A Study of Microstructure and Mechanical Properties of Grade 91 Steel A-TIG Weld Joint

In the present study, A-TIG welding was carried out on grade 91 steel plates of size 220 × 110 × 10 mm using the in-house developed activated flux to produce butt-joints. The room-temperature impact toughness of the A-TIG as-welded joint was low due to the presence of untempered martensite matrix despite the low microinclusion density caused by activated flux and also low δ-ferrite (<0.5 %) content. Toughness after postweld heat treatment (PWHT) at 760 °C-2 h was 20 J as against the required value of 47 J as per the specification EN: 1557:1997. However, there was a significant improvement in impact toughness after PWHT at 760 °C for 3 h. The improvement in toughness was attributed to softening of martensite matrix caused by precipitation of carbides due to tempering reactions. The precipitates are of type M₂₃C₆, and they are observed at grain boundary as well as within the grains. The A-TIG-processed grade 91 steel weld joint was found to meet the toughness requirements after PWHT at 760 °C-3 h. Observations of fracture surfaces using SEM revealed that the as-welded joint failed by brittle fracture, whereas post-weld heat-treated weld joints failed by decohesive rupture mode.     

Effect of post-weld heat treatment on the wear resistance of hardfacing martensitic steel deposits

The effect of different post-weld heat treatments on the microstructure and wear resistance of martensitic deposits were studied. The deposit was welded using a metal-cored tubular wire, in the flat welding position, on a 375 × 75 × 19 mm SAE 1010 plate, using 98% Ar–2% CO₂ shielding gas mixture and with an average heat input of 2.8 kJ/mm. The samples were heat treated at temperatures between 500 and 680°C for 2 h. Chemical composition, Vicker's microhardness and wear properties with AMSLER tests in a sliding condition were determined. In the as welded condition, the microstructure was principally composed of martensite and retained austenite. Significant variations in wear resistance and hardness were measured for different tempering temperatures. For the different heat-treated conditions, it was observed that the decomposition of retained austenite to martensite and carbide precipitation was associated with the tempering of martensite. A secondary hardness effect was detected with maximum hardness of 710 HV for 550°C heat treatment temperature. The best performance in wear test was obtained for this condition. Wear rates for the different conditions were obtained and mathematical expressions were developed. For each case, wear mechanisms were analyzed.     

Study of Steel P92 Microstructure and Mechanical Properties after Different Heat Treatment Regimes

Metal Science and Heat Treatment - The effect of normalizing in the temperature range 950 – 1150°C with subsequent tempering for 2 h at 760°C on the microstructure of Cr – Mo...     

Correlation of precipitate stability to increased creep resistance of Cr–Mo steel welds

An order of magnitude decrease (from 16.0 × 10^?4 to 4.1 × 10^?4% h^?1) in steady-state creep rate was observed in the fine-grained heat-affected zone (HAZ) of a Cr–Mo steel weld by the reduction of the pre-weld tempering temperature from 760 °C (HTT) to 650 °C (LTT). The microstructure during each stage of the manufacturing path, including pre-weld temper, thermal cycling and post-weld heat treatment, was characterized using a suite of characterization techniques. The techniques included simulated thermal cycling, dilatometry and electron microscopy, as well as time-resolved X-ray diffraction using Synchrotron radiation. Both LTT and HTT steels before welding contain M₂₃C₆ (M = Cr, Fe) and MX (M = Nb, V; X = C, N) precipitates in a tempered martensite matrix. During simulated HAZ thermal cycling with different peak temperatures, changes in M₂₃C₆ carbide characteristics were observed between the HTT and LLT conditions, while MX precipitates remained stable in both conditions. Simulated post-weld heat treatment samples show larger M₂₃C₆ in the HTT condition than in the LTT condition. The results provide a solution to extending the life of Cr–Mo steel welded structures used in power plants.     

Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 2 – Mechanical properties

Abstract

Three thick pipes of modified 9Cr–1Mo steel were welded using three different levels of heat inputs: low heat input 1·1 kJ mm^?1, medium heat input 2·3 kJ mm^?1 and high heat input 3·16 kJ mm^?1. Two types of heat treatments were employed, namely subcritical post-weld heat treatment and normalising/tempering treatment. The influence of heat input and post-weld heat treatment on the microstructure of boiler steel P91weld joints has been investigated in the previous paper. In the present work hardness, tensile, impact toughness and stress rupture properties were evaluated. Our results show a great influence of heat input and heat treatment on mechanical properties. The best combination of properties was obtained in low to medium range of heat input, between 1 and 2·8 kJ mm^?1 for both treatments and a 90% increase in time to rupture was observed for normalising/tempering treatment compared to subcritical post-weld treatment.     

Characterization of microstructure of HAZs in as-welded and service condition of P91 pipe weldments

Steels 9-12% Cr, having the high creep rupture strength are advocated for the modern low polluting thermal power plants. In the present investigation, the P91 pipe weldments have been characterized for microstructural responses in as-welded, post-weld heat treatment (PWHT) and ageing conditions. The PWHT of welded samples were carried out at 760 °C for time of 2 h and ageing at 760 °C for 720 h and 1440 h, respectively. The effect of time has been studied on precipitates size, distribution of precipitates and grain sizes present in various zones of P91 steel weldments. The impact toughness and hardness variation of heat affected zone (HAZ) have also been studied in as-welded condition as well as at different heat treatment condition. A significant change was observed in grain size and precipitates size after each heat treatment condition. The maximum impact toughness of HAZ was obtained after PWHT at 760 °C for 2 h. The main phase observed in weld fusion zone in as-welded, PWHT and ageing conditions were M₂₃C₆, MX, M₇C₃, Fe-rich M₃C and M₂C. The unwanted Z-phase (NbCrN) was also noticed in weld fusion zone after ageing of 1440 h.     

Effects of Different Heat Treatments on the Microstructures and Creep Properties of GH4169 Superalloy

Experiments were conducted to evaluate the effects of different heat treatments on the microstructures and creep properties of GH4169 alloy. In order to study the strengthening effect of the δ phase, three different heat treatments were performed in this research. (1) High standard treatment (namely, HST), i.e., 960 °C × 2 h/air cooling (AC) + 720 °C × 8 h/furnace cooling (FC) at 55 °C/h to 620 °C + 620 °C × 8 h/AC. (2) Standard treatment (namely, ST), i.e., 960 °C × 1 h/AC + 720 °C × 8 h/FC at 55 °C/h to 620 °C + 620 °C × 8 h/AC. (3) Direct aging treatment (namely, DA), i.e., 720 °C × 8 h/FC at 55 °C/h to 620 °C + 620 °C × 8 h/AC. In HST specimen, it is observed that the δ phases exist not only at the boundaries but also locate inside the grains. However, few δ phases can be observed in DA specimen. All the samples were creep tested to failure under the condition of 650 °C/725 MPa. The DA alloy exhibited the best creep property and the creep rupture life was as long as 169 h. The creep rupture life of the ST alloy was the shortest, which is only about 88 h. The reasons can be summarized as the amount of the strengthening phases, the compactness of the boundaries, the fixing effect of the δ phase and the favorable strength coordination. All the points are closely related with the δ phase. It is reasonable to conclude that the quantity and distribution of δ phase have great influence on the creep properties of GH4169 alloy. Moreover, the fracture modes of the three samples are also discussed in this paper.     

Assessment of nickel alloy 625 weld overlays deposited by the electroslag process

Nickel-base alloy weld overlays are commonly used in the oil and gas industry to extend the life of equipment under aggressive corrosion environments, since the overlays improve the corrosion resistance without a significant increase in the cost of manufacture when compared to massive equipment. Usually, the joints are welded by SMAW, GMAW or GTAW processes. In this respect, the electro slag welding process (ESW), which promotes high heat inputs and low dilution welds, can be an interesting option for this application as it provides high productivity as only one layer is necessary. The present work evaluates mechanical, microstructural and corrosion properties of an Alloy 625 weld overlay deposited on ASTM A516 Grade 70 carbon steel by the ESW process. The deposition was done with one and two layers in plates of dimension 50 mm?×?400 mm?×?400 mm, in the flat position and an average energy welding 11.7 kJ/mm. After welding, a post-weld heat treatment at 620 °C for 10 h was performed, this condition being compared with the as-welded condition. Bending tests showed no evidences of cracks. Microstructural evaluation performed using both optical (OM), scanning (SEM) and transmission electron microscopy techniques showed an austenitic microstructure of the weld deposit with a low proportion of secondary phases for all conditions, and the post-welding heat treatment did not promote significant changes in the mechanical properties. At the coarse grain heat-affected zone (CGHAZ), the occurrence of proeutectoid ferrite, pearlite and bainite were found for one layer deposit and refined pearlite and ferrite for two layer deposits where the microstructural constituents were observed due to the low cooling rate. In addition, no evidence of partially diluted zones (PDZ) were verified. Corrosion tests conducted on samples removed from the top layer of the weld overlay cladding deposit according to ASTM G 48 Method A Standard were considered satisfactory once no evidence of pittings was verified and the loss of mass was very much reduced.     

Optimised post-weld heat treatment procedures and heat input for welding 17–4PH stainless steel

Abstract

Post-weld heat treatment (PWHT) procedures and heat input during welding of 17–4PH stainless steel, using matching chemistry consumables, have been optimised in relation to its microstructural condition before welding based on room temperature tensile properties. The 17–4PH stainless steel was welded in two different prior microstructural conditions, namely, condition A (solution treated) and condition H1150 (overaged), using three different heat inputs of 0·27, 0·48, and 0·72 kJ mm^-1, and post-weld heat treated to condition H900 (aged) or condition H1150 (over aged), using different heat treatment procedures. Room temperature tensile tests were carried out to study the combined effects of prior microstructural condition, heat input during welding, and PWHT procedures.     

Modification of Low-Alloy Steel Surface by High-Temperature Gas Nitriding Plus Tempering

The low-alloy steel was nitrided in a pure NH₃ gas atmosphere at 640 ~ 660 °C for 2 h, i.e., high-temperature gas nitriding (HTGN), followed by tempering at 225 °C, which can produce a high property surface coating without brittle compound (white) layer. The steel was also plasma nitriding for comparison. The composition, microstructure and microhardness of the nitrided and tempered specimens were examined, and their tribological behavior investigated. The results showed that the as-gas-nitrided layer consisted of a white layer composed of FeN_0.095 phase (nitrided austenite) and a diffusional zone underneath the white layer. After tempering, the white layer was decomposed to a nano-sized (α-Fe + γ′-Fe₄N + retained austenite) bainitic microstructure with a high hardness of 1150HV/25 g. Wear test results showed that the wear resistance and wear coefficient yielded by the complex HTGN plus tempering were considerably higher and lower, respectively, than those produced by the conventional plasma nitriding.     

Friction stir welding of dissimilar A319 and A356 aluminium cast alloys

Abstract

In the present investigation, the microstructure and mechanical characteristics of dissimilar A319 and A356 cast Al alloys plates joined by friction stir welding (FSW) were evaluated. The effect of tool rotational and welding speeds as well as the post-weld heat treatment (PWHT) on such properties was investigated. Post-weld heat treatment was carried out at a solutionising temperature of 540°C for 12 h followed by aging at 155°C for 6 h. For the as welded specimens, the welded zone (WZ) exhibited higher hardness values when compared with the A319 and A356 parent alloys. The peak hardness at the WZ was found to increase by increasing the tool rotational speed and/or reducing the welding speed. In contrast, the post-weld heat treated (PWHTed) specimens exhibited lower hardness values at the WZ than the parent alloys. For PWHTed specimens, the peak hardness at the WZ was found to decrease by increasing the tool rotational speed and/or reducing the welding speed. Tensile tests results demonstrate that, for the as welded specimens, the tensile fracture took place on A356 side where the hardness was minimal. While for PWHTed specimens, the fracture took place at the WZ. Increasing the tool rotational speed reducing both tensile and yield strengths, but increases the ductility of the joint.     

Effect of post-weld heat treatment on microstructure and property of linear friction welded Ti17 titanium alloy joint

Abstract

In this study, post-weld heat treatment of linear friction welded Ti17 (Ti–5Al–2Sn–2Zr–4Mo–4Cr) titanium alloy joints was performed at 530, 610 and 670°C for 4 h followed by air cooling. Results show that with increasing treatment temperature, the recrystallisation extent of the α and β phases in the weld and deformation zones increases significantly. The overall property of the joint is remarkably improved, and the fracture behaviour of the tensile and impact samples changes from brittle failure to a ductile one. After treatment at 670°C, the impact toughness of the joint is 93·3% of the parent metal, and the failure of the tensile samples occurs in the parent metal far away from the weld. According to these findings, a treatment temperature slightly lower than 670°C but higher than 610°C would be a good value for linear friction welding Ti17 joints.     

Effect of Cr Contents and Heat Treating on Reverted Austenite in Maraging Steel Weldments

By conducting flux cored arc welding (FCAW) on maraging steels with Cr contents of 1.4 and 5.2 wt%, this study observed the effects of Cr content and heat treating on reverted austenite formation in welded maraging steel. Aging treatment was carried out at the temperatures of 450, 480 and 530 °C for 3 h in each condition. As the aging temperature increased, reverted austenite was formed along the interdendritic and intercellular grain boundaries, and the proportion of reverted austenite increased with increasing Cr addition. The aging process led to the segregation of Ti and Mo along the interdendritic and intercellular grain boundaries. Some of the welded specimens were subjected to solution heat treatment at 820 and 1250 °C for 1 h after welding, resulting in a decrease in reverted austenite fraction.     

Effect of Aluminum on Microstructure and Properties of Martensitic Wear-Resistant and Heat-Resistant Steel

The effect of aluminum on microstructure and properties of martensitic wear-resistant and heat-resistant steels was investigated. The results indicate that as-cast microstructure of the specimens is composed of ferrite, pearlite and carbides. After quenching at 1,000 °C and tempering at 600 °C, the microstructure of the specimens consists of tempered sorbite and Fe–Cr–Al intermetallic compounds which distribute directionally in the matrix and increase with increase in Al content. The additions of Al enhance the ambient tensile strength which reaches the peak at 1,230 MPa when the specimens contain 1.97 % Al. Oxidation notably decreases with the increase of Al and the average oxidation rates reduce to 0.0095 g m^?2 h^?1 at 650 °C and 0.0285 g m^?2 h^?1 at 800 °C, respectively. Wear resistance of the specimens containing Al obviously increased when compared to the Al-free specimens.     

轧制工艺对14Cr1MoR钢板组织与性能的影响

采取光学显微镜、扫描电镜及拉伸、冲击试验机对板厚60 mm的14Cr1MoR热轧钢板正火+回火态和模拟焊后态的组织与性能进行了研究。结果表明:一阶段控轧与两阶段控轧的钢板相比,终轧温度高,轧后冷却速度慢,钢板铁素体晶粒尺寸粗大,珠光体含量多;钢板的强度低,伸长率高,冲击性能低。两阶段控轧的钢板经655 ℃保温3 h模拟焊后热处理,屈服强度下降44 MPa,抗拉强度下降24 MPa,冲击吸能能量降低;模拟焊后保温时间延长到12 h,强度和冲击性能变化不大。两阶段控轧的14CrMoR钢板,经正火+回火或再经过655 ℃模拟焊后热处理,钢板的力学性能优良。     

Influence of Heat Treatments on Microstructure and Toughness of 9%Ni Steel

The 9%Ni low-carbon steel is applied to utilities and processes at temperatures as low as ??196 °C. However, the microstructural features play an important role on the mechanical properties. Notably, the cryogenic toughness and mechanical strength are strongly dependent on the final heat treatment. In this paper, the microstructure of a 9%Ni low-carbon steel was modified by different heat treatments. The hardness and cryogenic toughness were measured and correlated to microstructural features. The material shows a temper embrittlement with intergranular cracking and minimum cryogenic toughness after tempering around 400 °C. Austempering at 480 °C also produced very low toughness results. On the other hand, excellent cryogenic toughness was obtained with single tempering at 600 °C after quenching or normalizing. Even higher toughness was obtained with the double tempering at 670 °C/2 h plus 600 °C/2 h. The amount of reversed austenite and its morphology in the specimen quenched and tempered at 600 °C were shown in the paper.     

Effect of minor change in composition on toughness of weldmetal for repair of turbine blades made of martensitic stainless steel

Abstract

Repair welding procedure for cracked turbine blades, made of 13Cr–2˙6Ni–1˙1Mo martensitic stainless steel, has been developed using gas tungsten arc welding process and a twin wire filler metal. The twin wire consists of a 1˙5 mm diameter ER 16-8-2 and a 2˙0 mm diameter ER 410 filler wires tack welded along the length of the two filler wires. A two stage post-weld heat treatment at 675°C for 2 h and 615°C for 4 h, such that the first heat treatments is above the Ac ₁ temperatures of the weld metal and the second is just below its Ac ₁ temperature; has been found to be suitable for obtaining good mechanical properties for the weldment. The weldment has a good combination of transverse weldment strength and weldmetal toughness, with its room temperature yield strength and Charpy V notch impact toughness being similar to that of the turbine blade material.     

Hot cracking in cast alloy 718

Hot cracking susceptibility of the Fe–Ni-based precipitation hardening cast superalloy Alloy 718 was studied by Varestraint weldability testing. The effect of two hot isostatic pressing (HIP) treatments commonly employed in the aerospace industry was investigated in reference to the as cast condition. It was found that the heat affected zone (HAZ) liquation cracking susceptibility increased for samples with pre-weld HIP treatments. The as cast condition disclosed the best response for liquation cracking followed by HIP-1120 (1120°C/4h (HIP)?+?1050°C/1h and furnace cooling to 650°C/1h in vacuum?+?950°C/1h) and HIP-1190 (1190°C/4h (HIP)?+?870°C/10h and furnace cooling to 650°C/1h in vacuum?+?950°C/1h). The amount of the secondary precipitates and base metal grain size was found to be important parameters influencing the cracking susceptibility. Regarding solidification cracking susceptibility, the three conditions appear to behave similarly.     

Effect of ruthenium on tensile properties of a single crystal Ni-based superalloy

The tensile properties of two single crystal Ni-based superalloys with and without added Ru (0 and 3 wt%) were investigated under a constant strain rate of 3.3×10^?4 /s at 20 °C, 760 °C, 800 °C and 1000 °C, respectively. The deformation mechanisms could be divided into two temperature regimes. From room temperature to 800 °C, the deformation mechanism is caused by the shearing of ?á? particles by anti-phase boundaries (APB) or stacking faults. At 1000 °C, the deformation mechanism is caused by the bypassing of ?á? particles by dislocations. At 20 °C and 800 °C, ?á? particles were sheared by APB. Due to smaller ?á? particles, the yield strength was decreased with addition of 3 wt% Ru. Additionally, work hardening is less pronounced in the alloy without Ru, hence the ultimate tensile strength was not decreased with the addition of 3 wt% Ru. At 760 °C, ?á? particles were sheared by stacking faults. Since the formation of stacking faults was promoted, the yield strength was decreased due to a 3 wt% Ru addition. However, the ultimate tensile strength was significantly increased when 3 wt% Ru was added. This is due to the markedly stronger work hardening caused by large numbers of stacking faults. At 1000 °C, deformation occurred by dislocations bypassing ?á? particles. Due to wider ?? channels, the yield strength was decreased by 3 wt% Ru addition. Moreover, Alloy 3Ru has smaller ?á? particles and a volume fraction as well as less pronounced work hardening, so the ultimate tensile strength was decreased when 3 wt% Ru was added.     

焊后热处理对AA7075铝合金DP-MIG焊接接头组织及力学性能的影响

采用双脉冲熔化极精性气体保护焊（DP-MIG）工艺方法焊接AA7075-T651铝合金,焊接试板采用固溶处理（480℃×50 min）后水淬,再进行（80℃×24 h）+（120℃×24 h）两级人工时效热处理,通过金相观察、扫描电镜观察、X射线衍射分析、拉伸试验以及硬度测试,研究焊后热处理（PWHT）工艺对焊接接头显微组织及力学性能的影响.结果表明,焊缝区经热处理后,晶粒由枝晶向等轴晶转变,晶界处非平衡第二相溶解,晶界变细,焊缝显微组织特性改善明显;焊接接头经热处理后,抗拉强度由342.5 MPa提高到490 MPa,接头强度系数为0.872,焊缝软化区硬度得到较大改善,焊接接头力学性能有显著提升.