mn–18Cr–3Ni–0.62N, was isothermally annealed from 750 to 900 °C for different times to investigate the thermostability of its microstructures. Results show that the precipitates were Cr₂N and initially formed along the grain boundary. The time–temperature–precipitation diagram was established, according to which the critical cooling rate of this material with less than 0.1–0.5 vol.% of precipitated Cr₂N was specified as 30 °C/min. The microhardness of the matrix in samples annealed at different temperatures decreased with the diffusion of nitrogen from the matrix to grain boundaries.     

Micro-hardness, microstructures and thermal stability of (Ti,Cr,Al,Si)N films deposited by cathodic arc method

(Ti,Cr,Al,Si)N films were deposited by cathodic arc method using TiCrAlSi alloy cathodes. It was found that the microstructures of (Ti,Cr,Al,Si)N were closely related to (Al+Si) content. The crystal structure of (Ti,Cr,Al,Si)N was NaCl-type structure up to the (Al+Si) content of 0.60, where it changed to a hexagonal structure. The maximum hardness of 33 GPa was obtained at the lowest (Al+Si) content of 0.56, still in the cubic structure. The micro-hardness decreased down to 28 GPa as the crystal structure changed from NaCl-type to wurtzite-type.To investigate the thermal stabilities of (Ti,Cr,Al,Si)N, the films were annealed in a vacuum furnace. In Ti_0.20Cr_0.20Al_0.55Si_0.05N with cubic structure, the phase segregation occurred by annealing at over 900 °C, while Ti_0.22Cr_0.22Al_0.44Si_0.12N remained in cubic phase up to 1000 °C. The micro-hardness of Ti_0.20Cr_0.20Al_0.55Si_0.05N increased and that of Ti_0.22Cr_0.22Al_0.44Si_0.12N decreased at 1000 °C. Ti_0.20Cr_0.11Al_0.58Si_0.11N with a cubic and hexagonal mixture phase held its (c,h)-mixture phase up to 1000 °C, while there was an indication of an increase both in micro-hardness and in cubic ratio after annealing.In this paper, the micro-hardness and microstructure of (Ti,Cr,Al,Si)N are discussed as a function of annealing temperature and investigated by X-ray diffraction and electron microscopy.     

Synthesis and mechanical properties of high-purity Cr2AlC ceramic

High-purity and dense Cr₂AlC has been successfully fabricated by hot-pressing, using Cr, Al and graphite as raw materials. Delamination, kink bands, monolamellar kink, transgranular crack and transgranular fracture of bulk Cr₂AlC are found during the room-temperature test. The density, Vickers hardness, flexural strength, Young's modulus, compressive strength and fracture toughness of the Cr₂AlC are 5.17 g/cm³, 4.9 GPa, 469 ± 27 MPa, 282 GPa, 949 ± 22 MPa and 6.22 ± 0.26 MPa m^1/2, respectively. The strength of Cr₂AlC could be greatly improved by second phase of Cr₇C₃. And the slipping of basal planes and slip system cold be hindered by Cr₇C₃, thus resulting in a lower toughness.