The microstructure and mechanical properties of electron beam welded joints of reduced activation ferritic-martensitic steel in the as-welded and post-weld heat treatment (PWHT) states have been ...explored. The as-received base metal (BM) was in normalised and tempered condition. The PWHTs employed include post-weld direct tempering (PWDT) at 760°C/90min/air cooling and (ii) re-austenitizing at 980°C/30min/air cooling+ tempering at 760°C/90min/air cooling (PWNT). The BM microstructure was composed of fully tempered lath martensite with prior austenite grain and martensite lath boundaries decorated with M23C6 type carbides whereas intra-lath regions majorly displayed MX type carbides. In the as-welded state, the fusion zone (FZ) contained martensite in coarse grains and small amount of δ-ferrite with no evidence for precipitation of M23C6 and MX either in intra- or inter-granular regions. The heat affected zone (HAZ) was made up of martensite in fine grains without any δ-ferrite and with subtle variations in microstructure across the HAZ. The as-welded joints exhibited high hardness in the FZ and HAZ due to the occurrence of martensite during the weld thermal cycle. The impact toughness of the as-welded joint was inferior compared to that of the BM due to the combined influence of the martensite, coarse grains and presence of δ-ferrite in the weld zone. Tensile strength of as-welded joint was higher than that of BM. PWHTs were beneficial in decreasing the hardness in the FZ and HAZ. PWDT could not fully eliminate the pronounced variation of hardness observed in the transverse section of welded joint. Though the impact toughness of the weld joint was improved marginally compared to as-welded state after PWDT, it was much lower than that recorded in the case of BM. PWNT treatment minimised the variation in hardness across the transverse section of weld joint and the impact toughness surpassed than that achieved in BM. The tensile properties of BM, welded joints in as-welded and in PWHT conditions were determined at room temperature and correlated with the prevailing microstructures.
Bead-on-plate friction stir welding was conducted on 6mm thick plate of Reduced Activation Ferritic–Martensitic Steel employing polycrystalline cubic boron nitride tool with rotational speeds of 200, ...300, 500 and 700rpm and traverse speed of 30mm/min. The interface temperature between shoulder bottom and top surface of the plate was monitored by non-contact in-line thermography which served to identify the peak temperature attained in the stir zone (SZ). This temperature for 200, 300 and 500, and 700rpm was respectively below Ac1, between Ac1 and Ac3, and above Ac3. In the base metal (BM), the prior austenite grain and martensite lath boundaries were decorated with chromium and tungsten rich M23C6 precipitates while intra-lath regions revealed Ta and V rich MX type carbides. Rotational speeds greater than 300rpm led to martensite formation and simultaneous recovery, recrystallization and grain growth in SZs with wide distribution in grain size whereas SZ of 200rpm and BM possessed similar distribution. The grain boundary M23C6 dissolved and very fine needles of Fe3C precipitated in all SZs. The hardness of all SZs was unacceptably higher compared to the BM. The 200rpm weld exhibited higher impact toughness in the absence of martensite in SZ.
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•Effects of rotational speed during friction stir welding of a ferritic–martensitic steel were investigated.•Distribution of grain size in stir zones varied as a function of rotational speed.•Grain boundary M23C6 precipitates were dissolved while Fe3C formed in stir zones at all rotational speeds.•High rotational speeds promoted martensite occurrence in the stir zones with a drastic reduction in impact toughness.•Peak temperatures below Ac1 in the stir zone enabled impact toughness matching with the base metal.
The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), ...thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M
23
C
6
on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe
3
C precipitates were found at both the rotational speeds in SZ. The Fe
3
C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M
23
C
6
on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed.
In this paper the microstructure and hardness of electron beam weld joints of 16NCD13, low alloy steel in the as-welded and post weld heat treated (PWHT) conditions have been explored. The ...as-received base metal (BM) was in annealed condition. The BM microstructure depicted ferrite and carbides (Fe3C). In the as-welded condition the fusion zone(FZ) contained martensite in coarse grains and small amount of ferrite. The as-welded joints exhibited high hardness in the FZ and heat affected zone (HAZ) due to the occurrence of martensite during the weld thermal cycle. The PWHT employed is low temperature tempering commercially designed to preserve hardness and strength of as quenched martensite while relieving the internal stresses and providing small increase in toughness.The PWHT did not cause any significant reduction in hardness in the FZ and HAZ as tempering is carried out at 150 °C for 2 h and air cooled.The BM and FZ were characterised using optical microscopy, field emission scanning electron microscopy (FE-SEM), electron back scattered diffraction (EBSD) and Xray diffraction techniques. The current study is pursued with the aim of obtaining a comprehensive understanding of the fusion zone (FZ) and heat affected zone (HAZ) developed during electron beam welding process in as-welded and post weld heat treated conditions.
Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic-martensitic steel at tool rotational speeds of 500 and 900 rev min
−1
. Comparator welds at 500 rev min
...−1
were also produced in 6 mm thick reduced activation ferritic-martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermo-mechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.
