User:Sumit Kumar 96

Introduction Modified 9Cr-1Mo steels (P91) have been widely used for thermal power plant applications in view of their excellent creep strength. creep resistant steels are critical to the performance of boiler tubes and power generating plants in general[3]. Components made from these materials are mainly joined by welding. The use of creep resistant steels for the fabrication of components such as boiler tubes and fittings in power generation plants due to their high temperature strength properties has become the norm in recent years. The fabrication and repair of these components has however led to challenges regarding the degradation of mechanical properties and the formation of undesirable microstructures in the welded steels. The operating conditions of creep resistant steels induce large thermal and mechanical stresses on these components. Creep resistant steels such as P91 and 10CrMo9-10 are introduced due to their attractive mechanical performance at elevated temperatures, especially creep resistance. These properties depend on the chemical composition and microstructure of the steel. In order to manufacture and repair the various components using these materials, permanent joining through fusion welding processes is required. Welding in this case is a necessary compromise since no other fabrication process will be as cost effective or practical. The various effects of intense heat during welding on the microstructure of creep resistant steels[5] [2], the inclusion of filler material in the weld joints, the formation of heat affected zones (HAZ) and post weld heat treatments (PWHT) have been shown to have an impact on the mechanical and thermal performance of creep resistant steel weldments. The HAZ formed from the welding process has been shown to be a problematic area due to the development of crack defects in the weldment leading to failure. The challenges faced with the welding of steels such as P91 and 10CrMo9-10 has therefore led to the need for an investigation into the mechanical property changes and microstructural evolution of welded P91 and 10CrMo9-10 steels as a result of thermal exposure during welding. Joining Methods of P-91 Steel Various welding processes such as flux-shielded welding processes namely flux-cored arc welding[7] , shielded metal arc welding and also flux less shielded welding processes namely gas tungsten arc welding , and activated-tungsten inert gas welding . There are two issues associated with usage of P91 steel fabricated by the welding process. One is cracking observed in the inter-critical heat affected zone of weldment . cracking is formed by the thermal cycle imposed during welding on the base metal. There was reduction in life of components up to 30% as reported elsewhere. Another issue related to welding was degradation in toughness of flux shielded welds as compared with gas tungsten arc (GTA) welds. The toughness of fusion zone was less than in flux shielded welds as compared with toughness in GTA welds. Hence there is a need to improve the toughness of flux-shielded welds by employing alternative welding processes.[1] 1.Metal-Core Arc Welding-[6]

One such process is metal-cored  arc  welding  process that employs tube cored with metal powders and deoxidizers . MCW process is attracting more attention and is now being used in real applications. Development of MCW consumables suitable to joining of P91 steel are on recent focus due to the following advantages as outlined by various authors in their previous studies . There is a growing interest in this process for the application of joining of modified 9Cr-1Mo (P91) steel. This process has advantages such as superior finish and high productivity with less deslagging time hence amenable for automation of the process. Since the flux is absent, there was a thin oxide layer on the weld surface and also less micro inclusion content. In general, shielding gas composition has considerable influence on micro inclusion content of welds[10]. Less oxidizing shielding gas (pure argon) generates low volume fraction of micro inclusion content than highly oxidizing shielding gas[8], for example, pure carbon dioxide. Earlier studies reported that the following variables influence the toughness of P91 steel welds namely volume fraction of -ferrite, size and volume fraction of micro inclusion content.  In the present investigation, MCW process was carried out in order to understand the influence of process variables in joining of P91 steel. In addition, microstructure and mechanical properties were studied in detail to understand the factors influencing the toughness of metal-cored arc welds[9]. 

1.1. Experimental work In the present study, as-received material having chemical composition (Table 1) conforming to ASTM specification A335M-11 in the normalized (1080 °C – 1 h) and tempered (760 °C – 2 h) condition was used as base material in the preparation of weldments.

The as-received plates were cut into size of 220 mm × 100 mm × 12 mm for the preparation of weld pads. The weld groove details . Root face: 1.5 mm; Root gap 1.5 mm; Included angle 70° (each side angle being 35°). An inverter based, automated power source was used during welding trials. Direct current electrode positive (DCEP) was the polarity used during welding trials. Two kinds of metal-cored wires being used in the present investigation each being designated as ‘A’ and ‘B’ respectively and its chemical composition. The diameter of metal-cored wire is 1.2 mm. During welding the plates are joined with rigid clamps to avoid distortion during welding. Copper plate with provision for back purging was used during welding trials. Pure argon (99.7% purity) was used as backing gas to avoid oxidation during welding. The heat input can be calculated. (1) Heat input (kJ/mm)=n*I*V*60 /S The designation of variables used in the above mentioned Eq. are as follows: ‘n’ is the arc efficiency considered as 0.5 in the present study; ‘I’ is the arc current; ‘V’ is the arc voltage and ‘S’ is the traveling speed (mm/min).

Table 2. Parameters used for the preparation of metal-cored arc weldments. Process Number of layers Current (I) amps Voltage (V) volts Travel speed (S) (mm/min) Average heat input (kJ/mm) Root layer was completed using GTAW process and subsequent layers by MCW process. During welding a minimum amount of oxide layer is formed on the surface of weld due to absence of flux ingredients that does not require any cleaning. The appearance of the weld is smooth and shiny. Two types of shielding gases are used. One is 80% argon + 20% CO2 (designated as ‘A1’ and ‘B1’) and the other is pure argon (‘A2’ and ‘B2’). Shielding gas flow rate of 21 l/min was maintained during welding trials. Less spatter generation and spray mode of metal transfer was observed over a wide range of welding process parameters. Preheating of 200 °C and interpass temperature of 250 °C were maintained during welding. The problem with MCW process was difficulty in producing overhead welds. This has been due to absence of slag, since slag is required to support the molten weld pool during overhead (‘4G’ position) welding . The arc can be clearly observed due to fewer fumes generated during welding. The chemical composition of welds was analysed using optical emission spectroscopy (OES) technique.[14] The oxygen content was evaluated using CHNO analyser. After welding, the weldment was cooled to room temperature by natural air convection. Samples were collected at three different places along the length of the weld to arrive the statistical estimate of oxygen content of welds. Subsequently the drilled chips were immediately loaded into the CHNO analyser to know the oxygen content of welds. Weldments were subjected to radiography as per ASME section V to know the acceptance of weldments. The radiographically qualified weldments were then post-weld heat treated at 760 °C for various durations. The samples were heat treated in a pre-calibrated muffle furnace where the rate of heating and cooling were controlled. Using scanning electron microscope (SEM), in the unetched condition micro inclusion analysis was carried out. Micro inclusion count was carried out on images of as-weld, unetched microstructures for nearly ten images. The micro inclusion content reported was the average of the same. Microstructural characterization was carried out in the as-weld condition as well as post-weld heat treated, 760 °C – 2 h, and 760 °C – 5 h conditions. The samples were etched using Villella's reagent for 30 s. Micro inclusion content (using SEM) and its chemical composition (wt.%) were completed using SEM-EDS analysis. Further microstructural characterization was carried out using TEM[13]. The samples were thinned using electrolyte jet polishing technique. The electrolyte used was a mixture of 20% perchloric acid and 80% acetic acid maintained at a temperature of 20 °C. For toughness evaluation, Charpy V-notch specimens were prepared from the transverse cross-section of weldments. The V-notch is located in the fusion zone of the weldment and oriented parallel to the welding direction. In the present work, thermo-calc windows software used to generate property diagram (Temperature Vs Mole-fraction of phases) for the weldments studied. From the property diagram, equilibrium critical transformation points of the welds were determined. The database used was TCFE6 suitable for the compositions considered in the study. 2.Stir Welding of P91 Steel- Stir welds were made on P91 alloy with low and high rotational speeds(100and1000 RPM) to study their effects on weld microstructural changes and impression creep behaviour[11]. Temperatures experienced by the stir zone were recorded at the weld tool tip. Different zones of welds were characterized for their microstructural changes, hardness and creep behaviour (by impression creep tests). The results were compared with submerged arc fusion weld. Studies revealed that the stir zone temperature with 100 RPM was well belowAc1 temperature of P91 steel while it was above Ac3 with 1000 RPM. The results suggest that the microstructural degradation in P91 welds can be controlled by low temperature friction stir welding technique.[12] 2.2 Experimental The chemical composition (wt%) of P91 sheet used is as follows: Cr-8.91; Mo-0.98; C-0.09; Mn-0.42; Si-0.31; V-0.21; Nb-0.07; Fe-rest. Bead-on-plate friction stir welds with argon gas shielding (Fig. 1) were made on a 3 mm thick P91 steel (Normalized and tempered). The welding experiments were performed at Mega Stir Technologies LLC, Provo. A convex scrolled shoulder tool design made from a grade of polycrystalline cubic boron nitride (PCBN) weld tool was used with a small shoulder diameter and a tapered pin. The welding involved 3 stages: (1) Plunging stage: Tool rotational speed: 800 RPM; Plunge depth: 2.5 mm with tool feed rate of 76 mm/ min in Z-direction. (2) Dwell stage: Plunge depth is increased from 2.5 mm to 2.8 mm in Z-direction. Once plunge depth reached 2.8 mm, the RPM was reduced to 100 from 800, the tool’s dwell time (with no tool movement) was set at 5 sec with 2500 N axial force. (3) Weld stage: After 5 sec dwell time, the tool started traversing with 100 RPM, 55 mm/min traverse speed with 2500 N force. Friction stir welding experiment was repeated on a similar sheet of P91 with 1000 RPM rotational speed, keeping all other conditions same. The temperature of the stir zone was measured at the tip of the tool by inserting K-type thermocouple. For comparative purpose, a submerged arc weld (SAW) (3.75 kJ/mm heat input, post weld heat treated at 760 °C/16 hrs) was included in the study. The specimens were cut, polished and etched in the cross sectional direction of the weld to include stir (weld) zone, HAZ and base metal portions.The polished samples were etched using solution containing 1 g Picric acid + 5 mL HCl and 100 mL Ethanol. Preliminary impression creep tests were conducted to assess the relative creep behavior of different welds. Flat specimens with dimensions 20 × 10 × 3 mm were used for impression creep test with the following test conditions: Test temperature: 650 °C; Indenter: Tungsten carbide, 1 mm diameter, cylindrical, flat bottom; Punching stress: 280 MPa; Vacuum level: 10−3 Torr and test time: about 100 hrs (until attaining steady state). During impression creep testing, the displacement (i.e., depth of impression) was continuously monitored (at 10 minute intervals) as a function of test time. Testing was terminated after going well into the secondary creep regime[15].

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