Oxyfuel gas welding and cutting

1. Oxyfuel gas welding and cutting

2. Use of gas flame Welding Brazing Cutting Gouging Heating Straightening Blasting Spraying

3. OFW - Principle of operation

4. OFW fuel gasses Requirements for fuel gasses: high flame temperature high rate of flame propagation adequate heat content minimum chemical reaction of the flame with base and filler metals

5. Flame temp.with O2 Common fuel gasses acetylene (C2H2) propane (C3H8) methylacetylene-propadiene stabilised (MPS - C3H4) hydrogen 3087°c 2526°c 2927°c 2660°c

6. OFW flame Bright nucleus Outer envelope Inner cone 3100 Temperature (°C) 1000 300 Distance from the tip of the nozzle 2÷5 mm

7. Combustion reactions In the outer envelope - secondary burning 2CO + O2 2CO2 + Heat 2H2 + O2 2H2O + Heat In the bright nucleus: C2H2 2C + H2 + Heat In the inner cone - first burning stage: 2C + H2 + O2 2CO + H2 + Heat CO and H2 have reducing effect (no oxides are forming)

8. Flame types

9. Flame types Oxy acetylene flame Oxidising flame Neutral flame Carburising flame • welding of most materials (including cast iron, Cu, Mg) • brazing • cutting • excess O2 can combine with many metals to form hard, brittle oxides welding of bronze and brasses • reducing flame • welding of cast iron (slightly reducing!), Al, Ni and high C steels hardfacing with Cr and W carbide

10. OFW fluxes Why? to remove oxides that fail to flow from the welding zone to protect the weld pool from the atmosphere When? • when the oxides have a higher melting point than the parent metal • applied during preheating and welding • not required for carbon steels • required for Al and brazing operations How? • dry powder • paste or thick solution • preplaced coating on the welding rod

11. Acetylene cylinder (section) Oxyfuel gas welding equipment Low pressure valve Acetone Porous material Steel container Fusible safety plugs

12. Acetylene generators Lime Electric furnace smelting + 2H2O  C2H2 + Ca(OH)2 2CaC2 Coke 0,07-1,05 bar What is the acetylene pressure? <0,07 bar Low pressure Medium pressure Stationary type Acetylene generators Portable type Carbide-to-water type Intermittent contact type Water-on-carbide type

13. OFW torch Injector Oxygen valve Handle Oxygen connection Mixing tube Welding nozzle Union (mixer) nut Fuel gas valve Fuel gas connection Torch inset

14. OFW torch C2H2 in contact with Cu forms acetylides (explosive!) if the pressure of fuel gas > 14 kPa  a medium pressure type mixer is used if the pressure of fuel gas < 14 kPa  an injector type mixer is used Union (mixer) nut Mixing chamber Handle Oxygen valve Mixing nozzle/tube Injector Fuel gas valve

15. OFW torch

16. OFW torch

17. Regulators Fuel gas regulator Oxygen regulator Single stage • used when slight rise in delivery pressure from full to empty cylinder condition can be tolerated Regulator type Two stage • used when a constant delivery pressure from full to empty cylinder condition is required

18. Single stage regulator Gas from cylinder Flexible diaphragm Bonnet Secondary spring Adjusting screw Cylinder pressure gauge Working pressure gauge Primary spring Gas to torch Regulator body Capsule

19. Single stage regulator Adjusting screw Regulator body Primary spring Gas from cylinder Flexible diaphragm Gas to torch Secondary spring

20. Two stage regulator Regulator body Gas from cylinder Secondary spring Bonnet Cylinder pressure gage Adjusting screw Capsule Primary spring Primary spring Capsule Bonnet Flexible diaphragm Gas to torch Working pressure gage

21. Two stage regulator Adjusting screw Regulator body Primary spring second stage Gas to torch Adjustable second stage Gas from cylinder Flexible diaphragm Secondary springs Primary spring first stage Pre-set first stage

22. Flashback arrestors Flashback - recession of the flame into or back of the mixing chamber Built-in check valve stops reverse flow Built-in check valve Flashback flame quenched at the flashback barrier Flame barrier Normal flow Reverse flow Flashback

23. OFW parameters Type of parent/filler Thickness of plates to be welded Tip size (Diameter of hole) Joint preparation Welding technique Flame type Approximate gas pressure Filler rod diameter Approximate gas consumption

24. Weld quality OFW quality (left to right) - OK; torch too high; torch too low; travel speed too slow; travel speed too fast

25. Oxyfuel gas cutting process A jet of pure oxygen reacts with iron, that has been preheated to its ignition point, to produce the oxide Fe3O4 by exothermic reaction.This oxide is then blown through the material by the velocity of the oxygen stream Different types of fuel gases may be used for the pre-heating flame in oxy fuel gas cutting: i.e. acetylene, hydrogen, propane. etc By adding iron powder to the flame we are able to cut most metals - “Iron Powder Injection” The high intensity of heat and rapid cooling will cause hardening in low alloy and medium/high C steels  they are thus pre-heated to avoid the hardening effect

26. Oxyfuel gas cutting process

27. Oxyfuel gas cutting equipment The cutting torch Neutral cutting flame Neutral cutting flame with oxygen cutting stream

28. Cutting tip Torch head Tip nut Cutting oxygen Mixed preheat gases Universal pressure mixer Handle Cutting oxygen lever Preheat oxygen valve Preheat fuel gas valve Oxygen inlet Fuel gas inlet Oxyfuel gas cutting equipment Torch-mix cutting torch • fuel and oxygen for preheat flame are mixed within the torch • can be injector type (for pressure less than 14 kPa) or medium pressure type

29. Tip nut Tip mix Three tube design Preheat fuel gas Cutting oxygen Preheat oxygen Cutting oxygen lever Handle Preheat valves Oxyfuel gas cutting equipment Tip mix cutting torch • fuel and oxygen for preheat flames are mixed in the tip of the torch

30. Oxyfuel gas cutting related terms

31. Oxyfuel gas cutting quality Good cut - sharp top edge, fine and even drag lines, little oxide and a sharp bottom edge • Cut too slow - top edge is melted, deep groves in the lower portion, heavy scaling, rough bottom edge • Cut too fast - pronounced break in the drag line, irregular cut edge

32. Oxyfuel gas cutting quality Good cut - sharp top edge, fine and even drag lines, little oxide and a sharp bottom edge • Preheat flame too low - deep groves in the lower part of the cut face • Preheat flame too high - top edge is melted, irregular cut, excess of adherent dross

33. Oxyfuel gas cutting quality Good cut - sharp top edge, fine and even drag lines, little oxide and a sharp bottom edge • Nozzle is too high above the works - excessive melting of the top edge, much oxide • Irregular travel speed - uneven space between drag lines, irregular bottom with adherent oxide

34. Mechanised oxyfuel cutting can use portable carriages or gantry type machines  high productivity accurate cutting for complicate shapes

35. Mechanised oxyfuel cutting Cutting and bevelling head

36. OFW/C advantages/disadvantages Advantages: Disadvantages: 1) No need for power supply  portable 1) High skill factor 2) Wide HAZ 2) Versatile: preheat, brazing, surfacing, repair, straightening 3) Safety issues 4) Slow process 3) Low equipment cost 5) Limited range of consumables 4) Can cut carbon and low alloy steels 6) Not suitable for reactive & refractory metals 5) Good on thin materials

37. Special oxyfuel operations Gouging • Rivet cutting

38. Special oxyfuel operations Thin sheet cutting • Rivet washing