The pearlite-austenite growth interface in an Fe-0.8 C-12 Mn alloy

The interphase boundary structure and interface processes at the pearlite-retained austenite growth interface in Fe-0.8 wt% C-12 wt% Mn alloy have been investigated by transmission electron microscopy. Facetting, misfit correcting dislocations, and ledge defects are all observed at the previously assumed disordered boundary. Hot stage electron microscopy revealed that the ledge defects are mobile, indicating the migration of the growth interface occurs by the lateral movement of steps. It is found that the growth ledges are continuous across the growth interfaces of the pearlitic ferrite and cementite. This provides a mechanism by which the interface processes of the two pearlite phases may be coupled.     

Ultrafine-grained microstructures and mechanical properties of alloy steels

Ultrafine-grained microstructures can be developed in a variety of alloy steels by coldworking followed by annealing in theα +γ region. Because the annealing temperatures are relatively low and the recrystallized structure is two-phase, grain growth is restricted. Specimens with grain sizes in the range 0.3 to 1.1 μ.m (ASTM 20 to 16) were obtained in manganese and nickel steels by annealing 1 to 400 hr at temperatures between 450° and 650°C (840° to 1200°F). The expected improvement in yield strength through grain refinement was observed in almost all alloys. Other tensile properties depend on factors such as grain size, austenite stability, and specimen geometry, that determine which of three types of plastic behavior will occur. Transformation of austenite during straining improves the mechanical properties of ultrafine-grained specimens.     

Pearlite in ultrahigh carbon steels: Heat treatments and mechanical properties

Two ultrahigh carbon steel (UHCS) alloys containing 1.5 and 1.8 wt pct carbon, respectively, were studied. These materials were processed into fully spheroidized microstructures and were then given heat treatments to form pearlite. The mechanical properties of the heat-treated materials were evaluated by tension tests at room temperature. Use of the hypereutectoid austenite-cementite to pearlite transformation enabled achievement of pearlitic microstructures with various interlamellar spacings. The yield strengths of the pearlitic steels are found to correlate with a predictive relation based on interlamellar spacing and pearlite colony size. Decreasing the pearlite interlamellar spacing increases the yield strength and the ultimate strength and decreases the tensile ductility. It is shown that solid solution alloying strongly influences the strength of pearlitic steels.     

Effect of aluminum content on low temperature tensile properties in cryogenic Fe/Mn/AI/Nb/C steels

Metallurgical and Materials Transactions A -     

Texture and anisotropy of mechanical properties of spray-formed alloy tool steels

The initial texture and anisotropy of alloyed tool steel specimens are studied by constructing pole figures and conducting compression tests. Scattered initial axial crystallographic growth texture and weak anisotropy of yield stress and ultimate strength are established. A relationship between deformability and texture is determined. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 26–31, 2007.     

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low-alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple-phase high-strength high-formability low-alloy steels.     

Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low alloy steels

In the previous papers, a new heat treatment for improving the lower temperature mechanical propertise of the ultrahigh strength low alloy steels was suggested by the authors which produces a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite through isothermal transformation at 593 K for a short time followed by water quenching (after austenitization at 1133 K). In this paper, two commercial Japanese ultrahigh strength steels, 0.40 pct C-Ni-Cr-Mo (AISI 4340 type) and 0.40 pct C-Cr-Mo (AISI 4140 type), have been studied to determine the effect of the modified heat treatment, coupled above new heat treatment withγ ⇆ α′ repctitive heat treatment, on the mechanical properties from ambient temperature (287 K) to 123 K. The results obtained for various test temperatures have been compared with those for the new heat treatment reported previously and the conventional 1133 K direct water quenching treatment. The incorporation of intermediate four cyclicγ ⇆ α′ repctitive heat treatment steps (after the initial austenitization at 1133 K and oil quenching) into the new heat treatment reported previously, as compared with the conventional 1133 K direct water quenching treatment, significantly improved 0.2 pct proof stress as well as notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel at similar fracture ductility levels from 287 to 123 K. Also, this heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved both 0.2 pct proof stress and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel with increased fracture ductility at 203 K and above. The microstructure consists of mixed areas of ultrafine grained martensite, within which is the refined blocky, highly dislocated structure, and the second phase lower bainite (about 15 vol pct), which appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify the new heat treatment reported previously so as to raise 0.2 pct proof stress to a higher level and keep notch toughness at the same level. The improvement in the mechanical properties is discussed in terms of metallographic observations and the modified law of mixtures and so forth.     

锰质量分数对Fe-Mn-C钢热物性参数的影响

 为促进Fe-Mn-C钢连铸技术的发展,对不同锰质量分数的Fe-Mn-C钢铸锭的热物性参数进行了研究。结果表明,0Mn钢的导热系数高于3Mn钢。低于750 ℃时,6Mn钢的导热系数最低;高于900 ℃时,6Mn钢的导热系数最高。3个钢种的平均线膨胀系数为1.0×10－5～1.6×10－5 ℃－1。以Z＜60%作为判据,6Mn钢的第III脆性区为600～800 ℃,3Mn钢和0Mn钢的第III脆性区分别为600～850 ℃和600～900 ℃。在6Mn钢和3Mn钢中,大量生成的形变诱导铁素体（DIF）导致低温区热塑性的恢复。然而,由于连铸矫直过程的应变速率较低,不能生成大量的DIF。因此在连铸过程中,低温区6Mn钢和3Mn钢的热塑性不能恢复。     

The long-term aging embrittlement of Fe-12Cr steels below 773 K

When aged for long times at elevated temperatures, Fe-12Cr steels can experience a significant decrease in the fracture toughness, as observed by an increase in the fracture appearance transition temperature (FATT). The mechanism responsible for this decrease in toughness has never been unequivocally explained, but it has been generally attributed to the precipitation of second phases or impurity segregation. The objective of this study was to characterize the microstructural changes in conventional and super clean, electroslag remelted (high-purity) M152 steel aged between 616 K and 783 K up to 50 K hours for correlation to the toughness behavior. Analytical electron microscopy techniques were used to characterize the microstructure and impurity segregation. After aging at 727 K, conventional M152 contained large quantities of alpha prime (Cr-rich, bcc structure) in addition to Cr-rich M₂₃C₆ carbides and complex Cr-Fe-Mo-Ni-Si-rich precipitates. High-purity M152 aged at 727 K had a similar microstructure as the conventional material, except for the absence of the Si-rich precipitates. Aging at 616 K revealed the same phases as in the as-tempered material. Aged conventional M152 possessed segregated Sn on the prior austenitic grain boundaries, with equivalent levels of P segregation in both the conventional and high-purity materials. A “deembrittling” treatment of 873 K for 2 hours restored much of the original toughness, correlated with a decrease in hardness and a reduction in the amount of alpha prime phase. These results reveal that the degradation in toughness of M152 upon aging up to ∼753 K is primarily due to the combination of alpha prime formation and Sn segregation. These results are consistent with temper embrittlement mechanisms, where impurity segregation and second-phase formation synergistically embrittle NiCr steels. Improvements in the long-term aging embrittlement resistance of M152 below 773 K require ultralow levels of embrittling elements, especially Sn, and a reduction in the alloy Cr content to prevent alpha prime formation.     

粉末冶金低合金钢的制备和性能研究进展

从粉末制备、成形工艺、烧结工艺和组织控制等方面,系统的介绍了国内外粉末冶金低合金钢的制备技术,并对其研究方向和应用前景进行了展望。     

Influence of Mn and Si contents on structure and mechanical properties of ferritic-pearlitic HSLA steels

Laboratory melts of microalloyed low carbon steels with an increased silicon content of about 1 % and manganese contents between 0.3 and 1.3 % were thermomechanically rolled in a laboratory two-high rolling stand to plates of about 10 mm in thickness. The influence of chemical composition and finish rolling temperature (FRT) on the ferrite-pearlite structure of the plates (tensile and Charpy impact tests) was investigated. The choice of the temperature range of finish rolling with respect to the γ/α transformation start temperature strongly influences the ferrite-pearlite structure and the mechanical properties. The most fine-grained and homogeneous ferrite-pearlite structure and the best combination of strength and toughness have been obtained with steels containing about 1 % Mn and 1 % Si rolled with a finish rolling temperature of about 850°C.     

Nanocrystallization and magnetic properties of Fe-30 weight percent Ni alloy by surface mechanical attrition treatment

A nanostructured surface layer was formed in Fe-30 wt pct Ni alloy by surface mechanical attrition treatment (SMAT). The microstructure of the surface layer after SMAT was investigated using optical microscopy, X-ray diffraction, and transmission electron microscopy. The analysis shows that the nanocrystallization process at the surface layer starts from dislocation tangles, dislocation cells, and subgrains to highly misoriented grains in both original austenite and martensite phases induced by strain from SMAT. The magnetic properties were measured for SMAT Fe-30 wt pct Ni alloy. The saturation magnetization (M _s ) and coercivity (H _c ) of the nanostructured surface layers increase significantly compared to the coarse grains sample prior to SMAT. The increase of M _s for SMAT Fe-30 wt pct Ni alloy was attributed to the change of lattice structure resulting from strain-induced martensitic transformation. Meanwhile, H _c was further increased from residual microstress and superfined grains. These were verified by experiments on SMAT pure Ni and Co metal as well as liquid nitrogen-quenched Fe-30 wt pct Ni alloy.     

Mn含量对粉末冶金铁铜碳低合金钢组织与力学性能的影响

采用铜粉、石墨粉和铁粉为原料,以Fe-74.8Mn-6.9C中间合金粉的形式加入Mn元素,制备粉末冶金Fe-x Mn-(2-x)Cu-0.3C(x=0,0.2,0.4,0.6,0.8,1。质量分数,%)低合金钢,研究Mn含量对该合金组织与力学性能的影响。结果表明,合金组织由铁素体和珠光体构成。加入含Mn中间合金粉对混合原料粉末的压制性能没有明显影响。随Mn含量增加,合金中孔隙的数量增多,尺寸变大;合金密度先升高后降低,Mn含量为0.4%时合金密度最大,达到7.24 g/cm~3;合金硬度先升高后降低,Mn含量为0.6%时硬度最大;合金抗弯强度下降,冲击韧性升高,Mn含量超过0.4%时二者变化均较小。因此Fe-0.6Mn-1.4Cu-0.3C合金具有较好的综合性能,硬度(HRB)和冲击韧性分别达到57.4和8.80 J/cm~2,比Fe-2Cu-0.3C合金分别提高5.3和0.82 J/cm~2,材料呈部分韧性断裂特征。