Jan Henrik Sällström
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Metals exposed to hydrogen can after some time suffer from hydrogen embrittlement, which will result in a significant reduction of strength in the material. The step loading technique in ASTM F1624 allows that a test for evaluating a material´s susceptibility to hydrogen embrittlement can be performed in a couple of days instead of several months.
Hydrogen embrittlement is a problem that affects metals. This means that a metal subjected to hydrogen risks absorbing the hydrogen. If the metal is loaded, cracks can slowly start to grow and cause failure with a brittle type of fracture – for that reason the name hydrogen embrittlement. The hydrogen can come from the surrounding environment or from surface treatments. The susceptibility to hydrogen embrittlement of a metal depends on the alloy composition. Generally, high strength steels are more susceptible than low strength steels.
A considerable problem involving hydrogen embrittlement is that it can appear first after long time in service, which leads to large costs and difficulties. To ensure that selected materials will not suffer from hydrogen embrittlement, testing might be required. Testing methods taking several months will cause problems in the design process.
Test specimens with a notch are loaded in a test rig for four-point bending in a tensile testing machine. The bending moment is then constant over the notch between the two inner supports. Initially, the nominal ultimate strength stress is determined for at least three test specimens without being affected by hydrogen. To test with hydrogen, test specimens are continuously charged with hydrogen by use of electrolysis. This is accomplished by attaching a cylinder over the notch, so an open container is formed holding the electrolyte.
The loading is increased stepwise until fracture occurs. Each step has a duration between 1 to 4 hours (depending on steel hardness), to allow for the hydrogen to affect the material. The step load levels are at first determined from non-hydrogen charged tests and thereafter from the preceding hydrogen charged test. This increases the accuracy of the step loading, and after only three tests a good estimation of the fracture stress in a hydrogen environment is achieved.
Ten specimens are enough to perform the testing. The size of the specimens can be adjusted somewhat, with the length around 300 mm, the width 50 mm and the thickness 10 mm.
The results are delivered in a report including a description of the method.
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Resistance against hydrogen embrittlement with the step loading technique - ASTM F1624
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