Characterization and mechanical properties of ultrahigh boron steels produced by powder metallurgy

The present work is part of an investigation into the use of rapid solidification and powder metallurgy techniques to obtain iron-boron alloys with good mechanical properties. Two Fe-B binary alloys and two ultrahigh boron tool steels were gas atomized and consolidated by hot isostatic pressing (HIP) at temperatures ranging from 700 °C to 1100 °C to have a fine microstructure. Optimum properties were achieved for the binary alloys at low consolidation temperatures, since the solidification mi-crostructure from the original powders is eliminated and, at the same time, fine microstructures and low porosity are obtained in the alloys. At high temperatures and low strain rates, three of the four alloys exhibited low stress exponents, but only the Fe-2.2 pct B alloy showed tensile elongations higher than 100 pct. At low temperatures, only the Fe-2.2 pct B alloy deformed plastically. This alloy showed values of tensile elongation and ultimate tensile strength that were strongly dependent on testing and consolidation temperatures. J.A. JIMéNEZ, Postdoctoral Fellow, formerly with Centra Nacional de Investigaciones Metalúrgicas, C.S.I.C     

Liquid phase sintering,heat treatment and properties of ultrahigh carbon steels

Abstract

Thermo-Calc modelling was employed to predict liquid phase amounts for Fe–0·85Mo–(0·4–0·6)Si–(1·2–1·4)C in the temperature range of 1285–1300°C and such powder mixes were pressed and liquid phase sintered. In high C steels, carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat treated. To assist the break-up of these continuous carbide networks, 0·4–0·6% silicon, in the form of silicon carbide, was added. After solution of processing problems associated with the formation of CO gas in the early part of the sintering cycle, and hence large porosity, densities in excess of 7·75 g cc^?1 were attained. A spheroidising treatment resulted in microstructures having the potential of producing components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains up to 16% were attained.     

珠光体对铁素体动态再结晶的影响

利用Gleeble 1500热模拟实验机进行单轴压缩实验,研究了工业纯铁和两种不同含碳量的低碳钢在700℃、不同应变速率条件下的热变形行为.实验结果表明,变形组织中的珠光体对铁素体动态再结晶行为具有重要影响,即增加钢中珠光体含量,可以促进铁素体动态再结晶过程的发生和发展,使得可以发生铁素体动态再结晶的应变速率范围变宽.     

真空热压超高碳超高铬模具钢研究

采用真空热压技术制备了含有超高碳和铬（2.6%C,26%Cr,质量分数）的模具钢。基于差示扫描量热分析曲线,选取两个远低于熔点的温度（1100 ℃和1150 ℃）进行热压,分别制造出几乎完全致密、粉末之间冶金结合良好的块体钢,密度为7.45～7.47 g?cm?3;对应热压温度1100 ℃ 和1150 ℃,热压态钢中平均碳化物尺寸分别为3.5 μm和5.5 μm,最大碳化物尺寸分别为6.0 μm和8.5 μm。经1150 ℃淬火、500 ℃回火,1100 ℃ 和1150 ℃热压钢的平均硬度分别为HRC 62.6和HRC 60.8,平均三点弯曲强度分别为2060 MPa和1850 MPa;经1150 ℃淬火、550 ℃回火,1100 ℃ 和1150 ℃热压钢硬度分别为HRC 55.2和HRC 53.6,平均三点弯曲强度分别为2490 MPa和2320 MPa。在相同淬火和回火条件下,1100 ℃热压钢的三点弯曲强度较高,原因是淬火回火后钢中碳化物尺寸较小。     

Heat treatments for improved mechanical properties of CuNiFe spinodal alloys

Nonconventional heat treatments were designed to produce spinodal microstructures in two CuNiFe alloys for the improvement of mechanical properties with a reduction in total aging time. The microstructures were characterized by measuring the Curie temperature to determine the composition of the NiFe rich phase, and by TEM to measure the spinodal wavelength. In most cases the material failed intergranularly. However, it was only after the longer aging times that any discontinuous grain boundary coarsening was observed. The measured yield strength of both alloys was found to be proportional to the difference in lattice parameter between the two phases and independent of wavelength and nearly independent of volume fraction.     

Mechanical behavior of superplastic ultrahigh carbon steels at elevated temperature

Ultrahigh carbon (UHC) steels have been investigated for their strength and ductility characteristics from 600 to 850°C. It has been shown that such UHC steels, in the carbon range 1.3 to 1.9 pct C, are superplastic when the microstructure consisted of fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles. The flow stress-strain-rate relations obtained at various temperatures can be quantitatively described by the additive contributions of grain boundary (superplastic) creep and slip (lattice diffusion controlled) creep. It is predicted that superplastic characteristics should be observed at normal forming rates for the UHC steels if the grain size can be stabilized at 0.4 μm. The UHC steels were found to be readily rolled or forged at high strain-rates in the warm and hot range of temperatures even in the as-cast, coarse grained, condition. BRUNO WALSER, formerly with Department of Materials Science and Engineering, Stanford University     

Prospects for using carbonitride-forming elements in carbon steels to increase their mechanical properties

The effect of carbonitride-forming elements on the structure and mechanical properties of a carbon steel (0.65% C) is studied. A thermodynamic calculation is applied to plot solubility curves for the carbides and nitrides of titanium, aluminum, niobium, and vanadium. The optimum concentrations of these elements are determined to form dispersed carbide and nitride particles of the required number and sizes to impede austenite grain growth. Data on the effect of additions of various carbonitride-forming elements on the austenite grain growth and the impact strength are received. The intrduciton of carbonitride-forming elements in carbon steel in certain combinations and concentrations is shown to improve its mechanical properties.     

Heat treatment for improvement in lower temperature mechanical properties of 0.40 pct C-Cr-Mo ultrahigh strength steel

In the previous paper, it was reported that isothermal heat treatment of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel (AISI 4340 type) at 593 K for a short time followed by water quenching, in which a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite is produced, results in the improvement of low temperature mechanical properties (287 to 123 K). The purpose of this paper is to study whether above new heat treatment will still be effective in commercial practice for improving low temperature mechanical properties of the ultrahigh strength steel when applied to a commercial Japanese 0.40 pct C-Cr-Mo ultrahigh strength steel which is economical because it lacks the expensive nickel component (AISI 4140 type). At and above 203 K this new heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved the strength, tensile ductility, and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel. At and above 203 K the new heat treatment also produced superior fracture ductility and notch toughness results at similar strength levels as compared to those obtained by usingγ α′ repetitive heat treatment for the same steel. However, the new heat treatment remarkably decreased fracture ductility and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel below 203 K, and thus no significant improvement in the mechanical properties was noticeable as compared with the properties produced by the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment. This contrasts with the fact that the new heat treatment, as compared with the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment, dramatically improved the notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel, providing a better combination of strength and ductility throughout the 287 to 123 K temperature range. The difference in the observed mechanical properties between the above two ultrahigh strength steels is discussed on the basis of the effect of nickel content, fracture profile, and so forth.     

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.     

Effects of dynamic strain aging on the subsequent mechanical properties of carbon steels

Five carbon steels, AISI 1008, 1020, 1035, renitrogenized 1010 and 1522, were dynamically strain aged in the temperature range 100 to 600°C (373 to 873 K), with strains from 3 to 9 pct and strain rates from 2 × 10^-4 to 2 × 10^-2/s. Following this, changes in tensile, notch impact and fatigue properties were determined. The data obtained indicate that for a commercial combined straightening and strengthening operation on the cooling beds of a bar mill, the temperature during straining should be between 200 and 400° C, the strain should be less than 5 pct and the strain rate is unimportant, up to at least 10^-1/s. After such treatment the tensile properties will be:      

The improvement of cryogenic mechanical properties of Fe-12 Mn and Fe-8 Mn alloy steels through thermal/mechanical treatments

An investigation has been made to improve the low temperature mechanical properties of Fe-8Mn and Fe-12Mn-0.2 Ti alloy steels. A reversion annealing heat treatment in the two-phase (α+ γ) region following cold working has been identified as an effective treatment. In an Fe-12Mn-0.2Ti alloy a promising combination of low temperature (-196°C) fracture toughness and yield strength was obtained by this method. The improvement of properties was attributed to the refinement of grain size and to the introduction of a uniform distribution of retained austenite (γ). It was also shown that an Fe-8Mn steel could be grain-refined by a purely thermal treatment because of its dislocated α martensitic structure and absence of ε martensite. As a result, a significant reduction of ductile to brittle transition temperature was obtained. formerly with the Lawrence Berkeley Laboratory, University of California.     

Influence of sulfide inclusions and pearlite content on the mechanical properties of hot-rolled carbon steels

Sulfur content and sulfide shape are known to have a marked influence on the tensile ductility and notch toughness of plate steels. To investigate the initiation and growth of fractures at inclusions during plastic straining, a detailed study was conducted with a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels containing either 0.004 or 0.013 pct sulfur with and without rare-earth additions. This paper describes the results of this study and evaluates the influence of sulfur content and sulfide shape on the anisotropy in tensile ductility and notch toughness in the steels and assesses the influence of other factors, such as pearlite content, affecting the ductility and toughness. Both globular and stringered sulfide inclusions had a detrimental effect on reduction of area, shelf energy, and transition temperature, which was particularly evident in deterioration of through-thickness properties and which was much more severe for stringered inclusions than for globular inclusions. Increased pearlite content was more detrimental to reduction of area and transition temperature when stringered inclusions were also present, whereas its effect on shelf energy appeared to be about the same regardless of the presence of inclusions or their morphology.     

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.     

High-strength carbon steels with hereditary fine crystal structure. III. Effect of the structure on the mechanical properties of powdered carbon steels

Conclusions By using powdered metals with the corresponding substructure forming in vibration grinding, and by locking the polygonized substructure in the compacts on dislocations of carbon atoms, it is possible by the method of powder metallurgy to put into effect the idea of thermomechanical strengthening of the alloy Fe + C, greatly to increase its strength and ductility.The pure high carbon steels with hereditary finely crystalline structure thus obtained are in regard to structural strength (the combination of the characteristics k_1c and _y) on the level of some steels subjected to thermomechanical treatment and of high alloy steels. This may make it possible to increase considerably the load-bearing capacity of industrial carbon steels, to ensure their reliability and durability. The principal factor responsible for the low brittle strength of hypereutectoid low tempered carbon martensite is insufficient purity as regards admixtures in the initial material.Translated from Poroshkovaya Metallurgiya, No. 1(289), pp. 91–95, January, 1987.     

Strength and fatigue properties in medium carbon duplex steels

The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔK_TH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness K_IC, showing that for high m values K_IC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK.     

Effect of carbon content on the mechanical properties and weather resistance of high performance bridge steels

 The influence of carbon content on the mechanical properties of high yield strength bridge steel has been investigated. The results show that, the excellent mechanical properties and corrosion resistance were obtained for the steel with carbon content between 0.03 wt.%-0.05 wt.%. According to these results, a weathering bridge plate steel with 0.045 wt.% carbon content has been developed. The appropriate control cooling process has to be taken due to the results of CCT and TTT to ensure both microstructure and mechanical properties. Continuous cooling transformation (CCT) curve of the new developing steel presents that when accelerated cooled is faster than 5℃/s, the intermediate transformation products can be formed. The isothermal transformation test (TTT) displays that the intermediate transformation temperature range in 600℃~530℃. Yield strength of the new developing steels reached 500MPa, the elongation and toughness of which are both excellent.     

Stability and mechanical properties of some metastable austenitic steels

The relation between austenite stability and the tensile properties, as affected by testing temperature and processing, was studied for a series of alloys of increasing compositional complexity, viz., the Fe-Ni, Fe-Ni-C, and Fe-Ni-Cr-Mn-C systems. The “stress” and “strain induced” modes of transformation to martensite differed significantly in their influence on the shape of the stress-strain curve. Under certain testing conditions, unusually low yield strengths and high work hardening rates were observed in some of these alloys. Maxima in yield strengths were observed for all austenitic alloys containing carbon that were processed at deformation temperatures between 200° and 300°C. Evidence gleaned from electron microscopy and magnetic and mechanical testing suggested that the maxima were due to the formation of carbon atmospheres on dislocations during processing. The influence of austenite stability on the mechanical properties of steels, varied by systematic changes in test temperature (22° to -196°C), composition (8 pct, 12 pct, 16 pct, and 21 pct Ni) and deformation temperature (25° to 450°C), was evaluated quantitatively. An erratum to this article is available at .     

Microstructures and mechanical properties of melt-quenched ferritic stainless steels

Using the twin-roller type melt-quenching technique, the effects of rapid solidification on the microstructures and mechanical properties of ferrite have been examined in 17 pct Cr and 17 pct Cr-M steels where M = Ti, Nb, Si, or Al. The melt-quenched 17 pct Cr steels have the columnar and equiaxed grains which become finer with decreasing of specimen thickness. But the yield strength of these steels, which is about 0.48 GPa, remains almost unchanged with grain size refinement, contrary to predictions based on Hall-Petch’s relationship. The columnar structure of the melt-quenched 17 pct Cr-M steels is very fine on alloying with Ti and Si, while it is slightly coarse on alloying with Nb and Al. The hardness, yield strength, and fracture strength of these 17 pct Cr-M steels increase with increased amount of alloying element, these maximum values being about 460 DPN, 1.0 GPa, and 1.35 GPa, respectively, for 17 pct Cr- 6 pct Si steel, in the ductile compositional range. These are suitable values for a fine-gauge material having high hardness and strength as well as good ductility.