Influence of nickel on microstructure and mechanical properties of medium-manganese steels

To combine the advantage of density reduction and excellent performance, nanometer-sized B2 particles were introduced into the δ ferrite matrix of high-aluminium low-density steel by the addition of nickel (Fe–0.2C–11Mn–6Al–4/8Ni). Compared to Fe–0.2C–11Mn–6Al (0Ni) steel, the hardness and tensile strength of 4Ni and 8Ni steels are significantly improved. The improvement of tensile strength in 4Ni and 8Ni steels was primarily contributed by the precipitation strengthening or solution strengthening of B2 particles in δ ferrite. At the higher annealing temperature, the original dislocation density in δ ferrite is lower. However, dislocation multiplication during tensile deformation was more significant in the sample annealed at higher temperature, which was responsible for a higher work hardening rate.     

Influence of hydrogen on mechanical properties of nitrogen supersaturated austenitic stainless steels

Abstract

Alloying austenitic stainless steels with nitrogen up to a concentration of 1 wt-% improves yield strength, tensile strength, and ductility. Further increase in the nitrogen concentration results in chromium nitride precipitation at the grain boundaries and a decrease in the ductility with a change in the fracture mode from ductile to intergranular. Hydrogen charging causes high reversible dilatation in the lattice and remarkable reduction in the ductility. The ductility losses caused by hydrogen are more pronounced at higher nitrogen concentrations and a change of the fracture mode from intergranular to transgranular is observed in steels with more than 1 wt-% nitrogen. Chromium nitride precipitates are shown to have an insignificant role in the hydrogen embrittlement. Hydrogen charging steels with nitrogen concentrations of below 1 wt-% enhances the strengthening effect of nitrogen but, at higher nitrogen concentrations, hydrogen is shown to be detrimental to the strength.     

Effects of aluminum additions on the microstructure and mechanical properties of alumina-forming austenitic stainless steels

The mechanical properties, including tensile and impact properties at different testing temperatures of alumina-forming austenitic steels (25 % nickel, 20 % chromium) with different aluminum contents (0, 2.5 %, 5 % and 8 %) were investigated. Scanning and transmission electron microscopy together with tensile and impact properties tests were conducted. The results showed that the tensile strength of steels at 298 K increased obviously along with aluminum contents increasing, while plasticity decreased at the same, which attributed to the higher volume fraction and number density of spherical NiAl precipitation together with main ferrite in matrix. In addition, spherical NiAl particles dispersed easily in ferrite. In particular, the ultimate tensile strength of the sample with 8 % aluminum could reach 1398 MPa, with the elongation of 14 % at 298 K. However, NiAl precipitations would lose strengthening effects at high temperatures, but the plasticity could be improved. In addition, the sample with 5 % aluminum showed better comprehensive properties by comparison to other samples, and the ultimate tensile strength was 1018 MPa and 491 MPa at 298 K and 973 K with the elongation of 26 % and 43 %, respectively, enabling it to be promising material for industrial application in advanced nuclear systems.     

Effect of aluminum on microstructure,mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels

Effect of aluminum on microstructure, mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels has been investigated. Aluminum can significantly increase the ratio of equiaxed crystal grains, but the promotion effect has great relation with aluminum content. Aluminum can stabilize ferrite phase and significantly reduce recrystallization temperature. Increased aluminum content can also lead to the precipitate of AlN and Al₂O₃ at higher temperature. The increased amount of AlN may partly contribute to the reduced nitrogen element to form austenite at high temperature, hence the high temperature phase transformation of α + γ → α occurs. The fine and large number of Al₂O₃ particles can refine grain size and then promote recrystallization. The highest intensity of γ-fiber texture {1 1 1}〈1 1 2〉 is observed in the steel with 0.19 wt.% aluminum, which can improve the formability of steels. With the increase of aluminum content, the tensile strength increases linearly but the elongation and plastic strain ratio first increase then decrease, the working hardening index varies slightly among the steels. Appearance of Al₂O₃ inclusions with small size and decreased content of MnS benefit pitting corrosion resistance. However, the large dimension Al₂O₃ inclusions have significantly negative influence on pitting corrosion resistance.     

Effect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints

Influence of heat input on the microstructure and mechanical properties of gas tungsten arc welded 304 stainless steel (SS) joints was studied. Three heat input combinations designated as low heat (2.563 kJ/mm), medium heat (2.784 kJ/mm) and high heat (3.017 kJ/mm) were selected from the operating window of the gas tungsten arc welding process (GTAW) and weld joints made using these combinations were subjected to microstructural evaluations and tensile testing so as to analyze the effect of thermal arc energy on the microstructure and mechanical properties of these joints. The results of this investigation indicate that the joints made using low heat input exhibited higher ultimate tensile strength (UTS) than those welded with medium and high heat input. Significant grain coarsening was observed in the heat affected zone (HAZ) of all the joints and it was found that the extent of grain coarsening in the heat affected zone increased with increase in the heat input. For the joints investigated in this study it was also found that average dendrite length and inter-dendritic spacing in the weld zone increases with increase in the heat input which is the main reason for the observable changes in the tensile properties of the weld joints welded with different arc energy inputs.     

Prediction and optimization of friction welding parameters for super duplex stainless steel (UNS S32760) joints

Friction welding finds widespread industrial use as a mass production process for joining materials. Friction welding process allows welding of several materials that are extremely difficult to fusion weld. Friction welding process parameters play a significant role in making good quality joints. To produce a good quality joint it is important to set up proper welding process parameters. This can be done by employing optimization techniques. This paper presents a multi objective optimization method for optimizing the process parameters during friction welding process. The proposed method combines the response surface methodology (RSM) with an intelligent optimization algorithm, i.e. genetic algorithm (GA). Corrosion resistance and impact strength of friction welded super duplex stainless steel (SDSS) (UNS S32760) joints were investigated considering three process parameters: friction force (F), upset force (U) and burn off length (B). Mathematical models were developed and the responses were adequately predicted. Direct and interaction effects of process parameters on responses were studied by plotting graphs. Burn off length has high significance on corrosion current followed by upset force and friction force. In the case of impact strength, friction force has high significance followed by upset force and burn off length. Multi objective optimization for maximizing the impact strength and minimizing the corrosion current (maximizing corrosion resistance) was carried out using GA with the RSM model. The optimization procedure resulted in the creation of nondominated optimal points which can aid the process operator to fix the input control variables. The selection of a point from the Pareto front will always be a trade-off between the corrosion resistance and impact strength of the weld depending on the application.     

TWIP钢激光拼焊板的力学性能及微观组织分析

为评价TWIP钢的焊接性能,对1.2 mm厚的Fe-Mn-C系TWIP钢进行了激光拼焊实验,用线扫描分析了锰元素的分布,对拼焊板进行了拉伸试验,用扫描电镜观察了拉伸断口形貌,采用背散射电子分析技术分析了拉伸前后的组织变化及孪晶形成,测试了维氏硬度的分布曲线.实验结果表明:焊缝区未发生Mn元素的挥发,室温下为显著的柱状枝晶铸态组织;断裂发生在存在气孔等微小缺陷的焊缝区,拼焊板强度与母材接近,韧性显著降低;拉伸过程中有大量的形变孪晶形成,拉伸前后均为全奥氏体组织.     

Influence of the microstructure on the toughness of a duplex stainless steel UNS S31803

The microstructure of a duplex stainless steel UNS S31803 was varied by high temperature treatments (1300°C) followed by different cooling rates. A wide range of microstructures, with differents morphologies and phase proportions, were obtained by this way. Some samples were solution treated at 1000°C and fast cooled after the high temperature treatment. The impact toughness in all conditions were evaluated by reduced size (2.5 mm) Charpy impact tests. The highest toughness was obtained in the samples cooled in furnace from 1300 to 1000°C and then air cooled to room temperature. The microstructure at this condition was very fine with 55.4% of austenite. The lower toughness value was obtained in the water cooled sample, which presented only 17.1% of austenite and large grains of ferrite. The toughness of these and other microstructures was improved by the solution treatment.     

On the microstructure and mechanical properties of high-speed steels

The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. The role of the primary carbides has been reported to be of no importance in strengthening the steels, due to their large size and large interparticle spacing. The present authors have studied the role of the primary carbides on the wear of high-speed steels and found them to be of no importance, and under certain conditions contributing to higher wear rates. It has been shown analytically and experimentally that in quenched and tempered high-speed steels, the precipitation of the secondary hardening carbide (cubic M₂C type) is the main reason for the improved strength and wear resistance. This shows that the secondary hardening phenomenon of high-speed steels is a direct result of the hardening caused by the precipitation of the cubic M_2C-type carbide. The present study has estimated that at peak hardness the volume fraction of secondary hardening carbides is approximately 20%. The measured strength of high-speed steels was found to be lower than the theoretically calculated strength due to non-homogeneous precipitation of the secondary hardening carbides. Areas which were observed to be free from secondary hardening carbides are real and are not artefacts. It has been shown that the strength of high-speed steel in the region of peak hardness depends primarily on the precipitation of the secondary hardening carbide and secondarily on martensitic strengthening.     

Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750

This paper investigates the weldability, metallurgical and mechanical properties of the UNS 32750 super-duplex stainless steels joints by Gas Tungsten Arc Welding (GTAW) employing ER2553 and ERNiCrMo-4 filler metals. Impact and tensile studies envisaged that the weldments employing ER2553 exhibited superior mechanical properties compared to ERNiCrMo-4 weldments. Microstructure studies performed using optical and SEM analysis clearly exhibited the different forms of austenite including widmanstatten austenite on the weld zone employing ER2553 filler. Also the presented results clearly reported the effect of filler metals on strength and toughness during the multi-pass welding. This research article addressed the improvement of tensile and impact strength using appropriate filler wire without obtaining any deleterious phases.     

Grain size dependence of mechanical, corrosion and tribological properties of high nitrogen stainless steels

Austenitic stainless steels have been indispensable for the progress of technology during the last 80 years. Due to the cost of nickel and to the prospective of allergic reactions caused by this element, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low nickel content. In order to maintain the austenitic microstructure, nickel reduction is balanced with nitrogen addition. Nitrogen addition to austenitic stainless steels is also very effective for improving yield strength and corrosion resistance without reducing ductility and toughness. In order to further increase the strength, it is possible to combine the effect of nitrogen addition and grain refining. The purpose of this study is to examine the relationship between microstructures and mechanical, corrosion and tribological properties of a high nitrogen stainless steel with an ultrafine grained structure.     

Effect of welding speed on microstructure and mechanical properties of friction stir welded copper

The 3-mm-thick copper plates were friction stir welded at a low tool rotation rate of 600 rpm. The influence of welding speed on microstructure and mechanical properties of the joints was investigated. As the welding speed increased, the grain size of nugget zone first increased and then decreased, the thermo-mechanically affected zone became narrow and the boundary between these two zones got distinct, but the heat affected zone was almost not changed. The ultimate tensile strength and elongation of the joints increased first and decreased finally with increasing welding speed, but the effect was little when the welding speed is in the range of 25–150 mm/min. The defect-free joints were produced at lower welding speeds, and the fracture locations were outside the nugget zone on the retreating side. With increasing welding speed, the average hardness of nugget zone decreased first and then increased, but welding speed had little effect on the hardness of the other regions within the joints.     

Effect of backplate diffusivity on microstructure and mechanical properties of friction stir welded joints

Series of welds were made by friction stir welding (FSW) with various backplates made out of materials ranging from low diffusivity granite to high diffusivity copper in order to reveal the effect of backplate diffusivity on the joint microstructure and properties. The temperature, microstructure, microhardness and tensile properties of joints were compared and discussed. Results show that the backplate with high diffusivity effectively decreases the heat input to the workpiece during FSW. With decreasing the backplate diffusivity the sizes of equiaxed recrystallized grains in the nugget zone increase obviously, while the hardness of the nugget zone also increases a little. The interface between the thermo-mechanically affected zone and nugget zone at the retreating side disappears under the granite backplate. Moreover, the ductility of the joint is more excellent under the copper backplate, but under the granite backplate the failure has mixed fracture characteristics of quasi-cleavage and dimples.     

Optimization of process parameters of friction welding of UNS S31803 duplex stainless steels joints

This article presents an approach based on Taguchi method and Grey relational analysis to optimize process parameters of friction welding of UNS31803 duplex stainless steel. The main objective is to maximize mechanical properties like tensile strength, hardness and impact toughness and to minimize corrosion rate. Heating pressure, heating time, upsetting pressure and upsetting time were the four process parameters taken each at three levels. According to Taguchi quality design concept, an L9 orthogonal array was selected for experiments. The best combination of process parameters was found by both Taguchi method and Grey relational analysis. The influence of the process parameters on overall quality characteristics of the friction welding process was evaluated by the analysis of variance (ANOVA) method. The confirmation test results with optimal parameters confirmed the effectiveness of the proposed method in this study. Later, comparison was done between Taguchi method and Grey relational analysis on the basis of improvement in multiresponse signal to noise (S/N) ratio over initial process parameters. Grey relational analysis was proved to be a better technique than Taguchi method for optimization of multiple responses.     

Microstructure,mechanical and corrosion properties of friction stir welded high nitrogen nickel-free austenitic stainless steel

Friction stir welding (FSW) was applied to a 2.4 mm thick high nitrogen nickel-free austenitic stainless steel plate using tungsten–rhenium (W–Re) tool. The high-quality weld was successfully produced at a tool rotational speed of 400 rpm and a traveling speed of 100 mm/min. The microstructure, mechanical and corrosion properties of the weld were studied. The nitrogen content of the weld was almost identical to that of base metal (BM). FSW refined grains in the stir zone (SZ) through dynamic recrystallization and led to increase in hardness and tensile strength within the SZ, while the ductility was slightly decreased. The failure of tensile specimens occurred in the BM. TEM results revealed precipitates of Cr₂₃C₆ of size ~ 1 μm in the SZ, although their content was small. The precipitation of Cr₂₃C₆ and increase in δ-ferrite in the SZ led to small decrease in both pitting and intergranular corrosion resistance.     

焊接参数对铁素体不锈钢搅拌摩擦焊接头组织及性能的影响

对4mm厚T4003铁素体不锈钢进行搅拌摩擦焊接工艺实验,研究焊接参数对接头组织特征、硬度分布及常温和低温冲击韧性的影响。结果表明:接头搅拌区和热力影响区由铁素体和马氏体双相组织构成;接头搅拌区组织沿试样厚度方向存在非均质性,且随转速的降低及焊接速率的增加越发显著;转速从150r/min增加至250r/min,前进侧热力影响区组织呈现小梯度过渡趋势,无明显变形拉长特征。焊缝硬度分布相对均匀,其最高硬度为290HV,约为母材的1.87倍。焊接参数和温度对接头的冲击吸收功有较大影响:常温(20℃)下,热影响区为母材的90%～92%,搅拌区为母材的85%～103%;低温(-20℃)下,热影响区为母材的87%～97%,搅拌区为母材的82%～95%,表明焊缝区仍具有较好强韧匹配。     

Effect of long term aging on mechanical properties and microstructure of nickel base weld

Abstract

The mechanical properties of a Ni base weld have been examined after long term aging in the temperature range 600–900°C. Impact testing revealed a substantial decrease in toughness after heat treatment at 600 and 700°C. In particular, after aging at 700°C there was a marked loss of ductility, which was associated with elongated particles appearing in the fracture surfaces. The concomitant microstructural changes occurring have been investigated using analytical electron microscopy. In material heat treated at 600°C for 10000 h, seven phases were identified: M₂₃C₆, MC, η, γ′, γ″, G, and δ. At 700°C, the following phases were distinguished after 3000 h: M₂₃C₆, MC, and η. After longer testing times G, γ′, and γ″, were formed. Whereas G, γ′, and γ″ formed intragranular needles, η formed intragranular laths or plates at 700°C. The minimum in ductility observed in material aged at 700°C can be explained in terms of copious intragranular precipitation of γ′, γ″, and η.

MST/1131     

Effect of TiO2-containing fluxes on the mechanical properties and microstructure in submerged-arc weld steels

This work presents a study of the effect of TiO₂ additions in fluxes on the mechanical properties and microstructure of the weld metal formed during Submerged-Arc Welding (SAW) of ASTM A-36 steel plates. Four fluxes with about 9, 12, 15 and 18% Ti were used with a low-carbon electrode. The welding conditions were kept constant. The microstructure of the weld metal for each flux consisted mainly of equiaxed ferrite and acicular ferrite. The increase in the percentage of acicular ferrite and a decrease in its length were observed with an increase in titanium content. The increase in titanium content in fluxes also improved the toughness and ductility of the welds.