Einfluß des Anlassens auf Gefüge und Härte warmfester 9%Cr-Stähle

Influence of tempering on microstructure and hardness of high-temperature 9%Cr-steels The influences of temperature and duration of tempering on hardness and microstructure were investigated at high-temperature martensitic and low-carbon steels with 9% chrome and the further alloying elements molybdenium, vanadium, niobium and partially tungsten. After austenitizing and subsequent air cooling the steels were tempered at temperatures below, at and above Ac_1b for different times and finally a hardness test was performed. Making use of the temperature dependence of the hardness tempering diagrams were constructed and the Hollomon-Jaffe-Parameter on the three steels was determined within its application limits. Micrographs of the structure shows the formation of the carbides and the martensite. At tempering temperatures below Ac_1b a decrease of hardness occurs, above Ac_1b, a hardness rise due to the partial austenitizing was obtained. While hardening below Ac_1b, the tempering quality increases from P 91, NF 616 to E 911.     

高温耐磨PTFE复合材料的力学性能和摩擦性能

在室温～250℃宽范围内研究了温度对PTFE基复合材料硬度、剪切强度、压缩模量及磨损率的影响。结果表明,温度升高,聚四氟乙烯(PTFE)复合材料的剪切强度和压缩模量呈二次抛物型曲线快速降低,而硬度呈线性递减关系;所制纳微协同增强PTFE复合材料承载时200℃附近才出现磨损率拐点,明显优于国内外水平;基于室温环境的聚合物力学、磨损性能以及高温下的力学性能,建立了聚合物磨损预测方程,可以快捷地预测出高温下PTFE复合材料的磨损率。     

几种不同硬度和热导率的钢中的绝热剪切带特征

研究了几种不同硬度和热导率的钢铁材料在约束爆破条件下的绝热剪切带特征.研究结果表明:钢的高硬度和低热导率有利于形成相变带;反之,则倾向于形成形变带;对于具有高硬度和高热导率的钢,由于高温相变带区域的热量迅速传递到周围的基体组织而使其温度升高、硬度降低,形成的相变带会向形变带转化.     

热作模具钢热磨损和摩擦系数的研究

用Ａ．Ｔｈｏｍａｓ的试验方法,在相同的工况条件下,对比了六种热作模具钢的热塑性摩擦系数,以及润滑对摩擦系数的影响。同时还研究了模具钢的高温强度、延伸率、抗氧化性能与摩擦系数的关系,用失重法研究了五种热作模具钢的热磨损损次数与磨损量的对应关系。用金相、扫描电镜等手段,分析了经热磨损试样的端面硬度变化,磨损表面形貌和次表层组织的变化。     

Genesis of High Hardness in Hot-Rolled Coils of Cold Reducers’ Grade Steels

A significant quantity of cold-reducible hot-rolled (HR) coils is produced every year. These are basically low-carbon steels; however, the quality of HR coils with respect to cold reducibility had not been entirely satisfactory. The hardness of HR coil was generally higher (≥65 HR_B) than desirable for attaining satisfactory cold reducibility. A systematic study was, therefore, undertaken with the objective to control the hardness to ≤55 HR_B by modifying the existing chemistry, finish rolling temperature (FRT), and the coiling temperature (CT). To find the optimum conditions for lower hardness, trial rolling of slabs of cold reducers’ grade of selected chemistry was conducted under varied conditions. An assessment of hardness across the width and of the microstructure was carried out. The evolution of microstructure in the HR band at different locations from the edge of the strip and the genesis for the resulting hardness profile across the width of the HR coil were examined in detail. The influence of grain size on hardness was also analyzed. The paper outlines the role of finish rolling temperature, coiling temperature, chemistry, and grain size in restricting the hardness to values below 55 HR_B.     

Austenitic grain size evolution and continuous cooling transformation diagrams in vanadium and titanium microalloyed steels

The evolution of the austenitic grain size in medium carbon steels microalloyed with vanadium and titanium was studied as a function of reheating temperature, heating rate, and titanium content. High resolution dilatometric techniques were used to determine the continuous cooling transformation (CCT) diagrams for two different austenitization temperatures. The microstructure and hardness were determined for different cooling rates. The results revealed a significant effect of titanium concentration on the austenitic grain growth control. The smallest grain size was found in the steel with a Ti concentration = 0.019 wt%. Low heating rates produced smaller grain sizes than high heating rates although an abnormal grain growth took place. In these steels, at temperatures above 1050 °C the influence of the reheating temperature on their hardness for cooling rates around 2 °C · s^–1 was negligible. The higher reheating temperatures caused a slight increase in their hardenability. Finally, it was found that the greater the titanium content, the greater the hardness of these steels, but only when the titanium percentages were higher than 0.020 wt%.     

Diffusion kinetics of borided AISI 52100 and AISI 440C steels

In this study, the case properties and diffusion kinetics of AISI 440C and AISI 52100 steels borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of 1123, 1173 and 1223 K for 2, 4 and 8 h.The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe₂B and CrB compounds.The thickness of boride layer increases by increasing boriding time and temperature for all steels. The hardness of the boride compounds formed on the surface of steels AISI 52100 and AISI 440C ranged from 1530 to 2170 HV_0.05 and 1620 to 1989 HV_0.05, respectively whereas Vickers hardness values of untreated steels AISI 440C and AISI 52100 were 400 HV_0.05 and 311 HV_0.05, respectively. The activation energies (Q) of borided steels were 340.426 kJ/mol for AISI 440C and 269.638 kJ/mol for AISI 52100. The growth kinetics of the boride layers forming on the AISI 440C and AISI 52100 steels and thickness of boride layers were also investigated.     

Material behaviour of a dual hardening steel under thermomechanical loading

Dual hardening steels are a group of metals, which reach their material properties through a combination of strengthening via carbides and intermetallic precipitates. Because of their combination of mechanical properties, dual hardening steels are a promising alloying concept for hot‐work applications. The applied materials for hot‐work applications have to meet certain requirements, such as high hardness, high thermal strength, thermal stability, and fracture toughness. In this paper, a dual hardening steel in different heat treatment conditions was tested under out‐of‐phase thermomechanical loading conditions. All tests were done under full reverse strain control and the minimum temperature was kept constant. In the thermomechanical fatigue tests, solution annealed samples reached higher lifetimes compared with aged specimens. The hardness measurements show that the starting procedure of the thermomechanical fatigue leads to an increase of the hardness approximate to the values of the specimens with the ageing heat treatment. Cyclic softening can be observed in the test with the highest maximum temperature of 600°C. An increase of the maximum temperature also causes a decrease of the lifetime.     

Microstructural changes in austenitic stainless steels during long-term aging

Abstract

The microstructural changes, precipitation behaviour, and mechanical properties of typical austenitic stainless steels (304 H, 316 H, 321 H, 347 H, and Tempaloy A–1) have been examined after long-term aging. The steels were aged statically in the temperature range 600–800°C for up to 50000 h. The microstructural changes were observed by optical and transmission electron microscopy, and the extracted residue was identified using X-ray analysis. Time–temperature precipitation diagrams were made for each steel. The amount of σ-phase was measured in samples aged at 700°C. The hardness and impact-value changes, and the tensile properties of aged samples were measured.

MST/358     

Effects of Mo on high-temperature strength of fire-resistant steel

A series of Fe–Mo–C steels with Mo addition from 0.1 to 0.8 wt.% have been prepared to study the effects of Mo on high-temperature strength of fire-resistant steel. The high-temperature hardness tests were carried out to investigate the strengthening mechanisms of Mo in fire-resistant steel. The results show that the hardness of Fe–Mo–C steels increases with the increase of Mo content at a given temperature, and the strengthening effect of Mo becomes remarkable when the temperature is on the rise. Theoretical analysis indicates that the solid-solution strengthening of Mo is the dominant high-temperature strengthening mechanism in fire-resistant steel, but this strengthening effect becomes relatively weak when Mo content is more than or equal to 0.5 wt.%. Moreover, the bainite strengthening plays an important role in improving the high-temperature strength of fire-resistant steel. Furthermore, the analysis indicates that the ferrite grain size has less effect on high-temperature strength of fire-resistant steel. The present results also provide fundamentals to design low-cost fire-resistant steels with excellent high-temperature properties and the most reasonable range of Mo addition is 0.2–0.3 wt.%.     

Elevated temperature creep of pearlitic steels: an experimental–numerical approach

Pearlitic steels are well known for their high strength and hardness. This makes them the natural choice for applications in which structural integrity and minimum irreversible deformation over time are required. Although their room-temperature mechanical response has been intensively studied in the past, little information can be found in the literature regarding the effect of temperature on the mechanical response of pearlitic steels. In this paper, an experimental–numerical approach is used to study the mechanical response of pearlitic steels in the temperature range 20–500 °C. A finite-strain thermo-viscoplastic model is presented together with a set of elevated temperature tests (tensile and creep tests). The aim of the tests is twofold: first, to provide insight into the elevated-temperature mechanical response of the material; and second, to provide the data required to identify the corresponding material parameters. Furthermore, the model and the experimental data are instrumental in showing that the influence of temperature on the mechanical behavior of pearlitic steels becomes significant for temperatures above 350–400 °C.     

Relationship between hardness and tensile properties in various single structured steels

Abstract

An attempt has been made to establish a relationship between hardness and tensile properties for various single structured steels: ferrite, pearlite, bainite, and martensite. It is found that the proportionality constant A _Y of hardness to yield strength changes from 5.79 to 3.17 and is highest for the ferrite steel and lowest for the tempered martensitic steels. A less pronounced change was found in the proportionality constant A _T of hardness to tensile strength (from 3.97 to 2.72). A dependence on microstructure of the proportionality constant at 8% strain A _0.08 was found as well. This difference in A was found to be attributable mostly to the effect of different work hardening behaviours owing to different microstructures. Regression analysis shows that hardness can be expressed as a function of accessible material parameters such as composition, grain size, and transformation temperatures for various single structured steels within a certain degree of accuracy.     

Nondestructive measurement of stress in ferromagnetic steels using harmonic analysis of induced voltage

A method for nondestructively measuring mechanical stresses in ferromagnetic steels was explored by applying a sinusoidal magnetic field to the materials and analysing the harmonic content of the resulting voltage induced in a sensing coil. The uniaxial stress dependence of the third harmonic amplitude was investigated along with the feasibility of separating stress effects from the effects of variations in material properties such as hardness, grain size and tempering temperature. The results obtained from SAE 4340 steel and AISI 410 stainless steel with various grain sizes, hardnesses and yield strengths showed that it is feasible to reduce the effects of material property variations by using the anisotropy in the harmonic amplitude as a stress indicator and thus to measure stresses in components made of ferromagnetic steels.     

Effectiveness of Ni-based diffusion barriers in preventing hard zone formation in ferritic steel joints

A numerical procedure based on finite difference method was used to simulate the formation of ‘hard’ and ‘soft’ zones, in dissimilar weldments of 9Cr–1Mo and 2¼Cr–1Mo steels during high temperature exposure. Kinetic analysis of the calculated diffusion profiles showed that the activation energy for carbon diffusion in Cr–Mo steels is marginally higher than that in Fe–C system. Calculations were extended to incorporate the effect of Ni-based interlayers between 2¼Cr–1Mo and 9Cr–1Mo ferritic steels. The presence of a diffusion barrier was found to reduce the propensity for formation of hard and soft zones, which is related to the interaction parameter \( \varepsilon_{\rm C}^{\rm M}. \) Thickness of the interlayer required to suppress the formation of hard zone was optimized by the calculations. Transition joints of ferritic steels with Inconel 182 as the interlayer of thickness close to that predicted by the computations were fabricated and exposed to elevated temperature. Microstructural studies and hardness measurements further confirmed the effectiveness of Ni-based interlayers in preventing hard zone formation.     

Wear behavior of light-weight and high strength Fe-Mn-Ni-Al matrix self-lubricating steels

The good combination of mechanical and tribological properties for self-lubricating materials is crucial. In this work, novel self-lubricating Fe-16.4 Mn-4.8 Ni-9.9 Al-xC(wt%) steels containing graphite phase were fabricated using mechanical alloying and spark plasma sintering. The compositions of the steels were designed by using thermodynamic calculation, and the effect of carbon addition on the microstructure was further investigated. The steel possesses high hardness of 621 HV, high yield strength of 1437 MPa and good fracture toughness at room temperature. The yield strengths are still above 600 MPa at 600?C.The tribological behavior and mechanical properties from room temperature to 800?C were studied, and the wear mechanisms at elevated temperatures were discussed. The steel has a stable friction coefficient of 0.4 and wear rate in a magnitude of 10~(-6) mm~3/N·m below 600?C. The good tribological properties of the steels were mainly attributed to the high hardness, lubrication of graphite and stable surface oxide layer.     

Effect of tempering temperatures on microstructures and properties of 0.28C–0.22Ti wear-resistant steel

The microstructures and properties of a 0.28C–0.22Ti low-alloy wear-resistant steel at different temperatures from 200 to 600°C was experimentally studied. It is shown that the wear resistance of the steel is not monotone changing with its hardness and strength. With the increase of the tempering temperature, the tensile strength and the hardness of the steels were gradually declined; however, the wear resistance was first decreased and then increased. The TiC particles can be divided into two classes: the small TiC particles (about 0.3–0.4?µm in diameter) and the coarse TiC particles (1–5?µm in diameter). The small TiC particles can improve the yield strength of the steels, and the coarse TiC particles can improve the wear resistance of the tested steels.     

The design of advanced performance high strength low-carbon martensitic armour steels: Microstructural considerations

Neither a higher hardness nor higher mechanical properties (yield strength, ultimate tensile strength, impact energy, and %elongation) appear to be exclusive or even reliable criteria for predicting the ballistic performance of martensitic armour steels, as shown in our previous work [K. Maweja, W.E. Stumpf, Mater. Sci. Eng. A (February), submitted for publication]. An alternative design methodology for tempered martensitic armour steels is, therefore, proposed which is based on the effect of retained austenite on the ratio of the yield to ultimate tensile strength (YS/UTS), the microstructure of the tempered martensite and its martensite start temperature M_s. This approach was developed using 6 mm thick armour plates and later was successfully applied to the design of eight experimental armour steels with plate thicknesses ranging from 4.7 to 5.2 mm and tested by the standard R4 (5.56 mm rounds) ballistic test.     

Recrystallisation of austenite grain when non-isotherm steel working: Effects of thermal kinetics and deformation-based mechanisms

The effects of high temperature deformation on the recrystallisation of austenite grains and hardening occurring during hot forging of steels were studied. Three commercial steels containing various carbon weight percentages were heated beyond the austenitising temperature and free forged up to desired deformation ratios. The specimens were then air-, or oil-cooled. Two zones were distinguished according to the grain-size: a zone with fine grains, associated with highest plastic deformation and, a zone with coarse grains located within the subsurface layers. Unexpectedly, the highest values of microindentation hardness were obtained in the coarse-grain zone. Consequently, the interaction between the grain-size gradient induced by thermal kinetics of cooling and the local hardening governed by dislocation kinetics was studied by means of microindentation hardness inspections. Analysis of stress–strain curves confirmed that while forging enhances mechanical strength, it has a detrimental effect on ductility of steel.     

Struktur und Eigenschaften nichtrostender Stähle nach einer Plasmaimmersionsionenimplantation und einer Plasmanitrierung

Structure and properties of stainless steels after plasma immersion ion implantation and plasma nitriding Stainless steels can be nitrided at temperatures ≤ 400 °C to increase their hardness and wear resistance without a decreasing of their excellent corrosion resistance. Structure and properties of the surface layers produced by plasma nitriding and plasma immersion ion implantation in this temperature range were tested. There are negligible differences in the structure of the produced surface layers in spite of different interaction principles of the used technologies. However there are clear differences between the case of different steels. The case of ferritic chromium steels mainly consists of ε-nitride. Whereas the cases of austenitic and ferritic austenitic steels are characterized by expanded austenite. The corrosion resistance of the steels is reduced by nitriding only, if evident CrN-formation occurs.     

Kinetics of borided gear steels

In this study, the case properties and diffusion kinetics of GS18NiMoCr36 (GS18), GS22NiMoCr56 (GS22) and GS32NiCrMo6.4 (GS32) gear steels borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of 1123, 1173 and 1223 K for 2, 4 and 6 h. The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe₂B, CrB and Cr₂B compounds. The thickness of the boride layer increases by increasing boriding time and temperature for all steels. The hardness of the boride compounds formed on the surface of the steels GS18, GS22 and GS32 ranged from 1624 to 1905 HV_0,05, 1702 to 1948 HV_0,05, and 1745 to 2034 HV_0,05 respectively, whereas Vickers hardness values of the untreated steels GS18, GS22 and GS32 were 335 HV_0,05, 358 HV_0,05 and 411 HV_0,05, respectively. The activation energies (Q) of borided steels were 228.644 kJ/mol for GS18, 280.609 kJ/mol for GS22 and 294.359 kJ/mol for GS32. The growth kinetics of the boride layers forming on the GS18, GS22 and GS32 steels and the thickness of boride layers were also investigated.