Arc Welding of Specific Steels and Cast Irons

Arc Welding of Heat-Resistant Low-Alloy Steel
High-temp.
high-pressure
boilers
Oil
industry
Synthetic
chemical
industry
High-temp
high-pressure
hydrotreating
industry
Heat resist.
high alloys
Heat resist.
high alloys
18-8
stainless
18-8
stainless
18-8
stainless
18-8
stainless
Service temperature (℃)
Carbon
steel
Carbon
steel
Carbon
steel
Carbon
steel
Fig. 1.1 — Various steels and alloys and their applications
as a function of service temperature
At normal atmospheric temperatures, gaseous molecular hydrogen does not readily
permeate steel, even at high pressures. At elevated temperatures, however, molecular
hydrogen dissociates into the atomic form, which can readily enter and diffuse through the
steel. Under this condition, the diffusion of hydrogen in the steel is more rapid. The diffused
hydrogen in the steel may react with the carbon in the steel to cause either surface
decarburization or internal decarburization and fissuring. This form of hydrogen damage is
referred to as the high-temperature hydrogen attack.
Desulfurization reactors, for example, are subject to pressurized hydrogen in operation at
high temperatures. The hydrogen partial pressure and service temperature are the factors
which determine the suitable type of low-alloy steel, as shown in Fig. 1.2. This figure, known
as “Nelson Curve,” suggests the limits of application of each type of steel for high-temperature,
high-pressure services. Temperatures and pressures above the limiting curve for the
individual steel can cause the hydrogen attack. The Nelson Curve clearly shows that higher
temperature and pressure require higher-alloyed steel in order to resist the hydrogen attack.
2-3