X120管线钢回火过程中断口分离问题研究

针对一种高强度X120管线钢高温回火条件下出现的夏比冲击试样断口分离现象。利用扫描电镜、透射电镜及冲击试验机等,从磷、硼、锰等元素的偏析,晶界处第二相如碳化物、合金析出物等的析出及长大角度进行讨论分析。研究结果表明：随回火温度升高,断口分离加剧,分离平均长度及数量增加。夏比冲击功降幅达60％;断口分离先于主断口形成,导致试验钢表面能增加,刚度、主断口裂纹形成功及裂纹扩展功下降;磷、硼、锰等元素在晶界偏聚引起的回火脆性导致分离裂纹产生,夹杂物对分离起促进作用,尺寸较大的第二相弱化了晶界强度。     

Effect of Heating Rate Before Tempering on Reversed Austenite in Fe-9Ni-C Alloy

 The alloy was reheated to 580 ℃ for tempering at rates of 2, 5, 10, 20, and 40 ℃/s, respectively, after quenching. The amount, distribution, and stability of reversed austenite were investigated by X-ray diffraction (XRD) and electron back scatter diffraction (EBSD). The microstructure and cryogenic impact energy were studied by scanning electron microscope (SEM), transmission electron microscope (TEM) and Charpy V-notch (CVN) tests. The results showed that when the sample was heated at 10 ℃/s, the volume fraction of reversed austenite exhibited maximum of 8%; the reversed austenite was uniform along all kinds of boundaries; the reversed austenite contained higher concentration of carbon which enabled it to be more stable. The cryogenic toughness of the alloy was greatly improved when heated at 10 ℃/s, as the fracture surface observation showed that it mainly fractured in ductile rupture mode, which was consistent with the results of cryogenic impact energy.     

调质低碳贝氏体钢的组织和性能

研究了C-Mn-Mo-Cu-Nb-Ti-B系低碳微合金钢915℃淬火和490~640℃回火的调质工艺对钢的组织及力学性能的影响.用扫描电镜和透射电镜对实验钢的组织、析出物形态和分布以及断口形貌进行观察,采用X射线衍射仪分析钢中残余奥氏体的体积分数.结果表明:调质后,实验钢获得贝氏体、少量马氏体及残余奥氏体复相组织,贝氏体板条宽度只有250 nm,残余奥氏体的体积分数随着回火温度的升高而降低,经淬火与520℃回火后残余奥氏体的体积分数为2.1%.调质后析出物的数量激增,6~15 nm的析出物占70%以上.实验钢经过915℃淬火与520℃回火后,其屈服强度达到915 MPa,抗拉强度990 MPa,-40℃冲击功为95 J.细小的析出物及窄的板条提高了钢的强度.板条间有残余奥氏体存在,改善了实验钢的韧性.      

Mechanisms of tempered martensite embrittlement in low alloy steels

An investigation into the mechanisms of tempered martensite embrittlement (TME), also know as “500°F” or “350°C” or one-step temper embrittlement, has been made in commercial, ultra-high strength 4340 and Si-modified 4340 (300-M) alloy steels, with particular focus given to the role of interlath films of retained austenite. Studies were performed on the variation of i) strength and toughness, and ii) the morphology, volume fraction and thermal and mechanical stability of retained austenite, as a function of tempering temperature, following oil-quenching, isothermal holding, and continuous air cooling from the austenitizing temperature. TME was observed as a decrease in bothK _Ic and Charpy V-notch impact energy after tempering around 300°C in 4340 and 425°C in 300-M, where the mechanisms of fracture were either interlath cleavage or largely transgranular cleavage. The embrittlement was found to be concurrent with the interlath precipitation of cementite during temperingand the consequent mechanical instability of interlath films of retained austenite during subsequent loading. The role of silicon in 300-M was seen to retard these processes and hence retard TME to higher tempering temperatures than for 4340. The magnitude of the embrittlement was found to be significantly greater in microstructures containing increasing volume fractions of retained austenite. Specifically, in 300-M the decrease inK _Ic, due to TME, was a 5 MPa√m in oil quenched structures with less than 4 pct austenite, compared to a massive decrease of 70 MPa√m in slowly (air) cooled structures containing 25 pct austenite. A complete mechanism of tempered martensite embrittlement is proposed involving i) precipitation of interlath cementite due to partial thermal decomposition of interlath films of retained austenite, and ii) subsequent deformation-induced transformation on loading of remaining interlath austenite, destabilized by carbon depletion from carbide precipitation. The deterioration in toughness, associated with TME, is therefore ascribed to the embrittling effect of i) interlath cementite precipitates and ii) an interlath layer of mechanically-transformed austenite,i.e., untempered martensite. The presence of residual impurity elements in prior austenite grain boundaries, having segregated there during austenitization, may accentuate this process by providing an alternative weak path for fracture. The relative importance of these effects is discussed. Formerly with the Lawrence Berkeley Laboratory, University of California.     

The effect of heat treatment on microstructure and cryogenic fracture properties in 5Ni and 9Ni steel

Heat treatments were utilized in 5Ni and 9Ni steel which resulted in the development of tempered microstructures which contained either no measurable retained austenite (<0.5 pct) or approximately 4 to 5 pct retained austenite as determined by X-ray diffraction. Microstructural observations coupled with the results of tensile testing indicated that the formation of retained austenite correlated with a decrease in carbon content of the matrix. Relative values ofK _IC at 77 K were estimated from slow bend precracked Charpy data using both the COD and equivalent energy measurements. In addition, Charpy impact properties at 77 K were determined. In the 9Ni alloy, optimum fracture toughness was achieved in specimens which contained retained austenite. This was attributed to changes in yield and work hardening behavior which accompanied the microstructural changes. In the 5Ni alloy, fracture toughness equivalent to that observed in the 9Ni alloy was developed in grain refined and tempered microstructures containing <0.5 pct retained austenite. A decrease in fracture toughness was observed in grain refined 5Ni specimens containing 3.8 pct retained austenite due to the premature onset of unstable cracking. This was attributed to the transformation of retained austenite to brittle martensite during deformation. It was concluded that the formation of thermally stable retained austenite is beneficial to the fracture toughness of Ni steels at 77 K as a result of austenite gettering carbon from the matrix during tempering. However, it was also concluded that the mechanical stability of the retained austenite is critical in achieving a favorable enhancement of cryogenic fracture toughness properties. Formerly with Union Carbide Corporation, Tarrytown, NY     

低成本工程机械用Q690E级高强钢的热处理工艺与工业化

  以低成本、高附加值Q690E级调质板开发为目标,研究了亚温热处理与调质热处理工艺参数对试验钢显微组织与力学性能的影响。结果表明：在进行810℃亚温淬火处理的前躯体中存在大块状的铁素体时,易导致试验钢的低温冲击韧性恶化;以板条马氏体为前躯体经相同亚温淬火后,显微结构为更加细小的马氏体和以条状形态呈平行趋势分布在马氏体之间的铁素体两相混合组织,试验钢的-40℃冲击功值高达247J,但强度较低;常规调质热处理后的试验钢具有良好的综合力学性能,采用修正后的工艺参数,工业试制6~60mm规格产品的强韧性能均明显超过相关标准要求。     

Effect of intercritical temperature and cold-deformation on the kinetics of austenite formation during the intercritical annealing of dual-phase steels

The objective of this investigation was to study the effect of the intercritical temperature and percentage of cold-deformation on the kinetics auf austenite formation during the intercritical annealing in the alpha + gammy (α + γ) phase field of the iron-carbon phase diagram. This investigation was carried out on an Fe–0.11 C–1.58Mn–0.4 Si ferritic-pearlitic alloy with different structures of 0% (hot-rolled), 25% and 50% cold-deformation. The intercritical annealing temperatures were 735, 750°C and the intercritical annealing time ranged from 15 to 1815 s. It has been observed that recrystallization of the deformed ferrite was completed before any austenite formation. Surprisingly, it was noted that the recrystallized ferrite grain size was independent of percentage cold-deformation. Furthermore, it was expected that cold-deformation accelerates the kinetics of austenite formation. Nevertheless, the amounts of austenite formed from pearlite dissolution were mostly equal, irrespective of the starting condition. As has been previously reported, increasing the intercritical annealing temperature was found to increase the amount of austenite.     

原始组织对690 MPa级海工钢亚温淬火后强韧性的影响

 为了稳定亚温淬火工艺与工业化生产,通过力学性能分析及显微组织观察,对比了正火+亚温淬火+回火、在线淬火+亚温淬火+回火、离线淬火+亚温淬火+回火3种热处理工艺对690 MPa级海洋工程用钢板组织性能的影响。结果表明,采用离线淬火+亚温淬火+回火工艺结果最理想,能够大幅度提高钢板的低温冲击性能和伸长率。同时,还能够获得较低的屈强比,断口形貌全部为韧窝,呈明显的韧性断裂,而且随着亚温保温时间的增加,强度逐渐提高,当保温时间达到30 min以后,强度及条片状铁素体基本不发生变化;采用直接淬火态+亚温淬火+回火虽然可以保证高强度低屈强比,但是冲击功表现较为离散,稳定性欠佳,断口形貌为混合型,以韧性断裂为主;采用正火态+亚温淬火+回火工艺效果最差,尤其是不能保证钢板低温韧性,断口形貌全部为解理,呈明显的脆性断裂,其中片条状铁素体形貌是决定优良低温冲击性能的关键因素。     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

The Evaluation of Tempered Martensite Embrittlement in 4130 Steel by Instrumented Charpy V-Notch Testing

Tempered martensite embrittlement (TME) was studied in vacuum-melted 4130 steel with either 0.002 or 0.02 wt pct P. TME was observed as a severe decrease in Charpy V-notch impact energy, from 46 ft-lb. at 200 °C to 35 ft-lb. at 300 °C in the low P alloy. The impact energy of the high P alloy was consistently lower than that of the low P alloy in all tempered conditions. Fracture was transgranular for all specimens; therefore, segregation of P to the prior austenitic grain boundaries was not a factor in the occurrence of TME. Analysis of load-time curves obtained by instrumented Charpy testing revealed that the embrittlement is associated with a drop in the pre-maximum-load and post-unstable-fracture energies. In specimens tempered at 400 °C the deleterious effect of phosphorus on impact energy became pronounced, a result more consistent with classical temper embrittlement rather than TME. A constant decrease in pre-maximum-load energy due to phosphorus content was observed. The pre-maximum-load energy decreases with increasing tempering temperature in the range of 200 °C to 400 °C, a result explained by the change in work hardening rate. Carbon extraction replicas of polished and etched as-quenched specimens revealed the presence of Fe₂MoC and/or Fe₃C carbides retained after austenitizing. Ductile crack extension close to the notch root was related to the formation of fine microvoids at the retained carbides.     

The use of a boron addition to prevent intergranular embrittlement in Fe-12Mn

Fe-12 Mn alloys undergo failure by catastrophic intergranular fracture when tested at low temperature in the as-austenitized condition, a consideration which prevents their use for structural applications at cryogenic temperatures. The present research was undertaken to identify modifications in alloy composition or heat treatment which would suppress this embrittlement. Chemical and microstructural analyses were made on the prior austenite grain boundaries within the alloy in its embrittled state. These studies failed to reveal a chemical or microstructural source for the brittleness, suggesting that intergranular brittleness is inherent to the alloy in the as-austenitized condition. The addition of 0.002 to 0.01 wt pct boron successfully prevented intergranular fracture, leading to a spectacular improvement in the low temperature impact toughness of the alloy. Autoradiographic studies suggest that boron segregates to the austenite grain boundaries during annealing at temperatures near 1000 °C. The cryogenic toughness of a Fe-12Mn-0.002B alloy could be further improved by suitable tempering treatments. However, the alloy embrittled if inappropriate tempering temperatures were used. This temper embrittlement was concom-itant with the dissolution of boron from the prior austenite grain boundaries, which reestablishes the intergranular fracture mode.     

Effects of Pre-tempering on Intercritical Annealing in Fe-2Mn-0.3C Alloy

As-quenched martensite was pre-tempered at 623 K and 923 K (350 °C and 650 °C), and then it reverted to austenite by intercritical annealing at 998 K (725 °C) in a Fe-2Mn-0.3C alloy. Pre-tempering at 623 K (350 °C) accelerates austenite formation, while pre-tempering at 923 K (650 °C) significantly retards it. It is proposed that austenite nucleation is accelerated by increasing the number density and particle size of cementite during tempering, whereas austenite growth is retarded by Mn enrichment in cementite during tempering at high temperature, leading to opposite effects of pre-tempering on reversion kinetics.     

超细晶钒微合金化双相钢性能

研究了钒微合金化对高强双相钢微观组织及性能的影响。与Fe-0.186C-1.5Mn-0.3Si-0.008N参照钢相比,加入0.14%V带来如下效果:(1)在冷轧及退火状态铁素体晶粒高度细化;(2)严重推迟在连续退火过程中铁素体向奥氏体转变的初始动力学;(3)慢冷条件下铁素体开始转变温度稍微提高,但珠光体和贝氏体转变被抑制,导致淬透性提高;(4)在临界退火温度≤740℃时观察到未溶渗碳体;(5)750℃/180 s退火后铁素体相中发现大量V(C,N)析出(平均直径7.4 nm),而马氏体(奥氏体)中析出物稀少,尺寸更大(平均直径13.4 nm);(6)不含钒参照钢抗拉强度随马氏体体积分数增量为~16 MPa/%,而含钒钢由于晶粒细化和铁素体选择强化,强度随马氏体含量变化增量相当低(~4 MPa/%),在马氏体体积分数45%变软。     

The stability of precipitated austenite and the toughness of 9Ni steel

A correlation was confirmed between the good low temperature Charpy toughness of 9Ni steel and the stability of its precipitated austenite against the martensitic transformation. Changes in the microstructure during isothermal tempering were studied in detail. The austenite/martensite interface is originally quite coherent over ∼100 A distances. With further tempering, however, the dislocation structure at the austenite/martensite interface changes, and this change may be related to the increased instability of the austenite particles. The reduction in austenite carbon concentration does not seem large enough to account for the large reduction in austenite stability with tempering time. The strains inherent to the transformation of austenite particles create dislocation structures in the tempered martensite. The large deterioration of the Charpy toughness of overtempered material is attributed, in part, to these dislocation structures.     

Correlation of microstructure and tempered martensite embrittlement in two 4340 steels

This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K _Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation fields ahead of cracks, suggesting thatK _Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels studied can be explained quantitatively using different fracture models.     

热处理工艺对5 mm 9Ni钢板性能的影响

研究了淬火+亚温淬火+回火(QLT)、淬火+回火(QT)、正火+正火+回火(NNT)3种工艺对5 mm9Ni钢板低温韧性的影响.结果 表明,采用NNT工艺,钢板低温韧性良好,逆变奥氏体含量8.0％;通过工艺试制,5mm 9Ni钢板,采用3.3mm厚度规格试样,-196℃冲击吸收能量≥40 J;采用2.5mm厚度规格试样...     

Low- Temperature improvement of mechanical properties of aisi 4340 steel through high-temperature thermomechanical treatment

The low-temperature mechanical properties of AISI 4340 ultrahigh-strength steel can be dramatically improved by high-temperature thermomechanical treatment (HTMT). A comparison was made with the mechanical properties developed by the conventional heat-treatment (CHT). When the steel was hot forged by 50 pct reduction at 1177 K followed by direct oil quenching and subsequent tempering at 423 K, the slow-bending fracture energy of fatigue-precracked steels was significantly improved, with notably increased strength, owing to a moderate increase in ductility over the temperature range of 123 to 293 K, and the ductile-to-brittle transition temperature (DBTT) in the Charpy impact test was remarkably lowered. The marked development was achieved in the slow-bending fracture energy, with moderate improvement in strength, owing to significantly increased ductility over the temperature range of 123 to 293 K, when the steel was deformed by 50 pct reduction at 1473 K followed by direct oil quenching and subsequent tempering at 453 K. The treatment also improved the shelf energy and DBTT in the Charpy impact test. The improved mechanical properties are attributed to the subcell structure introduced in austenite by processing HTMT and inherited by martensite. The present work shows that an HTMT steel is attractive for low-temperature ultrahigh-strength applications when suitable combinations of deformation temperatures with tempering conditions are applied to the steels.     

Austenite stabilization from direct cementite conversion in low-alloy steels

Tr ansformation i nduced p lasticity (TRIP) effects associated with austenite dispersions in low alloy Fe-Mn-Si steels can be enhanced by austenite stabilisation. Austenite which forms during conventional intercritical annealing does not possess the required stability in order to exhibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite conversion which occurs under paraequilibrium conditions, i.e with partition of carbon but with no partition of substitutional alloying elements. In this way the austenite inherits the manganese content of cementite and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn-enriched cementite dispersion in ferrite is formed. In the second step, intercritical annealing, austenite nucleates on the cementite particles, which are consumed to form austenite. It was experimentally determined that this austenite has been enriched in manganese and carbon and, therefore, is stabilised. The conversion reaction is followed by the conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of the two-step annealing treatment is a significant austenite stabilisation relative to simple intercritical annealing, indicating a potential for enhanced TRIP effects in this class of steels.     

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M₂C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.     

Alloy Design Based on Computational Thermodynamics and Multi-objective Optimization: The Case of Medium-Mn Steels

A new alloy design methodology is presented for the identification of alloy compositions, which exhibit process windows (PWs) satisfying specific design objectives and optimized for overall performance. The methodology is applied to the design of medium-Mn steels containing Al and/or Ni. By implementing computational alloy thermodynamics, a large composition space was investigated systematically to map the fraction and stability of retained austenite as a function of intercritical annealing temperature. Alloys exhibiting PWs, i.e., an intercritical annealing range, which when applied satisfies the given design objectives, were identified. A multi-objective optimization method, involving Pareto optimality, was then applied to identify a list of optimum alloy compositions, which maximized retained austenite amount and stability, as well as intercritical annealing temperature, while minimized overall alloy content. A heuristic approach was finally employed in order to rank the optimum alloys. The methodology provided a final short list of alloy compositions and associated PWs ranked according to their overall performance. The proposed methodology could be the first step in the process of computational alloy design of medium-Mn steels or other alloy systems.