Fig.Robotic welding system requires very good fit-up
Welding designs
Welding designs assumes lot of importance in product manufacture.
Proper edge preparation based on material characteristics and thickness of the members requires special attention. Welding distortion study and control techniques, effect of residual stresses need to be attended to.http://www.aws.org
The designs to avoid cracking tendency are essential for critical structures, pressure vessels, submarines and rocket shells. Dynamically loaded members have to be designed for giving maximum life before failure. Normally wrong designs cause maximum damage though correct materials and processes are selected.http://www.asme.org
Design for welding is an important aspect in manufacturing.Welding practitioners should know the significance of each type of joint, edge preparations, welding symbols, pertaining codes and standards for product manufacture.
http://www.app.aws.org
Also it is necessary to understand residual stresses,distortion,and to minimise.
1.Welding symbols
Correct use of welding symbols and proper representation in fabrication drawings is essential for welding design engineers.http://www.aws.org
2.Design for welding
Apart from material selection and shape,the design of welds
should include the factors as below.
1.Method of preparation
2. Welding process,Position of welding
3.Welding consumables, auxiliary materials
4.Preheating, heat input
5.Weld build up,number of passes
6.Weld sequencing
7.Grooving of root
8.Post weld heat treatment
Guidelines for weld joint design
1.Minimum weld metal criteria.
2.Strength and service requirements based selection.
3.Lap and fillet joints may be used if lower fatigues resistance is acceptable.
4.Double V or double U or Single U in thick plates to minimise weld metal to reduce distortion.
5.Ready accessibility for welding.
Sub assemblies
1.Two or more sub assemblies can be welded simultaneously.
2.Better access for welding and possibility of automation.
3.Distortion in the finished assembly is easier to control.
4.Large sized welds may be deposited under less restraint.Residual stress can be minimised in final weld.
5.Facilitates machining to close tolerances in sub assemblies,and stress relieving of some sections before final assembly.
6.Testing of chamber components for leaks and painting before final assembly.
7.In process inspection and repair possible.
8.Lower handling costs.
Design of weld joints
Types
Butt,Lap,corner,T joint,Edge joint(Plates)
Butt joints:Groove welds ;single -square -groove, single -bevel -groove,single -V groove,single -V groove with backing,single V groove on surface,single J groove,single U groove,single flare bevel groove,single flare V groove.Double groove welds-square, Bevel,V,U,J,flare bevel,flare V.
Fillet joints:single,double.
Pipe joints:Circular,T,K,Y,Box,Combination,and other types.
Design considerations
Service performance ,Material weldability,Processes,Consumables,Access,NDT,Quality standards,Position of joint.
3.American Welding Society (AWS) Standard A5.1
The AWS Standard is less complicated, consisting of a prefix E followed by only two sets of digits:
E xxx
xx Exx10 Exx11 Exx12 Exx13 Exx14 Exx15
Electrode
Two or three digits indicating the tensile strength in 10000 psi units.
Two digits indicating the coating type and application as follows: Cellulosic, DC +ve. Deep penetration, all position
As Exx 10, but usable on AC
Rutile, AC/DC, flat and HV positions
Rutile, AC/DC, all positions
Iron powder rutile, high speed
Basic low-hydrogen, DC +ve, all positions
Exx16 Exx18 Exx20 Exx24 Exx27 Exx28
Basic low-hydrogen, AC/DC, all positions
As Exx16, with iron powder for improved recovery Mineral oxide/silicate, for flat and HV positions
Similar to Exx 12, but with iron powder, flat and HV As Exx20, but with iron powder
Low-hydrogen basic with 50% iron powder, flat and HV
A suffix (e.g. Al, B2) may be added to indicate the chemical composition of low-alloy-steel weld metal
4.Design of welds against brittle fracture
One of the principal reasons why the subject of brittle fracture occupies a key place in the design of steel fabrications is that the ferritic steels change their fracture behaviour with temperature, from being notch brittle at lower temperatures to being notch ductile at higher temperatures. What is more, the temperature at which this change takes place depends on the chemical composition and metallurgical structure of the steel.The basis of the approach to design and fabrication to prevent brittle fracture occurring then lies in appropriate material selection and welding procedure development.