I have a situation in which a 1020 steel plate has been MIG welded to an 8620 bushing that was carburized. The weld area of the bushing was coated with a layer of copper plating to prevent carburization in the area of the MIG weld. If the copper is not removed prior to MIG welding is there a potential of hot shortness in the weld or other weld quality concern?
The best way to answer your question is "it depends". Copper cracking is a very real concern and will depend on the amount of copper that is in the resulting weld deposit. What is in the deposit depends on what the components of each of the items that are being melted – wire, base material (both sides, and with the coating). The thickness of the coating is a big variable and will weigh heavily on the final outcome.
To keep with a robust manufacturing solution it would be recommended to remove the coating prior to welding, thus removing it as a variable.
We are having cracking problems when welding SAE 4140 steel. Can you suggest filler metals and or welding practices that can prevent these problems?
The first step in making decisions about welding SAE 4140 is to know what you are going to do with the weldment after it is done, and the strength you need out of the connection to meet your service requirements. Your question does not give me much of an idea of the thickness, application or what you want to do with this weld after welding so it is impossible to give you a direct answer.
It is advisable to use the lowest strength filler metal that is known to be a low-hydrogen metal that meets the service requirements of the weldment. The filler metal should be selected either from a hermetically sealed container or from stock that has been stored properly. With most joining applications, you can use a good E7018 stick electrode. This would be good for applications in which you will use the weldment as-is or with a stress relief – 1150 deg F for 1 hour per inch of thickness – post-weld heat treatment.
If you are going to use a postweld solution, treat the weldment at 1600 to 1700 deg F for one hour per inch of thickness and quench, then follow with a tempering heat treatment and another quench. With a tempering heat treatment, the choice of filler metal adds an additional variable. Typical joining filler metals that are subjected to this heat treatment will lose 30 percent to 40 percent of their strength and will not return to their original strength through subsequent heat treatment. If you want to match or have properties similar to the parent metal after heat-treating, you must use a filler metal with more carbon and alloy content. Keep in mind that these filler metals are more crack sensitive and may require higher preheat and inter-pass temperatures.
The next keys to success are to prepare your parent materials properly and to use the appropriate pre-heat and inter-pass temperatures during the welding process. Both material preparation and appropriate preheat and inter-pass temperatures should be should be good, low-hydrogen practices.
Proper material preparation that is designed to minimize hydrogen means that the surface should be clean, dry and free of rust, mill scale, grease, oil and other any contaminants that could produce hydrocarbons.
Preheat and inter-pass temperatures will depend on the thickness and restraint of the weld you are making, but they should be in the range of 550 degrees to 650 degrees F for materials greater than 1/8 in. Allowing the weld to cool slowly also is important.
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This column is sponsored by Penton and the Lincoln Electric Co., Cleveland. Dave Barton is a senior welding engineer in the Application Engineering Group of The Lincoln Electric Co. He oversees welding procedure development for both new technology and existing products, performs failure analyses for customers, and serves as a consultant on welding application problems. Barton has been with Lincoln Electric for 21 years. Send your questions for Mr. Barton in care of WDF by e-mail to: firstname.lastname@example.org.