Swing back in kinetics nearM s in hypereutectoid steels

The kinetics and metallography of isothermal transformations in four hypereutectoid steels (0.85 to 1.80 wt pct C) have been studied in the temperature range between 623 and 333 K. Isothermal transformation diagrams for each steel were constructed by means of dilatometry and microscopy. It was found that the C curve, the reverse in kinetics, which has been called “Swing Back”, appeared at the temperature near the in each steel. This paper focuses on the swing back phenomena appearing in four steels. The nose temperatures of C curves nearin 0.85, 1.10, 1.45, and 1.80 wt pct C steels were determined to be about 520, 440, 400, and 375 K, respectively. It was clarified that the C curves near in 1.45 and 1.80 wt pct C steels were associated with the formations of lower bainite with midrib (LBm), thin-plate isothermal martensite (TIM), and leaf-like isothermal martensite (LIM), but those in 0.85 and 1.10 wt pct C steels were related to the formation of LBm. On the basis of the kinetics and metalography, the temperatureJcarbon-contentJtransformation diagram was constructed in the hypereutectoid range of steel.     

1C－1．5Cr钢等温马氏体的TEM观察及其对残余奥氏体的影响

１Ｃ－１．５Ｃｒ钢奥氏体化后在稍低于Ｍｓ点的热浴中等温，可获得较常规工艺更多的残余奥氏体组织。随等温时间延长，因等温马氏体的生成，使室温组织中残余奥氏体量相应增加，当等温进入下贝氏体转变区后，由于下贝氏体的形成降低了奥氏体的稳定性，使残余奥氏体量又趋下降。通过ＴＥＭ观察表明，等温马氏体形成时，将优先以原马氏体片共格长大的方式进行。     

Application of Rietveld Refinement to Investigate the High Chromium White Cast Iron Austempered at Different Temperatures

 The effect of austempering temperature on the microstructure and properties of a high chromium white cast iron was investigated with the Rietveld refinement method. The result shows that the upper bainite exists in the sample austempered at 623 K and the martensite, lower bainite, M7C3, and retained austenite exist in the samples austempered at 563 K and 593 K. The relative content of the retained austenite increases with increasing the austempering temperature from 563 K to 623 K. The higher hardness, impact toughness and impact abrasive wear resistance can be obtained for the specimen austempered at 593 K.     

38CrMoAl钢表面超细贝氏体组织的研究

研究了38CrMoAl钢表面获得超细贝氏体组织的热处理工艺及其组织结构特征。首先利用自行设计的滴注式可控气氛全自动渗碳炉,在930℃对该钢渗碳8h,使钢表面的碳的质量分数达到0．85％,渗层深度为2mm。将经过渗碳热处理的试样加热到930℃保温20min进行奥氏体化处理。然后直接淬入温度分别为230、255和280℃的...     

Isothermal martensite transformation in a 1.80 Wt Pct C steel

The linear products formed isothermally at 373 K in a 1.80 wt pct C steel (Ms = 346 K) were examined by means of transmission electron microscopy. They were first reported as “black line products” by Greninger and Troiano. The isothermal product was of a thin plate with about 0.5 μm width, and it contained {112}_b transformation twins and revealed a habit plane of {3 15 10}_f. The orientation relationship between austenite and product was close to the Nishiyama relationship. These crystallographic data were in good agreement with those calculated by the phenomenological theory of martensitic transformation. Consequently, the product was determined not to be lower bainite, but isothermal martensite. The black color of isothermal martensites resulted from the fact that they were easily etched by the precipitates of epsilon carbide formed during the isothermal holding.     

Bainite morphologies in a 0.2 C–1.5 Mn steel

The isothermal transformation products of austenite over a wide range of temperatures and times in the bainitic range in a 0.2 wt.% C–1.5 wt.% Mn steel have been studied by transmission electron microscopy in order to characterise the bainitic microstructures in low-carbon low-alloy steels. Widmanstätten ferrite has formed with alternate layers of austenite (martensite) as a transition product at 600 and 500°C that has finally transformed on further isothermal transformation to either pearlite (at 600°C) or upper bainite (at 500°C). This type of transformation product was referred to as BI bainite by earlier investigators, but on the basis of the present investigation it is concluded that such ferrite-austenite (martensite) structures are not bainitic as this is not the final transformation product either at 600 or 500°C. Both upper bainite and lath-type lower bainite are formed at 450°C while the transformation product has been only lath-type lower bainite at 400°C.     

Effect of Boron on the Isothermal Bainite Transformation

The role of Boron on the isothermal bainitic transformation in low-C, lean-alloyed steel was investigated. B clearly affected both the transformation kinetics and the morphology of isothermally transformed bainite. The effect of B was more noticeable in the high-temperature range of the bainitic transformation. The microstructure of bainite formed at 773 K (500 °C) consisted of a bainitic ferrite matrix and the martensite/austenite constituent. While the martensite/austenite constituent had an elongated morphology in B-free steel, the martensite/austenite constituents in the B-added steel had a granular morphology. Two types of bainite unit nucleation were considered: the initial nuclei and the nuclei formed on previously formed units. Electron backscattered diffraction (EBSD) analysis showed that the initial bainitic ferrite nuclei were formed at austenite grain boundaries with a Kurdjumov-Sachs (K-S) crystallographic orientation relationship with respect to one of the neighboring austenite grains, revealing the importance of interfacial energy reduction in the nucleation stage. The nuclei of the bainite transformation in the B-added steel were confined to the austenite grain interior, and the bainitic ferrite nuclei had crystallographic orientations limited to K-S variants within the same Bain variant. The characteristic bainite microstructure in B-added steel is due to the inhibition of the bainitic ferrite nucleation at austenite grain boundaries.     

Chi-carbide in tempered high carbon martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide(x), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (0) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to theX-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of κ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of κ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest that κ is the dominant carbide in the tempering temperature range examined.     

Chi-Carbide in Tempered High Carbon Martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide (χ), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (θ) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to the χ-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of χ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of χ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest thatχ is the dominant carbide in the tempering temperature range examined.     

On Various Aspects of Decomposition of Austenite in a High-Silicon Steel During Quenching and Partitioning

Using a Gleeble thermomechanical simulator, a high-silicon steel (Fe-0.2C-1.5Si-2.0Mn-0.6Cr) was laboratory hot-rolled, re-austenitized, quenched into the M _s–M _f range, retaining 15 to 40 pct austenite at the quench stop temperature (T _Q), and annealed for 10 to 1000 seconds at or above T _Q in order to better understand the mechanisms operating during partitioning. Dilatometer measurements, transmission electron microscopy, and calculations showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperature. While isothermal martensite formation starts almost immediately after quenching with the rate of volume expansion dropping all the time, the beginning of bainite formation is marked by a sudden increase in the rate of expansion. The extent of its formation depends on the partitioning temperature following TTT diagram predictions. At the highest partitioning temperatures martensite tempering competes with partitioning. Small fractions of bainite and high-carbon martensite formed on cooling from the partitioning temperature. The average carbon content of the austenite retained at room temperature as determined from XRD measurements was close to the carbon content estimated from the M _s temperature of the martensite formed during the final cooling.     

低于Ms温度变形对等温贝氏体相变动力学和组织的影响

摘要：变形和等温热处理是高强贝氏体钢主要生产工艺,已有研究表明低于马氏体相变起始温度（Ms）的等温热处理可以促进贝氏体相变动力学,低温奥氏体预变形也可以加速贝氏体相变。研究了低于Ms温度变形对后续等温贝氏体相变动力学和组织的影响,结果表明,并未出现预想的加速相变叠加效应,反而,变形温度低于Ms温度时,贝氏体相变动力学减弱,等温贝氏体相变孕育期延长。低于Ms温度等温相变时,贝氏体铁素体与母相奥氏体位向关系接近K-S关系,变形试样虽然获得了一部分先马氏体,且能提高贝氏体形核率,但并非所有的胚核都能发生长大,变形改变母相奥氏体取向,使贝氏体原本的位向关系遭受破坏,导致有效形核率降低。     

Improvement in lower temperature mechanical properties of 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel with the second phase lower bainite

A study has been made of the effect of the second phase lower bainite on lower temperature mechanical properties from ambient temperature (287 K) to 123 K of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel corresponding to AISI 4340. When 25 vol pct lower bainite, which appeared in acicular form so as to partition prior austenite grains, was associated with martensite at 473 K, it provided a better combination of strength and ductility than that achieved using 1133 K direct water quenching irrespective of the test temperature. With the lower bainite, notch tensile strength was dramatically improved over the temperature region studied about 2150 MPa even at 123 K; whereas, in the case of 1133 K direct water quenching, it remained at about 1700 MPa. Similar trends were observed in the relationship between the lower bainite and the Charpy V-notch impact energy at and above 238 K. The lower bainite also produced superior fracture ductility and notch toughness results with decreased temperature of testing as compared to those obtained using a y γ α′ ’ repetitive heat treatment for the same steel. The above beneficial effects of the second phase lower bainite on lower temperature mechanical properties are briefly discussed in terms of metallographic examinations, the law of mixtures, and so on.     

Heat treatment of tool steel with mixed martensite-bainite transformation of austenite

Heat treatment with mixed martensite-bainite transformation of austenite is optimized in terms of strength and fatigue-crack resistance. For the manufacture of components in impact mechanisms, the best combination of strength, hardness, and crack resistance is obtained for steel in which 40% martensite is formed on intermediate cooling, while isothermal heating converts the residual supercooled austenite to lower bainite. This increases the strength by 200–250 N/mm² in relation to quenching + tempering + isothermal quenching (with comparable crack resistance) and increases the resistance to fatigue-crack propagation by a factor of 1.3–2 (with analogous strength).     

On the bainite structure in Cr-Mo-V rotor steel

The morphology of continuously cooled and isothermally transformed bainite structures formed in a Cr-Mo-V rotor steel has been studied using transmission electron microscopy. The samples were austenitised at 955°C for an hour followed by air cooling to room temperature. The isothermal transformation reaction was carried out at 450°C for up to 100 000 s. The microconstituents observed are predominantly lower bainite with very small amount of upper bainite and martensite (formed from untransformed austenite due to water quenching). Analysis of the selected area diffraction patterns confirm that the carbide in bainite is orthorhombic cementite and the orientation relationship between ferrite and cementite is consistent with that of Bagaryatskii. The carbide particles in isothermally transformed bainite are coarser than those of continuously cooled bainite. Tempering one hour at 670°C of continuously cooled steel samples exhibited formation of fine spheroidal MC type carbides. In addition tempering leads to the enrichment of prior austenite grain boundaries by cementite particles. Tempering ten hours at 670°C exhibited microstructures almost identical to those observed in one hour tempering.     

The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mechanical Stability of Retained Austenite at Elevated Temperatures

The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T) process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at 473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ε-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained austenite.     

Transformation of austenite into bainitic ferrite and martensite

The stress induced martensitic transformation in the upper metastable intermediate state of γ-α transformation in ferrous materials, structured as ferritic bainite, is discussed. The fibrous structured ferritic bainite consists of retained austenite and ferrite platelets growing in the [111]α//[101]γ direction. The ferrite growth Induces carbon enrichment of the adjacent austenite at the phase boundaries. Strengthening at high stress levels up to the yield point causes dislocation tangles in the ferrite fibre and the formation of shear bands crossing each other in the retained austenite. At lower carbon contents of the austenite, lath martensite precipitates at the shear band intersections and at high shear band densities martensite blocks are observed. In carbon enriched austenite martensite lenses formed by shear processes have been observed. At alternating loading conditions, exceeding the stress level for athermic martensite formation, various shear planes are activated forming characteristic patterns of plate martensite.     

Improved lower temperature fracture toughness of ultrahigh strength 4340 steel through modified heat treatment

A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (K _IC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593 K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly oil-quenched 4340 steel increased theK _IC values by 15 to 20 MPa • m^1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,K _IC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichK _IC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340 steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorK _IC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inK _IC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the lower bainite just ahead of the current crack.     

热处理工艺对高C-Cr高强耐磨钢组织和性能的影响

设计了一种高C-Cr高强耐磨钢,测试了末端淬透性,研究了不同热处理工艺对微观组织演变和性能的影响,测试了力学性能和耐磨损性能,利用SEM和TEM表征了微观组织.试验结果表明,随距离淬火端距离的增大,试验钢的硬度呈现单调递减的趋势,0～2.5 cm为硬度平缓降低阶段,布氏硬度在63～65HRC的区间.马氏体淬火十回火处理...     

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part I. transformation,microstructure, and room-temperature mechanical properties

Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300 °C and 500 °C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450 °C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.