Weldability
Weldability is an essential part of welding technology. Process , material and service weldability need to be established for any given application. Welding metallurgy pertaining to metals and alloys requires understanding. Metallurgical and mechanical behavior analysis forms the basis for performance evaluation of any engineering structure or component. Defects , sizing and acceptance criteria requires using of appropriate NDT http://www.ndt.orgmethods. Fracture toughness studies to make fail safe designs are now mandatory for nuclear and defence applications. Cold cracking and hot cracking behavior , low and high temperature behavior of welded joints requires special attention.
The effect of gases (Hydrogen , oxygen and nitrogen) on weld metal properties has to be analyzed critically for evaluating service behavior.http://www.twi-global.com
Welding consumables selection plays a very important role:http://www.weldability-sif.com
Thermal cycle during welding influences width of HAZ.There are
some important processes which give narrow HAZ.As heat input
controls the micro structure in HAZ, low heat input Process selection or techniques play an important role.http://www.worldwidescience.org
Weld simulators are used by specialists to find metallurgical behaviour and to aid in Weld parameter settings.
Weldability can be established by conducting most relevant tests.
There are various tests for hot cracking, cold cracking which reveal susceptibility of metals under certain critical conditions.
Preheat and post heating
Pre- and post-heating help to avoid defects such as cracking. These techniques are recommended – and indeed specified – by many standards and codes of practice. Pre-heating is usually applied immediately prior to cutting while both pre- and post-heating are used in welding.
Successful pre-heating is achieved through precise temperature control and uniform heat distribution across the thickness of the material. The temperature depends on the type and thickness of the material, and the subsequent process. Correct measurement is essential and requires special care.
Weldability of various steels requires use of correct temperature(deg c)preheat,inter pass temperature and post weld heat treatment.Below are the guide lines for various steels.
Type of steels. Preheat. Inter pass Post weld HT
1.C-Mn steels. 100-250. ———. ———-
2.Med.carbon st. 250-350. ————- ————
3.HSLA. steel. 250-300. ————- ————-
4.C-Mo steel. 150-300. ————- 650-700
5.Medium carbon
low alloy steel. 200-350. 200-350. 550-600
6.Cr-Mo steels. 250-300. ————— N 870-980
T 680-770
A 840-900
SR 700-730
7.Cryogenic steels
9Ni,5Ni. -——— ———— ————-
8.Martensitic stainless
steels,410,420 250 250-350 700-750
9.Austenitic SS ———- ————- ————
Factors for dissimilar metals welding:
1.Difference in physical and mechanical properties
2.Inter metallic compounds formation
3.Dilution
4.Cracking
5.Testing methods
6.Service conditions and life prediction
Weldability tests http://www.onlinelibrary.Wiley.com
1.Cold cracking susceptability
2.Hot cracking susceptability
There are many tests which quantify cracking tendency.There are some good references which describe the tests in detail.
The test results will be very useful to develop welding procedures for preventing weld failures in service.
Tests for cold cracking
1. Modified implant test
This test is very useful to find the influence of stress Hydrogen,and HAZ microstructure on embrittlement.Since helical notch is provided to locate the notch in HAZ,this test provides less scatter in results and embrittlement index can be determined.
2.CTS test
3.Y groove test
Hot cracking test
Hot cracking susceptibility can be determined by Vare strain test.
The weld is subjected to strain during weld solidification.Cracks developed are quantified with respect to strain to find the influence of composition on cracking tendency.
There are other tests to determine cracking tendency
1.Ductility dip cracking
2.Reheat cracking
3.Strain age cracking
4.Liquation cracking
5.Lamellar tearing
Carbon equivalent(CE) which is given by
%C + Mn/6 + (Cr+Mo+V)/5 + ( Ni +Cu)/15 provides a measure of weldability of steels
Electrode coating and need of control of moisture
The absorption of excessive moisture by the coating, either from the atmosphere, condensation, or from other sources, can cause:
• weld metal porosity
• excessive spatter
• arc instability
• poor weld contour
• undercut
• difficulty with slag removal
• blistering of the flux coating, especially with cellulose types
• increased risk of lamellar tearing
• increased risk of hydrogen induced cracking.
Weldability of stainless steels
Austenitic stainless steels are widely used and hence require attention regarding
weldability.AISI 316,316L,347,321,304.Hot cracking problem can be solved by selecting welding consumables with FN(Ferrite number) between 2 and 12.Austenitic stainless steels require less heat input compared to carbon steels.
Distortion is a problem due to low thermal conductivity ,high coefficient of thermal expansion and can be controlled by fixturing, back step welding, and intermittent welding.
Sensitisation in HAZ is another major problem due to depletion of chromium as chromium carbide forms at grain boundary.Stabilised grades,Low carbon grades
to be used to avoid this problem and improve corrosion resistance.
Welding duplex stainless steels:
These steels are to be welded with low heat input and austenite and ferrite to be balanced.Cooling rates to be controlled for HAZ microstructure to be balanced.
Pre heating and post heating need not be done.Matching fillers only to be added to the weld pool.Autogenous welds are not recommended.
Welding PH stainless steels
Cracking tendency to be understood and welding procedure to be developed for martensitic and austenitic grades.Welding to be carried out in solution annealed condition.Precipitation hardening to be done after welding to get required mechanical properties.Only matching fillers to be used.