The Process of Welding a Steel Pipe into a Stainless Steel Pipe
Stainless steel | Wednesday, 13 September 2017
People who work in the welding industry prefer using base materials that are of chemical composition and have the same mechanical properties. However, because of economic restrictions, it becomes necessary to weld two dissimilar metals or alloys. In a fabrication industry, it is not uncommon to see welding steel pipe with a stainless steel pipe. There are two very important factors that determine the success of welding dissimilar materials. The first important factor is the strict adherence to the designated welding procedures. The slightest of deviations can lead to some major problems. The second factor that people need to bear in mind is the selection of filler materials. Once again, a minor mistake in the selection can prove to be quite costly later on.
The Service Temperature
The choice of the filler material depends heavily on the service temperature. When it comes to high temperature applications, the difference in the thermal expansion coefficients of the filler and the base material could lead to disaster in the form of fatigue cracking. The mechanical properties and the corrosion resistance of the weld joint would be severely compromised under the current circumstances. This entire mishap can be attributed brittle and inter-metallic phase that has a very high level of hardiness (this is also commonly referred to as the sigma phase in the fabrication industry).
Three Strategies Used to Avoid Fatigue Cracking
There are mainly three strategies that are adopted to avoid fatigue cracking. These are:
a) The restrained joints are restricted because they cause high stress
b) In the case that the joints are highly restrained, usage of medium heat and preheating works wonders in prolonging the cooling time
c) The weld joints are immediately insulated without any delay right after the completion of the last pass. This results in slow cooking and drastically lowers the chances of thermal cracking.
In certain fabrication sites, the use of nickel based filler is also taken into consideration. The reason for that is because the nickel based filler produces weld joints that have lower thermal expansion coefficients than stainless steel. Therefore, the risk of fatigue cracking is significantly lower.
Ensuring Protection against Corrosion
One of the things that the people involved in fabrication are always careful about is protecting the base materials and the weld joints from corrosion. If the weld joints are left unprotected, then there is a very good probability that corrosion will take place. When this problem persists for an extended period of time, the mechanical properties of the weld joints are altered. Therefore, it is standard practice in the industry to sheathe the steel pipes with protective coating or paint. The stainless steel pipes on the other hand are kept unprotected because they are corrosion resistant. As far as the weld beads are concerned, they require protection as well.
Welding, as you can clearly see is a challenging job that requires detailed planning and flawless execution. Hence, the lack of expertise or experience in this field could be the difference between unacceptable and picture perfect fabrication.
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