Interface Migration between Martensite and Austenite during Quenching and Partitioning （Q＆P） Process

An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.     

Si对中锰钢淬火配分组织和性能的影响

将20Mn5钢和20Mn5Si2钢进行淬火和配分(Q&P)工艺处理,用扫描电镜观测其微观组织,用X射线法测量残余奥氏体量,研究了Si对其微观组织和力学性能的影响.结果表明,试验钢中的奥氏体含量明显高于传统的TRIP钢和Q&P 工艺处理钢;在相同Q＆P工艺条件下,20Mn5Si2钢比20Mn5有较多的残余奥氏体,析出物数...     

Interface migration during partitioning of Q&P steel

Abstract

The quenching and partitioning (Q&P) process for heat treatment of steel has previously been shown yield to good combinations of strength and ductility owing to the presence of martensite and austenite. Interface mobility has been discussed in previous literature, mostly related to local driving forces. The present work considers the migration of the martensite/austenite interface in two steels (containing CMnSiMo or CNiSiMo). Experimental data show clear evidence of interface migration in the CMnSiMo steel during partitioning treatments at temperatures between 200 and 400°C for times ranging from 30 to 1000 s; conversely, the interface in the CNiSiMo steel was stationary during the same partitioning treatments. The different behaviours observed are considered in the context of differences in interface mobility and driving forces, and it appears that interface crystallography differences could influence the partitioning behaviour of Q&P steels.     

Study of heat treatment parameters and kinetics of quenching and partitioning cycles

Abstract

Cold rolled sheets of a low carbon quenching and partitioning (Q&P) steel grade were subjected to heat treatment cycles, which were designed by dilatometric experiments and optimised with respect to the quenching temperature, partitioning temperature and partitioning time. Characterisation of the retained austenite was carried out by electron backscattered diffraction, whereas the carbides were studied by scanning electron microscopy (SEM) and differential scanning calorimetry. The mechanical properties were evaluated by tensile testing and linked with retained austenite fractions and carbon contents, determined by X-ray diffraction. Conclusions are drawn concerning the influence of the kinetics of partitioning on the microstructure in terms of optimal austenite fraction in the martensitic matrix, its C content and ensuing mechanical properties.     

A medium manganese steel designed for water quenching and partitioning

An aluminium-containing medium manganese steel has been designed to undergo intercritical annealing followed by quenching in water and subsequent partitioning. Water quenching, replacing the quenching temperature (QT) between 150 and 300°C in conventional quenching and partitioning steels, is therefore adopted in QP alloys, in order to guarantee the precise QT in practice. The low intercritical annealing temperature of 750°C refines both ferrite and prior austenite grains into submicron size. The large fraction of ultra-fine ferrite, as well as the transformation-induced plasticity effect of retained austenite, improves the overall ductility of this water-quenched and partitioned steel. The alloy has achieved excellent mechanical properties of 1130?MPa ultimate tensile strength combined with 19.2% total elongation.     

Prüfung der mechanischen Eigenschaften von “Quenching und Partitioning”‐Gefügen mit berührungsloser Dehnungsmessung

The “Quenching and Partitioning” (“Q&P”) concept was designed to fill the gap between the first and second generation of Advanced High Strength Steels (AHSS). It aims at a multiphase microstructure of retained austenite in a matrix of carbon depleted martensite. The martensitic components enhance the strength properties. The ductility is improved by the TRIP effect. This work investigates the “quenching and partitioning” response of a nickel and silicium alloyed TRIP steel. After “quenching and partitioning” processing the mechanical properties are evaluated by tensile testing. An adapted specimen geometry and the contact free measurement of the elongation by a laser speckle system are used. The mechanical properties of the “quenching and partitioning” microstructure are compared to the fully martensitic state and reviewed with respect to published data. Additional tests are stopped after a well defined plastic deformation. Subsequently the retained austenite fraction is measured magnetically in the test length. As a result the TRIP effect can be evaluated. The “quenching and partitioning” processing leads to tensile strengths of around 1300 MPa at elongations of more than 10 %. The martensitic microstructure exhibits a higher tensile strength and lower elongation values. The decreasing fraction of retained austenite with plastic deformation implies the TRIP effect. Comparable mechanical properties are reported in the published literature. The proposed method of annealing and adapted testing shows effective for the investigation of sophisticated heat treatment procedures.     

低碳硅锰钢I&Q&P处理中C,Mn元素配分综合作用EI北大核心CSCD

采用EMPA,SEM和XRD等手段,研究低碳硅锰钢在双相区保温淬火(I&Q)、双相区保温+奥氏体化+盐浴配分(I&Q&P)和奥氏体化+盐浴配分(Q&P)工艺中的C,Mn元素配分行为及对残余奥氏体的综合作用。结果表明:经I&Q工艺处理后,得到马氏体、铁素体加少量残余奥氏体混合组织,C,Mn在马氏体中出现了富集,并且C富集程度高于Mn;经I&Q&P工艺处理后,C,Mn在板条马氏体中呈现不均匀分布,C的局部富集现象更明显,按C,Mn含量的不同,马氏体可分为"高C高Mn"、"高C低Mn"和"低C低Mn"3种;相比较Q&P工艺中只有C配分作用稳定残余奥氏体,I&Q&P工艺在C,Mn配分综合作用下,能得到更多的残余奥氏体。     

Austenite stability for quenching and partitioning treated steel revealed by colour tint-etching method

Abstract

The complex microstructures of quenching and partitioning treated (QP980) steel have been investigated using two-step colour tint-etching method and further verified by X-ray diffraction, electron backscattering diffraction, magnetisation measurements and Mössbauer spectroscopy. The colour tint-etching method can quantitatively discriminate the ferrite, martensite and retained austenite by obviously colour differences. It is found that retained austenite was observed inside both martensite and ferrite, and the fraction of retained austenite in martensite was statistically higher but more scattering than that in ferrite. Moreover, the retained austenite in martensite is a little bit more stable than that in ferrite by comparing the change of volume fraction retained austenite in both phases after tension.     

Quenching and partitioning steel produced through hot rolling,direct quenching and annealing

A novel type of quenching and partitioning steel was developed using direct quenching after hot finishing rolling, followed by intercritical annealing, quenching and partitioning (DQ–Q&P) process. The desirable combination of strength and ductility was obtained. The effect of various intercritical annealing temperatures on the microstructures and mechanical proprieties was studied. With the decreasing intercritical annealing temperature, the amount of acicular retained austenite increased, which exhibited a good work-hardening behaviour resulting in enhanced tensile strength and total elongation. After annealing at 740°C, superior mechanical properties, which were the ultimate tensile strength of 1015?MPa and total elongation of 32.22%, were achieved.     

Enhancement of tensile properties by room-temperature quenching and partitioning of 0.2C–10Mn–2Al steel

ABSTRACT

Processing conditions better than those of conventional quenching and partitioning process are suggested for 0.2C–10Mn–2Al steel. The steel can retain 24% of austenite on quenching to room temperature and effectively partition carbon from martensite to austenite at 200°C. The resulting tensile properties were comparable to those produced by conventional quenching and partitioning. Moreover, the suggested processing condition resolves an issue of intercritically annealed medium Mn steels by improving the yield strength and eliminating yield point phenomenon as well as serrated flow.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

The approach to equilibrium during tempering of a bulk nanocrystalline steel: an atom probe investigation

A local electrode atom probe has been used to analyze the solute partitioning during bainite transformation in a novel, nanocrystalline bainitic steel. Atom probe results show the absence of any partitioning of substitutional elements between the phases involved. The results are fully consistent with the diffusionless transformation of austenite to bainite. However, substitutional elements are expected to redistribute approaching an equilibrium phase boundary as the mixture of bainitic ferrite and retained austenite is tempered. The compositional analysis of the austenite/ferrite interface by atom probe tomography indicates that retained austenite decomposes during tempering before equilibrium is reached at the interface.     

An ultrahigh strength steel with ultrafine-grained microstructure produced through intercritical deformation and partitioning process

In this work, a novel design scheme in which deformation-induced ferrite transformation (DIFT) was applied to produce fine-grained steel and the quenching is controlled by quenching and partitioning (Q&P) process has led to the development of a new kind of steel. This steel possesses excellent mechanical properties and the ductility can be further improved without compromising strength because the refined microstructure contains martensite, retained austenite and deformation-induced ferrite. The highest elongation of 15% allied with strength of 1700 MPa is obtained through hot deformation followed by Q&P treatment at 300 °C. The microstructure evolutions are discussed in terms of the current knowledge of the Q&P process and the experimental observations. The results show that the designed multiphase steels are a promising candidate for the development of the third generation of advanced high strength steels.     

真空分级淬火对8Cr4Mo4V钢组织和性能的影响

在真空条件下对航空轴承用8Cr4Mo4V钢进行不同温度的分级淬火并采用扫描电镜观察其微观组织、用XRD谱进行相分析并测试洛氏硬度、冲击性能和旋转弯曲疲劳性能,研究了真空分级淬火对其微观组织和力学性能的影响。结果表明,真空分级淬火后的8Cr4Mo4V钢其微观组织由下贝氏体、马氏体/残余奥氏体和碳化物组成;随着分级淬火温度的提高,淬火和回火态钢中析出碳化物的数量增加,残余奥氏体的含量降低。分级淬火温度为580℃时淬火态钢中贝氏体的含量最高(达到13.87%),残余奥氏体的含量为28.59%。回火后析出碳化物的含量和洛氏硬度均为所有分级温度中的最大值,分别为4.37%和62.38HRC。真空分级淬火能提高8Cr4Mo4V钢的综合力学性能。与未分级真空淬火相比,进行580℃×10 min真空分级淬火的8Cr4Mo4V钢的冲击韧性提高了23.3%,旋转弯曲疲劳极限提高了110 MPa。     

Research on Retained Austenite for Advanced Aluminum-containing Hot-rolled TRIP Steel

1. IntroductionTransformation induced plasticity (TRIP) steels havebeen well known for the well balanced comprehen-sive mechanical properties with high strength and highplasticity[1~13] and can be used to press form thevehicle's parts to increase the safety and save the oilby increasing the strength and reducing the weight forautomobile[14~17]. Silicon is one of the common andcheap elements to be added into the steel to increase re-tained austenite stability[18,19]. However, silicon tendsfor…     

A novel ultra-strong hot stamping steel treated by quenching and partitioning process

In the present study, we design a novel hot stamping steel containing high amounts of C and Si and micro-alloying element (i.e. Nb). The steel was subjected to quenching and partitioning (Q&P) process. The new Q&P treated hot stamping steel exhibits a significantly improved mechanical property in terms of strength, ductility and impact toughness compared with the traditional 22MnB5 hot stamping steel. The influence of partitioning time on the microstructure and mechanical properties was investigated. The retained austenite (RA) fraction and the carbon content of RA significantly increased with higher partitioning times. With increasing partitioning time, the uniform elongation, total elongation, strength-ductility balance and impact energy was also remarkably enhanced. The maximum strength-ductility balance achieves around 23?GPa %.     

前驱体对含Cu低碳钢I&Q&P处理后组织性能的影响

采用IQP工艺和EPMA、SEM和XRD等手段,研究了3种前驱体对含Cu低碳钢残余奥氏体含量及力学性能的影响。结果表明,双相区保温初期试验钢奥氏体长大由C配分控制,后期由合金元素Mn、Cu配分控制;双相区保温奥氏体化后,双相区配分后形成弥散分布的局部高浓度Mn、Cu区域仍保留富集效果,在随后的淬火-碳配分阶段易于形成残余奥氏体。经IQP处理后,前驱体为P+F的钢室温组织中马氏体板条较粗,原始奥氏体晶界并不明显;前驱体为F+M钢得到的马氏体板条有序细密;前驱体为M的钢室温组织中马氏体板条更加细密。其中,前驱体组织为M的钢中残余奥氏体量最高,延伸率为24.1%,强塑积可达25 338 MPa·%,综合性能最好。     

Multiphase matrix structure of unalloyed austempered ductile iron

A multiple phase structure characterised by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and film retained austenite (RA) of an unalloyed ductile iron is developed. The designed austempering consists of initial rapid quenching to 210, 200 and 180°C respectively and finally austempering at 220°C for 240 min. The optimum mechanical properties, with a tensile strength of 1330?MPa, an elongation of 3.1% and 422HB, can be achieved by controlling the volume fraction of PM to 12.3% and the RA content to 18.1%. This is mainly attributed to PM that can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies.     

Annealing temperature effects on super duplex stainless steel UNS s32750 welded joints. I: microstructure and partitioning of elements

Welding of austenitic-ferritic stainless steels is a crucial operation and all the materials and parameters used in this process must be optimized in order to obtain the suitable corrosion and mechanical properties. Since a great part of super duplex stainless steels is used in very aggressive environment, their corrosion resistance, referred in particular to pitting and crevice corrosion, is an all-important facet in production and processing of this type of steels. Pitting corrosion resistance of super duplex stainless steels welded joints depends on several aspects: microstructure of the bead, elemental partitioning between ferrite and austenite, and the possible presence of secondary phases. For these reasons, a post-weld annealing is generally performed to homogenize the microstructure. The annealing temperature is the most important parameter to be optimized in this heat treatment. In the present work, a comparison between the as-welded and solution-treated joints is carried out. An effort has been made to correlate the main factors that affect pitting corrosion of the welded joints (microstructure, secondary phases, chemical composition of single phases) with the experimental data obtained from corrosion tests. In this first part of the work the results regarding microstructure and partitioning of elements are presented. The phase balance and the austenite morphology are locally upset during submerged-arc welding of UNS S32750. In the fusion zone, the two phases (ferrite and austenite) result to have approximately the same composition regarding Cr, Mo, and Ni content, while nitrogen is heavily concentrated in austenite. After annealing treatment, the austenite volume fraction increases and the partitioning ratios of elements reach the equilibrium values. The base material results to be less sensitive to annealing treatment than the fusion zone, and the partitioning of elements in the base material is in agreement with previous works reported in the literature.     

Carbon partitioning during bainite transformation in low alloy steels

Abstract

Carbon partitioning in untransformed austenite during bainite transformation has been studied using high speed dilatometry. It was found that in specimens partially transformed to bainite, during subsequent quenching to ambient temperature two martensite start temperatures M _s can be registered. Because M _s depends directly on a carbon content in austenite, the obtained results may indicate that the carbon concentration trapped in films of austenite between parallel subunits of bainitic ferrite is much larger than in the blocks of austenite. It would indicate the necessity of a substantial modification of bainite and martensite regions on the time–temperature–transformation (continuous cooling) diagrams.     

A Mixed-control Mechanism Model of Proeutectoid Ferrite Growth under Non-equilibrium Interface Condition in Fe-C Alloys

By combining the α/γ interface migration and the carbon diffusion at the interface in Fe-C alloys, a mathematical model is constructed to describe the mixed-control mechanism for proeutectoid ferrite formation from austenite. In this model, the α/γ interface is treated as non-equilibrium interface, i.e., the carbon concentration of austenite at γ/α interface is obtained through theoretical calculation, instead of that assumed as the local equilibrium concentration.For isothermal precipitation of ferrite in Fe-C alloys, the calculated results show that the rate of interface migration decreases monotonically during the whole process, while the rate of carbon diffusion from γ/α interface into austenite increases to a peak value and then decreases. The process of ferrite growth may be considered as composed of three stages: the period of rapid growth, slow growth and finishing stage. The results also show that the carbon concentration of austenite at γ/α interface could not reach the thermodynamic equilibrium value even at the last stage of ferrite growth.