Penetration phenomena of nuggets in resistance spot welding of high strength stainless steels for railway passenger cars has been investigated. The results obtained were as follows:
(1) Welding current (I
4.0) required to produce a nugget of 4mm in diameter decreased with increasing cold-rolling reduction of plates.
(2) For materials which had different cold-rolling reductions, welding currents (I
4.0) required to produce the same size of nugget were different, but the electrical resistance heats required were equal.
(3) For material which had a high cold-rolling reduction and a low welding current (I
4.0), the increase of electrical resistance during welding time compensated electrical resistance heat.
(4) The sheet to sheet contact resistance did not contribute to electrical resistance heat and the electrode to sheet contact resistance did not depend on cold-rolling reduction.
(5) The conductor resistance and the spreading resistance depended on the specific resistance of material. The specific resistance increased with increasing the cold-rolling reduction. The decrease of welding current (I
4.0) with increasing the cold-rolling reduction was due to the increase of the conductor resistance and the spreading resistance. The increase of the conductor resistance and the spreading resistance with increasing the specific resistance compensated the electrical resistance heat.
(6) The increase of specific resistance with increasing the cold-rolling reduction depended on the amount of deformation-induced martensite.
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