Effects of dew point on selective oxidation of TRIP steels containing Si,Mn, and B

The selective oxidation of Si, Mn, and B on TRIP steel surfaces is a widely known phenomenon that occurs during heat treatment. However, the relationship between oxide formation and the annealing factors is not completely understood. This study examines the effect of the annealing conditions (dew point and annealing temperature) on oxide formation. A low dew point of −40 °C leads to the formation of Si-based oxides on the surface. A high dew point of −20 °C changes the oxide type to Mn-based oxides because the formation of Si oxides on the surface is suppressed by internal oxidation. Mn-based oxides exhibit superior wettability due to aluminothermic reduction during galvanizing.     

Ti对高硼钢显微组织和高温力学性能的影响

采用SEM和XRD研究了Ti对高硼钢显微组织的影响。采用冲击试验机、热力学模拟实验机、氧化增重法分析了Ti对高硼钢室温冲击及850℃高温力学、抗氧化性能的影响。结果表明,添加Ti后,基体内硼化物形态圆整、呈离散状分布,尺寸大幅减小。这种硼化物形态、分布的优化提高了高硼钢的室温冲击韧性。高硼钢中添加Ti后在基体内形成了TiC析出相,并使基体由单一奥氏体转变为奥氏体+铁素体双相组织。添加Ti元素后,B含量较低时提高B含量可以提高材料的高温力学性能;但B含量较高时,高温力学强度变化不大。B含量为0.33%(质量分数)时,材料的高温力学性能最佳。添加Ti前后高硼钢的850℃氧化测试结果均符合GB/T 13303-1991中2级"抗氧化性"标准,Ti的加入有利于提高高硼钢高温抗氧化性能。     

Effect of dew point on the surface selective oxidation and subsurface microstructure of TRIP-aided steel

In this work we examined the effect of dew point on the surface selective oxidation behavior and the subsurface microstructure of TRIP-aided steel mainly by XPS and FE-SEM. It is shown that the variation of the dew point changed the morphology and chemical composition of the surface oxides. Furthermore, as the dew point increased, ferrite was found in the subsurface region of TRIP-aided steel. Considering the severe degree of internal oxidation, decarburization and consumption of minor alloy elements are proposed to be responsible for this ferrite band.     

Effect of prior heat treatment on microstructure and mechanical properties of 0·23% carbon dual phase steel

Abstract

The present study is aimed at understanding the effect of prior heat treatment on the microstructure and mechanical properties of 0·23% carbon dual phase steel. The as received steel was subjected to annealing, normalising and hardening treatment before actual dual phase heat treatment. These steels along with the as received steel were intercritically annealed at 745°C followed by water quenching in order to produce a dual phase microstructure consisting of varying quantities and morphologies of ferrite and martensite. Mechanical properties were evaluated by tensile testing and hardness measurement. It was found that prior heat treatment affects the final dual phase microstructure in terms of size and relative amount of ferrite and martensite. It was also found that dual phase treatment resulted in improved mechanical property. Optimum combination of strength and ductility was obtained with the steel subjected to prior hardening (DPLA-3) treatment. The maximum in ultimate tensile strength (762 MPa) was obtained with the steel subjected to prior annealing treatment (DPLA-1). The X-ray diffraction study confirmed the presence of carbides in DPLA-1 steel, which contribute to the increased strength by precipitation hardening.     

Microstructure and mechanical properties of AZ91-Ca magnesium alloy cast by different processes

The microstructure and mechanical properties of magnesium(Mg) alloys are significantly influenced by the casting process. In this paper, a comparative study on microstructure and mechanical properties at ambient and elevated temperatures of AZ91-2 wt.% Ca(AZX912) Mg alloy samples prepared by gravity casting(GC), squeeze casting(SC) and rheo-squeeze casting(RSC), respectively, was carried out. The results show that α-Mg grains in SC and RSC samples are significantly refined compared to the GC sample. The average secondary dendritic arm spacing of AZX912 alloy samples decreases in the order of GC, SC and RSC. As testing temperature increases from 25 °C to 200 °C, strength of AZX912 alloy samples is reduced, while their elongation is increased continuously. Compared to GC and SC processes, RSC process can improve the mechanical properties of AZX912 alloy at both ambient and elevated temperatures. The enhancement of mechanical properties of RSC sample over GC and SC samples mainly results from grain refinement in the as-cast microstructure of AZX912 alloy.     

低合金耐硫酸露点腐蚀用钢板的试制及其性能研究

开发和使用低合金耐硫酸露点腐蚀钢板,对提高设备耐腐蚀性能、延长使用周期及降低材料成本非常重要。介绍了鞍钢低合金耐硫酸露点腐蚀用钢板的试制情况,本次试制通过添加Cu、Cr、Sb、Ti等合金元素并结合合理的轧制工艺,开发出4~12 mm厚耐硫酸露点腐蚀钢板。对试制钢板的显微组织以及力学性能进行了检测,同时采用硫酸浸泡腐蚀试验对试制钢板与Q235B钢板的腐蚀速率、锈层组成进行了研究。试验结果表明:试制钢板的组织以铁素体+珠光体为主、含有少量的贝氏体,其力学性能优异,远超性能设计要求;在相同腐蚀条件下,试制钢板表面生成一层均匀且致密的内锈层,其耐硫酸腐蚀性能是Q235B钢板的7倍左右。     

Calculation of Selective Oxidation in Grain and Grain Boundary

The aim of this paper is to identify selective oxidation of high strength steels under different conditions. FISHER model is introduced to simulate diffusion and precipitation of chemical elements in metallic matrices and grain boundary, a scheme of its analytical solution is given. Effects of grain size, dew point and annealing temperature related to concentration of oxygen during annealing treatment are presented. The results are consistent well with literatures. The calculation shows that the selective oxidation of CMnSi TRIP steel at ferritic annealing in a low dew point N₂-10%H₂ atmosphere of ?30 °C result in the formation of internal MnO in the matrix and SiO₂ particles in the subsurface. The calculations agree well with the experimental results.     

Effects of austempering conditions on the microstructures and mechanical properties in Fe-0.9%C-2.3%Si-0.3%Mn steel

In this study, the effect of the microstructure and mechanical properties of austempered high-carbon (0.9 %C) high-silicon (2.3 %Si) cast steel were investigated. The specimens were austenitised for 60 min. at 900 °C, and austempered at 260 °C, 320 °C, and 380 °C for periods of time ranging from 30 min to 240 min. After receiving this heat treatment, the mechanical properties were measured using both a tensile test and hardness test. To analyze the microstructure, an optical microscope was used and an X-ray diffraction (XRD) analysis was carried out. In this study, high carbon high silicon cast steel without graphite and with higher tensile strength (1300 MPa to 2200 MPa) and elongation (∼25 %), when compared to austempered ductile cast iron (ADI), was developed. When the austempering temperature was at 260 °C, the microstructures were low ausferrite, but at 380 °C, an upper ausferrite structure was formed. As the austempering temperature increased from 260 to 380 °C, the ultimate tensile strength and hardness decreased, but the elongation and retained austenite volume fraction increased. In addition, the microstructures were coarser.     

Effect of Tempering Temperature on the Microstructure and Properties of Fe-2Cr-Mo-0.12C Pressure Vessel Steel

To obtain the high-temperature strength and toughness of the medium–high-temperature–pressure steel, the microstructure evolution and mechanical properties of Fe-2Cr-Mo-0.12C steel subjected to three different tempering temperatures after being normalized were investigated. The results show that the microstructure of the sample, tempered in the range 675-725 °C for 50 min, did not change dramatically, yet the martensite/austenite constituents decomposed, and the bainite lath merged together and transformed into polygonal ferrite. At the same time, the precipitate size increased with an increase in tempering temperature. With the increase in the tempering temperature from 675 to 725 °C, the impact absorbed energy of the Fe-2Cr-Mo-0.12C steel at ?40 °C increased from 257 to 325 J, and the high-temperature yield strength decreased; however, the high-temperature ultimate tensile strength tempered at 700 °C was outstanding (422-571 MPa) at different tested temperatures. The variations of the properties were attributed to the decomposition of M/A constituents and the coarsening of the precipitates. Fe-2Cr-Mo-0.12C steel normalized at 930 °C and tempered at 700 °C was found to have the best combination of ductility and strength.     

Challenges in hot-dip galvanizing of high strength dual phase steel: Surface selective oxidation and mechanical property degradation

In this work, the two challenges in hot-dip galvanizing of high strength dual phase steel such as surface selective oxidation and mechanical property degradation were discussed. It was shown that the external selective oxidation occurred at low dew point − 30 °C caused the full coverage of oxides, which is a problem for reactive wetting by molten zinc. Although increase of dew point to + 10 °C decreased the surface coverage of oxides, the formation of internal oxides consumed the alloying element and accumulated in the subsurface area in a large scale. Furthermore, the heat treatment parameters of hot-dip galvanizing, i.e. cooling rate after inter-critical annealing and the 460 °C-isothermal soaking time influenced the mechanical properties.     

Kinetics of Internal Oxidation of Mn-Steel Alloys

Internal oxidation of three Mn-steel alloys with 1.7, 3.5 and 7.0 wt% Mn concentration at 950 °C in a gas mixture composed of nitrogen, hydrogen and water vapor with a dew point of +10°C was evaluated. For these alloys, the kinetics of internal oxidation are diffusion-controlled and obey parabolic growth rate law. The diffusion coefficient of oxygen and manganese determined from the observed internal oxidation kinetics are 3.35 × 10^?7 and 4.14 × 10^?12 cm²/s at 950 °C, respectively. The formed internal oxide precipitates are mainly composed of MnO. The solubility product of MnO in an austenitic iron matrix is estimated to be (7.66 ± 0.18) × 10^?9 mol fraction² at 950 °C. The numerical simulation of concentration depth profiles of precipitated oxygen is in agreement with depth profiles determined with image analysis and X-ray microanalysis. Validity of the numerical simulation in case of the phase transformation was also tested. When a 1.7 wt% Mn-steel alloy is oxidized at 850 °C (instead of 950 °C) with a dew point of +12 °C partial phase transformation from austenite to ferrite takes place due to the Mn depletion. The associated precipitated oxygen concentration depth profile can be predicted accurately with numerical simulation.     

Effects of environmental factors on atmospheric corrosion of aluminium and its alloys under constant dew point conditions

Environmental factors, such as chloride deposition rate, dew point and the addition of sulphate ions, were investigated under constant dew point conditions. Corrosion mass losses of AA1100 increased with increasing chloride deposition rates, dew points and with the absence of sulphate ions. Sulphate ions were shown to have an inhibition effect on pitting corrosion of aluminium alloys. High-purity aluminium was not sensitive to an increase of chloride deposition or dew point. As dew point increased from 5 to 28 °C, the corrosion mass loss of AA6061 decreased. The pitting susceptibility of AA6061 was rather high under low dew point conditions.     

Structural features and mechanical properties of austenitic Hadfield steel after high-pressure torsion and subsequent high-temperature annealing

Mechanisms of structure fragmentation and strengthening of single crystals of a Hadfield steel after warm torsion under high-pressure torsion (HPT) and subsequent annealing in a temperature range of 400?C800°C have been studied. Multiple twinning and formation of ultrafine carbides upon HPT at 400°C (P = 5 GPa) promote rapid fragmentation of the microstructure. They are responsible for the high mechanical properties of the steel after HPT and the thermal stability of the microstructure up to an annealing temperature of 500°C.     

Influence of heat treatment on mechanical properties of 51CrV4 high strength spring steel

Abstract

The durability of the springs is limited by plastic deformation, fatigue and fracturing. From this point of view, the use of spring steel with following properties is recommended: high ductility and toughness at operation temperatures from ?40°C to +50°C, good hardenability that provides required mechanical properties even at maximum dimensions. For the manufacturers of springs, the information relating to the heat treatment of specific spring steel is important. This paper describes the influence of heat treatment parameters on tensile strength R_m, yield strength R_p0·2, fracture toughness K_Ic, impact toughness, Charpy-V as a function of tempering temperature in the range from 350 to 700°C for a specific austenitising temperature. Also the difference between the properties given by the mathematical modelling of heat treatment using the computer software Hardenability and the properties obtained by testing the heat treated samples are presented.     

多孔钯块体材料的制备及其表面处理

采用等离子脉冲放电烧结(SPS)法制备多孔钯块体材料,并对其进行表面氧化还原处理。结果表明:采用该方法在550℃时可以制备出结构稳定性与力学性能好、孔隙率高(87.88%)的多孔钯块材。此种多孔钯块材在通过氧化还原处理以后,其表面碳杂质含量能够得到有效降低,表面粗糙度与表面积能够得到一定程度的提高。     

Mechanical characterisation and investigation of thermal and thermomechanical aging mechanisms of martensitic stainless steel 15-5PH

Abstract

The mechanical properties and microstructure of 15-5PH martensitic stainless steel were investigated after solution treatment, quenching and tempering. Aging at 350 and 400°C leads to a marked drop of the impact strength, an increase in the ductile to brittle transition temperature and an increase of both yield strength and hardness. Evolution of these mechanical properties during aging can be linearly correlated to allow aging in service to be determined via hardness measurements. During long term aging at intermediate temperatures, below 400°C, the martensite matrix of the 15-5PH suffers spinodal decomposition into chromium enriched and iron enriched domains. This specific embrittlement is reversible by an appropriate heat treatment performed on the aged alloy. However, this regeneration treatment is only effective if the interconnected network formed during aging has not reached its decomposition limit.     

Small Punch Tests at Oxide Scales Surface of Structural Steel and Low Silicon Steel

This work is focused on the study of the behavior of oxide scale layers grown on two types of steel: plain carbon steel and silicon-alloyed steel. The small punch test method was chosen to study the mechanical properties of oxide scale layers. Several specimens were prepared from both materials and exposed to 1,000 °C for different oxidation times to obtain the desired scale layers. When the oxidation process was complete, oxide layers of varying thickness, composed mainly of hematite and magnetite, were created. The specimens were used for testing at temperatures between 600 and 900 °C.     

Effects of cooling rate and annealing treatment on sensitization of austenitic-ferritic duplex stainless steel

Duplex stainless steel has higher corrosion resistance and better mechanical properties than conventional type 300 series stainless steel. The corrosion behavior of duplex stainless steel is strongly dependent on the ratio, shape, size and distribution of austenite and ferrite phase in the microstructure. The relationship between the microstructure and the corrosion behavior of the duplex stainless steel was studied. For this purpose, the duplex stainless steel samples were solution heal treated at 1150°C followed by either cooling at various rates (water quenching, air coooling, furnace colling with door opened and door closed) to 820°C and then water quenching to room temperature, or quenching to room temperature and annealing heat treatment at 840°C for various lengths of time. A double loop electrochemical polentiodynamic reactivation (EPR) test was carried out to examine the effect of various cooling procedures or annealing treatment on the sensitization of duplex stainless steel. The grain size, shape, and distribution of the two phases were examined under microscope. From the test results, the relationships were discussed among heat treatment, electrochemical properties and microstructure.     

Microstructure,Mechanical Properties,and Unlubricated Sliding Wear Behavior of Air-Cooled MnSiCrB Cast Steels

Two medium carbon low-alloy MnSiCrB cast steels containing different Si contents (0.5 and 1.5 wt.%) were designed, and the effects of Si contents on the microstructure, mechanical properties, and unlubricated sliding wear behavior of the cast steels after air-cooling from 850 °C and subsequent tempering at 220 °C was studied. The results show that the microstructure of the cast steel containing 0.5 wt.% Si consists of granular bainite and lower bainite/martensite multi-phase. In the cast steel containing 1.5 wt.% Si, granular bainite was not observed. The microstructure consists of carbide-free bainite/martensite multi-phase. Excellent hardenability can be obtained at both low and high Si levels. The cast steel containing 0.5 wt.% Si exhibits excellent combination of strength, ductility, and impact toughness superior to the cast steel containing 1.5 wt.% Si. Also, the wear-resistance of the former steel is better than that of the latter in the unlubricated sliding wear condition. The air-cooled MnSiCrB cast steel containing low Si levels, with excellent mechanical properties and wear-resistance, is a potential high-performance and low-cost wear-resistant cast steel for unlubricated sliding wear condition.