Welding-HAZ Problem

It appears that the sheet your working on belongs to the higher tensile strength grades which is usually the case such as pre-coated sheets. One has to incorporate pre-heating to 150⁰C before subjecting to resistance welding. Also Cooling after welding should be slow.

In order to minimize the size of the HAZ, you want to use as short a welding pulse as possible. 0.5 cycles if your machine can do it. High current for short time is the way to achieve a small HAZ. If 1/2 cycle is still too long, consider a capacitor discharge welder. We routinely weld .004 mm thick steel using 1/2 cycle.

The second recommendation is the way to handle this situation. Pulsing the weld cycle will help. I suggest that the amperage may require a higher amplitude for the weld itself.

What is the carbon content? What is the grade? If the weld is intact and the failures are indeed HAZ, then a post weld anneal or stress relief is usually the answer.

Not sure that weaken is the proper description, less ductile is probably the actual mecahism of separating during your coating process.

Probable mechanism is untempered martensite in HAZ due to air quenching.

milo

HAZ weaken its properties, make it tear off during coating process.

Does it mean the HAZ area is off around the weld area (or) cracking but the weld nugget / weld is intact..

If so the reasons could be plenty.

1. It could due to excessive welding current selected for the thickness of the metal used. It causes excessive indentation or nugget penetration or burning of the metal in and around welded and resulting of metal shearing.
2. it could be more hold time, i.e., time after welding current is off. The faying surfaces will retain heat for a longer time and result in burning of metal where the it joined together.
3. It could be due to excessive pressure (electrode force) used for holding the sheets which affects contact resistance. More the force moderate should be the current. Other wise it will burn the metal.
4. The max. Penetration in the nugget should be between 40 to 50 % as optimum or 70 % of the thickness of the sheet. Current excess to it will increase nugget penetration and do not add to joint strength.
5. If the dia. of the electrode is less for the current selected, the nugget size gets reduced and increasing its penetration.
6. Generally 0.20 mm size thick sheet requires 16 mm electrode size with 8 mm size dia. at the tip and app.14,000 amps and can provide 7 mm size nugget with 1.0 to 1.2 mm penetration.
7. A proper welding schedule has to be prepared controlling squeeze time, welding current, electrode force, hold and off time.
8. Any variation in above will have considerable influence on weld quality.
9. All above are guide lines and fixed only by trial & error method. Thre is no single formula in Resistance Spot Welding.
10. In general there can not be any problem with sheet including its chemistry .

C Sridhar, Advance Institute of Welding Technology, Chennai, India.

Dear sir,

I think it can be possible through our capacitor discharge welding machine.if you need more details pls. contact me on shanmuganand_indian@yahoo.co.in and artec1@vsnl.com

thank you.

Please provide chemical composition (check analysis) of the CRC plate to give appropriate suggestions. AWS