Chandan Pandey

Effect of Groove Design and Post-Weld Heat Treatment on Microstructure and Mechanical Properties of P91 Steel Weld

The martensitic creep-resistant steel designated as ASTM A335 for plate and as P91 for pipe is primarily used for high-temperature and high-pressure applications in steam power plants due to its excellent high-temperature properties such as high creep strength, high thermal conductivity, low thermal expansion, and so on. However, in the case of welded joints of such steels, the presence of an inter-critical heat-affected zone (IC-HAZ) can cause the joint to have lower creep strength than the base metal. In the present study, the effect of post-welding heat treatment (PWHT) and weld groove designs on the overall microstructure and mechanical properties of P91 steel pipe welds produced by the gas tungsten arc welding process was studied. Various regions of welded joints were characterized in detail for hardness and metallographic and tensile properties. Sub-size tensile samples were also tested to evaluate the mechanical properties of the weld metal and heat-affected zone (HAZ) with respect to PWHT. After PWHT, a homogenous microstructure was observed in the HAZ and tensile test fracture samples revealed shifting of the fracture location from the IC-HAZ to the fine-grained heat-affected zone. Before PWHT, the conventional V-grooved welded joints exhibited higher tensile strength compared to the narrow-grooved joints. However, after PWHT, both narrow- and V-grooved joints exhibited similar strength. Fractography of the samples indicates the presence of carbide precipitates such as Cr23C6, VC, and NbC on the fracture surface.

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.

Microstructure characterization and charpy toughness of P91 weldment for as-welded, post-weld heat treatment and normalizing & tempering heat treatment

The effect of weld groove design and heat treatment on microstructure evolution and Charpy toughness of P91 pipe weldments was studied. The P91 pipe weldments were subjected to subcritical post weld heat treatment (760 °C-2 h) and normalizing/tempering conditions (normalized-1040 °C/40 min, air cooled; tempered 760 °C/2 h, air cooled) were employed. The influence of subsequent PWHT and N&T treatment on the microstructure of various zone of P91 pipe weldments were also investigated. The present investigation also described the effect of PWHT and N&T treatment on hardness, grain size, precipitate size, inter-particle spacing and fraction area of precipitates present in each zone of P91 pipe weldments. The result indicated great impact of heat treatment on the Charpy toughness and microstructure evolution of P91 weldments. The N&T treatment was found to be more effective heat treatment compared to subsequent PWHT. Charpy toughness value was found to be higher for narrow-groove design as compared to conventional V-groove design.

Ductilizing of cast hypereutectic Al–17%Si alloy by friction stir processing:

In present research work, multi-pass friction stir processing was utilized for the as-cast Al–17%Si alloy. The multi-pass friction stir processing reproduced the aluminum (Al) matrix with a uniform distribution of the ultra-fine silicon (Si) particles. The multi-pass friction stir processing also resulted in the formation of refining cast microstructure with negligible porosity. The significant reduction in silicon particle size was measured and it reduced from 204 µm to 0.86 µm after the first pass and up to 0.30 µm after two pass friction stir processing. The frequency of fine Si particles was increased after two pass friction stir processing as compared to single pass friction stir processing. The engineering stress–strain curves revealed a significant enhancement in ductility and strength after one and two passes of friction stir processing as compared to the as-cast alloy. After one and two passes of friction stir processing, the ultimate tensile strength of as-cast alloy was enhanced by 24% and 31% and ductility was increased by 300% and 500%, respectively. The secondary electron micrograph of fracture surfaces of tensile specimens was taken before and after friction stir processing. The fractographs revealed the transformation from brittle mode to ductile dimples fracture after the multi-pass friction stir processing process.

Mechanical Properties and Wear Behavior of Zn and MoS 2 Reinforced Surface Composite Al- Si Alloys Using Friction Stir Processing

Friction stir processing (FSP) is a novel process for refinement of microstructure, improvement of material’s mechanical properties and production of surface layer composites. In present investigation, composite reinforced with using Zn/ MoS2 powder in as-cast alloy were developed at tool rotational speed of 664 rpm and tool transverse speed of 26 mm/min using FSP. Microstructural observation of MoS2/Zn reinforced composites confirmed the fine and equiaxed grains in the stir zone (SZ) and distribution of fine reinforced particles of MoS2/Zn in SZ. Moreover, agglomeration of MoS2/Zn particles were not observed. The ultimate tensile strength was measured to be 113 ± 9 and 82 ± 7 MPa for MoS2and Zn reinforced Al-Si alloy, respectively. The sliding wear was studied using pin-on-disk tribometer and it was found that FSP enhanced the wear resistance of the as-cast alloy. The MoS2 reinforced composite showed superior wear resistance than Zn reinforced composite and base material. To understand the acting wear mechanism, the field emission scanning electron microscope (FESEM) of worn out surfaces were performed.

Achieving optimized tungsten inert gas butt welding conditions of thin cold rolled steel sheets by response surface methodology and artificial neural networks

This paper describes the multiresponse optimization of tungsten inert gas welding for an optimal parametric combination to predict the weld characteristics of thin cold rolled steel sheets. The interaction effects of tungsten inert gas welding process parameters such as welding current, arc length, and traverse speed have been observed on the weld characteristic responses such as weld width, heat-affected zone width, and under-bead depression. Full factorial design of experiment was followed for determining the combinations of the experimental runs. Regression analysis was carried out to develop the mathematical models for the welding control factors and responses. Analysis of variance was used to check the adequacy of the developed mathematical model. The confirmatory tests were conducted to validate the accuracy of mathematical model. Sensitivity analysis was also done to analyze the effect of each individual process parameter on the weld responses. The full factorial experimental data was further utilized for multi-response optimization of the tungsten inert gas process parameters. It was observed that weld responses like weld width and heat-affected zone width could be optimized by regression modeling technique while the under-bead depression showed uncertain behavior. The under-bead depression ranged from 0.0 to 0.15 mm and was observed only when the arc traverse speed was at the lowest level (4 cm/min) for all values of the welding current and arc length. The experimental data were also modeled using the artificial neural network technique for the prediction of weld responses and the results were compared with that from the regression analysis.

Characterisation of dissimilar P91 and P92 steel welds joint

ABSTRACT In the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised.

Study on Effect of Process Variables on Distributed Compositional Characteristics in Metallurgically and Mechanically Bonded Claddings

In surface modification applications, cermets such as Cr3C2–NiCr are used to enhance the wear resistance of the substrate. Friction surfaced cladding is a recently developed solid-state surface modification process in which the deposition of the clad layer is accomplished below it’s melting temperature, so that degradation of the substrate is minimized. The present investigation discusses the friction surfaced cladding of Cr3C2–NiCr using consumable die steel rod. It was observed from the micrographs, that with the suitable combination of tool plunging rate (plunging speed), tool rotational speed and traverse rate, successful cladding of Cr3C2–NiCr was achieved. A plan of full factorial experiment was followed for determining the process variables for the successful deposition of Cr3C2–NiCr clad layer with the substrate. A mathematical model was developed to predict the composite clad dimensions and microhardness with respect to the process variables. Depending upon the process parameter, it was observed that the hardness of the clad and clad geometrical feature varied. The microstructural analysis exhibited sound and uniform bonding of clad layer to the substrate and near uniform distribution of Cr3C2–NiCr. Desirability based multi-response optimization procedure was followed to arrive at optimized Cr3C2–NiCr clad.

Study on Hydrogen-Assisted Cracking in High-Strength Steels by Using the Granjon Implant Test

In present research work, the modified Granjon implant test was performed to evaluate the susceptibility of AISI 8620 and AISI 304 steels toward the hydrogen-assisted cracking. To measure the diffusible hydrogen level (HD) in deposited metal, glycerin method was employed. Shielded metal arc welding process with basic type electrode was used to deposit the metal. The hydrogen was intentionally introduced for the plate of the material AISI 304 by using an oil of grade Society of Automotive Engineers 10, which is of very low viscosity. The heat-affected zone susceptibility was quantified by finding the lower critical stress for a measured hydrogen content.