First, we embed an interesting video on Inconlel 8″ pipeline welding.
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One of our customers asked a few technical queries on interpass temperature of Inconel / Incoloy materials welding and fabrication. The alloys are as follows:
(I) SB444 UNS number N06625 (Nominal composition: 60Ni-22Cr-9Mo-3.5Cb)
INCONEL Alloy 625:
(ii) SB407 UNS number N08800 (Nominal composition: 33Ni-42Fe-21Cr) INCOLOY Alloy 800:
(iii) SB729 UNS number N08020 (Nominal composition: 35Ni-35Fe-20Cr-Cb) INCO Alloy 20:
(iv) SB423/SB705 UNS number N08825 (Nominal composition: 42Ni-21.5Cr-3Mo-2.3Cu) INCOLOY Alloy 825:
The technical queries on interpass temperature of Inconel / Incoloy welding are:
Question (1): During welding of these alloys, what is the maximum permissible interpass temperature against each of the above mentioned material?
We consulted our welding expert and could provide solutions to their problems as shown:
Control of interpass temperature is an important component of a weld procedure (WPS) but is not as critical for high nickel alloys as for carbon steel, low alloy and stainless
steels. Generally it is a recommended practice that control of interpass temperature is included as an integral part of the weld procedure and a maximum interpass temperature of 175 0C is used as a guideline in many cases. Sometimes, tight interpass temperature control from 500C to 650C is used to control the exposure to interpass temperature within the sensitization range to a minimum.
Welding engineers are aware of some cases where the temperature should be controlled at lower levels at about 950C, and applications requiring higher temperatures of about 2300C with acceptable performance. The qualification and execution of weld procedures on these materials must take into consideration all the above in addition to the base material, thickness, condition, final application of the component etc. It is normally the responsibility of the welding engineer within an organization, as he has access to all relevant information.
Question (2): Is there any reason for restriction of interpass temperature to such limit?
Answer: Even the stabilized alloys when held at intermediate temperatures long enough will suffer degradation. Control of interpass temperature during welding also minimizes the risk of grain growth in heat affected zone (HAZ), in the parent material. It can also reduce the formation of surface oxides on weldments that can be retained in the molten weld pool as inclusions. Such inclusions can result in NDT or premature component failure. Formations of surface oxides on underlying weld beads should be removed or cleaned otherwise they may add to molten weld pool inclusions.
Before we discuss further, we embed an interesting welding video.
(Welding video by high speed camera from youtube.com)
Our customer further enquired as shown:
Question (3): Since the specified four alloys are stabilized and not getting sensitized, why interpass temperature should be restricted to 1750C? Why it should not be higher, say 2500 C? Is there any other associated problem? Kindly explain.
We contacted our expert again and fond the solution as explained here:
The four alloys mentioned are among the non-stabilized ultra low carbon alloys that require very restrictive interpass temperatures. For example, C-276 has very low carbon and is
not stabilized such that its TTT diagram has a C curve that is a few minutes
away from zero time at about 7050C to 8350C. It is logical that with lower interpass temperature, the alloy will cool faster through the sensitizing temperature range and allow the alloy to
maintain its corrosion resistance. Alloys 625, alloy 20, and INCOLOY 825
are all stabilized and additionally, INCOLOY 825 is given a stabilizing anneal before it
leaves the mill.
However, INCOLOY 800 has a small addition of Ti, but it does not require stabilization against sensitization as it is normally not used in oxidizing acids, where the grain boundary sensitization can be damaging. Generally 1750C is used as interpass temperature for this alloy without damage.
The next question from our customer was as follows:
Question (4): For above four alloys, during welded pipe manufacturing by SAW process, how do you control interpass temperature? SAW is a continuous welding process generating high heat input. If you decide to wait after every pass till temperature is lowered, you have to wait for a long time due to poor cooling rate. This will affect productivity. Is there a way out?
Our welding experts opined as:
The mass of the pipe may normally be great enough to take care of the heat input of SAW and not present a large interpass temperature problem. If this type of problem is present, it would be proper to do the first welding pass to seal the joint, subsequently fit the pipe with end caps and flow water through the pipe as the balance of the joint is welded. For gas shielded processes, one should be careful that free convection currents in welding do not lift the shield gas and result in oxidation of the weld beads. Shrinkage stresses from the higher interpass temperatures can result in some distortion of the pipe, unless the pipe is restrained during welding and cooling.