Hydrogen is not ideal for metal
In welding, hydrogen is generated from dissociating water vapour or hydrocarbons in the welding arc. At or near their melting temperatures, metals such as steel and aluminium diffuse hydrogen at a very high rate. Therefore, the molten weld metal can rapidly pick up hydrogen from the hot gas in the arc. Once in the weld metal, hydrogen atoms can diffuse swiftly into the base metal’s heat-affected zone (HAZ) as diffusible hydrogen because their diameter is much smaller than the lattice size of the metals.
Metals disperse diffusible hydrogen during cooling and phase transformation, at which it settles at microstructural dislocations and voids in the matrix. These atoms will combine to form diatomic or molecular hydrogen in the voids and the energy released causes pressure to increase. This induces localized tensile stresses that add to residual tensile stresses.
Case study:
pROBLEM
Diffusible hydrogen contributes to hydrogen cracking (also known as underbead, cold, or delayed cracking). Hydrogen cracking occurs when welding carbon and low-alloy steel. The probability of hydrogen cracking in the weld metal and HAZ depends on their composition, diffusible hydrogen content, and stress level. It generally occurs at a temperature below 150°C immediately upon cooling or after several hours with the combined presence of susceptible microstructure, high amounts of diffusible hydrogen, and high tensile stress.
Solution
1) To determine the net hydrogen contents produced by the electrode’s flux
To determine the net hydrogen contents produced by the electrode’s flux, this test first requires the electrodes to be used to produce welds of pre-determined length under laboratory-controlled temperature and humidity. The welds are carefully kept at sub-zero temperatures during post-welding processing to prevent loss of hydrogen via diffusion up until they are loaded into a gas chromatography chamber. The coupons are heated to a pre-determined temperature and held for a fixed duration to release the diffusible hydrogen gas within the weld. The hydrogen gas collected can be separated from the equipment’s inert carrier gas and quantified by the molecular weight difference.
2) To determine diffusible hydrogen content via Gas Chromatography
The determination of diffusible hydrogen content via Gas Chromatography is primarily used as a characterization test for electrode manufacturers to certify a batch of electrodes as being low hydrogen contents according to AWS electrode specifications. Still, it can be specified as a more realistic batch verification test for end users to determine whether a stored batch of the electrode is still fit for purpose as part of the quality control or troubleshooting process.
Application of Hydrogen Embrittlement Testing via Gas Chromatography:
AWS A4.3
BS EN ISO 3690
Common Test Method
To certify a batch of electrodes as being low hydrogen contents
Uses hydrogen analyzing only equipment
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Our state-of-the-art hydrogen embrittlement testing lab can conduct HISC testing on nickel alloys and duplex stainless steels to investigate how hydrogen influences materials’ behaviour under mechanical stress.
For more information about how we perform Hydrogen Embrittlement (HE) Testing, call us or send us an enquiry today.
