Recently, we received a request to produce a bi-metal pipe configuration created from a bond of copper to stainless steel. This is a fairly exotic combination even for a company like ours that is used to dissimilar metal combinations. When a unique request comes our way, we follow a development process to prove the success.
The first step was to verify material chemistry and sizing. In this case, the copper pipe was made from Schedule 40 C12200 alloy in 9’ and 12’ lengths in a 3” diameter. The request was to bond couplers made from Schedule 40 304L stainless steel pipe in 6” lengths to each end of the copper.
Would this chemistry work for friction welding and would these sizes fit into our equipment? Yes and Yes.
Test standards were reviewed with the client, and it was agreed to use bend testing and tensile pull testing that showed the weld to be equal or greater than parent material strength. Our staff would conduct the destructive testing and send the test pieces along with several whole coupons for the client to analyze in their approval process.
Next, we set up a development weld phase using small sections of the material as “coupons”. The weld development follows a weld, then test, and repeat sequence. The testing includes cutting strips and bending them at the weld zone. When the weld zone is shown to exceed the strength of the parent material, that weld parameter is selected for use in making the coupons under AWS weld spec C6.2. In this image, you can see the weld coupon and bend test strip cut from it.
It is interesting to note that in the photo of the coupon, we can see that the copper gave up more material known as “weld flash” than the harder stainless steel did. This view shows flash removed from the outside diameter but remaining on the inside diameter. The finished pipe design called for removing both.
Once successful parameters were established and recorded, we implemented them using the computer controlled friction welding machine to make a series of test coupons. One of the coupons was designated for cutting into strips to use for tensile pull testing.
It was established in the prior step that our weld parameter passed the bend test. The tensile pull also proved our parameter to be successful. You can see in the tensile pull photo that the parent material broke outside the weld zone. Look for the copper still fused to the stainless on the right side.
At this point, we sent the destructive sections and test strips along with whole coupons to the client. They applied their own analysis to verify acceptance of the welding process.
The client confirmed their acceptance of the weld results shown in the coupons and test pieces, so we moved on to production of the final weldments. Once the stainless steel couplers were welded to each end of the 9’ and 12’ pipes, the assemblies were run through a lathe to remove the outside and inner weld flash.
In this photo, you can see the shiny band where the flash was removed. You can also see swirl marks in the gel applied for Ultrasonic Inspection. The handheld Ultrasonic scanner slides around on the lubricating gel. In this case, a diagonal shear beam is used to read through the pipe wall. We routinely apply ultrasonic inspection to verify that no contamination or voids flow from the material into the weld zone. With welds verified as matching development established parameters and free from internal defects, they were ready to ship.
Into the crates they go!
You might also find this resource on The Process of Friction Welding useful.
For conversation… What material combinations does your company encounter?