American Welding Society Forum
I need recommendations for welding 4140 round bar, 2inch diameter, to a 2 inch thick 4140 collar by GMAW.
Pre/Post heat, interpass temperatures, wire/gas recommendations?
This steel has a very high hardenability; its carbon equivalent is CE=0.95 (calculated from the formula CE=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15), then it has a high susceptibility of cold cracking. The selection of method to avoid hydrogen cracking depends of the required mechanical properties of your weldment (wich depends of the service conditions). I suggest you use the Isothermal Transformation Method (developed by The Welding Institute - TWI), wich is suitable for steels of high carbon content or steels with high hardenabilty like your case. With this method you will obtain a softer HAZ microestructure than martensite after welding. The method require a knowledge of the isothermal transformation characteristics of the steel wich are usually found in a diagram called TTT or CCT. However, there are other methods for avoid cold cracking obtaining the best toughness, when this is a requeriment.
Description of method: The object of method is control the cooling of your weldment keeping a holding temp. after welding so that the HAZ (austenitic yet) transform in an approximately isothermal manner and produces a softer microestructure than martensite (bainite generally); then, the holding temp. is selected, using your specific TTT (or CCT) diagram, over the Ms (martensite start) temperature at a level required to produce bainite. The minimun holding time after welding should be at least twice that indicated on the diagram for 100% transformation to bainite. The holding temp. must be used as preheat and interpass temp. during the application.
Summary of the method: apply a preheat temp. and start the weld keeping the minimun interpass temperature and then control the cooling at a holding temp. during a such time for ensure the total transformation to bainite and finally cool to room temp. The temperatures and holding times must be selected before the welding from the TTT diagram of the particular steel.
For increase the assurance that the selected procedure is safe, is a good practice to do a simulation test in wich you apply the welding conditions calculated over a real coupon and after the weld is finished you examine it with NDE (ultrasonic and/or radiografic testing) and, finally, you may do selective sectioning of the coupon in order to determine whether cracking has occurred.
If you have some difficulty with this metallurgical concepts you may contact me.
I hope this help you.
There's only one thing I can add to the brilliant explanation given by my friend Jorge Giraldo.
The Welding Institute is a British organization and their site, where you'll find plenty of information, is
Once in the site, follow the instrucions you'll find therein.
Giovanni S. Crisi
Sao Paulo - Brazil
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