Alternative PWHT to Improve High-Temperature Mechanical Properties of Advanced 9Cr Steel Welds

Creep-resistant 9Cr steels are extremely important in thermal power generation industry due to their marked resistance to creep and corrosion. The weldability of these alloys is critical since they are used in welded construction equipment. The required mechanical properties are achieved after post-weld heat treatment. This study examined the effect of different post-weld heat treatments on microstructure and mechanical properties of creep strength-enhanced 9Cr steel welding deposits. It was obtained with an experimental flux-cored arc welding wire used under protective gas (Ar-20% CO₂). The heat treatments used were: (1) tempering (760 °C?×?2 h), (2) solubilizing (1050 °C?×?1 h)?+?tempering (760 °C?×?2 h) and (3) solubilizing (1150 °C?×?1 h)?+?first tempering (660 °C?×?3 h)?+?second tempering (660 °C?×?3 h). All-weld metal chemical composition was analyzed, and hot tensile tests were carried out at different temperatures. Charpy-V impact tests and Vickers microhardness measurements were also performed. Microstructures were studied using x-ray diffraction and optical and scanning electron microscopy. In all cases, a martensitic matrix with intergranular and intra-granular precipitates was detected. In the as-welded condition, δ-ferrite was also found. Microhardness dropped, and the impact energy increased with post-weld heat treatments. The highest hot tensile strength result was achieved with samples submitted to austenization at 1150 °C and double tempering at 660 °C.     

Effect of tempering temperature on the microstructure,corrosion resistance,and antibacterial properties of Cu-bearing martensitic stainless steel

In this study, the effects of tempering temperature on the microstructure, hardness, antibacterial performance and corrosion resistance of Cu-bearing 5Cr15MoV martensitic stainless steel (5Cr15MoV-Cu MSS) were investigated using an optical microscope, a scanning electron microscope, X-ray diffraction, a transmission electron microscope, an antibacterial test, electrochemical measurements, and the salt spray test. The results showed that the hardness curve had a saddle shape and its values reached the peak value after tempering at 500°C, due to the secondary hardening effect by the precipitation of tiny secondary carbides and Cu-rich precipitates. In addition, the antibacterial results also showed excellent antibacterial performance against Escherichia coli at 500°C because of the formation of Cu-rich precipitates. Results of corrosion tests indicated that the corrosion resistance decreased gradually with an increase of the tempering temperature. In particular, the passivation did not occur when the tempering temperature was above 500°C, which may be related to the Cu-rich precipitates and Cr depletion.     

Effect of carbon and oxygen on microstructure and mechanical properties of Ti–25V–15Cr–2Al (wt%) alloys

The effect of carbon additions on microstructure and mechanical properties of alloys with different levels of oxygen was studied in β titanium alloys of the general composition Ti–25V–15Cr–2Al (all compositions are in wt% unless otherwise indicated). The microstructures were studied using optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that titanium carbides with vacancy-ordered structure formed in all alloys with C additions of over 1000 w.p.p.m. Grains were refined by carbides. Wavelength-dispersive X-ray (WDX) analysis showed that oxygen was much higher in carbides than in β matrix. After long-term exposure at 550°C α precipitation was significantly reduced in samples with titanium carbides compared with those without. A significant improvement in room temperature tensile ductility was achieved by the addition of carbon to the alloys. Elongations of ∼10% were obtained in samples which were exposed at 550°C for 500 h following heat treatments at 1050 and 700°C.     

Effect of Annealing Treatments on the Microstructure,Mechanical Properties and Corrosion Behavior of Direct Metal Laser Sintered Ti-6Al-4V

An experimental investigation on the effects of post-annealing treatments on the microstructure, mechanical properties and corrosion behavior of direct metal laser sintered Ti-6Al-4V alloys has been conducted. The microstructure and phase evolution as affected by annealing treatment temperature were examined through scanning electron microscopy and x-ray diffraction. The tensile properties and Vickers hardness were measured and compared to the commercial Grade 5 Ti-6Al-4V alloy. Corrosion behavior of the parts was analyzed electrochemically in simulated body fluid at 37 °C. It was found out that the as-printed parts mainly composed of non-equilibrium α′ phase. Annealing treatment allowed the transformation from α′ to α phase and the development of β phase. The tensile test results indicated that post-annealing treatment could improve the ductility and decrease the strength. The as-printed Ti-6Al-4V part exhibits inferior corrosion resistance compared to the commercial alloy, and post-annealing treatment can reduce its susceptibility to corrosion by reducing the two-phase interface area.     

Physical Simulation of Hot Deformation and Microstructural Evolution of Fe-0.05C-0.13P Steel

High-phosphorus steels are important for structural applications where corrosion resistance is required and are subjected to hot deformation processing. Therefore, hot deformation behavior of Fe-0.05C-0.13P steel is studied by conducting hot compression tests in the temperature range 750-1050 °C after austenitization at 1050 °C for 10 s. The strain rates ranged from 0.001 to 10 s^?1. Optical and scanning electron microscopy was performed to determine the microstructural evolution. EBSD measurement on selected samples was used to determine the microstructural changes in the ferrite phase. Processing windows were determined using modified dynamic material model in order to determine the safe hot working domains and these are correlated with the microstructural developments.     

Effect of secondary aging on microstructure and properties of cast Al-Cu-Mg alloy

To obtain better comprehensive properties of cast Al-Cu-Mg alloys, the secondary aging (T6I6) process (including initial aging, interrupted aging and re-aging stages) was optimized by an orthogonal method. The microstructures of the optimized Al-Cu-Mg alloy were observed by means of scanning electron microscopy and transmission electron microscopy, and the properties were investigated by hardness measurements, tensile tests, exfoliation corrosion tests, and intergranular corrosion tests. Results show that the S phase and θ′ phase simultaneously exist in the T6I6 treated alloy. Appropriately increasing the temperature of the interrupted aging in the T6I6 process can improve the mechanical properties and corrosion resistance of Al-Cu-Mg alloy. The optimal comprehensive properties (tensile strength of 443.6 MPa, hardness of 161.6 HV) of the alloy are obtained by initial aging at 180 °C for 2 h, interrupted aging at 90 °C for 30 min, and re-aging at 170 °C for 4 h.

     

Effect of nitrogen on microstructure and corrosion resistance of Cr15 super martensitic stainless steel

ABSTRACT

To investigate the influence of nitrogen on structure and corrosion resistance of Cr15 super martensitic stainless steels (SMSS), two types (N-free and N-0.12%) of specimens were quenched at 1050°C and tempered at different temperatures, and then, optical microscope, transmission electron microscopy, X-ray diffraction, potentiodynamic polarisation, immersion experiments and Kelvin Probe Force Microscope were used to characterize its microstructures and corrosion properties. The experimental results show that the microstructure in the N-free Cr15 super martensitic stainless steel is a biphasic tissue with alternating martensite and austenite distribution while quenched at 1050°C and tempered between 600 and 700°C. The nitrogen addition increases the content of austenite, and changes the austenite morphology significantly into the coarse block and strip distribution. What’s more, micro-galvanic corrosion is formed between austenite and martensite, which deteriorates the corrosion resistance of the SMSS.     

Effects of heat treatment on microstructure and mechanical properties of Cr–V–Mo steel processed by recrystallization and partial melting method

The phase segregation of semisolid processed products resulted in an inhomogeneous microstructure and poor mechanical properties of such products. Optimal subsequent heat treatments including quenching and tempering with various processing parameters were conducted to improve the quality of RAP (recrystallization and partial melting) processed Cr–V–Mo steel. The microstructure characteristics and mechanical properties, such as hardness, tensile strength, elongation, impact toughness, and resistance to high-temperature wear, of specimens subjected to various heat treatments were investigated. When the RAP-processed specimen was quenched from 1050 °C after isothermal holding for 480 s and then tempered twice at 560 °C for 2 h, microstructural evolution took placed in both former solid-phase and liquid-phase regions. The weakening of phase segregation, the redistribution of carbides, and the release of residual stress occurred during this heat treatment strategy caused a good combination of mechanical properties.     

Microstructure,hardness, toughness and abrasive wear resistance of 16.0 wt. % chromium white iron after continuous and cyclic destabilisation treatment

Destabilisation of as-cast chromium white iron with 16 wt-% chromium are performed by continuous destabilisation treatment for 4 h and short duration (0.66 h) cyclic destabilisation treatment at 900, 950, 1000, 1050, and 1100 °C. Continuous destabilisation causes secondary carbides precipitation from austenite which on slow cooling transforms to pearlite matrix. Cyclic destabilisation treatment causes similar precipitation of finer secondary carbides following shorter period austenitisation and a matrix containing martensite and retained austenite on forced-air cooling. After continuous destabilisation, hardness falls below the as-cast value (HV622); whereas it rises to HV950 after cyclic destabilisation treatment. The as-cast notched impact toughness (4.0 J) increases to 8.5 J or more after both continuous and cyclic destabilisation at 1050 and 1100 °C. Abrasive wear resistance after continuous destabilisation improves only at higher wear load (49.0 N), while after cyclic destabilisation it supersedes the as-cast and Ni-Hard IV performance at both low (19.6 N) and high (49.9 N) wear load.     

Non-vacuum sintering process of WC/W2C reinforced Ni-based coating on steel

Ni-based composite coatings containing varied contents of tungsten carbides on low carbon steel were fabricated. Effects of sintering temperature and tungsten carbides contents on the surface, interface, microstructure and wear resistance of the coatings were investigated using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Vickers microhardness tester, bulk hardness tester and pin-on-disc tribometer. The results indicated that with appropriate sintering temperature (1230 °C), smooth coating surfaces can be achieved. Favorable interfaces about 200 μm can be got that both the chemical composition and property of the interfacial region showed gradual transitions from the substrates to the coatings. Microstructure of the coatings consists of tungsten carbides and M7C3/M23C6 in the matrix. With excessive sintering temperature, tungsten carbides tend to dissolve. Ni-based coatings containing tungsten carbides showed much higher level of bulk hardness and wear resistance than ISO Fe360A and ASTM 1566 steels. With increasing contents of tungsten carbides from 25% to 40%, bulk hardness of Ni-based coatings gradually increased. Ni-based coating with 35% tungsten carbides performed the best wear resistance.     

The Effect of Nb and Ti on Structure and Mechanical Properties of 12Ni-25Cr-0.4C Austenitic Heat-Resistant Steel after Aging at 900 °C for 1000 h

Austenitic heat-resistant steels are particularly suitable for applications where service conditions comprise high temperature. The demand for better performance has motivated developments in these steels. In this work, Ti and Nb were added to austenitic heat-resistant steels, Fe-12Ni-25Cr-0.4C, wt.% simultaneously. Microstructural changes were studied via scanning electron microscopy equipped with energy dispersive spectrum (EDS), optical microscopy, and x-ray diffraction (XRD) in as-cast condition and after aging in 900 °C for 1000 h. Mechanical properties were measured using tensile tests, impact energy, and Vickers hardness. It was observed that by formation of NbC and TiC, the level of fragmentation of the chromium carbides increased, as a positive aspect for mechanical properties. XRD and EDS results show increasing the amount of Ti can inhibit G-phase transformation.     

Effect of homogenization temperature on microstructure and mechanical properties of low-carbon high-boron cast steel

The effects of quenching treatment on the microstructure, hardness, impact toughness, and wear resistance of low-carbon high-boron cast steel (LCHBS) containing 0.15–0.3 %C, 1.4–1.8 %B, 0.3–0.8 %Si, 0.8–1.2 %Mn, 0.5–0.8%Cr, 0.3–0.6%Ni, and 0.3–0.6%Mo have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and via an electron probe microanalyzer (EPMA), X-ray diffraction (XRD) analysis, impact tester, hardness tester, and wear tester. The as-cast matrix of LCHBS consists of pearlite and ferrite. There is 8–10 vol.% Fe₂(B, C) type borocarbides in the matrix. The micro-hardness of Fe₂(B, C) is 1430–1480 Hv. Fe₂(B, C) shows no obvious change and the matrix completely transforms into lath martensite upon quenching at 900 °C to 1100 °C. The microhardness of the matrix and the macrohardness of the LCHBS sample show a slight increase with an increase of homogenization temperature. When the homogenization temperature exceeds 1050 °C, no distinct change in the hardness is observed. The change of homogenization temperature has no apparent effect on the impact toughness of LCHBS. The mass losses of LCHBS increase distinctly when the wear load increases. The homogenization temperature is less than 1000 °C and the wear rate of LCHBS decreases with an increase of temperature. The wear rate does not display any obvious change after exceeding a homogenization temperature of 1000 °C.     

Enhanced Strength in Cu-Ag-Zr Alloy by Combination of Cold Working and Aging

In this investigation, the effect of different degree of cold rolling and post-aging treatment on the microstructure and mechanical properties of a Cu-3wt.%Ag-0.5wt.%Zr alloy was studied by means of hardness measurement, tensile tests, optical and electron microscopy. The alloy was subjected to cold rolling up to 80% followed by aging in the temperature range of 400-500 °C. The yield strength, ultimate tensile strength and hardness were found to increase as degree of cold rolling increased, but at the expense of ductility. Aging of cold rolled samples in the studied temperature range has resulted in different combinations of strength and ductility. However, aging of cold rolled samples at 400 °C for 1 h has resulted in a combination of high strength and moderate ductility. A yield strength and ultimate tensile strength of 511 and 560 MPa, respectively with a ductility of 12% were achieved for 80% cold rolled and aged (400 °C for 1 h) sample. The high strength achieved after 80% cold rolling and aging is mainly attributed to precipitation of fine silver precipitates.     

Application limits and sensitisation behaviour of the manganese- and nitrogen-alloyed austenitic stainless steel P2000 (X13CrMnMoN18-14-3)

Nickel-free high-nitrogen-alloyed stainless steels like the P2000 (X13CrMnMoN18-14-3) were developed to enhance the strength and corrosion resistance of austenitic stainless steels like 304 and 316 while keeping the typical high ductility. The mechanical and corrosive properties of P2000 were investigated and compared with 304 and 316 to highlight the application opportunities of this new alloy. The microstructure of the solution-annealed condition was characterised by electron backscatter diffraction and the mechanical properties were studied by uniaxial tensile tests, Charpy impact tests and hardness measurements. The passivation behaviour was analysed using the electrochemical potentiodynamic reactivation, whereas the pitting corrosion resistance was compared by pitting potentials and pitting temperatures. However, secondary thermal influences or suboptimal heat treatment can impair the corrosion resistance due to the precipitation of secondary phases and the resulting sensitisation. Thermodynamic calculations and artificial ageing treatment in the range of 500–900°C for up to 100 h were used to determine critical time–temperature parameters for sensitisation. The microstructure of the various aged states was evaluated by scanning electron microscopy and compared with the degrading corrosion resistance characterised by the KorroPad method.     

Effects of Destabilization Temperature on the Microstructure and Mechanical Properties of High Chromium Cast Iron

The effect of destabilization temperature on the microstructure and impact toughness of high chromium cast iron was investigated. The result showed that the microhardness of matrix decreased from 860 to 332 HV with the increase of destabilization temperature from 950 to 1100 °C. The impact toughness of the alloy increased from 5.3 to 8.1 J/cm² with the increase of destabilization temperature from 950 to 1050 °C, while it decreased with further increase of destabilization temperature. The former change of impact toughness was found to be due to the increase in volume fraction of retained austenite from 12.6 to 56.5%, whereas the latter was attributed to the increase of carbon content in retained austenite by analysis of fracture morphologies. The wear resistance decreased with the increase of destabilized temperature from 950 to 1050 °C and increased only slightly for the temperature from 1050 to 1100 °C.     

On Microstructure and Microhardness of Isothermally Aged UNS S32760 and the Effect on Toughness and Corrosion Behavior

This paper investigates the microstructure and secondary phase precipitations obtained in UNS S32760 super duplex stainless steel and their effect on impact toughness and corrosion resistance. The heat treatment included first solution annealing at 1150 °C followed by water quenching, then isothermal heating at different temperatures from 350 to 950 °C for different times, ranging from less than 1 min to 600 min, followed by water quenching again. Microscopic investigation, microhardness tests, and x-ray diffraction (XRD) analysis were used to identify the microstructure and secondary phase precipitations formed by heat treatment. The study indicates a fair correlation between the microscopic observations and microhardness results, while XRD analysis defined the phase’s chemistry and confirmed the microscopic and hardness results. In addition to the austenite (γ) and ferrite (α) phases of the duplex structure, secondary phases of (σ, χ, and chromium nitrides) are observed at a high temperature range, while (?) and (aged ?) are observed at a lower temperature range. It is concluded that the microhardness test can be used to identify the phases appearing in the microstructure, which results in fair prediction for the TTT diagram and σ-phase range. The variation of toughness and corrosion resistance by heat treatment differs depending on the secondary phase formation.     

Effects of substrate preheating during direct energy deposition on microstructure,hardness, tensile strength,and notch toughness

This study examined the effects of substrate preheating for the hardfacing of cold-press dies using the high-speed tool steel AISI M4. The preheating of the substrate is a widely used technique for reducing the degree of thermal deformation and preventing crack formation. We investigated the changes in the metallurgical and mechanical properties of the high-speed tool steel M4 deposited on an AISI D2 substrate with changes in the substrate preheating temperature. Five preheating temperatures (100-500 °C; interval of 100 °C) were selected, and the changes in the temperature of the substrate during deposition were observed. As the preheating temperature of the substrate was increased, the temperature gradient between the melting layer and the substrate decreased; this prevented the formation of internal cracks, owing to thermal stress relief. Field-emission scanning electron microscopy showed that a dendritic structure was formed at the interface between the deposited layer and the substrate while a cellular microstructure was formed in the deposited layer. As the preheating temperature was increased, the sizes of the cells and precipitated carbides also increased. Furthermore, the hardness increased slightly while the strength and toughness decreased. Moreover, the tensile and impact properties deteriorated rapidly at excessively high preheating temperatures (greater than 500 °C). The results of this study can be used as preheating criteria for achieving the desired mechanical properties during the hardfacing of dies and molds.     

降碳增氮对马氏体不锈钢淬火组织和性能的影响

采用激光扫描共聚焦显微镜、扫描电镜、硬度计、X射线衍射仪和盐雾试验机,研究了不同温度(950、1000、1050、1100℃)下30Cr13和30Cr14N钢在马弗炉中空淬后,氮含量对30Cr13钢显微组织、碳化物、硬度和耐蚀性能的影响。结果表明,淬火温度相同,30Cr14N钢比30Cr13钢硬度高、碳化物少和耐蚀性能好。氮不仅影响马氏体不锈钢的显微组织及硬度,还能通过降碳增氮,避免因碳化物过多的析出而引起的晶间腐蚀,而FeNiN的析出不会像Cr₂₃C₆析出造成显著的晶间腐蚀。因此,降碳增氮是改善马氏体不锈钢组织和性能的一种有效途径。     

Microstructure Characterization and Corrosion Properties of Nitrocarburized AISI 4140 Low Alloy Steel

Plasma nitrocarburizing treatments of AISI 4140 low alloy steel have been carried out in a gas mixture of 85% N₂-12% H₂-3% CO₂. All treatments were performed for 5 h at a chamber pressure of 4 mbar. Different treatment temperatures varying from 520 to 620 °C have been used to investigate the effect of treatment temperature on the corrosion and hardness properties and also microstructure of the plasma nitrocarburized steel. Scanning electron and optical microscopy, x-ray diffraction, microhardness measurement, and potentiodynamic polarization technique in 3.5% NaCl solution were used to study the treated surfaces. The results revealed that plasma nitrocarburizing at temperatures below 570 °C can readily produce a monophase ε compound layer. The compound layer formed at 620 °C is composed of two sub-layers and is supported by an austenite zone followed by the diffusion layer. The thickest diffusion layer was related to the sample treated at 620 °C. Microhardness results showed a reduction of surface hardness with increasing the treatment temperature from 520 to 620 °C. It has also been found that with increasing treatment temperature from 520 to 545 °C the corrosion resistance increases up to a maximum and then decreases with further increasing treatment temperature from 545 to 620 °C.     

Preliminary Evaluation of the Thermally Affected Metallurgical Condition of Extruded and Drawn CuFe2P Tubes

The influence of heat treatment conditions on the mechanical behavior and microstructure of CuFe2P (ASTM C19400) in comparison to deoxidized-high-phosphorus (DHP-Cu/ASTM C12200) tubes was investigated. The aim of this study was the enhancement of understanding of microstructure/thermal treatment/strength relationships which could be further utilized for the manufacturing of components exhibiting superior performance and reliability for refrigeration and heat exchanger applications. Microstructural examination employing optical metallography and scanning electron microscopy is used for the evaluation of the recrystallization progress and grain growth processes. In addition, tensile testing was conducted to CuFe2P and DHP tubes following the application of heat treatment cycles, in accordance to the EN 10002-1 specifications. Mechanical properties and microstructure evaluation showed that CuFe2P material is fully recrystallized at 740 °C and DHP at 400 °C for 20 min. Recrystallization initiation varies within the range of 640-660 °C for CuFe2P and below 400 °C for DHP tubes. The tensile strength of the CuFe2P tube decreased from 513 to 367 MPa, the hardness was reduced from 144 to 126 HV, while tensile elongation was significantly improved from 3 to 17%. At 640 °C, only isolated recrystallized areas were evident mainly at the Fe-based intermetallic particle/copper matrix interface areas.