309LNb

Consumables specifically designed for use where a niobium-stabilized weld metal is required for overlays or inserts on carbon-manganese (C-Mn) or low alloy steels. A minimum niobium content of 0.7% in undiluted weld metal ensures a fully stabilized deposit with a composition similar to grade 347 in the first layer on medium and low carbon steels. This material can be used for the initial pass when welding clad steels in 321 or 347, before completion with 347 type weld metal. It is not recommended as an alternative to type 309L for dissimilar weld joints.

A low carbon 309 type weld metal with a minimum niobium level of 0.7%.

In the as-welded condition the microstructure consists of austenite with a ferrite content of 8-20FN.

There are no comparable parent materials and it's normally used for overlays only.

Preheating varies according to the hardenability of the base material: it is not required for mild steels, but may reach up to 200 °C for hardenable steels with 0.4% carbon. With a typical dilution of 25-30% on a medium carbon steel, these materials can produce a fully austenitic weld deposit. It is known that niobium-containing weld metals are particularly prone to hot cracking when the ferrite amount is insufficient. Therefore, it is advisable to minimize dilution in the first layer of overlays by carefully controlling parameters and bead overlap, aiming for a 50% overlap. If post weld heat treatment (PWHT) is applied, embrittlement of the weld metal may occur, although ductility should remain acceptable after standard times and temperatures. However, fusion line edge embrittlement could be more significant, and acceptability must be established through representative procedural tests.