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Materials processing and joining

In welding high-strength steel, preheating is used to prevent cold cracking (hydrogen cracking), which appears in the heat-affected zone and elsewhere within minutes to days after welding. Which statement correctly describes the effect of preheating?

  1. AIt slows the cooling of the weld so that hard martensite is less likely to form, and gives diffusible hydrogen time to escape
  2. BIt speeds solidification of the weld metal, preventing hot cracking from impurities gathering at the end of solidification
  3. CIt quenches the weld rapidly, hardening it to make it resistant to cracking
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Answer: A. It slows the cooling of the weld so that hard martensite is less likely to form, and gives diffusible hydrogen time to escape

Cold cracking occurs when three things coincide: diffusible hydrogen that has entered the weld metal, a hard, low-toughness microstructure (martensite), and tensile stress from restraint of the weld. Preheating lengthens the cooling time through the range of 800 to 500°C, suppressing hardening, and also lengthens the time held at high temperature, making it easier for hydrogen to diffuse out. Using low-hydrogen electrodes after drying them and postheating right after welding are countermeasures with the same purpose. Hot cracking (solidification cracking) is the tearing of liquid films enriched in segregated elements such as P and S at the final stage of solidification, and both its mechanism and its countermeasures differ. Rapid quenching to harden the weld actually makes cold cracking more likely.

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