Microstructural and Finite Element Analysis of Creep Failure in Dissimilar Weldment Between 9Cr and 2.25Cr Heat-Resistant Steels

Microstructural analysis and the creep failure mechanism of dissimilar weldment between ASTM A213 T92 (9Cr1.5W0.5MoVNbTi) and T22 (2.25Cr1Mo) heat-resistant steels are reported. The low-Cr part that has high carbon activity shows a depletion of C during postweld heat treatment. In particular, the soft carbon-depleted zone (CDZ) with the lowest hardness is surrounded by strong weld metal (WM) and the T22 heat-affected zone (HAZ). Load-displacement curves obtained by nanoindentation experiments are used to extract true stress–strain curves of the WM, the CDZ, and the T22 HAZ by using finite element methods (FEMs). Because of the mechanical properties of each region, the soft CDZ confined between harder regions is exposed to multiaxial stress. Therefore, creep voids actively form and coalesce in this CDZ and lead to macroscopic brittle fracture.     

Structure and Properties of Heat-Resistant Steels Obtained from Powders after Mechanical Activation

The effect of mechanical activation of heat-resistant steel 55Kh20N4AG9 powders on the structure and properties of high-density material obtained by dynamic hot pressing with extrusion elements was investigated. Mechanical activation improves the chemical homogeneity of the highly alloyed material, decreases the particle size of the strengthening carbonitride phase, promotes its interaction with dislocation tangles and increases their thermal stability. Mechanical activation of powders with intermediate anneals promotes the accumulation of thermally stable defects in the crystal structure and leads to the accelerated formation of a cellular fragmented structure in the austenite, characterized by a high dislocation density in block walls. The developed cellular fragmented substructure is retained after dynamic hot pressing and subsequent heat treatment, and elevates the mechanical properties of the high-density heat-resistant powder metallurgy steels.     

Creep Strain Modeling of 9 to 12 Pct Cr Steels Based on Microstructure Evolution

Creep deformation is simulated for 9 pct Cr steels by using the Norton equation with the addition of back stresses from dislocations and precipitates. The composite model is used to represent the heterogeneous dislocation structure found in 9 to 12 pct Cr steels. Dislocation evolution is modeled by taking capturing and annihilation of free dislocations into account. Recovery of immobile dislocations is derived from the ability of dislocation climb. In spite of the fact that the initial dislocation density is high and is reduced during creep, primary creep is successfully modeled for a P92 steel. Subgrain growth is evaluated using a model by Sandström (1977). The long time subgrain size corresponds well to a frequently used empirical relation, with subgrain size inversely proportional to the applied stress.     

Microstructural Evolution and Mechanical Properties of Nb-Ti Microalloyed Pipeline Steel

 The correlation between microstructures and mechanical properties of a Nb-Ti microalloyed pipeline steel was investigated. The results revealed that with decreasing the finish rolling temperature and the cooling stop temperature, the matrix microstructure was changed from quasi-polygonal ferrite to acicular ferrite, as a result of improvement of both strength and low temperature toughness. By means of electron backscattered diffraction observation, an effective acicular ferrite packet contained several low angle boundaries or subboundaries plates which made important contributions to improvement of strength. It was found that many fine quasi-polygonal ferrite grains with high angle boundaries as the toughening structure were introduced into the acicular ferrite matrix to refine effective grain size and improve the toughness.     

淬火冷却速度对耐热钢1Cr12Ni3Mo2VN组织和性能的影响

研究了1040℃1h油冷、炉冷(5℃/min)、1℃/min、0.5℃/min冷却后耐热钢1Cr12Ni3Mo2VN的组织和该钢1040℃1h不同冷却速度淬火+565℃2h空冷后的力学性能。试验结果表明,该钢4种冷却速度淬火均可得到马氏体组织,但油冷+回火的A_(KV2)值为156.5 J,而5～0.5℃/min冷却+回火时为40.5～16.5 J。残余奥氏体发生热失稳分解是导致试验钢淬火缓冷后冲击韧性显著下降的主要原因;在淬火缓冷过程中720～820℃这一温度段,由于原奥氏体晶界上碳化物的大量析出,使残余奥氏体中合金元素和碳含量的显著减少,造成淬火组织中的残余奥氏体稳定性大幅度下降。     

高温回火对1Cr12Ni3Mo2VN耐热钢力学性能和组织的影响

研究了1Cr12Ni3Mo2VN钢1010℃1 h油冷,2次580～620℃1 h回火空冷后的力学性能和组织。试验结果表明,在620℃回火拉伸试样中,回火马氏体板条界析出大量聚集的粗化的块状M_(23)C_6碳化物,试样塑性变形以孪晶方式进行,塑性较低;580℃回火拉伸试样塑性变形以滑移方式进行,塑性较好。该钢最佳回火工艺为两次580℃1 h空冷。     

Mechanical Properties and Microstructural Evolution of Friction-Stir-Welded Thin Sheet Aluminum Alloys

Friction stir welding of thin aluminum sheets represents a potential goal for aircraft and automotive industries because of the advantages of using this new technological process. In the current work, the microstructural evolution and mechanical behavior of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin friction-stir-welded joints were investigated. Uniaxial tensile testing at room temperature, 443 K, 473 K, and 503 K (170 °C, 200 °C, and 230 °C) was used to determine the extent to which these ultra-thin joints can be used and deformed. The tensile stress–strain curves showed a decrease of the flow stress with increasing temperature and decreasing strain rate. The ductility of 6082T6-6082T6 joints generally improved when deformed at warm temperatures. It was almost constant for the 6082T6-2024T3 and reached the higher value in the 2024T3-2024T3 when deformed at 443 K and 473 K (170 °C and 200 °C) when compared with the room temperature value. Tensile specimens fractured in the middle of the weld zone in a ductile mode. The precipitation and growth of S’ type phases strengthens 2024T3-2024T3 joints during deformation. In the 6082T6-6082T6, β″ precipitates show some increase in size but give a lower contribution to strength. At 503 K (230 °C), recovery mechanisms (dislocation reorganization inside the deformed grains) are initiated but the temperature was not enough high to produce a homogeneous subgrain structure.     

Directed Energy Deposition of Low-Alloyed Steels: An Insight on Microstructural and Mechanical Properties

Low-alloyed steels are used for a variety of different applications like bearings or gears. Additive manufacturing technologies like directed energy deposition (DED-LB/M) allow for a fast and close-to-contour fabrication of sophisticated products without excessive waste of material. However, the DED-LB/M process cannot be considered as state-of-the-art for this group of materials. This study presents findings on the material properties of the additively manufactured low-alloyed steel Bainidur AM by means of DED-LB/M. This includes studies on the mechanical properties (hardness, compression strength) as well as the microstructural properties (scanning electron microscopy [SEM]). The microstructure in the as-built state appears like a bainitic–martensitic one with shares of retained austenite which is not fully transformed during cooling. As a differentiation is barely possible from the SEM images, a plethora of investigations is further used to assess the microstructure. As-built samples possess a good combination of ductility and hardness. Furthermore, the specimens are characterized by a good tempering stability up to 600 °C. This tempering stability is characterized by a homogeneous hardness of around 400 HV1 for all temperatures. In contrast, the conventionally hardened specimens show a drop-off in material hardness, further indicating the excellent material properties of additively manufactured Bainidur AM.     

The Effect of Cooling Rate,and Cool Deformation Through Strain-Induced Transformation,on Microstructural Evolution and Mechanical Properties of Microalloyed Steels

In this article, a detailed study was conducted to evaluate the microstructural evolution and mechanical properties of microalloyed steels processed by thermomechanical schedules incorporating cool deformation. Cool deformation was incorporated into a full scale simulation of hot rolling, and the effect of prior austenite conditioning on the cool deformability of microalloyed steels was investigated. As well, the effect of varying cooling rate, from the end of the finishing stage to the cool deformation temperature, 673 K (400 °C), on mechanical properties and microstructural evolution was studied. Transmission electron microscopy (TEM) analysis, in particular for Nb containing steels, was also conducted for the precipitation evaluation. Results show that cool deformation greatly improves the strength of microalloyed steels. Of the several mechanisms identified, such as work hardening, precipitation, grain refinement, and strain-induced transformation (SIT) of retained austenite, SIT was proposed, for the first time in microalloyed steels, to be a significant factor for strengthening due to the deformation in ferrite. Results also show that the effect of precipitation in ferrite for the Nb bearing steels is greatly overshadowed by SIT at room temperature.     

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     

9Cr马氏体耐热钢发展及其蠕变寿命预测

9Cr马氏体耐热钢是目前热电厂关键设备制造的主选钢种,其最大的特点是600℃左右高温服役条件下良好的持久强度,较好的抗腐蚀性能。本文主要阐述了9Cr马氏体耐热钢的发展及其研究的最新进展,从材料的组织结构、蠕变特性和蠕变寿命预测等多方面叙述了有关9Cr马氏体耐热钢的研究动态。