Care should be taken not to overheat the metals to the degree that they become softened or metallurgically altered. Often, simply directing the arc toward the higher conductivity base metal can solve problems brought about by differences in base-metal conductivities.
In the case of steels welded to copper-base alloys, it is usually the copper alloy that should receive most of the heat because the copper alloys' conductivities can be several times higher than steel's. On the other hand, some copper alloys, like copper-nickels, have thermal conductivities that are as low as those of highly alloyed or stainless steels. Preheating may not be necessary when joining such combinations of metals.
Coefficient of Thermal Expansion.
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When there are large differences in thermal expansion between two base metals and between base metals and the filler metal high stresses can arise during and after welding, and during service. The stresses can be high enough to cause cracking. On cooling from the welding process, other factors being equal, the metal with the higher coefficient of thermal expansion will shrink more than the metal with the lower coefficient.
Since the metals are rigidly bound together, this differential shrinking induces tensile stresses in the metal with the higher coefficient of thermal expansion and compressive stresses in the other. Again, tensile stresses are more damaging than compressive stresses because tensile stresses lead to cracking. The magnitude of the thermal stresses is determined by the geometry of the assembly and by the difference in thermal expansion coefficients of the dissimilar metals. The higher the difference the coefficients, the higher the stresses.
It is easily possible for these thermal stresses to exceed the tensile strength of one of the metals involved including the filler metal , and cause cracking. Cracking sometimes occurs during or immediately after welding. This is called hot cracking or hot tearing, and it results from the normal weakness of metals at high temperatures. Cracking can also occur at room temperature if the stresses are not relieved. Cracking of this type is accelerated when the metals are cycled between low and high temperatures during service.
There are two accepted methods to avoid cracking caused by differences in thermal expansion. The first method is to select a filler metal with a thermal expansion coefficient that is intermediate between those of the two base metals.
To use our previous example, the coefficient of thermal expansion of Type stainless steel is 9. The difference in this case is not large and the risk of cracking from this cause is low. On the other hand, if we had to weld a low alloy steels with a coefficient of thermal expansion as low as 6. The second "fix" is to apply preheat and, if necessary, post-heat in order to reduce the level of thermal stresses during and after welding.
The use of heat in this manner can only provide partial relief of residual stresses since the choice of temperature will be a compromise between those needed to anneal both base metals. It is important not to overheat one base metal in an attempt to fully anneal the other.
Out-of-position welds are difficult because of the filler metals' high fluidity; however Wisconsin Wire Works welding experts can provide you with assistance if necessary. They've been welding for over three years and has a degree in Welding Technology. They love TIG welding and primarily do TIG welding on stainless and mild steel at work and on projects, even when it's not the faster option!
You can reach Cory through the Hack Factory metal shop email for questions. Add to Calendar. View Map View Map. Find out more about how your privacy is protected.
Nov Sold Out. Event description. Description Who is this Class For? What to Expect This class is now five hours long to give everyone as much time with each type of metal. Physical Space The floors are concrete going to be harder on joints and there is seating available. There will be earplugs as well About the Instructor Cory Gabrielle works as a welder at a small production and fabrication welding shop in Seward. Cory Gabrielle metalshop tcmaker. Read more Read less.
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Welding Dissimilar Metals
If you still have questions, Please call or email us, and we will be happy to help! Let AXIS prepare your procedure test assemblies.
Please forward us your Welding Data Sheets, Code reference, and if available, CWB correspondence letter, outlining testing requirements. Phone: Fax: E-mail: info axisinspection. Let AXIS help make your welding as costs as efficient as possible. Option 3 — Fillet Weld Test.