Microstructural Evolution of 11Al-3Mg-Zn Ternary Alloy-Coated Steels During Austenitization Heat Treatment

This study details the microstructural evolution of a commercial hot-dip 11Al-3Mg-Zn-coated steel during austenitization. After 5 minutes of austenitization at 1173 K (900 °C), the ternary alloy coating transformed to consist of a nearly pure Zn as the major layer, a Fe-Al alloy layer at the interface, and a thin oxide overlay. The Fe-Al alloy layer effectively acted as the inhibition layer to prevent Fe from diffusing and reacting with Zn, which in turn retained the molten Zn layer and the integrity of the surface oxide layer. Moreover, the potential difference between the 11Al-3Mg-Zn coating and the steel substrate remained similar after austenitization, signifying the resulting coating kept its sacrificial protection capability.     

Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.     

Transformation of Oxide Inclusions in Type 304 Stainless Steels during Heat Treatment

Heat treatment of Type 304 stainless steel in the range of 1273 K (1000 °C) to 1473 K (1200 °C) can transform manganese silicate inclusions to manganese chromite (spinel) inclusions. During heat treatment, Cr reacts with manganese silicate to form spinel. The transformation rate of inclusions depends strongly on both temperature [in the range of 1273 K to 1473 K (1000 °C to 1200 °C)] and inclusion size. A kinetic model, developed using FactSage macros, showed that these effects agree quantitatively with diffusion-controlled transformation. A simplified analytical model, which can be used for rapid calculations, predicts similar transformation kinetics, in agreement with the experimental observations.     

Microstructural Evolution and Mechanical Properties of Short-Term Thermally Exposed 9/12Cr Heat-Resistant Steels

The microstructural evolution during short-term (up to 3000?hours) thermal exposure of three 9/12Cr heat-resistant steels was studied, as well as the mechanical properties after exposure. The tempered martensitic lath structure, as well as the precipitation of carbide and MX type carbonitrides in the steel matrix, was stable after 3000?hours of exposure at 873?K (600?°C). A microstructure observation showed that during the short-term thermal exposure process, the change of mechanical properties was caused mainly by the formation and growth of Laves-phase precipitates in the steels. On thermal exposure, with an increase of cobalt and tungsten contents, cobalt could promote the segregation of tungsten along the martensite lath to form Laves phase, and a large size and high density of Laves-phase precipitates along the grain boundaries could lead to the brittle intergranular fracture of the steels.     

Transformation of Inclusions in Linepipe Steels During Heat Treatment

Metallurgical and Materials Transactions B - The effect of heat treatment on the transformation of inclusions in linepipe steels was investigated experimentally and theoretically. During heat...     

Transformation Induced Martensite Evolution in Metal Forming Processes of Stainless Steels

Because of their high corrosion resistance and deformation characteristics, the industrial application of stainless steels is of high importance. During deep drawing processes, phase transformation of austenite to martensite occurs, which leads to an increased strain hardening of the material. The phase transformation depends on alloying constituents, transformation temperatures, stresses and strains. Consequently, in the design of deep drawing processes of stainless steels the phase transformation has to be considered. This paper presents a mathematical model for the calculation of the martensite evolution depending on temperatures, stresses and strains. The precise simulation of deep drawing processes of stainless steels can be enabled by the implementation of this model into commercial FE‐programs.     

Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon,Manganese Transformation-Induced-Plasticity Steel

Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 1 mass pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 3 mass pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 3 mass pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 1 GPa with a total elongation over 25 pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.     

Phase Transformations and Microstructural Evolution of Mo-Bearing Stainless Steels

The good corrosion resistance of superaustenitic stainless steel (SASS) alloys has been shown to be a direct consequence of high concentrations of Mo, which can have a significant effect on the microstructural development of welds in these alloys. In this research, the microstructural development of welds in the Fe-Ni-Cr-Mo system was analyzed over a wide variety of Cr/Ni ratios and Mo contents. The system was first simulated by construction of multicomponent phase diagrams using the CALPHAD technique. Data from vertical sections of these diagrams are presented over a wide compositional range to produce diagrams that can be used as a guide to understand the influence of composition on microstructural development. A large number of experimental alloys were then prepared via arc-button melting for comparison with the diagrams. Each alloy was characterized using various microscopy techniques. The expected δ-ferrite and γ-austenite phases were accompanied by martensite at low Cr/Ni ratios and by σ phase at high Mo contents. A total of 20 possible phase transformation sequences are proposed, resulting in various amounts and morphologies of the γ, δ, σ, and martensite phases. The results were used to construct a map of expected phase transformation sequence and resultant microstructure as a function of composition. The results of this work provide a working guideline for future base metal and filler metal development of this class of materials. An erratum to this article can be found at     

Influence of Heat Treatment on the Microstructure and Low-Temperature Performance of Low-Carbon Steel

In the present work, 20L and 20GL steels are investigated with a view to increase in their impact strength at negative temperatures, by adjusting the structural state in final heat treatment of valve and pipeline components. Stable impact strength KCV may be established in 20L steel at –40°C and in 20GL steel at ?60°C by three-stage heat treatment: double quenching in water and subsequent high tempering. The other three heat treatments considered proved ineffective. The greater strength of 20GL steel components in comparison with 20L steel components may be attributed to its finer structure and greater bulk content of acicular α phase, on account of the higher content of manganese, which deters the ferrite transformation of austenite.     

一种高硅Cr—Ni不锈钢热处理后的显微组织转变

研究了一种铸态σ相强化的高硅Ｃｒ－Ｎｉ不锈钢热处理后的显微组织转变。试验结果表明，在１１５０℃高温固溶状态下钢具有α＋γ双相组织；在固溶后冷却及８００℃附近时效过程中，α相和γ相不稳定，发生３种相变反应，即α→α＋σ、α→γ′＋σ及γ→γ＋σ。长期时效后，通过上述反应，最终形成以γ相为基体的γ＋σ双相组织。根据试验结果分析讨论了该钢的组织不稳定性、由α＋γ双相组织转变成γ＋σ组织的相变过程、σ相的形成及强化机制。     

相变诱发塑性钢研究开发的进展

介绍了高强度低合金相变诱发塑性钢的研究现状,残余奥氏体形态、尺寸和体积分数对钢强塑性的影响以及为改善钢的热镀锌性能所进行的以铝代硅的成功开发;以热力学、动力学为工具进行的TRIP钢的成分设计以及相变塑性钢高速冲击拉伸性能的特点;800～1200MPa级微合金TRIP钢的研发和激光焊接试验以及具有优良焊接性能和热镀锌性能的新型相变塑性钢的开发进程.     

1Cr17Ni2钢的热处理工艺研究

1Cr17Ni2钢在生产应用过程中其裂纹缺陷及热处理工艺影响着材料的综合性能,通过热处理工艺的优化,可获得优良的综合性能,研究表明,不同回火温度下该材料的碳化铬析出相的数量存在差异,从而导致冲击韧性的起伏。     

Local Composition Migration Induced Microstructural Evolution and Mechanical Properties of Non-equiatomic Fe40Cr25Ni15 Al15Co5 Medium-Entropy Alloy

A newly designed composition of non-equiatomic Fe₄₀Cr₂₅Ni₁₅Al₁₅Co₅ medium-entropy alloy (MEA) was produced by induction melting (IM). The as-cast alloy was found to consist of a two-phase microstructure of BCC (2.87 ± 0.01 Å) and ordered B2 (2.88 ± 0.02 Å) type phases. The structures of these phases were confirmed through X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It was observed that the Ni-Al-enriched ordered B2 phase of cuboidal shapes (~ 100 to 200 nm) is homogeneously distributed in Fe-Cr-rich BCC matrix with a cube-on-cube orientation relationship. The formation of the columnar dendrites (width 50 to 100 μm) was identified through optical microscopy (OM). The structural and microstructural stability of the alloy was investigated by heat-treating the alloy through different schedules. Heat-treated samples at different temperatures (< 1273 K) exhibit a similar type of two-phase microstructure with columnar dendrites. However, compositional rearrangement takes place during long time exposure to develop polymorphically related phases. The alloy was observed to possess a high compressive yield strength and hardness, i.e., ~ 1047 MPa and 391 ± 9 HV, respectively, at room temperature. Heat-treated samples at 600 °C and 900 °C (873 K and 1173 K) showed an increase in yield strength and ultimate strength with a significant increase in plasticity due to the increase in volume fraction of B2 phase and softening of the BCC matrix phase. The thermal stability and high strength of this alloy may open new avenues for high-temperature applications.

     

Precipitation Reactions during the Heat Treatment of Ferritic Steels

The precipitation reactions in two ferritic steels, 9Cr-1Mo-V-Nb and 12Cr-1Mo-V-W, were studied. Analytical electron microscopy, optical microscopy, electrolytic extractions, and hardness measurements were used to determine the types, amounts, and effects of precipitates formed as a function of the heat treatment. The effect of variations in the austenitizing treatment was ascertained. In addition to variations in the austenitizing time and temperature, different cooling rates after austenitization were also used. Air cooling after austenitization (normalization) resulted in little precipitation in both alloys. Precipitation in the 12Cr-1Mo-V-W alloy after furnace cooling was found in all cases examined. Under certain conditions precipitation was also found after furnace cooling the 9Cr-1Mo-V-Nb alloy. However, when compared to the amount of precipitate in the fully tempered state, the 9Cr-1Mo-V-Nb showed a much greater variation in the degree of precipitation following furnace cooling. In addition, the matrix microstructure of the 9Cr-1Mo-V-Nb alloy was very sensitive to cooling rate. The precipitation reactions during tempering after a normalizing treatment were followed as a function of tempering treatment. Tempering temperatures were varied from 400 to 780 °C. The carbide precipitation was essentially complete after one hour at 650 °C for both alloys. Analytical microscopy was used to identify the precipitates. In the 9Cr-1Mo-V-Nb alloy, a combination of chromium-rich M₂₃C₆ and vanadium-niobium-rich MC carbides was found. The carbides in the 12Cr-1Mo-V-W alloy were identified as chromium-rich M₂₃C₆ and vanadium-rich MC. The results give an indication of the sensitivity of these alloys to heat treatment variations. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Fatigue Behavior of Four High-Mn Twinning Induced Plasticity Effect Steels

Fatigue behavior of four high-Mn (18 to 22 wt pct Mn) steels, after cold rolling and soft annealing, were investigated in reversed plane bending in a high-cycle regime. The surfaces of fatigued specimens were examined using an optical microscope, a scanning electron microscope (SEM), and an atom force microscope (AFM). It was discovered that the chemical composition of the steel had only a minor influence on fatigue behavior, and the fatigue limit (FL), i.e., the stress amplitude leading to more than 2·10⁶ cycles to failure, of all steels is about 400 MPa. This is about 42 to 48 pct of their tensile strength whose value is comparable to that of austenitic stainless steel and 780 MPa transformation induced plasticity (TRIP) steel. Mechanical twinning does not occur in the course of cycling; however, intense slip bands are formed. Fatigue cracks tend to nucleate at quite an early stage of fatigue life and most commonly on grain and annealing twin boundaries due to intersections of slip bands and boundaries.     

Characterization of Transformation Stasis in Low-Carbon Steels Microalloyed with B and Mo

In the present study, bainite transformation kinetics was examined in low C-Mn steels with the addition of small amounts of B and Mo. This addition delays the onset of the bainite transformation. Mo addition causes transformation stasis at temperatures between 873 K and 823 K (600 °C and 550 °C) just below the bainite-start (B _s) temperature, resulting from an incomplete bainite transformation. Post-stasis transformation after a prolonged hold proceeds by the formation of ferrite with a low dislocation density, and in Mo-containing alloys, often the formation of carbides. The volume fraction at which the transformation stops is higher for lower carbon contents and lower transformation temperatures. By contrast, at 773 K (500 °C), the bainite transformation accompanying cementite precipitation occurs regardless of microalloying and is completed after shorter hold times. EDX measurement performed on the Mo-added 0.15 pct C alloy with aberration-corrected STEM revealed that segregation at the bainite/austenite interphase boundary is small for Mn and negligible for Mo in the early stages of stasis, which does not support the incomplete transformation mechanism based on the solute drag theory for the alloys used.     

Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel

Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 °C and 600 °C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 °C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M _s ) and martensite finish (M _f ) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the M _s temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 °C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed.