Copyright Goodheart-Willcox Co., Inc. Chapter 8 Manual Welding Techniques 147 Craters. Formed at the end of a weld bead due to a lack of weld metal fi ll or weld shrinkage. How to correct: • Do not stop welding at end of joint (use tabs). • Fill craters to the proper crown height and reduce amperage slowly. Cracks. Caused by cooling stresses in the weld and/or base metal. How to correct: • Use wire with lower tensile strength or different chemistry. • Increase joint preheat to slow the weld cooling rate. • Allow joint to expand and contract during heating and cooling. Increase size of the weld. Porosity. Caused by entrapped gas that did not have enough time to rise through the melt to the surface. How to correct: • Remove all heavy rust, paint, oil, or scale on joint before welding. • Remove oxide fi lm from previous passes or layers of weld. • Protect welding area from wind. • Remove all grease and oil from the fi ller metal. Handle fi ller metal with clean gloves. • Check gas supply for fl ow rate, possible contamination, loose connections, dirty torch nozzle. Linear porosity. Forms in a line along the root of the weld at the center of the joint where penetration is very shallow. How to correct: • Make sure root faces are clean. • Increase current. • Decrease arc length. • Increase amperage. • Decrease travel speed.