Porosity of common process defects in welded joints

Update:05 Mar 2021

Porosity refers to the cavity formed when the bubbles i […]

Porosity refers to the cavity formed when the bubbles in the molten pool fail to escape during solidification and remain. According to the different places where the pores are generated, they can be divided into internal pores and external pores; according to the distribution, they can be divided into single pores, ridged pores and dense pores; according to the different causes and conditions of the pores, their shapes are spherical, oval, Whirlpool and caterpillar.

1. Cause
The gas that forms the pores mainly comes from:
(1) Atmosphere: If the air humidity is too high, over 90%, moisture will decompose, hydrogen and oxygen will invade; arc closing is too fast and the protection is not good, and N2 gas in the air will invade; the arc is too long and N2 gas in the air will invade.
(2) Gas dissolved in the base material, welding wire and steel core of the welding rod, water and gas in the coating and flux: the drying temperature of the welding rod is too low, the holding time is too short; the welding rod expires and fails; the purity of argon is not enough, and the protection is poor ; The electrode drying temperature is too high, the coating composition is deteriorated, the protective effect is lost, the current is too large, the coating becomes red and loses the protective effect, the N2 gas in the air invades; the welding core is corroded, the welding wire is not cleaned, and the flux is mixed Dirt.
(3) Oil, rust, water, paint and other dirt on the welding material and base metal will decompose to produce gas.
(4) Porosity caused by operation: the speed of transporting the strip is too fast, and the bubbles are too late to escape; the welding wire is not uniformly filled and air invades; during submerged arc welding, the arc voltage is too high, the network voltage fluctuates too much, and the air invades.

2. Preventive measures
(1) Strictly control the drying temperature and holding time of the electrode.
(2) Do not use expired welding consumables, and use shielding gas (argon, etc.) that meets the requirements of the standard.
(3) Thoroughly clean the oil, rust, water, paint and other dirt on the groove and welding wire.
(4) The arc length should be appropriate to prevent the intrusion of N2 gas. In particular, short arcs should be used for alkaline electrodes.
(5) Do a good job of joints and arc closing: Fully preheat the joints and establish the first molten pool to eliminate the gas at the last arc closing; the arc closing should be slow, fill the arc crater, and use the "reflow method" Etc., so that the gas and slag can fully protect the molten pool to prevent N2 gas from intruding; the joints of each layer of the multi-layer and multi-pass welding should be staggered to prevent the pores from being dense (up and down).
(6) Appropriately increase the heat input and reduce the welding speed to facilitate the escape of bubbles.