Class 5 Notes: Forge Welding

3.37 (Class 5) Review: • • • Metals have high surface energies o Share electrons from several levels below the atomic surface o Longer range distance of attraction Friction welding has high interfacial shears o For good friction welder need a good brake so that don’t break the weld when it stops o Inertial friction welding uses large flywheel o Extrudes contamination out of the joint Ultrasonic welding has low macroscopic displacements but only buries the contamination o A form of friction welding, essentially the same, except displacements much smaller Today: Question: What is a wetting angle? • If put a liquid on a surface, it is the contact angle of the liquid with the surface • Magic Sand, children’s toy Forge welding and other processes used for joining railroad rails • Forging is one of the oldest native processes • Still use it in different processes • Used to join railroad rails (diagram on the board) o If bolted, even tightly, get wear on the ends of the rail, more maintenance o Most track is now welded o Different types of rail welds Forge welding using oxyacetylene torches with hydraulic ram • Shear off the hot flash • 5-10 minutes to heat the rails up Flash Welding • Tractor trailer with electric generator • Bring rails together then pull them apart slightly to form arc • Slowly bring back together as arc melts, • Then ram squeezed them together Some do stick welding • Not a good process for rails o Mostly care about a good weld on the head • Rails for a large telescope or microwave radar o 1’ thick, 2-3’ wide • o Want to avoid the wear and bounce o Tolerance is very tight o Wanted to try welding o Using narrow groove arc welding Thermit Welding o See diagram on board o Fe2O3 +2Al Al2O3 + 2Fe + Heat o Raises to about 2000 degC o Start with a small blasting cap to start the chemical reaction o Molten iron flows down, makes a small casting o Get about 50% defective welds o Cheap, equipment on the order of $100 o See rounded, cone-shaped surface o Story: MIT prank, thermit welding trolley car wheels o Navy: Steel hull, Build aluminum superstructures so not top heavy Used explosive bonding to bond the plates together Get aluminum-steel transition Get aluminum in the presence of rust, start of a thermit reaction Aluminum will burn • British ship lost • Belknap disaster o Jet fuel ignited thermit reaction o Aluminum superstructure Explosive Bonding o After WW II o Take two plates, say one steel plate, with another stainless or nickel alloy plate on top, at an angle (see diagram) o Put explosive on top to drive top plate onto the other plate o Drives out contaminant o Can put a very thin layer of expensive plate on top of steel to make process vessel, etc. o Dupont did this after WW II, now out of this business o Firm out of Boulder, in a canyon, “Explosive Fabricators” o Can use with tube sheet (12” thick plate) heat exchanger o Explosive weld all the way down the length o Can do explosive welding in a vacuum so no noise o Not used much here anymore, rolling process has been worked out o Airbag o Sodium azide (sp?) explodes in approx 1ms o Do this inside a fairly heavy cylinder to reduce force of impact o Generate approx 200psi Question: Stresses in joined materials? o Railroad rails o Diurnal heating and cooling (day and night) o How to avoid these thermal stresses Expansion joint Curves in the track o Concrete o Pretension or posttension of steel reinforcing elements o Sometimes need to repair, even tear down building Question: Changing metallurgical properties of the rails? o High carbon steels o Used to be mostly hot-rolled rails, these don’t really degrade o Some heat-treated rails o Starting to use 1% Cr rails (not heat treated) o Some starting to be more sophisticated (like for the specialized telescope track described above) Question: Use joint for rails with more joining surface area? o Asymmetric loading kills this Full penetration joint o Back gouging o Need to grind or air arc gouging to burn out defects at the root