Original Research Article
A brief study on δ-ferrite evolution in dissimilar P91 and P92 steel weld joint and their effect on mechanical properties

https://doi.org/10.1016/j.acme.2017.12.002Get rights and content

Abstract

Ferritic/martensitic 9Cr-1Mo-V-Nb steel also designated as ASTM A335 used in construction of several components of power plants operating in temperature range of 600–650 °C. In present investigation, dissimilar weld joint of P91 and P92 steel were prepared using the autogenous tungsten inert gas (A-TIG) welding and multi-pass gas tungsten arc welding (GTAW) process. A comparative study was performed on evolution of δ-ferrite patches in weld fusion zone and heat affected zones (HAZs) of welded joints. The evolution of δ-ferrite patches was studied in as-welded and post-weld heat treatment (PWHT) condition. PWHT was carried out at 760 °C for tempering time of 2 h and 6 h, for both A-TIG and GTA weld joints. It was observed that presence of higher content of ferrite stabilizer in P92 steel promote the formation of δ-ferrite patches in weld fusion zone as well as HAZs. To study the effect of welding process and PWHT, Charpy V impact energy and microhardness tests were performed. For microstructure characterization, field-emission scanning electron microscope (FESEM) and optical microscope were utilized.

Introduction

Modified 9Cr-1Mo (P91) steels are developed as first wall and blankets structure of ITER test modules [1]. In present years, the weldability of P91 steel is a serious issue. It has been reported that, compared to that of weld, a higher creep rupture life of P91 base metal is observed regardless of the arc welding process and heat treatment used [2]. It occurs due to presence of non-equilibrium structure in heat affected zone (HAZ). The non-equilibrium structure formed because of high heating and cooling rate in the weld thermal cycle. P91 steel can be welded by any of the commonly used arc welding processes such as manual metal arc welding (MMA), gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), electron beam welding (EBW), and submerged arc welding (SAW) [3], [4], [5]. A lot of efforts have been made to develop P91 and P92 steel electrode that provide optimum strength and toughness to weldments. The initial problem faced during welding of P91 steel is selection of matching filler wire/rod that provide optimum composition in weld fusion zone. Composition of Ni and Mn in filler wire required more attention due to their strong influence on temperature Ac1, Mf, and Ms. In P91 base metal, maximum 1%(Ni + Mn) is allowed. For 1.4% (Ni + Mn), Mf temperature decreased lower than 200 °C (preheat temperature) that led to formation of retained austenite in filler metal. Santella et al. [6] reported a numerical equation to calculate the Ac1 temperature based on Mn and Ni contents. The equation is given below:Ac1(°C)=854.5±0.643.9±1×(Mn+Ni)9±0.4(Mn+Ni)2

Increase in Ni and Mn content in filler metal decrease the Ac1 for weld metal. The V and Nb are the other elements in filler wire that affects the mechanical properties of weld fusion zone. Arivazhaga et al. [7] studied the effect of V + Nb content in weld fusion zone on Charpy toughness of weld fusion zone. Higher Charpy toughness was observed for the low V + Nb content in weld zone. Fusion welding process introduces the heterogeneity in microstructure by its nature. Single phase tempered martensitic structure in P91 weld zone provides optimum strength and toughness compared to two phase of tempered martensite and δ-ferrite. In P91 weldments, formation of δ-ferrite in weld fusion zone lead to reduction in mechanical properties of weldments. A schematic evolution of sub-zone in multi-pass welded P91 joint is shown in Fig. 1.

As per Schaeffler diagram, high chromium and low nickel equivalent in the composition of base metal and weld zone might be also promote the formation of δ-ferrite [8], [9]. The δ-ferrite formation occurs during the unconventional mechanical and thermal treatment. Formation of δ-ferrite are reported commonly in P91 and P92 steel welds joint. In autogenous tungsten inert gas (TIG) welding process, composition of base metal, heat input and cooling rate plays an important role in δ-ferrite formation while in multi-pass gas tungsten arc welding (GTAW) process, cooling rate, filler composition, heat input and number of passes play an important role in deciding the formation of δ-ferrite.

The δ-ferrite formation in weld fusion zone have a noticeable effect on mechanical properties. δ-Ferrite formation in P91 steel weldments are generally reported at highest peak temperature [10], [11]. The region exposed at temperature more than 1200 °C leads the formation fine prior austenite grains (PAGs) because of δ-ferrite nucleation on preferential sites of prior austenite grain boundaries (PAGBs). The δ-ferrite nucleation limits the growth of austenite and resulting in fine PAGs formation. Chandravathi et al. [11] reported that formation of soft δ-ferrite in microstructure led to reduction in hardness. Schafer [9] studied the effect of δ-ferrite formation on tensile properties and observed that δ-ferrite upto 8% have no any significant effect on the strength. However, a different opinion was reported related to effect of δ-ferrite on toughness and ductility. Some author(s) [10], [11] have reported positive effect of retained δ-ferrite on toughness while others [12] emphasize the negative influence. Schafer [9] reported that combined effect of δ-ferrite and brittle ‘dendritic’ carbides of M23C6 types (M  65%Cr + 30%Fe + rest) reduced the toughness by encapsulate the δ-ferrite. However, pure and soft δ-ferrite increased the toughness and ductility. Anderko et al. [13] have also reported the negative influence of δ-ferrite on impact toughness of fusion zone. Presence of δ-ferrite in base metal, weld zone or HAZ have found a negative influence on creep strength of P91 steel [14], [15]. Presence of δ-ferrite accelerated the recovery rate of martensite and growth rate of intermetallic phase like laves phase (Fe2W or Fe2Mo) and ultimately reduced the creep and fatigue strength of steel.

In present case, effect of A-TIG and GTAW welding process on δ-ferrite evolution have been studied for as-welded and PWHT condition. PWHT was performed at 760 °C for varying tempering time of 2–6 h. The welded samples were characterized using the Charpy V impact energy test, hardness test and field emission scanning electron microscope (FESEM). For phase analysis X-ray diffraction analysis was performed.

Section snippets

Experimental methodology and material

Creep strength ferritic/martensitic P91 and P92 steel were supplied in normalized and tempered condition, as per manufacturer. Normalizing was performed at 1040 °C for 40 min, followed by air cooling and then tempered at 760 °C for 2 h and finally air cooled. Schematic microstructure evolution during the normalizing and tempering process is shown in Fig. 2.

Normalizing was performed to develop the martensitic microstructure with high dislocation density. Tempering process leads to tempering of

As-received material and mechanical properties

Typical micrographs of ‘as-received’ P91 and P92 steel are shown in Fig. 3(a) and (b), respectively. Microstructure consists of martensite equiaxed laths, lath blocks, prior austenite grain boundaries (PAGBs), and lath boundaries. The stability of tempered microstructure is derived from the precipitates that decorated along the PAGBs, lath blocks, lath packets and inside the intra-lath region. Precipitates located along the boundaries and lath packets having size in range of 50–140 nm are

Conclusions

  • P91 and P92 steel plate of thickness 5 mm were welded successfully using autogenous tungsten inert gas (A-TIG) welding process and plates of 8 mm thickness were welded using the multi-pass gas tungsten arc welding (GTAW) process.

  • A-TIG and GTAW process were found a noticeable effect on microstructure evolution and mechanical properties of P91 and P92 dissimilar weld joints.

  • In A-TIG weld joint, coarse martensitic microstructure was formed due to high peak temperature as compared to GTA welds joint.

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