August 26, 2014
Q: We manufacturer dual submerged arc welded (DSAW) pipe for API use. Some jobs require that the weld hardness be below 250 Vickers. We are having trouble meeting that requirement. What can we do?
A: Weld hardness restrictions generally are found in pipelines designated for sour service and are designed to prevent sulfide stress cracking (SSC). SSC is a failure of steel caused by stress and the simultaneous absorption of hydrogen resulting from corrosion by aqueous hydrogen sulfide. SSC can occur from transporting wet, sour products or from external contamination by sulfate-reducing bacteria found in water or soil around the pipe. Hard steels appear to be more susceptible than soft steels, and hardness limits have proven to be effective requirements for pipelines in sour service.
In pipe manufacture, spiral-seam and straight-seam welds are often produced with submerged arc welding. The maximum hardness in these [pipes usually is found in the heat-affected zone (HAZ) and is a function of the chemistry of the base metal and the thermal cycles induced by welding. Many people mistakenly believe that changing their consumables will alter the hardness in the HAZ, but in fact, HAZ hardness is most effectively reduced by preheating the steel to slow the postweld cooling rate.
While the maximum hardness is generally found in the HAZ, it also can be found in the weld metal. Hardness problems in the weld metal can be minimized by altering the consumables to avoid the use of active fluxes or decrease the alloy level in the wire. Active fluxes contribute elements to the weld metal that cause the chemistry to be richer, producing a higher hardness. Decreasing the alloy level in the wire gives a leaner composition that lowers hardness. Unfortunately, decreasing the alloy level in the weld metal also can cause a decrease in Charpy impact toughness, so this must be taken into consideration.
The maximum weld hardness often is found in the reheated zone of the first inside diameter (ID) weld pass. Using unalloyed (EM12K) wire for the ID pass and ½ molybdenum (EA2 or EA3K) wire for the outside diameter (OD) pass will reduce your ID pass hardness and ensure that the weld metal with the lowest hardness is closest to the product being transported through the pipe. This also keeps the alloy level in the weld high enough to maintain good Charpy toughness.
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