For joining type 316LN austenitic stainless steel to modified 9Cr–1Mo steel for power plant application, a trimetallic configuration using an insert piece (such as alloy 800) of intermediate thermal coefficient of expansion (CTE) has been sometimes suggested for bridging the wide gap in CTE between the two steels. Two joints are thus involved and this paper is concerned with the weld between 316LN and alloy 800. These welds were produced using three types of filler materials: austenitic stainless steels corresponding to 316, 16Cr–8Ni–2Mo, and the nickel-base Inconel 182 1 . The weld fusion zones and the interfaces with the base materials were characterised in detail using light and transmission electron microscopy. The 316 and Inconel 182 weld metals solidified dendritically, while the 16–8–2 (16%Cr–8%Ni–2%Mo) weld metal showed a predominantly cellular substructure. The Inconel weld metal contained a large number of inclusions when deposited from flux-coated electrodes, but was relatively inclusion-free under inert gas-shielded welding. Long-term elevated-temperature aging of the weld metals resulted in embrittling sigma phase precipitation in the austenitic stainless steel weld metals, but the nickel-base welds showed no visible precipitation, demonstrating their superior metallurgical stability for high-temperature service.

Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800

A comparative evaluation of welding consumables for dissimilar welds between 316LN austenitic stainless steel and Alloy 800

The effect of the prior solution treatment temperature on the δ-ferrite transformation in 2205 duplex stainless welds after aging at 850 °C has been studied. Microstructural examination showed that the σ-phase and M23C6 chromium carbides precipitate at the δ/γ interfaces and within the δ-ferrite grains. Increasing the solution treatment temperature from 1050 to 1250 °C delays the σ-phase formation and favours the precipitation of intragranular secondary austenite γ2. The simulation of the σ-phase precipitation kinetic in the base metal, HAZ and weld metal, indicates a good agreement between the experimental fitted data and the modified Johnson–Mehl–Avrami model. The results indicate a marked sensitivity of the σ-phase precipitation kinetic to the solution treatment temperature. A high precipitation rate corresponds to a fine grained structure with ferrite enriched in σ forming elements (Cr, Mo).

Effect of solution treatment temperature on the precipitation kinetic of σ-phase in 2205 duplex stainless steel welds

Transition metal joints for steam generators—An overview

Microstructural characterization of dissimilar welds between Incoloy 800H and 321 Austenitic Stainless Steel

Microstructures of welds of AISI type 316 stainless steel have been examined with a view to establishing the changes that occur during postweld heat treatments in the temperature range 900 to 1100°C. The ferrite content of the weld zone has been estimated using a point count technique in the as-welded condition as well as after heat treatments. In addition, the compositional changes occurring in the ferrite phase have been determined using an electron microprobe analyzer. The ferrite content of the weld decreases continuously with an increase in the aging time. At the same time, the morphology of the ferrite changes from a dendritic network into a distribution of isolated near spherical particles. An analysis of the kinetics of the dissolution and the spheroidization of ferrite suggests that both these processes are controlled by interface diffusion; activation energies for these processes are 132 and 140.5 kJ/mol respectively. The influence of the heat treatment variables on the morphology and composition of the ferrite phase have been discussed.

The influence of post weld heat treatments on the structure, composition and the amount of ferrite in type 316 stainless steel welds

A transition metal joint between type 304 stainless steel and 2·25Cr–1Mo steel, with Alloy 800 as the transition piece, is being developed for application in the steam generator circuit of the 500 MW prototype fast breeder reactor. As part of this programme, the hot cracking susceptibility of Inconel 82/182 and of 16–8–2 welding consumables were compared and the microstructure and mechanical properties of butt welds between type 304 stainless steel and Alloy 800, welded by the two consumables, were studied to select the appropriate welding consumables for this joint. It is recommended that the 16–8–2 consumable should be used for welding this joint because of its lower microfissuring tendency and reduced mismatch in the coefficient of thermal expansion across the joint, although this would mean a slight adverse effect on the elevated temperature mechanical properties. Further, to select the optimum post-weld heat treatment (PWHT) of the joint between Alloy 800 and 2·25Cr–1Mo steel, welded with Inc...

V. Seetharaman

Papers

The influence of post weld heat treatments on the structure, composition and the amount of ferrite in type 316 stainless steel welds

Development of transition metal joint for steam generator circuit of prototype fast breeder reactor