Consumables Corner: Preheat and interpass temperature defined, Part II

Q: I have seen many questions and discussions about preheat and interpass temperatures lately. It appears there is confusion about how and when to measure both of these values. Can you expand on these welding terms?

A: In Part I, we defined preheat and interpass temperature, discussed appropriate measurement locations, and briefly discussed the effects of preheat and interpass temperatures. We will discuss the reasons for preheat and interpass control and discuss their effects in greater depth in this installment.

A common misconception in the welding industry is that the primary purpose of preheat is to remove any water from the surface of the steel to be welded. This is incorrect. The primary purpose of preheat is to slow the cooling rate of the weld zone after the arc has passed. Removing water from the steel's surface is a secondary benefit.

Slowing the cooling rate of the weld zone is necessary when the steel being welded has a composition that allows for martensite—which is a hard, brittle microstructure—to form during rapid cooling. Martensite formation needs to be avoided as it is prone to cold cracking and sudden, brittle failure under loading.

A rule of thumb for plain carbon steels is carbon content of 0.30% or more requires preheat and interpass control. Preheat/interpass temperatures must be increased for steels with higher carbon content and thicker sections. For low-alloy steels, a carbon equivalent may be calculated to determine the need for preheat (see AWS D1.1 Annex B).

While proper preheat and interpass control provides benefits, excessive preheat and interpass temperatures can cause problems. High preheat/interpass temperatures—especially when welds are made with high heat input—can lead to excessive grain growth in the heat-affected zone (HAZ), which can reduce impact toughness significantly. Controlling the maximum interpass temperature prevents excessive toughness degradation in the HAZ.

Quench and tempered steels get their mechanical properties through a process of intentional rapid cooling (quenching) to create martensite, followed by heating to a specified temperature (tempering) to transform the martensite into a microstructure that is less brittle but still strong. Welding quench and tempered steels using excessively high preheat and interpass temperatures can result in undesirable softening of the HAZ. Steel manufacturers provide preheat/interpass and heat input recommendations for some quench and tempered steels. These recommendations should be followed.