铁素体不锈钢焊接接头的再结晶现象

考察了新型铁素体不锈钢焊接接头中的再结晶现象，分析了其形成特点及相关影响因素。     

Evolution of microstructure and mechanical properties for 9Cr18 stainless steel during thixoforming

Hot compression tests were carried out in the semi-solid state of 9Cr18 stainless steel on Gleeble-1500 thermal simulation testing machine to investigate the effects of thixoforming parameters on its microstructure and mechanical properties. In this paper, microstructure was observed by scanning electron microscopy (SEM) and analyzed using energy dispersive spectrometer (EDS), and true stress–stain curves of the specimens with different initial microstructures after thixoforming were obtained to study the deformation mechanism. The results showed that thixoforming parameters such as reheating temperature and the strain rate had a significant influence on microstructure and mechanical properties evolution of 9Cr18 semi-solid billet. With increasing reheating temperature or decreasing strain rate, average size of carbides decreased from 2 μm to 0.5 μm, and the phenomenon of liquid extrusion during thixoforming became more obvious. During thixoforming, carbon atoms diffused to molten metal from austenite in the centre of specimens. When thixoforming temperature reached 1300 °C, martensitic transformation occurred after rapid cooling. Flow stress of semi-solid billet was lower than traditional ingot casting and hot rolled state steel, when reheated to the semi-solid range, due to their different original microstructure.     

Processing Map of AZ31-1Ca-1.5 vol.% Nano-Alumina Composite for Hot Working

A processing map for extruded AZ31-1Ca-1.5NAl composite has been developed, which exhibited four important domains for hot working. The corresponding temperatures and strain rates associated with these domains are: (1) 250–350°C and 0.0003–0.01 s^?1; (1A) 350–410°C and 0.0003–0.01 s^?1; (2): 410–490°C and 0.002–0.2 s^?1; and (3) 325–410°C and 0.6 s^?1 to 10 s^?1. Dynamic recrystallization (DRX) occurred in all the four domains although different slip mechanisms and recovery processes are involved. Basal slip and prismatic slip dominates deformation in Domains 1 and 1A, respectively, with recovery occurring by climb that is lattice self-diffusion controlled. However, because of the high strain rates in Domain 3, recovery occurs through a climb process, controlled by grain boundary self-diffusion. The recovery mechanism in Domain 2 is cross-slip assisted by pyramidal slip along with basal and prismatic slip. The grain size has a linear relation with Zener–Hollomon parameter in all the domains. At high strain rates, the composite undergoes shear fracture at lower temperatures and intercrystalline fracture at higher temperatures. All of the identified DRX domains are suitable for conducting bulk metal forming processes although the one with the highest strain rates (Domain 3) is preferred for achieving high productivity.     

钪对2618合金挤压前后组织及性能的影响

主要研究了添加Ｓｃ对２６１８合金挤压前后及挤压加淬火时效后组织与性能的影响，同时也考察了Ｃｕ含量的影响。结果表明，添加Ｓｃ进行合金化可以改善２６１８合金的显微组织，提高组织稳定性，有利于获得亚结构，实现预形变热处理；可以明显提高合金最终强度、塑性以及热稳定性。分析认为这主要归功于Ａｌ３Ｓｃ的沉淀硬化、Ｓ相的细化以及强烈的挤压效应。     

Effect of the extrusion conditions on microstructure evolution of the extruded Al–Mg–Si–Cu alloy rods

Hot extrusion experiment was conducted using an Al–Mg–Si–Cu alloy and the effect of the extrusion conditions on microstructure and texture changes through the radial direction was investigated by using SEM/EBSP analysis method. In the surface layer where severe frictional shear deformation is predominant, the recrystallized 1 1 0//ED grains surrounded by high angle grain boundaries are formed in spite of the existence of some peripheral overcoarse grains. Strong 1 0 0//ED and 1 1 1//ED fiber textures evolve in the center where axisymmetric deformation along the extrusion direction is intensive. As the extrusion ratio increases, number of 1 1 1//ED grains remarkably decreases while the number of 1 0 0//ED grains apparently increases. It is also found that the 1 0 0//ED grains surrounded by low angle grain boundaries form orientation colonies in the center of the extruded rods.     

Failure Analysis of an 18% Cr Ferritic Stainless Steel in a Simulated Exhaust Condensate Containing Urea

Exposure in the laboratory simulating a selective catalytic exhaust system component using urea injection for NO _x reduction and an artificial exhaust condensate in a diesel application has been performed. The hot, cyclic corrosion test was designed to simulate the conditions seen during use in a commercial vehicle application. The sample was subjected to a cyclic thermal gradient from ambient to a high of 500 °C and a 5% solution of urea containing sulfates, chlorides, and nitrates. During material evaluation for a selective catalytic exhaust component, a perforation was observed in a 439 ferritic stainless steel component. Examination of the sample found multiple modes of failure and multiple types of deposits from the urea test solution.     

Microstructural Evolution of Rapidly Solidified ZK60 Powders during Extrusion

The microstructural evolution of rapidly solidified(RS) ZK60 powders extruded at 250 C was investigated.It was shown that formation of new ultrafine grains took place through continuous dynamic recrystallization(CDRX),accompanied by the perfect bonding of powders via severe plastic deformation.At a low strain level,a well-defined structure made up of equiaxed and elongated subgrains was developed.Simultaneously,the operation of basal and non-basal dislocation slip led to the formation of low-angle dislocation cells(LADC) within the elongated subgrains.With increasing strain,the number and average misorientation of LADC increased,resulting in fragmentation of original elongation subgrains into a finally homogeneous fine-grained structure.Almost full-recrystallized structure with an average grain size of 0.4 μm was finally evolved after large cumulative strain.The results suggested that structural change was connected with thermal strain,where dislocation activities dominated this process.     

Recrystallization behavior of a supersaturated Al-Mn alloy

The effect of concurrent precipitation on recrystallization behavior during the isothermal annealing of a supersaturated and deformed Al-Mn alloy was investigated. It is found that concurrent precipitation strongly affects the recrystallization behavior of this alloy. At low temperatures, concurrent precipitation retards recrystallization and results in large flat grains. The size of recrystallized grains decreases significantly with increasing temperature. The kinetics of recrystallization was determined by measurements of hardness. The JMAK exponent decreases from 3.0 to 0.8 as the annealing temperature increases from 371 °C to 427 °C. The activation energy for recrystallization of the alloy is about 456 kJ/mol. Concurrent precipitation enhances the activation energy for recrystallization of aluminum alloys.     

Creep properties of 12% Cr and improved 9% Cr weldments

Martensitic Cr-alloyed high-temperature materials offer interesting opportunities for design and construction of advanced power plants. An extensive research programme has been carried out at the Research Centre of the Belgian Welding Institute and Laborelec on martensitic 12% Cr steel for gaining a better understanding of the failure mode and the deformation mechanism of welded joints under uniaxial and multiaxial loads. A large number of pipe girth welds were realized by three Belgian manufacturers (Cockerill Mechanical Industries, Fabricom and Mannesmann-Carnoy). Different filler metals were used and the influence of the welding regime (austenitic and martensitic), the post-weld heat treatment (PWHT) (single or double) and the base metal wall thickness on the high-temperature properties of the different weldments was evaluated. It was found that the creep properties of a 12% Cr weldment are not influenced by the welding regime and the base metal wall thickness. As would be expected, the creep strengths of the original 12% Cr base metal as well as the temperature of the PWHT have some effect. The existence of a typical failure in the intercritical zone (type IV region) is demonstrated and explained. The consequences for the design of welded 12% Cr components are indicated. More recently the research was extended towards improved 9% Cr steel (T91). A rather small preliminary programme for the orientation of further research showed a similar failure location as for 12% Cr steel, although the observed loss in strength of the weldment compared to the base metal tended to be considerably lower. The so-called ‘half-tempering’ treatment was tried out and the effect on the creep strength of the weldment is shown. A more fundamental national research programme on P91 steel has been established and is actually running.     

Microstructure and Properties of 3.5 vol.% TiBw/Ti6Al4V Composite Tubes Fabricated by Hot-hydrostatic Extrusion

Tubes of 3.5 vol,% TiB whiskers reinforced Ti6Al4V matrix composites （TiBw/Ti6Al4V） were successfully fabricated by a two-step hot-hydrostatic extrusion process： （3 extrusion at 1100 ℃ and subsequent near-β extrusion at 950℃. The dimensions of tubes were about 7 mm in diameter and 2 mm in thickness. A refined basket-weave structure in Ti6Al4V matrix was achieved at ambient temperature after the extrusion process. Besides, the original network structure formed by TiB whiskers synthesized was broken, while the TiB whiskers were preferentially aligned in the extruding direction. Meanwhile, a fibrous texture was evolved finally, resulting from partial dynamic recrystallization during the β extrusion and the involvement of α phase during the near-β extrusion. The tensile and compressive tests results showed that both the strength and ductility of the tubes were significantly improved. In particular, the tubes exhibited good mechanical properties at elevated temperatures.     

Influence of Die Angle on Containerless Extrusion of Commercially Pure Titanium Tubes

Containerless tube extrusion has been investigated with commerically pure titanium at room temperature and a strain rate of 0.07 s^-1 using 20 conical dies of five different strains and four different angles with MoS₂ lubricant. Theoretical punch pressures have been calculated using appropriate equations from slab analysis of the process and compared with experimentally determined punch pressures. It is found that there exists an optimum angle at which the punch pressure is the least at a given strain.     

Microstructures and Toughness of Weld Metal of Ultrafine Grained Ferritic Steel by Laser Welding

3 mm thick 400 MPa grade ultrafine grained ferritic steel plates were bead-on-plate welded by CO2 laser with heat input of 120～480 J/mm. The microstructures of the weld metal mainly consist of bainite, which form is lower bainite plates or polygonal ferrite containing quantities of dispersed cementite particles, mixed with a few of low carbon martensite laths or ferrite, depending on the heat input. The hardness and the tensile strength of the weld metal are higher than those of the base metal, and monotonously increase as the heat input decreases. No softened zone exists inheat affected zone (HAZ). Compared with the base metal, although the grains of laser weld are much larger, the toughness of the weld metal is higher within a large range of heat input. Furthermore, as the heat input increases, the toughness of the weld metal rises to a maximum value, at which point the percentage of lower bainite is the highest, and then drops.     

Microstructures and Toughness of Weld Metai of Ultrafine Grained Ferritic Steel by Laser Welding

3 mm thick 400 MPa grade ultrafine grained ferritic steel plates were bead-on-plate welded by CO2 laser with heat input of 120-480 J/mm. The microstructures of the weld metal mainly consist of bainite, which form is lower bainite plates or polygonal ferrite containing quantities of dispersed cementite particles, mixed with a few of low carbon martensite laths or ferrite, depending on the heat input. The hardness and the tensile strength of the weld metal are higher than those of the base metal, and monotonously increase as the heat input decreases. No softened zone exists in heat affected zone (HAZ). Compared with the base metal, although the grains of laser weld are much larger, the toughness of the weld metal is higher within a large range of heat input. Furthermore, as the heat input increases, the toughness of the weld metal rises to a maximum value, at which point the percentage of lower bainite is the highest, and then drops.     

Optimization of Process Parameters of Hot Extrusion of SiC/Al 6061 Composite Using Taguchi's Technique and Upper Bound Technique

Significance of improving the material processing techniques for SiC/Al composite has been felt due to its enormous application in various key industries. In this research study, SiC reinforced that Al 6061 composite has been developed specially through stir casting route and the same was admitted to hot extrusion process to convert the round geometry into the hexagonal section. Totally, nine experiments were conducted based on L9 orthogonal array emphasized by Taguchi's technique, and the optimum levels were predicted based on the average response graph method. During the experiments, ram speed, temperature of the billet, and a friction between the die and the billet were considered as the process variables, thereby considering the extrusion force as the response variable. Additionally, the analysis of variance has been applied to determine the most significant factor to influence the response. At last, confirmation test was carried out to validate the results of the optimized model. In order to enhance the degree of validation, very famous analytical method of the upper bound technique was also employed to compare the results of the optimized model. Results of the upper bound technique and confirmation test were deviated in zero tolerance with the predicted one.     

高速钢高温变形再结晶规律研究

本文研究了W9Mo~3Cr~4V钢的热变形奥氏体动态再结晶规律。结果表明:由于形变诱发析出碳化物的影响,W9Mo~3Cr~4V钢热变形奥氏体动态再结晶方式与低碳低合金钢的晶界弓出机构和晶界生核机构不同,是通过亚晶合并形成再结晶晶粒的,即亚晶合并机构。当形变速率较低时,可由亚晶粒直接形成再结晶晶粒。形变温度和形变速率对W9Mo~3Cr~4V钢热变形奥氏体动态再结晶影响比较复杂,不能简单地用Z参数来统一描述。     

Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel

In this study,the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic(RAFM) steel friction stir welded joints were investigated.When a W-Re tool is used,the recommended welding parameters are 300 rpm rotational speed,60 mm/min welding speed and10 kn axial force.In stir zone(SZ),austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths.The microstructure in SZ contains lath martensite with high dislocation density,a lot of nano-sized MX and M_3C phase particles,but almost no M_(23)C_6 precipitates.In thermal mechanically affect zone(TMAZ)and heat affect zone(HAZ),refinement of prior austenite and martensitic laths and partial dissolution of M_(23)C_6 precipitates are obtained at relatively low rotational speed.However,with the increase of heat input,coarsening of martensitic laths,prior austenite grains,and complete dissolution of M_(23)C_6 precipitates are achieved.Impact toughness of SZ at-20?C is slightly lower than that of base material(BM),and exhibits a decreasing trend with the increase of rotational speed.     

热轧工艺对Cr12钢表面起皱的影响机制

研究了热轧工艺对Cr12钢表面起皱相关的组织和织构演变的影响,重点关注其对变形组织不均匀和晶粒簇形成的改变。结果表明：随着热轧终轧温度由960降低到850℃,在变形组织中带状晶粒减薄,剪切变形比重增大,含有剪切带的晶粒增多,由表层向中心层逼近,中心层附近光滑带状晶粒减少;在退火组织中由含有剪切带晶粒形成的高r值晶粒簇增...     

T91铁素体耐热钢强化新途径

T91钢以其优异的综合性能成为高Cr铁素体耐热钢的代表钢种,被广泛应用于超临界发电厂锅炉管道上,同时也是开发更高使用温度的铁素体耐热钢材的研究基准.为进一步提高其耐热温度进而提高电厂热效率,在对T91钢组织结构及强化机理深入分析的基础上,探讨了形变热处理工艺通过优化组织来提高其性能的可行性,指出了高Cr铁素体耐热钢新的研究方向.