Secondary Tempering Research Articles

Fatigue Properties at Elevated Temperatures of Secondary Tempered Nanostructured Bainite: A Study of Deformation Mechanisms

Nanostructured bainitic steel obtained through austempering or bainitic transformation of high Si steels consists of bainitic ferrite and carbon‐enriched retained austenite. Secondary tempering of those microstructures tailors their mechanical properties through its unique nanostructure and austenite stability. Herein, fatigue and tensile properties of a nanobainitic steel in different secondary tempering conditions are presented and the results are discussed based on the deformation mechanisms taking place due to the microstructure evolution during secondary tempering. Influence of secondary tempering and elevated testing temperature on tensile and fatigue properties at room temperature and 250 °C is presented. Based on the present deformation, mechanism results are discussed supported by microstructure evolution results obtained through dilatometry, X‐ray diffraction, and microscopy. Depending on secondary tempering temperature, main influencing mechanisms are martensite formation after secondary tempering during cooling, transformation‐induced plasticity effect, and precipitation hardening obtained by secondary tempering.

Electrodeposition of Dy and its grain boundary diffusion behavior of sintered NdFeB magnets

This paper presents the electrodeposition of a Dy layer onto the surface of sintered NdFeB magnets to serve as a diffusion source. Subsequently, the sintered NdFeB magnets were subjected to grain boundary diffusion treatment through electrochemical deposition at the solution temperature of 45 °C and the solution pH (hydrogenii) of 4.0. The results indicated that the coercivity of the Dy-diffused magnet increased from 1073 kA/m to 1418 kA/m compared to the original magnet after annealing at a primary diffusion heat treatment temperature of 900 °C for 6 h and subjected to the same secondary tempering heat treatment at 500 °C for 2 h, while the remanence remained relatively unaffected. SEM (Scanning Electron Microscopy) revealed that the Dy coating exhibited a compact and flat surface with a small grain size. Energy spectrum analysis confirmed a relatively uniform distribution of Dy elements in the electrodeposition coating, without any significant agglomeration. SEM-EDS (Scanning Electron Microscopy-Energy Dispersive Spectrometer) line scan demonstrated that the diffused Dy elements along the grain boundary phase primarily concentrated in the grain boundary phase itself and the epitaxial layer of the main grains. BSE (Backscattered Electron imaging) revealed the presence of a thin and continuous grain boundary phase structure between the main phases following grain boundary diffusion. XRD (X-ray diffraction) analysis demonstrated a general shift toward higher angles in the main peak of NdFeB after Dy diffusion. The remarkable increase in coercivity of the diffused Dy magnets can be attributed to the optimization of microstructure, grain boundary phase composition, and the formation of the (Dy,Nd)2Fe14B structure. The coercivity mechanism in diffusion Dy magnets was still governed by a nucleation reversal mechanism, as supported by the Kronmüller-Plot curve fitting. Tafel tests have revealed that Dy-diffused magnets have a much higher corrosion resistance with a more positive equilibrium potential, Ecorr, and a lower corrosion current density, Icorr.

Numerical simulation and correlation research of multi-track overlapping laser quenching process for 40Cr steel

Laser quenching has a short process cycle and high production efficiency, and it plays an important role in automobile, ship, machinery manufacturing, and other fields. The surface hardness of 40Cr steel laser quenching parts is uneven by unreasonable set on the quenching overlap zone. This will affect the microstructure of the quenched layer, resulting in cracking, corrosion during service, and other hazards that ultimately reduce reliability. Numerical simulations provide an effective way to quantitatively reveal the transient evolution of the multi-field coupling between temperature field, stress field, and phase transition field in quenching, which directly determines the extent of the overlap zone and quenching properties. The quenched phase transition layer profile is predicted to effectively determine the extent of the secondary tempering softening zone and solve the bottleneck problem of uneven surface hardness in quenching. The innovation of this paper is to establish a multi-field coupled numerical model of the 40Cr steel multi-track laser quenching process. The transient quenching temperature, phase transition hardening, and stress distribution were numerically calculated, and the size of the tempering zone under different overlapping rates was evaluated, revealing the internal coupling mechanism and correlation between multi-fields during the laser quenching. The quenching temperature, microstructure, and hardness distribution of 40Cr steel were tested by an infrared thermometer, Axio Vert A1 Zeiss microscope, Thermo ScientificTM Apreo scanning electron microscope, and Q10M microhardness tester, which verified the effectiveness of numerical simulation. The research can provide an important theoretical basis for optimizing quenching process parameters in production.

The Effect of Tempering Duration on the Creep Behavior of the P91 Steels at 600℃

High performance martensitic heat resistant steels are widely used in fossil fuel power plant industry due to because of their good creep resistance at high temperatures. In-depth understanding of the high temperature inelastic deformation mechanism of such steels is crucial to ensure the reliable, safe and efficient operation of the power plant [1]. The martensitic steels have a complex microstructure with a hierarchical arrangement, including a collection of packets in the prior austenite grain, blocks in the packet and laths along with dispersed nanoscale strengthening phases (e.g., MX precipitates and carbides). The purpose of this paper is to study the creep mechanisms with regard to the microstructure of the P91 martensitic steel at high temperature by means of experimental characterisation and finite element simulation. In the present paper, the effect of tempering duration on the creep response of the P91 steel at 600 oC was experimentally examined with the precipitate size identified. Apart from the standard heat treatment (holding at 1060℃ for 40 min and then at 760℃ for 2 h) for the as-received P91 steels, secondary tempering treatment has been applied with different tempering time periods, e.g., 0 h (T0), 10 h (T10) and 20 h (T20) at 780℃. In order to simulate the creep response, crystal plasticity based micromechanical finite element model was developed based on the measured microstructure [2]. The crystallographic slip at the block level is accounted for using an exponential type of constitutive flow rule and the precipitate size effect is represented in the crystal plasticity model through the use of an internal variable in association with the slip resistance. The results show that the secondary tempering duration can significantly affect the creep behavior of the martensitic steel (see Fig. 1). It is found that with increasing tempering time period the precipitate size increases, which could lead to detrimental effect on material’s creep resistance. Similar effect of the precipitate size on the creep response has also been shown in literatures [3]. The measured creep response of the P91 steels with different heat treatments at different stress levels can be predicted by the developed micromechanical model. The micromechanical modelling results show (see Fig. 2) that the accumulated equivalent plastic strain during the creep is strongly heterogeneous and the levels of the strain localization depend on the tempering duration.

Influence of Deep Cryogenic Treatment and Secondary Tempering on Microstructure and Mechanical Properties of Medium-Carbon Low-Alloy Steels

Deep cryogenic treatment (DCT) and secondary tempering for 40CrNiMoA steel were carried out to obtain the desirable microstructures corresponding to excellent mechanical properties suiting for the preparation of flex spline of harmonic drive reducer. The effects of DCT and secondary tempering on microstructural characteristics and mechanical properties were investigated by x-ray diffraction, scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy, in association with property measurements. The results show that the DCT promotes the transformation of retained austenite to martensite, the precipitation and homogeneous distribution of carbides as well as the refinement of martensitic substructures. The structural and morphological changes significantly improve the hardness, yield and tensile strength of steels, but slightly lower the elongation of them. Further secondary tempering at special temperatures can successively increase the elongation and fracture toughness of the DCT-treated 40CrNiMoA steels at the cost of slight decrease in hardness and strength due to the reduction in dislocation density. Therefore, the DCT in combination with secondary tempering can improve the comprehensive mechanical properties of 40CrNiMoA steels to produce the flex spline with a higher lifetime.

Quantification of retained austenite by X-ray diffraction and saturation magnetization in a supermartensitic stainless steel

The good performance of supermartensitic stainless steels is strongly dependent on the volume fraction of retained austenite at room temperature. The present work investigates the effect of secondary tempering temperatures on this phase transformation and quantifies the amount of retained austenite by X-ray diffraction and saturation magnetization. The steel samples were tempered for 1h within a temperature range of 600–800°C. The microstructure was characterized using scanning electron microscopy and electron backscatter diffraction. Results show that the amount of retained austenite decreased with increasing secondary tempering temperature in both quantification methods.

Microstructure evolution and mechanical properties of X12CrMoWVNbN10-1-1 steel during quenching and tempering process

The effect of heat treatments was investigated on the microstructure evolution and mechanical properties of X12CrMoWVNbN10-1-1 steel. The as-received steel was austenitized at 1080°C for 16h and cooled in furnace, following tempered in two steps, i.e. at 570°C for 18h and then at 690°C for 24h. The austenitized sample had a typical lath martensite structure with some retained austenite and no delta ferrite was observed after austenitization. After the first tempering at 570°C, there was no retained austenite remained but it resulted in the forming of precipitates, such as Cr-rich M7C3, Cr-rich M23C6, Cr-rich M2N, Nb-rich MN, and the subgrain, as well as the reduction in dislocation density. However, microstructure observation after secondary tempering proved the further arrangement of dislocation, subgrain growth and the coarsening of precipitates. Hardness and impact tests were carried out for establishing a correlation between microstructure and mechanical properties. Finally, a careful fractographic analysis of impact samples had been done using SEM and EDS.

Effects of heat treatment on mechanical properties of h13 steel

Heat treatment on the mechanical properties of H13 hot working die steel for die casting is discussed. The H13 steel for die casting was treated by different temperatures of vacuum quenching, tempering, and secondary tempering to investigate its mechanical properties. Strength, plasticity, hardness, and impact toughness of the H13 hot working die steel for die casting were measured. Microstructure, grain size, and carbide particle size after heat treatment have a great impact on the mechanical properties of H13 hot working die steel for die casting. The microstructure of the H13 was analyzed by scanning electron microscopy (SEM) and by a metallographic microscope. It is found that H13 exhibits excellent mechanical properties after vacuum quenching at 1050°C and twice tempering at 600°C.

Hardness–fracture toughness–creep rupture strength relationship for welds in P91 main steam pipe

An experimental study on the impact toughness, hardness, fracture toughness and creep rupture strength of P91 steel weld metals under two conditions (R and D) has been carried out. The results show that weld metal having higher hardness, lower impact toughness and lower fracture toughness (sample R) has higher creep rupture strength. The creep rupture strength is closely related to creep life of component. Thus, assessment of the weld hardness of P91 steel is of value in assessing the creep life of pipes in service. The lower toughness of sample R is attributed mainly to the higher welding current and input line energy used. The lower hardness and strength of sample D, which received a second post-weld heat treatment, partially result from too high a secondary tempering temperature being employed.

ＴｉＣ被覆工具鋼材の曲げ疲労強度へ及ぼすＱＴ処理の影響

In the previous study, it was found that heat treatments of quenching and tempering after TiC coating improved the fatigue strength, and also the fatigue life of TiC coated steels depended on the hardness of substrate.In this study, the authors investigated in detail the effect of condition of quenching and tempering after TiC coating on the fatigue strength of tool steel. Four kinds of quenching and tempering conditions after TiC coating were selected, but TiC coating condition was all the same. Fatigue tests were conducted using the smooth specimens of a flat plate. The residual stresses were measured by an X-ray diffraction method. As the results, it was found that the condition of heat treatment of quenching and tempering influenced the hardness of substrate and consequently the fatigue strength which was not necessarily dominated by the hardness of substrate, and there existed an optimal hardness of substrate in relation to the fatigue strength. And it was reconfirmed that tensile stress existing in the top part of substrate, which was balanced by the compressive stress in TiC layer, was not only the main factor controlling the fatigue strength, and the secondary tempering condition played some role.

Temper brittleness of steel 30Kh2N2SMK5

1. Steel 30Kh2N2SMK5 has little susceptibility to temper brittleness. The ductile-brittle temperature shifts by 30°C-from −60° (ductile condition) to −30°C (brittle condition). 2. The range of temper brittleness is 450–550°C; the temperature of maximum brittleness is ∼500°C. 3. The range of temper brittleness and the temperature of maximum embrittlement can be reliably determined by means of secondary tempering. The beginning of softening of the quenched and tempered steel after secondary tempering coincides with the upper limit of the temper brittleness range.

Formation of the hardened layer during burnishing of steel 55SM5FA

1. The hardening that occurs during burnishing of steel 55SM5FA under a pressure ≤210 kgf/mm2 is due to precipitation hardening in the surface layer, which leads to higher dislocation density and block refining. 2. With burnishing under a pressure of 210–260 kgf/mm2 the hardening increases due to phase strain hardening. 3. At pressures >260 kgf/mm2 the hardening effect decreases due to heating and also to exhaustion of the resistance of the material to the accumulation of plastic deformation. Phenomena associated with the formation of secondary tempering products are possible at this stage. 4. It is recommended that rollers of steel 55SM5FA be burnished under a pressure of 240–250 kgf/mm2 and feed rate 0.1–0.12 mm/rev. They should be burnished with a ball 5–8 mm in diameter.

Temper brittleness of forged steel 30Kh2N2SMK5

1. The susceptibility to temper brittleness of steel 30Kh2N2SMK5 (EP 822) was determined by means of primary and secondary tempering and from the shift of the ductile-brittle transition temperature. 2. It was found that forgings 315 mm in diameter are susceptible to reversible temper brittleness. The difference between the ductile-brittle transition temperature after quenching + tempering and after maximum embrittlement is around 200°.

Determining susceptibility to reversible temper brittleness

1. For determining the susceptibility of structural steel to reversible temper brittleness, particularly for revealing the degree of embrittlement, tempering should be conducted at the temperature of maximum embrittlement for a given steel. 2. Slow cooling after tempering (in the primary and secondary temepering methods) induces additional embrittlement during cooling in the brittleness range and does not make it possible to determine the effect of the tempering temperature alone. 3. The secondary tempering method is recommended for determining the maximum embrittlement temperature and the boundaries of the brittleness range.

軸受鋼の時効変形に関する研究(III)

The author analysed the quantjty of retained austenite employing the integrated intensity of X ray diffraction lines and Hurlbut counter to quantify the results of dimensional stability of ball bearing steel mentioned in Report No.1 and 2. The results were as follows:1) The retained austenite increased with higher quenching temperature, and repeated quenching increased it too.2) The retained austenite increased by tempering immediately after quenching.3) The retained austenite.decreased about 3-4% by one day of air cooling after quenching and prior to tempering, or by employing secondary tempering, compared with specimens tempered immediately after quenching.4) The retained austenite decreased by subzero treatment and even after marquenching.5) The quantity of spheroidized carbide decreased by higher quenching temperature and also by repeated quenching.6) The crystal structure before quenching affects the diffusion of spheroidized carbide into matrix. Increase of the retained austenite was proportional to the degree of spheroidized carbide diffusion into matrix. The dimensional stability was also influenced by the crystal structure.7) The quantity of retained austenite was influenced by the cooling rate according to the mass of specimens.The above results can quantitatively prove the experiments on dimensional stability of ball bearing steel introduced by the previous two reports. Moreover, he studied relations between the .dimensional changes by heat treatment and the retained austenite and magnetic properties, for further considerations on ball-bearing steel problems.

Societies and Academies

PARIS. Academy of Sciences, Sept. 29.—E. L. Bouvier: A new type of ceratocampian Saturnioid.—Mile. Marie Charpentier: The Peano points of a differential equation of the first order.—Paul Montel: Some consequences of Rolle's theorem.—A. Rosenblatt: Linear equations with total differentials.—Miron Nicolesco: The extension of the theorem of Gauss to harmonic functions of p order.—Podtiaguine: The upper limit of the canonical product of infinite order.—L. Escande and M. Teissie-Solier: The chronophotographic study of the flow [of a fluid] round a plate normal to the current. The velocity measured on the surface of discontinuity is constant and equal to the velocity at infinity, agreeing with that indicated by theory. The velocities found esperimentally at various points give a law of retardation in good agreement with the theoretical law.—L. Goldstein: The principle of exclusion and intramolecular statistics.—Pierre Chevenard and Albert Porte vin: The secondary tempering of hyper-tempered steels and the stability of austenite.—Jean Lugeon: Simultaneous investigation by atmospherics at Zurich and in the Sahara. On the basis of three years' records, the following conclusion can be drawn: out of every 100 atmospherics recorded in a year, 20 per cent are of distant origin, some thousands of kilometres; 70 per cent have a range of between 100 and 1000 kilometres; 10 per cent are local, with a range of less then 100 kilometres. The Kennelly-Heaviside layer is always higher at El-Golea (Sahara) than at Zurich.—Jean Piveteau: Contribution to the study of the fossil Ganoid fishes: the family of the Catopteridese.—Louis Baudin: The variation of the respiratory exchanges of fishes as a function of the barometric pressure. By experiments at Lausanne and at Concarneau it has been established that fish are very sensitive to changes in the barometric pressure, and this sensibility is measured by large differences in the gaseous exchanges and in the respiratory coefficient.