Interpass and preheat temperatures are used for steels containing high levels of which elements near maximum permitted by the steel specification?

• A. Carbon, manganese, chromium, nickel
• B. Copper, zinc
• C. Aluminum, magnesium
• D. Silicon, phosphorus

Answer: (A)

Preheat and interpass temperatures are used to control the cooling rate of a weld and the heat-affected zone to prevent cracking in steels that have high hardenability due to alloying elements. When a steel contains elevated levels of carbon along with manganese, chromium, and nickel (especially near the maximum permitted by the specification), the weld metal and HAZ can become very hard if cooled quickly. Heating the base metal before welding and keeping the surface warm between passes slows the solidification and diffusion processes, reduces the peak hardness, lowers residual stresses, and helps hydrogen to diffuse out of the weld area. All of this lowers the risk of hydrogen-induced and brittle cracking that is more likely in these high-alloy, high-carbon steels. The other element combinations don’t present the same cracking risk profile that necessitates preheat/interpass control in the same way.