Effects of annealing and quenching on fatigue behaviour in type 444 ferritic stainless steel

In order to understand the effects of annealing and quenching on fatigue behaviour in type 444 stainless steel, fully reversed axial fatigue tests have been performed using smooth specimens of heat‐treated materials in laboratory air and 3%NaCl aqueous solution. Three materials subjected to different heat treatments, annealing at 960 and 1000 °C, and water‐cooling at 960 °C, were prepared. In laboratory air, the fatigue limit of the annealed specimens was lower than that of the as‐received specimen and decreased with increasing annealing temperature. The subsequent grain coarsening from the heat treatments was primarily responsible for the lower fatigue strength in the annealed specimens. The fatigue strength of the water‐cooled specimen was lower than that of the corresponding annealed specimen. In the annealed specimens, cracks were generated within ferritic grains, while in the water‐cooled specimen, at or near grain boundaries. In 3%NaCl solution, the fatigue strengths of all specimens decreased compared with those in laboratory air. Only in the water‐cooled specimens, crack initiation at grain boundary and intergranular crack growth were observed, indicating the most sensitive to corrosion environment.     

Effect of hot working on microstructure evolution of as-cast Nickel Aluminum Bronze alloy

In this work, influences of temperature and hot working on microstructure evolution of a Nickel Aluminum Bronze alloy (NAB) were studied. First, as-cast NAB alloy was annealed and subsequently cooled in air for obtaining homogeneous structure. Microstructure and mechanical properties of NAB specimens before and after annealing were characterized by tensile test, hardness test, optical microscopy and scanning electron microscopy. Then, annealed NAB samples were heat treated at different temperatures between 750 °C and 1000 °C and rapidly cooled down to room temperature. The results showed that amounts and types of emerged microstructures and corresponding hardness strongly depended on the applied temperatures. Additionally, hot compression tests during the temperature range of 800 °C and 950 °C were performed for the annealed NAB alloy. After forming, specimens were cooled down with two different cooling rates of 40 °C/s and 100 °C/s. Developed microstructure and resulting hardness of the deformed NAB alloy were discussed regarding to the heat treating conditions.     

Effects of inter-critical temperatures on martensite morphology,volume fraction and mechanical properties of dual-phase steels obtained from direct and continuous annealing cycles

Homogenizing and normalizing heat treatments were performed on low carbon–manganese steel. Then, direct and continuous annealing heat treatments were carried out at 800 °C, 770 °C, 750 °C and 725 °C. Finally; dual phase ferrite–martensite steel was obtained. Thereafter, hardness and tensile tests were applied at ambient temperature, and impact tests for the initial sample and the dual-phase steels obtained from continuous and direct annealing heat treatment in the temperature ranges of (−67 to +70), (−70 to +60), (−70 to +29), respectively, were accomplished. The ductile–brittle transition temperature (DBTT) and the fracture modes of the samples were obtained, and the fracture surface of the steel was observed through scanning electron microscopy (SEM). The results revealed that the best mechanical properties in dual-phase steels, like impact toughness and flexibility, appear at the inter-critical temperature of 725 °C for both continuous and direct annealing cycles. The (DBTT) for the specimens obtained from direct and continuous annealing and the initial sample were −49 °C, −6 °C, and −34 °C, respectively. The dual-phase specimen achieved through the direct annealing method had better toughness and impact properties than the initial specimen or the one obtained through continuous annealing.     

电动客车用余热回收型热泵的换热器优化及性能实验研究

本文针对热泵空调系统在冬季低温工况下制热能力衰减问题,通过换热器设计优化,研发出基于喷射补气的余热回收型热泵空调系统,并进行了性能实验研究.结果表明:研制的准二级压缩电动客车热泵空调系统在低温条件下具有较好的制热性能.在环境温度为-20℃,车内温度为20℃,余热量为1.8 kW的制热工况下,相比于无余热回收工况,系统制...     

Effect of annealing parameters on the magnetic properties of NiZn ferrite thin films

Ni_0.5Zn_0.5Fe_2.0O_4.0 thin films (NZFs) were deposited on Si (100) substrate by a sol–gel method, and the effects of annealing parameters on the structure and magnetic properties of the proposed films were investigated. Moderate heating rate was beneficial to the nucleation of NZFs. When the heating rate was 2 °C/min the saturation magnetization (M _s) achieved its maximum and the coercivity (H _c) reached its minimum. Both the crystallization and M _s of NZFs enhanced with increasing annealing time; however, H _c changed contrarily. High quenching temperature produced a large stress and consequently deteriorated magnetic properties. The optimal annealing parameters of NZFs were annealed at 700 °C, heating rate 2 °C/min, annealing time 1 h, and gradually cooled to room temperature. Finally, NZFs showed a high magnetization of 320 emu/cm³ and low coercivity of 86 Oe.     

High‐temperature testing in a Charpy impact pendulum using in‐situ Joule heating of the specimen

In this paper, an innovative approach to high‐temperature testing of subsize Charpy V notched specimens is introduced. The design concept is to heat the specimen on the specimen piece supports up to the moment of impact by flowing AC electric current through it. This approach allows a very accurate centring of the specimen with respect to the anvils and the control of their temperature up to the moment of impact. The temperature profile measured by using the in‐situ heating device on ferritic steel specimen over the notch temperature range of 400°C < T < 750°C is presented. The impact energy was measured at different temperatures going through the eutectoid phase transformation of the ferritic steel specimens, with different carbon composition, to investigate the validity of the instrumented in‐situ heating method. The method is particularly appropriate to estimate the ductile brittle transition that occurs at high temperature in some metallic alloy systems. Also, its wide range of specimen heating rate provides new research tools for studying, for example, the intermediate temperature embrittlement of metals and alloys.     

Study on formation and morphology of sigma-phase in continuous annealing furnace roller

Investigations were revealed that roller fracture was caused by sigma-phase embrittlement and an increase of precipitations in the microstructure. In order to decreasing the failure, homogenization heat treatment at 1100 °C for 2 h followed by air cooling was used. In this study, optical microscopy metallography, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) examinations were performed, in order to investigation of formation and morphology of the sigma-phase before and after homogenization treatment. It was found that by transformation of delta-ferrite, first M₂₃C₆ carbides with lamellar morphology and also secondary austenite were formed and after completion of lamellar precipitation, the sigma-phase was formed at foreside of the precipitates. The formation of sigma-phase with dendritic morphology resulted in decrease in ductility and failure of the continuous annealing furnace roller. Also, homogenization heat treatment at 1100 °C for 2 h followed by air cooling can led to change in the morphology of sigma phase from dendritic structure to globular structure. Finally, it was found that the formation of sigma-phase with globular structure led to remarkable increase in the impact energy and ductility of heat resistant steels and therefore, the failure of continuous annealing furnace roller was decreased.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

Processing and properties of a new biodegradable Mg−Zn−Ca−Zr alloy

Magnesium alloys are used for degradable orthopaedic and cardiovascular implants due to their favourable mechanical and biological properties, degradation ability in physiological environment and stimulatory effect on the new bone formation. The research challenges are related to the increase of biological and mechanical compatibility. For the present study, a magnesium based alloy design was conducted to the following chemical composition: Mg?2.7Zn?1Ca?0.6Zr (wt.%). A complex thermomechanical processing route was applied: a plastic deformation by extrusion at various temperatures and deformation degrees (400 °C–480 °C, ? = 20 %–40 %), followed by various final heat treatments at 200 °C–400 °C for 10 min–60 min. Further, the influence of processing parameters upon the structure, mechanical properties and biological response was studied. Processed specimens were characterized by scanning electron microscopy (secondary electron imaging and energy dispersive spectroscopy) and mechanically by tensile tests. The most representative results were obtained for the samples extruded at 450 °C/? = 20 %, followed by a final heat treatment at 350 °C/15 min, air cooling. Further, for samples which revealed promising results, in‐vitro testing was developed. Biocompatibility testing of the Mg?2.7Zn?1Ca?0.6Zr (wt.%) alloy was realized by indirect contact studies using the Vero (ATCC® CCL‐81?, American Type Culture Collection) cell line. Cells morphologies, cell viability and proliferation were evaluated.     

The effects of pre- and post-weld heat treatment variables on the strain-age cracking in welded Rene 41 components

Rene 41, a precipitation-hardened nickel-base superalloy, is mostly used for hot section parts of jet aircraft engines. When cracks occur in these parts, welding is frequently utilized for crack repair to extend their service life. Rene 41 is very susceptible to strain-age cracking during welding and post-weld heat treatment because of its strengthening mechanism. The strengthening mechanism depends on the existence of gamma prime (γ′) phase and the morphology of carbides both of which are affected by heat treatment. In this study, the effects of pre- and post-weld heat treatment variables such as heating rate, holding temperature and time, and cooling rate on the strain-age cracking of Rene 41 during welding and post-weld heat treatment were investigated. An optimum combination of heat treatment variables was investigated to obtain a microstructure that would be less susceptible to strain-age cracking during welding and post-weld heat treatment. Pre- and post-weld solution heat treatments at 1080 °C for 30 min with 20 °C/min heating and 34 °C/min cooling rates produced optimum microstructures, and no cracks were observed in the Rene 41 samples after welding. MC carbides were dissolved lightly and distributed homogenously within the grains. M₆C carbides precipitated as globules within the grains and in the grain boundaries. M₂₃C₆ carbides precipitated as chains of globules in the grain boundaries.     

Zeitstandversuche an einem mittels Induktivbiegeverfahren hergestellten Rohrbogen aus X 20 CrMoV 12 1

Investigations on the Creep Behaviour of a X 20 CrMoV 12 1 Bend, Fabricated by an Inductive Pipe Bending Machine A bend with a bend radius of 1700 mm ?350 mm outer diameter and 25 mm wall thickness – of a X 20 CrMo V 12 1 steel, fabricated by an inductive pipe bending machine at 1050–900 °C deformation temperature was investigated. Heat treatment after fabrication: air cooling from 1050 °C, following a 2 hour annealing at 770 °C with air cooling. In addition to the usual mechanical and technological tests at ambient temperature and 530 °C, creep tests were conducted at 530 °C with unnotched and notched specimens. For comparison a straight pipe with the same dimensions was proofed, too.     

Effects of external operating environments on fatigue of heat‐exchanger copper tubes

The purpose of this study is to analyze the effects of surface defects (eg, notches) and external environment conditions (eg, operating temperature, the number of re‐weldings) on the static strength and fatigue of C1220T‐O copper tubes used in the heat exchangers of air conditioners. Instead of using standardized specimens, as is done in general rotary bending fatigue tests, special specimens were fabricated in this study by inserting metal plugs on both ends of the copper tubes to perform fatigue tests on the actual tube product, and then the fatigue characteristics were evaluated using stress‐life (S‐N) curves. Regarding the welding conditions (maximum 1000°C and 10 seconds), the grain size grew (grain size number decreased), and the hardness decreased as the number of re‐weldings increased. The effects of the operating temperatures on the fatigue life were examined at a room temperature of 25°C and a heat exchanger operating temperature of 125°C, resulting in the same fatigue limit (70.21 MPa) at both room and operating temperatures. However, the fatigue limit of 37.46 MPa measured in the notched specimens (radius of 3 mm, depth of 0.2 mm) was lower than that obtained from those without notches. The material constant (1.07) used in the Peterson equation was then computed from the fatigue notch factor (1.87 = 70.21/37.46), and the stress concentration factor (2.18) of the notched tube specimens was obtained from the structural analysis. This material constant can be used to predict a decrease in the fatigue limit over varying notch sizes in copper tubes (C1220T‐O).     

Structure and mechanical properties of the annealed TZAV-30 alloy

The Ti–30Zr–5Al–3V (wt.%, TZAV-30) alloy having good mechanical properties is a potential structural material to apply in the aerospace industry. The microstructure and mechanical properties of ZTAV-30 alloy underwent various annealing heat treatments were investigated. The specimens annealed from 500 to 800 °C are composed of α and β two phases. No compound is detected in specimens annealed in that temperature range. The microstructure of annealed specimens is characterized as a typical basketweave microstructure. Three microstructural parameters, thickness of plate α phase, relative fraction of β phase and aspect ratio of α grains, were measured in those annealed specimens. As the alloy annealed in the range from 500 to 800 °C, the average thickness of plate α grains increases with the increasing annealing temperature from 500 to 700 °C but decreases while annealed at 800 °C. The fraction of retained β phase increases with annealing temperature. And the aspect ratio of plate α grains decreases firstly but increases while the annealing temperature is higher than 700 °C. As the variation of those three microstructural parameters, the strength of examined alloy varies from 1269 to 1355 MPa for tensile strength and from 1101 to 1190 MPa for yield strength, inversely, the elongation changes in the range from 12.7% to 8.4%. The strengthening and toughening mechanism of the TZAV-30 alloy with basketweave microstructure is also discussed in this paper.     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Microstructure and creep behavior of a new developed nickel‐base single‐crystal superalloy

A new developed nickel‐base superalloy is employed to prepare single‐crystal castings. The as‐cast and heat‐treated microstructure, the creep behavior in 900 °C/ 500 MPa and 1100 °C / 140 MPa are investigated. The dendrites are well developed in the as‐cast microstructure. Segregation and γ′ precipitation morphology difference exist between dendrite core and γ/γ′ eutectic pool. After heat treatment the segregation is significantly decreased and the γ′ precipitations of all regions become much more uniform and cubic. Creep curves in varied conditions both present three stages, while the specimen in 1100 °C exhibits lower creep rate and longer life span. The fracture analysis shows that fracture style changes from mixed fracture to ductile when the temperature increases from 900 °C to 1100 °C. Porosity is the crack resource to form the facet, which is observed in both specimens. The γ′ precipitations raft in both specimens, while the further development of γ′ rafting is found in that of 1100 °C / 140 MPa, as well as the surface recrystallization with coarser rafted γ/γ′ microstructure.     

Very high cycle fatigue behavior of a low strength welded joint at moderate temperature

Very high cycle fatigue (VHCF) behavior of a low strength weldment was investigated by fully reversed axial tests in air at room temperature and 370 °C. The role of non-metallic inclusions in the VHCF was addressed in terms of experimental results and finite element simulations. The higher potential for interior crack nucleation at higher temperature was ascribed to matrix softening, surface oxidation and surface compressive residual stress. A new model for interpretation of the role of inclusion in the transition of crack initiation modes was developed.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

Effect of temperature on the fracture behaviour of heat‐treated Al–Cu–Li alloy laser welds under low‐cycle fatigue loading

The paper presents the results of the studies of the effect of temperature on the fracture behaviour of Al–Cu–Li alloy laser welds under low‐cycle fatigue loading. The mechanical properties and the microstructure of the welded joints without and after postweld heat treatment (PWHT) were investigated. The tensile strength and the low‐cycle fatigue resistance of the welded joints were studied at various test temperatures (20°C, 85°C and ? 60°C). It was been found that heating up to 85°C and cooling down to ?60°C reduced the maximum number of loading cycles of the welded joints after PWHT by 1.5–2.0 times compared with that at a test temperature of 20°C.     

Texture evolution in Fe–3% Si steel treated under unconventional annealing conditions

The present work investigates texture evolution stages in grain-oriented steel heat-treated using unconventional conditions. The Fe–3%Si steel taken after final cold rolling reduction from an industrial line was subjected to a laboratory isothermal annealing at different temperatures. The annealing temperatures were varied in a range of 850–1150 °C. During the annealing each specimen was heated at 10 °C/s and kept at the stated temperature for 5 min. Development of microstructure and texture in the annealed specimens were followed by the DC measurements of magnetic properties. The grain oriented steel, taken from the same industrial line after final box annealing was also analyzed and compared with the laboratory annealed specimens. It was shown that there is an optimal temperature region that, with combination of a fast heating rate, led to the best conditions of a drastically reduced development time of the {110} < 001 > crystallographic texture in the cold rolled grain-oriented steel. Materials heat treated below the optimum temperature region account for a primary recrystallization, while applying heat above this region leads to a secondary recrystallization without abnormal grain growth. Moreover, in the optimum temperature range, there was a particular temperature leading to the most optimal microstructure and texture. The magnetic properties, measured after the optimal heat treatment, were close to that measured on specimens taken after the final box annealing. The electron back scattered diffraction measurement technique revealed that sharpness of the {110} < 001 > crystallographic texture, developed at the optimum temperature is comparable to the steel taken after the industrial final box annealing. This fact is evidence that there is a temperature where the abnormal grain growth proceeds optimally.     

The effect of thermomechanical process on metallurgical and mechanical properties of 38MnVS6 micro alloyed steel

This study aims to have the proper mechanical properties without applying any heat treatment process after hot forging by using 38MnVS6 micro alloyed steel in tow hook manufacturing which is used in automobiles. The effect of forging temperature and cooling rate on metallurgical and mechanical properties of micro alloyed steel is investigated. The samples of micro alloyed steel are forged at 800 °C, 850 °C, 900 °C, 1050 °C and cooled down at the rates of 0.75 °C/s and 1.5 °C/s. The forging temperature and the cooling rate have a significant effect on toughness by the experimental and numerical studies. The amount of ferrite increases with a decreasing forging temperature, and the toughness is improved by increasing the amount of ferrite. Also, by increasing the cooling rate after forging at 1050 °C, the acicular ferrite is observed which has a positive effect on both the toughness and strength of the micro alloyed steel.