Which of the following practices can help prevent weld cracks?

Preheating the base metal is a vital practice in welding that helps to prevent weld cracks. When base metals are heated before welding, the thermal stress associated with the cooling of the weld bead is reduced. Preheating increases the overall temperature of the material, which can help in expanding different parts of the joint more evenly and minimizing the risk of rapid cooling and subsequent brittle fracture. This is particularly important for thicker materials or those that are high in carbon content, which are more prone to cracking.

In contrast, using a thicker electrode may not necessarily contribute to preventing cracks; it may instead lead to an uneven weld bead or increased heat concentration. Welding in cooler temperatures can increase the likelihood of weld cracking because the cooler environment can cause the weld metal to solidify too quickly and lead to thermal stresses. Using a lower current can sometimes help with controlling the heat input, but it does not directly address the thermal stresses that lead to cracking as effectively as preheating does.