4.09.4.1 Low-Alloy Steels

Low-alloy steels generally have good weldability with the main concern being liquation cracking near impurities (Figure 8), the propensity of some grades (esp. Cr-Mo steels and some pressure vessel grades) to reheat-type cracking (discussed earlier) and to hydrogen-induced cracking.1,91,92 The suscep­tibility to hydrogen-induced cracking is controlled by four major considerations:

1. The composition of the steel

2. The mobile hydrogen concentration

3. The stresses in the weldment

4. The thermal management of the weld

In general, the more hardenable the steel (i. e., the more easily martensite is formed), the more suscep­tible it is to cracking. Since hardenability generally increases with carbon content and alloying additions, several parameters have been developed to gauge susceptibility to hydrogen-induced cracking, includ­ing the carbon equivalent (Ceq) in eqn [2] and the Ito- Bessyo ‘cold cracking’ parameter (Pcm) (eqn [3]).93 The concentrations in eqns [2] and [3] are for weight percentage.

Cr + Mo + V Mn Ni + Cu

——— 5——— + — + — І5- и

V Mo

5 x B + C + To+15 +

Si Ni

+ 30 + 60

The hydrogen concentration in the weldment can be minimized by proper cleanliness, preheat, and post­weld heat treatment.92,94 Additionally, microstruc­tural hydrogen traps can provide significant benefit by preventing hydrogen redistribution to regions of high stress.37 As the mobile hydrogen concentration is most detrimental, significant work has gone into standards to accurately assess the amount of diffusible hydrogen in steels.37,92,94 Minimizing residual stres­ses and avoiding geometric stress concentrators (e. g., notches) in the weldment also impart resistance to hydrogen-induced cracking.

Thermal management of the weldment (e. g., pre­heating, interpass temperature, bead tempering, and postweld heat treatment) is also critical to the mitiga­tion of hydrogen-induced cracking. Preheating of components and interpass temperature control act to outgas hydrogen or hydrogen-bearing compounds (e. g., water) and lower the cooling rate. Additionally, careful control of heat input can produce hydrogen — resistant microstructures (i. e., bead tempering). Postweld heat treatment can act to lower residual stresses, produce beneficial hydrogen traps, and remove dissolved hydrogen from the weld.3,4,95,96 Several sources provide guidelines to mitigate hydro­gen cracking in specific grades of steel.95-97