Which steel can normally be welded with the fewest special precautions?
- Low-carbon (mild) steel
- Medium-carbon steel
- High-carbon tool steel
- Gray cast iron
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A. Low-carbon (mild) steel
Low-carbon steel does not harden much when it cools quickly, so in ordinary thicknesses it welds readily without preheat. Medium- and high-carbon steels and cast iron form hard, brittle structures next to the weld unless preheat and controlled cooling are used.
Reference: Welding metallurgy: weldability of carbon steels
What is the main reason for preheating a hardenable steel before welding?
- To slow the cooling rate of the weld and heat-affected zone
- To speed the cooling rate of the weld and heat-affected zone
- To burn the mill scale off so that the arc starts easily
- To raise the tensile strength of the finished weld metal
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A. To slow the cooling rate of the weld and heat-affected zone
Preheat reduces the temperature difference between the weld and the surrounding metal, so the joint cools more slowly. Slower cooling limits the formation of hard, brittle martensite and gives hydrogen time to escape. It is not a cleaning step.
Reference: Welding metallurgy: purpose of preheat
What is interpass temperature?
- The peak temperature of the molten puddle while a pass is being made
- The temperature of the shop air measured between welding shifts
- The furnace temperature held during stress relief after welding
- The temperature of the weld area just before the next pass is started
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D. The temperature of the weld area just before the next pass is started
Interpass temperature is measured on the weldment at the joint immediately before the next pass in a multipass weld. A minimum keeps the benefit of preheat; a maximum is often set so the joint does not get so hot that strength or toughness suffers.
Reference: Welding metallurgy: preheat and interpass temperature control