Intro4u2u

4.2 Bend Tests
Bend tests apply strain to all parts of the welded joint, and are useful for the exploitation and detection of defects and embrittlement. When bend tests are specified for welder approval testing, the intention is to assess the ability of the welder to make sound joints. The bend test also reveals small cracks and embrittlement which arise from the impurities within the steel. The phenomena of burning or the related overheating, which occur close to the fusion line, are likely causes of bend test failure when impure steels are welded. The welder could be unnecessarily rejected due to bad material!
4.3 Impact Tests
The number of impact tests required and the test temperature depends on the standard being followed. The typical locations shown are intended to sample the toughness of all relevant zones at heat inputs around 3kJ/mm. At higher heat inputs, the HAZ will be wider and other locations will be relevant. Note that heat input is an example of an essential variable which cannot be increased without qualification testing when impact properties are specified.
When high heat inputs are used, loss of toughness is most likely in the CGHAZ of the steel. Here, close to the fusion boundary, almost all the carbide, nitride and sulphide particles are dissolved. Impurities (such as
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sulphur and phosphorus) involved in burning and overheating are particularly detrimental in coarse structures, but have been significantly reduced in modern, clean, steels. Soft, low strength ferrite around the prior austenite grain boundaries also reduces impact toughness and control of the transformation (to give a fine ferritic structure) becomes especially important at high heat inputs.
Some special titanium refined steels contain very stable nitride particles which restrict grain growth, and thereby reduce the grain size and width of the CGHAZ. Other low carbon special steels (developed along the lines of weld metals) contain oxide particles, which promote nucleation of fine and therefore tough intragranular ferrite (which is similar to the acicular ferrite of weld metals).
Strain ageing caused by welding, forming or thermal correction of distortion can also cause loss of toughness at almost any location.